Ambient mass spectrometry in metabolomics

Abstract

Since the introduction of desorption electrospray ionization (DESI) mass spectrometry (MS), ambient MS methods have seen increased use in a variety of fields from health to food science. Increasing its popularity in metabolomics, ambient MS offers limited sample preparation, rapid and direct analysis of liquids, solids, and gases, in situ and in vivo analysis, and imaging. The metabolome consists of a constantly changing collection of small (<1.5 kDa) molecules. These include endogenous molecules that are part of primary metabolism pathways, secondary metabolites with specific functions such as signaling, chemicals incorporated in the diet or resulting from environmental exposures, and metabolites associated with the microbiome. Characterization of the responsive changes of this molecule cohort is the principal goal of any metabolomics study. With adjustments to experimental parameters, metabolites with a range of chemical and physical properties can be selectively desorbed and ionized and subsequently analyzed with increased speed and sensitivity. This review covers the broad applications of a variety of ambient MS techniques in four primary fields in which metabolomics is commonly employed.

Figure 1

MS-based untargeted and targeted metabolomics workflows.

Figure 2

Endoscopy experimental setup. A) The polypectomy snare was equipped with an additional T-piece in order to establish direct connection between the electrode tip and the mass spectrometer for the transfer of electrosurgical aerosol. B) Resection of GI polyps by using a commercial snare. The polyp is captured with the snare loop, which is tightly fastened around its base. Electrosurgical dissection is performed and the generated aerosol is aspirated through the fenestrations created on the plastic sheath of the snare. Reproduced from Ref. with permission from John Wiley and Sons ©2015.

Figure 3

A) Mass spectra from gastric mucosa, gastric submucosa, and adenocarcinoma tissue recorded ex vivo using a modified Xevo G2-S Q-ToF mass spectrometer. Cancerous and healthy mucosal tissues feature mainly phospholipids in the m/z 600–900 region, whilst submucosa features triglyceride and phosphatidylinositol species in the m/z 850–1000 region. B) Comparison of the abundance of selected peaks showing significant differences between cancerous and healthy tissues in the range m/z 600–1000 using Kruskal–Wallis ANOVA, p<0.005. T=tumor, M=mucosa, S=submucosa. Reproduced from Ref. with permission from John Wiley and Sons ©2015.

Figure 4

NHBA content in the rat brainstem and whole-body distribution. (a) NHBA content in the brainstem of rats euthanized at various time points after dosing, as measured by quantitative LC-MS/MS in MRM mode. (b) Whole-body distribution of NHBA (40 mg/kg via intraperitoneal injection, followed by euthanasia 20 min later) acquired by AFADESI-MSI (MRM, m/z 374.2 → 242.0). Organ regions are outlined. Spatial resolution = 300 μm × 500 μm. ((Top panel) HE-stained, whole-body rat tissue section at 20 min after NHBA administration; (middle panel) optical image of rat tissue section; and (bottom panel) MSI image of rat tissue section. Reproduced from Ref. with permission from the American Chemical Society ©2015.

Figure 5

(A) Schematic of sampling from culture or patient throat swab and subsequent analysis by TS-MS. (B) Negative ionization mode TS-MS spectra of a single colony of S. pyogenes sampled from culture. Peaks of negative relative abundance, after subtraction, and those annotated by asterisks are attributable to background. The other peaks with positive relative abundance are tentatively identified as bacterial phospholipids. Reproduced from Ref. with permission from the Royal Society of Chemistry ©2014.

Figure 6

Analysis of microbial colonies directly from petri dishes using nanoDESI. Reproduced from Ref. with permission from the National Academy of Sciences ©2012.

Figure 7

(A) Positive-ion LAESI mass spectra from n = 6 to 8 oil gland cells (pooled from the ablation of the center of two glands) of a C. aurantium leaf (red trace on top) and n = 6 to 8 cells from the leaf away from the gland (pooled from two ablation spots) (black trace in the bottom). The inset shows a microscope image of an oil gland with the ablation mark (scale bar is 50 μm). The ablated spot is ∼30 μm in diameter. (B) S-plot produced by OPLS-DA of the spectra showed that many metabolites strongly correlated with either the oil gland cells (n = ∼25) or cells in the leaf away from the gland (n = ∼25). The 10 metabolites with serial numbers (SN) (solid squares) indicated in the figure are identified in Table S4 of the Supporting Information. Reproduced from Ref. with permission from the American Chemical Society ©2011.