A modified internal lock‐mass method for calibration of the product ions derived from sustained off‐resonance irradiation collision‐induced dissociation using a Fourier transform mass spectrometer

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Abstract
A modified internal lock‐mass calibration method is introduced for improving the mass measurement accuracy of the product ion spectra derived from sustained off‐resonance irradiation collision‐induced dissociation (SORI‐CID) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. This method involves an initial external calibration of the Fourier transform mass spectrometer to obtain the initial A‐ and B‐terms for the equation (fi = A/(m/z)i + B). The A‐term is adjusted by using an empirical relationship between the up‐shift of the A‐term and the pulse‐gas duration, whereas the B‐term is adjusted by using the mass of the unfragmented precursor ion from the SORI‐CID mass spectrum of the unknown sample as internal lock‐mass. These adjusted A‐ and B‐terms are then used to provide exact mass SORI‐CID calibration for the unknown sample. The modified internal lock‐mass method achieved average mass measurement accuracy of ∼3 ppm which is significantly better than that of the conventional internal lock‐mass calibration (∼9 ppm) and is approaching that of the internal calibration (∼2 ppm) and requires no addition of internal calibrant or instrumental modifications. Copyright © 2004 John Wiley & Sons, Ltd.

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