Discussion on the Use of Gas Chromatography Analytical Instruments

　　The gas path system of a gas chromatograph is a system in which the carrier gas runs continuously and the pipeline is sealed. The air tightness of the gas path system, the stability of the carrier gas flow rate, and the accuracy of flow measurement all have an impact on the results of chromatographic experiments and need to be carefully controlled.

　　During the gas chromatography analysis and detection process, the gas chromatograph has high requirements for the purity of the gas used. In order to meet the working requirements and extend the life of the instrument , the purity of the gas used must meet or slightly exceed the instrument's own requirements for gas purity; otherwise, if low-purity gas that does not meet the requirements is used, it will cause a series of adverse effects; in general, the selection of gas purity should follow the following principles, that is, trace analysis has higher requirements than constant analysis, capillary column analysis has higher requirements than packed column analysis, programmed temperature analysis has higher requirements than constant temperature analysis, concentration detectors have higher requirements than mass detectors, FIDs equipped with methane devices have higher requirements than single FIDs, and mid- to high-end instruments have higher requirements than low-end instruments. Commonly used carrier gases in gas chromatography are: hydrogen, nitrogen, helium, argon and air. Except for air, which can be supplied by air compressors, these gases are generally supplied by high-pressure cylinders. Usually, they must be purified, stabilized, controlled, and flow measured.

　　How to select gas sources with different gas purities as carrier gas and auxiliary gas for gas chromatograph is an old technical problem. However, it is difficult for users who have just come into contact with gas chromatograph to find comprehensive information on this aspect. Therefore, they always ask questions like what kind of gas with the best purity should be selected.

　　1. Gas purity requirements

　　It is indeed a complicated issue to choose the gas of what purity according to the specific type of instrument (high, medium, low-end) used by each user. In principle, the choice of gas purity mainly depends on: ① the object of analysis; ② the filler in the chromatographic column; ③ the detector. We recommend that you use gas with higher purity as much as possible under the premise of meeting the analysis requirements. This will not only improve (maintain) the high sensitivity of the instrument, but also extend the life of the chromatographic column and chromatograph (gas path control components, gas filter). Practice has proved that as a medium- and high-end instrument, it is very difficult to restore the high sensitivity of the instrument if a gas source of lower purity is used for a long time when it is required to analyze samples with low concentration and high precision requirements. For low-end instruments, the use of high-purity gas for constant or semi-micro analysis will increase the operating cost and sometimes increase the complexity of the gas path. Therefore, the purity of the selected gas should meet or slightly exceed the gas purity requirements of the instrument itself, so that it can meet the working requirements and extend the life of the instrument without increasing the operating cost of the instrument.

　　Generally speaking, the degree of carrier gas purification for trace analysis or capillary chromatography is higher than that for conventional analysis. Especially for electron capture and thermal conductivity detectors, the purity of carrier gas directly affects the sensitivity and stability, so it must be strictly purified.

　　2. Possible adverse effects caused by low gas purity

　　Depending on the analytical object, the type of chromatographic column, the grade of the operating instrument and the specific detector, if unqualified low-purity gas is used, the adverse effects may be as follows:

　　2.1 Sample distortion or disappearance: For example, H2O gas hydrolyzes chlorosilane samples;

　　2.2 Chromatographic column failure: H2O and CO2 make the molecular sieve column inactive, H2O gas decomposes the polyester stationary phase, and O2 breaks the chain of the PEG stationary phase.

　　2.3 Sometimes certain gas impurities interact with the stationary phase to produce false peaks;

　　2.4 Impact on column retention characteristics: For example, H2O will increase the retention index of hydrophilic stationary phases such as polyethylene glycol. When the oxygen content in the carrier gas is too high, the retention characteristics of both polar and non-polar stationary phase columns will change. The longer the use time, the greater the impact.

　　2.5 Detector: TCD: signal-to-noise ratio is reduced, zero adjustment is impossible, linearity becomes narrower, correction factors in the literature cannot be used, excessive oxygen content accelerates aging of components at high temperatures and reduces life; FID: especially when operating at Dt≤1×10-11/S, organic impurities such as CH4 will cause a surge in base flow, increase noise and make micro-analysis impossible;

　　2.6 When performing programmed temperature operation, some impurities in the carrier gas will be retained in the chromatographic column at low temperatures. When the column temperature rises, it will not only cause baseline drift, but may also cause relatively wide "false peaks" to appear on the spectrum.