Investigation of space charge effects and ion trapping capacity on direct introduction ultra‐high‐resolution mass spectrometry workflows for metabolomics

Abstract

Ultra-high-resolution mass spectrometry, in the absence of chromatography, is finding its place for direct analyses of highly complex mixtures, such as those encountered during untargeted metabolomics screening. Advances, however, have been tempered by difficulties such as uneven signal suppression experienced during electrospray ionization. Moreover, ultra-high-resolution mass spectrometers that use Orbitrap and ICR analyzers both suffer from limited ion trapping capacities, owing principally to space-charge effects. This study has evaluated and contrasted the above two types of Fourier transform mass spectrometers for their abilities to detect and identify by accurate mass measurement, small molecule metabolites present in complex mixtures. For these direct introduction studies, the Orbitrap Fusion showed a major advantage in terms of speed of analysis, enabling detection of 218 of 440 molecules (<2 ppm error, 500 000 resolution at m/z 200) present in a complex mixture in 5 min. This approach is the most viable for high-throughput workflows, such as those used in investigations involving very large cohorts of metabolomics samples. From the same mixture, 183 unique molecules were observed by FT-ICR in the broadband mode, but this number was raised to 235 when "selected ion monitoring-stitching" (SIM-stitching) was employed (<0.1 ppm error, 7 T magnet with dynamic harmonization cell, 1.8 million resolution at m/z 200, both cases). SIMstitching FT-ICR thus offered the most complete detection, which may be of paramount importance in situations where it is essential to obtain the most complete metabolic profile possible. This added completeness, however, came at the cost of a more lengthy analysis time (120 min including manual treatment). Compared to the data presented here, future automation of processing, plus the use of absorption mode detection, segmented ion detection (stepwise detection of smaller width m/z sections), and higher magnetic field strengths, can substantially reduce FT-ICR acquisition times.

Soil Organic Matter Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS): A Critical Review of Sample Preparation, Analysis, and Data Interpretation

“…Increasing magnetic field strength will increase mass resolution, dynamic range, and peak noncoalescence. , Increasing the time-domain acquisition period will also increase resolution; however if sample signal magnitude is low (i.e., sample ions are very low in abundance), instrument noise can also erroneously be assigned . Increasing ion accumulation times can increase sensitivity; however high ion densities in the ICR cell can produce space charge effects that coalesce peaks or alter ion frequencies, adversely affecting both formula assignment and mass accuracy. ,,, The limited ion capacity during FTICR MS measurement also means that intensity and m / z distributions are sensitive to instrumental parameters. Molecular weight distributions are better determined in low resolution MS systems like linear ion trap and time-of-flight that also capture lower molecular weights (<200 m / z ) efficiently. , Thresholds for S/N will also strongly affect peak detection and reproducibility.…”

“…20 Increasing ion accumulation times can increase sensitivity; however high ion densities in the ICR cell can produce space charge effects that coalesce peaks or alter ion frequencies, adversely affecting both formula assignment and mass accuracy. 20,125,148,149 The limited ion capacity during FTICR MS measurement also means that intensity and m/z distributions are sensitive to instrumental parameters. Molecular weight distributions are better determined in low resolution MS systems like linear ion trap and time-of-flight that also capture lower molecular weights (<200 m/z) efficiently.…”

Effect of the Ionization Source on the Targeted Analysis of Nickel and Vanadyl Porphyrins in Crude Oil

“…We believe that the differences between resolution and mass accuracy for the assignments shown in Figure 5 can be attributed to space-charge phenomena, which are closely related to the total ions and the number of species per nominal mass. 54 In a recent contribution, Hohenester et al 55 reported several analytical considerations for space-charge phenomena in FT-ICR using the Bruker SolariX equipped with the Paracell, the same instrument used in this work. The authors found that raising the number of ions within the cell due to increased accumulation times or use of concentrated samples affected the spectral mass accuracy.…”

Segment Scan Mass Spectral Acquisition for Increasing the Metabolite Detectability in Chemical Isotope Labeling Liquid Chromatography–Mass Spectrometry Metabolome Analysis

“…13 DI-MS with segment scan has also been used for metabolome analysis with improved coverage compared to full scan in various metabolomics applications. 9,11,12 In addition, segment scan has been shown to increase the accuracy of isotopic ion abundance measurement in a targeted metabolic flux analysis (e.g., by reducing the space-charge effect) with separate scans of two segments, instead of one full scan, in LC−Orbitrap−MS. 14 However, to our knowledge, there is no report of detailed study of segment scan mass spectral acquisition for increasing the metabolomic coverage in an LC−MS-based metabolome analysis.…”