Digital quadrature heterodyne detection for high-resolution Fourier transform ion cyclotron resonance mass spectrometry

The pursuit of ever higher mass-resolving power in Fourier transform ion cy clotron resonance mass spectrometry (FT-ICR MS) has driven a demand for hig her magnetic field strength and longer time-domain ICR signal lifetime, wit h proportionate increase in data set size in direct-mode detection. Heterod yne-mode detection thus becomes increasingly important for achieving ultrah igh-mass resolution from a fixed maximum-size data set. Unfortunately, as c onventionally performed (i.e., analog single phase), heterodyne detection r educes the S/N ratio by a factor of 2(1/2) relative to direct-mode detectio n. Here, we restore the factor of 2(1/2) by use of quadrature heterodyne de tection. In addition, replacement of analog by digital heterodyning elimina tes analog circuitry (and its associated noise). Finally, digital filtering of the time-domain discrete ICR signal not only eliminates the need for a bank of analog low-pass filters but also ensures that the Nyquist bandwidth and tilter bandwidth are always matched, for optimal noise reduction. With these features, digital quadrature heterodyne detection becomes the detect ion method of choice for high-resolution FT-ICR MS.