DESI-MS Imaging

an ionization technique developed by Cooks and coworkers [88] can be used for IMS analysis under atmospheric pressure with minimal sample preparation [89] (see Section 2.5.2). Eor DESI imaging, the sample is either placed onto a target (e.g., microscope glass slide) or analyzed in situ [12]. A lateral resolution of 40 pm could be achieved by DESI [90]. 

In typical DESI imaging experiments, the collected tissue sample is flash frozen in liquid nitrogen and subsequently cut in micron thin sections using a cryostat-microtome the thin tissue slices are thaw mounted onto glass microscope slides for analysis the ions generated by the DESI (Chapter 2) are transported fi om surfaces to the gas phase for mass analysis as the surface is being moved in order to cover the entire sample area a mass spectrum is acquired for each pixel on the surface and finally, tissue images are constructed to display the spatial intensity distribution of individual selected lipid ion. 

A list of studies of IMS of lipids by DESI-MS can be found in a recent review article [18]. Many of the studies by DESI-IMS have focused on identifying the differences of lipid distributions of tissue samples between disease and health states in an attempt to discover lipid biomarkers for disease [41, 91-96]. It indicates that most of the major lipid classes have been successfully detected by DESI. All lipid classes can be detected as intact ions with little fi agmentation except cholesterol, which is detected as the [M-I-H-H20] ion as usual. In comparison to MALDI-IMS [97], DESI-IMS of human lens [98] indicated that DESI allows detecting a wider variety of lipid classes in a single acquisition under typical conditions. For example, MALDI-IMS only detected SM, Cer, and cholesterol species, whereas DESI-IMS detected SM, Cer, ceramide-1-phosphate, PE, lysoPE, PS, LacCer, and cholesterol. 

The ability to use the same spray solution for analysis of lipid species in both positive-and negative-ion modes is an advantage of DESI for imaging with aid of a modifier in comparison to MALDI-IMS, which often needs different matrices for optimal desorption/ionization of different lipids. For example, in the presence of sodium or ammonium salts, TAG, PC, and SM species can be readily detected in the positive-ion mode [99], whereas PC and SM as well as other anionic lipids can be detected in the negative-ion mode [100]. 

A drawback of DESI-IMS analysis of lipids in tissue sections in comparison to that of MALDI or SIMS is its tissue damage to a much larger degree. This consequence makes the tissue samples after DESI analysis difficult to acquire the complementary information such as histological data for direct comparison between data sets. To resolve this issue, Eberlin et al. developed a method [29,104] by using binary solvent 

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An example of DESI MS imaging of pharmaceutical components is presented in Fig. 1.5 (67). Lung tissues were collected from animals 30 min after dosing with clozapine. A tissue section was imaged using DESI MS across the w/z range of 200-1,100 to evaluate the sample for clozapine distribution and its metabolites. A variety of lipids were detected as well as clozapine and its 27-desmethyl metabolite. This analysis and the results from LC-MS/MS analysis performed in a complementary study confirmed the presence of clozapine in the lung. 

Fig. 1.5. DESI imaging of a dmg in lung tissue, (a) Optical image, (b) Image of clozapine at m/z 327.1. (c) Image of desmethyldozapine at m/z 313.1. (d) Image of sodiated PC 16 0/16 0 at miz 756.4. (e) DESI-MS imaging and LC/MS/MS results. The signal response In each method was normalized to the maximum response in each experiment and plotted against the clozapine plasma concentration as determined by LC/MS. (Reprinted with permission from ref. (67).)...

PasiUs, S. P, Kertesz, V, Van Berkel, G. J., Schulz, M., Schorcht, S. (2008) HPTLC/DESI-MS imaging of tryptic protein digests separated in two dimensions. J Mass Spectrom, 43, 1627-1635. 

Hemalatha RG, Pradeep T. Understanding the molecular signatures in leaves and flowers by desorption eleetrospray ionization mass spectrometry (DESI-MS) imaging. J Agric Food Chem. 2013 61 7477-87. 

DESI has also been introduced into MS imaging. [39] SIMS is also used for surface imaging and depth profiling. [40]... 

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

FIGURE 15.14 Tissue imaging using DESI—MS/MS and autoradiography in mice dosed with propranolol, (a) Scanned optical image of a 40-pm-thick sagittal whole-body tissue section of a mouse dosed intravenously with 7.5 mg/kg propranolol and euthanized 60 min after dose, (b) Distribution of propranolol in the 94 mm X 30 mm tissue section presented in (a) measured by DESI—MS/MS. [Reprinted from Kertesz et al. (2008) with permission of American Chemical Society.]... 

Today, M S imaging is perceived as a rather new - but very promising - approach, not only for the basic sciences but also for clinical diagnosis [156]. Although other MS imaging methods based on desorption electrospray-ionization (DESI) MS [157] or secondaryion mas s spectrometry (SIMS)are currently available (for a timely review, see Ref. [158]), attention here will be focused exclusively on MALDI-MS imaging. The same technique is also discussed comprehensively in Chapter 4 of this book. 

Summarizing, DESI due to its ability to work under ambient conditions allows for convenient connection of simple and low-cost TLC separation and highly specific MS analysis. Of the currently popular MS imaging techniques, DESI seems to be the most comprehensive. This type of ion source does not cause extensive fragmentation of the sample as in the case of SIMS. Additionally, since there is no need to use any kind of matrix or high vacuum, as in the case of MALDI, the molecules of low molecular weight, such as drugs and their metabolites, may be easily analyzed. 

Desorption electrospray ionization (DESI) may serve as an example of the maiy atmospheric-pressure surface ionization technique that has recently been introduced [63, 76]. In DESI, the high-velocity spray of charged microdroplets from a (pneumatically assisted) electrospray needle is directed at a surface, which is mounted in front of the ion-sampling orifice of an API source (see Fig. 7.6). Surface constituents are released fiom the surface and ionized. These gas-phase ions can be introduced to and observed by MS [77]. In this way, DESI-MS enables for instance the analysis of dmgs in tablets or natural products in plant parts withont extensive sample pre-treatment or prior separation. In addition, DESI-MS and some of its related snrface ionization techniqnes enable chemical imaging of surfaces such as thin-layer chromatography (TLC) plates and tissue sections [78]. 

Several studies have combined results from different MSI approaches or to integrate them with other, non-MS-based imaging techniques. These multimodal studies have reported the successful combination of DESI-MALDI, SIMS-MALDI, and MALDI-MSI/MRI results (92-94). Three-dimensional analysis of tissues or similar samples (cell cultures or microbial colonies) is also getting increased attention (Figure 4B) (95). 

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