Routine part-per-million mass accuracy for high-mass ions: Space-charge effects in MALDI FT-ICR

The effect of ion space-charge on mass accuracy in Fourier transform ion cy clotron resonance mass spectrometry is examined. Matrix-assisted laser deso rption/ionization is used to form a population of high-molecular-weight pol ymer ions with a wide mass distribution. The density of the ions in the ana lyzer cell is varied using ion remeasurement and suspended trapping techniq ues to allow the effect of ion space charge to be examined independently of other experimental influences. Observed cyclotron frequency exhibits a lin ear correlation with ion population. Mass errors of 100 ppm or more in exte rnally calibrated mass spectra result when ion number is not taken into acc ount. By matching the total ion intensities of calibrant and analyte mass s pectra, the protonated ion of insulin B-chain, 3494.6513 Da, is measured wi th an accuracy of 0.07 ppm (average of 10 measurements, sigma = 2.3 ppm, av erage absolute error 1.6 ppm) using a polymer sample as an external calibra nt, Alternatively, the correction for space charge can be made by using a c alibration equation that accounts for the total ion intensity of the mass s pectrum. A calibration procedure is proposed and is tested with the measure ment of the mass of insulin B-chain, A mass accuracy of 2.0 ppm (average of 20 measurements, sigma = 4.2 ppm, average absolute error 3.5 ppm) is achie ved. Space-charge-induced mass errors are more significant for samples with many components, such as a polymer, than for single-component samples such as purified peptides or proteins.