Resolution: peak width definition

Orange Book entry

For a single peak made up of singly charged ions at mass m in a mass spectrum, the resolution may be expressed as m / ??????????????????????m, where ??????????????????????m is the width of the peak at a height which is a specified fraction of the maximum peak height. It is recommended that one of three values 50%, 5% or 0.5% should always be used. (Note that for an isolated symmetrical peak recorded with a system which is linear in the range between 5% and 10% levels of the peak, the 5% peak width definition is equivalent to the 10% valley definition. A common standard is the definition of resolution based upon ??????????????????????m being Full Width of the peak at Half its Maximum (FWHM) height.