Chemistry Reference
In-Depth Information
partially soluble in water [116]; also, very polar compounds can lead to competition between the aqueous
phase, SPME fibres, glass walls of the extraction vessel and the surface of polytetrafluoroethylene stir-bar
[117, 118]. Coating stir-bars with an amount of polydimethylsiloxane (PDMS) 50-250 times greater than that
typically used in SPME substantially increases the amount of extractant to be handled. Typically, the stir-bar
in SBSE is a magnetic cylinder covered with a glass layer that is turn coated with organic material (usually
PDMS). In recent years, however, sol-gel technology has been successfully used as an alternative choice for
stir-bar coating.
Similarly to SPME, SBSE dispenses with thermal desorption and the need for polluting organic solvents.
However, SBSE is slower than SPME owing to the larger amount of sorbent used; also, it requires cold
trapping to avoid analyte losses.
Although thermal desorption is the most environmentally friendly way to desorb the target compounds,
liquid desorption with a very low volume (100-200
l) of an organic solvent such as methanol or acetonitrile
is also usually effective. Extraction can be improved by applying ultrasound energy and the extract is most
often ready for injection into an HPLC or GC instrument.
One salient feature of SBSE is its availability for in situ derivatization, if required. This is especially
interesting when extracting relatively high-polar compounds, the extraction efficiency for which is diminished
by the non-polar nature of PDMS. Examples of in situ derivatization in SBSE include the extraction of
hydroxyl groups in phenols with acetic acid anhydride [119-121], carboxyl groups with ethyl chloroformate
[119-121] and carbonyl compounds with
O
-(2,3,4,5,6-pentafluorobenzyl)[122]. In situ derivatization also
improves sensitivity when used prior to GC. Simultaneous extraction and derivatization shortens analysis
times and increases throughput as a result. In addition, SBSE affords de-conjugation of biological samples
simultaneously with extraction (i.e. in situ de-conjugation). The advantages of this procedure are similar to
those of in situ derivatization (shorter sample preparation times, mainly).
μ
8.4.4 Continuous filtration
Filtration is a very simple and old separation technique outdated by modern analytical procedures. However,
it has regained some importance after its joint use with continuous flow systems. The ensuing dynamic
approaches have allowed downsizing equipment and reducing sample and reagent volumes as a result. In
addition, dynamic systems facilitate automatic filtration, thus suppressing or reducing operational and
handling hazards, and making analytical procedures safer for humans and environment. Filtration in
continuous systems can be done with or without a filtering device. In fact, filtering can be accomplished by
using an auxiliary form of energy such as ultrasound or a filterless device such as a knotted reactor (see
Figure 8.13(a)) [45] to separate phases. Continuous filtration is commonly used to remove solid particles
from a liquid sample or from the leachate of a solid sample-leachant system. It is also used after precipitation,
either to remove interferents from the sample or to isolate the analyte as a solid for indirect measurement or
subsequent dissolution prior to detection [45].
The filters used in continuous filtration systems are usually in the form of stainless-steel cylinders or
plates (Figures 8.13(b) and 8.13(c)), disposable membranes or packed beds. Stainless-steel filters are the
most widely used and have the advantage that they can be reused over long periods simply by alternating
filtration and cleaning. Filters can be cleaned by hand, flushing an appropriate solvent or applying
ultrasound.
Disposable membrane filters are usually made of nylon or cellulose. Unlike steel filters, their construction
requires too much energy and material for so short a useful life; so much so that filters must be replaced rather
frequently. This is also the case with packed-bed filters owing to their frequent need for refilling (usually with
polystyrene granules, cotton or filter paper pulp) and poor reproducibility; a result of the difficulty of packing
the material to the same degree of tightness every time.
