The quantification of Cyclosporin A (CsA) and related biological compounds by various mass spectrometric techniques is well established. It is precisely this fact that makes the use of CsA particularly appealing as a model system in evaluating the quantitative properties of electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). Utilizing the internal standard methodology, we have generated a linear calibration curve ranging from 5-250 ng/mL CsA described by the equation y = 0.00503x-0.00958, with a %RSD slope of 0.74% and a correlation coefficient (r 0 ) equal to 0.999 2 (N = 30). The internal standard (200 ng/mL Cyclosporin G) was found to be necessary to establish the reported dynamic range, two orders of magnitude based on an experimentally determined detection limit of 2.4 ng/mL CsA, and also significantly improved the overall precision of the method. The data represented in the standard curve were collected as 1.05 s single acquisitions (512 K data points at an ADC rate of 500 kHz) and unapodized prior to the Fourier transformation of the time-domain data. We report that full Hanning apodization has a statistically significant negative effect (F-test at 95% confidence level) on the regression statistics of the curve (i.e. increasing the %RSD slope to 1.33%). Finally, we report that acceptable quantitative properties of an FTICR-MS experiment can be realized as soon as the detection of a second isotopic beat for the targeted species is obtained. This should allow for the practical and effective coupling of on-line separation techniques for quantitative analysis using ESI-FTICR-MS. Copyright # 1999 John Wiley & Sons, Ltd.
Received 4 December 1998; Accepted 4 December 1998Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has been demonstrated to have impressive capabilities for the analytical characterization of biological materials. Recent reports have described such varied investigations as highly accurate molecular mass determinations of peptides and proteins, 1-4 high resolution tandem mass spectra of exceedingly large biomolecules (b10 kDa), 2,5,6 and structural verification of 50-to 100-mer DNA and RNA sequences. 7,8 The maturation of this increasingly useful technique in the qualitative analysis of biological compounds is well documented. Interestingly, one area of research that has been comparatively slow to develop is the exploration of electrospray ionization (ESI) FTICR-MS as a quantitative tool. Limbach et al. reported the first experimental demonstration of the potentially achievable detection limit ($177 ions at S/N = 3) and dynamic range (b4 orders of magnitude) for an FTICR mass spectrometer with a 1.875-in cubic trap housed within a 3.0 T superconducting magnet. 9 In a more empirical approach, Padley and co-workers quantified both singly and multiply charged ions according to their absolute peak intensities 10 utilizing ESI and observed linearity of $2-3 orders of magnitude for singly charged lysine (147 Da) and 1.5 or...