Estimating the precision of exact mass measurements on an orthogonal time-of-flight mass spectrometer

The combination of orthogonal TOF/ESI MS exact mass measurement and on-line chromatography represents a powerful analytical tool for identifying unknown components in complex mixtures and is being widely utilized. The precision of these mass data is often incorrectly estimated as the precision or mean deviation obtained for reference standards under standard conditions. But, the precision of a mass measurement is dependent on the number of ions sampled in the measurement and, thus, is likely to be different for every measurement. A simple procedure for correctly estimating the precision of a specific mass measurement is presented, the limits of the procedure are investigated, and the utility and validity of the procedure are demonstrated.