https://doi.org/10.1351/goldbook.12554
For a single peak corresponding to singly charged ions at mass \(m\) in a mass spectrum, the resolution may be expressed as \((m/z)/\Delta(m/z)\), where \(\Delta(m/z)\) 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 \(\pu{0.5\%}\) should always be used.
Note: For an isolated symmetrical peak recorded with a system which is linear in the range between \(\pu{5\%}\) and \(\pu{10\%}\) levels of the peak, the \(\pu{5\%}\) peak width definition is equivalent to the \(\pu{10\%}\) valley definition. A common standard is the definition of resolution based upon \(\Delta m\) being full width of the peak at half its maximum (FWHM) height.