Abstract
Electrospray ionization (ESI) operating in the negative mode coupled to high-resolution mass spectrometry is the most popular technique for the characterization of dissolved organic matter (DOM). The vast molecular heterogeneity and the functional group diversity of this complex mixture prevents the efficient ionization of the organic material by a single ionization source, so the presence of uncharacterized material is unavoidable. The extent of this poorly ionizable pool of carbon is unknown, is presumably variable between samples, and can only be assessed by the combination of analysis with a uniform detection method. Charged aerosol detection (CAD), whose response is proportional to the amount of nonvolatile material and is independent from the physicochemical properties of the analytes, is a suitable candidate. In this study, a fulvic acid mixture was fractionated and analyzed by high-pressure liquid chromatography–mass spectrometry in order to investigate the polarity and size distributions of highly and poorly ionizable material in the sample. Additionally, DOM samples of terrestrial and marine origins were analyzed to evaluate the variability of these pools across the land–sea aquatic continuum. The relative response factor values indicated that highly ionizable components of aquatic DOM mixtures are more hydrophilic and have lower molecular weight than poorly ionizable components. Additionally, a discrepancy between the samples of terrestrial and marine origins was found, indicating that marine samples are better represented by ESI than terrestrial samples, which have an abundant portion of hydrophobic poorly ionizable material.
Highlights
Dissolved organic matter (DOM) from aquatic environments is a complex and heterogeneous mixture characterized by material with a wide range of molecular weight (MW), polarity, structure, and stability
The mass spectrometric signal (MS) sensitivity was variable across the model compounds and exhibited a broad relative standard deviation among those which gave a response (RSD ≈ 46%), while the Charged aerosol detection (CAD) response was characterized by a more uniform response (RSD ≈ 16%) (Figure 1)
The final 6% was constituted by the material lost during handling and analysis of samples, whose Electrospray ionization (ESI) response is unknown. These results suggest that an impressive amount of material in the Suwannee River fulvic acid (SRFA) mixture is uncharacterized in routine ESIMS analysis performed in negative mode
Summary
Dissolved organic matter (DOM) from aquatic environments is a complex and heterogeneous mixture characterized by material with a wide range of molecular weight (MW), polarity, structure, and stability. The hydrophilic and mid-polarity organic material (RT < 10 min; log P < 1.79) was responsible for the majority of the MS signal, in contrast with the more hydrophobic material (RT > 10 min; log P > 1.79) poorly ionizable compared to the abundance observed by the CAD (Figure 3A) One explanation for this result might be that the relative response factor decreases with hydrophobicity, but this is unlikely as hydrophobic compounds are more likely to be at the surface of the droplets in the ESI spray and to enter the gas phase, and hydrophobicity has been shown to relate poorly to ionization efficiency.[6]. The fjord showed a stable and higher relative response factor, almost constant throughout the elution range (Figure 5); differently from the terrestrial samples, an increase in the relative response factor profile was observed for the more retained material (RT > 8 min; log P > 0.77), suggesting the predominance of highly ionizable DOM throughout the polarity gradient. Three main caveats are necessary to understand our interpretations of the data presented:
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