Non-target screening of leaf litter-derived dissolved organic matter using liquid chromatography coupled to high-resolution mass spectrometry (LC-QTOF-MS)

Abstract

Leaf litter-derived dissolved organic matter (DOM) is an important source of organic matter entering the mineral soil, but characterization of leaf litter DOM is often not detailed enough to understand DOM dynamics and processes at the molecular level. (Ultra-) high-resolution techniques such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), Orbitrap mass spectrometry and quadrupole time-of-flight (QTOF) mass spectrometry have brought a new level of detail to (D)OM research by providing molecular formulae and information on molecular structures. We present a novel approach for leaf litter DOM characterization that includes non-target screening using liquid chromatography (LC) coupled to high-resolution QTOF-MS. A method validation test showed that out of 26 spiked naturally occurring DOM molecules in a leaf litter DOM sample, 23 were found as features. After implementation of additional filtering to exclude certain combinations of heteroatoms (that are difficult to resolve even by the newest high-resolution MS instruments), including molecular formulae with P atoms, 22 molecular formulae were correctly assigned. Although a large part of the annotated molecular formulae was unique for the respective litter sources, the molecular compound class distribution was similar for deciduous, coniferous and mixed leaf litter DOM. Most intense masses were detected in the 100–300 Da mass range, showing a bias of QTOF-MS towards lower masses compared to FT-ICR-MS. The use of LC in combination with QTOF-MS for leaf litter (D)OM characterization provides, together with Orbitrap-MS, a more widely available and lower cost high-resolution MS alternative to FT-ICR-MS. Novel approach to the characterization and analysis of leaf litter derived DOM. Non-target screening using LC-QTOF-MS and the patRoon R package. Similar molecular compound class distribution for three leaf litter sources. High-resolution DOM characterization alternative to FT-ICR-MS. Highlights: Novel approach to the characterization and analysis of leaf litter-derived DOM. Non-target screening using LC-QTOF-MS and the patRoon R package. Similar molecular compound class distribution for three leaf litter sources. High-resolution DOM characterization alternative to FT-ICR-MS.