Quantitation of ion abundances in fourier transform ion cyclotron resonance mass spectrometry

To improve the analytical usefulness of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), an extensive survey of various methods for quantitation of peak magnitudes has been undertaken using a series of simulated transient response signals with varying signal-to-noise ratio. Both peak height (five methods) and peak area (four methods) were explored for a range of conditions to determine the optimum methodology for quantitation. Variables included dataset size, apodization function, damping constant, and zero filling. Based on the results obtained, recommended procedures for optimal quantitation include: apodization using a function appropriate for the peak height ratios observed in the spectrum (i.e., Hanning for ratios of about 1:10, three-term Blackman-Harris for ratios of ∼1:100, or Kaiser-Bessel for ratios of ∼1:1000); zero filling until the peaks of interest are represented by 10–15 points (generally obtained with one order of zero filling); and use of the polynomial y = (ax² + bx + c)ⁿ and the three data points of highest intensity of the peak to locate the peak maximum, Y_max = (−b²/4a + c)ⁿ. In this peak fitting procedure, which we have termed the “Comisarow method,” n is 5.5, 9.5, and 12.5 for the Hanning, three-term Blackman-Harris, and Kaiser-Bessel apodization functions, respectively. Accuracy of quantitation using an optimal peak height determination is about equal to that for peak area measurements. These recommendations were found to be valid when tested with real FTICR-MS spectra of xenon isotopes.