Stability assessment of organic sulfur and organosulfate compounds in filter samples for quantification by Fourier Transform-Infrared Spectroscopy

Abstract

Abstract. Organic sulfur and sulfate compounds, tracers for sources and atmospheric processes, are not currently measured in national monitoring networks such as the Interagency Monitoring of Protected Visual Environments (IMPROVE). The goal of this paper is to begin to assess the stability of organic sulfur and sulfate containing compounds on polytetrafluoroethylene (PTFE) filters and the suitability of Fourier-transform infrared (FT-IR) spectroscopy to measure these compounds. Stability assessment is needed because PTFE samples collected by IMPROVE are typically stored 6–9 months prior to analysis. For this study, two organosulfur compounds, methanesulfonic acid (MSA) and hydroxymethanesulfonate ion (HMS), and two organosulfate compounds, methyl sulfate (MS) and 2-methyltetrol sulfate (2-MTS), are collected individually on PTFE filters. Gravimetric mass measurements is used to assess mass stability over time. FT-IR spectra are evaluated to assess the capability of measuring the compound from PTFE filters by assessing the compound stability or chemical changes over time. Ion chromatography (IC) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) are used as an additional tool to assess stability or chemical changes over time. MS has the highest potential to be measured by FT-IR in IMPROVE samples. For MS, a simple organosulfate, the mass changes are within measurement uncertainty and FT-IR spectra indicate no compositional change over a 4-month period, suggesting MS can be measured using FT-IR. IC and ICP-OES support the conclusion that MS is stable on the filter. However, for 2-MTS, the other organosulfate measured in this study, spectral changes after a month on the filter suggests it decomposes into other organosulfates or an inorganic sulfate. MSA in IMPROVE samples can be measured, but only as a lower bound, due to volatility off of the filter as indicated by FT-IR and gravimetry. FT-IR and IC both show that MSA is not chemically changing over the course of the study. Measurements by all methods indicate HMS is unstable on PTFE filter and IC and FT-IR indicate that it likely converts to inorganic sulfate. Future work includes the evaluation of these compounds in as ambient aerosol sample matrix to determine any differences in stability and identify interferences that could limit quantification.