Petroleum Analysis: Methodology for Quantitative and Qualitative Assessment of Oil Spill Harry S. Hertz', Willie E. May, Stephen N. Cheder, and Barry H. Gump Trace Organic Analysis Group, Bioorganic Standards Section, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
An integrated chromatographic technique for petroleum analysis compatible with long-term studies of oil spills is presented. Dynamic headspace sampling and the complementary analytical techniques of gas chromatography and coupled-column liquid chromatography are utilized for quantitation of petroleum containing samples. Gas chromatography-mass spectrometry is employed for identification of individual components in these samples. Analytical data obtained from a major oil spill are presented and discussed.
In the year 1970 alone, approximately 500 000 metric tons of oil were catastrophically spilled in the waterways of the world ( 1 ) .The effects of such spills on the marine environment are of current great concern due to the biological and economic losses which could be sustained. To properly understand the biological damage caused by an oil spill, it is necessary to know the magnitude and composition of the petroleum in water, tissue, and sediment samples collected in the spill area. Furthermore, the analytical methodology developed for this purpose must be compatible with long-term studies in which low levels, as well as the initial high levels, of hydrocarbon pollution are encountered. This paper details an integrated chromatographic technique (developed as a result of receiving samples from a major oil spill) utilizing dynamic headspace sampling which meets the above criterion. Dynamic headspace sampling is the use of a flowing stream of inert gas to purge a sample of volatile organic components and sweep these components onto an appropriate adsorbent. Through the advent of efficient adsorbents, this technique is receiving considerable current interest for environmental and biomedical trace analysis ( 2 ) . Gas chromatography (GC) and coupled-column liquid chromatography (LC) using a novel internal and external standard technique are employed for quantitation of petroleum containing samples. Gas chromatography-mass spectrometry (GC-MS) is employed for identification of individual components. The two quantitative techniques are uniquely complementary in that they utilize different molecular weight regions and compound classes (aliphatic vs. aromatic hydrocarbons) to arrive a t a quantitative answer. Furthermore, an indication of the degree of weathering that has occurred in various samples may be determined by examining the n-pentadecaneln-undecane gas chromatographic peak height ratios. Experimental Samples from the site of a catastrophic oil spill (>50 000 tons of a light Saudi Arabian crude spread along -100 miles of shoreline) were collected and frozen in the field. Portions of these samples along with a sample of oil taken directly from the tanker after the spill (unweathered spill oil) were shipped to NBS for corroborative analyses. These samples consisted of an oil-water emulsion (M-1), two sediments (S-1and S-2), a control sediment (S-3)collected beyond the spill area, and a homogenized mussel tissue (T-1).In the laboratory these samples were thawed and transferred under clean conditions to separate headspace sampling flasks containing hydrocarbon-free water. 900
Environmental Science & Technology
Quantitation was facilitated by adding known amounts of a solution of internal standards to the weighed quantities of each sample (1mg to 100 g depending upon the oil content of the sample). The internal standard consisted of a pentane solution of naphthalene and phenanthrene each present a t a known concentration (approximately 2 pg per pl). One port of each flask was connected to a prepurified nitrogen line which directs a stream of gas across the liquid interface at a flow rate of -180 ml/min. A second port contained the exit line, which is connected to a 6 X 0.6 cm stainless steel Swagelok-fitted column packed with Tenax-GC (Applied Science Laboratories, Inc., State College, Pa.) used for trapping the purged organics. This column was mounted in a cylindrical jacket which was chilled by a steady stream of cold air. Samples were purged for 4 h, first at room temperature, and then while gradually raising the temperature to 70 OC. At the end of this period, the nitrogen gas was diverted from the flasks directly to the Tenax columns for 2 h to dry the columns. The dried columns were capped and taken to the instrument laboratory for analysis. Following headspace sampling, the less volatile components were removed by pumping the liquid remaining in the flasks through a liquid chromatographic precolumn packed with Bondapak C18 (Waters Associates, Inc., Milford, Mass.). The Tenax-GC column from headspace sampling was installed as a precolumn to the analytical column in the gas chromatograph (GC). A heating block was clamped around the precolumn and heated to 375 "C with carrier gas flowing. Just prior to and during this flashing operation, a stream of liquid nitrogen was directed a t the head of the analytical column, thus thermally focusing the sample on the GC column. The oven was temperature programmed, and data were acquired. GC-MS data were acquired in an analogous fashion with mass spectra being recorded every 4 s during the course of the entire chromatogram. The Bondapak C18 columns were coupled to an analytical column packed with pBondapak C18 (microparticulate bonded phase LC packing material) column, and the organic constituents were elution-focused onto the head of the latter using a water-acetonitrile gradient. The gradient was programmed to increase the percentage of acetonitrile in the mobile phase. The effluent from the analytical column was passed through a UV-photometer (254 nm), and the chromatogram was recorded. Details of these headspace sampling and chromatographic procedures are presented elsewhere (3, 4).
Additional analyses were performed by Soxhlet extraction to obtain quantitative comparison values. Portions of each sample were Soxhlet extracted for a minimum of 6 h using diethyl ether. The residue remaining in the thimble was weighed to give a total ether-extractable value. Methanol extracts of separate portions of the samples were used for Karl Fischer titrations to determine the percentage of water. Treatment of Data Quantitation of Petroleum by GC a n d LC. Gas chromatograms were quantitated by summing the peak heights of the four aliphatic peaks, n-pentadecane to n-octadecane (Figure 1)and dividing this sum by the product of the peak
height of the naphthalene internal standard and the sample weight used. The value so obtained was then divided by the corresponding value from the unweathered spill oil sample. Upon multiplying by 100,the weight percent oil (wet weight basis) in the sample was obtained (Equation 1).
(
h,)
WC * hCNAp
n=15
- 100 = wt % oil in sample
(1)
where
h, = peak height of normal aliphatic hydrocarbon of carbon number n in sample chromatogram hc, = corresponding peak height from unweathered spill oil sample chromatogram hNAp = peak height of naphthalene from internal standard in sample chromatogram hcNAp = corresponding naphthalene peak height in unweathered spill oil sample chromatogram
W = weight of sample analyzed Wc = weight of unweathered spill oil sample used as external standard An essentially analogous method was used for liquid chromatographic quantitation. The sum of the peak heights of the three aromatic hydrocarbon peaks (numbered in Figure 2) characteristic of the previously headspace-sampled unweathered spill oil was referenced to the internal standard phenanthrene (Equation 2). Since both the unweathered spill oil and the sample were previously headspace-sampled, any loss of phenanthrene does not affect LC quantitation.
where
hp = peak height of aromatic hydrocarbon of peak number p in sample chromatogram
Table 1. Hydrocarbon Content of Samples Obtained from Catastrophic Oil Spill Percent hydrocarbons by varlous methodsa
Sample
Type
M-1
Oil-water emulsion Sediment Sediment Control sediment
S-1 S-2 S-3
Tissue Unweathered spill oil
T-1 a
Soxhlet extraction corrected for water content (Karl Flscher), %
GC, Yo
7.7 ( 1)
50 f 20 (4)b 0.2 f 0.06 (3) 9 f 3 (3) 0.0002 f 0.00004 ( 3) 0.03f 0.01 (3)
...
...
67 ( l)b 0.2 ( 1) 16 ( 7 )
Trace ( 1)
See text for discussion of methods used.
Value in parentk
3s
indicates number of
Coupled-column LC, %
Weathering factor (ratlo of n-Cl5In-Ci i peak helghts)
Percent water by Karl Flscher
80 f 12 (3)b 0.7 f 0.07 (2) 10 f l ( 2 )
