Gas Chromatography Detectors - The Flame Ionization detector > Page 36

This will result in a differential flow through the horizontal tubes with a consequent change in the output from the sensors. As the differential flow will be proportional to the pressure difference between the right-hand column of pure carrier gas and the center column full of vapor, the output from the sensor filaments will be proportional to the vapor density of the solute and consequently be related to the molecular weight. In fact with a second detector that measured the concentration of the solute, the gas density balance can be used to determine molecular weight of an eluted solute. This device has about the same sensitivity and linearity as the katharometer but, unfortunately, is no longer commercially available. It was one of the very few simple and inexpensive methods available for measuring the molecular weight of an eluted solute.

 

The Flame Ionization detector

Without doubt, the Flame Ionization Detector (FID) is the most useful GC detector available and by far that most commonly used in GC analyses. The FID has a very wide dynamic range, a high sensitivity and (with the exception of a few low molecular weight compounds) will detect all substances that contain carbon. The first FID was described about the same time by Harley and Pretorious (12), and McWilliams and Dewer (13). The FID is an extension of the flame thermocouple detector and is physically very similar, the fundamentally important difference being that the ions produced in the flame are measured as opposed to the heat generated. Hydrogen is mixed with the column eluent and burned at a small jet. Surrounding the flame is a cylindrical electrode and a relatively high voltage is applied between the jet and the electrode to collect the ions that are formed in the flame. The resulting current is amplified by a high impedance amplifier and the output fed to a data acquisition system or a potentiometric recorder. The detector usually requires three separate gas supplies together with their precision flow regulators. The gases normally used are hydrogen for combustion, helium or nitrogen for the carrier gas and oxygen or air as the combustion agent. The detector is normally thermostatted in a separate oven; this is not because the response of the FID is particularly temperature sensitive but to ensure that no solutes condense in the connecting tubes.