9 compared to LPG.
Investigating the Source of Residue Contamination
As mentioned previously, an added benefit of gas chromatography/FID is that it provides information of the hydrocarbon range of the residue, which can then be used as a troubleshooting tool to investigate the source of the contamination. For this part of the study, an investigation was carried out to determine if there are losses or discrimination of the light boiling point contaminants of the residue by Method D2158 which could lead to erroneous data. This was done by taking the residue material in the graduated tube left over from Method D2158, dissolving it in a solvent and analysing it by GC/FID via liquid injection. The resultant chromatogram is shown on the left in Figure 8. The same sample was also analysed by this new gas chromatography method. However, instead of an FID, the analysis was performed by GC- MS so that specific components, such as the phthalates and other compounds, could be identified with a high degree of certainty. It’s important to emphasise that the hydrocarbon profile using GC-MS will be consistent with FID detection. The total ion chromatogram (TIC) is displayed on the right of Figure 8. The results of this experiment demonstrate the discrimination of the lighter components using Method D2158 which could possibly lead to inaccurate results. In addition, it will not provide the speciation data available with the chromatographic method.
Conclusion
All objectives of the thermal desorption investigation have been met, with good accuracy, precision, recoveries and detection capability being achieved for all hydrocarbons from C6
to C40 on a single
tube. Additionally, it has been shown that there are no interferences of compounds below C5
since the majority of C5 minus
is not retained thus allowing for the quantitation of C6
plus. It offers the added
benefit of the tubes being portable and very easy to transport back to the lab for analysis. This means remote sampling in the field can be carried out with more convenience
Figure 8: Comparison of the chromatogram of a GC injection of the residue after the evaporation procedure described in ASTM Method D2158 (left) and a chromatogram of the thermal desorption sample introduction approach of the same sample measured by GC-MS (right)
and safety and, as discussed, a more cost effective solution than the traditional way of sampling LPG cylinders. As a result, this methodology has proved itself to be rugged enough that it is now been designated as ASTM Method D7828. Additionally, if there is a need to detect lower levels below 10 µg/g, the ASTM test method can be modified to achieve a 50x enhancement, or a detection capability of 0.2 µg/g.
Acknowledgement
The authors would like to thank Timon Huybrighs, Product Specialist at PerkinElmer for his help and Robert Thomas from Scientific Solutions for his expertise in drafting the article.
Further Reading
1. Good Practices for the Care and Custody of Propane in the Supply Chain, A Report From Energy and Environmental Analysis, Inc on PERC Docket 11352, Propane Education and Research Council, Washington, DC, June 2005
2. ASTM D1835: Standard Specification for Liquefied Petroleum Gases (LPG), ASTM Website:
http://www.astm.org/Standards/ D1835.htm
3. ASTM D2158: Standard Test Method for
Residues in Liquefied Petroleum Gases (LPG), ASTM Website:
http://www.astm.org/ Standards/
D2158.htm
4. ASTM D7828: Standard Test Method for Determination of Residue Composition in Liquefied Petroleum Gas (LPG) Using Automated Thermal Desorption/Gas Chromatography (ATD/GC), ASTM Website:
http://www.astm.org/Standards/D7828.htm
5. A Single-Method Approach for the Analysis of Volatile and Semi Volatile Organic Compounds in Air Using Thermal Desorption Coupled with GC/MS: R. Provost, L. Marotta, R. Thomas; LCGC North America, October, 2014, http://www.
chromatographyonline.com/single-method- approach-analysis-volatile-and-semivolatile- organic-compounds-air-using-thermal-desorp
6. Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-17 Determination of Volatile Organic Compounds in Ambient Air Using Active Sampling onto Sorbent Tubes, Center for Environmental Research Information, Office of Research and Development, U.S. EPA, 1999,
http://www.epa.gov/ttnamti1/files/ ambient/airtox/to-17r.pdf
When contacting companies directly from this issue please tell them where you found their information.
Thank you
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