Imaging by Mass Spectrometry

Over the past 10 years, methods have been optimized for the direct analysis of individual cells, groups of cells, and small tissue sections [8-27]. Peptide profiling of complex mixtures by MALDI TOP MS has been performed without previous molecular separation to show differences between cell types and physiological conditions, identify novel peptides, assess post-translational processing, and demonstrate peptide localization within the tissues. Additional experiments have demonstrated sub-cellular protein localization within intact cells [23,28]. Efforts have also been made to characterize bacterial strains based on intact peptide and protein profiles [12,15,18]. Most of this early work involved analysis of peptides and low-molecular-weight proteins, typically from established cell lines. 

Imaging involves collecting MS data in a regular pattern or array across a matrix-coated tissue section (Fig. IB). This approach typically provides a higher resolution molecular picture of the tissue from data collected from hundreds or thousands of pixels across the tissue surface. An image displaying the distribution 

Applications presented in this chapter have employed a Voyager DE-STR MALDl TOP mass spectrometer and a QStar Pulsar i QqTOF mass spectrometer (Applied Biosystems, Foster City, CA). The Voyager instrument utilizes a 337 mn nitrogen laser with a 2.5 ns pulse and a repetition rate of 2 Hz. The instrument was operated in linear mode under delayed extraction conditions. The laser spot size on target is —50 xm in diameter. The QStar Pulsar instrument is equipped with a MALDl source and a 337 mn nitrogen laser operating at a 20 Hz repetition rate. The laser spot size is —200 p,m in diameter on target. 

have also been used to remove salts and other contaminants from the sections before matrix deposition [35,36]. This procedure enhances the protein profile quality by increasing the signal-to-noise ratio for many ions, but it should be used with caution as some molecules may be removed during this process. 

Depending on the experimental goal, different methods for matrix deposition may be applied including depositing discrete large (nL) droplets regionally and coating the entire tissue section with matrix. Fig. 2 presents a visual comparison of three matrix deposition methods (described below in more detail). 

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Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometres), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. 

Code, D. and Volmer, D.A. 2013. Lipid imaging by mass spectrometry—A review. Analyst, 138 1289-1315. 

Code D, Volmer DA. Lipid imaging by mass spectrometry—a review. Analyst. 2013 138 1289-315. 

Ifa DR, Manicke NE, Dill AL, et al. Latent fingerprint chemical imaging by mass spectrometry. Science. 2008 321 805-5. 

Trimpin, S. Herath, T.N. Inutan, E.D. Wager-Miller, J. Kowalski, R Claude, E. Walker, J.M. Mackie, K. Automated solvent-free matrix deposition for tissue imaging by mass spectrometry. Anal. Chem. 2010, 82, 359-367. 

Figure 8. Image and diffraction pattern from an (100) epitaxial. specimen of gold prepared in an unbaked UHV evaporator by depo.sition onto KOI and then transfer onto amorphous carbon. Here water vapour was the dominant residual gas (determined by mass spectrometry). The particles are square pyramidal single crystals.

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

Figure 3.1. Protein expression mapping using 2-D electrophoresis and mass spectrometry. The purpose is to compare protein expression patterns between cell types or in the same cell type under different growth conditions. Proteins are extracted from the different cell types and separated by 2D gel electrophoresis. Image analysis programs are used to compare the spot intensities between gels and identify proteins that are differentially expressed. The protein of interest is excised from the gel and its identity is determined by mass spectrometry. The power of the method increases greatly if the identity of a large number of proteins on the gel is known and present in a database because information can then be obtained without further mass spectrometry.

However, phosphate salts are not volatile. We must constantly remember that mass spectrometry is a gas-phase experiment. Materials to be examined by mass spectrometry must ultimately be made gaseous. Figure 19.14 shows the atmospheric pressure ionization source chamber of a mass spectrometer after infusion of a 20 mM potassium phosphate-containing mobile phase into the instrument for a few hours. The accumulation of phosphate salts on the striker plate is evident. Visual evidence of salt accumulation is also apparent on the back wall of the source chamber, above the striker plate. The overall haziness of the image is not the result of poor photography, but rather due to the coating of dust on the inner walls of the chamber and all surfaces within. 

Enjalbal, C. Maux, D. Combarieu, R. Martinez, J. Aubagnac, J.-L. Imaging Combinatorial Libraries by Mass Spectrometry From Peptide to Organic-Supported Syntheses. J. Comb. Chem. 2003, 5, 102-109. 

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

Altelaar, A. F., van Minnen, J, Jimenez, C. R., Heeren, R. M., and Piersma, S. R. (2005). Direct molecular imaging of Lymnaea stagnalis nervous tissue at subcellular spatial resolution by mass spectrometry. Anal. Chem. 77 735-741. 

Khatib-Shahidi, S., Andersson, M., Herman, J. L., Gillespie, T. A., and Caprioli, R. M. (2006). Direct molecular analysis of whole-body animal tissue sections by imaging MALDI mass spectrometry. Anal. Chem. 78 6448-6456. 

Thiery G, Shchepinov M, Southern E, Audebourg A, Audard V, Terries B, Gut I (2007) Multiplex target protein imaging in tissue sections by mass spectrometry - TAMSIM. Rapid Commun Mass Spectrom 21 823-829. doi 10.1002/rcm.2895... 

Lemaire R, Stauber J, Wisztorski M, Van Camp C, Desmons A, Deschamps M, Proess G, Rudlof I, Woods A, Day R, Salzet M, Fournier I (2007) Tag-mass specific molecular imaging of transcriptome and proteome by mass spectrometry based on photocleavable tag. J Proteome Res 6 2057-2067. doi 10.1021/rp0700044... 

Chaueand, P., Capeioli, R. M. (2002). Direct profiling and imaging of peptides and proteins from mammalian cells and tissue sections by mass spectrometry. Electrophoresis 23, 3125-3135. 

Cellular Metabolites. - A review of methods for the measurement of ml has been produced with 95 references. It examines the quantitative measurement of ml by mass spectrometry and in vivo NMR. The NMR chemical shifts and /-coupling values of 35 metabolites which can be detected by in vivo or in vitro investigations of the mammalian brain have been published. The principles and recent applications of dynamic nuclear polarisation, which combines the sensitivity to oxygen of EPR and the tractability of NMR imaging, have been reviewed with 244 references. A review of studies of intermediary metabolism, including the use of NMR in the analysis of substrate selection under in vivo conditions, has been produced. A review has been produced, with 74 references, on the study of metabolic flux and subcellular transport of metabolites using NMR. " ... 

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