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be mainly a function of the fast desorption rate on the supply side and the
rate of microbial degradation and partitioning into benthic infaunal lipid on
the sink side. Recalcitrant compounds in the rapidly desorbing fraction may
overwhelm the capacity of microorganisms to degrade it and render this
fraction more likely to accumulate in benthic biota (Fry and Istok, 1994). The
slowly desorbing fraction may be effectively scavenged by microorganisms
as quickly as the contaminants desorb, thus reducing to negligible the effec-
tive dose of recalcitrant compounds realized by higher benthic species. This
effect has been termed biostabilization (Talley, 2000). Although this is cur-
rently an active research area, there is little published information on the
relationships between rates of recalcitrant compound desorption from sed-
iment, pore water recalcitrant compound pool size, rates of recalcitrant com-
pound biodegradation, and rates of recalcitrant compound accumulation
into benthic biota.
In summary, the bioavailable portion of recalcitrant compounds associ-
ated with sediments is a subset of that which is solvent extractable. The
rapidly desorbing recalcitrant compounds (aqueous) fraction from sediment
is roughly equivalent to the bioavailable fraction. Microorganisms effectively
degrade a large, but sediment-specific, variable fraction of the rapidly des-
orbing fraction. The potential for bioremediation of a recalcitrant com-
pound-contaminated soil is generally limited to the fast desorbing fraction.
If the potential rate of recalcitrant compound biodegradation is greater than
the slow recalcitrant compound desorption rate, then the residual recalcitrant
compounds bound in the sediment may present little environmental risk.
2.9 Thermal desorption mass spectrometry of recalcitrant
compounds
Within an analysis time of 10 minutes, thermal desorption mass spectrometry
(TD-MS) can provide information on the identity of the recalcitrant com-
pounds present in sediment, recalcitrant compound levels, and the energy
with which the individual are being sequestered. Many common recalcitrant
compounds such as PAHs, PCBs, and pesticides are thermally stable, semi-
volatile organic compounds well suited for TD-MS. For TD-MS analysis, a
sample of dried sediment (1 to 10 mg) is placed in a glass vial, weighed, and
then placed on a direct probe (Figure 2.4). The probe is inserted through a
vacuum lock into the ion source of a mass spectrometer and heated according
to a specified program. Thermally desorbed recalcitrant compounds are
ionized by electron impact, and the resulting ions are directed into the mass
analyzer using electronic lenses.
Recalcitrant compounds are identified on the basis of their molecular
weight and by mass fragmentography when tandem mass spectrometry is
employed. Molecular ion or base peak area is indicative of the amount of a
particular recalcitrant compound thermally desorbed from the sediment. The
thermal desorption profile (Figure 2.5) shows the heat energy required to
