A GC-MS Method to Detect Naphthenic Acids in Natural Waters by Selected Ion Scanning
Mireya Merlin1†, Selma E. Guigard1 and Phillip M. Fedorak2
1
Department of Civil and Environmental Engineering,
2
Department of Biological Sciences
University of Alberta
Edmonton, Alberta
†
Corresponding author; phone: (780) 492-4434; fax: (780) 492-9234;
e-mail: mmerlin@ualberta.ca.
1
Abstract
Naphthenic acids (NAs) are naturally occurring acids in petroleum, and they are also
found in the Athabasca oilsands in Northern Alberta, Canada. NAs are compose of a mixture of
aliphatic and cycloaliphatic carboxylic acids that fit into the formula C nH2n+ZO2, where n
indicates the carbon number, and Z specifies the hydrogen deficit resulting from the ring
formation. The acyclic structures are highly branched, and the cyclic structures are fused or
bridged rings of cyclopentane or cyclohexane which bear an alkyl chain. NAs are released into
tailing waters after bitumen extraction and are moderately to highly toxic to fish, plants and
mammals. NAs can be characterized by analyzing their the t-butyldimethylsilyl esters by gas
chromatography-mass spectrometry (GC-MS), and the composition of each mixture can be
plotted in a three-dimensional graph (Fig.1) which showed abundance of specific ions
characteristics of each mixture (Holowenko et al. 2002).
Two methods are currently used to detect NAs in natural waters. One involves extracting
the acids into dichloromethane and measuring the absorbance of the carboxylic groups in the
extract using Fourier transform infrared spectroscopy (Jivraj et al. 1995). The other method
involves derivatizing the carboxylic acid group to detect this by high performance liquid
chromatography with a UV-visible detector (Yen et al. 2004). Any carboxylic acids in a sample
have the potential to be detected by these two non-specific methods. The objective of this study is
to apply a GC-MS method to specifically detect NAs in natural waters.
Examination of GC-MS data from 37 samples of NAs from various sources showed that
three selected ions were often abundant and these ions were specific for NAs. Selected ion
scanning of the data from GC-MS analyses of NAs yields “humps” with retention times between
2
15 and 21 min in the chromatogram, because each nominal ion in the mixture has many isomers
that cannot be completely separated from each other in the GC column.
To further evaluate this method, 22 water samples were analyzed by GC-MS. Among
these were water samples that were in contact with petroleum or oilsands and as expected, these
analyses indicated that NAs were present. In addition, a variety of surface waters and ground
waters collected from locations that were distant from the oilsands were analyzed, and most of
these analyses showed no sign of NAs. However, unexpectedly, the selected ion scans of three of
these samples indicated that NAs were present. However, the three-dimensional plots of these
three samples showed unusual "fingerprints" not seen in any of the NAs mixtures. When a strong
anion exchange solid-phase extraction (SPE) column was used to clean up these “false positive”
sample extracts, the humps in the selected ion scans were eliminated.
This GC-MS method can detect NAs as low as 10 µg/L in river water. The data at this
point suggested that the selected ion scan can provide a specific and sensitive detection of NAs in
natural waters, but further studies are required to evaluate the usefulness of the SPE column.
Keywords: Naphthenic acids, Athabasca oilsands, GC-MS analyses, selected ion scanning
References
Holowenko, F.M., MacKinnon, M.D., and Fedorak, P.M. 2002. Characterization of naphthenic
acids in oil sands wastewaters by gas chromatography-mass spectrometry. Water Research 36:
2845-2857.
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Jivraj, M.N., MacKinnon, M., and Fung, B. 1995. Naphthenic acids extraction and quantitative
analyses with FT-IR spectroscopy. Syncrude Analytical Methods Manual. 4th Edition. Syncrude
Canada Ltd. Research Department. Edmonton, AB. Canada.
Yen, T.-W., Marsh, W., MacKinnon, M.D., and Fedorak., P.M. 2004. Measuring naphthenic
acids concentrations in aqueous environmental samples by HPLC. Journal of Chromatography A.
1033: 81-88.
4
9
8
7
Percentage
6
5
4
3
2
1
0
5
7
9 11
13 15
17 19
12
21 23
Ca rb
25
on nu
27 29
mb er
31
33
6
-Z family
0
Fig. 1. The distribution of ions corresponding to various carbon numbers and Z families in the
complex naphthenic acids mixtures extracted from the Mildred Lake Settling Basin from
Syncrude. The bars represent the percentage (by number of ions) of naphthenic acids in the
mixture that account for a given carbon number in given Z family (corresponding to specific
m/z values from GC-MS analysis). The sum of all the bars equals 100%.
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