Routine Part-per-Million Mass Accuracy for High- Mass Ions:  Space-Charge Effects in MALDI FT-ICR

Abstract

The effect of ion space-charge on mass accuracy in Fourier transform ion cyclotron resonance mass spectrometry is examined. Matrix-assisted laser desorption/ionization is used to form a population of high-molecular-weight polymer ions with a wide mass distribution. The density of the ions in the analyzer cell is varied using ion remeasurement and suspended trapping techniques to allow the effect of ion space charge to be examined independently of other experimental influences. Observed cyclotron frequency exhibits a linear correlation with ion population. Mass errors of 100 ppm or more in externally calibrated mass spectra result when ion number is not taken into account. By matching the total ion intensities of calibrant and analyte mass spectra, the protonated ion of insulin B-chain, 3494.6513 Da, is measured with an accuracy of 0.07 ppm (average of 10 measurements, σ = 2.3 ppm, average absolute error 1.6 ppm) using a polymer sample as an external calibrant. Alternatively, the correction for space charge can be made by using a calibration equation that accounts for the total ion intensity of the mass spectrum. A calibration procedure is proposed and is tested with the measurement of the mass of insulin B-chain. A mass accuracy of 2.0 ppm (average of 20 measurements, σ = 4.2 ppm, average absolute error 3.5 ppm) is achieved. 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.

Mass measurement errors caused by “local” frequency perturbations in FTICR mass spectrometry

“…The role of the ion space charge becomes increasingly important for LC-FTICR studies because both the total ion population and specific ion abundances may vary in a wide range. A systematic study of the mass accuracy under such conditions [9] has demonstrated that the frequency correction based on the overall intensity can improve the FTICR mass measurement accuracy over a wide mass range for externally calibrated spectra.…”

“…However, the mass accuracy in an FTICR experiment depends crucially on the number of ions used for the measurement [9,[11][12][13][14][15][16][17][18]. When online separations are used, the analyte ion production rates vary widely, and the ion population in the trap cannot be easily or precisely controlled.…”

“…When online separations are used, the analyte ion production rates vary widely, and the ion population in the trap cannot be easily or precisely controlled. Although mass accuracies in the sub-ppm level have been reported using internal calibration [10], external calibration typically doesn't provide accuracies better than several ppm, unless the ion population (and achievable dynamic range) is severely constrained, or when the ion population for the measurement differs significantly from the ion population used for the calibration [9]. In FTICR, the highest MMA have been obtained with small ion populations, often with the use of summation (or signal averaging) of many spectra and with the application of internal calibration.…”

Parts-Per-Billion Mass Measurement Accuracy Achieved through the Combination of Multiple Linear Regression and Automatic Gain Control in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

“…22 For example, Amster and co-workers developed a calibration curve to account for the difference between ion populations of the calibration spectrum and subsequent spectra resulting in <10 parts-per-million MMA. 23 calibration law which afforded <5 ppm MMA. 26 Amster and co-workers recently developed stepwise-external calibration, in which calibration spectra are acquired at low trapping voltages, and experimental spectra are acquired at more routine trapping voltages.…”

In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases

“…Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) [15][16][17][18][19][20] is known for its unparalleled mass accuracy and mass resolution and has become an important tool in many areas of analytical life sciences [21,22]. For this reason, the FT-ICR MS technique has been combined with the soft ionization technique MALDI using internal [23][24][25][26][27][28] as well as external [29 -37] ion sources. One of the striking advantages of the MALDI FT-ICR technique is high sensitivity as well as high throughput capability.…”