25th International Meeting on Organic Geochemistry IMOG 2011

P-154
Adaptation of TO-17 thermal desorption protocol for CSIA of
volatiles in air
Tomasz Kuder 1 , Paul Philp 1 , Thomas McHugh 2
1 University of Oklahoma, Norman, United States of America, 2 GSI Environmental Inc., Houston, United
States of America (corresponding author:tkuder@ou.edu)
Identification of sources of volatile organic
compounds (VOCs) in vapor intrusion studies is often
complicated, due to the potential of the same VOCs to
be introduced from various household chemicals [1].
Compound-specific isotope analysis (CSIA) permits
determination of isotope ratios of those target
compounds and thus makes it possible to directly
compare the isotope ratios of, e.g., trichloroethylene
(TCE) present in indoor air, soil gas and various
household products to determine whether the soil gas
is a feasible source of the VOCs present in indoor air.
CSIA requires relatively large mass of the analyte
(tens to hundreds on nanograms), often (in particular
for indoor air samples) requiring preconcentration of
VOCs from large air volumes (>100L). Use of sorbent
tubes similar to those utilized in USEPA TO-17
method allows such preconcentration to be conducted
on site. Precise and accurate determination of isotope
ratios of VOCs after preconcentration on sorbent
tubes requires that one of the following criteria is met:
1) the process does not introduce isotope
fractionation; 2) if isotope fractionation does occur, its
magnitude has to be predictable. Isotope fractionation
would results from a difference in recovery of VOC
molecules with variable isotope substitution. Ideally, a
TO-17 sampling process with 100% mass recovery
would assure that no isotope fractionation is present.
The main objective of the present study was to
investigate the performance of the selected types of
sorbents as applied to sampling of a large volume of
air, i.e., under conditions that are conducive to isotope
fractionation.
The sorbent tubes have been spiked with standard
solutions of TCE and PCE and then flushed with air
volumes equal to the expected sampling volumes for
indoor air. Additional parameters, such as humidity
and occurrence of other VOCs were manipulated to
simulate the conditions of actual sampling. Some of
the tubes were analyzed immediately, while other
tubes were refrigerated and analyzed at later date to
simulate the effects of sample storage. The spiked
tubes were subjected to CSIA and the isotope ratios
obtained for TCE and PCE were compared to the
ratios determined independently. The magnitudes of
isotope fractionation, if any, were recorded and
interpreted in terms of the sampling and analytical
conditions.
Most of work to date centered on a combination of
Tenax GR and Carboxen 569 (Supelco). The carbon
isotope ratios of TCE were adversely affected by the
sampling volume, resulting with artifactual enrichment
of 13 C in the recovered TCE. The significance of other
sampling process parameters was limited. Unlike
TCE, PCE showed very limited fractionation caused
by the sorption-desorption process. While the
fractionation of TCE was predictably tied to the
sampling volume and could be corrected in data
calibration, the presence of such fractionation
increases the analytical uncertainty for the determined
isotope ratios. The observed effects can be
rationalized by the properties of Tenax GR and
Carboxen 569 – specifically, the fractionation of TCE
may be attributed to strong retention of TCE on the
Carboxen 569 bed (i.e., the recovery of TCE was