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Elemental Compositions from Exact Mass Measurements and Accurate Isotopic Abundances

Introduction

Exact masses have been used for decades to calculate elemental compositions for known and unknown molecules. The traditional approach calculates all possible combinations of user-specified atoms that fall within a given error tolerance of a measured mass. The number of possible combinations increases dramatically with increasing mass and as more atoms are included in the search set. In many cases, it is not possible to determine a unique composition based on mass alone.

A common source of error in measuring isotopic abundances with scanning mass spectrometers is related to fluctuations in ion current during measurement. The AccuTOF family of mass spectrometers overcomes this problem by analyzing all of the isotopes formed at the same instant. Combined with a high-dynamic-range detector, this provides highly accurate isotopic abundances. It has been shown that accurately measured isotopic abundances can be combined with measured exact masses to dramatically reduce the number of possible elemental compositions for an unknown. It is often possible to deduce a unique elemental composition, facilitating the identification of unknown substances.

Experimental

Samples in this report were measured with the AccuTOF-DART™ mass spectrometer. Similar procedures can be used with other members of the AccuTOF mass spectrometer family. Calibrated mass spectra were centroided and saved as JEOL-DX (JCAMP) text files. These text files were processed with the Elemental Composition Workshop from the Mass Spec Tools™ software suite distributed with AccuTOF mass spectrometers. The program permits automated isotope matching for measured mass spectra and provides a visual comparison between the measured and theoretical isotopic abundances for each hit (Figure 1).

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