Abstract: How to choose a gas source with different gas purity as carrier gas and auxiliary gas for operating a gas chromatograph. For users who are new to gas chromatographs, it is difficult to find comprehensive information in this regard. They always ask everywhere what they choose This type of problem has the best gas purity.
In principle, when selecting the purity of the 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 the entire instrument (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, it is sometimes 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 not only increases operating costs, but also sometimes increases the complexity of the gas path, and is more prone to leakage or other problems that affect the normal operation of the instrument.
In addition, for some special analysis purposes, some “impurities†are intentionally added to the carrier gas, for example: when analyzing polar compounds, adding appropriate amount of water vapor, when operating the flame photometric detector, in order to improve the sensitivity of sulfide analysis While adding trace sulfur. The operation of the helium ionization detector requires that the content of neon must be 5 ~ 25ppm, otherwise it will produce negative peaks or "W" shaped peaks when analyzing hydrogen, nitrogen and argon. This article will not discuss it in detail here.
One. The usual principles for the selection of gas purity 1) From an analytical point of view, trace analysis is more demanding than constant analysis, that is, the impurity content in the gas must be lower than the content of the analyzed component. If 10 ppm CO is analyzed by TCD, the carrier gas The total content of impurities must not exceed 10ppm, because 99.999% purity gas contains 0.001% impurities, which is equivalent to 10ppm to deal with 10ppm trace analysis, the purity of the carrier gas should be higher than 99.999%; to deal with FID gas, the hydrocarbon content must be very low As long as a large amount of oxygen impurities in the carrier gas are affected by the wrong column, it will not affect the FID performance. While the ECD is dominated, the oxygen and water content in the carrier gas must be very low.
2) Capillary column analysis is more demanding than supplementary column analysis.
3) Step temperature analysis is more demanding than constant temperature analysis.
4) Concentration type detectors have higher requirements than quality type detectors.
5) The FID equipped with a methane device is much higher than the trace CO in the carrier gas of a single FID, and the CO2 requirements are much higher.
6) From the perspective of instrument life and high instrument retention, medium and high instruments are more demanding than low instruments.
two. The adverse effects of low gas purity Depending on the analysis object, the type of chromatographic column, the level of operation of the instrument and the specific detector, if undesirable low purity gas is used, the adverse effects may have the following possibilities:
1) The sample is distorted or disappears: if H2O gas hydrolyzes the chlorine silicon sample;
2) Chromatographic column failure: H2O and CO2 deactivate the molecular sieve column, H2O gas decomposes the polyester fixative, and O2 breaks the PEG chain.
3) Sometimes some gas impurities interact with the fixing liquid to produce false peaks;
4) The effect on the retention characteristics of the column: for example: H2O will increase the retention index of hydrophilic fixing solutions such as polyethylene glycol, etc. When the oxygen content in the carrier gas is too high, whether it is polar or nonpolar fixing solution The retention characteristics of the column will change, the longer the use time, the greater the impact.
5) Detector:
TCD: The signal-to-noise ratio is reduced, the zero cannot be adjusted, the linearity is narrowed, 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 at Dt ≤1X10ˉ⒒ / sec, organic impurities such as CH4 will cause the base current to surge, and the noise will be increased.
ECD: Oxygen and water in the carrier gas have the greatest impact on the normal operation of the detector. In different power supply operation modes, pulse power supply has a greater impact than DC voltage power supply, and fixed base current pulse modulation power supply has a greater impact than pulse power supply. This is why at present, when operating a fixed base flow pulse modulation ECD, when the carrier gas purity is low, the carrier gas purity selection switch must be set from the "standard nitrogen" to the "general nitrogen" position. You will find that operating under this condition not only lowers the sensitivity, but also narrows the linearity. Practice has proved that when operating ECD, the water content in the carrier gas is less than 0.02ppm, and the oxygen can reach the ideal performance when it is less than 1ppm. It is worth noting that we have repeatedly found that the secondary pollution of the carrier gas caused by the contamination of the instrument's regulating gas system has caused the ECD fundamental frequency to increase significantly and the signal-to-noise ratio to decrease.
Commonly used detectors such as FPD and NPD, because they are selective detectors, according to the analysis requirements when operating, pay special attention to the removal of impurities in the sensitive substances being tested.
6) When doing temperature-programming operation, some impurities in the carrier gas will remain in the chromatographic column at low temperature. When the temperature is increased, not only will the baseline drift but also a relatively wide "false peak" on the spectrum .
7) Instrument influence
a. Accelerated failure of various filters;
b. The control valve (stabilizing valve, steady flow valve, needle valve) is contaminated, the air resistance is blocked, and the adjustment accuracy is reduced or malfunctions;
c. The air circuit system is contaminated. If you want to restore the instrument to operate under high sensitivity, sometimes it needs to be purged for a long time (may be more than one week). If the pollution is serious, sometimes it cannot be recovered.
d. The life of the detector. Practice has shown that it has the most obvious impact on the life of ECD and TCD, and should be paid special attention to by users.
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