Gas Chromatography FID Detector Common Faults and Troubleshooting

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There are twenty or thirty kinds of gas chromatography detectors studied so far, but only the TCD, FID, ECD, FPD, TID, and PID detectors are commonly used in commercial chromatographs. Among them, the FID (hydrogen flame ionization detector) is It is the most commonly used detector in gas chromatography. It has features such as high sensitivity, wide linear range, wide application range, easy to handle, etc. It is especially suitable for capillary gas chromatography. In the daily use of the FID detector, there are often no peaks, small signals, large baseline noise, etc. The author will briefly discuss the detector structure, common faults and troubleshooting methods. 

1FID structural characteristics

Hydrogen flame ionization detector (referred to as hydrogen flame detector) has high sensitivity to most organic compounds, generally 3 orders of magnitude higher sensitivity than thermal conductivity detector, and can detect 10 to 9 grades of traces. Amount of organic substances, suitable for trace organic analysis. It consists of an ion block, an ion head, a polarization coil, a collector, and a gas supply. The ion head is a key part of the detector. 

After the trace organic components are carried into the detector by the carrier gas, they are ionized under the action of a hydrogen flame. The generated ions move directionally under the external electric field of the emitter and collector to form micro-currents. The ionization efficiency of organics in hydrogen flames is extremely low, and it is estimated that only one pair of ions is produced per 500,000 carbon atoms. The number of ions generated by ionization is proportional to the mass of the measured component entering the detector per unit time within a certain range.

The weak ion current is converted into a voltage signal by a high resistance (108 to 1011 Ω). After being amplified by the amplifier, the chromatographic outflow curve is obtained by the terminal signal acquisition. In the case of normal ignition, the size of the FID signal is affected by ionization and collection effects. The factors affecting the ionization effects are sample properties (different material correction factors) and flame temperature (affected by the flow ratio of several gases); the effects of the collection effect include polarization voltages and nozzles, polarization poles, collectors The relative position. Therefore, to obtain high sensitivity for the same sample, the flow ratio of the optimal hydrogen, carrier gas, and air must be selected; the optimal nozzle, polarization pole, relative position of the collector, and appropriate polarization voltage. The flow rate of hydrogen, carrier gas, and air can be optimized by experiments. The general theoretical ratio is 30:30:300. 

2FID common faults and troubleshooting methods

2.1 The causes of chromatographic peak failure after sample introduction and its exclusion methods are as follows:

(1) If there is no ignition, first place a cold, bright iron plate on the top of the detector. If there are tiny beads of water, then it proves that the fire has been lit. On the other hand, it proves that the fire is not lit. At this time, check the hydrogen, Nitrogen, air sealing is intact, there is a leak. Secondly, it is necessary to measure the flow rate with soap foam flowmeter, properly increase the flow rate of hydrogen gas, reduce the flow rate of carrier gas and air, and then adjust the flow rate to the optimum flow velocity position after ignition.

(2) Interruption of signal output Check the connection of the signal line from the chromatograph to the workstation, and observe that there is no contact or disconnection. In addition, after sampling, use a multimeter to measure the chromatographic signal output and observe whether there is a signal output. If there is no signal output, it is proved that this fault is caused by the chromatograph and further inspection is required.

(3) Poor collector insulation The resistance of the collector and instrument housing should be greater than 1013Ω.

(4) Other reasons include damage to the sample pad, broken column (more common capillary column), and damage to the micro sample injector. 

2.2 Baseline noise fluctuations

(1) Reasons for electrical appliances First disconnect the detector signal line and observe the baseline operation under the acquisition status. If the baseline fluctuates greatly, it can be determined that the fault is the cause of the electrical appliance. At this time, it is necessary to further check whether the instrument grounding is Good (grounding resistance should be less than 5Ω), circuit boards and plug-ins are loose and so on.

(2) Measurement system pollution After the signal line is disconnected, the baseline operation is checked under the acquisition status. If the baseline operation is normal, the measurement system is contaminated. Need to check whether the column is invalid (requires activation treatment), whether the column inlet is contaminated (replacement of glass filaments, glass liners, etc.), detector contamination, mainly due to contamination of the ion head, because there will be impurity carbon knots at high temperatures, so it is necessary to carefully remove The lower detector is cleaned with a neutral solvent. 

2.3 Air peak masking component peak When analyzing trace components, such as analyzing the total hydrocarbon content of liquid oxygen, the oxygen signal peak retention time is the minimum, followed by methane, ethane, ethylene, etc. If the adjustment is not good, there will be oxygen covering methane or The oxygen peak was misclassified as a methane peak. The method of elimination is to gradually reduce the flow rate of hydrogen gas. In order to observe the gradual drop of the oxygen peak, the adjustment is satisfactory.

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