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2.1 Sediments and Sediment Cores (Extraction and Processing)
Samples of surface sediment or specific depth interval sections of
sediment cores were solvent-extracted and fractionated according to an
ambient temperature solvent drying and solvent extraction procedure
based on that of Brown et al. (1980) as revised by Atlas et al. (1981)
and Boehm et al. (1981). The procedure, involving methanol drying and
ambient temperature extraction with a methylene chloride/methanol
azeotrope, is illustrated in Figure 2.1. The concentrated extract is
displaced with hexane and charged to a glass absorption chromatography
column (1 cm i.d.) containing 10 g fully activated (150°C) 80-100 mesh
silica gel topped with 1 g 5% deactivated alumina and 1 g activated
(i.e. acid washed) copper powder. The column, which is wet packed in
methylene chloride, is rinsed with this solvent followed by hexane. A
0.5 ml volume of extract is charged to the column and eluted with
hexane (17 ml, f]_) » hexane: methylene chloride (21 ml, f 2) , and
methanol (20 ml, f 3) . The fractions are collected separately, reduced
in volume, desulfurized using an activated (1 N HC1) copper powder
slurry, and an aliquot weighed on a Cahn electrobalance. The f^ and
f2 fractions are then analyzed by fused silica capillary gas chromatography
(FSCGC flame ionization detector) and a selected set further
scrutinized by gas chromatographic mass spectrometry. FSCGC analysis
determined the overall composition of the sample by appraisal of the
distribution of resolved (peaks) and unresolved (hump) features,
as well as the specific quantities of individual n-alkane (C^g to C32)
and isoprenoid (C15 to C20) compounds. GC/MS/computer analyses focused
on the list of saturated and aromatic compounds presented in Table 2 to
confirm the identities of compounds or to quantify minor, but important
"marker" compounds.
Table 3.
Details of the GC and GC/MS analytical procedures are presented in
Quantification of GC traces was according to the internal standard
method wherein quantities of individual hydrocarbons are computed.
Several other GC-derived parameters were routinely calculated on sample
data. One of these was the n-alkane to isoprenoid ratio (ALK/ISO) in
the C13 - C19 range:
ArK/Tsn =
'
n ~ C 14 + "-Cis + n-Ci e. + n C, 7 + n-Ci a
1380 + 1450 + 1650 + 1710 + 1812 a
a GC retention indices of isoprenoids: 1450 = farnesane, 1710 =
pristane, 1812 = phytane.
This ratio, beginning at ^7 in the reference oil is quickly
decreased due to preferential bacterial degradation of n-alkanes
versus the branched isoprenoids.
The carbon preference index (CPI) , the ratio of odd chain alkanes
to even chain alkanes in the n-C26 to n-c 31 range, is defined as
follows:
CPI =
2(n-C ?7 + n-Coq)
n-C 26
'+ 2n-C 2 Q
+ n-C 30
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