Let’s discuss the core components of gas phase: the column and the detector?

The information I have available shows that FID does not respond to the following substances: noble gases, nitrogen oxides, silicon halides, heterocyclic compounds, H2O, CO, O2, CO2, N2, HCOH, HCOOH, NH3, CS2, H2 COS

The thermal conductivity cell detector (TCD) is the most widely used in gas analysis. The key thermal conductivity element in the thermal conductivity cell is made of tungsten-rhenium wire, whose diameter is typically only 15μ–30μ. This material is prone to oxidation; once oxidized or contaminated, its resistance value changes or it breaks, which disrupts the symmetry of the measurement bridge in the thermal conductivity cell and prevents the instrument from functioning properly. There are many factors that can cause damage to the heat conduction element; the precautions are summarized as follows: 1. When using a heat conduction cell with parallel dual gas paths, two chromatography columns must be installed in parallel. Gas must flow through both paths simultaneously. If only one column is installed while the other path lacks a column and no gas flows through it, then applying power will burn out the tungsten wire element. 　　2. After the instrument is shut down, outside air often flows back into the heat conduction cell and column system; therefore, it is necessary to supply the carrier gas for more than 10 minutes before turning the power on again. The longer the instrument has been shut down, the longer the time required to supply the carrier gas before restarting it, otherwise the oxygen present in the residual air in the system can oxidize or burn out the heat conduction elements. 　　3. The purity of the carrier gas used in a thermal conductivity detector must be at least four 9s (99.99%); a high oxygen content in the carrier gas is particularly undesirable. Impure carrier gas can affect the service life of the thermal conductivity element as well as reduce the detection sensitivity, so the carrier gas must be deoxygenated and purified. 　　4. When replacing the chromatography column, it is necessary to check for leaks to ensure airtightness; leaks at the connections of the chromatography column can cause damage to the thermal conductivity element. The outlet end of the column must be filled with glass wool and stainless steel mesh to prevent the column support from entering the TCD. 5. After multiple sampling analyses, the silicone rubber gasket on the sampler should be replaced promptly. It is too late to replace it only after the gasket has been punctured by the injection needle multiple times and air leakage occurs, because once the gasket leaks, the carrier gas escapes and air gets in, which can damage the heat-conducting element. During the analysis process, when replacing the silicone rubber gasket, it is necessary to turn off the heat conduction power supply first, then replace the gasket quickly; after replacement, gas must be passed through for a few minutes before the power supply for the heat conduction cell can be turned on again. 　　6. When using a planar six-way valve for gas injection, the valve must be positioned at one of the two extreme positions; it should not be turned to the middle position, as this will cut off the flow of carrier gas, which is very dangerous and can lead to damage to the thermal conductivity cell due to the lack of carrier gas flow. 　　7. When performing high-temperature aging of the chromatography column, it is necessary to turn off the power supply for the thermal conductivity cell, as well as the temperature control for that cell. Additionally, the connection between the column outlet and the thermal conductivity cell inlet must be disconnected, so that the carrier gas (N2) used for high-temperature aging can flow into the column chamber. This prevents contamination of the thermal conductivity cell and the tungsten-rhenium wire components due to column aging. 　　8. The setting of the bridge current for the heat conduction cell must be 20–30°C higher than the highest boiling point of the components in the sample being analyzed, in order to prevent high-boiling-point components in the sample from condensing within the heat conduction cell and contaminating the tungsten-rhenium wire elements. 　　9. When setting the bridge current for the heat conduction cell, it is necessary to take into account the type of carrier gas used, the operating temperature, and the cold resistance of the tungsten-rhodium wire element. The following principle should be kept in mind: ① For light carrier gases (H2, He), the bridge current can be high, while for heavy carrier gases (N2, Air), the bridge current must be low ； ②At high operating temperatures of the thermal conductivity cell, the bridge current should be reduced; at lower operating temperatures, the bridge current can be increased ； ③The resistance values of the tungsten-rhodium wire elements in the thermal conductivity cells vary among different manufacturers; as a result, the bridge current used also differs. For elements with higher resistance values, a lower bridge current should be set. The specific value for the bridge current can be found in the instruction manual.

Regarding packed columns: Chromatography columns that have a stationary phase packed inside them. They are classified into packing columns for analysis, packing columns for preparation, micro-packing columns, packed capillary columns, etc. Packed columns are simple to prepare, come in various types, have wide applications, and can be used in preparative chromatography. The packing material of the packed column can be a porous granular binder, or a very thin film of stationary phase uniformly coated on the surface of inert carrier particles. Filling columns commonly use metal or glass tubes with an inner diameter of 2–5 mm and a length of 0.5–10 m. Packed columns are simple to prepare; there are a wide range of available carriers, stationary phases, and adsorbents, which provides broad selectivity and facilitates the separation and analysis of various components, making them widely used. Furthermore, the packing column can handle a large sample load and is suitable for preparative chromatography; its disadvantages are low column permeability, high mass transfer resistance, and the inability to make the column too long, resulting in lower separation efficiency. The choice of column efficiency depends on the components of the sample; many analyses do not require a high degree of separation efficiency, so packed columns still have broad application prospects. For the analysis of substances such as *** in industrial wastewater and benzene derivatives, packed-column gas chromatography is sufficient to meet the analytical requirements. This is from Baidu; who has more practical experience?:)

In FID applications, problems caused by contamination and water accumulation are quite common； Pollution: Inappropriate samples or sample sizes are often used, coupled with inadequate temperature and carrier gas pressures, which leads to carbon buildup and sample residues, thereby causing contamination or damage to the detector ； Water accumulation: This phenomenon is caused, firstly, by a lack of understanding of the equipment, by temperatures that are not high enough for ignition, and by improper ratios of the various gases. It can also be due to improper design of the instrument’s structure; in such cases, careful observation by the user is required. For example, in the old HP5890 model, the circular hole in the cover plate was directly above the ion chamber, which meant that condensation water would inevitably drip into the ion chamber. In such cases, it was necessary to lift the cover plate, and the problem would be resolved. Some chromatographs use a closed ion chamber; an external conduit is attached above the ion chamber. It seems fine at first, but if the operator fails to adjust the gas ratios properly and the pressure is low, the water generated during ignition flows slowly, resulting in a gas blockage that can cause the ignition to go out. This might happen once or twice, but if it occurs frequently, the ion chamber will be damaged beyond repair. In TCD applications, damage is usually caused more by issues with the operating program and bridge current. For TCD, it is important to first supply the carrier gas and then heat it. It sounds simple, but not many people actually manage to do it. Many devices just start working as soon as gas is supplied, with no room for any buffering; over time, their lifespan doesn’t increase much ； This is called hidden damage! Also, after the temperature stabilizes, bridge current is added – there are too many things happening simultaneously. It’s wrong to say so, but it’s not wrong either, because the instrument isn’t broken. If it breaks, it can’t be entirely his fault; everyone does it, and the law doesn’t punish the crowd. Another thing is the shutdown of the carrier gas; there’s no room for negotiation at all. I would suggest that if you use a carrier gas, keeping it flowing continuously is the best way to protect the instrument. For the thermal conductivity during prolonged shutdowns, the purge pressure of the carrier gas must be increased gradually, with a smooth transition; otherwise, the instrument will become very unstable after heating, and this is caused by the resurfacing of deposits accumulated in the pipelines. I don’t understand chromatography theory; these are just things to keep in mind in my daily work. It’s written in books; only by combining books with practice can something truly complete be achieved.

The heart of chromatography is the chromatographic column; without it, chromatography would not be possible, as samples can only be analyzed after they have been separated

For FID detector contamination, I usually add some ethanol or propane at the nozzle and then ignite it for burning. For TCD contamination, I have a dedicated hollow chromatography column connected to it; the temperature is set to 120 degrees for the detector as well, 150 degrees for the vaporization chamber. By injecting 10 ml of propane or ethanol each time at the sample inlet, repeated several times, the problem of tungsten wire contamination can generally be resolved

After being used for a period of time, the hydrogen flame detector needs to have its temperature increased for burning in order to eliminate the residues. After some time, remove the detector and soak it in organic solvents such as alcohol or acetone, then use ultrasound; this will basically remove any remaining residues.

This information is drawn from experiences shared on the Instrument Information Forum, and I’d like to share it with everyone: During chromatography operations, the detector can sometimes become contaminated due to the loss of the stationary phase, as well as by high-boiling-point components, substances that are prone to decomposition, or corrosive materials in the sample, which prevents it from functioning properly. This has led to questions regarding how to clean the detector. If the contaminating substances are limited to high-boiling-point components, it is usually possible to remove them by heating the detector to its maximum operating temperature and then introducing a carrier gas. Extra care must be taken when heating detectors that contain radioactive sources. For example, electron capture detectors made with tritium sources generally should not be heated above 200 degrees, and it is also important to ensure that the heating temperature does not damage the detector’s insulating materials. If heating is not suitable, a pure propylene carbonate solution can also be injected from the sample inlet (tens of microliters at a time) for cleaning, which is effective when the degree of contamination is mild. If the above methods fail to resolve the contamination issue, the detector should be removed for a more thorough cleaning. First, select an appropriate solvent that can dissolve the contaminants without damaging the detector, and then use a syringe to inject it into the measurement cell for cleaning. If possible, it is better to use ultrasonic cleaning; note that the cleaned area must not be touched by hand. I. Cleaning of the thermal conductivity detector: Fill the measurement cell of the detector 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, then install it in the instrument, heat the detector, and after flushing with a carrier gas for several hours, it is ready for use. II. Cleaning of the hydrogen flame ionization detector: When the contamination is not severe, it is not necessary to remove the detector for cleaning. In such cases, it is sufficient to remove the chromatography column, connect the injection port to the detector using a tube, then introduce carrier gas and raise the temperature of the detector’s furnace to over 120 degrees. First, inject about 20 microliters of distilled water into the injection port, and then use several dozen microliters of a solvent such as propane or Freon 113 to carry out the cleaning. Keep at this temperature for 1–2 hours to check whether the baseline is stable; if it is still not satisfactory, repeat the above procedure or remove it for cleaning. When the contamination is severe, it must be removed and cleaned. First, remove the collector, anode, nozzle, etc. If the nozzle is made of quartz, soak it in water overnight first. If the nozzle is made of materials such as stainless steel, it can be carefully polished together with the electrode using fine sandpaper (300–400#), followed by treatment with an appropriate solvent (such as a 1:1 mixture of methanol and benzene). Ultrasonic cleaning can also be used, after which the nozzle should be rinsed with methanol and dried in an oven. Be careful not to use halogen-containing solvents (such as chloroform, dichloromethane, etc.). To avoid interaction with polytetrafluoroethylene 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 30 minutes first, then ignite to raise the temperature of the detection chamber; it is best to maintain the temperature at 120 degrees for several hours before raising it to the operating temperature. III. Cleaning of the electron capture detector: The electron capture detector contains a radioactive source, usually Ni63, so special care must be taken. First, remove the source foil from the detector, and then wash the metal and PTFE parts of the detector with a solution of sulfuric acid, nitric acid, and water in a 2:1:4 ratio. Once the cleaning solution is clean, wash it with distilled water, then with acetone, and finally dry it in an oven at around 100 degrees Celsius. For H3 source foils, rinse them first with hexane or pentane; never wash them with water. The waste liquid must be diluted with a large amount of water before being discarded. Extra care must be taken with the Ni63 source; it must never come into contact with the skin, and it should only be handled using long forceps. First, wash with ethyl acetate and sodium carbonate or with benzene, then soak in boiling water for 5 minutes, take it out and dry it, before placing it in the analyzer. After loading it into the instrument, pass the carrier gas for 30 minutes, then raise it to the operating temperature, and leave it ready for use after a few hours. The remaining waste liquid must be diluted with a large amount of water before it can be discarded. It generally cannot be disassembled for cleaning due to the risk of radioactive sources. It’s fine to leave a 10cm distance. The thermal cleaning method is generally used: that is, the detector is baked at a high temperature of 300-350 degrees. The hot steam method involves raising the detector to a high temperature of 300-350 degrees after connecting it to an empty column; 10-15 uL of water is introduced each time, and this process is repeated 50-100 times. The hydrogen baking method involves using hydrogen as the carrier gas, with a flow rate of 30-40 ml/min, and the detector is baked at 300-350 degrees for 18-24 hours. If none of these methods work, contact the manufacturer