Improved mass accuracy and isotope confirmation through alignment of FT-ICR-MS data of complex natural mixtures.

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is one of the state-of-the-art methods to analyze complex natural organic mixtures. The precision of detected masses is crucial for molecular formula attribution. Random errors can be reduced by averaging multiple measurements of the same mass, but because of limited availability of ultrahigh-resolution mass spectrometers, most studies cannot afford analyzing each sample multiple times. Here we show that random errors can be eliminated also by averaging mass spectral data from independent environmental samples. By averaging the spectra of 30 samples of our 15 Tesla instrument we reach a mass precision comparable to a single spectrum of a 21 Tesla instrument. We also show that it is possible to accurately and reproducibly determine isotope ratios with FT-ICR-MS. Intensity ratios of isotopologues were improved to a degree that measured deviations were within the range of natural isotope fractionation effects. In analogy to δ¹³C in environmental studies, we propose Δ¹³C as an analytical measure for isotope ratio deviances instead of widely employed C deviances. In conclusion, here we present a simple tool for post-detection data processing that significantly improves mass accuracy and the precision of intensity ratios of isotopologues at no extra costs.