Programmed Temperature Vaporization (PTV) injection



Abstract The introduction of larger sample volumes is an attractive approach to improve the detection limits in capillary gas chromatography. The use of large volume injection can greatly improve the sensitivity of an analytical measurement. This, however, is not the only advantage of the use of large volume injection.

LevelBasic
LARGE VOLUME SAMPLING USING PTV INJECTION SYSTEMS


Other than to improve the detection limits in capillary gas chromatography or its effect on the sensitivity, large volume injection can also be used as an alternative for an intermediate evaporative preconcentration step in an extraction procedure of a complex sample. This could, for example, be of advantage in the analysis of solid samples, such as soil or sediment samples or polymeric samples:

  • The first step in the analysis of these solid samples is usually a liquid extraction. In this step a relatively small amount of the solid sample is extracted with a relatively large volume of a liquid. The extraction can be carried out in different ways, e.g. by shaking, sonication or in a Sohxlet extractor. Regardless of which procedure has been used, the result of the extraction is a relatively diluted extract that has to be preconcentrated prior to injection of a 1 to 3 ┬Ál aliquot into the GC instrument.
  • The most widely employed procedure for preconcentration is evaporation of the solvent. Also this step in the sample preparation can be carried out in various ways. Each of these methods, however, is time-consuming. Moreover, irrespective of which method of evaporation is used, there is a serious risk that volatile components are lost in the evaporation step. Unstable solutes can decompose, especially if the evaporation is carried out at elevated temperatures. Finally, evaporative preconcentration is difficult to automate.

It will be evident from what has been stated above, that two important application areas of large volume injection are the improvement of detection limits in trace analysis and the reduction in the overall analysis time that can be obtained due to the elimination of the time consuming and laborious solvent evaporation step.
A third application area of large volume sampling is encountered in the on-line combination of sample pretreatment and chromatographic analysis. This direct combination is greatly simplified if the gas chromatograph can accept larger sample volumes than the standard microliter volumes. The advantages of on-line coupling of sample pretreatment and chromatographic analysis are very diverse. On-line systems are easier to automate, more sensitive, more rugged, more reliable and reduce the costs per sample.

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