The Challenges of Texas 1005 TPH

by Kristel Fijolek, Quality Control Chemist

The Texas TPH sample provided by APG is comprised of regular grade gasoline, diesel fuel, and 10W-30 motor oil, all of which are n-alkanes.  None of these individual components are particularly troublesome to analyze for alone, but when the three are combined things get complicated.

The first challenge is the actual sample preparation.  The solvent is Pentane, and the samples fizz like a freshly opened can of soda when the ampules are opened.  The contents of each ampule must be carefully mixed to reduce fizzing before diluting to avoid RSD issues.

Once the sample is prepared, it runs by GC-FID.  The temperature program starts at 30˚C (86˚F) and ends at 325˚ (617˚F). The temperature must start this low to achieve proper separation between the solvent, Pentane (nC5), and nC6. The temperature must end this high because of the boiling point of motor oil. Because of this extreme temperature difference, it takes a long time for the GC to cool down at the end of a run.  This extended cool down time turns a 35 minute run into almost an hour from injection to injection. A typical QC run for WP Texas TPH high level and Texas TPH low level together takes about 30 hours.

Data analysis for Texas TPH can be difficult as well. It is reported as carbon ranges: nC6 - nC12, nC12 - nC28, and nC28 -nC35. The trouble with this is that there is overlap between the carbon ranges. On the chromatogram, diesel, the middle compound, overlaps into both the gasoline and the motor oil ranges. Since this product is reported in carbon ranges, only a portion of each compound is used in the calculation. NC6 – nC12 is 64% of the gasoline raw amount plus 13% of the diesel raw amount. nC12 – nC28 is 87% of the diesel raw amount plus 61 % of the motor oil raw amount, and nC28 – nC35 is 34% of the motor oil raw amount. It is impossible to look at a chromatogram of a mix of these compounds and make those determinations. The portion of the chromatogram that falls beyond C35 is not used at all.  To ensure the correct portions of the chromatogram are used for each carbon range, retention time markers must be run with each analytical batch. These markers contain the individual carbons; nC6, nC12, nC28, and nC35. Because the baseline does not hold steady in this analysis, is must be forced with a baseline hold to allow constant integration throughout the chromatogram. The amount for each carbon range is calculated by integrating   each as a sum beginning just after the initial carbon marker and at the end of the second retention time marker. 

Texas TPH is a state specific sample for those customers seeking accreditation with the state of Texas.  The way the sample is run and reported is based upon TNRCC Method 1005 (Texas Natural Resources Conservation Commission).      

References: Texas Natural Resources Conservation Commission: Total Petroleum Hydrocarbons TNRCC Method 1005, Revision 03. June 1, 2001. Jeffrey A. Saitas, P.E., Executive director.

For more information please contact Kristel Fijolek at k.fijolek@apgqa.com.