Because all ions in the same experimental conditions undergo the same shift induced by the electric field associated with the charge of the ions in the analyzer cell in MALDI/FFMS, the constants A and B in calibration equation(m/z=A/f+B/f²)are amended by the shift of matrix peak observed frequency.The mass obtained by the amended calibration equation is therefore immune to space-charge effects on mass accuracy.Comparison of prediction performance of the calibration models constructed by multivariate regression and simplex method was studied.Simplex method, in which maximum relative error acts as objective function, is superior on solidity of ion mass prediction.And the relative errors of ion mass measured all are less than 5x10^-6.The IRMPD fragment ions of clarithomycin were determined by this method, and the fragmentation patterns were discussed.

Key words: MALDI, FYMS, mass calibration