7
can be seen that there is no discrimination (within experimental error) using the ATD approach for this residue range.
Calibration and Chromatographic Separation
To cover the full residue range, a calibration standard containing a mixture of hexane, heptane, iso-octane, and toluene was used for the lower boiling point region (gasoline range), while diesel was used for the mid- range and compressor oil was used for the higher boiling point region.
In order to demonstrate that the gasoline surrogate components could be separated from each other and in particular, that the hexane could be separated from pentane, and the diesel and compressor groups were distinquishable, a chromatogram of the residue standard containing the gasoline surrogate components, diesel and compressor oil was collected using the set up previously described. This was done by making up a stock solution in pentane and then diluting with LPG in a cylinder. The chromatogram of the residue standard is shown in Figure 4, while an expanded view of the low end (red box) showing the lighter components is seen in Figure 5. It can be clearly seen that the pentane peak is well resolved from hexane, and there is also no interference from propane or butane.
A calibration plot was then generated by transfering fixed amounts of the standard residue dissolved in pentane onto the sorbent tubes, which represented 11 concentrations in total, ranging from 3 to 1500 µg. Figure 6 shows this calibration plot, which gave a correlation coefficient of 0.9990. For calibration, a timed group area of the residue is used as shown in Figure 4, which is the response of each standard, taken from the time immediately after the elution of pentane (C5
the elution of the compressor oil (C40
Figure 5: An expanded view of the low end (red box) showing the ligher hydrocarbon components are well-separated from the pentane solvent (for safety reasons, benzene was used for separation purposes, but not for calibration)
Compound n-Hexane Iso-octane n-Heptane Toluene
C22 C30 C36 C40
Liquid Injection % Recovery 99.8
105.2 104.3 113.2 100.0 108.9 105.7 105.5
Thermal Desorber Injection % Recovery 94.2 99.5
100.3 104.2 100.0 112.2 106.9 102.7
Table 3: Recoveries (%) of the hydrocarbon standard using conventional liquid injection compared to thermal desorption.
second tube in line will be blank (or less than 5%) because the front tube was able to retain the residue. Figure 7 exemplifies the result from that breakthrough experiment. The chromatogram of the
first tube is seen in black, which shows all the residue hydrocarbon peaks. Whereas the chromatogram of the second tube is seen in blue, which shows an absence of all the signature peaks except for the pentane
) through to the end of ).
Breakthrough Experiment
The prevention of breakthrough is a very important aspect of any adsorbent. According to the EPA, it is defined as the volume sampled when the amount of analyte collected in a backup sorbent tube reaches 5% of the total amount collected by both sorbent tubes (6). Therefore a breakthrough experiment was performed to ensure the adsorbent was able to retain the target analyte range of residue, by connecting two sorbent tubes together while sampling the LPG. If breakthrough does not occur, the
Figure 6: Calibration plot of weight of residue versus peak area of chromatogram shown in Figure 4, which gave a correlation coefficient of 0.9990
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