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 cyclotron resonance mass spectrometry (FT-ICR MS) has driven a demand for higher magnetic field strength and longer time-domain ICR signal lifetime, with proportionate increase in data set size in direct-mode detection. Heterodyne-mode detection thus becomes increasingly important for achieving ultrahigh-mass resolution from a fixed maximum-size data set. Unfortunately, as conventionally performed (i.e., analog single phase), heterodyne detection reduces the S/N ratio by a factor of 2(1/2) relative to direct-mode detection. Here, we restore the factor of 2(1/2) by use of quadrature heterodyne detection. In addition, replacement of analog by digital heterodyning eliminates 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 detection method of choice for high-resolution FT-ICR MS.