| Abstract
| - Few systematic investigations have addressed the use offreezing for applications in analytical chemistry. Here, wetested its potential to preserve groundwater samples andto improve headspace quantification limits for compound-specific isotope analysis. Analysis of compound concentrations, as well as stable carbon isotope ratios, confirmedthat trichloroethene was preserved in frozen suspensionsof nanoscale zerovalent iron. In contrast, storage at 7 °Cwas ineffective, and complete degradation of TCE occurred in 4 weeks. Hence, freezing may stop even abioticchemical reactions that would not be prevented by coolingor traditional preservation agents. In the absence of iron,we found that headspace concentrations of 14 organiccontaminants were considerably higher over frozen solutions than at 25 °C, likely reflecting a freezing-out effectgoverned by Raoult's law. The observed enhancementdepended on the salinity of the samples and was strongestfor water-soluble, volatile compounds (values in bracketsindicate the minimum observed effect out of six replicates): tert-butyl alcohol (TBA, 35-fold), methyl tert-butylether (MTBE, 14-fold), 1,2-dichloroethane (10-fold), orbenzene (7-fold). In contrast, little enhancement wasobserved for less water-soluble compounds, such astetrachloroethene. Although standard deviations of themeasurements were too high for the method to be usedfor quantitative analysis of total compound concentrations,since we found that freezing introduces no measurablecarbon isotope effect for TBA, MTBE, 1,2-dichloroethane,and benzene, the method is an effective way of increasingthe sensitivity of compound-specific isotope analysis,particularly of water-soluble organic contaminants.
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