Techniques for PTV-LVI

Abstract The PTV injector is a very flexible injector that can be used for a wide range of different sample introduction techniques. Large volume injection using a PTV can be performed in five different ways.

There are five techniques for PTV-LVI:
  1. Multiple injection
  2. Direct or 'at-once' injection
  3. Speed controlled sample introduction
  4. Multiple 'at-once' injection
  5. PTV on-column injection

The first four of the above mentioned techniques for PTV large volume injection are based on the so-called solvent vent or solvent elimination technique. With these techniques the solvent is evaporated in the liner of the injector and the solvent vapour is discharged via the split exit of the injector. Technique number five is slightly different. PTV on-column large volume injection is largely similar to the standard on-column large volume injection technique described in paragraph 3.2. The only difference is that in the PTV on-column injection the on-column injector is replaced by a PTV injector equipped with a special on-column liner. With this liner the sample can be injected directly into the chromatographic column. PTV on-column large volume sampling has the same advantages and disadvantages as the 'standard' on-column large volume injection technique. From the theoretical point of view it is an excellent technique. For practical reasons, however, one will often try to avoid the use of the method. It is only if none of the other PTV large volume injection methods work, for example because the substances that have to be analysed are extremely volatile or highly unstable, will the PTV on-column technique be the method of choice. For day to day work, this means that the PTV injector is truly a universal interface for large volume injection. With only one injector one can perform various PTV methods for large volume sampling as well as the on-column large volume injection technique. In subsequent paragraphs the various PTV large volume sampling techniques will be discussed in more detail. Particular emphasis will be devoted to the multiple injection technique, 'at-once' injection and speed controlled sampling. As the technique of solvent venting holds a key position in each of the PTV methods for large volume injection, this technique will first be discussed in more detail in a separate paragraph. In all cases it is assumed that the solvent has a lower boiling point than the components of interest. From this it should not be concluded that PTV large volume cannot be used if this is not the case. By carefully exploiting polarity difference or differences in affinity for an adsorbent, also large volumes of samples containing components that are more volatile than the solvent can be successfully analyzed using techniques for PTV large volume sampling. This, however, is outside the scope of the present course. More detailed discussions of the PTV methods can be found in recent literature.

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