Rapid separation of isomeric perfluoroalkyl substances by high-resolution differential ion mobility mass spectrometry

Ezaz Ahmed; K M Mohibul Kabir; Huixin Wang; Dan Xiao; John Fletcher; William A Donald

doi:10.1016/j.aca.2019.01.038

Rapid separation of isomeric perfluoroalkyl substances by high-resolution differential ion mobility mass spectrometry

Anal Chim Acta. 2019 Jun 13:1058:127-135. doi: 10.1016/j.aca.2019.01.038. Epub 2019 Jan 31.

Authors

Ezaz Ahmed¹, K M Mohibul Kabir¹, Huixin Wang¹, Dan Xiao², John Fletcher², William A Donald³

Affiliations

¹ School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia.
² School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW, 2052, Australia.
³ School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia. Electronic address: w.donald@unsw.edu.au.

PMID: 30851846
DOI: 10.1016/j.aca.2019.01.038

Abstract

The analysis of persistent organic pollutants, such as perfluoroalkyl substances (PFAS) including perfluorooctanoic acid (C₇F₁₅COOH, PFOA) and perfluorooctane sulfonate (C₈F₁₇SO₃^-, PFOS) by hyphenated chromatography-mass spectrometry methods is crucial in ensuring water quality. One of the challenges in PFOA and PFOS analysis is the separation of linear and branched isomers that have the same mass-to-charge ratio but can have different toxicological properties. Current methods that are used for separating isomeric PFAS require relatively long analysis times (min to h) that can limit throughput. An emerging technique for the direct analysis of isomeric compounds is differential mobility spectrometry (DMS), which can rapidly separate gas phase ions prior to detection by mass spectrometry (MS). However, an ion mobility-based method for the analysis of PFAS has not been reported in the literature. Herein, high-resolution DMS-MS is used to separate and detect isomeric PFAS compounds for the first time in a separation process that occurs in milliseconds. The resolution of isomeric peaks increased by over 200% and 500% for PFOA and PFOS isomers, respectively, using a DMS carrier gas composed of 50:50% He:N₂ by volume compared to 100% N₂, which was crucial in the separation of PFAS isomers under these conditions. Linear, secondary-branched, and tertiary-branched isomers of PFOA and PFOS including those that differ by the position of a single perfluoromethyl group can be resolved by DMS-MS. The DMS compensation field required to transmit different isomers increases as the extent of branching increases. For isomeric PFASs with a single branching point, the compensation field for optimal transmission also increases as the perfluoromethyl group is positioned closer to the carboxylate and sulfonate groups under these conditions. These results indicate that high-resolution DMS-MS should be a useful approach for the rapid analysis of isomeric PFAS.

Keywords: Differential ion mobility spectrometry; Field asymmetric ion mobility spectrometry; Isobaric separation; Perfluoro-compounds; Perfluorooctanesulfonic acid; Perfluorooctanoic acid.