Читать книгу Process Gas Chromatographs - Tony Waters - Страница 31

A chromatographic separation cannot produce a measurement. Chromatography is merely a separating technique; it doesn't measure anything. To measure the concentration of the analytes, the analytical instrument must estimate the quantity of selected molecules as they elute from the column. It follows that every gas chromatograph needs a device to generate a signal proportional to the number of sample molecules exiting the column. This is what a detector does.

In any gas chromatograph, two things are happening in series. First the column separates the analytes, and then the detector measures them. To improve your troubleshooting ability, keep that distinction in mind.

Many detectors are available for gas chromatography, most developed for applications that require selective measurement or enhanced sensitivity.

The thermal conductivity detector (TCD) was the first gas chromatograph detector, and after much improvement it is still popular today. The TCD responds to the difference in thermal conductivity between pure carrier gas and carrier gas that contains sample molecules. So, when a TCD is used, the carrier gas is chosen to maximize the difference in thermal conductivity between the carrier gas and the analytes. The TCD is a general‐purpose detector that will respond to any analyte.

Most other detectors are selective; they respond only to certain kinds of molecules and often do so with very high sensitivity. For instance, the flame ionization detector (FID) responds only to compounds containing both carbon and hydrogen, so it's very useful in the analysis of hydrocarbons. The flame photometric detector (FPD) is also very sensitive, but only to sulfur or phosphorus compounds. It's most used to measure sulfur compounds in fuels and stack emissions to ensure compliance with environmental regulations.

Generally, detectors operate in the differential mode. When pure carrier gas is passing through a detector, its output signal should be constant. The analytical instrument reads that signal and offsets it to a value close to zero. We call that the baseline. Then, when the detector responds to the presence of analyte molecules, the instrument outputs a change in signal level proportional to the concentration of that component.

Chapter 10 provides a detailed review of the three detectors most used in process gas chromatographs (TCD, FID, and FPD) and briefly mentions some other detectors that are common in laboratory instruments but only occasionally deployed for online process applications.