Pervaporation, analytical selectivity

The resulting spectra from El usually contain a number of fragments, providing extensive structural information about the analyte. A disadvantage of the observed fragmentation is eventually occurring isobaric overlay from different compounds in the analysis of sample mixtures, which often requires a separation step prior to the MS analysis. For this purpose the coupling of a GC with the ion source of the mass spectrometer via capillary inlet is a well established technique. Volatiles can be selectively introduced into El mass spectrometers via pervaporation membranes. The principle and application of this technique, called membrane introduction (MI) MS was recently reviewed [45]. The accuracy of intensity ratio measurements by El MS is about 0.1 -0.5% [4,8]. 

The use of silicone membranes as an interface in MIMS for direct extraction and analysis by MS has fostered their implementation for extraction purposes that can be combined off-line or on-line with other analytical instrumentation, such as GC. The technique of membrane extraction with sorbent interface (MESI) (Figure 4.2) employs the pervaporation principle in a nonporous polymeric membrane unit, where the membrane is used as a selective barrier for the extraction of VOCs and SVOCs in gaseous or liquid samples. 

Although clean-up and preconcentration help to improve the selectivity of dissolution and offset the dilution effeot, they lengthen the analytical process. This drawback should always be borne in mind in view of the growing tendency to shorten the analytical process so as to analyse as many samples as possible in the shortest time. It is always preferable to use selective steps such as leaching, pervaporation or headspace to remove the analytes from a solid sample. However, very frequently, they fail to provide quantitative results owing to inadequate efficienoy and (or) preoision. In this situation, USASD is an effective alternative to ensure complete transfer of analytes to a liquid phase and hence the quality in the results. 

As a rule, permeability in glassy polymers (e.g. cellulose) is lower than in rubbery polymers (e.g. polydimethylsiloxane, PDMS) on the other hand, selectivity is dictated by the molecular dimensions of the permeating species [167]. The polymers used as membranes in analytical pervaporation are similar to those employed for gas separation and possess a dense, non-porous macroscopic structure. The difference between the two lies in the transport mechanism and arises mainly from a large affinity difference between the permeating molecules and the polymer membrane. 

This section deals with the basic analytical properties (sensitivity, selectivity and precision) of pervaporation-based methods. 

In multideterminations, and also when the target volatile analytes possess different boiling points, the pervaporation unit can be used at different temperatures to ensure selective separation of the species of interest. Use of a heating system allowing reproducible control of time and temperature is obviously mandatory here. 

The separation of methanol from ethylene glycol is an important industrial process in the synthesis of poly(ethylene terephthalate). The methanol/ethylene glycol system has been extensively studied using various analytical methods and pervaporation experiments as well. The methanol selectivity reaches up to 250 at low methanol concentrations, however, the total flux decreases in this region. 

Selectivity depends on the molecular dimensions of the permeating species. Thus, the hydrophobic membranes for analytical pervaporation are usually made of polytetrafluoroethylene and they are similar to those used in processes like ultrafiltration and gas diffusion. 

The analytical features of pervaporation, concerning the basic analytical properties, namely, sensitivity, selectivity, and precision, strongly rely on experimental variables. Sometimes, a given variable can have an opposite effect on two of the analytical properties, so a compromise between them is needed. 

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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