Comparative study of organic matter chemical characterization using negative and positive mode electrospray ionization ultrahigh-resolution mass spectrometry.

The chemical characterization of dissolved organic matter (DOM) is critical for understanding carbon sequestration processes in soils. This work evaluated the use of electrospray ionization in both negative ion mode (ESI-) and positive ion mode (ESI+) for the characterization of DOM extracted from nine terrestrial sources using Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS). The compositing of the peaks from ESI- to ESI+ modes increased the total assigned formulas from 23 to 63 % as compared to the traditional use of ESI- alone for DOM characterization. In general, there was a preferential increase in the number of assignments for the aliphatic and carbohydrate-like DOM components in the ESI+ mode. The soil-extracted DOM specifically exhibited greater increases in the aliphatic and carbohydrate-like DOM components with the combined use of ESI- and ESI+ modes likely due to the greater presence of aromatic DOM molecules that suppressed the ionization of these entities in ESI- mode. On the basis of these findings, we show that improved characterization of DOM is possible through the combined use of ESI- and ESI+ modes for FT-ICR-MS analysis, especially for samples rich in condensed aromatic and aromatic molecules.