Molecular formula assignment for dissolved organic matter (DOM) using high-field FT-ICR-MS: chemical perspective and validation of sulphur-rich organic components (CHOS) in pit lake samples

摘要

Molecular formula assignment is one of the key challenges in processing high-field Fourier transform ion cyclotron resonance mass spectrometric (FT-ICR-MS) datasets. The number of potential solutions for an elemental formula increases exponentially with increasing molecular mass, especially when non-oxygen heteroatoms like N, S or P are included. A method was developed from the chemical perspective and validated using a Suwannee River Fulvic Acid (SRFA) dataset which is dominated by components consisting exclusively of C, H and O (78 CHO). In order to get information on the application range and robustness of this method, we investigated a FT-ICR-MS dataset which was merged from 18 mine pit lake pore waters and 3 river floodplain soil waters. This dataset contained 50 CHO and 18 CHOS on average, whereas the former SRFA dataset contained only 1.5 CHOS. The mass calculator was configured to allow up to five nitrogen atoms and up to one sulphur atom in assigning formulas to mass peaks. More than 50 multiple-formula assignments were found for peaks with masses > 650 Da. Based on DBE -aEuro parts per thousand O frequency diagrams, many CHO, CHOS1, CHON1 and CHON1S1 molecular series were ultimately assigned to many m/z and considered to be reliable solutions. The unequivocal data pool could thus be enlarged by 523 (6.8 ) CHOS1 components. In contrast to the method validation with CHO-rich SRFA, validation with sulphur-rich pit lake samples showed that formulas with a higher number of non-oxygen heteroatoms can be more reliable assignments in many cases. As an example: CHOS molecular series were reliable and the CHO classes were unreliable amongst other molecular classes in many multiple-formula assignments from the sulphur-rich pit lake samples.