Desorption Electrospray Ionization-Mass Spectroscopy Imaging

DESI is an ambient ionization technique used for the mass spectrometric imaging of samples (Garza et al., 2018; Lewis et al., n.d.; Takats, Wiseman, Gologan, & Graham Cooks, 2004). There are different types of ion sources available. DESI ionizes compounds with little sample preparation and without a vacuum system, and it involves spraying electrically charged solvent droplets onto a sample surface, accompanied by a high-velocity gas jet. The generally accepted model for DESI analyte ionization involves analyte dissolving in a thin layer of the DESI solvent before being ejected in secondary droplets by impact from the primary spray. The charged secondary droplets evolve into gas-phase ions similar to the theorized electrospray mechanism due to electrostatic and pneumatic forces. The desorbed gas-phase ions are then sucked into a mass spectrometer for detection through a custom-built electrically charged metal inlet. With a soft-ionization mechanism, DESI can be used to see molecular ions of larger biological molecules as well as multiply charged ions sampled directly from intact biological tissue. DESI could ionize both polar and nonpolar molecules such as alkaloids, peptides, and proteins that are present on varied surfaces including metals, polymers, and minerals. It could be applied for both quantification and qualification of molecules (Araujo et al., 2017; Garza et ah, 2018).

DESI relies on a soft ionization technique, which delivers mass spectra with very low fragmentation in either positive or negative ionization mode. The DESI source can be connected to existing mass spectrometers. It is called DESTMSI. DESI-MSI utilizes the original principle of electrospray ionization, but in this case solvent droplets are desorbed directly on the sample surface. It requires basic sample preparation and allows repeated measurements on samples. Then it results in a simplified analytical procedure for a rapid spatial and temporal identification of chemicals in samples all under ambient conditions (Griffiths, Kocurek, & Cooper, 2018). The workflow of DESI is that high-velocity ionized solvent droplets desorb the analytes directly from the sample surface. Solvents are electro-sprayed under high voltage through an emitter capillary producing charged “primary” droplets, and then directed toward the sample. Metabolites located on the sample surface are desorbed into gaseous “secondary” droplets delivering molecular ions entering the MS inlet where m/z values are measured (Mohana Kumara et al., 2016; Towers, Karancsi, Jones, Pringle, 8c Claude, 2018).

According to the polarity, different spray solvents such as MeOH or ACN with the addition of 2-5% ЕГО can be used for imaging, and different solvent compositions have been employed according to the specific needs and application type (Takats et al., 2005). However, in some cases, ionization efficiency and its selectivity should be increased by using specific ionization-aiding treatments. A reactive reagent should also be added to the spray solvent to selectively improve the ionization efficiency of analytes (Dong et al., 2016). In addition to solvents, salt adducts to solvents have shown to have a high degree of tolerance for formation and ionization suppression (Takats et al., 2005, 2004). The correct adjustment of different parameters is going to affect the analysis and the final imaging resolution.

The most critical parameters are the electrospray solvent flow rate and voltage, and the distance of sample surface to the DESI spray nozzle tip and to the MS-inlet, as well as their respective incident and collection degree angles (a and (3) (Takats et al., 2005,2004).

Sample preparation for DESTMSI may change depending on sample types. Sample preparation from plant tissues is more challenging than those for mammalian tissues (Dong et al., 2016). There is no need for preparation of samples with smooth and regular surfaces, while cryosections can be prepared in non-flat surfaces as freezing. In plants the sample preparation step is different according to sample type. Since flowers and leaves are soft, irregular, and have a very absorbent surface, signal during imaging is low and instable (Schwartz, Reyzer, 8c Caprioli, 2003). The thickness of sample is also important for the imaging process. For mammalian tissues, section thickness between 5 and 20 pm is recommended for the analysis of low molecular weight molecules, and <5pm thickness for high molecular weight proteins (m/z > 9000) (Dong et al., 2016).

DESI-MSI is carried out by directly scanning the unmodified sample in the x- and у-directions through an impinging spray of charged droplets; the chemical information obtained can then be plotted as two-dimensional images recording the abundance of specific ions (Eberlin et al., 2011). DESTMSI contains some information and thus requires intensive analysis for data extraction, visualization, and interpretation. For that reason, advanced software and computational data analysis techniques are needed to extract results from the data. Some software packages are BioMap (Novartis, Basel, Switzerland), Datacube Explorer (FOM-AMOLF, Amsterdam, Netherlands), Fleximaging and

(Continued3)

ClinProTools (Bruker Daltonik, Bremen, Germany), HDI (high-definition MALDI MS imaging) coupled to MassLynx and MarkerLynx (Waters, Manchester UK), ImageQuest (Thermo Scientific, Waltham, MA, USA), MALDIVision (PREMIER Biosoft), Metabolite Imager (University of Texas), MIRION (Justus Liebig University), MSiReader (North Carolina State University), OpenMSI (Lawrence Berkeley National Lab, CA, USA, http:// openmsi.nersc.gov), SCiLS Lab (SCiLS Bremen, Germany), and TissueView (AB Sciex, based on BioMap) (Boughton et al., 2016).

Due to its easy workflow and versatility, DESI-MSI has successfully been applied to many different research fields (Gerbig 8c Takats, 2010). Herein we discuss the present applications of DESI-MSI in food.