ISOTOPIC BEAT PATTERNS IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY - IMPLICATIONS FOR HIGH-RESOLUTION MASS MEASUREMENTS OF LARGE BIOPOLYMERS

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry ti me domain signals from multiply charged biopolymer ions exhibit charac teristic and predictable beat patterns due to the closely spaced cyclo tron frequencies of the various isotopic constituents. Isotope beat fe quencies can readily and accurately be predicted from the difference i n cyclotron frequencies of neighbouring isotope peaks. The nature of t hese signals has important implications for high resolution mass analy sis, particularly in instances where rapid spectral acquisition is des irable such as with on-line analysis of chromatographic/electrophoreti c effluents. Due to the pulsed nature of the frequency information in these transients, resolution improvements are effectively realized in a stepwise nature. As will be demonstrated, the application of apodiza tion functions can have deleterious effects on signal-to-noise and res olution when beats are present only near the beginning and end of the transient. Additionally, in instances where the length of FTICR data a cquisition is critical (such as in conjunction with on-line separation s or in the analysis of very high molecular weight species), it is cru cial to choose data acquisition parameters based on the predicted beha vior of the time domain signal for the efficient and accurate acquisit ion of mass spectra.