The PTV injector

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The problems of syringe discrimination and decomposition in the vaporization techniques described so far could obviously be reduced if much lower sample introduction temperatures were to be employed. One way of avoiding syringe discrimination is to cool the immediate region surrounding the syringe needle by a secondary air flow. The programmed temperature vaporizer (PTV), is a complete injector which can be rapidly heated and cooled allowing the sample to be introduced in liquid form at cool temperatures and then to be vaporized rapidly. Numerous varieties of this type of injector are commercially available most of which enable both split and splitless modes, and in some cases, on-column injection, depending on the type of liner chosen. Injector designs for the PTV are usually similar to non-programmable versions except for the inclusion of rapid heating and cooling facilities. These facilities may include the ability to use different programming ramps and cryogenic cooling. PTV injectors are often operated with packed liners depending on the type of application. Thus the packing may consist of glass or silica wool, or adsorbent material to selectively remove certain materials.

PTV Injector

Clearly the programmed method gives far less discrimination than the normal hot split injection procedure, almost certainly due to needle discrimination as described earlier. The discrimination rapidly worsens above C18 which boils at 318 °C and if attention is restricted to the range C10- C18 then the hot split injection mode is probably satisfactory. Obviously the use of programmable injectors is advantageous in the analysis of very wide boiling range mixtures but the merits should be thoroughly assessed by the potential user in relation to his or her particular application as in most cases perfectly acceptable results are achievable using the conventional injectors so far described.

Split injection with a PTV must also be applied with care in view of the increased vaporization time. It is unlikely to be successful unless temperature programming conditions are applied to the column in order to utilize the focusing effect of the programme. Particular situations where programmable injection may offer advantages include the separation of very high boiling or labile compounds, and in removing excessive amounts of solvent. In the latter case the solvent is allowed to vaporize at the PTV cold temperature and hence be removed before the migration of sample components starts. Alternatively the split can be operated to remove solvent vapor and then turned off before the sample components can be lost. PTV injection has proven to be successful in so-called large volume sampling techniques.

Discrimination PTV injector

Because there is no explosive evaporation of the solvent, discrimination in PTV is minimized. In experiments a 100 % recovery was found for compounds between C10 and C30. With less volatile solvents, such as toluene, this effect is further reduced.

Injector discrimination

General advantages of PTV are:

Reduced needle – and injector discrimination (cold needle injection)
Large volume injection (LVI) possible
Removal of the sample solvent prior to injection
Holding back of non volatile sample components in the liner
Good accuracy and reproducibility
Cold trapping of samples in the injector and the top of the (pre) column

A disadvantage is that the PTV requires special hardware and a control unit, which makes the system relatively expensive.