Theory And Instrumentation Of Gc Detectors
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Theory and Instrumentation of GC GC Detectors
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Wherever you see this symbol, it is important to access the on-line course as there is interactive material that cannot be fully shown in this reference manual.
Aims and Objectives Aims • • •
• • •
To highlight the various detector types available for GC To describe the performance characteristics associated with a range of common detectors for GC To explain the working principles behind the following detectors: o Flame Ionisation o Electron Capture o Nitrogen Phosphorous o Thermal Conductivity o Flame Photometric To describe the optimisation process for each of the detector types and typical uses To illustrate the various performance characteristics of each of the detector types To highlight the use of various other detector types and give brief descriptions of them
Objectives
At the end of this Section you should be able to: • • • •
Explain the parameters and performance measures by which GC detectors are characterised Describe the components and working principle of a number of common GC detectors Give suggestions of how each detector type might be optimised and demonstrate an understanding of each detector type in a practical context Choose the correct detector type for a variety of analyte and application types
Content Overview of GC Detectors Concentration versus Mass Flow Selective versus Universal Destructive versus Nondestructive Characteristics of GC Detectors Noise Signal to Noise Ratio The Flame Ionisation Detector Overview Operating and Optimising Uses and Performance The Nitrogen Phosphorous Detector (NPD) Overview Operating and Optimising Use and Performance The Electron Capture Detector (ECD) Overview Operating and Optimising Uses and Performance The Thermal Conductivity Detector (TCD) Overview Operating and Optimising Uses and Performance Other GC Detectors Flame Photometric Detector (FPD) Photoionisation Detector (PID) Electrolytic Conductivity Detector (ELCD) Mass Spectrometer (MS)
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Overview of GC Detectors There have been over 60 detectors designed for GC applications since the inception of the technique. Probably 10-12 detectors are used commonly in routine GC applications and of these only 3-4 are used in non-specific applications. All GC detectors monitor a physical property of the analyte –the property chosen must significantly change, and therefore cause a large detector response, when the eluent gas flowing into the detector is ‘contaminated’ by the analyte. Several properties have been used in GC detectors and these are outlined in the next table. Detectors that exhibit an enhanced response to certain analyte types are known as ‘selective detectors’. Table 1. Selected GC detectors Detector type Detector (Property) Ionisation Flame Ionisation Detector (FID) Thermal Conductivity Detector (TCD) Electron Capture Detector Nitrogen Phosphorous Detector (NPD) Photoionisation Detector (PID) Emission Flame Photometric Detector (FPD) Plasma Atomic Emission (AED) Electrochemical Hall Electrolytic Conductivity (HECD) Others Chemiluminescent Mass Spectrometer (MSD) Fourier Transform Infra-red (FTIR)
Selective No No Halogens N, P, Halogens Aromatics S, P Metal, Halogen, C, O S, N, Halogen S Yes Yes
There are three important classifications of GC detector and these are: Concentration versus Mass Flow Concentration sensitive detectors respond to the concentration of the analyte in the detector flow cell / chamber. If the flow to the detector were interrupted the signal (peak) height would not decrease, however the peak area would change. Mass Flow sensitive detectors respond to the amount of analyte in the detector at any time, irrespective of the volume of carrier gas. If the carrier flow is reduced –the detector response (peak height) will decrease but the peak area will remain constant. The diagram below shows the effect of altering carrier flow on the two detector types:
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Therefore, when using GC temperature programming and operating in constant pressure mode Concentration Sensitive Detectors (TCD, ECD) will have a variable peak area response as the flow rate changes, but Mass Flow sensitive detectors (FID, NPD) will show a constant peak area response as the flow rate changes. In reality, the use of constant flow with temperature programs is recommended to ensure constant peak widths. Selective versus Universal This classification describes the number of analytes that a detector will respond to. A Universal detector will theoretically respond to all solutes, whilst Selective detectors respond to specific types or classes of compound. Universal detectors are good for screening unknown mixtures and for situations where it is important to ensure all solutes are detected and quantified. Selective detectors usually show enhanced sensitivity to particular compounds and are able to ‘simplify’ complex chromatograms by only detecting certain classes of compound.
MS in Total Ion mode –universal detector
MS in Selected Ion mode –responds to only compounds producing certain fragment ions. The chromatogram is simplified compared to the top example due to the specificity of the detection mode. Destructive versus Nondestructive Nondestructive detectors are required where the separated intact analyte components need to be recovered for further analysis or characterisation. This can be accomplished using destructive detectors (where the analyte is destroyed) by splitting the carrier effluent stream just prior to the detector entrance.
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Characteristics of GC Detectors There are several important detector characteristics that apply to all Gas Chromatography detectors and help to describe the detector performance. The important characteristics of GC detectors include: • • •
Sensitivity (the signal output per unit concentration or mass of analyte) Minimum Detectability (usually measured by signal to noise ratio) Linearity (the range of concentrations over which reliable quantitative measurements can be made)
These characteristics will be explored further using some of the terms outlined in the brief definitions above. Noise Noise is the background signal produced by the detector in the absence of analyte. It arises from electronic noise, stray environmental signals and contamination or leaks. Good design and shielding can help to reduce the inherent noise. Noise is a rapid random variation in output, whereas drift is a slow systematic change in detector output. Signal to Noise Ratio The ratio of the detector signal to the inherent background noise is a useful measure of detector performance to convey information about the lower limit of detection. Conventionally the lowest signal to noise ratio that can be attributed to an analyte would have a signal to noise (S/N) ratio of 2. From the figure opposite showing a S/N value of 2, the signal is only just perceptible from the background noise. Different regulatory authorities in various industries may specify limit of detection as a signal to noise value – the actual value will vary from authority to authority but will usually be either 2 or 3.
Schematic representation of the determination of detector (FID) noise using the ASTM method
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Schematic representation of a peak representing a signal to noise ratio value of 2
The Flame Ionisation Detector Overview The Flame Ionisation Detector is the most widely used GC detector and was invented specifically for GC applications. It’s high sensitivity and linear range for carbon-containing compounds make it very popular in organic analysis. A typical FID design is shown opposite. The column effluent is mixed with hydrogen and a make-up gas (for capillary systems) before exiting via a small orifice (jet-tip) which is surrounded by a high flow of air. The Hydrogen is combustible in air and can be lit via a remote glow plug. The stiochiometric ratio’s of the air / hydrogen / carrier and make-up gas are important and will be studied further.
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As the column effluent is burned in this flame, ions are created which form a small current when a potential difference is applied. When no analyte (carbon containing compounds) are being burned, a small background current (10-20 picoamperes) arises from impurities in the carrier and detector gases. Conventionally the jet forms the anode and a cylindrical electrode held just above the flame is the cathode. A voltage of between 200 and 300V across these components is usually optimal but depends on detector design. The exact mechanism of ion production is not well characterised, however the formation of carbon ions via pyrolysis and the formation of small organic fragment ions produced via high energy combustion products are popular theories. The flame ionisation detector produces a proportional response to the number of carbon atoms in a molecule. One suggested reason for this constant response factor is the conversion of all solute carbon molecules to methane in the combustion process. When heteroatoms are present within the analyte molecule the sensitivity of the detector is much reduced. A calibration curve should be constructed for each analyte prior to quantitative analysis to take into account response variations due to detector settings.
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Operating and Optimising Since water is a product of combustion, the FID detector should be maintained at temperatures above 125 oC to prevent condensation of water and high boiling sample species. Typically most FID detectors will operate at 250 oC or above –depending upon the application. Temperature does affect the sensitivity of the detector although in practice, the detector temperature is rarely a parameter that is important in optimising an analysis. If required –start with a detector temperature of 250 oC and raise and lower the temperature in 20 oC steps to investigate the temperature / sensitivity relationship for particular analytes or compound groups. For efficient operation the ratio (stoichiometry) of the gases used must be correct. This is usually in the order of 1 effluent (carrier): 1 fuel (hydrogen): 10 oxidiser (air) –for capillary applications this is usually in the order of 300-400 mL/min. For capillary column applications where the carrier flow is less than 30ml/min. a makeup gas is used to increase the column effluent volume –this serves to dilute the effluent stream (to keep within the detector linear range) and propels the analyte up into the body of the flame. The makeup gas is usually chosen to be different from the carrier –nitrogen is the most popular as it’s viscosity ensures good mixing with the effluent stream. The ratio of fuel to makeup + carrier is vital in determining the sensitivity of the instrument –this ratio can be optimised for each analysis if required. The air flow (ratio) is not so important (as can be seen opposite) so long as a minimum flow is established. The makeup (nitrogen in this case) and hydrogen ratio have an affect on the sensitivity of the detector – this is demonstrated on the plot. Each makeup flow (actually carrier + makeup) is plotted against a range of fuel (hydrogen) flow rates. As can be seen below there is a maximum value for each combination representing a possible increase of around 10% response by optimising the gas flow rates.
Once past a critical minimum value the actual flow of the oxidiser seems to make little difference to the detector sensitivity. Typically this value will be between 300 and 400mL/min. The critical ratio for sustaining a flame is 8-12% hydrogen in air.
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Uses and Performance The FID detector shows excellent sensitivity to carbon containing compounds –the response being proportional to the number of carbon atoms in the molecule. Compounds containing halogens show a reduced response in FID detection and analytes not containing organic carbon do not burn and are therefore not detected. These include some significant compounds in organic analysis and are listed below –perhaps the most significant being water. Table 2. Compounds giving little or no response in FID Detection NH3 He CS2 Ar COS CO Kr H2S CO2 Ne SO2 H2O Xe NO SiCl4 O2 N2O SiHCl3 N2 NO2 SiF4 Flame Ionisation Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 10-11g (~50ppb) Response –organic carbon containing compounds, exceptions shown above Linearity –107 (very wide dynamic range) Stability –excellent stability, virtually no response change on flow or temperature fluctuation Temperature Limit ~ 400oC (instrument / column dependant) Gases –Combustion: Hydrogen and air, Makeup: helium or nitrogen
The presence of water in samples often produces very large tailing solvent peaks and induces tailing in analyte peaks. Having a detector that effectively ‘ignores’ the water is sometimes useful at simplifying the chromatogram. Other solvents (such as carbon © Crawford Scientific
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disulphide) are also ‘ignored’ by the detector and have utility when dealing with very early eluting analytes that may otherwise elute beneath the solvent peak. The general uses of FID are too widespread to categorise fully here –open any column manufacturers catalogue to find a host of relevant applications. A few typical examples have been shown cited opposite for general reference. The FID performance ‘figures of merit’ are shown–highlighting just why this detector is the most popular GC detector in use today. It’s excellent sensitivity, linearity and wide applicability make it the foremost detector in general organic analysis.
Sulphur containing compounds analysis. HP-PlotQ 6890. 53m×0.30mm×40μm Where: 1. Hydrogen sulphide 2. Carbonyl sulphide 3. Ethanelthiol Analysis conditions. Inlet Oven Split flow Detector Concentrations
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4. iso- propyl mercaptan 5. n- propyl mercaptan 6. n- butyl mercaptan
250oC, split mode, 0.25cc injection Temperature programmed 100mL/min FID 250oC Mercaptans less than 10%
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Blood analysis compounds on a DB-ALC1 30m×0.32mm×1.8μm
Where: 1. Methanol 2. Acetaldehyde 3. Ethanol Headspace conditions. Oven: 70oC Loop: 80oC Transfer line: 90oC Vial equil time: 10oC Press time: 0.20min
4. Isopropanol 5. Acetone 6. 1-Propanol
Loop fill time: Loop equil time: Inject time: Sample loop size: Samp comp:
0.20min 0.05min 0.1-0.20min 1.0mL 0.1% ethanol
Analysis conditions. Carrier Oven Injector Detector
Helium at 69 cm/sec, measured at 40oC 40oC Isothermal Headspace to split inlet, split 1:10, 250oC injection FID 300oC, nitrogen makeup gas at 23mL/min
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The Nitrogen Phosphorous Detector (NPD) Overview Although the nitrogen phosphorous detector (NPD) is based on a similar design to the FID, and whilst belonging to the family of ‘ionising detectors’ it works on a different principle to the FID. First introduced in 1964 by Karmen and Giuffrida this detector is also known as the Thermionic detector and is popular because of it’s enhanced sensitivity and selectivity towards compounds containing nitrogen and/or phosphorous, as its’ name suggests. Whilst the main structure of the detector looks essentially similar to the FID, the main difference is the addition of a resistively heated bead just above or in the vicinity of the jet. The bead contains or is coated with an alkali metal salt –usually caesium or rubidium silicate. When heated this bead emits thermionic electrons which migrate to the collector electrode and form the background current. The hydrogen flow into the detector is appreciably less than used in the FID and is too low to sustain a flame at the jet tip. Rather a ‘plasma’ of fuel (hydrogen), oxidiser (air), carrier effluent and makeup passes over the bead –at which point partial combustion occurs due to the heating filament. When a suitable analyte is eluted into the plasma, the partially combusted nitrogen or phosphorous materials are adsorbed onto the surface of the bead. This reduces the work function of the bead surface –effectively making it easier to emit electrons at the applied voltage / temperature. Thus the emitted electron density increases, which causes an increase in the current in the detector which is amplified and becomes the chromatographic peak.
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Operating and Optimising Typically NPD detectors are run at slightly higher temperatures than FID detectors –260 to 350 oC being typical. The temperature of the detector does not have a radical effect on response but higher temperatures do extend bead lifetime. The voltage applied to the bead (and hence the resulting temperature) does have a marked effect on the detector response. If unsure –start at 2V and make 10mV adjustments until the optimum response is obtained. It is essential to ensure that the hydrogen flow rate is low enough NOT to sustain a flame at the jet tip –otherwise measurement of nitrogen containing compounds will not be possible. The detector is sensitive to variations in the hydrogen flow rate and a constant flow of hydrogen is recommended to ensure steady baselines. The main disadvantage of this detector is that its performance deteriorates with time – usually seen via the need to increase the bead voltage in order to generate a signal. The water formed from combustion of hydrogen hydrolyses the alkali silicate to the metal hydroxide and silica. As the alkali metal hydroxide is volatile under typical operating conditions, the rubidium or caesium is constantly lost from the bead –ultimately leaving an inert bead of silica. To conserve bead life it is possible to turn off the detector (conventionally by interrupting the hydrogen flow) whilst solvent peaks elute or between injections when the GC oven is cooling. As the detector is flow sensitive –it is recommended that constant flow mode is used during temperature programming.
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Extending the life of the bead • • • • • •
Use the lowest practical adjust offset or bead voltage. Run clean samples. Turn the bead off when not in use. Keep the detector temperature high (320 to 335°C). Turn the hydrogen flow off during solvent peaks and between runs. If your NPD is Off for a long time in a high-humidity environment, water may accumulate in your detector. To evaporate this water: o Set the detector temperature at 100°C and maintain it for 30 minutes o Set the detector temperature to 150°C and maintain it for another 30 minutes
Use and Performance The NPD detector is a factor of around 500 times more sensitive to nitrogen and phosphorous containing compounds than to carbon containing compounds in general – this makes the detector effectively selective towards nitogenated and phosphorylated compounds. The NPD has a very high sensitivity, i.e., about an order of magnitude less than that of the electron capture detector (ca.10-12 g/mL for phosphorus and 10-11 g/mL for nitrogen). The high selectivity of the detector leads to it’s high sensitivity and the ability of the detector to ‘simplify’ chromatograms by effectively ‘ignoring’ other compound classes. The specific response of the NPD to nitrogen and phosphorus, coupled with its relatively high sensitivity, makes it especially useful for the analysis of many pharmaceuticals and in environmental samples containing herbicides. Employing appropriate column system – traces of herbicides at the 500 pg level can easily be determined. Nitrogen Phosphorous Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 10-11g (~10pg) Response –selective to nitrogen and phosphorous containing compounds Linearity –106 (very wide dynamic range) Stability –excellent stability, hydrogen flow and bead temperature should be carefully controlled Temperature Limit ~ 400oC (instrument / column dependant) Gases –Combustion: Hydrogen and air, Makeup: helium or nitrogen
Nitrogen/Phosphorous containing Pesticides (carrier gas 30cm/sec He), EPA method 507 © Crawford Scientific
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Where: 1. Prometon 2. Atrazine
3. Prometryne 4. Ametryne
Oven and detector parameters. Oven Initial temp: 80oC st o 1 ramp: 30 C/min to 178oC 2nd ramp: 2oC/min to 205oC 3rd ramp: 30oC/min to 310oC Final temp: 310oC for 4min
Detector Temperature: Makeup gas: Hydrogen:
290oC He, 30mL/min 4mL/min
Injection and pressure programming. Pressure programming
Injection parameters Temp: Vol injected: Pneumatics: Purge delay:
250oC 1μL Splitless mode 1min
Initial head pressure: 13.6 psi 0.5psi/min to 21psi 1st ramp: 1psi/min to 31psi 2nd ramp: Void time: 1.63 min
Determination of carbaryl
Conditions. Injection parameters
Pressure programming
Oven: 70oC (1min) to 230oC (5min) at 20oC/min Column: HP-5, 30×0.32mm×0.25mmμL N2, 15mL/min Makeup gas:
Column flow: 2mL/min Detector temp: 330oC Detector: 3mL/min of H2, air at 60mL/min Inlet: SL mode, 60mL/min purge 0.75min Inlet temp: 250oC
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Determination of triazines in pond water extract Where: 1. 2. 3. 4. Conditions. Oven Initial temp: 1st ramp: 2nd ramp: Final temp:
Propazine Atrazine Simazine Terbuthylazine
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170 C for 1min to 195oC at 2oC/min to 230oC at 2oC/min 230oC for 3min
5. 6. 7. 8.
Prometryne Ametryne Simetryne Terbutryne
Detector: NPD 250oC Makeup gas: He at 28mL/min Injector: Split 1:10, 250oC Column: DB-35 30m×0.25mm×0.25μm
The Electron Capture Detector (ECD) Overview The invention of the Electron Capture Detector is attributed in Lovelock in 1961. The detector measures the electrical conductivity of the effluent gas stream resulting from exposure to ionising radiation from a radionulclide. It is a selective detector that responds to compounds capable of ‘capturing electrons’ –more especially, halogenated compounds. The radionuclide is 63Nickel which emits beta particles (low energy electrons). 63
Ni → β −
These negatively charged particles collide with carrier gas molecules and produce further, higher energy, electrons (remember that beta particles are themselves low energy electrons). © Crawford Scientific
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β − + N 2 → 2e − + N 2+ The electrons formed by this process establish a high standing current between the anode (usually the inlet tube or detector body) and the cathode (usually a cylindrical electrode in the centre or top of the detector). The potential difference applied is usually 20-100V dc). When an electronegative analyte elutes, the analyte molecules capture some of the ‘background’ electrons and this results in a reduction of the standing (background) current.
A + e − → A− The negative analyte ions formed are slow moving and are not collected by the anode. The extent of electron absorption, and hence the reduction in standing current, is proportional to the concentration of the analyte.
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Operating and Optimising The carrier gases used for ECD operation should be pure and dry. Oxygen and water are both electronegative and as such contribute to a noisy baseline if they are present in the carrier or makeup gases even in trace amounts. When the detector is used with capillary columns, a makeup gas is generally required –in this case it is convenient to use less expensive (but high quality) nitrogen as the makeup gas and the more expensive helium as the carrier. It is recommended that ultrapure gases are used at all times when operating an ECD detector. © Crawford Scientific
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63
Ni sources are preferred to 3H (tritium) sources as they can be operated at higher temperatures (up to 400 oC vs 220 oC), and are inherently less radioactive, however they produce a less linear response. Improved performance and linearity can be obtained by operating the detector in a ‘pulsed’ mode. A square wave pulse (amplitude 50V, width 1 m s at intervals of 20-50 μs) is applied at a frequency that maintains a constant current in the detector cell –in order to maintain the current in the presence of an analyte the pulse frequency has to be increased. The signal is generated in proportion to the frequency of the applied pulse. The cleanliness of the detector needs to be maintained at all times, which often means care with sample preparation. Chromatographic peaks obtained from a dirty ECD detector have a distinctive shape and the sensitivity of the detector can often increase as detector performance deteriorates.
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As the ECD is a non-destructive detector, contamination builds up on the inner surfaces of the detector. As a result the magnitude of the standing current decreases –although the decrease in the current per analyte molecule remains the same. Unusually this may lead to an increase in the instrument sensitivity as the detector becomes dirtier. This behaviour is less likely when the detector is operated in the pulsed mode. Typically, when the detector performance in deteriorating a marked deviation away from the expected linear range is seen –especially at higher analyte concentrations as shown. Further, a typical chromatographic symptom od detector performance loss is the formation of a negative ‘dip’ either before or after each peak. Uses and Performance One drawback of the ECD is the necessity to use a radioactive source, which may require a license or at least regular radiological testing. The sensitivity of the electron capture detector is variable, depending upon the electron affinity of the analyte. For compounds of high electron affinity, such as halogenated compounds, minimum detectability of picograms on column are not untypical –with the added benefit that chromatograms are much simplified due to the very high selectivity. The linearity of the detector is limited, although as discussed the use of the detector in pulsed mode does extend the minimum detectability, and hence the dynamic range. Ranges of X50 are typical for 63Ni sources. It is vital that any analysis is carried out against a multi-point calibration curve that covers the anticipated range of sample concentrations. The detector is perhaps, one of the more tricky to operate in practice. It will also require periodic cleaning. Given that it contains a radioactive source –this may need to be carried out by the equipment manufacturer. Electron Capture Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 10-9 - 10-12g Response: highly selective towards analytes with electronegative atoms (especially halogens) Linearity: 103 - 104 (limited dynamic range- better in pulsed mode) Stability: fair –but susceptible to impurities in gases used and leaks in the detector body Temperature Limi:t ~ 400oC (63Ni) / 220oC (3H) Gases Carrier: Helium, Makeup: nitrogen or 5% methane in argon (all gases must be ultra high purity)
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Organochloride pesticides Where: 1. Trifluralin 2. Chloroneb 3. Propachlor 4. HCB 5. α-BHC 6. r-BHC 7. β-BHC 8. Chlorothalonil 9. Heptachlor Conditions. Oven Initial temp: 1st ramp: Dwell: 2nd ramp: Final temp:
10. δ-BHC 11. Alachlor 12. Aldrin 13. DCPA 14. Heptachlor epoxide 15. r-Chlordane 16. o,p’-DDE 17. α-Chlordane
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140 C for 2min to 240oC at 10oC/min 240oC for 5 min to 265oC at 5oC/min 265oC for 10min
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18. 19. 20. 21. 22. 23. 24. 25. 26.
Endosulfan I p,p’-DDE Dieldrin o,p’-DDD Chlorobenzilate Endrin o,p’-DDT p,p’-DDD Endosulfan II
27.p,p’-DDT 28.Endrin aldehyde 29.Endosulfan sulphate 30.Dibuthyl chlorendate 31.Methoxychlor 32.HBB
Detector: ECD 325oC Makeup gas: N2 at 30mL/min Carrier: He at 35cm/sec measured, 250oC Column: DB-5 30m×0.53mm×1.5μm
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Haloacetic acid methyl esters (optimized for resolution)
Where: 1. Chloroacetic acid 2. Bromoacetic acid 3. Dichloroacetic acid 4. Dalapon 5. Trichloroacetic acid 6. Bromochloroacetic acid Conditions. Oven Initial temp: 1st ramp: Dwell: 2nd ramp: Dwell: 3rd ramp: Final temp:
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35 C for 10min to 75oC at 5oC/min 75oC for 15 min to 100oC at 5oC/min 100oC for 5 min to 135oC at 5oC/min 135oC for 5min
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7. 1,2,3,-Trichloropropane (ISTD) 8. Dibromoacetic acid 9. Bromodichloroacetic acid 10. Chlorodibromodichloroacetic acid 11. 2,3,-Dibromopropionic acid (SSTD) 12. Tribromoacetic acid
Detector: ECD 260oC Makeup gas: N2 at 30mL/min Injector: Splitless, 200oC, 30sec purge Column: DB-1701 30m×0.25mm×0.25μm
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Purgeable halocarbons (EPA method 502.1) Where: 1. 2. 3. 4. 5.
Fluorotrichloromethane Methylene chloride trans-1,2-Dichloroethylene 1,1-Dichloroethylene 2,2- Dichloroethane, cis-1,2 Dichloroethylene 6. Bromochloromethane 7. Chloroform 8. 1,1,1-Trichloroethane 9. Carbon tetrachloride 10. 1,2-Dichloroethylene 11. 1,2-Dichloropropane Conditions. Oven Initial temp: 35oC for 5min Ramp: to 135oC at 4oC/min
12. Bromodichloromethane 13. 1,1,2-Trichloroethane, 1,3dichloropropane, tetrachloroethylene 14. Dibromochloromethane 15. 1,2-Dibromoethane (ECB) 16. 1,1,1,2-Tetrachloroethane 17. Bromoform 18.1,2,3-Trichloropropane, 1,1,2,2tetrachloroethane 19. Pentachloroethane 20. bis-(2-chloroisopropyl) ether 21. 1,2-Dibromo-3-chloropropane
Detector: ECD 300oC Makeup gas: N2 at 40mL/min Injector: Purge and trap (desorption 8mL/min) Column: DB-624 30m×0.53mm×3.0μm
The Thermal Conductivity Detector (TCD) Overview The Thermal Conductivity Detector (TCD) was used on most early GC instruments and was developed from a traditional gas analyser. It’s simplicity, robustness and ability to monitor almost any analyte make it still popular today –especially for the analysis of inorganic compounds and permanent gases. The detector is a heated metal (usually stainless steel) block drilled with two passageways or cavities. Each cavity contains a filament made from a high temperature coefficient of resistance metal such as Rhenium or Tungsten. These two filaments are heated and connected to the arms of a Wheatstone Bridge as shown opposite. Pure carrier (or reference gas) is allowed to flow over one filament (the reference cell) whilst the column effluent flows over the other (the analysis cell). © Crawford Scientific
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When an analyte elutes into the analytical cell –the thermal conductivity of the gas changes which causes a change in the rate of heat loss from the analytical filament. The Wheatstone Bridge becomes unbalanced and current needs to be supplied in order to restore the balance. Whilst the gas flowing over the two filaments is equal –the thermal conductivity will be equal and the filaments will have the same rate of heat loss –the Wheatsone bridge will be balanced and the signal will be zero. If an analyte elutes with the carrier, the thermal conductivity of the gas in the analysis cell changes, the rates of heat loss of the two filaments will not be balanced and a current will have to be applied to balance the Wheatstone bridge. It is this current which is recorded as the signal. The size of the applied current will be related to the concentration of analyte in the effluent gas. The size of the cell is critical in determining the sensitivity of the detector –with 140 μL being typical in a detector used for packed column GC. These devices cannot be used with narrow bore capillary columns and the cell volume must be drastically reduced. Several manufacturers offer ‘single cell’ devices of very low volume (~5μL) for use with capillary columns in which the reference and analysis streams are switched onto the single filament in the order of 5-10Hz. Ultra-small detector devices etched onto silica wafers are also available but are not used routinely.
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Thermal conductivity detector. In order to reduce volume –single cell TCD’s are used with capillary columns –this design has a cell volume of around 5μL. The switching occurs around to times/second
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Operating and Optimising It is important that the filaments are held at a temperature ABOVE that of the body of the detector so that the effects of filament temperature change are optimised. The larger the heating current applied to the filament, the greater the temperature differential and the greater the detector sensitivity. However, high filament temperatures also mean shorter filament life and the potential that the filament will burn out if no gas is flowing in the cell. The detector is susceptible to changes in the detector body, the filament temperature (and hence the current supplied), and the nature and flow of the carrier gas. Therefore, all of these parameters should be kept as stable as possible. This usually means covering the detector with a thermal cladding and ensuring that constant flow mode is used during temperature programming analysis. These factors inhibit the inherent stability of the detector. The sensitivity of the detector very much depends upon the difference in thermal conductivity between the eluting analyte and the carrier / reference gas. Interestingly this precludes the use of Area Normalisation as a method of quantitation! Hydrogen and helium have a much larger thermal conductivity value than most common analytes and as such they are preferred as the carrier. The use of nitrogen or argon as the carrier severely impairs the sensitivity of the detector. Table 3. Thermal Conductivities and TCD Response Values for Selected Compounds. Item Compound Thermal Relative Molar Conductivity Response Carrier gases Argon 12.5 Carbon Dioxide 12.7 Helium 100 Hydrogen 128 Nitrogen 18 Samples Ethane 17.5 51 n-Butane 13.5 85 n-Nonane 10.8 177 i-Butane 14.0 82 Cyclohexane 10.1 114 Benzene 9.9 100 Acetone 9.6 86 Ethanol 12.7 72 Chloroform 6.0 108 Methyl iodide 4.6 96 Ethyl Acetate 9.9 111 Thermal Conductivity values relative to Helium Relative Molar Response in Helium: Standard Benzene = 100
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Uses and Performance The thermal conductivity detector is capable of detecting almost any species, as long as their thermal conductivity is differentiated from that of the carrier gas used, therefore it’s universality is excellent. By default, the selectivity of the detector is poor, however it is possible to use an interfering species as the carrier to increase selectivity. An example is the analysis of oxygen in argon, which are virtually impossible to separate chromatographically. However by using argon as the carrier, the argon peak is eliminated and the oxygen peak can be clearly seen in the chromatogram. Sensitivity of the detector again depends upon the equipment set-up and the differential in thermal conductivity between the analyte and the carrier gas. It is one of the few detectors that can be used to detect the inorganic gases (such as H2O, CO, CO2, H2 etc.) and therefore it is the detector of choice for permanent gases. As the detector is non-destructive, it is often used in series with other detectors to detect gaseous or highly volatile sample components. The use of the TCD alongside an FID for the analysis of Transformer Oil Headspace gas is highlighted opposite. The linearity of the detector is widest when the differential between the carrier and analyte thermal conductivity is widest. The dynamic range can be extended by using micro or nano cell devices or by employing low volume single cell instruments with stream switching as described previously. Detector stability is reasonable but precautions should be taken to avoid temperature drift or fluctuation and a consistent flow of carrier should be established prior to analysis. Thermal Conductivity Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 10-9g (~10ppm) Response: all compounds Linearity: 104 (limited range) Stability: moderate Temperature Limit: ~ 400oC Gases – Carrier: (usually) Helium / Makeup: Same as carrier
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Fuel cell gas extract analysis Where: Conditions. Carrier gas: Helium Inlet: Purged packed 55oC Sample loop: 0.1mL TCD: 180oC
Oven Initial temp: 50oC for 10min Ramp: to 120oC at 10oC/min
Permanent gas analysis (GS-Molosieve, 30m×0.53mm I.D.) Where: 1. Neon 2. Oxigen 3. Nitrogen Conditions. Carrier gas: Helium at 35 cm/sec (4.6mL/min) Oven: 50oC isothermal
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4. Methane 5. Carbon monoxide
Injector: Detector: Makeup:
Split 1:10, 250μL, 100oC TCD 125oC He at 10mL/min
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Inorganic gases (GS-GasPro, 30m×0.32mm I.D.)
Where: 1. 2. 3. 4.
Nitrogen CO2 SF6 COS
5. H2S 6. Ethylene oxide 7. SO2
Conditions. Carrier gas: Helium at 53 cm/sec Oven: 25oC for 3 min o o 25-200 C (10 C/min) -200oC hold
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Injector: Split 1:50, 50μL, 200oC Detector: TCD 250oC
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C1-C10 Hydrocarbons (GS-Q, 30m×0.53mm I.D.) Where: 1. 2. 3. 4. 5. 6.
Methane Ethylene Ethane Propylene Propane Isobutane
Conditions. Carrier gas: Oven:
7. 1-Butene 8. n-Butane 9. cis-2-Butene 10. trans-2-Butene 11. Isopentane 12. n-Pentane Helium at 8 mL/min 70oC for 3 min 70-180oC (20oC/min) 180oC hold for 3 min 180-230oC (5oC/min) 230oC hold for 10 min
Injector: Detector:
13. n-Hexane 14. n-Heptane 15. n-Octane 16. n-Nonane 17. n-Decane
Split 1:50, 250oC TCD 300oC
Other GC Detectors Flame Photometric Detector (FPD) Compounds are burned in a hydrogen-air flame very similar to an FID detector. Sulphur and phosphorous containing compounds produce light emitting species (sulphur – 394nm / phosphorous – 526nm). A monochromatic filter allows only one of the wavelengths to pass and a photomultiplier tube is used to measure the amount of incident light and a signal is generated. A different filter is required for each detection mode.
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Flame photometric detector Flame Photometric Detector Characteristics • • •
• • •
Minimum Detectability (LOD): ~ 10-100pg (sulphur); 1-10pg (phosphorous) Response: Sulphur or phosphorous containing compounds (one at a time) Linearity: The response in the phosphorus mode is linear over four orders of magnitude. The sulphur response, varies such that the square root of the response is proportional to the concentration and is linear on a log–log scale over three orders of magnitude. Stability: moderate Temperature Limit: ~ 300oC Combustion gases: Hydrogen and Air. Makeup: Nitrogen
Photoionisation Detector (PID) Compounds eluting into a cell are bombarded with high-energy photons emitted from a lamp. Compounds with ionisation potentials below the photon energy are ionised. The resulting ions are attracted to an electrode, measured and a signal is generated.
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Photoionisation Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 25-50pg (aromatics); 50-200pg (olefins) Response: Depends on lamp characteristics, conventionally used for aromatics and olefins (10 eV lamp) The response is linear over six or seven orders of magnitude. Stability: good Temperature Limit: ~ 200oC Makeup gas: Same as carrier
Electrolytic Conductivity Detector (ELCD) Compounds are mixed with a reaction gas and passed through a high temperature reaction tube. Specific reaction products are created which mix with a solvent and pas through an electrolytic conductivity cell. The change in the electrolytic conductivity of the solvent is measured and a signal generated. Reaction tube temperature and solvent determine the type of reactant detected.
Electrolytic Conductivity Detector Electrochemical Conductivity Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 5 - 10pg (halogens); 10-20pg (sulphur); 10-20pg (nitrogen) Response: Halogens, sulphur or nitrogen containing compounds (one at a time) Linearity: Sulphur 103 - 104; Nitrogen 104 - 105; Halogens 105 - 106. Stability: good Temperature Limit: ~ 800 - 1000oC (halogens); 850 - 925oC (nitrogen); 750 - 825oC (sulphur) Gases: Hydrogen (halogens and nitrogen); air (sulphur)
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Mass Spectrometer (MS) The detector is maintained under vacuum. Compounds eluting from the GC column are bombarded with electrons (EI) or reagent gas ions (CI) in order to ionise them. Compounds fragment into characteristic charged ions or fragments and the resulting charged species are focussed and accelerated into a mass analysing device –typically a Quadrupole mass analyser or Ion Trap mass analyser. The mass analyser selectively allows ions of a selected mass to charge ratio to pass through the analyser to the electron multiplier where a signal due to that specific mass to charge ration ion is generated. The mass filter can be adjusted to allow specified ions to pass (selected ion monitoring mode) or to quickly scan over a range of mass to charge values (scanning mode). The abundance (total ion count) versus time is plotted to form the chromatogram. A mass spectrum can be obtained for each ‘scan’ which plots the number of ions at each different mass to charge ratio in the range selected.
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Mass Spectrometer Mass Spectrometer Characteristics • • • • • •
Minimum Detectability (LOD): ~ 1-10ng (SCAN); 1-10pg (SIM) Response: Any compound that produces fragments (or ions) within the selected mass range of the analysing device. May be an inclusive range of mass to charge ratios (SCAN mode) or selected ions (Selected Ion Mode (SIM)) Linearity: 105 - 106 (good dynamic range) Stability: good Temperature Limit: ~ 250oC Carrier gas: Helium
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Wherever you see this symbol, it is important to access the on-line course as there is interactive material that cannot be fully shown in this reference manual.
Aims and Objectives Aims • • •
• • •
To highlight the various detector types available for GC To describe the performance characteristics associated with a range of common detectors for GC To explain the working principles behind the following detectors: o Flame Ionisation o Electron Capture o Nitrogen Phosphorous o Thermal Conductivity o Flame Photometric To describe the optimisation process for each of the detector types and typical uses To illustrate the various performance characteristics of each of the detector types To highlight the use of various other detector types and give brief descriptions of them
Objectives
At the end of this Section you should be able to: • • • •
Explain the parameters and performance measures by which GC detectors are characterised Describe the components and working principle of a number of common GC detectors Give suggestions of how each detector type might be optimised and demonstrate an understanding of each detector type in a practical context Choose the correct detector type for a variety of analyte and application types
Content Overview of GC Detectors Concentration versus Mass Flow Selective versus Universal Destructive versus Nondestructive Characteristics of GC Detectors Noise Signal to Noise Ratio The Flame Ionisation Detector Overview Operating and Optimising Uses and Performance The Nitrogen Phosphorous Detector (NPD) Overview Operating and Optimising Use and Performance The Electron Capture Detector (ECD) Overview Operating and Optimising Uses and Performance The Thermal Conductivity Detector (TCD) Overview Operating and Optimising Uses and Performance Other GC Detectors Flame Photometric Detector (FPD) Photoionisation Detector (PID) Electrolytic Conductivity Detector (ELCD) Mass Spectrometer (MS)
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Overview of GC Detectors There have been over 60 detectors designed for GC applications since the inception of the technique. Probably 10-12 detectors are used commonly in routine GC applications and of these only 3-4 are used in non-specific applications. All GC detectors monitor a physical property of the analyte –the property chosen must significantly change, and therefore cause a large detector response, when the eluent gas flowing into the detector is ‘contaminated’ by the analyte. Several properties have been used in GC detectors and these are outlined in the next table. Detectors that exhibit an enhanced response to certain analyte types are known as ‘selective detectors’. Table 1. Selected GC detectors Detector type Detector (Property) Ionisation Flame Ionisation Detector (FID) Thermal Conductivity Detector (TCD) Electron Capture Detector Nitrogen Phosphorous Detector (NPD) Photoionisation Detector (PID) Emission Flame Photometric Detector (FPD) Plasma Atomic Emission (AED) Electrochemical Hall Electrolytic Conductivity (HECD) Others Chemiluminescent Mass Spectrometer (MSD) Fourier Transform Infra-red (FTIR)
Selective No No Halogens N, P, Halogens Aromatics S, P Metal, Halogen, C, O S, N, Halogen S Yes Yes
There are three important classifications of GC detector and these are: Concentration versus Mass Flow Concentration sensitive detectors respond to the concentration of the analyte in the detector flow cell / chamber. If the flow to the detector were interrupted the signal (peak) height would not decrease, however the peak area would change. Mass Flow sensitive detectors respond to the amount of analyte in the detector at any time, irrespective of the volume of carrier gas. If the carrier flow is reduced –the detector response (peak height) will decrease but the peak area will remain constant. The diagram below shows the effect of altering carrier flow on the two detector types:
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Therefore, when using GC temperature programming and operating in constant pressure mode Concentration Sensitive Detectors (TCD, ECD) will have a variable peak area response as the flow rate changes, but Mass Flow sensitive detectors (FID, NPD) will show a constant peak area response as the flow rate changes. In reality, the use of constant flow with temperature programs is recommended to ensure constant peak widths. Selective versus Universal This classification describes the number of analytes that a detector will respond to. A Universal detector will theoretically respond to all solutes, whilst Selective detectors respond to specific types or classes of compound. Universal detectors are good for screening unknown mixtures and for situations where it is important to ensure all solutes are detected and quantified. Selective detectors usually show enhanced sensitivity to particular compounds and are able to ‘simplify’ complex chromatograms by only detecting certain classes of compound.
MS in Total Ion mode –universal detector
MS in Selected Ion mode –responds to only compounds producing certain fragment ions. The chromatogram is simplified compared to the top example due to the specificity of the detection mode. Destructive versus Nondestructive Nondestructive detectors are required where the separated intact analyte components need to be recovered for further analysis or characterisation. This can be accomplished using destructive detectors (where the analyte is destroyed) by splitting the carrier effluent stream just prior to the detector entrance.
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Characteristics of GC Detectors There are several important detector characteristics that apply to all Gas Chromatography detectors and help to describe the detector performance. The important characteristics of GC detectors include: • • •
Sensitivity (the signal output per unit concentration or mass of analyte) Minimum Detectability (usually measured by signal to noise ratio) Linearity (the range of concentrations over which reliable quantitative measurements can be made)
These characteristics will be explored further using some of the terms outlined in the brief definitions above. Noise Noise is the background signal produced by the detector in the absence of analyte. It arises from electronic noise, stray environmental signals and contamination or leaks. Good design and shielding can help to reduce the inherent noise. Noise is a rapid random variation in output, whereas drift is a slow systematic change in detector output. Signal to Noise Ratio The ratio of the detector signal to the inherent background noise is a useful measure of detector performance to convey information about the lower limit of detection. Conventionally the lowest signal to noise ratio that can be attributed to an analyte would have a signal to noise (S/N) ratio of 2. From the figure opposite showing a S/N value of 2, the signal is only just perceptible from the background noise. Different regulatory authorities in various industries may specify limit of detection as a signal to noise value – the actual value will vary from authority to authority but will usually be either 2 or 3.
Schematic representation of the determination of detector (FID) noise using the ASTM method
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Schematic representation of a peak representing a signal to noise ratio value of 2
The Flame Ionisation Detector Overview The Flame Ionisation Detector is the most widely used GC detector and was invented specifically for GC applications. It’s high sensitivity and linear range for carbon-containing compounds make it very popular in organic analysis. A typical FID design is shown opposite. The column effluent is mixed with hydrogen and a make-up gas (for capillary systems) before exiting via a small orifice (jet-tip) which is surrounded by a high flow of air. The Hydrogen is combustible in air and can be lit via a remote glow plug. The stiochiometric ratio’s of the air / hydrogen / carrier and make-up gas are important and will be studied further.
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As the column effluent is burned in this flame, ions are created which form a small current when a potential difference is applied. When no analyte (carbon containing compounds) are being burned, a small background current (10-20 picoamperes) arises from impurities in the carrier and detector gases. Conventionally the jet forms the anode and a cylindrical electrode held just above the flame is the cathode. A voltage of between 200 and 300V across these components is usually optimal but depends on detector design. The exact mechanism of ion production is not well characterised, however the formation of carbon ions via pyrolysis and the formation of small organic fragment ions produced via high energy combustion products are popular theories. The flame ionisation detector produces a proportional response to the number of carbon atoms in a molecule. One suggested reason for this constant response factor is the conversion of all solute carbon molecules to methane in the combustion process. When heteroatoms are present within the analyte molecule the sensitivity of the detector is much reduced. A calibration curve should be constructed for each analyte prior to quantitative analysis to take into account response variations due to detector settings.
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Operating and Optimising Since water is a product of combustion, the FID detector should be maintained at temperatures above 125 oC to prevent condensation of water and high boiling sample species. Typically most FID detectors will operate at 250 oC or above –depending upon the application. Temperature does affect the sensitivity of the detector although in practice, the detector temperature is rarely a parameter that is important in optimising an analysis. If required –start with a detector temperature of 250 oC and raise and lower the temperature in 20 oC steps to investigate the temperature / sensitivity relationship for particular analytes or compound groups. For efficient operation the ratio (stoichiometry) of the gases used must be correct. This is usually in the order of 1 effluent (carrier): 1 fuel (hydrogen): 10 oxidiser (air) –for capillary applications this is usually in the order of 300-400 mL/min. For capillary column applications where the carrier flow is less than 30ml/min. a makeup gas is used to increase the column effluent volume –this serves to dilute the effluent stream (to keep within the detector linear range) and propels the analyte up into the body of the flame. The makeup gas is usually chosen to be different from the carrier –nitrogen is the most popular as it’s viscosity ensures good mixing with the effluent stream. The ratio of fuel to makeup + carrier is vital in determining the sensitivity of the instrument –this ratio can be optimised for each analysis if required. The air flow (ratio) is not so important (as can be seen opposite) so long as a minimum flow is established. The makeup (nitrogen in this case) and hydrogen ratio have an affect on the sensitivity of the detector – this is demonstrated on the plot. Each makeup flow (actually carrier + makeup) is plotted against a range of fuel (hydrogen) flow rates. As can be seen below there is a maximum value for each combination representing a possible increase of around 10% response by optimising the gas flow rates.
Once past a critical minimum value the actual flow of the oxidiser seems to make little difference to the detector sensitivity. Typically this value will be between 300 and 400mL/min. The critical ratio for sustaining a flame is 8-12% hydrogen in air.
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Uses and Performance The FID detector shows excellent sensitivity to carbon containing compounds –the response being proportional to the number of carbon atoms in the molecule. Compounds containing halogens show a reduced response in FID detection and analytes not containing organic carbon do not burn and are therefore not detected. These include some significant compounds in organic analysis and are listed below –perhaps the most significant being water. Table 2. Compounds giving little or no response in FID Detection NH3 He CS2 Ar COS CO Kr H2S CO2 Ne SO2 H2O Xe NO SiCl4 O2 N2O SiHCl3 N2 NO2 SiF4 Flame Ionisation Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 10-11g (~50ppb) Response –organic carbon containing compounds, exceptions shown above Linearity –107 (very wide dynamic range) Stability –excellent stability, virtually no response change on flow or temperature fluctuation Temperature Limit ~ 400oC (instrument / column dependant) Gases –Combustion: Hydrogen and air, Makeup: helium or nitrogen
The presence of water in samples often produces very large tailing solvent peaks and induces tailing in analyte peaks. Having a detector that effectively ‘ignores’ the water is sometimes useful at simplifying the chromatogram. Other solvents (such as carbon © Crawford Scientific
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disulphide) are also ‘ignored’ by the detector and have utility when dealing with very early eluting analytes that may otherwise elute beneath the solvent peak. The general uses of FID are too widespread to categorise fully here –open any column manufacturers catalogue to find a host of relevant applications. A few typical examples have been shown cited opposite for general reference. The FID performance ‘figures of merit’ are shown–highlighting just why this detector is the most popular GC detector in use today. It’s excellent sensitivity, linearity and wide applicability make it the foremost detector in general organic analysis.
Sulphur containing compounds analysis. HP-PlotQ 6890. 53m×0.30mm×40μm Where: 1. Hydrogen sulphide 2. Carbonyl sulphide 3. Ethanelthiol Analysis conditions. Inlet Oven Split flow Detector Concentrations
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4. iso- propyl mercaptan 5. n- propyl mercaptan 6. n- butyl mercaptan
250oC, split mode, 0.25cc injection Temperature programmed 100mL/min FID 250oC Mercaptans less than 10%
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Blood analysis compounds on a DB-ALC1 30m×0.32mm×1.8μm
Where: 1. Methanol 2. Acetaldehyde 3. Ethanol Headspace conditions. Oven: 70oC Loop: 80oC Transfer line: 90oC Vial equil time: 10oC Press time: 0.20min
4. Isopropanol 5. Acetone 6. 1-Propanol
Loop fill time: Loop equil time: Inject time: Sample loop size: Samp comp:
0.20min 0.05min 0.1-0.20min 1.0mL 0.1% ethanol
Analysis conditions. Carrier Oven Injector Detector
Helium at 69 cm/sec, measured at 40oC 40oC Isothermal Headspace to split inlet, split 1:10, 250oC injection FID 300oC, nitrogen makeup gas at 23mL/min
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The Nitrogen Phosphorous Detector (NPD) Overview Although the nitrogen phosphorous detector (NPD) is based on a similar design to the FID, and whilst belonging to the family of ‘ionising detectors’ it works on a different principle to the FID. First introduced in 1964 by Karmen and Giuffrida this detector is also known as the Thermionic detector and is popular because of it’s enhanced sensitivity and selectivity towards compounds containing nitrogen and/or phosphorous, as its’ name suggests. Whilst the main structure of the detector looks essentially similar to the FID, the main difference is the addition of a resistively heated bead just above or in the vicinity of the jet. The bead contains or is coated with an alkali metal salt –usually caesium or rubidium silicate. When heated this bead emits thermionic electrons which migrate to the collector electrode and form the background current. The hydrogen flow into the detector is appreciably less than used in the FID and is too low to sustain a flame at the jet tip. Rather a ‘plasma’ of fuel (hydrogen), oxidiser (air), carrier effluent and makeup passes over the bead –at which point partial combustion occurs due to the heating filament. When a suitable analyte is eluted into the plasma, the partially combusted nitrogen or phosphorous materials are adsorbed onto the surface of the bead. This reduces the work function of the bead surface –effectively making it easier to emit electrons at the applied voltage / temperature. Thus the emitted electron density increases, which causes an increase in the current in the detector which is amplified and becomes the chromatographic peak.
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Operating and Optimising Typically NPD detectors are run at slightly higher temperatures than FID detectors –260 to 350 oC being typical. The temperature of the detector does not have a radical effect on response but higher temperatures do extend bead lifetime. The voltage applied to the bead (and hence the resulting temperature) does have a marked effect on the detector response. If unsure –start at 2V and make 10mV adjustments until the optimum response is obtained. It is essential to ensure that the hydrogen flow rate is low enough NOT to sustain a flame at the jet tip –otherwise measurement of nitrogen containing compounds will not be possible. The detector is sensitive to variations in the hydrogen flow rate and a constant flow of hydrogen is recommended to ensure steady baselines. The main disadvantage of this detector is that its performance deteriorates with time – usually seen via the need to increase the bead voltage in order to generate a signal. The water formed from combustion of hydrogen hydrolyses the alkali silicate to the metal hydroxide and silica. As the alkali metal hydroxide is volatile under typical operating conditions, the rubidium or caesium is constantly lost from the bead –ultimately leaving an inert bead of silica. To conserve bead life it is possible to turn off the detector (conventionally by interrupting the hydrogen flow) whilst solvent peaks elute or between injections when the GC oven is cooling. As the detector is flow sensitive –it is recommended that constant flow mode is used during temperature programming.
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Extending the life of the bead • • • • • •
Use the lowest practical adjust offset or bead voltage. Run clean samples. Turn the bead off when not in use. Keep the detector temperature high (320 to 335°C). Turn the hydrogen flow off during solvent peaks and between runs. If your NPD is Off for a long time in a high-humidity environment, water may accumulate in your detector. To evaporate this water: o Set the detector temperature at 100°C and maintain it for 30 minutes o Set the detector temperature to 150°C and maintain it for another 30 minutes
Use and Performance The NPD detector is a factor of around 500 times more sensitive to nitrogen and phosphorous containing compounds than to carbon containing compounds in general – this makes the detector effectively selective towards nitogenated and phosphorylated compounds. The NPD has a very high sensitivity, i.e., about an order of magnitude less than that of the electron capture detector (ca.10-12 g/mL for phosphorus and 10-11 g/mL for nitrogen). The high selectivity of the detector leads to it’s high sensitivity and the ability of the detector to ‘simplify’ chromatograms by effectively ‘ignoring’ other compound classes. The specific response of the NPD to nitrogen and phosphorus, coupled with its relatively high sensitivity, makes it especially useful for the analysis of many pharmaceuticals and in environmental samples containing herbicides. Employing appropriate column system – traces of herbicides at the 500 pg level can easily be determined. Nitrogen Phosphorous Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 10-11g (~10pg) Response –selective to nitrogen and phosphorous containing compounds Linearity –106 (very wide dynamic range) Stability –excellent stability, hydrogen flow and bead temperature should be carefully controlled Temperature Limit ~ 400oC (instrument / column dependant) Gases –Combustion: Hydrogen and air, Makeup: helium or nitrogen
Nitrogen/Phosphorous containing Pesticides (carrier gas 30cm/sec He), EPA method 507 © Crawford Scientific
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Where: 1. Prometon 2. Atrazine
3. Prometryne 4. Ametryne
Oven and detector parameters. Oven Initial temp: 80oC st o 1 ramp: 30 C/min to 178oC 2nd ramp: 2oC/min to 205oC 3rd ramp: 30oC/min to 310oC Final temp: 310oC for 4min
Detector Temperature: Makeup gas: Hydrogen:
290oC He, 30mL/min 4mL/min
Injection and pressure programming. Pressure programming
Injection parameters Temp: Vol injected: Pneumatics: Purge delay:
250oC 1μL Splitless mode 1min
Initial head pressure: 13.6 psi 0.5psi/min to 21psi 1st ramp: 1psi/min to 31psi 2nd ramp: Void time: 1.63 min
Determination of carbaryl
Conditions. Injection parameters
Pressure programming
Oven: 70oC (1min) to 230oC (5min) at 20oC/min Column: HP-5, 30×0.32mm×0.25mmμL N2, 15mL/min Makeup gas:
Column flow: 2mL/min Detector temp: 330oC Detector: 3mL/min of H2, air at 60mL/min Inlet: SL mode, 60mL/min purge 0.75min Inlet temp: 250oC
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Determination of triazines in pond water extract Where: 1. 2. 3. 4. Conditions. Oven Initial temp: 1st ramp: 2nd ramp: Final temp:
Propazine Atrazine Simazine Terbuthylazine
o
170 C for 1min to 195oC at 2oC/min to 230oC at 2oC/min 230oC for 3min
5. 6. 7. 8.
Prometryne Ametryne Simetryne Terbutryne
Detector: NPD 250oC Makeup gas: He at 28mL/min Injector: Split 1:10, 250oC Column: DB-35 30m×0.25mm×0.25μm
The Electron Capture Detector (ECD) Overview The invention of the Electron Capture Detector is attributed in Lovelock in 1961. The detector measures the electrical conductivity of the effluent gas stream resulting from exposure to ionising radiation from a radionulclide. It is a selective detector that responds to compounds capable of ‘capturing electrons’ –more especially, halogenated compounds. The radionuclide is 63Nickel which emits beta particles (low energy electrons). 63
Ni → β −
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β − + N 2 → 2e − + N 2+ The electrons formed by this process establish a high standing current between the anode (usually the inlet tube or detector body) and the cathode (usually a cylindrical electrode in the centre or top of the detector). The potential difference applied is usually 20-100V dc). When an electronegative analyte elutes, the analyte molecules capture some of the ‘background’ electrons and this results in a reduction of the standing (background) current.
A + e − → A− The negative analyte ions formed are slow moving and are not collected by the anode. The extent of electron absorption, and hence the reduction in standing current, is proportional to the concentration of the analyte.
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Operating and Optimising The carrier gases used for ECD operation should be pure and dry. Oxygen and water are both electronegative and as such contribute to a noisy baseline if they are present in the carrier or makeup gases even in trace amounts. When the detector is used with capillary columns, a makeup gas is generally required –in this case it is convenient to use less expensive (but high quality) nitrogen as the makeup gas and the more expensive helium as the carrier. It is recommended that ultrapure gases are used at all times when operating an ECD detector. © Crawford Scientific
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63
Ni sources are preferred to 3H (tritium) sources as they can be operated at higher temperatures (up to 400 oC vs 220 oC), and are inherently less radioactive, however they produce a less linear response. Improved performance and linearity can be obtained by operating the detector in a ‘pulsed’ mode. A square wave pulse (amplitude 50V, width 1 m s at intervals of 20-50 μs) is applied at a frequency that maintains a constant current in the detector cell –in order to maintain the current in the presence of an analyte the pulse frequency has to be increased. The signal is generated in proportion to the frequency of the applied pulse. The cleanliness of the detector needs to be maintained at all times, which often means care with sample preparation. Chromatographic peaks obtained from a dirty ECD detector have a distinctive shape and the sensitivity of the detector can often increase as detector performance deteriorates.
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As the ECD is a non-destructive detector, contamination builds up on the inner surfaces of the detector. As a result the magnitude of the standing current decreases –although the decrease in the current per analyte molecule remains the same. Unusually this may lead to an increase in the instrument sensitivity as the detector becomes dirtier. This behaviour is less likely when the detector is operated in the pulsed mode. Typically, when the detector performance in deteriorating a marked deviation away from the expected linear range is seen –especially at higher analyte concentrations as shown. Further, a typical chromatographic symptom od detector performance loss is the formation of a negative ‘dip’ either before or after each peak. Uses and Performance One drawback of the ECD is the necessity to use a radioactive source, which may require a license or at least regular radiological testing. The sensitivity of the electron capture detector is variable, depending upon the electron affinity of the analyte. For compounds of high electron affinity, such as halogenated compounds, minimum detectability of picograms on column are not untypical –with the added benefit that chromatograms are much simplified due to the very high selectivity. The linearity of the detector is limited, although as discussed the use of the detector in pulsed mode does extend the minimum detectability, and hence the dynamic range. Ranges of X50 are typical for 63Ni sources. It is vital that any analysis is carried out against a multi-point calibration curve that covers the anticipated range of sample concentrations. The detector is perhaps, one of the more tricky to operate in practice. It will also require periodic cleaning. Given that it contains a radioactive source –this may need to be carried out by the equipment manufacturer. Electron Capture Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 10-9 - 10-12g Response: highly selective towards analytes with electronegative atoms (especially halogens) Linearity: 103 - 104 (limited dynamic range- better in pulsed mode) Stability: fair –but susceptible to impurities in gases used and leaks in the detector body Temperature Limi:t ~ 400oC (63Ni) / 220oC (3H) Gases Carrier: Helium, Makeup: nitrogen or 5% methane in argon (all gases must be ultra high purity)
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Organochloride pesticides Where: 1. Trifluralin 2. Chloroneb 3. Propachlor 4. HCB 5. α-BHC 6. r-BHC 7. β-BHC 8. Chlorothalonil 9. Heptachlor Conditions. Oven Initial temp: 1st ramp: Dwell: 2nd ramp: Final temp:
10. δ-BHC 11. Alachlor 12. Aldrin 13. DCPA 14. Heptachlor epoxide 15. r-Chlordane 16. o,p’-DDE 17. α-Chlordane
o
140 C for 2min to 240oC at 10oC/min 240oC for 5 min to 265oC at 5oC/min 265oC for 10min
© Crawford Scientific
18. 19. 20. 21. 22. 23. 24. 25. 26.
Endosulfan I p,p’-DDE Dieldrin o,p’-DDD Chlorobenzilate Endrin o,p’-DDT p,p’-DDD Endosulfan II
27.p,p’-DDT 28.Endrin aldehyde 29.Endosulfan sulphate 30.Dibuthyl chlorendate 31.Methoxychlor 32.HBB
Detector: ECD 325oC Makeup gas: N2 at 30mL/min Carrier: He at 35cm/sec measured, 250oC Column: DB-5 30m×0.53mm×1.5μm
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Haloacetic acid methyl esters (optimized for resolution)
Where: 1. Chloroacetic acid 2. Bromoacetic acid 3. Dichloroacetic acid 4. Dalapon 5. Trichloroacetic acid 6. Bromochloroacetic acid Conditions. Oven Initial temp: 1st ramp: Dwell: 2nd ramp: Dwell: 3rd ramp: Final temp:
o
35 C for 10min to 75oC at 5oC/min 75oC for 15 min to 100oC at 5oC/min 100oC for 5 min to 135oC at 5oC/min 135oC for 5min
© Crawford Scientific
7. 1,2,3,-Trichloropropane (ISTD) 8. Dibromoacetic acid 9. Bromodichloroacetic acid 10. Chlorodibromodichloroacetic acid 11. 2,3,-Dibromopropionic acid (SSTD) 12. Tribromoacetic acid
Detector: ECD 260oC Makeup gas: N2 at 30mL/min Injector: Splitless, 200oC, 30sec purge Column: DB-1701 30m×0.25mm×0.25μm
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Purgeable halocarbons (EPA method 502.1) Where: 1. 2. 3. 4. 5.
Fluorotrichloromethane Methylene chloride trans-1,2-Dichloroethylene 1,1-Dichloroethylene 2,2- Dichloroethane, cis-1,2 Dichloroethylene 6. Bromochloromethane 7. Chloroform 8. 1,1,1-Trichloroethane 9. Carbon tetrachloride 10. 1,2-Dichloroethylene 11. 1,2-Dichloropropane Conditions. Oven Initial temp: 35oC for 5min Ramp: to 135oC at 4oC/min
12. Bromodichloromethane 13. 1,1,2-Trichloroethane, 1,3dichloropropane, tetrachloroethylene 14. Dibromochloromethane 15. 1,2-Dibromoethane (ECB) 16. 1,1,1,2-Tetrachloroethane 17. Bromoform 18.1,2,3-Trichloropropane, 1,1,2,2tetrachloroethane 19. Pentachloroethane 20. bis-(2-chloroisopropyl) ether 21. 1,2-Dibromo-3-chloropropane
Detector: ECD 300oC Makeup gas: N2 at 40mL/min Injector: Purge and trap (desorption 8mL/min) Column: DB-624 30m×0.53mm×3.0μm
The Thermal Conductivity Detector (TCD) Overview The Thermal Conductivity Detector (TCD) was used on most early GC instruments and was developed from a traditional gas analyser. It’s simplicity, robustness and ability to monitor almost any analyte make it still popular today –especially for the analysis of inorganic compounds and permanent gases. The detector is a heated metal (usually stainless steel) block drilled with two passageways or cavities. Each cavity contains a filament made from a high temperature coefficient of resistance metal such as Rhenium or Tungsten. These two filaments are heated and connected to the arms of a Wheatstone Bridge as shown opposite. Pure carrier (or reference gas) is allowed to flow over one filament (the reference cell) whilst the column effluent flows over the other (the analysis cell). © Crawford Scientific
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When an analyte elutes into the analytical cell –the thermal conductivity of the gas changes which causes a change in the rate of heat loss from the analytical filament. The Wheatstone Bridge becomes unbalanced and current needs to be supplied in order to restore the balance. Whilst the gas flowing over the two filaments is equal –the thermal conductivity will be equal and the filaments will have the same rate of heat loss –the Wheatsone bridge will be balanced and the signal will be zero. If an analyte elutes with the carrier, the thermal conductivity of the gas in the analysis cell changes, the rates of heat loss of the two filaments will not be balanced and a current will have to be applied to balance the Wheatstone bridge. It is this current which is recorded as the signal. The size of the applied current will be related to the concentration of analyte in the effluent gas. The size of the cell is critical in determining the sensitivity of the detector –with 140 μL being typical in a detector used for packed column GC. These devices cannot be used with narrow bore capillary columns and the cell volume must be drastically reduced. Several manufacturers offer ‘single cell’ devices of very low volume (~5μL) for use with capillary columns in which the reference and analysis streams are switched onto the single filament in the order of 5-10Hz. Ultra-small detector devices etched onto silica wafers are also available but are not used routinely.
© Crawford Scientific
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Thermal conductivity detector. In order to reduce volume –single cell TCD’s are used with capillary columns –this design has a cell volume of around 5μL. The switching occurs around to times/second
© Crawford Scientific
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Operating and Optimising It is important that the filaments are held at a temperature ABOVE that of the body of the detector so that the effects of filament temperature change are optimised. The larger the heating current applied to the filament, the greater the temperature differential and the greater the detector sensitivity. However, high filament temperatures also mean shorter filament life and the potential that the filament will burn out if no gas is flowing in the cell. The detector is susceptible to changes in the detector body, the filament temperature (and hence the current supplied), and the nature and flow of the carrier gas. Therefore, all of these parameters should be kept as stable as possible. This usually means covering the detector with a thermal cladding and ensuring that constant flow mode is used during temperature programming analysis. These factors inhibit the inherent stability of the detector. The sensitivity of the detector very much depends upon the difference in thermal conductivity between the eluting analyte and the carrier / reference gas. Interestingly this precludes the use of Area Normalisation as a method of quantitation! Hydrogen and helium have a much larger thermal conductivity value than most common analytes and as such they are preferred as the carrier. The use of nitrogen or argon as the carrier severely impairs the sensitivity of the detector. Table 3. Thermal Conductivities and TCD Response Values for Selected Compounds. Item Compound Thermal Relative Molar Conductivity Response Carrier gases Argon 12.5 Carbon Dioxide 12.7 Helium 100 Hydrogen 128 Nitrogen 18 Samples Ethane 17.5 51 n-Butane 13.5 85 n-Nonane 10.8 177 i-Butane 14.0 82 Cyclohexane 10.1 114 Benzene 9.9 100 Acetone 9.6 86 Ethanol 12.7 72 Chloroform 6.0 108 Methyl iodide 4.6 96 Ethyl Acetate 9.9 111 Thermal Conductivity values relative to Helium Relative Molar Response in Helium: Standard Benzene = 100
© Crawford Scientific
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Uses and Performance The thermal conductivity detector is capable of detecting almost any species, as long as their thermal conductivity is differentiated from that of the carrier gas used, therefore it’s universality is excellent. By default, the selectivity of the detector is poor, however it is possible to use an interfering species as the carrier to increase selectivity. An example is the analysis of oxygen in argon, which are virtually impossible to separate chromatographically. However by using argon as the carrier, the argon peak is eliminated and the oxygen peak can be clearly seen in the chromatogram. Sensitivity of the detector again depends upon the equipment set-up and the differential in thermal conductivity between the analyte and the carrier gas. It is one of the few detectors that can be used to detect the inorganic gases (such as H2O, CO, CO2, H2 etc.) and therefore it is the detector of choice for permanent gases. As the detector is non-destructive, it is often used in series with other detectors to detect gaseous or highly volatile sample components. The use of the TCD alongside an FID for the analysis of Transformer Oil Headspace gas is highlighted opposite. The linearity of the detector is widest when the differential between the carrier and analyte thermal conductivity is widest. The dynamic range can be extended by using micro or nano cell devices or by employing low volume single cell instruments with stream switching as described previously. Detector stability is reasonable but precautions should be taken to avoid temperature drift or fluctuation and a consistent flow of carrier should be established prior to analysis. Thermal Conductivity Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 10-9g (~10ppm) Response: all compounds Linearity: 104 (limited range) Stability: moderate Temperature Limit: ~ 400oC Gases – Carrier: (usually) Helium / Makeup: Same as carrier
© Crawford Scientific
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Fuel cell gas extract analysis Where: Conditions. Carrier gas: Helium Inlet: Purged packed 55oC Sample loop: 0.1mL TCD: 180oC
Oven Initial temp: 50oC for 10min Ramp: to 120oC at 10oC/min
Permanent gas analysis (GS-Molosieve, 30m×0.53mm I.D.) Where: 1. Neon 2. Oxigen 3. Nitrogen Conditions. Carrier gas: Helium at 35 cm/sec (4.6mL/min) Oven: 50oC isothermal
© Crawford Scientific
4. Methane 5. Carbon monoxide
Injector: Detector: Makeup:
Split 1:10, 250μL, 100oC TCD 125oC He at 10mL/min
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Inorganic gases (GS-GasPro, 30m×0.32mm I.D.)
Where: 1. 2. 3. 4.
Nitrogen CO2 SF6 COS
5. H2S 6. Ethylene oxide 7. SO2
Conditions. Carrier gas: Helium at 53 cm/sec Oven: 25oC for 3 min o o 25-200 C (10 C/min) -200oC hold
© Crawford Scientific
Injector: Split 1:50, 50μL, 200oC Detector: TCD 250oC
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C1-C10 Hydrocarbons (GS-Q, 30m×0.53mm I.D.) Where: 1. 2. 3. 4. 5. 6.
Methane Ethylene Ethane Propylene Propane Isobutane
Conditions. Carrier gas: Oven:
7. 1-Butene 8. n-Butane 9. cis-2-Butene 10. trans-2-Butene 11. Isopentane 12. n-Pentane Helium at 8 mL/min 70oC for 3 min 70-180oC (20oC/min) 180oC hold for 3 min 180-230oC (5oC/min) 230oC hold for 10 min
Injector: Detector:
13. n-Hexane 14. n-Heptane 15. n-Octane 16. n-Nonane 17. n-Decane
Split 1:50, 250oC TCD 300oC
Other GC Detectors Flame Photometric Detector (FPD) Compounds are burned in a hydrogen-air flame very similar to an FID detector. Sulphur and phosphorous containing compounds produce light emitting species (sulphur – 394nm / phosphorous – 526nm). A monochromatic filter allows only one of the wavelengths to pass and a photomultiplier tube is used to measure the amount of incident light and a signal is generated. A different filter is required for each detection mode.
© Crawford Scientific
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Flame photometric detector Flame Photometric Detector Characteristics • • •
• • •
Minimum Detectability (LOD): ~ 10-100pg (sulphur); 1-10pg (phosphorous) Response: Sulphur or phosphorous containing compounds (one at a time) Linearity: The response in the phosphorus mode is linear over four orders of magnitude. The sulphur response, varies such that the square root of the response is proportional to the concentration and is linear on a log–log scale over three orders of magnitude. Stability: moderate Temperature Limit: ~ 300oC Combustion gases: Hydrogen and Air. Makeup: Nitrogen
Photoionisation Detector (PID) Compounds eluting into a cell are bombarded with high-energy photons emitted from a lamp. Compounds with ionisation potentials below the photon energy are ionised. The resulting ions are attracted to an electrode, measured and a signal is generated.
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Photoionisation Detector © Crawford Scientific
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Photoionisation Detector Characteristics • • • • • •
Minimum Detectability (LOD) ~ 25-50pg (aromatics); 50-200pg (olefins) Response: Depends on lamp characteristics, conventionally used for aromatics and olefins (10 eV lamp) The response is linear over six or seven orders of magnitude. Stability: good Temperature Limit: ~ 200oC Makeup gas: Same as carrier
Electrolytic Conductivity Detector (ELCD) Compounds are mixed with a reaction gas and passed through a high temperature reaction tube. Specific reaction products are created which mix with a solvent and pas through an electrolytic conductivity cell. The change in the electrolytic conductivity of the solvent is measured and a signal generated. Reaction tube temperature and solvent determine the type of reactant detected.
Electrolytic Conductivity Detector Electrochemical Conductivity Detector Characteristics • • • • • •
Minimum Detectability (LOD): ~ 5 - 10pg (halogens); 10-20pg (sulphur); 10-20pg (nitrogen) Response: Halogens, sulphur or nitrogen containing compounds (one at a time) Linearity: Sulphur 103 - 104; Nitrogen 104 - 105; Halogens 105 - 106. Stability: good Temperature Limit: ~ 800 - 1000oC (halogens); 850 - 925oC (nitrogen); 750 - 825oC (sulphur) Gases: Hydrogen (halogens and nitrogen); air (sulphur)
© Crawford Scientific
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Mass Spectrometer (MS) The detector is maintained under vacuum. Compounds eluting from the GC column are bombarded with electrons (EI) or reagent gas ions (CI) in order to ionise them. Compounds fragment into characteristic charged ions or fragments and the resulting charged species are focussed and accelerated into a mass analysing device –typically a Quadrupole mass analyser or Ion Trap mass analyser. The mass analyser selectively allows ions of a selected mass to charge ratio to pass through the analyser to the electron multiplier where a signal due to that specific mass to charge ration ion is generated. The mass filter can be adjusted to allow specified ions to pass (selected ion monitoring mode) or to quickly scan over a range of mass to charge values (scanning mode). The abundance (total ion count) versus time is plotted to form the chromatogram. A mass spectrum can be obtained for each ‘scan’ which plots the number of ions at each different mass to charge ratio in the range selected.
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Mass Spectrometer Mass Spectrometer Characteristics • • • • • •
Minimum Detectability (LOD): ~ 1-10ng (SCAN); 1-10pg (SIM) Response: Any compound that produces fragments (or ions) within the selected mass range of the analysing device. May be an inclusive range of mass to charge ratios (SCAN mode) or selected ions (Selected Ion Mode (SIM)) Linearity: 105 - 106 (good dynamic range) Stability: good Temperature Limit: ~ 250oC Carrier gas: Helium
© Crawford Scientific
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