Characterisation of the semi-volatile component of Dissolved Organic Matter by Thermal Desorption – Proton Transfer Reaction – Mass Spectrometry

Abstract

Proton Transfer Reaction – Mass Spectrometry (PTR-MS) is a sensitive, soft ionisation method suitable for qualitative and quantitative analysis of volatile and semi-volatile organic vapours. PTR-MS is used for various environmental applications including monitoring of volatile organic compounds (VOCs) emitted from natural and anthropogenic sources, chemical composition measurements of aerosols, etc. Here we apply thermal desorption PTR-MS for the first time to characterise the chemical composition of dissolved organic matter (DOM). We developed a clean, low-pressure evaporation/sublimation system to remove water from samples and coupled it to a custom-made thermal desorption unit to introduce the samples to the PTR-MS. Using this system, we analysed waters from intact and degraded peat swamp forest of Kalimantan, Indonesian Borneo, and an oil palm plantation and natural forest in Sarawak, Malaysian Borneo. We detected more than 200 organic ions from these samples and principal component analysis allowed clear separation of the different sample origins based on the composition of organic compounds. The method is sensitive, reproducible, and provides a new and comparatively cheap tool for a rapid characterisation of water and soil DOM.

Distribution and cycling of terrigenous dissolved organic carbon in peatland-draining rivers and coastal waters of Sarawak, Borneo

“…4.4). Photochemical POC solubilisation could also produce DOC in situ (Kieber et al, 2006;Mayer et al, 2006). However, estimated rates of photochemical POC solubilisation under realistic condi-tions of light penetration are only around 4-6 mmol m −2 d −1 (Kieber et al, 2006;Riggsbee et al, 2008), which is too low to explain the DOC increase we observed, given the likely short transit time of water through the Rajang Delta (see Sect.…”

“…Human disturbance has been shown to increase the loss of DOC from peatlands at field sites in central Borneo (Moore et al, 2013), and depending on water table height, peatland oil palm plantations in Sarawak can also export old DOC (Cook et al, 2018). Differences in DOM quality between agricultural and natural peatland sites on Borneo have also been noted (Harun et al, 2016;Materić et al, 2017). While it is thus clear that land use can impact DOC export from South-East Asian peatlands, our study was not designed specifically to determine the effect of land use on DOM concentrations or quality in the rivers; more field work would be needed to do so.…”

Validity and limitations of simple reaction kinetics to calculate concentrations of organic compounds from ion counts in PTR-MS

“…However, new-generation instruments substantially gained sensitivity by using advanced ion optics between the reactor and mass analyser at the cost of the transmission of the whole system being less well constrained -especially in the lower mass range. In addition, poor tuning and/or ageing of the ion detection system can cause deviations from the expected behaviour at the high mass range (Müller et al, 2014). So, fast and robust methods for retrieving the transmission are needed for quantitative measurements.…”

Presence of nanoplastics in rural and remote surface waters

“…the same vials and the same syringes and lters were used. Samples (0.5 ml) were then subjected to thermal desorption proton transfer-reaction mass spectrometry (TD-PTR-MS) using the method described in Materić et al (2017 and2020) 56,56 . This method provides detailed information on the molecular composition of volatile and semi-volatile dissolved organic matter in the sample; it gives both the molecular weight and the concentration of molecules up to a size of ~500 m/z.…”