The analysis of dissolved organic matter (DOM) by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has gained wide interest recently, driven primarily by its ultrahigh resolving power and mass accuracy. Accurate calibration of mass spectra is a key step to successfully decipher the DOM components. We propose a simple and accurate method to internally calibrate the peaks in the complex spectra without the need to add a calibrant. Mass spectra of DOM samples from the Dismal Swamp, Virginia, and the lower Chesapeake Bay display the presence of naturally occurring fatty acids which can be readily recognized and calibrated with accuracies <0.1 ppm. Once calibrated with fatty acids, approximately 80% of all peaks in the DOM mass spectra can be assigned unique molecular formulas with accuracies <0.4 ppm. Although the formula errors for the assigned molecular formulas do increase with increasing m/z, the dynamic range of the fatty acids used as calibrants is sufficient because high m/z values (>600) have formulas with an average error of <0.6 ppm. Because fatty acids are ubiquitous components of most DOM, this approach is applicable to a large variety of DOM samples.

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