Gschwend%20thesis.pdf

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trations, as demonstrated by hex anal in Fig. 3. Except
for such uncertain features, related trends in temporal
variations of the VC were not apparent.
This situation may simply reflect inadequacies in
the amount or spatial and temporal resolution of
these preliminary data. Alternatively, it may be that
individual organic compounds typically have rather
specific biological sources and sinks. In that case, biologically
relevant parameters such as nutrient concentrations.
chlorophyll level. and temperature. which are
highly non-specific, would not show strong correlations
with individual VC levels; instead. compound
levels would correlate best with their specific sources
and sinks. These may frequently be the metabolic activities
of individual genera, species, or even subpopulations
of the same speies. The likelihood of
these and other such complexities has been discussed
(Blumer. 1975). Despite these potential problems, the
variations observed in VC levels suggest that they are
intimately involved in various coastal marine processes.
VC al1d VOC
Although it was not our purpose to assess the
amount of seawater organic matter which is 'volatile',
the rather low levels of individual and total VC that
ocur in the coastal waters of Cape Cod is rather
striking. Early Russian work (Vityuk, 1965 quoted in
Skopintsev, i 966) had suggested that the VOC repre-
sented i 2% of the TOC in . seawater. MacKinnon
(1977, personal communication), utilizing an
approach which he felt provided an upper limit to
the amount of volatile carbon in seawater, stripped
HgCli-poisoned unfiltered samples with Ni using
varying temperatures and times. Subsequently, he
determined the Tenax adsorbable volatiles by thermal
desorption and combustion analysis. He found about
I % of the coastal TOC was volatile by this procedure
at 35°C. Thus MacKinnon recovers about 10-30 ¡ig
VOC/kg. .
In contrast. we find only about 0.2- 1.0 ¡ig/kg by
summing the concentrations of the VC and converting
to VOc. There are several possible explanations
for this discrepancy:
(i) Volatiles are non-uniformly distributed and data
cannot be intercom Pared.
(2) Volatiles do not survive some steps in our procedure
(charcoal trap, active or hot GC surfaces).
(3) Low boiling volatiles are included in MacKinnon's
measurement, while they are masked by the solvent
peak in ours.
(4) MacKinnon's procdure provides an overestimate.
While all of these factors maybe involved to some
degree, the first is unlikely to be responsible for the
. major part of the discrepancy, as we both see a
moderate degree of spatiotemporal homogeneity
within our respective methods. We doubt that factor
two is dominant, as we obtain chromatograms using
thermally desorbed Tenax similar to our routine char-
IUi. I ~ (.,
Volatile organic compounds in coastal seawater
coal trap chromatograms. Also we have found that
some standards diffcult to determine by low-level GC
(polysulfides and aldehydes) survive our techniques
at low levels. However, by prolonged stripping
MacKinnon may measure relatively polar compounds
which we do not determine. Also, low-boiling compounds
may exist in seawater at high levels; our CSi
peak obscures compounds boiling below 100°C. We
have observed large amounts of carbon dioxide, pentane,
hexane, methylene chloride, acetone, and ben-
zene in samples analyzed by the 'Tenax' method.
While much of these materials is likely to be laboratory
contamination, micrograms of these compounds
may come from the sample. In summary, it is diffcult
to compare our results with the results of other
workers who use different methodological
approaches.
SUMMARY AND CONCLUSIONS
The 'stripping' method is a viable and useful
105
approach for studying the volatile organic components
in coastal seawater samples, marine marshes,
etc. It has been feasible to attain the blanks, recoveries,
reproducibility and sensitivity required for
quantitation and identification of a structurally varied
set of organic compounds. Potentially this set includes
compounds within the boiling range
I1-C,-I1-C18 that are not too water soluble or reactive.
The method is relatively trouble-free, rapid. and
shows good repr(Kucibility under realistic environmental
sampling conditions. Incomplete recovery of
volatiles from highly turbid samples may complicate
interpretation, but also yields information about VC
adsorption and formation processes.
About 50 compounds have been identified in
samples from the coastal waters of Cape Cod,
Massachusetts with individual abundances of
- I-i 00 ng/kg; individual levels above 20 ng/kg are
rare. These compounds include alkanes, alkenes, aromatic
and alkyl-aromatic hydrocrbons, l1-aldehydes.
methyl sulfur compounds, and some halogenated
compounds. No ketones, esters, or terpenoid hydrocarbons
were recovered. With the possible exception
of compounds attributed to petroleum contamination,
l1-alkanes in the C,-Cii range, branched alkanes, and
alkenes were also absent or present only in trace
quantities.
Some sources of individual compounds have been
suggested. Within the biological realm, there are
examples of compounds that are probably derived
from different classes of benthic algae (n-C i 5. n-C 17)'
marsh algal mats (alkene Nos. 405, 413, 505, 509) and
bacterial procsses (e.g. DMDS and DMTS). Several
classes of compounds that might be expected to have
an atmospheric source were not found, such as
branched saturated hydrocarbons from gasoline and
terpenoid compounds from terrestrial vegetation.
The time variability of the data suggest that sources
and sinks may change volatiles levels within the