Pervaporators manifolds

The analytical pervaporator can be used in combination with a flow-injection manifold, either in the upper chamber when the pervaporated species must be derivatized for adaptation to the detector and/or in the lower chamber for the pervaporation of analytes from liquid samples or slurries. Alterations of either the auxiliary dynamic manifold or the pervaporator itself are required when the pervaporation step is assisted by focused microwaves, the separation step assists in the continuous monitoring of an evolving system, untreated solid samples are used or pervaporation is integrated with detection. 

Fig. 4.18. Continuous iscrete approach to implementing analytical pervaporation of solid samples. The dotted line corresponds to a potential derivatization reaction of the pervaporated species and the dashed lines represent the continuous manifold used for automatic insertion of liquid samples. P peristaltic pump, AS acceptor stream, IV injection valve, SV switching valve for changing between continuous and discrete insertion of sample into the pervaporator, R reagent, DS donor-sample stream, S sample, RC reaction coil, PM pervaporation module, M membrane, D detector, W waste.

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. 

The qualifiers continuous and discrete as applied to pervaporation refer to different aspects of the process. In fact, analytical pervaporation is a continuous technique because, while the sample is in the separation module, mass transfer between the phases is continuous until equilibrium is reached. Continuous also refers to the way the sample is inserted into the dynamic manifold for transfer to the pervaporator. When the samples to be treated are liquids or slurries, the overall manifold to be used is one such as that of Fig. 4.18 (dashed lines included). The sample can be continuously aspirated and mixed with the reagent(s) if required (continuous sample insertion). Discrete sample insertion is done by injecting a liquid sample, either via an injection valve in the manifold (and followed by transfer to the pervaporator) or by using a syringe furnished with a hypodermic needle [directly into the lower (donor) chamber of the separation module when no dynamic manifold is connected to the lower chamber]. In any case, the sample reaches the lower chamber and the volatile analyte (or its reaction product) evaporates, diffuses across the membrane and is accepted in the upper chamber by a dynamic or static fluid that drives it continuously or intermittently, respectively, to the detector — except when separation and detection are integrated. 

The relative procedure is typically used in optimization experiments in order to accommodate the sensitivity (usually by maximizing it), but also to ensure the best possible conditions for derivatization reactions (prior and/or subsequent to pervaporation) and dispersion along the continuous system, among others. No special alterations of the manifold other than those resulting from the optimization process are required in this case. 

Fig. 4.19. Simplified scheme of a hydrodynamic manifold for evaluation of pervaporation efficiency. Note that, when a derivatization reaction is required prior to or after analyte separation, one or more additional channels for the reagent solution(s) must be included in the donor and acceptor stream, respectively. AIV auxiliary injection valve, MIV main injection valve, PM pervaporation module, m membrane, a merging point, D detector, W waste, AUX auxiliary channel containing acceptor solution, S sample, AS acceptor solution, DS donor solution. (Reproduced with permission of Wiley Sons.)...

Multideterminations involving pervaporation [180] can be implemented either by altering parts or parameters affecting the overall approach or by coupling the dynamic manifold of the upper chamber to a highly discriminating separation technique such as gas chromatography. 

Nonchromatographic continuous separation techniques involving gas-liquid interfaces (e.g., gas diffusion, hydride generation, pervaporation) prior to detection by an atomic spectrometer have frequently been coupled to FI manifolds to develop excellent methods of interest, mainly to the clinical and industrial fields. 

Analytical pervaporation can be carried out in a continuous way if the sample reaches the pervapor-ator by means of a continuous manifold, or in a discrete mode if the sample is introduced by injection,... 

The simplest way of giving pervaporation the character of a continuous separation technique is its coupling to a dynamic manifold for assistance of both donor and acceptor chambers. In the first instance, when the samples are liquid, the coupling with the manifold (usually a flow-injection (FI) arrangement) is mandatory for driving the sample either by injection or aspiration to the donor chamber. In addition, (bio)chemical and/or physical steps, namely, reactions that convert the analyte into the most appropriate form for being evaporated, physical dispersion, etc., can also be developed in the manifold prior to the arrival of the sample to the donor chamber meanwhile, a detector can be located in postpervaporator position in order to monitor nonvolatile species. When the sample is a solid, the... 

The versatility of the design of an analytical pervaporation module, attributable to its changeable donor volume and the air gap present above the sample, has enabled its use with liquid, solid, and semisolid samples. Table 1 summarizes the applications of analytical pervaporation and the main fields in which it has been used. The manifold used and the way in which the sample is introduced in the system vary depending on whether it is liquid or solid. The main difference between the treatment of liquid and solid samples is the fact that with liquids the whole pervaporation process, from sample introduction to delivery of the results can be fully automated, while some operator s intervention is needed when dealing with solids. 

FIA manifolds for the determination of sulfide in (a) liquid and (b) solid environmental samples. ISE ion-selective electrode IV injection valve PM pervaporation module RE reference electrode. 

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