We used electrospray‐ionization mass spectrometry (ESI‐MS) to characterize, at the compound level, dissolved organic matter (DOM) composition and bioavailability in two streams. There was considerable consistency in the composition of the DOM between the two streams (unit mass resolution): ≫70% of the masses detected occurred in both streams. Approximately 40–50% of the bulk dissolved organic carbon in the stream water was bioavailable during a 12‐d microbial decomposition experiment. ESI‐MS compound level analysis identified which masses were used, which were not, and their patterns of utilization. In both streams, ~40% of the masses decreased in concentration, ~55% did not change, and ≪5% increased. Despite the complex system (≫1,500 DOM compounds and a natural consortia of bacteria), there was a high degree of similarity in which masses were used and the amount of each mass used between replicate flasks for a stream. There was also good agreement between the two streams in which masses were used and the amount of each mass used. This suggests that the selection by the microbial consortia of organic compounds in the complex and heretofore largely uncharacterized DOM pool is repeatable and, therefore, ultimately predictable.

The complex mixture of organic compounds in the atmosphere influences climate, air quality, and ecosystem processes. Atmospheric pressure electrospray ionization mass spectrometry (APESI-MS) was evaluated as a potential tool for direct measurement of the total suite of individual dissolved organic matter (DOM) compounds in rainwater. The APESI-MS response was linear to all DOM compounds of atmospheric significance examined as standard solutions. Urban precipitation samples from New Brunswick, NJ (USA) were analyzed by APESI-MS over the mass-to-charge (m/z) range 50-3,000. Over 95% of the m/z ions detected were in the low m/z range (50-500). Over 300 unique m/z ions were detected across the 11 rainwater samples indicating the complexity of the mixture of DOM in rainwater. Forty percent of the organic bases (positive mode detection) and 22% of the organic acids (negative mode) occurred in at least 6 of the 11 rainwater samples. Ions corresponding to the m/z of carboxylic acids standards (nonanedioic acid; 1,4-butanedicarboxylic acid; pentanedioic acid; hydroxybutanedioic acid; and butanedioic acid) and to reduced N standards (allylurea; caffeine; imidazole; and N-2-propenylurea) occurred in at least one of the 11 rainwater samples. Total dissolved organic carbon (DOC) estimated from the APESI-MS analysis and measured by standard DOC methods were not statistically different.

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