Gas Chromatography Detectors

The components eluted from the g.c. columns are detected by a detector, which emits a proportional electrical signal (voltage).

The detector of a gas chromatograph must have high sensitivity and reliability and a linear signal-concentration relationship over a wide range.

Which are the properties that a gas chromatography detector must have?

A list of these properties , in approximate order of importance, is as follows: sensitivity, stability, linearity, universality, selectivity, ease of use and cost.

Sensitivity is usually defined as the response of the detector per unit concentration of analyte usually as mV mg^-1 cm³.

The sensitivity determines the slope of the calibration graph and therefore to some extent the precision of analysis. High sensitivity very often means a low limit of detection, which means that you will able to detect and determine very small quantities of analyte.

Stability is the extent to which the output signal remains constant with time, given a constant input. Instability appears either as a rapid and random variation in the output signal –noise- or as a slow systematic variation called drift. Both noise and drift decrease the sensitivity of the detector because it is more difficult to see small peaks against a noisy background.

Linearity, is the extent of the range over which the signal is truly proportional to the concentration or amount of analyte. With a linear calibration curve the precision will be higher comparing to convex calibration curves.

Universality is the detector’s ability to detect all the components present in a mixture.

Selectivity is a measure of the response characteristics towards the various compounds. Some detectors respond to almost all compounds and are referred to as “universal”. Others only respond to certain types of compounds.

The following are the most common GC detectors:

The Flame Ionization Detector (FID)

The flame ionization detectorconsists essentially of a block in which hydrogen can be mixed with the effluent from a gas chromatography column and the mixed gases burned in air in a draught free enclosure. Two electrodes, maintained at a steady potential difference are placed in or near the flame and the current flowing between them is monitored. The current is approximately proportional to the amount of carbon in the form of volatile organic compounds which enter the flame in the column effluent, so that a graphical record of it, will take the usual form of a series of peaks superimposed on a steady baseline.

In general, the FID responds to compounds that yield electrically charged species on combustion  in a hydrogen/air flame.

Fig. 1: Schematic design of a flame ionization detector for gas chromatography

 

An advantage of the FID is that it is a universal detector as it responds to almost all organic compounds. Its operation is simple.

A disadvantage is that it is often too unspecific and insensitive for environmental analysis.

The FID is the most commonly used detector in GC mainly for the analysis of organic compounds. It is also used in quality-control analysis of pharmaceutical compounds.

The Electron Capture Detector (ECD)

The electron capture detector is an example of a selective detector. It provides selectivity for solutes with halogen, sulfur and nitro functional groups i.e. compounds that are able to “capture” electrons. Because of its high sensitivity and selectivity, it is much used for residue analysis – volatile halogenated hydrocarbons.

The detector consists of a beta emitter (a beta particle is an electron) such as ₆₃Ni. The emitted electrons ionize the mobile phase, which is usually N₂, resulting in the production of additional electrons that give rise to an electric current between a pair of electrodes (Figure 2).

If electrophilic molecules are introduced into the cell these absorb electrons and become negatively ionized. The electric current decreases.This decrease in electric current serves as the signal. 

Fig. 2: Schematic design of an ECD detector for gas chromatography

The main advantage of an ECD detector is its high selective toward solutes with electronegative functional groups, such as halogens, and nitro groups and for the fact that its detection limit is excellent.

The main disadvantage that its linear range extends over only about two orders of magnitude and  that is relatively insensitive to amines, alcohols, and hydrocarbons. 

The Thermal Conductivity Detector (TCD)

The thermal conductivity detector (TCD) is a universal GC detector in which the signal is a change in the thermal conductivity of the mobile phase. As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament’s electrical resistance depends on its temperature which in turn  depends on the thermal conductivity of the mobile phase - helium in this case. When a solute elutes from the column, the thermal conductivity of the mobile phase decreases and the temperature of the wire filament, and thus its resistance, increases. This change in temperature results in a signal.

A TCD detector has the advantage of universality since it gives a signal for any solute whose thermal conductivity differs for that of helium. However, it is used mainly for inorganic compounds.

A main advantage of the TCD detector is the fact that is nondestructive to analytes.

Fig. 3: Schematic design of a thermal conductivity detector

The Thermionic Detector (TID)

 The thermionic detector is selective toward organic compounds containing phosphorus and nitrogen.

 Compared with the flame ionization detector, the thermionic detector is approximately 500 times more sensitive to phosphorus-containing compounds and 50 times more sensitive to nitrogen bearing species. These properties make thermionic detection particularly useful for detecting and determining the many phosphorus-containing pesticides.

Mass Spectrometry (MS)

In GC-MS the effluent from the column is introduced directly into the mass spectrometer’sionization chamber in a manner that most of the carrier gas will be eliminated (Fig. 4). In the ionization chamber all molecules – solute, solvent and remaining carrier gas – are ionized and the ions are separated by their mass-to-charge ratio. A plot of ion’s intensities as a function of the ion’s  mass-to-charge ratios is called a mass spectrum and it offers qualitative information that can be used to identify the solute.

Fig. 4: Schematic design of a mass spectrometer

For more information on mass spectrometry see the post “How does a mass spectrometer work?”

REFERENCES

1. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
2. “Gas Chromatography”, J. Willett, John Wiley &Sons, 1987