Specification of Mass Accuracy

Measured accurate masses, when used to assign molecular formulas, should always be accompanied by their mass accuracies. [52], Ideally, this can be done by giving the mean mass value and the corresponding error in terms of standard deviation through several repeated measurements of the same ion. [40] This is definitely not identical to the error usually provided with a mass spectrometer s software, where the error is based on the difference of a single pair of calculated and measured values. The reduction of the average mass error goes with the square root of the number of determinations (Chap. 3.5.4) [39]. 

Example The [M-Cl] ion, [CHCy, represents the base peak in the El spectrum of chlorofoim The results of three subsequent determinations for the major peaks of the isotopic pattern are listed below (Fig. 3.26). The typical printout of a mass spectrometer s data system provides experimental accurate mass and relative intensity of the signal along with absolute and relative mass error as calculated for a set of suggested formulas. Here, the experimentally accurate mass values yield a root-mean-square of 82.9442 0.0006 u for the [ CH Cl2] ion. The conpara-tively small standard deviation of 0.0006 u corresponds to a relative error of 7.5 ppm. 

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