Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 439

molecules and enrich or deplete the ratio of the rare to the most abundant isotope. The natural
variations in the mass of the oxygen, hydrogen, and carbon atoms on the 1,4-dioxane molecule can
potentially be used to interpret its fate in the subsurface, as discussed further in Section 9.2.5 .
Ratios a re referenced to an inter national standa rd mater ial, such as Vienna Standa rd Mean Ocean
Water for oxygen and hydrogen; the Peedee Belemnite (PDB), a South Carolina limestone forma-
tion, for carbon; and seawater for chlorine (Fritz and Clark, 1997). By convention, the ratio of two
isotopes of the same element is described by δ notation and expressed in units of per mil (‰), as

shown for carbon:

( R sample 1 ) × 1000,

_____

δ

13 C

=

R std

− (9.5)

where R sample is the 13 C/ 12 C ratio for a compound in a sample and R std is the 13 C/ 12 C ratio for the
international CO 2 standard (PDB) (Fritz and Clark, 1997). A detailed treatment of isotope hydrol-
ogy and CSIA is beyond the scope of this topic; the reader is directed to the leading texts and online
resources on this subject. *
CSIA is accomplished with in-line gas chromatography-isotope ratio mass spectrometry
(GC-IRMS). CSIA can be effective for differentiating solvents that have been through processes
causing isotopic fractionation (i.e., enrichment or depletion, shifting the initial isotope ratio).
Applications include characterizing, monitoring, and evaluating natural and engineered bioreme-
diation (Sturchio et al., 1998). For example, the analysis of deuterium ( 2 H) in TCE has been used to
distinguish two sources of TCE: the i rst due to biodegradation of PCE and the second due to a
direct release of TCE. This differentiation is possible because the deuterium isotope ratio ( δ

2 H) in

manufactured TCE is typically in the range of 681.9‰, whereas TCE that has been

dechlorinated from PCE typically has a deuterium isotope ratio in the range of -351.9‰ to -320.0‰
(Shouakar-Stash et al., 2003; Morrison et al., 2006).
Depletion of heavy isotopes in the solvent can also be expected from vaporizing solvents such as
will occur in a vapor degreaser or dry-cleaning operation. The enrichment of heavy isotopes in the
vapor phase is due to the higher vapor pressure for heavier compounds. This “inverse isotope effect”
is also attributed to a greater surface tension (i.e., intermolecular attraction) for lighter compounds.
The smaller volume and heavier mass in the liquid phase confer greater motion energy for 13 C
molecules in the liquid phase, which together with lower intermolecular cohesive forces, produces a
higher volatility for the molecules with the heavy isotopes (Bouchard, 2007). Studies have also
demonstrated a large enrichment of 13 C in the headspace above the liquid phase contained in closed
vessels for chlorinated solvents (e.g., Slater et al., 1999; Hunkeler and Aravena, 2000).
CSIA has also been applied to differentiate the manufacturer of origin for chlorinated solvents.
Isotopic differentiation between two grades of the same solvent produced at different manufac-
turers may occur because of variation in the method of solvent production, such as dehydrochlo-
rination and dehydrogenation reactions, temperatures, and durations of the different stages of
production, catalysts used, and isotopic variation in the original feedstock used to produce the
solvent (Morrison and Murphy, 2006). The utility of this isotopic differentiation may be limited
because carbon isotope ratios for the same solvent produced by the same manufacturer in different
years have shown substantial variation and because feedstocks and methods of production may

+ 466.9‰ to +

* Texts: (1) Clark and Fritz (1997). (2) Immanuel Mazor, 1996, Chemical and Isotopic Groundwater Hydrology . New York:
Marcel Dekker. (3) Pradeep K. Aggarwal, Joel R. Gat, and Klaus F.O. Froehlich, 2005, Isotopes in the Water Cycle: Past,
Present and Future of a Developing Science . New York: Springer-Verlag.

Websites: (1) University of Arizona, “Sustainability of Semi-Arid Hydrology and Riparian Areas (SAHRA)”: http://
www.sahra.arizona.edu/programs/isotopes/index.html (2) International Atomic Energy Agency, “Environmental
Isotopes in the Hydrological Cycle—Principles and Applications”: http://www.iaea.org/programmes/ripc/ih/volumes/
volumes.htm (3) R.J. Pirkle, 2006, “Compound Specii c Isotope Analysis: The Science, Technology and Selected
Examples from the Literature with Application to Fuel Oxygenates and Chlorinated Solvents”: http://www.microseeps.
com/pdf/csia.pdf