Multichannel Heterodyne Detection for Accurate Mass Selected Ion Monitoring in Fourier Transform Mass Spectrometry: Implementation and Comparison with Undersampling

A multichannel receiver was built to monitor simultaneously and at high mass resolution ion species of three different and widely spaced masses in a Fourier transform mass spectrometer. The method of detection, which is similar in intent to peak matching in sector mass spectrometry, was used to test the feasibility of performing accurate mass measurements with wide mass separation between test and reference ions. Sub-part-per-million mass measurement errors were obtained under conditions of low space charge by using reference ions separated from the test ions by approximately 50%. This capability is unique to Fourier transform mass spectrometry (FT-MS). An alternate FT-MS approach for simultaneous detection of peaks that are widely separated in mass is undersampling or “foldover”. The two approaches are compared by treating the noise for each, and it is found that undersampling is more susceptible to noise than are heterodyne techniques.