TIME-BASE MODULATION FOR THE CORRECTION OF CYCLOTRON FREQUENCY-SHIFTSOBSERVED IN LONG-LIVED TRANSIENTS FROM FOURIER-TRANSFORM ION-CYCLOTRON-RESONANCE MASS-SPECTROMETRY OF ELECTROSPRAYED BIOPOLYMERS

The occurrence of small shifts in the cyclotron frequency during the a cquisition of very long transients (in excess of 80 s) has been observ ed to be a limiting factor for ultrahigh-resolution mass measurements of protein ions performed with electrospray-ionization Fourier-transfo rm ion-cyclotron-resonance mass spectrometry. Resolution measurements were restricted to values less than 10(6) because of the frequency shi fts. Measurements of the frequency shifts, performed by sequentially t ransforming small segments of the transient, allowed the shift to be c haracterized and fitted to a 4th-order equation. The sampling rate of the acquired transient was then modulated (at a rate equal to the reci procal of the rate for the frequency shift) to allow ultrahigh resolut ion, greater than 2 X 10(6), and improved mass measurement and precisi on to be achieved for a small protein.