Analysis and detection of gas chromatograph and selection of high-purity gas

Abstract: When the new gas chromatograph is connected to the gas source, it must be aware of it, and it must not be arbitrarily connected, otherwise it will cause various losses, and eventually lead to serious distortion of the analysis data, losing the significance of the analysis and bringing serious work to the work. loss.

In the process of chromatographic analysis and detection, the gas chromatograph has high requirements on 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 be slightly higher than the instrument's own gas purity requirements; Otherwise, if you use low-purity gas that does not meet the requirements, it will cause a series of adverse effects; under normal circumstances, the selection of gas purity should master the following principles, that is, micro analysis is more demanding than constant analysis, capillary column analysis is more demanding than packed column analysis, Temperature-programmed analysis is more demanding than constant temperature analysis. Concentration detectors are more demanding than mass detectors. FID equipped with methane devices are more demanding than single FID. Mid-range and high-grade instruments are more demanding than low-grade instruments.

The gas path system of the gas chromatograph is a system in which the carrier gas runs continuously and the pipeline is closed. The air tightness of the gas circuit system, the stability of the carrier gas flow rate, and the accuracy of the flow measurement all have an impact on the chromatographic experiment results, and attention should be paid to control.

Carrier gases commonly used 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 steel cylinders. Usually, it has to be purified, stabilized, controlled, and measured for flow.

How to choose a gas source with different gas purity as carrier gas and auxiliary gas for a gas chromatograph, although it is an old technical problem, for users who are new to gas chromatographs, it is currently difficult to find comprehensive information in this regard, so they always It is a question of asking everywhere what kind of gas purity is best.

1 The requirement of gas purity depends on which type (high, medium and low-grade) instruments each user uses, and what kind of purity gas to choose is indeed a relatively complicated question. In principle, when choosing the purity of a gas, it mainly depends on: â‘  the analysis object; â‘¡ the packing in the chromatographic column; â‘¢ the detector. We recommend that the gas with higher purity be selected as much as possible on the premise of satisfying 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-to-high-end instrument, a gas source of lower purity is used for a long time, and once it is required to analyze a sample with a low concentration and high precision, it is very difficult to restore the high sensitivity of the instrument. For low-end instruments, for constant or semi-micro analysis, the use of high-purity gas will increase the operating cost and sometimes increase the complexity of the gas circuit. Therefore, the purity of the selected gas must meet or be slightly higher than the instrument's own gas purity. The requirements can be met, so that it can not only meet the working requirements, but also extend the life of the instrument, and it will not increase the operating cost of the instrument.

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

2Possible adverse effects due to low gas purity Depending on the analysis object, the type of chromatographic column, the grade of the operating instrument and the specific detector, if undesirable low purity gas is used, the adverse effects may have the following possibilities: 2.1 Distorted or disappeared samples: For example, H2O gas hydrolyzes the chlorine silicon sample; 2.2 Chromatographic column failure: H2O and CO2 deactivate the molecular sieve column, H2O gas decomposes the polyester fixative, and O2 breaks the chain of the PEG fixative.

2.3 Sometimes some gas impurities interact with the fixing liquid to produce false peaks; 2.4 The effect on the retention characteristics of the column: such as H2O, the retention index of hydrophilic fixing liquids such as polyethylene glycol will increase, and the oxygen content in the carrier gas When it is too high, the retention characteristics of the polar or non-polar fixed liquid column will change, the longer the use time, the greater the impact; 2.5 Detector: TCD: the signal-to-noise ratio is reduced, zero adjustment is impossible, and the linearity becomes narrow , The correction factor in the literature cannot be used, and the oxygen content is too large, which accelerates the aging of the component at high temperature and reduces the life; FID: especially when operating under Dt≤1 × 10-11 / S, organic impurities such as CH4 will cause the base The flow surge and the noise increase can not be trace analysis; 2.6 When doing temperature programming operation, some impurities in the carrier gas are retained in the chromatographic column at low temperature. When the column temperature increases, it not only causes baseline drift, but also may A broad "false peak" appears on the spectrum.

2.7 Influence of instrument 2.7.1 Accelerating failure of various filters; 2.7.2 Control valve (stabilizing valve, steady flow valve, needle valve) is contaminated, air resistance is blocked, adjustment accuracy is reduced or malfunction; 2.7.3 Air circuit system If it is contaminated, if you want to restore the instrument to operate under high sensitivity, sometimes you need to purge for a long time (may be more than a week). Sometimes when the pollution is serious, it can not be recovered.

2.7.4 The life of the detector For FID, water vapor will affect the analysis result until it affects the life of the detector; the life of the ECD and TCD is most obvious, which should be paid special attention to by the user.

3 General principles for the selection of gas purity 3.1 From an analytical point of view, trace analysis is more demanding than constant analysis, that is to say, the content of impurities in the gas must be lower than the content of the component being analyzed. If TCD is used to analyze 10 mL / m3 of CO , The total content of impurities in the carrier gas should not exceed 10mL / m3, because 99.999% purity gas contains 0.001% impurities, which is equivalent to 10mL / m3, so for trace analysis of 10mL / m3, the purity of the carrier gas should be higher than 99.999%; the gas used in FID, the hydrocarbon content must be very low, as long as the large amount of oxygen impurities in the carrier gas does not affect the performance of the FID as long as it does not affect the chromatographic column, while operating ECD, the oxygen and water content in the carrier gas Must be inferior.

3.2 Capillary column analysis is more demanding than packed column analysis; 3.3 Programmed temperature analysis is more demanding than constant temperature analysis; 3.4 Concentration detector is more demanding than mass detector; 3.5 FID equipped with methane device is more suitable for carrier gas than single FID The trace CO and CO2 requirements are much higher.

3.6 In terms of instrument life and maintaining the high sensitivity of the instrument, middle and high-end instruments have higher requirements than lower-end instruments.

4 Recommended gas purity for operating different detectors. We recommend the technical requirements for gas purity, which is usually used for routine analysis. For trace analysis with special high sensitivity, a gas with a higher purity should be used. If you do n’t care about the use of chromatographic columns and instruments Life time, or when the concentration of the analyzed sample component is very high, you can not use gas of too high purity. Due to the different settings of each gas plant, its impurity content will be different; in order to meet different use requirements, choose different manufacturers with different purity After the gas source, the gas purification process can meet the analysis requirements. What kind of purification methods and devices are used for gases with different impurities will be discussed later.

In summary, the new gas chromatograph must be aware of when it is connected to the gas source, and it must not be arbitrarily connected, otherwise it will cause the failure of the chromatographic column, shorten the life of the detector, damage to the methanation device, etc., and reduce the signal-to-noise ratio. Too small to use, etc., eventually causing severe distortion of the analysis data, losing the meaning of the analysis and causing serious losses to the work.

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