Characterization of the Spectral Accuracy of an Orbitrap Mass Analyzer Using Isotope Ratio Mass Spectrometry

Abstract

Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) source coupled to the Q Exactive Plus has been extensively used in untargeted mass spectrometry imaging (MSI) analyses of biological tissue sections. Although the Orbitrap is a high-resolution and accurate-mass (HRAM) mass analyzer, these attributes alone cannot be used for the reliable identification of unknown analytes observed in complex biological matrices. Spectral accuracy (SA) is the ability of the mass spectrometer to accurately measure the isotopic distributions which, when used with high mass measurement accuracy (MMA), can facilitate the elucidation of a single elemental composition. To investigate the effects of different ion populations on an Orbitrap's SA and MMA, a solution of caffeine, the tetrapeptide MRFA, and ultramark was analyzed using a Q Exactive Plus across eight distinct automatic gain control (AGC) targets. The same compounds from the same lot numbers were also individually analyzed using isotope ratio mass spectrometry (IRMS) to accurately determine the isotopic abundance of C,N, and S. We demonstrated that at optimum absolute ion abundances the Orbitrap can be used to accurately count carbons, nitrogens, and sulfurs in samples with varying masses. Additionally, absolute monoisotopic ion abundances required for high SA were empirically determined by using the expected (IRMS) and experimental (Orbitrap) isotopic distributions to calculate the Pearson chi-square test. These thresholds for absolute ion abundances can be used in untargeted MSI studies to shorten an identification list by rapidly screening for isotopic distributions whose absolute ion abundances are high enough to accurately estimate the number of atoms.

Distinguishing malignant from benign microscopic skin lesions using desorption electrospray ionization mass spectrometry imaging

“…It is noteworthy that some of the selected peaks originate from a pair of monoisotopic and 13 C1 isotopic forms of the same molecular ion, which might increase a relative weight of the ion in distinguishing between normal and diseased states. In cases when the imaging is performed with high mass resolution, the ratio of the peaks may predict the molecular composition of the ion (23).…”

A simplified strategy for molecular formula determination of chemical constituents in traditional Chinese medicines based on accurate mass, A + 1 and A + 2 isotopic peaks using Fourier transform ion cyclotron resonance mass spectrometry

“…All these criteria have been implemented in formula assignment software packages, and other techniques and software packages, such as spectral accuracy and MassWorks, have also been employed to improve the identification of small organic compounds using MS 1,12–17 . However, there is still no standard approach to data processing.…”

“…In recent years, various approaches and software, such as spectral accuracy and MassWorks, respectively, have been used to improve the identification of small organic compounds using MS 1,12–17 . In these approaches, isotope abundance information is the major parameter required for chemical formula determination 18,19 .…”

3D Imaging and metabolomic profiling reveal higher neuroactive kavalactone contents in lateral roots and crown root peels of Piper methysticum (kava)

“…Carbon counting based on spectral accuracy for 12 C, 13 C 1 was carried out for selected kavalactones to characterize the samples [ 34 , 35 ]. The “Isotope Count Heatmap” function in MSiReader was used to plot the estimated carbons for each of the target kava lactones [ 45 ].…”