Discussion on the application method of analysis a

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Discussion on the use method of gas chromatography analysis and detection

in the process of gas chromatography analysis and detection, the gas chromatograph has higher requirements for the purity of the gas used and some working parts that need to be in contact with extremely low temperature or even liquid nitrogen. In order to meet the working requirements and prolong the service life of the instrument, the purity of the gas used should meet or slightly higher than the requirements of the instrument itself for the purity of the gas; Otherwise, the use of low purity gas that does not meet the requirements will cause a series of adverse effects; In general, the selection of gas purity should grasp the following principles, that is, microanalysis requires higher requirements than constant analysis, capillary column analysis requires higher requirements than packed column analysis, temperature programmed analysis requires higher requirements than constant temperature analysis, concentration detector requires higher requirements than quality detector, FID equipped with methane device requires higher requirements than single FID, and medium and high-grade instruments require higher requirements than low-grade instruments

the gas path system of gas chromatograph is a system with continuous operation of carrier gas and closed pipeline. The gas 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 chromatographic experiment results, which need to be controlled

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

these gases are generally supplied by high-pressure steel cylinders, except that air can be supplied by air compressors. Generally, reopening the oil delivery valve to raise the test bench by 5-10mm, and then closing the oil valve requires purification, pressure stabilization, control and flow measurement

although it is an old technical problem how to choose gas sources with different gas purity as carrier gas and auxiliary gas for gas chromatograph, it is difficult to find comprehensive information about this aspect for users who have just come into contact with gas chromatograph, so they always ask everywhere to choose the best gas purity

1. Requirements for gas purity

it is indeed a complex problem to choose the purity of gas according to the type of instrument (high, medium and low) used by each user. In principle, the selection of gas purity mainly depends on: ① analysis object; ② Packing in chromatographic column; ③ Detector. We suggest that on the premise of meeting the analysis requirements, the gas with high purity should be selected as far as possible. This will not only improve (maintain) the high sensitivity of the instrument, but also prolong the life of the chromatographic column and chromatograph (gas path control components, gas filters). Practice has proved that as a medium and high-grade instrument, it is very difficult to restore the high sensitivity of the instrument once it is required to analyze samples with low concentration and high precision after long-term use of low purity gas source. For low-grade instruments, for constant or semi micro analysis, the selection of high-purity gas will increase the operation cost, and sometimes increase the complexity of the gas circuit. Therefore, the purity of the selected gas is required to meet or slightly higher than the requirements of the instrument itself, which can not only meet the working requirements, but also extend the service life of the instrument, without increasing the operation cost of the instrument

generally speaking, the carrier gas purification degree of trace analysis or capillary chromatography is higher than that of conventional analysis. In particular, electronic capture, thermal conductivity cell detector, carrier gas purity, the main body and force measuring device are transported to the concrete foundation, which directly affects the sensitivity and stability, and must be strictly purified

2. Possible adverse effects caused by low gas purity

according to the analysis 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 avoid risks 1. Sample distortion or disappearance: for example, H2O gas hydrolyzes silicon chloride sample

2.2 chromatographic column failure: H2O and CO2 make the molecular sieve column inactive, H2O gas decomposes the polyester fixed liquid, and O2 breaks the chain of PEG fixed liquid

2.3 sometimes some gas impurities interact with the fixed liquid to produce false peaks

2.4 effect on column retention characteristics: for example, H2O will increase the retention index of hydrophilic fixed liquids such as polyethylene glycol. When the oxygen content in the carrier gas is too high, the retention characteristics of both polar and nonpolar fixed liquids will change, and the longer the service time is, the greater the effect is

2.5 detector: TCD: the signal-to-noise ratio decreases, cannot be adjusted to zero, the linearity narrows, the correction factor in the literature cannot be used, and the oxygen content is too large, which accelerates the aging of the components at high temperature and reduces the service life; FID: especially when operating at DT 1/s, organic impurities such as CH4 will make the base flow surge, and the noise will increase, so microanalysis cannot be carried out

2.6 during temperature programmed operation, some impurities in the carrier gas will remain in the chromatographic column at low temperature. When the column temperature rises, it will not only cause baseline drift, but also may appear wide false peaks on the spectrum

2.7 instrument influence

2.7.1 accelerated failure of various filters

2.7.2 the regulating valve (pressure stabilizing valve, flow stabilizing valve, needle valve) is polluted, the air resistance is blocked, and the regulating accuracy is reduced or fails

2.7.3 if the air path system is polluted, it sometimes takes a long time (maybe more than a week) to restore the instrument to operate under high sensitivity. Sometimes it can't be recovered when the pollution is serious

2.7.4 detector life

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