The gas chromatograph has four basic injection methods, including conventional split and splitless injection (Split/Splitless), on-column large volume injection (On-column LVI), and temperature-programmed evaporation large volume injection (PTV LVI). This article is abbreviated as PTV). The first two general injection volumes are less than 2 μL, and the latter two can reach several hundred microliters. The maximum injection volume of a pure PTV-GC GC system can reach 50 μL.
Less sample volume, simpler sample preparation methods and higher sensitivity are the development direction of modern analytical chemistry. One of the ways to do this is through the Large Volume Injection Technology (LVI). LVI increases sensitivity by increasing injection volume, reduces extraction and concentration steps, and can be used as an interface between liquid chromatography and gas chromatography (LC-GC) or automated solid phase extraction and gas chromatography (SPE-GC). The PTV-GC gas chromatography system has the following three characteristics when it is analyzed by PTV injection:
(1) The detection limit can be reduced by large volume injection
Large volume injection can reduce the detection limit by increasing the injection volume. For example, when the PTV-GC gas chromatography system detects pesticides, the vegetable samples are processed according to the QUECHERS method, and 20 μL is injected (after GPC separation and finally reaches the PTV inlet). The amount of solvent is about 300 μL. In the SCAN mode, the quantitative limits of the method such as methamidophos, acephate, omethoate, monocrotophos, dimethoate, and pyrethroid can reach 0.01 mg/kg or less.
(2) For vegetables with sulfur-containing volatile substances, such as amaranth and other pesticide residues.
The PTV inlet is used to separate the characteristics of volatile substances according to the difference in boiling point. By reasonably controlling the time of the solvent venting valve, the volatile sulfur-containing substances in the samples such as amaranth can be eliminated to achieve the purpose of removing interference, and the PTV injection is used. Matrix interference is significantly less than conventional splitless injection.
(3) Improve the reproducibility of heat-labile pesticides.
When PTV is injected, the temperature of the inlet rises rapidly from low temperature to the vaporization temperature of all the components tested, which reduces the decomposition of the thermal instability components such as carbamate and dicofol. The effects of carbaryl, carbofuran, 3-hydroxyfurantan and dicofol on the amaranth, cauliflower, green vegetables, pepper, spinach and long kidney bean matrix addition solutions have obtained good reproducibility, carbofuran and 3- The retention time of hydroxyfuran has drifted, and the specific reasons need further study.
The following points should be noted in the application of the gas chromatograph large volume injection analysis system:
(1) The key to PTV injection technology is the initial temperature of the inlet and column. The initial temperature must be higher than the optimization of the solvent venting valve closing time when the boiling point of the solvent used is too long to measure the semi-volatile organic matter in the water.
(2) to ensure that the system can remove as much solvent as possible when switching to the analytical column; at the same time, the initial temperature should not be too high, so as to avoid the component with lower boiling point being excluded from the solvent. The initial temperature of the online system selected in this paper can be Meet these two requirements.
(3) Another key point of PTV injection technology is the opening time control of the solvent vent valve. Excessive opening time of the solvent venting valve will result in loss of some volatile components; time is too short, a large amount of solvent enters the chromatographic system, resulting in less purity and less sensitivity, and it will not meet the requirements for a long time. It is easy to cause the instrument to report an error and stop collecting data. The selection of this time should be based on the length of the injection and the amount of injection. Therefore, under the premise of ensuring the sensitivity of the gas chromatograph, the valve closing time should be extended as appropriate.
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