Mass Spec m/z Ratio Calculator

How it is calculated

The tool predicts the m/z of an ion made by adding or removing protons. With neutral mass M in daltons and charge z, a positive charge uses m/z = (M + |z| × 1.007276) ÷ |z| (adducts like [M+zH]z+), and a negative charge uses m/z = (M − |z| × 1.007276) ÷ |z| ([M−zH]z−), where 1.007276 Da is the proton mass. The charge sign only picks the mode; m/z itself is reported as a positive number, and the ion mass (M plus or minus the protons) is shown as well. The mass must be above zero, a charge of 0 gives no result, and so does a negative-mode case where the ion mass would drop to zero or below. Other adducts such as sodium or ammonium are not covered. Use whole-number charges; the tool rejects fractional charges.

Common mistakes

• Entering the m/z of a peak as the neutral mass. This tool goes from neutral mass to m/z; for a doubly charged peak at 501.0073 the neutral mass is 1000, not 501.
• Adding the proton mass once for a multiply charged ion. The correction is z protons, so for z = 2 it is 2.0146 Da added to the mass before dividing.
• Using the mass of a hydrogen atom (1.00783) instead of a proton (1.00728). Hydrogen includes an electron, and using it shifts a high-resolution m/z in the fourth decimal place.
• Choosing the wrong sign for the mode. A positive charge adds protons, a negative charge removes them, so −2 gives 498.9927 instead of 501.0073.
• Expecting sodium or potassium adducts. The tool only adds or removes protons, so an [M+Na]⁺ peak will not match its output.