So far, there are twenty or thirty kinds of gas chromatograph detectors, but only TCD, FID, ECD, FPD, TID, PID detectors are commonly used in commercial chromatographs, among which FID (hydrogen flame ionization detector) It is the most commonly used detector for gas chromatography. It has high sensitivity, wide linear range, wide application range and easy to master. It is especially suitable for capillary gas chromatography. FID detectors often show no peaks, small signals, and large baseline noise in daily use. This paper briefly discusses the structure, common faults and troubleshooting methods of the detector.
1 FID structural features
Hydrogen flame ionization detector (referred to as hydrogen flame detector) has high sensitivity to most organic compounds. It is generally three orders of magnitude higher than the thermal conductivity detector and can detect -9 grade trace organic substances. Suitable for the analysis of trace organics. It consists of an ion block, an ion head, a polarized coil, a collector, a gas supply, etc. The ion head is a key part of the detector.
After the trace organic component is carried into the detector by the carrier gas, it is ionized by the action of a hydrogen flame. The generated ions are orientated by the external electric field of the emitter and the collector to form a microcurrent. The ionization efficiency of organic matter in a hydrogen flame is extremely low, and it is estimated that only one pair of ions is generated per 500,000 carbon atoms. The number of ions produced by ionization is proportional to the mass of the component to be tested entering the detector per unit time within a certain range.
The weak ion current is converted into a voltage signal by high resistance (108~1011 Ω). After amplification by the amplifier, the chromatographic elution curve is obtained by the terminal signal acquisition. The FID signal size is affected by the ionization effect and the collection effect under normal ignition conditions. The factors affecting the ionization effect are the nature of the sample (different material correction factors) and the flame temperature (affected by the flow ratio of several gases); the factors affecting the collection effect are polarization voltage and nozzle, polarization pole, collector Relative position. Therefore, to obtain high sensitivity for the same sample, it is necessary to select the flow ratio of hydrogen, carrier gas, and air; the nozzle, the polarization pole, the relative position of the collector, and the appropriate polarization voltage. The flow rate of hydrogen, carrier gas and air can be explored by experiments, and the general theoretical ratio is 30:30:300.
2 FID common faults and troubleshooting methods
2.1 Chromatography does not produce peak after injection
The cause of the fault and its troubleshooting are as follows:
(1) If there is no ignition, first place a cold bright iron plate on top of the detector. If there is a small water droplet, it will prove that the fire has been lit; otherwise, it proves that the fire is not lit. At this time, it is necessary to check the hydrogen gas. Whether the sealing of nitrogen and air is in good condition and whether there is air leakage. Secondly, use the soap flow meter to measure whether the flow rate is normal, increase the flow rate of hydrogen gas appropriately, reduce the flow rate of the carrier gas and air, and adjust the flow rate to the ** flow rate position after the ignition.
(2) Signal output interruption Check the signal line connection from the chromatograph to the workstation to check for poor contact or disconnection. In addition, after the injection, the chromatographic signal output is measured with a multimeter, and the presence or absence of the signal output is observed. If there is no signal output, it is proved that the fault is caused by the chromatograph, and further inspection is required.
(3) The collector insulation is not good. The resistance of the collector and the instrument case should be greater than 1013 Ω.
(4) Other reasons include injection pad damage, column breakage (comparison of capillary columns), and micro-injector damage.
2.2 Baseline noise fluctuations are large
(1) Electrical reasons First, the detector signal line is disconnected, and the baseline operation is observed under the acquisition state. If the baseline fluctuation is large, it can be judged that the fault is an electrical cause. At this time, it is necessary to further check whether the instrument is grounded. Good (grounding resistance should be less than 5 Ω), board and plug-in is loose.
(2) After measuring the system pollution, disconnect the signal line, check the baseline operation in the acquisition state, and prove the measurement system pollution if the baseline operation is normal. Need to check whether the column is invalid (requires activation treatment), whether the column inlet is contaminated (replace glass filament, glass liner, etc.), detector contamination, mainly ion head contamination, because there will be impurity carbon junction at high temperature, need to be carefully removed The lower detector is cleaned with a neutral solvent.
2.3 Air peak masking component peak
When analyzing trace components, such as analysis of total hydrocarbon content in liquid oxygen, the peak retention time of oxygen signal is the smallest, followed by methane, ethane, ethylene, etc. If the adjustment is not good, oxygen will cover methane or the oxygen peak will be misidentified as methane. peak. The elimination method is to gradually reduce the hydrogen flow rate, and the oxygen peaks are gradually decreased after the injection, and the adjustment is satisfactory.
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