Analytical pervaporation

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. 

Preconcentration of pervaporated analytes is the most effective way of favouring mass transfer. The equilibrium between phases can be efficiently displaced by continuous removal of the transferred species from the upper chamber, followed by concentration (either in a minicolumn placed before the deteetor or in the flow-cell itself)-... 

An upper acceptor chamber fitted with inlet and outlet orifices through which the acceptor stream (liquid or gas) is circulated, and in which the pervaporated analyte (or its volatile reaction product) is collected. 

Continuous removal of the pervaporated analytes to the preconcentration column, if used, allows fresh portions of acceptor gas to come into contact with the diffused species, thus displacing the mass transfer equilibrium. 

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 analytical extraction systems related to points 1 and 2 are pervaporation-based techniques (such as those mentioned in Sections 4.3.1 and 4.3.2). Extraction based on the membrane separation of an aqueous phase and an organic phase (point 3 above) will be dealt with in Section 4.3.3. As the system concerning point 4 is very rarely used, it will not be considered here. 

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. 

Membranes can be classified as porous and nonporous based on the structure or as flat sheet and hollow fiber based on the geometry. Membranes used in pervaporation and gas permeation are typically hydrophobic, nonporous silicone (polydimethylsiloxane or PDMS) membranes. Organic compounds in water dissolve into the membrane and get extracted, while the aqueous matrix passes unextracted. The use of mircoporous membrane (made of polypropylene, cellulose, or Teflon) in pervaporation has also been reported, but this membrane allows the passage of large quantities of water. Usually, water has to be removed before it enters the analytical instrument, except when it is used as a chemical ionization reagent gas in MS [50], It has been reported that permeation is faster across a composite membrane, which has a thin (e.g., 1 pm) siloxane film deposited on a layer of microporous polypropylene [61],... 

Therefore, as noted in Chapter 2, most analytical methods involve some sample preparation step. Most often, such a step involves the transfer of the target analytes from the solid to a liquid phase. By exception, some techniques such as headspace or pervaporation involve the physical removal of volatile analytes from the solid sample. 

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. 

Gomez-Ariza J. L., Velasco-Aijona A., Giraldez 1., Sanchez-Rodas D., and Morales E. (2000) Coupling pervaporation-gas chromatography for speciation of volatile forms of selenium in sediments. Int. J. Environ. Analyt. Chem. 78, 427-440. 

Semenova SI, Ohya H, and Soontarapa K. Hydrophilic membranes for pervaporation An analytical review. Desalination 1997 110 251-286. 

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. 

One of the most salient, favourable aspects of pervaporation is its ability to integrate many steps of the analytical process, thereby facilitating miniaturization. On the other hand, the greatest shortcomings of this technique are its still incipient development and the resulting absence of commercially available equipment to implement it. 

Analytical pervaporation is the process by which volatile substances in a heated donor phase evaporate and diffuse through a porous hydrophobic membrane, the vapour condensing on the surface of a cool acceptor fluid on the other side of the membrane. Surface tension forces withhold the fluids from the pores and prevent direct contact between them. A temperature difference that results in a vapour pressure difference across the membrane provides a strong driving force for the separation, which also occurs in the absence of a temperature gradient. Evaporation will occur at the sample surface if the vapour pressure exceeds that at the acceptor surface. One important feature of pervaporation modules used for analytical purposes is the air gap between the donor phase and the hydrophobic membrane, which avoids any contact between them and reduces the problems associated with fouling of the membrane. 

The mechanism of transport by pervaporation can be described in the light of the sample diffusion model [159], which comprises the following steps (a) evaporation of the analyte into the air gap (b) sorption into the membrane on the sample side (c) diffusion of the sorbed component through the polymer matrix and (d) desorption into a liquid or gas phase on the acceptor side. The last three steps are also included in industrial pervaporation processes. 

Analytical pervaporation is a very mild process that can be operated at the required temperature and needs no high pressure or cross-flow velocity, and no additional chemicals. Because of its short life, the theoretical principles of analytical pervaporation have not yet been established except for liquid samples and gas-phase acceptors [160] there is, however, research in progress on solid and liquid samples processed with both liquid and gaseous acceptors [161]. 

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. 

Basically, an analytical pervaporator consists of the elements shown in Fig. 4.17A, namely an upper acceptor chamber (a) with inlets and outlets through which the acceptor stream is circulated and in which the gaseous analyte (or its reaction product if the analyte is not volatile) is collected a lower, donor chamber (d) that contains the solid sample or through which the feed stream of liquid or slurry sample is circulated a thin (ca. 1 mm) membrane support (b) made of polytetrafluoroethylene (PTFE) or metal and spacers (c) of variable thickness (2-10 mm) that can be placed below or above the membrane support in order to increase the volumes of the corresponding chambers. 

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. 

When the pervaporation unit is used for the continuous monitoring of a fermentation process with a view to determining analytes in samples containing suspended particles, or in slurries, the module is altered by increasing the diameter of both channels (inlet and outlet) of the donor chamber, which might otherwise be clogged. 

When separation and detection of volatile species occur simultaneously, a hole is drilled at the centre of the upper chamber top of the pervaporation cell in order to accommodate the sensor [149,165] with the aid of appropriate adaptors. This module can be used with both liquid [149,152] and solid samples [165]. The flow of the acceptor stream is stopped during measurement so as to allow the accumulation of the analyte released from the matrix during the process and hence to increase the sensitivity [166]. 

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. 

The membranes used for analytical pervaporation are hydrophobic membranes of the types usually employed in ultrafiltration and gas-diffusion processes. In practice, PTFE is the most frequently used membrane material, followed by hydrophobic polyvinylidene-fluoride (PVDF). Ultrafiltration membranes are very thin, which, in combination with the large surface area of both the donor and acceptor chamber, leads to their easy bending. This results in changes in the ffux of the permeating component through an altered membrane area and hence in changes in the efficiency of the process. As a result, membranes must be replaced fairly often. Because of their thickness, gas-diffusion membranes are not so easily bent, so the same membrane can be used over long periods. The pore size of the... 

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.

For solids, an appropriate amount of sample is weighed in the donor chamber, which is then connected to the rest of the pervaporator. The reagents, when necessary, are injected through the septum (at the inlet of the donor chamber) or septa (at both the inlet and outlet of the donor chamber) when several separate reagents are required. Then, the donor chamber is heated and the analytes pervaporate through the membrane and are accepted by the fluid on the other side of the membrane. 

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 efficiency of a given pervaporation process must be evaluated in order to act on the overall system so as to either boost or reduce mass transfer to the acceptor chamber as required. Such evaluation can be done either in (a) relative terms by comparing signals provided under different conditions by the analyte (or its reaction product), previously collected in the upper chamber and driven to the detector, or in (b) absolute terms by comparing the signal obtained under the working conditions with that corresponding to 100% mass transfer. 

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