Unwanted loss of volatile organic compounds (VOCs) during in situ chemical oxidation sample preservation: Mechanisms and solutions

To assess the performance of hazardous waste sites
remediation technologies like in situ
chemical oxidation (ISCO)
with persulfate (S2O82−) researchers must periodically
measure concentrations of target contaminants. Due to the presence of
relatively high concentrations of the
residual oxidant expected in many samples, the standard
analytical method requires the addition of a relatively high concentration
of ascorbic acid to prevent the oxidation process from
continuing after sample collection. We discovered that addition
of ascorbic acid quencher results in a radical chain
reaction that transforms two common halogenated solvents
(i.e., tetrachloroethene and hexachloroethane). To avoid
the artifact associated with the radical chain reaction, a small quantity
of n-hexane can be added to aqueous samples to extract target compounds
and protect them from the radical chain reaction initiated by addition of
the quencher. We recommend the use of this alternative sample
preservation method whenever high concentrations of residual
S2O82− are expected to be present in water samples that are contaminated
with halogenated solvents.