New Insights into the Seasonal Variation of DOM Quality of a Humic-Rich Drinking-Water Reservoir—Coupling 2D-Fluorescence and FTICR MS Measurements

Long-term changes in dissolved organic matter (DOM) quality, especially in humic-rich raw waters, may lead to intensive adaptions in drinking-water processing. However, seasonal DOM quality changes in standing waters are poorly understood. To fill this gap, the DOM quality of a German drinking water reservoir was investigated on a monthly basis by Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) measurements and 2D fluorescence for 18 months. FTICR MS results showed seasonal changes of molecular formula (MF) intensities, indicating photochemical transformation of DOM as a significant process for DOM quality variation. For an assessment of the two humic-like components, identified by parallel factor analysis (PARAFAC) of excitation–emission matrices (EEM), their loadings were Spearman’s rank-correlated with the intensities of the FTICR MS-derived MF. One of the two PARAFAC components correlated to oxygenrich and relatively unsaturated MF identified as easily photo-degradable, also known as coagulants in flocculation processes. The other PARAFAC component showed opposite seasonal fluctuations and correlated with more saturated MF identified as photo-products with some of them being potential precursors of disinfection byproducts. Our study indicated the importance of elucidating both the chemical background and seasonal behavior of DOM if raw water-quality control is implemented by bulk optical parameters

High-field FTICR-MS data evaluation of natural organic matter: Are CHON5S2 molecular class formulas assigned to ¹³C isotopic <em>m/z</em> and in reality CHO components?

The analysis of dissolved organic matter (DOM) using high-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) poses challenges in molecular formula assignment. The consideration of (13)C isotopes provides new insights into the consistent elemental formula solutions. Modern software helps to overcome misinterpretation, but false assignments of molecular classes to mass peaks have rarely been elucidated until now. It will be demonstrated that this can be important with formula assignments comprising exactly five nitrogen and two sulfur atoms in DOM data sets: the molecular class CHON5S2. The existence of such components in DOM under tripeptide Met-His-Cys formed with the formula C14H23O4N5S2 cannot be excluded; however, components containing 5 N and 2 S should be suspected to not be highly abundant. The true elemental compositions of such unusual &quot;N5S2 moieties&quot; were calculated using Suwannee River fulvic acid (SRFA) data from the literature and one data set from acidic pit lake pore water. The replacement of a H3N5S2 moiety with a (13)C1(12)C5O4 moiety explained more than 95% of the questionable &quot;N5S2 moieties&quot;. This finding was proved by calculation of &delta;(13)C&permil; values from relative peak intensities

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.

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&nbsp;% 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&nbsp;% CHO and 18&nbsp;% CHOS on average, whereas the former SRFA dataset contained only 1.5&nbsp;% 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&nbsp;% multiple-formula assignments were found for peaks with masses &gt; 650&nbsp;Da. Based on DBE&thinsp;-&thinsp;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&nbsp;%) 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. Graphical abstract An exemplary frequency versus DBE&thinsp;-&thinsp;O diagram. CHOS components but not CHO (and not CHON2 or CHON2S) components were considered here reliable