Pervaporation, analytical sample types

The manifold into which the upper chamber is inserted does not depend on the initial state of the sample, but only on the characteristics of the pervaporated analytes, the type of detector used and its position along the manifold. Depending on the particular type of detector used, auxiliary channels will have to be included to bring the pervaporated species into contact with appropriate reagents in order to obtain products to which the detector will respond. Integrated detection and pervaporation requires altering the pervaporator but simplifies the overall manifold. As shown below, preconcentration units, solid-phase reactors (mainly enzyme reactors) and various other devices can also be connected in-line in the manifold when required. 

So far, laboratory-scale pervaporators have been user-designed and built. Although pervaporation can be applied to liquid, solid and slurry samples, the basic separation unit is identical whichever the sample type, the sole difference as regards equipment requirements being the use of appropriate accessory units. An analytical pervaporator consists of two essential parts, namely the body of the separation module (including the devices for hindering gas losses) and the membrane. 

The techniques discussed in this chapter vary in automatability and frequency of use. Thus, while automatic hydride and cold mercury vapour generation are implemented in laboratory-constructed or commercially available dynamic equipment that is straightforward, easy to operate and inexpensive, automating laboratory headspace modes and solid-phase microextraction is rather complicated and commercially available automated equipment for their implementation is sophisticated and expensive. Because of its fairly recent inception, analytical pervaporation lacks commercially available equipment for any type of sample however, its high potential and the interest it has aroused among manufacturers is bound to result in fast development of instrumentation for both solid and liquid samples. This technique, which is always applied under dynamic conditions, has invariably been implemented in a semi-automatic manner to date also, its complete automatization is very simple. 

Pervaporators are amenable to coupling to any type of detector via an appropriate interface such as a transport tube, a microcolumn packed with adsorptive or ion-exchange material, or a gas liquid separator. The acceptor stream can be either liquid or gaseous depending on the characteristics of the detector. The detectors most frequently used are the spectroscopic - atomic or molecular, electroanalyti-cal (potentiometric, voltammetric), electron capture, and flame ionization types. The low selectivity of some of these detection techniques is overcome by that of the pervaporation step, endowing the overall analytical process with the selectivity required for the analysis of complex matrices. The potential use of the pervaporation technique for sample insertion into water-unfriendly detectors such as mass spectrometers or devices such as those based on microwave-induced plasma remains unexplored. 

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