Gas chromatograph FID (hydroquinone detector) has high sensitivity, small dead volume, fast response, wide linear range, and can be effectively combined with capillary column, which has become the most widely used detector for organic microanalysis. The FID detection gas chromatograph system is mainly composed of three parts: detector, detection circuit (amplifier) ​​and gas path. When a fault occurs or the analysis spectrum is abnormal, it should first be judged which part is the difference. The common abnormalities of the FID system gas chromatograph summarized by the technicians of Shandong Luchuang Analytical Instrument Co., Ltd. are:

1, can not ignite - the problem is mainly in the gas path or detector; 2, the base flow is large - the problem is mainly in the gas path or detector; 3, the noise is very large - gas path, detector and There are problems in the circuit; 4, the sensitivity is significantly reduced - the gas path, detector and circuit are not normal; 5, no peak - gas path, detector, circuit is not normal; 6 The chromatographic peak shape is not normal---the injector, gas path and detector are the main inspection objects; 7. The baseline drift is serious---the gas path and the detector are all possible; 8. Sometimes there are signals, sometimes no signal- -- The problem is mainly on the circuit.

First, check the gas path:

Check whether the flow rate of H2 (hydrogen), N2 (nitrogen), AIR (air) is normal, the air flow is too small, and the nozzle is seriously leaked, which will cause a large popping sound and cannot ignite; hydrogen is too small, too much nitrogen will cause ignition Difficult and easy to extinguish; nozzle leakage, column leakage will not only make ignition difficult, but also lead to reduced sensitivity, or even peak; hydrogen to nitrogen flow ratio will significantly affect sensitivity; large hydrogen flow will cause noise Larger; the gas path system is not clean, including injector contamination, detector contamination, or insufficient aging of the column can cause base flow, loud noise, and baseline drift. Please pay attention to the base flow size when igniting: Before ignition, the baseline position of the amplifier should be adjusted as close as possible to the zero position of the recorder. Under the condition of not rotating the zero potentiometer, after the ignition, the distance of the recording pen from the zero position can be Indicate the size of the base flow, which can be changed by changing the recorder range or the amplifier attenuation factor. Generally speaking, when the H2 gas is adjusted back to the normal working value after ignition, the base flow deviation is less than 1mV, indicating that the system is very clean, the base flow is less than 10mV, and generally Can be used, if the base flow is greater than tens of mV, it means that the system pollution is more serious, then the noise and drift are very large, and the instrument stabilization time is also longer. A simple way to check which part is contaminated is to separately increase the operating temperature of a certain part. If the base flow is significantly larger, the part is seriously polluted. Blockages and leaks in the gas path (including the injector) tend to cause abnormal peaks; the flattening of the liner in the injector also destroys the normal peak shape.

Second, check the detector:

Check the nozzle for air leaks, which will affect ignition, sensitivity, peak shape and baseline drift; check that the position of the polarizer and the nozzle is correct: the nozzle is higher than the plane of the polar circle, the sensitivity is significantly reduced, which is often loaded When the column tube is used, the column tube will be caused by the top of the quartz nozzle. If the anti-nozzle port is lower than the plane of the polarizing pole or the pole of the polarizer is in contact with the nozzle, the noise will increase. Check if the collector is well insulated, if the collector is insulated. If it is not good, the noise will be very large, the baseline will be unstable, and the drift will be serious. If the collector ion current signal line is in poor contact or disconnected, it will not cause a peak; if the detector is polluted, it can be determined by the temperature change of the base flow. The way to remove pollution is to remove and wash parts and perform high temperature aging.

Third, check the circuit:

When the gas chromatograph is not ignited and the collector plug is removed, the baseline can be used to judge and check whether the amplifier is normal. The light is taken from the baseline of the amplifier. Generally, the noise should be less than 5 uv and the drift should be less than 10 uv/0.5 u. If possible, a small current can be input to the amplifier, that is, a 109Ω high resistance is connected in series with a battery to the input end of the amplifier (collector ion line plug end), the other end of the battery is grounded, the amplifier gain is 109Ω, and the output should have About 100mv, if the amplifier gain is in the 108Ω range, the output should be about 10mv, which means that the amplifier works normally. In the absence of high impedance, it is used to refer to the input end of the light touch amplifier, and a large signal should appear at the end. This is the simplest and most rough way to judge whether the amplifier is normal. If the above check is not normal, the circuit should be further inspected. The high-resistance switching relay and the dummy solder joint of the AD549 integrated operational amplifier often cause the amplifier to malfunction. The soldering iron is soldered one by one at each spot weld to judge and check; the circuit inside the amplifier shielded iron box (mainly high resistance) will be seriously affected by the moisture; the collection of the pole ion signal wire is thin and easy to break, often causing the signal Nowhere and no peak; polarization pole-to-ground voltage (polarization voltage) is generally 220V-230V (some products are designed to be 250V- 300V) The high voltage regulator tube given the polarization voltage will damage the FID polarization voltage, resulting in no peak or chromatographic peak malformation. Use a multimeter to measure the DC voltage of the pole to ground to check the polarization voltage. Is it normal? The noise is sometimes generated from the high-voltage Zener diode that gives the polarization voltage. The method of judging is to remove the 220-230V polarization point and see if the noise is eliminated or reduced. In addition to replacing the high-voltage Zener diode, the polarization voltage. A 300KΩ resistor is connected in series on the 230V, and a 0.33uf/400V capacitor is connected to the ground, which also effectively filters out the noise from the high voltage Zener diode. If the amplifier has an output, but zeroing does not work, then the problem must be in the zero potentiometer or the corresponding connection line.