The AccuTOF makes it very easy to obtain exact mass measurements and determine elemental compositions. The mass spectra shown below were acquired with the electrospray ion source operated in negative-ion mode. The mass calibration was acquired the previous day in positive-ion mode. After changing the mass spectrometer polarity and retuning, a stearic acid impurity (present in the methanol) was used as a single-point drift correction “lock mass” to determine the elemental composition of the analyte peak at m/z 256.06242 and the palmitic acid impurity at m/z 256.23104. The error was less than 0.002 u.