Gas chromatography instrument troubleshooting methods

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The website recommended by a friend is great. Troubleshooting methods for gas chromatography instruments (cleaning of components) I. Cleaning of gas lines, injectors, and syringes: When cleaning the connections in the gas lines, it is necessary to first remove the connectors at both ends of the tube, then take that section of tubing out of the chromatograph. At this point, the dust on the outside of the tube should be wiped clean, so as to prevent contamination once the inside of the tube is cleaned. When cleaning the inner walls of the pipeline, anhydrous ethanol should be used first for cleaning; this helps to remove most of the particulate impurities in the pipeline, as well as the organic substances and moisture that can be dissolved by ethanol. During this clearance process, if it is found that the pipeline is blocked, a bulb can be used to blow under pressure; if this does not work, a thin wire needle can be considered to clear the pipeline. If this method does not clear the pipeline, an alcohol lamp can be used to heat the pipe, causing the blockage to carbonize at high temperatures and thus clearing it. After cleaning the gas circuit pipes with anhydrous ethanol, it is necessary to check whether there are contaminants on the inner walls of the pipes that are not easily dissolved by ethanol. If not, the pipeline can be heated and purged with dry gas, after which it can be reinstalled in its original gas circuit for use. If the analysis of the sample process indicates that there may be other contaminants on the inner walls of the gas path that are not easily soluble in ethanol, other cleaning solutions can be selected based on the solubility properties of those specific substances. The order of selecting cleaning solutions should be to use high-boiling-point solvents first, followed by low-boiling-point solvents for soaking and cleaning. Available cleaning solutions include naphthalene, N,N-dimethylformamide, methanol, distilled water, propane, ether, Freon, petroleum ether, ethanol, etc. The cleaning of the injector (including the vaporization chamber) should begin with unclogging. The blockages in the injector are usually fragments of the injection gasket or highly boiling substances that have carbonized within the sample; these solid impurities can be cleared using a stainless steel probe, followed by rinsing with ethanol or acetone. To achieve more thorough cleaning, a 2:1:4 mixture of H2SO4/HNO3/H2O can be used to clean the sampler first, followed by distilled water, and then acetone or ethanol. After cleaning, dry it, install it in the instrument and pass gas through it for half an hour; heat it to 120°C and wait for a few hours before it can operate normally. When assembling or disassembling the sampler, be careful not to break the heater wires or allow the wires to touch the housing ； The temperature sensing element should also be reinstalled at the original temperature measurement point after being put back into the sampler. Typically, the temperature sensing element and the heater of the sampler are in close contact; if the distance between them is too large, it will result in an excessively high vaporization temperature. Before use, the syringe can be cleaned with acetone to prevent contamination of the sample; however, it is best to clean the syringe itself once or twice with the sample that is to be injected. Only the sample should be drawn in during cleaning; when discharging the sample, it must be done outside the sample bottle. The syringe should be cleaned immediately after use to prevent it from being contaminated by high-boiling-point substances in the sample. Generally, the following solutions are used in sequence for cleaning: 5% sodium hydroxide solution, distilled water, acetone, chloroform, and finally the area is dried using a vacuum pump. II. Cleaning of the detector During the operation of a chromatograph, the detector can sometimes become contaminated by lost stationary phase, as well as high-boiling-point components, substances that are prone to decomposition, or corrosive materials present in the sample. At this point, the detector should be cleaned. There are three possible scenarios during cleaning: in one case, the contaminants consist only of high-boiling-point substances, and these can usually be removed by heating the detector to its maximum operating temperature and then introducing a carrier gas. The second scenario is when the detector has only mild contamination, in which case steam cleaning can be used. The process involves injecting several dozen microliters of distilled water or a solvent such as acetone at the injection port; after 1 to 2 hours, it is checked whether the baseline is stable. The third approach is a thorough cleaning method used when the two simple methods mentioned above are insufficient to solve the problem. This method requires disassembling the detector, and it is also necessary to select an appropriate solvent – one that can dissolve the contaminants without causing further contamination or damage to the detector. Do not touch the cleaned parts with your hands at this time. 1. Cleaning of the Thermal Conductivity Detector (TCD): Fill the detector’s measurement cell with solvents such as propane, ether, and decahydronaphthalene; soak it for a period of time (about 20 minutes), then pour out the solution. Repeat this process several times until the liquid poured out is relatively clean. When a single solvent is not sufficient for cleaning, depending on the nature of the contaminant, a high-boiling-point solvent can be used first for soaking and cleaning, followed by repeated cleaning with a low-boiling-point solvent. After washing, heat to remove the solvent, reinstall the detector on the instrument, and then heat with a carrier gas for several hours of flushing before it can be used. 2. Cleaning of the hydrogen flame ionization detector (FID): When the FID is not severely contaminated, it is not necessary to remove it for cleaning. In such cases, it is sufficient to remove the chromatography column, connect the sample inlet to the detector using a tube, and then use a carrier gas to raise the temperature of the detector to above 120°C. Inject another 20 microliters of distilled water through the injection port, and then clean it with several dozen microliters of ethanol or Freon 113 as a solvent (propane can also be used, but note that the nozzles in the hydrogen flame chamber of some chromatographs are not suitable for cleaning with propane). Maintain at this temperature for 1–2 hours to check whether the baseline is stable; if it remains unsatisfactory, repeat the above steps or follow the method below. When the contamination is severe, the detector must be removed for cleaning. The method is to first remove the collector, polarizing electrode, nozzle, etc.; if the nozzle is made of quartz, it should be placed in water and soaked overnight first ； If the nozzle is made of materials such as stainless steel, it can be carefully polished first with fine sandpaper grit 300–400 together with the electrode, and then soaked in an appropriate solvent (such as a 1:1 mixture of methanol and benzene). It can also be cleaned using ultrasonic waves, followed by rinsing with methanol, and then dried in an oven. Be careful not to use halogen-containing solvents such as chloroform and dichloromethane. To avoid interaction with polyethylene materials, which could lead to increased noise. The cleaned components should be picked up using forceps; do not touch them with your hands. Care must also be taken during assembly after drying, otherwise it will get dirty again. After loading the instrument, pass the carrier gas for half an hour first, then ignite to raise the temperature of the detection chamber; it is best to maintain the temperature at 120°C for a few hours before raising it to the operating temperature. Troubleshooting Methods for Gas Chromatography Instruments (Heat Conductivity Cell Detector) 1. Bridge current fault: With an inert gas flowing through the heat conductivity cell, turn on the bridge current switch and adjust the bridge current control knob. The bridge current should be able to be stably adjusted to the predetermined value. If it is found during the adjustment process that the current cannot be increased, especially when the thermal cell is at high temperature and the bridge current cannot reach its maximum rated value, this can be considered a fault where the bridge current fails to reach the desired level. This type of fault can occur for the following reasons: the connections of the heat conduction units are not properly made ； The heating wire in the heat conduction cell is broken or the leads are open-circuited ； There is a fault with the bridge circuit voltage regulator ； The bridge circuit configuration circuit is disconnected or the ammeter is faulty. 2. Baseline zeroing fault: After the bridge current is set and stabilized, adjust each knob for thermal conductivity zeroing separately to bring the baseline indication on the recorder back to zero. If, no matter how the knobs are adjusted, the baseline remains unchanged or cannot be set to zero, it is considered that there is a fault with the thermal conductivity zeroing. The reasons for the fault in which thermal conductivity cannot be zeroed out are as follows: asymmetric resistance of the heating wire or incorrect wiring ； The hot wire hits the wall or is severely contaminated ； Open circuit in the zeroing potentiometer leads ； The recorder is open-circuited or unresponsive ； The flow rates of the two gas circuits differ too much. To rule out the issue of non-zeroable thermal conductivity, follow these steps: (1) Attenuation setting test: When it is observed that the baseline is offset from zero, adjust the attenuation setting from low to high and watch whether the baseline deviation gradually decreases. (2) Check of the zero adjustment knob function: Rotate the coarse, medium, and fine adjustment knobs respectively to observe whether there is a response in the baseline. (3) Dual-channel flow inspection: On the basis of leak testing the gas lines, use a soap film flow meter to measure the flow rates of each of the two gas lines, and check whether there is a significant difference between them. (4) Check of errors in the resistance values of the heating wires: Measure the resistance values at the terminals of each heating wire in the thermal conductivity cell. Generally, the difference in resistance values between various groups of heating wires should not exceed 0.2–0.5 Ω; if this value is exceeded, it should be handled according to (6). (5) Hot wire touching the wall or becoming contaminated: The fact that the hot wire is touching the wall can be confirmed by measuring the insulation resistance between the hot wire and the tank. Severe contamination of the hot wire can be eliminated or partially eliminated by cleaning the thermal conductivity cell; the specific steps are outlined in the section on cleaning the detector. (6) Asymmetric hot wire or incorrect lead connection: This usually occurs after repairing the thermal conductivity cell circuit; in such cases, it is necessary to carefully check the connections between the hot wire leads. The correct connection is such that the four heating wires form a bridge circuit, with the heating elements on the two upper arms of this bridge located in the same gas path. (7) Handling of excessive differences in flow rates between the two channels or gas line leaks: Excessive differences in flow rates between the two channels can be addressed by adjusting the gas line control valves; however, there should be no leaks in the gas lines at this time. (8) The zero-setting circuit is open. (9) The recorder is open-circuited or unresponsive. 3. Baseline noise and drift. There are many reasons that can cause instability in the baseline of thermal conductivity detectors; there are roughly dozens of such reasons. Common ones include: (1) The supply voltage is too low or fluctuates too much, as well as large variations in the load on the power supply within the same phase ； (2) There is condensation or foreign matter in the gas outlet pipe ； (3) Poor grounding of the instrument ； (4) Unstable control of the column room temperature, or fluctuations or drifts in the detection room temperature ； (5) The carrier gas is not pure, the gas path is contaminated, there is a leak in the carrier gas path, or the pressure of the carrier gas is too low or it is about to run out ； (6) Poor control accuracy of pressure stabilizing valves and flow stabilizing valves ； (7) The difference in the air paths between the two columns is too large, resulting in poor compensation ； (8) There is wind at the carrier gas outlet, or there is soap solution in the soap film flow meter at the outlet ； (9) Loose column filler ； (10) Excessive mechanical vibration ； (11) The bridge circuit DC regulated power supply is unstable ； (12) Loss of stationary phase in the column ； (13) The carrier gas flow rate is too high ； (14) Poor contact of the potentiometer in the bridge circuit configuration ； (15) Thermal conductivity cell contamination ； (16) Local overheating of the thermosensitive element ； (17) Poor contact of power plug and leads, poor contact of shift band switch ； (18) The tungsten wire has not aged; the tungsten wire of the thermosensitive element is touching the wall ； (19) Bridge current is too high. When the chromatograph experiences an unstable baseline issue, the first thing to check is whether there is contamination in the gas path of the chromatograph. This is not only because impure airflow in the gas path can directly affect the stability of the baseline, but more generally, under conditions of impure gas flow, many factors that would have only a minor impact on baseline stability when the gas path is clean (such as changes in airflow rate or fluctuations in temperature control) can suddenly exert a greater influence on baseline stability. This is the interaction between gas path contamination and other instabilities. The steps below are a series of measures taken for the gas path, assuming that contamination exists in it. There are three reasons for this contamination: loss of the stationary phase, contamination of the gas path pipelines by impurities, and impure carrier gas. To further distinguish the root cause of the fault, the following inspection steps can be followed: (1) Lower the column temperature. The loss of stationary phase in the chromatography column is exponentially related to the column temperature. Therefore, lowering the column temperature will significantly reduce the loss of stationary phase. If the baseline stabilizes as the column temperature decreases, it indicates that the column loss was originally too high, and further adjustments are required based on the specific analysis conditions. (2) Whether larger loss of columns is allowed. Under the constraints of certain analytical methods, it is necessary to allow a certain degree of column loss; in such cases, improving the stability of other parts of the instrument can be considered to enable the entire analytical method to function. (3) Address the issue of large column loss. First, it is necessary to suspect whether the column has aged sufficiently; this can be confirmed by further aging the chromatography column under elevated column temperature and then observing whether the baseline improves at the operating temperature. If the aging treatment shows no effect, a cleaning test can be conducted by injecting several drops of distilled water at a column temperature of over 150°C (with each injection amount being around 10–20 microliters). After cleaning with steam, if it is effective, the chromatography column can be considered to be contaminated with impurities ； If cleaning with steam is ineffective, it is necessary to consider replacing the chromatography column. (4) Leak testing of the gas path behind the column. There should be no leaks in the tubing from the chromatography column to the thermal conductivity detector, including the gas path of the thermal conductivity detector itself. If there is a leak in that area, oxygen from the air will seep into the gas path through the leak, affecting the stability of the baseline; in severe cases, it can corrode the tungsten wire and cause permanent damage to it. The method for testing for leaks behind the column is very simple: simply block the outlet of the heat conduction cell and check whether the rotor of the flow meter in the corresponding gas path drops to zero. (5) Replace the filter and purifier. The filters and purifiers in the carrier gas circuit of the chromatograph need to be activated or replaced after being used for a period of time. It should be replaced promptly even more when the carrier gas source is not clean. Observe the changes in baseline stability after filtration and filter replacement. If the baseline improves significantly, it indicates that the purity of the carrier gas is insufficient, or that the filters and purifiers are not functioning properly. (6) Impure carrier gas: Although a gas source with low purity can be used as a higher-grade gas source with low impurity content after passing through a good filter or purifier. However, this will affect the service life of the filters and purifiers, and the more impurities are present in the air supply, the shorter the usable life of these filters and purifiers. Therefore, the ultimate solution is to use a carrier gas source with high purity, along with effective filtration and purification devices. This ensures that the baseline remains as stable as possible, with a normal service life of up to one year. (7) Clean the contamination in the gas circuit piping. To clean contaminants from the gas line, it is possible to first perform cleaning by injecting distilled water or ethanol. The method involves raising the temperature of the entire system to above 150°C, then injecting 10–20 microliters of distilled water or ethanol multiple times using a syringe into the injector; after the corresponding peaks have appeared, the stability of the baseline is observed. If the baseline improves significantly, it can be assumed that there is only mild contamination of the tubing, and it can still be used ； If there is no change or only a minor change in baseline stability, thorough cleaning of the pipeline should be considered. In the gas path, the injection port, the connection tube from the column to the thermal conductivity cell, and the chamber of the thermal conductivity cell are prone to contamination; therefore, they require special attention during cleaning. (8) Air intrusion into the detector. Minor leaks in the gas path behind the column are the root cause of oxygen from the air penetrating into the thermal conductivity detector. This mostly occurs at the connections between the pipe fittings and the tungsten wire elements; for methods to repair air leaks in these areas, refer to the aforementioned procedures for checking and eliminating air leaks in the gas circuit. Troubleshooting methods for gas chromatography instruments (hydrogen flame ionization detector): 1. Do not adjust the zero setting before ignition. After the amplifier has been preheated but before the hydrogen flame is lit, the baseline should be able to be adjusted to the zero point on the recorder. At this time, changing the attenuation ratio on the amplifier should not cause any deviation in the baseline. If, after performing these operations, it is found that no matter how much the microcurrent amplifier knob is adjusted, the baseline on the recorder cannot be brought back to zero, then it is considered a zero-setting issue. The reasons for the failure to zero before ignition are as follows: incorrect wiring ； Poor insulation in the ion chamber ； The lead cable has a short circuit ； Microcurrent amplifier damaged ； The recorder is malfunctioning. 2. Ignition failure: Under normal operating conditions of the chromatograph, press the igniter button; after a short while, a popping sound should be heard as the hydrogen-oxygen mixture is ignited, and at that point a shift in the baseline will be observed. After ignition, place an item such as a cool glass sheet or a shiny metal sheet directly above the gas outlet at the flame; after a short while, traces of water vapor condensation on the surface of the glass sheet or metal sheet can be observed. If the above phenomena occur, it indicates that the instrument is igniting properly. If no such signs of ignition occur during the ignition process, an attempt to ignite should be made again; if there is still no response after multiple attempts, it can be considered that an ignition failure has occurred. The reasons for the failure to ignite are as follows: malfunction of the ignition components ； The ignition power supply has no output ； Improper air-fuel ratio before and after ignition ； Hydrogen leakage ； There is a blockage in the gas line ； The wiring and connectors of the ignition circuit are open. For ignition failure, follow these steps to diagnose and resolve the issue: (1) Check the brightness of the ignition wire: It should emit a bright yellow-red light; if the ignition wire can be illuminated, it indicates that the ignition circuit is generally functioning properly ； If the spark shows no reaction at all, it indicates a problem with the ignition circuit; in this case, proceed to step (7) for further inspection. (2) Check of gas flow ratio in the gas circuit: During normal ignition, the hydrogen flow rate should be increased while the air flow rate should be reduced appropriately; the carrier gas or backflow gas should be set to a very low level or turned off. If the flow rates are not set correctly, adjustments should be made. (3) Hydrogen leakage check: After powering off, close all flow control valves except those for oxygen. Block the nozzle of the hydrogen flame ionization chamber with a silicone rubber gasket or a clean, soft rubber tip, applying slight downward pressure to prevent hydrogen from flowing out of the nozzle; at this point, the rotor in the hydrogen rotameter should gradually drop to zero. If the rotor does not descend, or descends but does not reach zero, it indicates a leak in the hydrogen supply; proceed according to step (4) ； If the rotor can be reduced to zero, proceed to (5) for processing. (4) Eliminate air leaks: Perform leak testing to locate the source of the leaks; if necessary, test each section of the gas pipeline separately. After locating the leak, appropriate action should be taken based on the specific circumstances; detailed methods are described in the section on checking and fixing gas line leaks. When troubleshooting hydrogen leakage, one thing to keep in mind is that leaks in the downstream section of the carrier gas circuit can also prevent the rotor of the hydrogen gas circuit from reaching zero position; this is due to the connection between the carrier gas and hydrogen gas circuits ahead of the nozzle. (5) Blockage in the gas path: A blocked gas path, especially one at the nozzle, is a common cause of failure to ignite or of the flame going out after ignition. Methods to eliminate blockages are described in the cleaning section for air path components. (6) Adjustment of gas path ratios: The inability to ignite or difficulty in ignition is often related to the flow ratio of various gases in the gas path during ignition. During ignition, the hydrogen flow rate should be increased several times, while the air flow can be slightly reduced. The nitrogen used as an inert gas should be decreased or even turned off, and then gradually increased after ignition. This adjustment can be repeated several times until a flame is achieved. (7) Inspection of the good contact of the ignition components. (8) Check of the output voltage of the ignition circuit: By directly measuring whether the output voltage of the ignition power supply is at the rated value, it can be determined whether there is a fault with the ignition power supply. (9) There is an open circuit in the connection wire and plug. (10) Poor contact of the detector. 3. No zero adjustment possible after ignition: The hydrogen flame ionization detector allows the baseline to be adjusted to zero before ignition, but it is not possible to bring the baseline back to its level before ignition once ignition has taken place. This phenomenon is known as the failure to adjust the zero level after ignition. The reasons for the failure to zero after ignition include water accumulation in the ion chamber ； The polarization voltage is reversed ； Gas path and detector contamination ； Severe column loss ； Improper airflow adjustment ； Baseline compensation has no effect. The troubleshooting of this type of fault can be carried out by following these steps: (1) Check the function of the baseline compensation knob: Note the direction in which the baseline deviates after ignition, and remove the hydrogen flame signal cable from the side of the ion chamber. At this time, by rotating the baseline compensation knob, it is possible to observe the direction and magnitude of the baseline compensation deflection. Under normal conditions, the direction of the baseline compensation should be opposite to the direction of the signal deviation; if it is in the same direction as the signal deviation, considering changing the polarity of the polarization voltage may be advisable. If the baseline does not respond after adjusting the baseline compensation knob, or responds but the deviation value is too small, proceed to step (9). (2) Detector temperature check: When ignited by a hydrogen flame, the temperature of the ion chamber must be above 100°C; otherwise, moisture will accumulate in the ion chamber, damaging the insulation of the collector and preventing the amplifier from being zeroed out. Another point to note is that right after the chromatograph is started, although the detector indicates a temperature of over 100°C, the ion chamber is at a certain distance from the central heating element; therefore, it is necessary to wait for some time until the actual temperature of the ion chamber reaches over 100°C before igniting it. (3) Is the flame too large? Observe directly whether the hydrogen flame after ignition is too large or too red, and whether it has reached the collection plate; if so, proceed according to (4). (4) Air flow adjustment: Adjust the flow rate of each air path to reduce the flame size, and set the optimal air flow ratio if necessary. If oxygen is used in place of air, care must be taken to increase the flow rate of nitrogen back-blow appropriately, up to the limit where it will not cause extinguishment. After adjusting the flow ratio of the gas supply, observe the hydrogen flame; it should be a small flame that emits a slight blue light or is colorless. (5) Test to see if the baseline can be zeroed out after lowering the column temperature: Reduce the temperature of the chromatography column to room temperature and observe whether the baseline can be zeroed out. If it can be zeroed out, it indicates severe column leakage. (6) Handling of severe column drift: In cases of severe column drift, it is first necessary to check whether the column has been subjected to aging treatment. If the column has aged but the baseline still cannot be zeroed out, it is necessary to consider changing the operating conditions or replacing the column with a new one. (7) Severe contamination of the gas path and detector: Severe contamination of the gas path and detector can be detected by the red or yellow color of the hydrogen flame; the thorough solution is to clean the gas path and detector. Another important cause of contamination in the gas circuit is insufficient purity of the gas source, as can be confirmed by the fact that the baseline can be reset to zero after replacing the filters and purifiers. (8) Treatment of water accumulation in the ion chamber: Extinguish the hydrogen flame and raise the temperature of the ion chamber; after 1 hour, the water accumulation in the ion chamber should be dried, after which normal ignition can be carried out. (9) Handling of reversed polarization voltage or faulty baseline compensation circuit: Once it is confirmed that the polarity of the polarization voltage is reversed, the polarity can be corrected by turning the polarization voltage polarity switch or reconnecting the polarization voltage connector ； When the baseline compensation circuit is ineffective or its effect is too weak, it is necessary to check whether the baseline compensation potentiometer is desoldered, whether the slider and other components are malfunctioning, whether the baseline compensation voltage value is correct, and whether there are any open circuits or short circuits in the baseline compensation circuit. 4. Troubleshooting of unstable baseline: When using a hydrogen flame detector to analyze samples, it is essential that the chromatography instrument have a stable and straight baseline. To achieve this, in addition to correctly selecting various operating conditions, it is often necessary to analyze and address the various factors and causes that lead to baseline instability. The reasons for baseline instability in hydrogen flame detectors are complex; at least the following common ones include: (1) An improper flow ratio of hydrogen, air, and carrier gas in the gas circuit ； (2) The hydrogen flame ionization chamber is damp, and the collector has poor insulation ； (3) Severe loss of the stationary phase in the chromatography column ； (4) The gas supply pressure is too low, with fluctuations in gas supply pressure ； (5) Contamination of hydrogen and air pipelines as well as carrier gas, or impure gas source ； (6) Temperature fluctuations and drifts in the column chamber and detection chamber ； (7) Hydrogen flame ionization chamber nozzle contamination ； (8) There is a leak in the gas supply system ； (9) Unstable polarization voltage, causing poor contact ； (10) Poor contact of the signal cable or excessive vibration ； (11) The supply voltage of the microcurrent amplifier is too low, the high-resistance components are damp, and the internal solder joints are loose ； (12) Unstable recorder, poor instrument grounding, power supply interference, excessive electrostatic field interference around the instrument ； (13) Poor contact of the attenuator contacts or solder joints ； (14) A strong wind is blowing at the outlet of the hydrogen flame ionization chamber ； (15) There is too much dust in the ambient air of the instrument. The above lists various possible causes of the unstable baseline issue in hydrogen flame detectors. Due to the numerous reasons, and in order to improve inspection efficiency, the following inspection steps are provided: (1) Environmental inspection: First, use visual methods to check whether there is too much dust in the environment where the instrument is located, whether strong winds are blowing at the exit of the ion chamber, whether there is a strong electrostatic field near the ion chamber, and whether the instrument’s workbench is experiencing significant vibrations. (2) Fire extinguishing check: After the hydrogen is turned off, the hydrogen flame goes out. At this point, the baseline recording of the instrument is observed; if the baseline recording improves and a relatively satisfactory valid baseline is achieved, it is determined that there is a fault in the gas supply system ； If the baseline recording is still unsatisfactory, it indicates a fault in the circuitry (including the detector circuit). (3) Gas circuit ratio check: In the gas circuit, the flow rates of hydrogen, air, and nitrogen – the relative proportions of these three are crucial for maintaining a stable flame. When the flame is unstable, both the base flow and noise increase ； When the flow ratios in each channel are appropriately set, the highest sensitivity and an ideal baseline can be achieved. If there is no significant improvement in the baseline after adjusting the flow rates of the air paths, or if it is not possible to achieve the optimal settings for any of the air paths, then further inspection should be carried out to gain more insights. (4) Baseline drift and fluctuation inspection: Check for signs of baseline instability to determine whether it is simple baseline drift and fluctuation or other forms of noise; if it falls under the former, proceed to (5), and if it falls under the latter, proceed to (14) and handle it as a baseline noise fault. (5) Verify whether all component peaks have appeared after injection. (6) Removal of high-boiling-point components: When certain components retain in the column for too long, affecting subsequent normal injections, the backflushing method via the gas path can be used to remove them. The backflush time of the gas path should be at least as long as the original peak emergence time for sample injection. Another approach is to appropriately increase the flow rate and column temperature to allow high-boiling-point components to escape more quickly, thereby shortening the injection cycle. (7) Inspection of temperature control trend: Observe the changes in the temperature of the detection column chamber and the detector temperature separately. During testing, special attention should be paid to whether the temperature change trends of the column chamber and the detector are consistent with those of the baseline drift, and it is necessary to verify that their cycles are the same. If there is synchronization between the two, it is a fault in the temperature control system ； If there is no observable change in temperature, or if a change does occur but it is not synchronized with the baseline drift, then inspection (9) should be performed. (8) Temperature control system failure: A decrease in temperature control accuracy is a typical fault in the temperature control system. (9) System air leakage check. (10) System leak repair. (11) Check for too low gas source pressure and pressure fluctuations. (12) Change the air supply source and increase air resistance: Adjust the flow rates of air and hydrogen to the optimal level. (13) Ion chamber and nozzle condensation: When the temperature in the ion chamber is lower than the column temperature or the boiling point of the condensates, it is possible for high-boiling-point substances or water vapor in the sample to condense in the ion chamber, especially in the nozzle. At this point, considering raising the temperature of the ion chamber to eliminate this condensation phenomenon should be considered. (14) Reducing the column temperature to observe baseline stability: Since there is an exponential relationship between the loss of the stationary phase in the chromatography column and the decrease in column temperature, if there is significant loss of the stationary phase, reducing the column temperature will result in a substantial drop in that value. This method can quickly determine whether column loss is excessive. (15) Treatment for excessive loss of the stationary phase: First, it is necessary to consider the maximum temperature at which the stationary liquid can be used; if this value is close to the column temperature being used, excessive loss will inevitably occur during operation ； If it still must be used at this time, it should be used on the low sensitivity setting ； If the analytical method permits, other types of chromatography columns can be used. Another common cause of column loss is insufficient aging of the column; if the baseline stabilizes after the column is aged at an elevated temperature for a period of time, this is the underlying reason. When a column is accidentally damaged during use, a new chromatography column must be replaced. (16) Flame color inspection: Block out the surrounding bright light and carefully observe the color of the hydrogen flame at the nozzle. Under normal conditions, the flame should be light blue or invisible; if there is a distinct color in the flame, such as yellow, red, or flashing spots, it is considered that the flame is contaminated. (17) Turn off the nitrogen and observe the noise: When nitrogen is used as the carrier gas, shut off the nitrogen flow control valve to temporarily reduce the carrier gas flow to zero; if nitrogen is used as an auxiliary gas (also known as backflow gas), then close the auxiliary gas control valve. Thereafter, observe whether the baseline stability improves; if it does, it indicates contamination in the nitrogen gas circuit. (18) Nitrogen gas line contamination: Nitrogen gas line contamination includes impure nitrogen and contamination of the entire pipeline. If the nitrogen gas source is impure, a new filtering and purifying element (such as a molecular sieve) can be installed; this will help stabilize the baseline in the short term, thereby providing confirmation. If the baseline noise disappears after replacing the filtration unit, it is necessary to consider using a pollution-free nitrogen gas source ； If the baseline noise remains unchanged after replacing the filter, the possibility of contamination in the piping system should be considered. Usually, when using nitrogen as the carrier gas, the tubing between the column and the ion chamber must be considered first. Of course, the gas path before the column can also become contaminated. One way to distinguish between the two is to carefully observe the pattern of the baseline noise; if there are irregular disturbances in the form of peaks along the baseline, it should be considered as contamination before the column, while if there are no such peak-like disturbances, it is considered as contamination after the column. The contaminated air pathways should be cleaned promptly. (19) Air and hydrogen pollution treatment: If the air and hydrogen gas lines become contaminated, it can also affect the baseline stability of the hydrogen flame. A method to confirm and distinguish which one, hydrogen or air, causes pollution is to fix the nitrogen level and gradually increase or decrease the hydrogen level, while observing whether a maximum base flow appears. If there is no maximum base flow even as the hydrogen level increases, that is, if the base flow keeps rising in one direction as the hydrogen level increases, it can be considered that the hydrogen gas path is contaminated ； Otherwise, it is considered that there is contamination in the air circuit. To determine whether gas line contamination is caused by impure gas supply or dirty gas lines, this can be done by observing the change in baseline noise after replacing the filter purifier. If the baseline remains stable for a short period after replacing the filter, it indicates that there is no contamination in the piping downstream of the filter; the contamination occurs in the piping upstream of the air supply or the filter. At this point, it is necessary to consider replacing the air source and cleaning the preceding pipelines. If it doesn’t work after replacing the filter, the hydrogen or air pipelines need to be cleaned. (20) Nozzle contamination: Generally speaking, aside from the significant change in flame color caused by large amounts of effluent flowing from the column, it is contamination at the nozzle that can alter the flame color. When the nozzle surface is covered with organic substances, the flame is affected as soon as it is ignited. At this point, the ionizer cover can be removed to clean the nozzle separately with ethanol; if necessary, the nozzle assembly can be taken out and soaked in ethanol for a few minutes, then gently wiped with a brush or silk cloth. After drying it with hot air, it can be put back in place. Three points should be noted at this time: first, do not touch the surface of the nozzle with your hands again, to prevent it from being contaminated once more ； Second, an appropriate sealing gasket must be replaced when reinstalling it, and after installation, the outlet should be blocked with a clean rubber plug to check for leaks ； Third, when installing the nozzle, be sure not to let the wrench touch the nozzle, otherwise it is easy for the nozzle to break or for its base to crack, resulting in air leakage! (21) Poor gas path ratio: It should be considered that the gas flow rate is not properly adjusted when a significant deviation is found between the gas path ratio and the ideal value. Another abnormal phenomenon that requires attention is the situation where, when adjusting the gas flow ratios, it is not possible to reach the desired values; for example, it may be impossible to find the baseline peak when adjusting nitrogen flow, or the saturation point may not be identifiable when adjusting air flow. In such cases, it is necessary to consider the possibility of contamination in the gas supply lines, and inspection according to step (14) should be carried out. (22) Inspection and troubleshooting of unstable circuits and detectors. (23) Improvement of environmental conditions: When abnormalities are detected in the environment surrounding the chromatograph, appropriate measures should be taken to eliminate them one by one. If too much dust is found indoors, identify the devices that generate it and find a way to isolate them. When a strong wind blows at the outlet of the ion chamber, the doors and windows, the blower, or the position of the chromatograph should be adjusted. To prevent electrostatic interference, especially that caused by charged clothing on staff, it is necessary to use work clothes that do not generate static electricity or to avoid staying near the exit of the ion chamber. Severe vibration of the workbench is usually caused by the use of wooden tables. Where conditions permit, chromatographs are placed on concrete workbenches for operation. Troubleshooting methods for gas chromatography instruments (non-repetitive retention times): There are only two likely reasons for non-repetitive retention times, one of which is unstable column temperature ； The other is that the flow rate changes. And detector failures do not cause non-repeatability in retention times. Other reasons for non-repetitive retention times include poor injection technique, excessive injection volume, and column damage. The steps to rule out the issue of non-repetitive retention times are as follows: (1) Repeated injection check: To further confirm the problem of non-repetitive retention times, the repeatability of injections should be checked first. When performing repeated injections, it is best for one person to carry out the operation independently, as this helps to address the issue of variability in injection time ； If the retention time still cannot be reproduced after repeated injections, proceed to the next step. (2) Check of temperature control accuracy and program heating repeatability: During isothermal analysis, it is first necessary to check whether the column chamber temperature remains stable at the set value required for the original analysis procedure. If necessary, check the stability of the column chamber temperature; if there are deviations between the set value and the actual column temperature from the original analysis conditions, the original analysis conditions should be followed ； If there are sudden fluctuations in the column chamber temperature during operation, a check and correction of temperature control faults should be carried out. In the case of program heating, it is necessary to check whether the start and end column temperatures as well as the heating rate during the program heating are consistent with the original analysis conditions. During inspection, it should be noted whether there is sufficient time to maintain a consistent starting temperature each time the temperature is raised again, especially when the starting temperature is very close to room temperature. The heating rate of programmed temperature rise can be verified by first measuring the time required between the starting and ending points of the temperature increase, and then dividing the difference between the final and initial temperatures by this time value. There is another situation during programmed temperature rise that is not easily detected by the operator. That is, during the heating process, the temperature changes unevenly, rising sometimes quickly and sometimes slowly. However, no change was observed in the overall heating rate. This phenomenon can be compared by recording the program temperature rise curve. If no automatic recording method is available, manual recording can be done section by section; once the programmed temperature increase is complete, the sections can be compared one by one. (3) Carrier gas flow rate check: Changes in the carrier gas flow rate are another important reason for inconsistent retention times. This can be confirmed by measuring the actual flow rate behind the column or after the detector using a soap film flow meter. In constant temperature analysis, the main focus is on detecting the deviation between the measured value and the predetermined value; if necessary, the set value is adjusted to ensure that the flow rate meets the specified requirements. For programmed temperature rise, it is necessary to check whether there is a significant change in the carrier gas flow rate at the initial and final temperatures. If the difference in flow rate between the starting and ending points exceeds 2 mL/s (when the column inner diameter is 4 mm), it is considered that the steady-flow characteristics are poor; in such cases, it is necessary to further check whether there are leaks in the system, and whether the operating pressures of the flow stabilizing valve and pressure regulating valve meet the required standards. System leakage is a factor that cannot be ignored, as it causes non-reproducible retention times in both programmed temperature chromatography and constant temperature chromatography. Leakage from the injection port gasket is a common issue in system leaks; when sampling frequently under high-temperature conditions, it is important to replace it promptly. (4) Column inspection: If there are no abnormalities in airtightness or carrier gas flow rate, the problem is likely to lie with the chromatography column itself, and the column should be inspected. First, check whether the chromatographic peak is tailing; if so, reduce the sample volume or dilute the sample concentration to avoid overloading the column. If the reproducibility of retention times improves after reducing the injection volume, it indicates that a small amount of the stationary phase in the column has been lost or that the packing was inadequate ； At this point, the original chromatography column can still be used. If the above methods also fail, it indicates that the chromatography column is damaged and must be replaced with a new one. Troubleshooting methods for gas chromatography instruments (poor quantitative repeatability). There are various reasons for the lack of quantitative repeatability, which can generally be classified into two main categories: one is the type of simple sensitivity variation, meaning that aside from the issue with quantitative repeatability, no other abnormalities are detected ； Another category is the type with accompanying sensitivity changes, meaning that in addition to changes in sensitivity, other abnormal phenomena also occur, including baseline instability, changes in peak retention times, and distortions in the peak shape. The causes of the first type of faults mainly include: poor sampling technique, leaks in the syringe, uneven sample preparation, accumulation of contaminants at the sampling port, and air leaks in the gas circuit. The causes of the second type of faults are mainly: changes in the carrier gas flow rate, detector contamination or overload, changes in column temperature, and changes in the operating conditions of the detector (such as hydrogen gas, polarization voltage, pulse voltage, etc.). Considering the likelihood of various types of failures and the ease of identifying them, the following detection plan has been formulated: (1) Injection technique inspection: Poor injection technique is the most likely cause of non-reproducible chromatographic peaks. It usually manifests as fluctuations in peak height/peak area, with these values changing in an irregular manner. The key to improving injection repeatability lies in maintaining repeatability in all steps of the injection process. This includes the sampling operation, the downtime from sampling to injection, the speed of inserting the needle, and the timing of removing the syringe. Typically, after undergoing sufficient training on injection repeatability, operators can meet the required standards. (2) Syringe inspection: Even after the operator’s sample injection technique improves, there is no significant change in the sensitivity of the chromatographic peaks; it is therefore necessary to carefully check whether the syringe itself has any blockages or leaks. If necessary, replace it with a good syringe and repeat the sampling test. (3) Sample homogeneity check: Uneven mixing of the prepared sample in the sample vial, contamination of the sample by the syringe during each sampling, as well as sample evaporation, can all affect the reproducibility of the peak sensitivity (this check must not be omitted). Quantitative non-repeatability is very likely to be caused by the above three reasons, all of which are closely related to the sampling procedure; therefore, they can be examined together. Only when no abnormalities are found after the aforementioned checks is it possible to proceed to the next inspection step. (4) Observation of accompanying phenomena: While checking the sensitivity, pay attention to whether any of the following abnormal phenomena occur, including whether the baseline is stable, whether the retention time of the peaks is consistent, and whether the peak shape is distorted. If any of these issues arise, address the corresponding fault first before conducting the quantitative repeatability test again. If no accompanying abnormal phenomena are detected, proceed to the next inspection step. (5) Inspection for injection port contamination and system leaks: After turning off the bridge current (for TCD), remove the injection port gasket and check whether there is any contamination or buildup inside the injection port; if so, it must be removed and cleaned. After cleaning and installing the gasket, it is necessary to perform a leak test on the gas circuit system: block the detector outlet and check whether the rotor in the rotameter can drop to zero; if not, it indicates that there is a leak in the gas circuit. Check (6) after confirming that there is no severe contamination in the injection port and no air leaks in the gas path. (6) Inspection for special reasons: For some detectors, the fault abnormalities associated with certain reasons are not very apparent and easy to overlook; therefore, inspections should be carried out according to this item in order to avoid missing any potential factors that could cause failures. a) For FID, a low polarization voltage and unstable hydrogen flow rate may cause changes in sensitivity without any other obvious abnormalities. To address this, the magnitude of the polarization voltage can be tested first to determine whether it is too low; both an excessively low polarization voltage and no polarization voltage constitute faults. Under normal conditions, the polarization voltage is 150~300V. If the polarization voltage is normal, one should switch to the sensitivity setting of the amplifier in order to observe the variations in the base current of the hydrogen flame; when the hydrogen flow is unstable, the base current should exhibit fluctuations and drifts. b) For any detector, the gradual condensation and accumulation of certain components in the sample within the detector will affect the sensitivity after the next injection; in severe cases, it can even cause blockages in the gas path. The usual solution is to appropriately increase the temperature of the detector in order to reduce or eliminate condensation in the sample chamber. Troubleshooting methods for gas chromatography instruments (no peaks appearing and reduced sensitivity): Under the selected operating conditions, when a specified sample is injected into the chromatograph and no corresponding peaks appear on the recorded spectrum, this condition is referred to as a problem of no peaks appearing. If peaks do appear, but their intensity differs significantly from that of the previously recorded spectra, it is considered a problem of abnormal sensitivity. Normally, the sensitivity decreases, which is also known as a low-sensitivity fault. According to this concept, both non-peak occurrence and low sensitivity faults are relative to known given operating conditions; therefore, their confirmation should be carried out after checking the predetermined operating conditions first. Although no peak generation and low sensitivity are two different types of faults, many of their causes are the same. Therefore, a fault diagnosis procedure applicable to both can be provided to resolve their troubleshooting issues in a more concise manner. The steps for troubleshooting issues of no peak during maintenance and low sensitivity are as follows: (1) Check for repeatability of operating conditions: First, it is necessary to verify whether the operating conditions are similar to those originally known ； This includes the flow rates of each gas path, the temperature values in various temperature zones (such as the vaporization temperature, column temperature, and detector temperature), the magnitude of the bridge current, whether the flame is lit, and whether the power supply is connected. If abnormalities are detected in the operating conditions, efforts should be made to bring the operating value close to the originally specified value, and the factors affecting the restoration of the operating value should be identified promptly. (2) Check whether the detector responds: This check is mainly carried out to address the issue of no peak formation. The method for checking the detector’s response should vary depending on the type of detector. a) The thermal conductivity detector can be tested using the simplest air path blockage and release method: specifically, one first blocks one of the outlets of the thermal conductivity detector by hand, then suddenly releases it after a short while, thereby creating an airflow fluctuation; under normal conditions, this fluctuation should also cause a variation in the baseline of the spectrum. After testing one channel of detectors, you can try another one. If there is a fluctuation at the baseline after the above tests, it indicates that the thermal conductivity detector is responding. b) For the hydrogen flame detector, the following simple method can be used to check for any reaction: one approach is to use handheld forceps to bring them close to the collector electrode of the detector and move them above it; as the electric field changes, the baseline should exhibit corresponding fluctuations ； Another method is to light it with a match and place it near the collector, then gently blow towards the collector side with your hand to observe whether there is a corresponding change in the baseline. (3) Inspection of syringes and sampling techniques: Leaks and blockages in the syringes, air being drawn in during sampling, and failure to proceed promptly after sampling leading to sample evaporation are the most common reasons for the absence of a peak or low sensitivity. A good syringe can be used to take a new sample, which can then be injected into the injection port to test its sensitivity again; if the peak formation remains the same, it is caused by other factors. (4) Inspection for blockages and leaks in the carrier gas: Perform a leak check on the carrier gas system, paying special attention to whether there are any leaks or blockages at the sample inlet gasket and from the back of the chromatography column to the detector inlet. Extensive experience shows that failures are most likely to occur in these two areas. (5) Injector installation check: Sometimes, even though the system has no air leaks, an improperly installed injection port, excessive dead volume, or improper flow of the carrier gas and sample can also result in low peak sensitivity or even no peaks at all. The main reason for this situation is forgetting to install the injection tube when using a capillary column, or installing it in the wrong position at the tip of the capillary column. (6) Pre-adjustment check: The pre-adjustment check of the instrument refers to the zero baseline adjustment performed after the instrument is turned on. (7) Inspection of detector wiring and operating conditions: Depending on the type of detector, the wiring and operating conditions vary. a) For the thermal conductivity detector, there are only two possible causes to consider: either the wiring of the heating wire is incorrect, or the surface of the heating wire is severely contaminated. b) For hydrogen flame detectors, first check whether the signal cable is properly connected and correctly wired. Furthermore, the size and position of the flame, as well as the relative position between the polarization ring and the collector, all affect the sensitivity of the peak detection; these factors should be carefully adjusted when necessary. The correct position is that the polarization ring is generally level with or slightly below the nozzle, while the collector is positioned above the nozzle, so that the flame after ignition lies between the nozzle and the collector.