imaging mass spectrometry Essay

Chapter 1
1. Introduction:
1.1 Ambient mass spectrometry
Mass spectrometry is an essential analytical tool in chemistry, biochemistry, pharmacy, medicine and many other related sciences. It is used in structural elucidation of unknown compounds using m/z ratio of fragmented ions. . ‘Ambient mass spectrometry’ allows direct analysis of samples in their native state in an open air environment. This leads to generation of ions and neutrals outside the vacuum system allowing unique experiments to be performed. 2 Conventional mass spectrometry requires many sample pretreatment procedures and involves introduction of the sample into the vacuum environment of mass spectrometer. Therefore, ‘Ambient mass spectrometry’ has a great prospect in the field of analytical chemistry as it involves analysis of real-life objects such as fingertip imprint, 3 a fruit seed, 4 the surface of a leaf, 5 a freshly cut vegetable slice etc. 6 Due to the above characteristics, more than thirty ambient ionization methods have been developed in last eight years. 7 Among these ambient ionization techniques, most number of publications were in the field of desorption electrospray ionization (DESI) and direct analysis in real time (DART). This makes DESI and DART, the two top ambient ionization techniques. 8
1.2 Imaging mass spectrometry (IMS)
Imaging mass spectrometry allows visualization of distribution of chemical compounds by creating 2D images. It does so by collecting mass spectra for each spot of the sample. Unlike traditional molecular imaging techniques, ‘IMS’ does not require labelling. With ‘IMS’, it is also possible to capture snapshots of small molecules such as lipids that is limited with conventional microscope and electronic microscopic techniques. Therefore, ‘IMS’ is a major discovery in the field of medical and clinical applications. Ionization technique such as DESI, secondary ion mass spectrometry (SIMS) and matrix−assisted laser desorption/ionization (MALDI) allows ‘IMS’ experiments to be performed. Unlike MALDI and SIMS, DESI uses ambient pressure for analysis of small molecules in the absence of a crystalline matrix {{1421 Watrous,Jeramie D. 2011;}}.
1.3 Desorption Electrospray Ionization (DESI)
DESI, an ambient ionization method, was introduced by Cooks and coworkers in 2004. 9 Like many other, ambient ionization methods, DESI follows the ESI (Electrospray Ionization) ion production mechanism. 10 Just like ESI, DESI can convert solution phase analyte into gas phase ions at atmospheric pressure. Main difference between DESI and ESI is that DESI can analyze intact samples without sample preparation step.
In 2006, Eberlin and group introduced desorption sonic spray ionization (DeSSI), 11 later renamed as easy ambient sonic spray ionization (EASI) in 2008. 12 DeSSI can produce gaseous ions at atmospheric pressure without requiring high voltage or heat. DeSSI, therefore, is claimed to be a softer technique than ‘ESI’ producing ions with low internal energy and low charge state.13
1.3.1 Principles of ionization
The primary ionization mechanism in DESI is believed to take place via a ‘droplet pick up’ mechanism. 2 This process is initiated by wetting the sampling surface with micron sized droplets from sprayed solvent. 14 These micron sized droplets are charged from the use of high voltage. These charged droplets produce a thin liquid film in which the solid phase analyte must dissolve (Fig. 1.1). Further collision of these primary droplets with the surface produces second generation of progeny droplets containing analyte.

Figure 1.1: Schematic of a typical DESI experiment. The sample solution is dried and deposited on to surface and the solvent is sprayed at a flow rate of typically 1.5 – 10.0 µL/min with application of high voltage (~4 kV). 9
The analyte acquires charge by a fast charge−transfer reaction with protonated solvent molecule during vaporization to gas phase. 15 The second generation droplets produce an ejection cone

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