Chemistry Reference
In-Depth Information
(a)
(b)
Waste
Acceptor
stream
To detector
Membrane
Acceptor (or
donor) stream
Membrane
Waste
To detector
Donor
stream
(sample)
Donor
(or acceptor) stream
(i)
(ii)
(iii)
Figure 8.1
Types of membrane-based units. (a) Sandwich membrane in various chamber designs:
(i) parallelepipedal, (ii) winding and (iii) spiral. (b) Tubular (hollow-fibre) type membrane.
taking place in a single step in the same unit. The volatile analyte - or its volatile reaction product - present in
a heated donor phase, evaporates through a porous membrane and is collected in an acceptor stream for
appropriate detection [14]. An air gap is left between the sample in the donor chamber and the membrane (see
Figure 8.2) in order to avoid clogging of membrane pores when processing dirty samples and also to permit
the presence in the donor chamber of species such as high-molecular-weight components, acids, bases,
organic solvents, and so on, which could damage the membrane on contact with it. This is the most salient
advantage of analytical pervaporation over its industrial counterpart and other membrane-based non-
chromatographic techniques such as dialysis or gas diffusion. As with gas-diffusion modules, the pervaporator
can be designed and constructed by the operators themselves as the need arises.
A dramatic reduction in acceptor volume - and a consequent increase in concentration - can be obtained
by using a spiral acceptor chamber with its outlet pointing to the bottom instead of a flat chamber with its
inlet and outlet facing each other (Figure 8.2 b). This chamber design is especially useful with a view to
implementing pervaporation prior to capillary electrophoresis (CE) separation since the small amount of
solution used, enriched with volatile analytes, suffices for insertion into the capillary. This combined
