Multi-isotope characterization of 4,4'-DDT isolated from sediment samples via Orbitrap-IRMS

Dichlorodiphenyltrichloroethane (DDT) is an environmental contaminant with a long lifetime and significant toxicity. Stable isotopes provide a tool to trace DDT through the environment. We present an Orbitrap Isotope Ratio Mass Spectrometry (Orbitrap-IRMS) method to measure the isotopic content of two DDT fragment ions: m/z = 235 [M⁺- CCl₃] and m/z = 316 [M⁺- HCl]. With 3 nmol sample sizes and commercial 4,4'-DDT standards, we observe: for the [M⁺- CCl₃] fragment, δ¹³C (0.13 ‰); δ³⁷Cl (0.07 ‰); δ¹³C³⁷Cl (0.19 ‰); δ³⁷Cl³⁷Cl (0.19 ‰); δ¹³C³⁷Cl³⁷Cl (0.39 ‰); and for the [M⁺- HCl] fragment, δ³⁷Cl (0.22 ‰); δ³⁷Cl³⁷Cl (0.26 ‰); δ³⁷Cl³⁷Cl³⁷Cl (0.52 ‰). We applied our method to 4,4'-DDT extracted from sediment near the White Point (WP) sewage outflow on the Palos Verdes Shelf, a site with substantial DDT contamination. Resulting precisions were worse for natural samples than standard by a factor of ≈4-5 due to their lower concentrations. Our results for [M⁺- CCl₃] show enrichment vs our standard in δ¹³C (1.87 ± 0.49 ‰) and δ³⁷Cl (1.23 ± 0.29 ‰) while δ¹³C³⁷Cl and δ³⁷Cl³⁷Cl follow the stochastic distribution. The multiply substituted results add confidence and are useful for fingerprinting. For [M⁺- HCl] we see no enrichment in δ³⁷Cl (0.31 ± 1.0 ‰) or δ³⁷Cl³⁷Cl (0.01 ± 1.25 ‰) and possible enrichment in δ³⁷Cl³⁷Cl³⁷Cl (7.81 ± 2.11 ‰). These results are within 2σ of our stochastic error bars and are not evidence of clumped isotope enrichment but should be targeted in future study. Finally, our results place an upper bound on the amount of DDT degradation of 66 %.