| Popis: |
A Fourier transform ion cyclotron resonance (FT/ICR) time domain signal (TDS) can be simply described by a superposition of exponentially decaying sinusoids. If the mass components decay at different rates, however, then the FT magnitude peak heights give erroneous ratios of actual ion abundances. A method is presented to circumvent this difficulty that divides the original TDS into shorter (overlapping) segments as a function of the time of the first point, T i , in each segment. For a particular mass component, the peak height maximum, M , of the FT magnitude spectrum of each segment is related to T i such that a plot of ln [ M ] versus T i is linear, with slope and intercept proportional to the decay constant (inverse of relaxation time) and initial amplitude (TDS amplitude at beginning of detection), respectively. Inclusion of an arbitrary apodization function in the derivation of the equation for M leaves the slope of the above plot invariant but introduces a decay constant-dependent shift in the intercept. Numerical simulations using this scheme generally predict initial amplitudes and damping constants to better than 0.1%. Electron-beam ionization experiments on Kr ( P = 3.3 × 10 −7 torr) are compatible with exponential damping and suggest that the dominant isotopic fractionation mechanism in FT/ICR is dependent on ion number rather than mass. Application of the segmenting procedure to the Kr experimental data yields isotope ratios with greatly reduced scatter and apparently improved accuracy. |
