The isotope masses and relative abundances for each element are fundamental chemical knowledge. Computing the isotope masses of a compound and their relative abundances is an important and difficult analytical chemistry problem. We demonstrate that this problem is equivalent to sorting $Y=X_1+X_2+\cdots+X_m$. We introduce a novel, practically efficient method for computing the top values in $Y$. then demonstrate the applicability of this method by computing the most abundant isotope masses (and their abundances) from compounds of nontrivial size.