RAPID CALCULATION OF ISOTOPE DISTRIBUTIONS

A new method is presented for calculating isotope distributions starting from a molecular formula and elemental isotopic abundances. The algorithm uses Fourier transform methods to do the multiple convolutions required to calculate molecular isotope distributions, In contrast to ''infinite resolution'' methods based on expansion of a polynomial expression, the present method produces peak profiles of finite resolution, and the peak profile function can be adjusted to match that of an experimental apparatus, The method is very fast, accurate, and economical in its use of computer memory and can be applied to extremely large molecules, In a sample calculation applied to a DNA oligomer of molecular mass > 123 kDa, the calculation completed in < 1 s at a resolution of similar to 400 000 while an array size of only 4096 double precision points was used. Calculation of the moments of the resulting distribution showed that the average molecular weight of the distribution was accurate to 0.013 parts per million and the standard deviation was accurate to 0.15 parts per million.