2.5. Sample clean-up and pre-concentration
Each sample was cleaned up using silica gel column prior to
analysis. About 2.0 g of silica gel was stirred in 10 mL of 65:35
(v/v) hexane–acetone mixtures, and then the slurry was transferred
to a disposable, 5 mL glass pipette. About 15 mg of glass
wool was used as a porous plug to confine silica to the pipette. Silica-gel
columns were preconditioned with 20 mL of 65:35 (v/v)
hexane–acetone mixtures. The amount extracted from Section 2.4
above was loaded onto the column using a Pasteur pipette, and
analytes were eluted with 30 mL of the hexane–acetone mixture.
The eluate was then collected in a glass test tube and the solution
was reduced to a volume of about 20 lL under a stream of nitrogen
at 30 C. The sample was transferred directly into a GC insert and
reconstituted in 120 lL of hexane–acetone mixture. Prior to analysis,
a constant amount of phenanthrene d10 (40 ng, 20 lL of
2 ng lL1
) was added as an internal standard (IS). The final volume
of sample solution was about 140 lL. A schematic diagram of the
experimental procedure is given in Fig. 1.