1999 Progress Report: Gas chromatography-isotope ratio mass spectrometry-A novel approach for monitoring the origin and fate of hydrocarbon contaminants in the environmentEPA Grant Number: R826178
Title: Gas chromatography-isotope ratio mass spectrometry-A novel approach for monitoring the origin and fate of hydrocarbon contaminants in the environment
Investigators: Philp, R. Paul , Smallwood, B.
Current Investigators: Philp, R. Paul , Kuder, T. , Smallwood, B.
Institution: University of Oklahoma
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1997 through September 30, 2000
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $313,743
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Objective:The major objectives of this proposal can be summarized in the following manner:
- To demonstrate that the isotopic compositions of individual compounds as determined by GCIRMS, may be used to correlate weathered and unweathered samples and monitor movement of pollutant species even when excessive biodegradation, water washing, or other weathering processes, may render the more conventional techniques such as GC and GCMS highly impractical or useless. This includes correlation of hydrocarbons that have accumulated in various wildlife species affected by oil spills.
- To show that even with very extensively weathered crude oil samples it is possible to isolate the asphaltene fractions, pyrolyse the same, and determine the isotopic composition of individual compounds in the pyrolysates. Comparison of these values with those from asphaltenes of the unweathered samples will provide an extremely powerful correlation tool.
- To determine whether or not it is possible to use isotopic compositions of individual compounds in different gasoline samples to determine the relative proportions of the gasolines in a mixture of the same. If so, this could become an extremely useful technique for determination of relative proportions of gasoline in a groundwater plume of hydrocarbons derived from leaking underground storage tanks. If successful, attempts would be made to extend the same approach to mixtures of three or more gasoline samples.
- Oxygenated additives have been used in a few cases to discriminate between gasoline samples from different sources. In this study, it is proposed to determine the isotopic composition of these additives in gasolines from different suppliers to determine whether this might be another viable method for differentiating possible origins for gasolines in groundwater samples.
Progress Summary:A large proportion of the work completed during this reporting period has been focused on the analysis of gasolines and the potential of GCIRMS as a source tracer for gasoline contamination, particularly from leaking underground storage tanks. In the latter half of the year a purge and trap sample concentrator was purchased, with funding from this project, to enable gasoline contaminated soils and groundwater to be analyzed reproducibly and efficiently.
Another part of our research this year has used high-performance liquid chromatography (HPLC) as a means to separate a complex standard mixture of aromatics into subfractions based on ring number prior to analysis on the GCIRMS.
Neat Samples. Nineteen gasoline samples from various locations throughout the United States (Table 1) have been analyzed by GCIRMS. Due to the wide ranging concentrations of individual hydrocarbon components within each gasoline, each sample had to be analyzed four times, twice with a small amount of sample (0.1 L) and twice with a larger amount (0.6 L). This procedure ensured that isotopic numbers for the most concentrated compounds and the least concentrated compounds were duplicated. In addition, to determine reproducibility for this procedure, one sample (DTX) was analyzed 14 times at various concentrations (0.1-0.6 L). Reproducibility ranged from 0.03 - 0.57 ? for 46, apparently fully resolved, compounds within the gasoline sample and was 0.51 and 0.36 for the deuterated standards n-nonane and n-decane, respectively (Figure 1, Table 2). Comparisons for all other gasolines were made on the basis of this data, although in some instances, due to compounds being absent from some of the gasolines, the total number of components actually used in statistical testing was less than 46.
Ultimately, 16 compounds that were ubiquitous to each gasoline were selected for more detailed statistical analysis. The range of isotope numbers for the sample group are shown in Figure 2. All of these compounds show a wide range of variability, the greatest being for benzene, 2,3-dimethylpentane, ethyl benzene, 1-methyl-2-ethylbenzene, and 1,2,3-trimethylbenzene, which have standard deviations greater than 2.40 from the mean for all samples. In addition to this analysis, a statistical test was carried out that basically compared the isotope numbers for each compound from each gasoline with every other gasoline. This test confirmed that of the 171 possible combinations of gasoline samples, 169 were significantly different from each other. The isotopic fingerprint of two pairs of samples were remarkably similar and it has been concluded that these gasolines, even though sampled from different gasoline companies, used the same supplier. The variability observed for the compound specific carbon isotopic distribution of gasolines within this sample group indicates the potential of using GCIRMS as a means to differentiate between fresh gasolines on a local and national scale.
Methyl Ter-Butyl Ether (MTBE). A survey of the isotopic composition of "neat" MTBE directly from suppliers throughout the United States proved to be impossible, mainly due to the manufacturers unwillingness to send samples of their product. Bulk isotopes of two samples that were purchased gave numbers of -30.74 ? and -30.52 ?. Ten gasolines contained MTBE (Table 1) and gave reproducible carbon isotope data with a range of numbers from -28.29 ? (DTX) to -30.35 ? (CTX). By comparing the isotopic composition of MTBE in every gasoline, in a similar statistical method to that used above, it was found that 26 of the 45 combinations were isotopically distinct.
Evaporation and Water Washing. Three gasoline samples (EOK, FOK, GOK) were evaporated for a period of 168 hours with aliquots taken after 24, 48, 120, and 168 hours. Gas chromatograms of sample EOK are shown in Figure 3; the gasoline after 168 hours is clearly different from the fresh gasoline sample and would, therefore, be impossible to use for spill to source correlations. The carbon isotopic composition for those compounds (naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene) remaining after 168 hours are similar to the initial sample and are deemed isotopically stable. A number of compounds, however, are not stable after a period of 24 hours and it is not thought that these can be used for spill to source correlations, but they can be used for remediation studies.
One gasoline sample (DTX) was washed with water to simulate gasoline in contact with groundwater. Analysis of the headspace of the water after a period of one week determined that the only compound present was MTBE and the isotopic composition of MTBE was similar to that of the fresh gasoline sample (standard deviation 0.40 ?). Twelve compounds present in the gasoline after water washing were isotopically stable, nine compounds were significantly different and three of these were compounds that were also different after the evaporation study. These results suggest that a number of compounds can be used to discern the source of gasoline contamination, while others can be used to determine how long the contamination has been present.
Purge and Trap. A Purge and Trap interfaced to our GCIRMS enables us to measure the isotopic composition of organic contaminants that are present in extremely low (15 ppb) concentrations in both groundwater and soil samples. A preliminary investigation of the use of purge and trap versus heated headspace has shown that isotopic numbers are reproducible to standard deviations of 0.4 ? versus 1.2 ?, for the same compound (MTBE). Gasoline contaminated water and soil samples have also been analyzed using this method, and reproducibility is less than 0.5 ?.
Subfractionation of Aromatic Compounds. Determination of the isotopic composition of aromatics in a crude oil is not feasible by analysis of the total aromatic fraction alone due to the complexity of the fraction; consequently, the aromatics have to be fractionated further. A standard mixture of aromatics has been prepared and separated according to ring number using preparative HPLC with a Spherisorb amino column.
Future Activities:A time series of gasoline sampled daily for a period of one month from one service station will be analyzed by GCIRMS to observe the trends in isotope numbers for each of the 46 compounds identified in the above survey. The results of this study will provide us with information regarding the variability of gasoline at one site.
Fractionation of the aromatics from an oil sample will be optimized and repeated on a wide variety of oil samples to determine if this is a viable method for oil spill to source correlations, especially in weathered samples.
A preliminary study into the isotopic fractionation effects during biological degradation (microbial, invertebrate feeding, i.e., by fish) of MTBE will be undertaken. If significant fractionation occurs this will enable us to have an excellent means of monitoring remediation of this compound within contaminated groundwater and soils.
Preliminary analysis of other oxygenated additives, (ethyl tertiary butyl ether, tertiary amyl methyl ether, tertiary butyl alcohol, and diisopropyl ether) and organic compounds perchloroethylene (PCE), trichloroethylene (TCE), creosoles etc., will be undertaken in a similar manner to the MTBE study to determine the fate and transport of these contaminants.