Flow, pumping method thermal effect

Nonmechanical pumping. Micropumps in this class are usually continuous and include the use of effects such as electrochemical displacement (bubble generation), thermal expansion, electrohydrodynamics, capillarity, and evaporation forces. The most commonly used nonmechanical pumping method is based on electrokinetic flow. In comparison with mechanical micropumps, field-induced flow is advantageous as it acts as both a valve and a pump, enabling both the direction and the magnitude of the flow to be controlled. 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

The flow tube, which is about 20 cm long, acts as a reaction region that is maintained at about 1 mbar by action of a vacuum pump attached to its exit. These conditions afford thermalization and effective proton transfer as the hydronium ions drift along together with the VOC-loaded air. As PTR-MS does not necessitate an additional buffer gas, the analyte air is not diluted any further, a feature that contributes to the high sensitivity of the method. Furthermore, the original mole fraction of the analytes in the air is maintained. 

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