DETECTORS Flame Ionization Detector - FID
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DETECTORS Flame Ionization Detector - FID Overview 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. (SRI Capillary FID GC with built-in Hydrogen Generator) Thermostatted Detector viewport heater block (remove Swagelok cap) Flameport Collector electrode Ceramic ignitor Electrode lead to amplifier 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 375oC. The optional built-in air compressor may be used to supply the air for the FID, eliminating bulky air cylinders. The built-in hydrogen generator is another option: the standard model can produce 20mL/min for use as both carrier gas and FID combustion gas at pressures up to 25 psi. Amplifier gain switc

Theory of 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...

DETECTORS Flame Ionization Detector - FID Fite Edit View Acquisition Help Column: 15m MXT-1 Carrier: Helium @ 10mL/min FID gain = HIGH FID temp = 150oC FID ignitor = -400 Temperature program: Initial Hold Ramp Final FID noise averages less than 1 QQ^V from peak to peak Sample: 1mL of 1000ppm C1-C6 Carrier: Helium @ 10mL/min FID H2 at 25psi = 25mL/min FID air at 6psi = 250mL/min FID temp = 150oC FID ignitor = -750 FID gain = HIGH Valve temp = 90oC

DETECTORS FID - Flame Ionization Detector The BTEX chemicals (Benzene, Toluene, Ethylbenzene, and Xylenes) are volatile monoaromatic hydrocarbons found in petroleum products like gasoline. Due to industrial spills and storage tank leakage, they are common environmental pollutants. Groundwater, wastewater, and soil are tested for BTEX chemicals in many everyday situations. The chromatogram below was obtained using an FID-equipped SRI GC. 1 pL 100ppm BTEX sample 15m MXT-VOL capillary column FID gain = HIGH FID temp = 150oC FID ignitor = -400 Results:...

DETECTORS FID - Flame Ionization Detector 1. Set the FID amplifier gain switch to HIGH for most hydrocarbon applications. If peaks of interest go off the scale (greater than 5000mV), set the gain to MEDIUM. When peaks of interest are 20 seconds wide or more at the base and extra noise immunity is desired, set the gain switch to HIGH (filtered). This setting broadens the peaks slightly. FID amplifier gain switch 2. Set the FID hydrogen flow to 25mL/min, and the FID air supply flow to 250mL/min. The approximate pressures required are printed in the gas flow chart on the right-hand side of the...

DETECTORS Flame Ionization Detector - FID FID Troubleshooting Whenever you experience problems with your FID, review your operating procedures: check the detector parameters, check to make sure you are on the correct channel of the data system display, check the mixture of hydrogen (25mL/min) and air (250mL/min), check gas pressures and connections, check the oven and detector temperatures, and all the other variables that compose your analysis. Having ruled out operating procedure as the source of the problem, there are two simple diagnostic tests you can perform. Detector problems can be...

DETECTORS FID - Flame Ionization Detector Cleaning the FID The FID detector rarely requires cleaning or servicing. It may develop a film or coating of combustion desposits in the flameport with extended use. Use the FID detector viewport to check for visible deposits. If you’re experiencing problems with your FID detector, try cleaning it, even if you can’t see deposits through the viewport. 1. Unscrew the viewport cap nut and examine the flameport interior for coatings or films. If residue is found, the collector electrode and the flameport will need cleaning. 2. Remove flameport assembly...

DETECTORS Flame Ionization Detector - FID Cleaning the FID continued 4. Remove the FID ignitor element a. The ignitor element is brittle and will break when stressed, so handle the ignitor carefully, mindful of any torque on the blades. While holding the ignitor by the ceramic body with one hand, loosen the 1/4” swagelok-type nut that holds it in place. There is a graphite ferrule inside this nut that secures the ceramic ignitor body when the nut is tightened. Ignitor blade Graphite ferrule Ignitor body b. Carefully pull the ignitor down out of the flameport. Disconnect the ignitor from the...