Transformative effects of higher magnetic field in Fourier transform ion cyclotron resonance mass spectrometry

Abstract

The relationship of magnetic field strength and Fourier transform ion cyclotron resonance mass spectrometry performance was tested using three instruments with the same design but different fields of 4.7, 7, and 9.4 tesla. We found that the theoretically predicted "transformative" effects of magnetic field are indeed observed experimentally. The most striking effects were that mass accuracy demonstrated ϳsecond to third order improvement with the magnetic field, depending upon the charge state of the analyte, and that peak splitting, which prohibited automated data analysis at 4.7 T, was not observed at 9.4 T. Numerous performance parameters in FT-ICR MS are predicted to improve with magnetic field, including linear improvements in mass resolving power and acquisition speed, and higher order improvements in mass accuracy, dynamic range, kinetic energy, and peak coalescence [2][3][4][5][6][7]. All of these parameters combine to determine the figure of merit of a mass analyzer, but not in a manner that is easily predicable. Improving FT-ICR MS performance is critical in many fields such as proteomics, petroleomics, and MALDI imaging, and in many cases enables, rather than improves, analytical capabilities. In practice, however, a component with no a priori field dependence, for example field homogeneity [8] vacuum strength, or acquisition speed, which set fundamental limits on resolution [2], or a phenomenon with a convoluted electrical and magnetic field dependence such as peak coalescence [9] or phase locking [7, 10 -14], can become the limiting factor and the determinant of performance. In addition, the relationship of important instrument performance metrics can be affected by factors with no-or convoluted field dependences. For example, the relationship between mass accuracy and resolution is blurred by anything that affects ideal peak shapes [15][16][17].There are isolated literature examples of what Marshall terms the "transformative effects" of higher magnetic fields. For example, post processing of 14.7 T FTMS spectrum to simulate a 7 T FTMS spectrum revealed that three compounds that were identifiable at 14.7 T appeared to be a single compound at 7 T [18]. In another example, the instrument's sensitivity for a complex and highly charged sample (29 kDa protein dissociation products) significantly increased when going from 6 T to 9.4 T FT-ICR [19]. On the other hand, despite the availability of 15 T instruments, the highest published resolving powers of ϳ8,000,000 (m/z 1148) [20], ϳ17,000,000 (m/z 1084.5) [21], ϳ1,000,000 (m/z 12,360) [22] were acquired (in one case 11 years ago [20]) at 9.4 T or 7 T, illustrating the importance of factors other than magnetic field. While the effects of field homogeneity upon instrument performance have been characterized [8], we are aware of no publication that systematically explores the relationship of FT-ICR MS performance and gross magnetic field. Here, we make an empirical determination of the merit of higher magnetic field, using the same instrument type, the same...

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