Want to know more about detectors for GC and when to use them...
In my opinion, there are three major strengths of gas chromatography over other common analytical separation techniques. The first is the total separation power afforded by long wall-coated open tubular columns. The second is the speed of separation afforded by the gas-phase separation process. The third, and topic of this article, is the range and capabilities of its detectors. Because the carrier gases used in GC are transparent to most detectors, background levels and interferences are very low. This transparency provides choices in detectors not possible with other separation techniques.
Detectors fall into two general categories: universal and selective . Universal detectors are able to detect all compounds (or most compounds) that elute. A selective detector only detects compounds with specific molecular, elemental or physical properties. Table 1 lists GC detectors in terms of their general attributes, including if they are considered universal or selective.
To compare and contrast GC detectors, we must first define a few metrics related to detection in general. The first is the detection limit . This is the minimum quantity of material than can be distinguished from background. The limit of detection (LOD), another way of saying detection limit, is a measure of the ability of the detector to differentiate a signal generated by an eluting compound (the peak) from the neighbouring background noise. The numerical measure of LOD is known as the signal-to-noise ratio (S/N). The larger the S/N for a given amount of solute and set of conditions, the better the detector. The LOD is usually specified as the amount (mass or concentration) of compound that can be detected with a S/N of 2 or 3. The lower the amount of analyte that generates this S/N, the better the detector.
Another metric relating to detector performance is its dynamic range . This is the usable (operating) range over which the detector will generate a changing signal as the amount of analyte changes. The low end of the dynamic range is limited by the LOD. The upper range is limited by saturation of the detector. Sophisticated GC operators in lab coats say the signal is “maxed out” when the signal does not increase as analyte amount increases. This effect is typically seen for the solvent peak, where the top of the peak appears flat and perhaps noisy. But saturation can sometimes go undetected for large analyte peaks, causing errors in quantification.
Table 1: Typical attributes and performance specifications of common GC detectors.
Another aspect related to dynamic range, is the range over which the detector response is linear (signal increases proportionally with amount of analyte). The linear dynamic range is a characteristic that was more important in the early days of chromatography than it is now because computers can easily deal with non-linear calibrations. Nevertheless, a large linear dynamic range simplifies calibration and data reduction and, therefore, is still a useful thing to have. So, the larger the number, the better. Dynamic and linear dynamic ranges are usually stated as powers of 10 (orders of magnitude). The linear operating range of GC detectors is often several orders of magnitude larger than typical LC detectors. Typical dynamic and linear operating ranges are listed in Table 1 for common detectors.
When comparing selective detectors, one is interested in their ability to detect the characteristic of interest and reject everything else. For example, a flame photometric detector (FPD) can be selective for sulfur-containing compounds. Selectivity is usually stated as the ratio of the amount of a compound that does not contain the selected property that generates the same signal as a compound with the selected functionality (e.g., sulfur). So, if 1 µg (10-6 g) of carbon (C) in benzene generates the same response as 10 pg S (10-11 g) in thiobenzene, then the selectivity ratio would be 10-6/10-11 = 105
For selectivity, the larger the number, the better. Some detectors, especially mass spectrometers, have extremely high selectivity.
Some selective detectors are selective for a category or class of compounds, as is the case for an electron capture detector (ECD). The ECD is selective for compounds containing atoms or functional groups that are electronegative (atoms that attract electrons). These functional groups , among other things include halogens (Group 7 in the Periodic Table, F, Cl, Br, I) and oxygen-containing functional groups, among other things. With these types of detectors, values for LOD, dynamic range and selectivity are usually different for each of the functionalities, and in the case of the ECD, they disproportionally differ based on how many atoms/groups are on each molecule. The differences can be very dramatic (chromatographers need a little drama in their work from time to time, no?). Because of this, manufacturers often choose to use the same compound when stating performance numbers so realistic comparisons can be made.
Another characteristic of detector response is whether it responds to the concentration of an analyte passing through it or to the mass of the analyte passing through it. A concentration-sensitive detector will generate a signal proportional to the concentration of an analyte in the mobile phase as it passes through the detector. The thermal conductivity detector (TCD) is the most common concentration-sensitive detector. If for example, a makeup gas flow rate equal to the column flow rate were added at the end of the column prior to the detector, the concentration of an eluting analyte would be cut in half and so would its signal. In contrast, if all else were equal and the column flow rate were doubled, the detector response would not change because the concentration of analyte passing through the detector would remain the same. LOD of concentration-sensitive detectors is most often stated in concentration terms (e.g., g/mL).
Mass-sensitive detectors respond to the mass of analyte passing through the detector in a given time. The flame ionization detector (FID) is an example of a mass-sensitive detector and its response is stated in mass per time (g/sec). Adding makeup gas at the end of the column would not change the detector response because the same mass of analyte would pass through the detector in the same time (it will just be more dilute). However, if the flow of the column were reduced to ½, then the same amount of analyte would generate ½ the signal because the same mass of analyte would take twice the time to pass through the detector.
One last consideration of GC detectors is if they are destructive or not. Non-destructive detectors such as the TCD can detect solutes without changing them chemically. This is a benefit if you are interested in collecting fractions, smelling (olfactory detection), or especially if putting detectors in tandem (e.g., passing effluent through a non-destructive detector and then through a second detector).
By comparing the metrics described in Table 1, one can see why GC detectors are key differentiators of GC over other separation techniques.
This series of articles is produced in collaboration with Dr Matthew S. Klee, internationally recognized for contributions to the theory and practice of gas chromatography. His experience in chemical, pharmaceutical and instrument companies spans over 30 years. During this time, Dr Klee’s work has focused on elucidation and practical demonstration of the many processes involved with GC analysis, with the ultimate goal of improving the ease of use of GC systems, ruggedness of methods and overall quality of results.