Mass spectrometry imaging under ambient conditions

Abstract

Mass spectrometry imaging (MSI) has emerged as an important tool in the last decade and it is beginning to show potential to provide new information in many fields owing to its unique ability to acquire molecularly specific images and to provide multiplexed information, without the need for labeling or staining. In MSI, the chemical identity of molecules present on a surface is investigated as a function of spatial distribution. In addition to now standard methods involving MSI in vacuum, recently developed ambient ionization techniques allow MSI to be performed under atmospheric pressure on untreated samples outside the mass spectrometer. Here we review recent developments and applications of MSI emphasizing the ambient ionization techniques of desorption electrospray ionization (DESI), laser ablation electrospray ionization (LAESI), probe electrospray ionization (PESI), desorption atmospheric pressure photoionization (DAPPI), femtosecond laser desorption ionization (fs-LDI), laser electrospray mass spectrometry (LEMS), infrared laser ablation metastable-induced chemical ionization (IR-LAMICI), liquid microjunction surface sampling probe mass spectrometry (LMJ-SSP MS), nanospray desorption electrospray ionization (nano-DESI), and plasma sources such as the low temperature plasma (LTP) probe and laser ablation coupled to flowing atmospheric-pressure afterglow (LA-FAPA). Included are discussions of some of the features of ambient MSI for example the ability to implement chemical reactions with the goal of providing high abundance ions characteristic of specific compounds of interest and the use of tandem mass spectrometry to either map the distribution of targeted molecules with high specificity or to provide additional MS information on the structural identification of compounds. We also describe the role of bioinformatics in acquiring and interpreting the chemical and spatial information obtained through MSI, especially in biological applications for tissue diagnostic purposes. Finally, we discuss the challenges in ambient MSI and include perspectives on the future of the field.

Figure 1

MSI imaging concepts and methods (a) In a typical MSI experiment the total area is subdivided (conceptually) into pixels that are individually inspected. (b) For each pixel a single mass spectrum or the average of several mass spectra is collected and stored together with its spatial coordinates. (c) After the entire surface is scanned, an average mass spectrum can be created. The distribution of specific ions can be visualized by the creation of chemical images where the color scale (false color) represents the normalized intensity of particular ions. Each pixel from the image is associated with the original mass spectrum/mass spectra acquired at the specific point. The numbers 1 and 2 on panel A, represent the steps desorption and ionization process. (d) The aim of imaging is to display the distribution of chemicals across a surface.

Figure 2

DESI imaging concepts and methods (a) DESI schematic; (b) Sequential analysis of the same coronal section of rat brain by different MS and chemical staining techniques (c) Optical image of (top) handwritten text on paper (. Chemical images of (middle) rhodamine 6G monitored by its fragment m/z 415 (neutral loss of ethylene), (bottom) by its fragment m/z 399 (neutral loss of CO₂). Panels a, b, and c are reproduced with permissions from (Ifa, et al., 2010), (Eberlin, et al., 2011), and (Ifa, et al., 2009), respectively.

Figure 3

DESI-MS images of various types of tissue (a) Negative ion mode tissue imaging of human bladder tissues including areas of cancer and adjacent normal tissue (i) Ion image of m/z 788.7, PS(18:0:18:1) (ii) Ion image of m/z 885.7, PI(18:0/20:4) (iii) H&E stained tissue sections of the tumor and normal tissues. (iv) Ion image of m/z 281.5, FA(18:1) (v) Ion image of m/z 563.5, FA dimer (vi) PCA-developed images, false color plot of PC1, PC2, and PC3. (b) Two types of distribution patterns of epinephrine and norepinephrine cells in the adrenal medullae of porcine and rabbit adrenal gland, examined using immunohistochemistry. Dotted (gray) area shows adrenal cortex, and white and black areas show epinephrine (E) producing cells and norepinephrine (NE) producing cells regions in the adrenal medulla. The distributions of epinephrine and norepinephrine in the adrenal gland of the porcine and rabbit obtained with DESI imaging are compared with the patterns obtained with immunohistochemistry. The white dashed circle represents the border of rabbit adrenal gland. The distribution patterns of A and NA cells obtained from using immunohistochemistry are adapted from (Suzuki & Kachi, 1996). (c) 3D models of the mouse brain showing the distribution of PS 18:0/22:6 (in green) and ST 24:1 (in red). Length scale bars correspond to 2 mm. Panels b and c are reproduced with permissions from (Wu, et al., 2010) and (Eberlin, et al., 2010) respectively.

Figure 4

LAESI MS imaging (a) Schematic diagram of the LAESI instrumentation. (C, capillary; SP, syringe pump; HV, high-voltage power supply; L-N2, nitrogen laser; M, mirrors; FL, focusing lenses; CV, cuvette; CCD, CCD camera with short-distance microscope; CE, counter electrode; OSC, digital oscilloscope; SH, sample holder; L-Er:YAG, Er:YAG laser; MS, mass spectrometer; PC-1 to PC-3, personal computers). (b) Variegated A. squarrosa leaf probed with LAESI-MS while rastering the surface with a focused infrared laser beam over three adjacent 4 mm by 4 mm areas. The live tissue surface was ablated from a circular area (350-μm diameter) with a 400-μm step size. (c) Contour plots show the distribution of ions detected at m/z 663 and m/z 493, respectively. (d) Mass spectra obtained at different regions of the leaf (top: green sectors, bottom: yellow sectors). Length scale bars correspond to 1 mm. Panel a is reproduced from (Nemes & Vertes, 2007) with permission. Panels b–d are reproduced from (Nemes, et al., 2008) with permissions.

Figure 5

(a) Schematic of the atmospheric pressure femtosecond laser desorption ionization imaging mass spectrometer (AP fs-LDI IMS). (b) Optical image of onion epidermis cells and ion images showing the distribution of deprotonated glucose (m/z 179.05) and triiodide (m/z 380.7). (c) Optical image of a “S” character dye pattern recorded under ambient conditions. Negative ion images of the distribution of triiodide (m/z 380.7), and citrate (m/z 191.1), are also shown. (d) Negative ion image of the distribution of m/z 179 generated by probing the onion epidermis tissue at a spatial resolution of 10 μm. The inset shows the corresponding optical image. (e) Schematic of the atmospheric pressure femtosecond laser electrospray ionization mass spectrometry (LEMS) setup. (f) Positive ion LEMS image of the distribution of oxycodone (m/z 316) on a metal slide. Panels a–d are reproduced from (Coello, et al., 2010)with permissions. Panels e and f are reproduced with permissions from (Brady, et al., 2009) and (Judge, et al., 2010), respectively.

Figure 6

IR-LAMICI and LTP MS imaging experiments. (a) Schematic diagram of the IR-LAMICI. (b) IR-LAMICI MS analysis of a Tylenol tablet, with the insets showing false-color scale IR-LAMICI MS images of the distribution of acetaminophen monomer (m/z 152) and dimer (m/z 303) ions on the tablet. (c) Analysis of calligraphy patterns using LTP-IMS. (d) Expanded view of the LTP probe. (e) Chemical image of the inkpad of seals on rice paper recorded using a LTP probe. Panels a and b are reproduced from (Galhena, et al., 2010) with permissions. Panels c–e are reproduced from (Liu, et al., 2010) with permissions.

Figure 7

DAPPI and DESI imaging data and methods (a) Schematic diagram of the DAPPI setup. (b) DESI-MS spectrum of mouse brain tissue section obtained from a single point using an average of three scans acquired on an high-resolution mass spectrometer. Detail of the m/z 700–900 region with dominant phospholipid peaks. Inset: Overall spectrum of the whole m/z 250–1800 region. (c) Two-dimensional distributions of selected lipid-originating ions in mouse brain tissue section, m/z 760.5850 (red) and m/z 810.6006 (green). (d) Distribution of an ion at m/z 430.3810 (red) on sage leaf and combined distributions of peaks at m/z 301.2166 (green) and m/z 315.0863 (red). Whereas m/z 301 is located more on the side of the leaf, m/z 315 is centered. The inset shows the part of the leaf that was imaged. Panels a is reproduced from (Haapala, et al., 2007) with permission. Panels b–d are reproduced from (Pol, et al., 2009) with permissions.

Figure 8

PESI-MS imaging (a) Simplified schematic of imaging PESI-MS system consisting of a PESI ion source, an auxiliary heated capillary sprayer, and a three-axis precision sample stage. P_I = ionization position and P_S = sampling position. b) Microscopic images of the mouse brain section taken (i) before the measurement, (ii) during the raster scan of the imaging PESI-MS, and (iii) after the completion of the measurement and (iv) close-up inspection of the holes made by the solid needle. (c) PESI ion images of the mouse brain section for different ions. (d) Positive ion mode PESI mass spectra obtained from two sampling spots from a rat brain. All panels are reproduced from (Chen, et al., 2009) with permissions.

Figure 9

(a) Schematic of LMJ-SSP/ESI. (b) Liquid microjunction formed between the probe and the surface of the sample plate. (c) Rat liver section with reserpine (+ sign), chemical image shown in the inset. (d) Laser ablation and capture of the ablated material by the liquid meniscus of the probe. (e) Chemical image of a fingerprint blotted on glass with ink. The blue circles show the regions of interest with an apparent resolution of 100μm. Panels a and c are reproduced from (Van Berkel, et al., 2008), panel d and e are reproduced from (Ovchinnikova, et al., 2011), and panel b is reproduced from (Van Berkel et al., 2009) with permission.