This page allows you to perform two calculations. The first section is used to calculate the mass accuracy of your assignment, when comparing a theoretical m/z and an experimentally observed m/z. The second section enables you to calculate the theoretical m/z of the monoisotopic peak for a given empirical formula.
When calculating the theoretical m/z, remember to include the charge carriers (e.g. add or remove hydrogen atoms and adjust the charge accordingly, to simulate gain or loss of protons) and, when setting the charge, to use a positive value for position ions or a negative value for negative ions. If there are specific elements that are not present in the list below and which you'd like to see added, let me know.
1) Mass accuracy (in ppm):
2) To calculate a theoretical m/z: