Chemistry Reference
In-Depth Information
number of samples in field analysis is relatively small. Here, the analyst can take advantage of human
superiority over the computer to recognize patterns, and can directly intervene in the analysis process by
identifying common points on both the reference and running electropherograms. This approach is illustrated
in Figure 9.3(b). Adding compounds #3 (sodium decanoate) and #9 (lactic acid) to the standards and sand
extract solutions makes it possible to correct the migration time axis of the raw electropherogram and facilitate
identification. This example uses an important class of analytes - phosphonic acids - which are of great
interest as degradation products of certain chemical weapons. In this case, the useful anchor compounds
could be found relatively easily. Considerable research and experimental design might be needed to develop
proper methods of field analysis for other compounds of interest (e.g. toxic industrial chemicals).
Correct and reproducible quantization is an even more complicated issue for field analysis using portable
CE instruments. Sampling by wiping or by extracting affects reproducibility. Reproducibility is also
influenced by the sampling process, because different operators cannot manipulate the syringes and vials in
exactly the same way. The author's experience indicates that peak area reproducibility can vary about 20
,
which is definitely not acceptable. The anchor compounds described above can be used to improve
reproducibility. The author's have found that using anchor compounds as internal standards can reduce the
standard deviation of the peak area measurements approximately five times.
The shortcomings of CE that result from the robustness needed for field analysis can therefore be partially
overcome by the systematic sophistication of the analysis process. This approach does require sacrificing one
feature of a portable instrument: operational simplicity. However, kits can be prepared for use beforehand, so
the process of analysis in the field still remains straightforward for the technician who is not a chemist.
CE has much higher detection and quantification limits than ordinary HPLC due to the smaller sample
capacity of the capillary columns. This adds one more element of sophistication to the development of field
CE instruments. If regulations require that the concentration of some pollutant in the environment be below
the detection limit of the portable CE instrument, then various methods of sample concentration (perhaps
solid phase extraction) would be necessary. Fortunately, solid phase extraction is a well-developed method of
sample preparation. The author does not foresee any major obstacles to its application in field analysis using
portable CE instruments.
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9.4
Sample preparation in CE
A microscale separation method will be greener than its larger-scale counterpart, but appropriate sample
preparation procedures must still be considered. How green is an analytical method if sample preparation
eliminates the desired outcome? Green approaches to this issue have been largely ignored until recently. Xie
and He researched a group of green sample-pretreatment techniques known as liquid-phase microextraction
(LPME), which is used as a method of sample preparation mainly in connection with gas and liquid
chromatography, but also for CE [9]. Single-drop microextraction (SDME) [60,61] is a popular LPME
technique. A droplet of several
L is suspended at the tip of a microsyringe, which serves as both the solvent
holder and sample injector for performing the extraction and extract injection procedures. The extract can
then be analysed off-line by CE. Various possibilities for microextraction in CE have been described [62].
Most of the methods of green sample preparation based on LPME and used in CE were developed for gas
and liquid chromatography and do not take advantage of the specific features of a CE apparatus. Those
methods are effectively described in a special issue of
Analytica Chimica Acta
[63] and in other chapters of
this topic, and we refer the reader to those descriptions. However, SDME, an LPME technique adapted for
CE, deserves a more thorough description. It takes full advantage of the CE column format: the droplet is
suspended on the top of the capillary instead of at the tip of the syringe needle. This ingenious approach
developed by Choi
et al.
[64,65]. According to that approach, SDME consists of four steps (Figure 9.7):
μ
