A universal method to isolate, enrich, and quantify atmospheric organosulfates

Abstract

Organosulfates (OSs) are ubiquitous compounds in ambient aerosols, being formed by multiphase chemistry.1,2 However, accurate quantification of unknown and suspected OSs remains difficult. One of the main reasons for this is the limited availability of authentic standards. Furthermore, a large amount of unknown and not yet identified OSs might remain undetected in ambient samples. To overcome the two main difficulties, we developed a new solid phase extraction (SPE) method to enrich and fractionate OSs compounds. Preliminary result shows that with this approach the majority of the OSs fraction was separated from the native sample matrix. Furthermore, we can easily enrich the native extract by a factor of ~300 and get good recovery of those targeted OS-compounds. Following this SPE-method, a charged aerosol detector (CAD) was employed for the quantification of the OSs fraction extracts after chromatographic separation. CAD, which has universal response as a prominent feature, is advantageous to the quantification of non-volatile species without the necessity for the preparation of authentic or surrogate standards.3  The sample preparation by SPE greatly reduced the complexity of both chromatograms detected with a mass spectrometer and the CAD, thereby increasing our confidence for the peak identification and quantification. Although, CAD has a sub-nanogram sensitivity, our classical extraction method for ambient filter samples most OSs are below the limit of detection. Therefore, the SPE enrichment of an ambient filter extraction is necessary for CAD detection. Moreover, the volatility for majority OSs compounds is very low, which perfectly match the CAD feature. With this method we can identify and quantify ~40 OSs compounds and nitrooxy-OSs in an ambient Chinese PM2.5 sample. This work emphasizes the potential of the SPE approach in combination with CAD to quantify the unknown and suspected OSs precisely in ambient air. The presented method is able to quantify the individual OS compounds without authentic or surrogate standards.