Page 435 - Multidimensional Chromatography

Forensic and Toxicological Applications 427

17 of the components are the main features here. However, it was noted that one of
the internal standards (heptadecanoic acid) co-eluted with major interferences, so
even with two-dimensional separation, complete separation of the analytes of inter-
est from a complex matrix is still difficult. The sample preparation used in this case
was typical solid phase extraction. These authors also showed reproducibility and
linear ranges that were easily competitive with those from traditional GC methods.
Solid phase micro-extraction (SPME) (41, 42) has also been employed by Gaines
et al. (43), along with comprehensive two-dimensional GC in the analysis of trace
components from aqueous samples. This combination fills the need for a rapid, high
sensitivity and high resolution analysis of complex mixtures. These authors exam-
ined the analysis of oxygenated and aromatic compounds from water. While these
are not strictly forensic analytes, they do provide effective models for other applica-
tions. As described above, Gaines and co-workers employed a two-column scheme,
with the first analytical column being non-polar and essentially separating com-
pounds by volatility, while the second (fast) column separated analytes of similar
volatility by their polarity. In this way, they were able to demonstrate the low ppb
analysis of various gasoline components spiked into water. The adaptation of this
method to the analysis of volatile and semi-volatile components from water, fire
debris, biological material and other forensic matrices would seem to be reasonably
straightforward.

15.5 ON-LINE SAMPLE PREPARATION

Although on-line sample preparation cannot be regarded as being traditional multidi-
mensional chromatography, the principles of the latter have been employed in the
development of many on-line sample preparation techniques, including supercritical
fluid extraction (SFE)–GC, SPME, thermal desorption and other on-line extraction
methods. As with multidimensional chromatography, the principle is to obtain a por-
tion of the required selectivity by using an additional separation device prior to the
main analytical column.
The coupling of supercritical fluid extraction (SFE) with gas chromatography
(SFE–GC) provides an excellent example of the application of multidimensional
chromatography principles to a sample preparation method. In SFE, the analytical
matrix is packed into an extraction vessel and a supercritical fluid, usually carbon
dioxide, is passed through it. The analyte matrix may be viewed as the stationary
phase, while the supercritical fluid can be viewed as the mobile phase. In order to
obtain an effective extraction, the solubility of the analyte in the supercritical fluid
mobile phase must be considered, along with its affinity to the matrix stationary
phase. The effluent from the extraction is then collected and transferred to a gas
chromatograph. In his comprehensive text, Taylor provides an excellent description
of the principles and applications of SFE (44), while Pawliszyn presents a descrip-
tion of the supercritical fluid as the mobile phase in his development of a kinetic
model for the extraction process (45).