Space charge effects in Fourier transform mass spectrometry. II. Mass calibration

Abstract

A new relatlonshlp between Ion mass and effective cyclotron frequency Is derived for Ions stored In a cublc cell and detected by uslng Fourler transform mass spectrometry. An assessment of colllslonal damping on mass measurement error Is also made. I t Is concluded that frequency perturbation by colllslonal damping, whlle predlcted by the model, Is negllglble at sufflclently low pressure. The mass callbration law is tested at a magnetlc fleld of 1.2 T by uslng major fragment Ions of 1,1,1,2-tetrachloroethane. WHh broad band "chirp" excltatlon of Ions, systematic rather than random errors were dlscovered. The magnitude of these systematic errors increased as the number of Ions Qtored In the cell was Increased. However, H Is predlcted from the calibration law that errors will decrease wHh the square of the magnetic field strength.The Fourier transform mass spectrometer (FTMS) (1) is recognized as a potentially useful analytical instrument because it is capable of high mass range and ultrahigh mass resolution. The demonstration of high resolving power with the FTMS, however, has predated by several years the development of methodology for exact mass measurement and elemental composition assignment.Early attempts to produce a mass calibration scheme sufficiently accurate for elemental composition assignments were not successful. For example, Ledford and McIver (2) reported measurement accuracy ranging from 7 ppm to 151 ppm over the mass range m/z 47 to m/z 264 using an ICR mass spectrometer with electrometer detection. The mass errors were systematic, caused by changes in space charge conditions in the analyzer cell as ions were sequentially observed. Comisarow (3) reported mass measurement accuracy ranging from 0.3 ppm to 80 ppm over the mass range m/z 69 to m / i 1166. The relative measurement errors were found to increase systematically with mass.Mass measurement accuracy of the a few parts per million or less (often sufficient for elemental composition assignment) was achieved in more recent studies. Ledford ( 4 ) et al. investigated mass measurement using a parabolic mass calibration law for cubic analyzer cells. Over a 4 m u mass range, errors of 0.8 ppm were typical, while average errors of 2 ppm over an 18 amu mass range were obtained when a three-parameter fit was used. Wanczek and Allemann (5) reported a novel side-band method for measuring the masses of trapped ions. Errors averaging 1.5 ppm were obtained over the mass range m J z 18 to m/z 170 amu.Although these latter methods (4,5) of calibration represent improvements over earlier work ( 2 , 3 ) , they have not proved to be routinely useful for exact mass determination. One reason is that ion space charge in the analyzer cell affects observed frequencies, and this must be accounted for in accurate measurements. Ledford et al. (4) recognized this in their efforts to develop a mass calibration scheme. They demonstrated that frequency shifts associated with changes in space charge were qualitatively similar to those caused by changes in trap voltage. Whit...

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