Selecting a suitable detector for Gas Chromatographic analysis

Gas Chromatography offers a wide range of detectors to meet your analysis applications. The two detectors which are in common use are the Flame Ionisation detector and Thermal Conductivity detector as these cover detection of majority of compounds encountered in routine applications. However, there are other detectors also available which are useful for specific applications.

Schematic of Flame Ionization Detector

Gas Chromatographic detectors fall under three distinct categories:

Non –selective-Respond to all compounds coming out of the chromatographic column except the carrier gas.

Selective– Respond to a range of compounds with common physical or chemical characteristics

Specific-respond only to a single chemical compound in the carrier stream

Desirable features of a detector

The desirable features in the detector selected for your application should cover some if not all of the following attributes:

• Reproducibility of response to changes in composition of eluting gas
• High sensitivity
• Large linearity of response
• Least peak broadening and resultant loss of resolution
• Preferably non-destructive

Detector types and their common applications

Flame Ionisation Detector (FID)

FID is the most popular of all detectors. It shows high sensitivity to most organic compounds. The advantages of FID are high sensitivity, large linear response, low noise and ruggedness. However, it is destructive in nature and the sample gets destroyed. Compounds with little or no response include inorganic gases such as CO, CO₂, NH₃, CS₂, NO_X, noble gases, halogenated compounds, formic acid and formaldehyde.

Thermal Conductivity detector (TCD)

TCD is also referred to as a universal detector as it responds to most compounds including inorganic gases such as O₂, N₂ and CO_2.It permits detection at both high percent and low ppm levels.

Nitrogen Phosphorus Detector (NPD)

The Nitrogen Phosphorus detector responds selectively to most organic compounds that contain phosphorus or organically bound nitrogen down to the picogram level. Its high level of sensitivity and specificity makes it suitable for analysis of low levels of drugs and pesticides.

Electron Capture Detector (ECD)

Electron Capture detector provides trace analysis of halogenated compounds, nitriles, nitrates and compounds with conjugated double bonds. It finds widespread use in analysis of environmental samples containing organochlorine pesticides, PCB’s etc.

Flame Photometric Detector (FPD)

Flame photometric detector is a highly sensitive detector for phosphorus   compounds or alkyl tin compounds. Sulfur compounds include mercaptans, alkyl sulphides, H₂S, CS₂, SO₂ andCO₂ in petroleum fractions and pulp-milling processes. Alkyl tin finds use in marine antifouling paints which are applied to boat hulls and ocean oil rigs. Such paints released into the sea pose an environment hazard.

Photoionization detector (PID)

The photoionization detector responds to compounds ionized by UV light .it is used to detect aromatic and unsaturated compounds in drinking water, waste water, soil and sludge samples. Other applications include detection of aromatics such as benzene, toluene, xylene and polynuclear aromatic hydrocarbons in petroleum fractions.

Detector Selection Summary

Detector	Typical Classes of Compounds	Detection limits	Dynamic range
FID	Majority of organic compounds	100 pg	107
TCD	Universal detector	1ng	107
ECD	Halides, nitrates, nitriles, peroxides, anhydrides, organometallics	50fg	105
NPD	N, P containing compounds	10pg	106
FPD	S,P containing compounds	100pg	103
PID	Aromatics, Polynuclear aromatic hydrocarbons	2pg	107