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There are only two possible causes of non-repetition of retention time. One is unstable column temperature; the other is variation in flow rate. The failure of the detector will not cause duplication of retention time. Other reasons for the non-repetition of retention times include poor injection techniques, excessive sample injection, and column damage.
The steps to eliminate non-recurring faults in retention times are as follows:
(1) Repeat injection test: In order to further confirm that the retention time does not repeat the fault, the repeatability of the injection should be checked first. It is advisable to have one person operate independently during repeated injections. This can better solve the problem of reproducibility of injection time; if the retention time after repeated injections cannot be repeated, it should be transferred to the next step.
(2) Temperature Control Accuracy and Program Temperature Repetition Check: In the constant temperature analysis, first check whether the column room temperature is stable at the set value required for the original analysis operation. If necessary, check the stability of column room temperature. If the set value and the actual column temperature deviate from the original analysis conditions, the original analysis conditions shall prevail; if the column room temperature suddenly jumps during operation, temperature control failure should be performed. Check and eliminate. When the application temperature rises, it is necessary to check whether the start and end of the program temperature increase process and the temperature increase rate are consistent with the original analysis conditions. Care should be taken during the inspection to ensure that there is sufficient time for the initial temperature to be consistent with each re-warming, especially if the initial temperature is close to room temperature. The temperature rise rate of the program can be verified by first determining the required time between the first and the last two points of the temperature rise, and then dividing the difference between the final temperature and the initial temperature by the time value. There is another condition in program warming that is not easily discovered by the operator. That is, during the warming process, the temperature changes very unevenly. However, the overall rate of warming shows no change. This phenomenon can be compared by recording the program warming curve. If no automatic recording method can be recorded by the manual method segment by segment, then the program can be controlled by paragraph by segment when the temperature rise ends.
(3) Carrier gas flow rate check: The change of carrier gas flow rate is another important cause of non-repetition of retention time. It can be confirmed by the actual flow rate after the column or after the detector can be measured with a soap film flow meter. For the isothermal analysis, the deviation between the measured value and the predetermined value is mainly detected, and if necessary, the set value is re-adjusted so that the flow rate reaches a predetermined value. For program warming, it is necessary to check whether there is a large change in the flow rate of the carrier gas at the beginning or end of the temperature. If the difference between the initial and final flow rate exceeds 2 mL/s (when the column diameter is 4 mm), the steady flow characteristics are considered to be poor. At this time, the system needs to be further checked for leaks, and the working pressure of the steady flow valve and the pressure-stabilizing valve Is it desirable? Leakage of the system, regardless of temperature-programmed chromatograms or constant-temperature chromatography, is a reason why retention values ​​are not repeated and should not be overlooked. Leakage of the septum in the inlet of the system is often generated in the event of system leakage.
(4) Column inspection: If there is no abnormality in airtightness and carrier gas flow rate, it should be suspected that the column itself has a problem and check the column. The first thing to notice is whether the chromatographic peaks are tailed. If the tails are trailing, reduce the sample volume or dilute the sample concentration to avoid overloading the column. If the repeatability of the retention value increases after reducing the injection volume, it indicates that the original column phase has a small amount of loss or poor filling; at this time, the original column can still be used. If the above method is also invalid, the column has been damaged and the new column must be replaced.
Gas chromatograph instrument troubleshooting method (poor quantitative repeatability)
There are many reasons that cause quantitative non-replication, which generally can be attributed to two major categories: one is a simple sensitivity change type, that is, in addition to the quantitative repeatability failure, other indicators have not been found abnormal; the other is the accompanying sensitivity Variants, that is, in addition to changes in sensitivity are also accompanied by other anomalies, including baseline instability, changes in peak retention time, and peak shape distortion and other anomalies. The reasons for the first type of failure are mainly due to poor sampling technique, plugging of the syringe, uneven sample preparation, accumulation of pollutants at the inlet, and leakage of gas in the gas path. The second type of failure is mainly caused by changes in the carrier gas flow rate, detector contamination, overload, column temperature changes, and detector operating conditions (such as hydrogen, polarization voltage, and pulse voltage).
Taking into account the possibility of the possibility of various failures and the convenience of fault identification, the following detection solutions were developed:
(1) Injection sample inspection: Poor injection techniques are the most likely causes of non-repetition of chromatographic peaks. It usually shows that the peak height/peak area is large and small, and the peak height/peak area size is irregular. The key to improving reproducibility of injection is to maintain the repeatability of each step of the injection operation. This includes the sampling operation, sampling to the idle time during injection, the speed of needle insertion, and the early and late withdrawal of the syringe. Usually the operator can achieve the required requirements after repeated injections of repeated training.
(2) Syringe inspection: After the operator has improved the sample injection technology, the sensitivity of the chromatographic peak is still not significantly improved. It is necessary to carefully check whether the syringe itself is clogged or leaking. Replace a good syringe if necessary and re-inject the test.
(3) Sample uniformity test: The prepared sample is not uniformly mixed in the sample bottle or the syringe contaminates the sample and volatilizes the sample during each sampling. This may affect the reproducibility of the peak sensitivity (cannot miss this check). There is a high possibility that the quantification is not repeated due to the above-mentioned three reasons, and both are closely related to the sample injection operation and therefore can be checked together. Only if there is no abnormality after the above inspection is it transferred to the next inspection step.
(4) Accompanying phenomenon observation: While checking the sensitivity, pay attention to whether the following abnormal phenomena occur, including whether the baseline is stable, whether the peak retention time is repeated, and whether the peak shape is distorted. If there is one, it should be First remove the faults that occurred, and then repeat the quantitative repeatability test. If no abnormality is found, it should be transferred to the following inspection procedure.
(5) Inlet contamination and system leak check: After shutting off the bridge current (for TCD), remove the inlet septum and observe if there is any contamination or deposits in the inlet. If so, clean it up. And cleaning. After the septum is cleaned, it is necessary to perform a test leak on the gas circuit system: block the detector outlet and observe that the rotor in the rotameter should be able to drop to zero, otherwise the gas line will leak. Make sure that (6) is checked without serious contamination of the inlet and no air leakage.
(6) Special reason check: For some detectors, the fault anomaly accompanying certain reasons is not obvious and can be easily ignored. Therefore, inspection should be carried out according to this item, so as not to overlook the factors that may cause failure.
a) For FIDs, lower polarization voltages and unstable hydrogen flow may result in sensitivity changes without other significant anomalies. The polarization voltage can be tested first to determine if the polarization voltage is too low, too low polarization voltage, and no polarization voltage is a fault. The normal polarization voltage is 150~300V. If the polarization voltage is normal, it should be transferred to the sensitive file of the amplifier to observe the change of the initial current of the hydrogen flame base. When the hydrogen flow is unstable, the base flow should exhibit the phenomenon of swing and drift.
b) For any detector, certain components of the sample gradually condense and accumulate in the detector, which will affect the sensitivity of the next injection. In severe cases, it will also result in blockage of the gas path. The usual solution is to increase the temperature of the detector appropriately to reduce or eliminate the condensation in the sample chamber.
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Gas chromatograph instrument troubleshooting method (retention time is not repeated)