Monitoring and evaluation of dechlorination processes using compound‐specific chlorine isotope analysis

Abstract

A simple, quick and sensitive method for the compound-specific stable chlorine isotope analysis of chlorinated solvents by conventional quadrupole gas chromatography/mass spectrometry (GC/MS) is presented. With this method, compound-specific stable chlorine isotope ratios of typical chlorinated solvents like tetrachloroethene (PCE) and trichloroethene (TCE) can be determined quantitatively within 30 min by direct injection. The chlorine isotope ratios of target substances are calculated from the peak areas of several selected molecular ions and fragment ions of the substances, using a set of unique mathematical equations. The precision of the method was demonstrated through reproducibility tests. An internal precision of +/-0.4 per thousand to +/-1.1 per thousand was obtained when analyzing PCE and TCE in the 10-1000 pmol range. The validity of the method was further demonstrated by determining the chlorine isotopic fractionation factor during the reductive dechlorination of TCE in a batch experiment using zero-valent iron. The chlorine isotopic fractionation factor was calculated as 0.9976 +/- 0.0011 with a correlation coefficient of 0.9469 (n = 38). The high correlation coefficient indicates that compound-specific stable chlorine isotope analysis can be performed with sufficient accuracy using conventional quadrupole GC/MS when significant fractionation takes place during a reaction. For the first time, the chlorine isotope fractionation factor of TCE during an abiotic anaerobic dechlorination process was determined using quadrupole GC/MS, without offline sample preparation.

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…Instead the GC effluent is transferred directly to a high-precision IRMS (Fig. 4 upper panel) [99] or a quadrupole MS [100][101][102]. Instead of species that contain only one chlorine atom (e.g., 37 [101,102], fragment ions [6,99], or a weighted combination of both [100].…”

A novel online approach to the determination of isotopic ratios for organically bound chlorine, bromine and sulphur

“…achieved continuous flow analysis by coupling a gas chromatograph to an IRMS instrument, but their method suffers from the need to specifically adjust the Faraday cups for each compound . An attractive online method was conceived by Sakaguchi‐Söder, Aeppli, and colleagues using gas chromatography coupled to quadrupole mass spectrometry (GCqMS) . However, their method involves substantial calculation to obtain isotope ratios from compound‐specific mass spectrometric data and demands repeated comparison with molecularly identical reference compounds .…”

Contrasting dual (C, Cl) isotope fractionation offers potential to distinguish reductive chloroethene transformation from breakdown by permanganate

“…Several studies have indicated that a dual element isotope approach has the potential to distinguish different transformation pathways Elsner et al, 2005;Fischer et al, 2008;Elsner, 2010). However, analytical methods for dual element isotope analysis of 13 C/ 12 C and 37 Cl/ 35 Cl of chlorinated ethenes have only relatively recently become available and are still being improved (Shouakar-Stash et al, 2005;Sakaguchi-Söder et al, 2007;Bernstein et al, 2011;Aeppli et al, 2010;Jin et al, 2011). Abe et al (2009) were the first to apply dual element (C, Cl) isotope fractionation to distinguish between microbial aerobic oxidation and microbial anaerobic reductive dechlorination of cis-DCE and VC.…”