Probing Molecular Composition of Soil Organic Matter with Nanospray Desorption Electrospray Ionization (nano-DESI) High-Resolution Mass Spectrometry

Abstract:

The standard techniques for chemical analysis of SOM often lack molecular detail necessary for characterization of the key classes of compounds in soil necessary for the development of predictive models. High-resolution mass spectrometry (HR-MS) combined with tandem mass spectrometry (MSn) is the technique of choice for structural characterization of individual molecules in complex environmental mixtures because HR-MS enables chemical characterization of complex samples with a level of detail that is not attainable using other techniques. However, the power of this technique for characterization of SOM has been demonstrated only recently. Nanospray desorption ionization (nano-DESI) bypasses the traditional SOM extraction steps and provides the unique ability to record SOM mass spectra from small whole-soil samples. Nano-DESI benefits from a short sample preparation time (dozens of samples can be analyzed in a matter of hours by simply swapping the samples on the sample holder and running a pre-programmed positioning stage protocol), fast analysis time (useful signal results from only a few seconds of contact between the sample and the liquid bridge), and highly sensitive detection (less than 10 ng of organic mass is required for analysis). The nano-DESI method is minimally destructive. Only a small spot (typically < 1mm) is in contact with the solvent bridge during the analysis, and the rest of the sample is unaffected. Sampling over multiple spots on the same sample in effect replaces the need to extract a large amount of SOM from a soil sample in order to homogenize it, as done in conventional soil analysis. Another critical advantage of nano-DESI is that it makes it possible to observe organic components that cannot be easily detected using traditional ESI method (69). Specifically, by minimizing the residence time of analyte in the spray solvent, it enables detection and structural characterization of chemically labile molecules in environmental samples, which tend to slowly react with or decompose in typical ESI working solvents.