Virtual Calibration Quantitative Mass Spectrometry Imaging for Accurately Mapping Analytes across Heterogenous Biotissue

Abstract

It is highly challenging to quantitatively map multiple analytes in biotissues without specific chemical labeling. Quantitative mass spectrometry imaging (QMSI) has this potential but still poses technical issues for its variant ionization efficiency across a complicated, heterogeneous biomatrices. Herein, a self-developed air-flow-assisted desorption electrospray ionization (AFADESI) is introduced to present a proof of concept method, virtual calibration (VC) QMSI. This method screens and utilizes analyte response-related endogenous metabolite ions from each mass spectrum as native internal standards (IS). Through machine-learning-based regression and clustering, tissue-specific ionization variation can be automatically recognized, predicted, and normalized region by region or pixel by pixel. Therefore, the quantity of analytes can be accurately mapped across highly structural biosamples including whole body, kidney, brain, tumor, etc. VC-QMSI has the advantages of simple sample preparation without laborious isotopic IS synthesis, extrapolation for those unknown tissues or regions without previous investigation, and automatic spatial recognition without histological guidance. This strategy is suitable for mass spectrometry imaging using a variety of in situ ionization techniques. It is believed that VC-QMSI has wide applicability for drug candidate's discovery, molecular mechanism elucidation, biomarker validation, and clinical diagnosis.

Recent Advances of Ambient Mass Spectrometry Imaging and Its Applications in Lipid and Metabolite Analysis

“…Analyte response-related endogenous metabolite ions were chosen as native internal standards to solve the difficult problem of evenly adding the IS in tissues. This method was demonstrated to be suitable for accurately mapping the quantity of analytes in the whole body and the linearity of the standard curve calibrated with VC was as good as using isotopic IS (Figure 7H) [140]. (2021) American Chemical Society.…”

“…(H) Standard curves obtained with different calibration methods. Reprinted with permission from the authors of [140]. Copyright (2019) American Chemical Society.…”

Spatially resolved absolute quantitation in thin tissue by mass spectrometry

“…Later, the same group demonstrated that virtual calibration is just as effective as using SIL ISs for pixelby-pixel calibration (Fig. 3) [116]. The presented method utilized analyte response-related endogenous metabolite ions from each mass spectrum as native ISs.…”

“…(h) Final quantitative result by virtual calibration-QMSI which is cross-validated with a previously reported tissue extinction coefficient-QMSI method. Reproduced from reference [116]. Copyright 2019 American Chemical Society extraction efficiency is dependent on sample properties, making quantitation difficult.…”

Natural Products, the Fourth Industrial Revolution, and the Quintuple Helix

“…Historically, quantitation of AMS data has proven to be challenging due to reproducibility, especially the uniform introduction of standards on to tissue surfaces and matrix ion suppression effects, 188,189 although improved techniques have been reported recently. 190,191 For the future, advances in database metabolite identification, the ability to quantify the data, and particularly to view drug distribution and metabolic profile changes over time in a continuous manner, would provide important information regarding catabolic and biosynthetic pathways, as well as the mechanisms and timing of metabolite translocation in plant and animal tissues. Enhancement in spatial resolution requires a corresponding increase in sensitivity.…”