Calibration procedure for secular frequency scanning in ion trap mass spectrometers

Rationale Mass spectra can be recorded using ion traps by scanning the frequency of an alternating current (ac) signal that corresponds to the secular frequency of a trapped ion. There is a considerable simplification in the instrumentation needed to perform such a scan compared with conventional scans of the radiofrequency (rf) amplitude. However, mass calibration is difficult. An algorithm that can be used to achieve mass calibration is investigated and the factors that affect ion mass assignments are discussed. Methods Time domain data, recorded using a commercial benchtop linear ion trap mass spectrometer, are converted to the m / z domain using ion Mathieu parameter q u values which are derived from the dimensionless frequency parameter β u expressed as a continuing fraction in terms of q u . The relationship between the operating parameters of an ideal ion trap and the ion m / z ratio is derived from the Mathieu equations and expressed as an algorithm which through successive approximations yields the Mathieu q u value and hence m / z values and peak widths. The predictions of the algorithm are tested against experiment by sweeping the frequency of a small supplementary ac signal so as to cause mass‐selective ejection of trapped ions. Results Calibration accuracy is always better than 0.1%, often much better. Peak widths correspond to a mass resolution of 250 to 500 in the m / z 100–1800 range in secular frequency scans. Conclusions A simple, effective method of calibration of mass spectra recorded using secular frequency scans is achieved. The effects of rf amplitude, scan rate, and ac amplitude on calibration parameters are shown using LTQ linear ion trap data. Corrections for differences in ion mass must be made for accurate calibration, and this is easily incorporated into the calibration procedure. Copyright © 2016 John Wiley & Sons, Ltd.