GC gas selection

During the chromatographic analysis, the gas chromatograph has a higher requirement for the purity of the gas used. It can reach the working requirement and extend the life of the instrument. The purity of the gas used must be at or slightly higher than the gas purity required by the instrument itself. Otherwise, the use of low-purity gases that do not meet the requirements will cause a series of adverse effects. Under normal circumstances, the purity of the gas should be selected in such a way that the microanalysis is more demanding than the constant analysis, and the capillary column analysis is more demanding than the packed column analysis. The program warming analysis is more demanding than the isothermal analysis. The concentration detector is more demanding than the quality detector. The FID equipped with the methane device requires higher than the single FID, and the medium-to-high-end instrument has higher requirements than the low-grade instrument.

The gas path system of the gas chromatograph is a system in which the carrier gas continuously operates and the pipeline is closed. The airtightness of the airway system, the stability of the flow rate of the carrier gas, and the accuracy of the flow measurement all have an influence on the chromatographic experimental results and need to be controlled.

The commonly used carrier gases in gas chromatography are: hydrogen, nitrogen, helium, argon, and air.

These gases, except air can be supplied by air compressors, are generally supplied by high-pressure cylinders. It is usually purified, regulated and controlled to measure the flow.

Gas chromatographs use gas sources with different gas purities as carrier gas and auxiliary gas. Although it is an old technical problem, it is difficult for users who have just been exposed to gas chromatograph to find comprehensive information on this aspect. It is the question of where to choose what kind of gas purity is best.

1 Gas Purity Requirements According to which type of user (high, medium, and low) instruments each user uses, what kind of purity gas is chosen is indeed a more complex problem. In principle, when choosing the gas purity, it mainly depends on: 1 Analytical object; 2 Filler in the column; 3 Detector. We propose to use gases with higher purity as far as possible, provided that the analytical requirements are met. This will not only increase (hold) the instrument's high sensitivity, but it will also extend the life of the column, chromatograph (air control components, gas filter). Practice has proved that as a medium-to-high-end instrument, long-term use of lower-purity gas sources, once it is required to analyze samples with low concentration and high precision, it is very difficult to restore the high sensitivity of the instrument. For low-grade instruments, for constant or semi-microanalysis, the use of high-purity gases will increase the operating costs and sometimes increase the complexity of the gas path. Therefore, the purity of the selected gas must meet or slightly exceed the gas purity of the instrument itself. Requirements can be, so that it can not only meet the requirements of work, but also extend the life of the instrument, but also does not increase the operating cost of the instrument.

In general, trace gas analysis or capillary chromatography provides a higher degree of carrier gas purification than conventional analysis. Especially for electron capture and thermal conductivity cell detectors, the purity of the carrier gas directly affects the sensitivity and stability and must be strictly purified.

2 Poor effects due to low gas purity Depending on the object of analysis, the type of column, the grade of the instrument being operated, and the specific detector, if unsatisfactory low-purity gases are used, adverse effects can be as follows:

2.1 Distortion or disappearance of the sample: If H2O gas hydrolyzes the silicon chloride sample;

2.2 Column failure: H2O and CO2 deactivate the molecular sieve column, H2O gas decomposes the polyester immobilized solution, and O2 breaks the PEG immobilized solution.

2.3 sometimes some of the gas impurities interact with the fixed solution and produce false peaks;

2.4 Effect on retention characteristics of the column: If the retention index of H2O to hydrophilic fixed liquid such as polyethylene glycol is increased, the oxygen content in carrier gas is too high, whether it is a polar or non-polar fixed liquid column Preserving the characteristics will produce changes. The longer the usage time, the greater the influence.

2.5 Detector: TCD: The signal to noise ratio is reduced, can't be zeroed, the linearity is narrow, the correction factor in the literature can't be used, the oxygen content is too large, the component accelerates aging at high temperature, reduces the life; FID: Especially in Dt When operating under ≤1×10-11/S, organic impurities such as CH4 will increase the base flow, and the noise will not increase.

2.6 During the program temperature-raising operation, certain impurities in the carrier gas remain on the column at low temperatures. When the column temperature rises, it not only causes the baseline to drift, but it may also show a relatively broad “false peak” on the spectrum. .

2.7 Impact of the instrument 2.7.1 Accelerated failure of various types of filters;

2.7.2 Regulating valves (regulating valves, steady flow valves, needle valves) are contaminated, air blockages are blocked, and adjustment accuracy is reduced or malfunctioning;

2.7.3 The gas system is contaminated. If you want to restore the instrument to operate at high sensitivity, it may sometimes be purged for a long period of time (perhaps more than a week). When the pollution is severe, it sometimes cannot be recovered.

2.7.4 Lifetime of the Detector For FID, water vapor will affect the analysis results until the life of the detector is affected; the life of the ECD and TCD is the most obvious, and this should cause the user to pay special attention.

3 General preferences for the selection of gas purity 3.1 From the analysis point of view, microanalysis is more demanding than the constant analysis, that is, the content of impurities in the gas must be lower than the content of the component being analyzed, if 10mL/m3 of CO is analyzed by TCD. The total impurity content in the carrier gas must not exceed 10 mL/m3 because 99.999% purity gas contains 0.001% impurities, which is equivalent to 10 mL/m3. Therefore, the purity of the carrier gas should be higher than 10 mL/m3 for trace analysis. 99.999%; When using gas in FID, the content of hydrocarbon must be very low. If there is a large amount of oxygen impurities in the carrier gas, the performance of FID will not be affected as long as it does not affect the column, while the operation of ECD, the oxygen and water content in the carrier gas Must be very low.

3.2 Capillary column analysis requires higher analysis than packed column;

3.3 program temperature analysis than constant temperature analysis requirements;

3.4 Concentration detectors are more demanding than mass detectors;

3.5 FIDs equipped with methane plants have much higher CO2 and CO2 requirements for carrier gases than single FID operations.

3.6 From the life of the instrument and maintaining the high sensitivity of the instrument, the medium-to-high-end instrument is more demanding than the low-grade instrument.

4 Operation Gas purity recommended by different detectors We recommend the technical requirements for gas purity, which are usually used for routine analysis. Higher specific purity gas should be used for special high-sensitivity trace analysis if you do not care about the use of columns and instruments. Lifetime, or analysis of high concentrations of sample components, you can also not use high-purity gas, due to different gas plant settings, the impurity content will be different; to meet the different requirements of the use of different manufacturers of different purity After the gas source, the gas purification process can be used to meet the analysis requirements. What kind of purification methods and devices are used for the gases of different impurities will be discussed later.

To sum up, the new gas chromatograph must be aware of the gas source when it is connected to the gas source, and it must not be randomly inserted, otherwise it will cause failure of the column, shorten the life of the detector, damage of the methanation device, etc., and reduce the signal to noise ratio. Being too small to use, etc., eventually lead to serious distortion of the analysis data, loss of the significance of the analysis, and serious losses for the work.