Programmed Temperature Vaporization (PTV) injection

Hans-Gerd Janssen, Unilever Research and Development Vlaardingen, the Netherlands

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.

Level: Basic

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 µl 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.

Techniques for LVI

Now that the advantages of large volume injection in capillary gas chromatography have been outlined, the various techniques that have been developed for large volume injection in GC will be briefly mentioned. In principle three (groups of) techniques for large volume injection can be distinguished.

• The on-column technique for large volume injection.
• Techniques using the so-called loop-type interface.
• Methods based on the use of the programmable PTV injector.

The PTV large volume sampling method is actually not just one method. PTV large volume sampling is basically a group of large volume injection methods that share the use of a common interface - the PTV injector. The principles of the on-column approach for large volume sampling will be briefly discussed. Loop-type injection is not treated here as this approach of large volume injection offers no advantages in comparison with PTV or on-column large volume injection. The various techniques for PTV large volume injection are described in detail under this link.

Selection of the PTV large volume injection mode

Despite the fact that several large volume injection techniques have been studied for over 10 years, their use is still not common practice in (routine) laboratories. The main reason for this is that most of the techniques require careful optimization, use expensive instrumentation and lack ruggedness. For widespread acceptance of large volume injection the technique should be simplified and made more rugged and the instrumentation used should be made cheaper. This can be achieved by using PTV based methods for large volume injection. By using PTV injectors equipped with packed 3.4 mm. i.d. liners, sample volumes up to some 150 µl can be injected 'at-once'. The optimization of the key parameter, the solvent vent time, requires only a very limited set of injections. Large volume PTV Rapid or “at-once” injection is rugged, both with regard to small fluctuations in instrumental parameters, as well as with regard to the introduction of non-volatile matrix constituents. This should be the first technique of choice in developing a new PTV large volume application. Speed controlled injection is slightly more complicated to optimize but shares the advantage of a good ruggedness. The table herunder shows an overview of the characteristics of PTV 'at-once' and PTV speed controlled injection is given. This table provides the guidelines for the selection of either of these two PTV large volume injection modes.

Comparison of PTV Rapid and PTV speed controlled large volume injection.

PTV Rapid versus PTV speed controlled

	PTV Rapid	PTV Speed controlled
Preferred liner i.d.	3-4 mm	1-4 mm
Sample volume (µl)	< 150	>150
Main optimization parameters Ease of optimization Ruggedness	Vent time + +	Injection speed +/- +
Applicability Volatility range Labile components Polar solvents 'dirty' samples	> n-C9 - + +	> n-C13 +/- + +