文摘
The pursuit of ever higher mass-resolving power inFourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has driven a demand for highermagnetic field strength and longer time-domain ICR signallifetime, with proportionate increase in data set size indirect-mode detection. Heterodyne-mode detection thusbecomes increasingly important for achieving ultrahigh-mass resolution from a fixed maximum-size data set.Unfortunately, as conventionally performed (i.e., analogsingle phase), heterodyne detection reduces the S/N ratioby a factor of 21/2 relative to direct-mode detection. Here,we restore the factor of 21/2 by use of quadrature heterodyne detection. In addition, replacement of analog bydigital heterodyning eliminates analog circuitry (and itsassociated noise). Finally, digital filtering of the time-domain discrete ICR signal not only eliminates the needfor a bank of analog low-pass filters but also ensures thatthe Nyquist bandwidth and filter bandwidth are alwaysmatched, for optimal noise reduction. With these features,digital quadrature heterodyne detection becomes thedetection method of choice for high-resolution FT-ICR MS.
