Cryogenic separation cooling step

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

The extract is collected by depressurizing the fluid into a sorbent trap or a collection solvent. A trap may retain the analytes selectively, which may then be selectively washed off by a solvent. This can offer high selectivity, but requires an additional step. The trap can be cryogenically cooled to avoid the loss of analytes. Using a collection solvent is more straightforward. The choice of solvents often depends on the analytical instrumentation. For example, tetrachloroethene is suitable for infrared determination, while methylene chloride and isooctane are appropriate for gas chromatographic separations. 

The Linde cycle is a simple cryogenic process based on Joule-Thompson effect. It is composed of different steps the gas is first compressed, then preliminarily cooled in a heat exchanger using liquid nitrogen, finally it passes through a lamination throttle valve to exploit the benefits of Joule-Thomson expansion. Some liquid is produced, and the vapour is separated from the liquid phase and returns back to the compressor through the heat exchanger. A simplified scheme of the overall process is reported in Fig. 2.9. 

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