The limits of quantitation (LOQs) defined as 3 times the LOD were analytedependent
and they were in the range from 6.1 μg L−1 (eicosane) to
38 μg L−1 (pentadecane).
Reproducibility of the method was examined by extracting five real
water samples from firefighting spiked at two different concentration
levels 40 μg L−1 and 200 μg L−1 under optimized experimental conditions.
As shown in Table 2, RSDs vary between 0.037 (hexadecane) and
0.14 (pentadecane) for the hydrocarbons at the 40 μg L−1 concentration,
and between 0.026 (o-xylene) and 0.10 (hexadecane) for the
hydrocarbons at the 200 μg L−1 concentration, indicating that the reproducibility
of this extraction method is satisfactory.
The mean recoveries obtained for the studied hydrocarbons
spiked at the two concentration levels 40 and 200 μg L−1 in the real
water samples are shown in Table 2. The relative recovery, as determined
by the peak area ratio of real sample to purified water sample
spiked with hydrocarbons at the same concentration level, was
applied because SPME is a non-exhaustive extraction technique instead
of an absolute recovery as used in exhaustive extraction techniques.
The recovery of analytes at the concentration level of
40 μg L−1 ranged between 98% (pentadecane) and 118% (hexane),
except for decane for which the recovery was 93%. For hydrocarbons
C6–C20 added to the water samples at the concentration level of
200 μg L−1, the recovery was in the range of 87% (undecane) to
117% (hexane). Fig. 4 displays the chromatograms obtained for
water from firefighting (a), spiked water from firefighting at the
concentration level 40 μg L−1 for each analyte (b) and spiked
water from firefighting at the concentration level 200 μg L−1 for
each analyte (c) using HS-SPME–GC–FID.
The confirmation of the developed methodology using an Equity
™-1 capillary column (30 m×0.25 mm×0.25 μm film thickness)
was performed, as identification based solely on retention times is insufficient.
In this experiment an apolar column was used. Conditions
and parameters during the HS-SPME process and final determination
by GC–FID were the same as outlined previously. From the obtained
results it can be stated that all hydrocarbons were sufficiently separated,
except p-xylene and m-xylene which were eluted from the column
at the same time.
3.5. Applications
The developed procedure was applied to determine the hydrocarbons
C6–C20 in two real aqueous samples. Firstly, tapwater contaminated
by unleaded petrol 0.1% (v/v) was used to examine the applicability
of the HS-SPME method for samples containing high concentrations of
different hydrocarbons. Approximately 2 μL of unleaded petrol was
added to 4 mL glass vial. The vial was then filled with a volume of tap
water up to 2 mL, which contained 10% (w/v) of sodium chloride salt
and extracted under optimized conditions. Fig. 5 shows the chromatogram
obtained for the contaminated tap water sample by 0.1% (v/v) of
unleaded petrol using the HS-SPME–GC–FID.