Overview of FID:
The Flame Ionization Detector responds to any molecule with a carbon-hydrogen bond, but its response is either poor or nonexistent to compounds such as H2S, CCl4, or NH3. Since the FID is mass sensitive, not concentration sensitive, changes in carrier gas flow rate have little effect on the detector response. It is preferred for general hydrocarbon analysis, with a detection range from 0.1ppm to almost 100%. The FID’s response is stable from day to day, and is not susceptible to contamination from dirty samples or column bleed. It is generally robust and easy to operate, but because it uses a hydrogen diffusion flame to ionize compounds for analysis, it destroys the sample in the process.
The SRI FID features a unique ceramic ignitor which can run hot continuously, and prevent the flame from extinguishing even with large water injections or pressure surges from column backflush. This ignitor is positioned perpendicular to the stainless steel detector jet and does not penetrate the flame. Opposite this flame is the collector electrode. This positively charged metal tube serves as a collector for the ions released as each sample component elutes from the column(s) and is pyrolyzed in the flame; it doubles as a vent for the FID exhaust gas. The FID is equipped with an electrometer amplifier which has HIGH, HIGH (filtered), and MEDIUM gain settings. On an SRI GC, the hydrogen and air gas flows are controlled using electronic pressure controllers, which are user adjustable via the GC’s front panel. A thermostatted aluminum heater block maintains a stable detector temperature which is user adjustable up to 375 degrees C. The built-in air compressor is used to supply the air for the FID, eliminating bulky air cylinders.
Theory of FID Operation:
In the SRI FID, the carrier gas effluent from the GC column is mixed with hydrogen, then routed through an unbreakable stainless steel jet. The hydrogen mix supports a diffusion flame at the jet’s tip which ionizes the analyte molecules. Positive and negative ions are produced as each sample component is eluted into the flame. A collector electrode attracts the negative ions to the electrometer amplifier, producing an analog signal for the data system input. An electrostatic field is generated by the difference in potential between the positively charged collector electrode and the grounded FID jet. Because of the electrostatic field, the negative ions have to flow in the direction of the collector electrode.
The FID hydrogen diffusion flame:
M.I. Phillips Geological Consultants, LLC
Since 1983 Phone +1.9407237608
Mark Phillips, Registered Geologist, MBA, PMP
M. I. Phillips uses SRI 8610 instrumentation with a dual FID configuration for gas chromatograph and total gas analysis in our mobile laboratory operations. The system is designed to provide a continuous reading of total hydrocarbons in a gas sample stream while periodically performing a chromatographic separation of the sample to determine the exact composition (speciation) of the sample stream. The cycle time for analyzing the first seven hydrocarbons (methane through normal pentane) is one minute.
The ratio of air to hydrogen in the combustion mixture should be approximately 10:1. If the carrier flow is higher than normal, the combustion ratio may need to be adjusted. Flow is user adjusted through the Electronic Pressure Controllers (EPC); the rates used to generate test chromatograms at the factory are printed on the right side of the GC in the flow rate chart. The FID temperature must be hot enough so that condensation doesn’t occur anywhere in the system; 150 degrees C is sufficient for volatile analytes; for semi-volatiles, a higher temperature is used. In addition to using the ignitor to light the flame, it is maintained at an intermediate voltage level to prevent flameout (-750 or 7.5 volts). The ignitor is very durable and will last a long time, even at high temperatures.
FID detector schematic: