Chemistry Reference
In-Depth Information
The major factors for laboratory efficiency are equipment costs, operating costs including energy, solvents,
chemicals and consumables, complexity of the methods, and level of automation. These factors determine the
cost per analysis. To achieve the highest possible gains in productivity, the laboratory process flow must
be carefully studied to identify the points at which automation would have a measurable impact. Sensitive
instruments with high productivity will improve the overall efficiency of the laboratory. Laboratory automation
can eliminate defects and errors in laboratory processes, and it can often be a way to minimize human errors.
The high costs of instrumental analysis may prohibit more extended studies of environmental screening
and field monitoring. Screening chemical residues is not easy because the methods must be capable of
achieving extremely low limits of detection (LOD). The task is also complex, because even when low LOD
values can be achieved, they vary widely depending on the analyte, the sampling technique, and the sample
preparation and matrix, which is especially important for food samples. It can also be difficult to achieve the
levels of sensitivity required for all the analytes with one pre-treatment method.
Legislation requires many different compounds and matrices to be monitored. Despite considerable activity
in this area of research, there are still many gaps in government-mandated laboratory monitoring plans for
certain matrices and residues. For this reason, multi-residue 'catch-all' methods, or even combinations of
methods for different drug residues using definitive techniques such as LC-MS, are highly appealing in terms
of their high throughput and sensitivity, and ability to meet legislated requirements. However, they are
expensive, and they require solvents and chemicals for sample pre-treatment and qualified specialists to
operate the laboratories.
A reasonable balance must be found between laboratory and field experiments. Field instrumentation in
particular needs serious improvement. Ideal analytical field instruments should (1) be able to produce the
required information on a real-time or near real-time basis, (2) be capable of in situ/at-site analysis and have
little or no need for sample preparation, (3) be portable for field use with a minimum requirement for power
(battery power is desirable), consumables such as gases or/solvents, and clean space for handling samples and
(4) be sufficiently sensitive and selective.
15.6.1
Miniaturization in sample treatment
Reducing the use of solvents is the main force driving the development of sample treatment methods.
Fortunately, solvent reduction is almost always accompanied by decreased energy consumption.
Miniaturization is one way to reduce solvent and energy use. Several attempts have been made to miniaturize
liquid-liquid extraction (LLE). Three of them will be discussed: single-drop microextraction (SDME), liquid-
phase microextractions (LPME) and electro membrane extraction (EME).
In SDME, a microdrop of solvent (ca. 1-3
l) is suspended from the tip of a conventional microsyringe
and then exposed to the headspace of the sample or immersed in a sample solution in which it is immiscible
[41, 42]. The analytes diffuse into this droplet. Once the extraction is complete, the drop is retracted into the
syringe and injected into the chromatographic system for analysis. The main advantage of SDME is its high
enrichment factor, which is due to the very small volume of the acceptor phase. The other advantages of the
single-drop technique include suitability to a wide range of solvents, minimal solvent consumption and low
cost. In addition, no preconditioning is required and memory effects are avoided.
Hollow fibre systems operate similarly to solvent drops, but the solvent is immobilized in the pores of the
fibre. The fibre can also function as a supported liquid membrane, and an acceptor can be placed inside
the lumen [43]. This system is called liquid phase microextraction (LPME). The advantage of LPME is the
presence of a fibre that supports organic solvent and hinders the dissolution or evaporation of the solvent that
occurs in SDME. High enrichment factors can also be achieved with this technique because of the relatively
small volume of the acceptor phase. The main disadvantage of all membrane techniques is that the extraction
tends not to be exhaustive. The recoveries are usually at a level similar to those in SPME, and precise
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