Dataset: Chemical composition of dissolved organic matter (DOM) determined by mass spectrometry from samples collected in Galveston Bay, Texas, USA in September 2017

Surface water samples were collected on five consecutive one-day trips aboard R/V Trident along a transect from the Port of Houston to the Galveston Bay entrance following Hurricane Harvey from September 4 to September 28, 2017. Samples were filtered on board through 0.2-micrometer (μm) Whatman-Nucleopore Q-TEC filters (Filtration Solutions) for DOC, optical, and chemical analysis.

Samples for mass spectrometry analyses (900 milliliters (mL)) were acidified to pH 2.5 using 6 moles per liter (mol L-1) sulfuric acid and extracted through Agilent PPL cartridges (1 gram) at 10 milliliters per minute (mL min-1). After extraction, cartridges were rinsed with 10 mL of deionized water acidified to pH 2.5 and dried for 30 seconds to remove residual water. The cartridges were eluted with 20 mL of methanol at 2 mL min-1, and the eluate was stored in glass vials at -20 degrees Celsius until analysis.

Electrospray ionization mass spectrometry (ESI-MS) experiments were performed using a Thermo Scientific Q Exactive Focus. Sample was injected into a 10-microliter (µL) loop and was transferred to the mass spec using a mobile phase containing 70% methanol and 30% water with 0.1% formic acid at a flow rate of 600 microliters per minute (µL/min). The Q Exactive Focus HESI source was operated in full MS in positive mode and negative mode. The mass resolution was tuned to 70000 FWHM at m/z 200. The spray voltage was set to 3.5 kilovolts (kV) for positive mode and 2.8 kV for negative mode. The sheath gas and auxiliary gas flow rates were set to 40 and 10 arbitrary units, respectively. The vaporizer and transfer capillary temperatures were held at 250 and 320 degrees Celsius, respectively. The S-Lens RF level was set at 50 volts. Exactive Series 2.11 /Xcalibur 4.2.47 software was used for data acquisition and processing of profile data. Mass spectral data processing for molecular formulae assignment followed procedures outlined by Hawkes et al. (2020). This included noise removal, internal calibration, and contaminant or rare peak removal. MATLAB code adapted from Fu et al. 2020) was used to assign formulae to noise-filtered and calibrated peaks. Assignment was performed allowing a mass tolerance of ± 2 parts per million (ppm).