Calculation of elemental composition of gaseous ions from stable nuclei signals using high resolution mass spectrometry (HRMS): Examination of the stages involved

- A simple method is presented for identifying isotopic fine structure of high resolution mass spectra.
- Isotopologues [M + 1 + Na]+ and [M + 2 + Na]+ of the sodiated molecule [M + Na]+ permit ready determination of elemental composition.
- The method is illustrated by sulfamethoxazole, C10H11N3O3S; it is composed of five elements that have stable isotopes.
- Comparison is made with bovine insulin that is derived from the same five elements.
- The calculation of relative abundances is simplified greatly when isotopologues are considered in turn.

A simple method is presented for identifying and confirming the components, or isotopic fine structure, of mass spectra obtained at ultra-high mass resolution with a Bruker SolariX XR Fourier transform ion cyclotron resonance (FTICR) mass spectrometer at a 7 T magnetic field. A sample compound, sulfamethoxazole, was chosen for illustration of the method because its elemental composition, C10H11N3O3S, contains the elements carbon, hydrogen, nitrogen, oxygen, and sulfur; all replaceable by a stable isotope. While multiple atoms of carbon, hydrogen, nitrogen, and oxygen are present in sulfamethoxazole, there is but one sulfur atom in the elemental composition. In the mass spectrum of protonated sulfamethoxazole, the major peaks in the region of the protonated molecule, [M + H]+, are represented by isotopologues [M + 1 + H]+ and [M + 2 + H]+, etc. Similarly, in the mass spectrum of sodiated sulfamethoxazole, the major peaks in the region of the sodiated molecule, [M + Na]+, are represented by isotopologues [M + 1 + Na]+ and [M + 2 + Na]+, etc. In a mass scan obtained at high mass resolution, [M + Na]+ remains as a single peak yet the isotopologues are observed to have fine structure. The method describes identification of isotopic fine structure components through consideration of mass differences and mass/charge ratios. Confirmation of identification is achieved by calculation of relative abundances of components that is simplified to the level of a hand-held calculator by considering each isotopologue in turn.

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