Large Molecules - Isotopic Patterns at Sufficient Resolution

3 Large Molecules - Isotopic Patterns at Sufficient Resolution 

Terms such as large molecules or high mass are subject to steady change in mass spectrometry as new techniques for analyzing high-mass ions are being developed or improved [66]. Here, the focus is on masses in the range of 10 -10 u. 

The calculation of isotopic patterns of molecules of several 10 u is not a trivial task, because slight variations in the relative abundances of the isotopes encountered gain relevance and may shift the most abundant mass and the average mass up or down by 1 u. In a similar fashion the algorithm and the number of iterations employed to perform the actual calculation affect the final result [24]. 

Note The calculation of relative molecular mass, M, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u [37]. 

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