ANALYSIS AND ELIMINATION OF SYSTEMATIC-ERRORS ORIGINATING FROM COULOMB MUTUAL INTERACTION AND IMAGE CHARGE IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE PRECISE MASS DIFFERENCE MEASUREMENTS

The effect of mutual Coulomb-mediated interactions between ions of two different mass-to-charge ratios (but equal ion cyclotron orbital radi i) on their Fourier transform ion cyclotron resonance (FT/ICR) mass sp ectral frequency difference is derived analytically and measured exper imentally. For a cylindrical ion trap, ion packets are modeled theoret ically as infinitely extended lines of charge, and contributions to cy clotron frequency difference due to direct Coulomb repulsion between t he line charges as well as the forces arising from image charge induce d on the trap electrodes by each line charge are calculated. A strikin g theoretical prediction is that the effect on ICR frequency differenc e of mutual Coulomb repulsion between ions in a mass doublet may be co mpensated by the image-charge effect. As a result, there is an optimal (calculable) ion cyclotron orbital radius at which the measured cyclo tron orbital frequency difference between ions of two different mass-t o-charge ratios is independent of mutual Coulomb-mediated interactions between the two components of the mass doublet! Moreover, if the two mass-doublet component ions are present in equal numbers, then the mea sured ion cyclotron orbital frequency difference is also independent o f all Coulomb-mediated interactions between the two types of ions! Thu s, the single largest systematic error in measurement of mass differen ce in a mass doublet by FT/ICR mass spectrometry may be virtually elim inated by appropriate control of ICR orbital radius and/or by performi ng measurements at various relative abundance ratios and extrapolating to equal relative abundance of the two mass-doublet components. We re port experimental tests and verification of these predictions for two different mass doublets: He-3+/H-3+ (cylindrical trap at 4.7 Tesla) an a (CH2+)-C-12-H-1/N-14+ (cubic trap at 7.0 Tesla). From the latter mea surement, we determine the mass of atomic nitrogen as m(N-14) = 14.003 074 014(19) u.