Molecular Structure Characterization of Riverine and Coastal Dissolved Organic Matter with Ion Mobility Quadrupole Time-of-Flight LCMS (IM Q-TOF LCMS)

Abstract

Deciphering molecular structures of dissolved organic matter (DOM) components is key to understanding the formation and transformation of this globally important carbon pool in aquatic environments. Such a task depends on the integrated use of complementary analytical techniques. We characterize the molecular structure of natural DOM using an ion mobility quadrupole time of flight liquid chromatography mass spectrometer (IM Q-TOF LC/MS), which provides multidimensional structural information on DOM molecules. Geometric conformation of DOM molecules is introduced into molecular-level analysis via the ion mobility (IM) in the system, and an actual measurement of isomers is achieved for the first time. Our data show that natural DOM molecules from several south Texas rivers and adjacent coastal waters have smaller geometric conformation compared with standard biomolecules. Furthermore, about 10% of all DOM molecules resolved within the detection limit of IM-MS had at least one but no more than four isomers. With acquired geometric and isomeric information, we established a multidimensional database containing 89 natural DOM compounds. This database provides a foundation to expand further, or compare, with DOM data from different seasons and locations.

Molecular Level Analysis Reveals Changes in Chemical Composition of Dissolved Organic Matter From South Texas Rivers After High Flow Events

“…The instrument and data acquisition methods followed our previous study (Lu et al, 2018). Briefly, for ESI+ mode, mobile phase A was H 2 O with 0.1% (v/v) formic acid, and B was acetonitrile.…”

“…Samples were run in duplication. The acquisition parameters followed the published protocol (Lu et al, 2018). Briefly, the MS acquisition rate was 4 spectra/s.…”

“…Data analysis also followed the published protocol (Lu et al, 2018). Briefly, MS data and MS/MS data were analyzed with MassHunter Qualitative Analysis (Version B.07.00, Service Pack 2), and IM-MS data was analyzed with MassHunter IM-MS Browser (Version B.07.01).…”

“…The mass inaccuracy tolerance was set at ≤1.5 ppm. Formulas of DOM were then calculated and assigned with the following rules (Kujawinski and Behn, 2006;Stubbins et al, 2010;Liu et al, 2011;Hertkorn et al, 2013;Lu et al, 2018) (7) Correct isotopic spacing and abundance. On average, 1818 peaks per sample were detected, and 1006 peaks were assigned with a formula under ESI+.…”

“…IM-MS provides insights into structural isomers of DOM molecules and thus structural diversity (Lu et al, 2018;Leyva et al, 2019), which adds another level of information on the top of accurate molecular formulas. The IM-MS spectra were calibrated for mass and ion mobility measurements using the Agilent Tuning Mix mass calibration standard (Tune Mix, G2421A, Agilent Technologies, Santa Clara, CA, United States).…”