Andre Venter

Ambient molecular imaging by desorption electrospray ionization mass spectrometry

Desorption electrospray ionization (DESI) allows the direct analysis of ordinary objects or pre-processed samples under ambient conditions. Among other applications, DESI is used to identify and record spatial distributions of lipids and drug molecules in biological tissue sections. This technique does not require sample preparation other than production of microtome tissue slices and does not involve the use of ionization matrices. This greatly simplifies the procedure and prevents the redistribution of analytes during matrix deposition. Images are obtained by continuously moving the sample relative to the DESI sprayer and the inlet of the mass spectrometer. The timing of the protocol depends on the size of the surface to be analyzed and on the desired resolution. Analysis of organ tissue slices at 250 μm resolution typically takes between 30 min and 2 h.

Internal energy distributions in desorption electrospray ionization (DESI).

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the “survival yield” method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7–1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves “droplet pick-up” (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters.

Mechanisms of Real-Time, Proximal Sample Processing during Ambient Ionization Mass Spectrometry

ion. It has also been postulated that compounds can undergo direct Penning ionization with excited plasma species such as helium metastable atoms. Different classes of analytes will preferentially ionize through different mechanisms. For instance, many polar analytes will produce spectra consisting of strictly protonated molecular ions, MH + , indicating they undergo proton-transfer ionization, whereas nonpolar species, such as polycyclic aromatic hydrocarbons (PAHs), yield odd-electron molecular ions, M +· . In addition, sufficiently electronegative molecules, including halogenated or nitrated compounds, preferentially form negative ions through the Page 33 of 61 ACS Paragon Plus Environment Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 34 mechanisms. In general, the observed ions and ionization pathways have been demonstrated to be similar to APCI and atmospheric-pressure photoionization. 163,164 Because these ionization processes rely on the formation of reagent species, differences in ionization with different plasmas can be extrapolated from background reagent-ion spectra. The DART source is an indirect plasma source which also filters any electrons and ions from the stream resulting in mostly lower energy reagent ions such as protonated water clusters, (H2O)nH + , and negative oxygen ions, O2 . 136 It was also shown that the reagent ions from DART could be altered by considerably changing source parameters, such as gas temperature and filter-electrode potentials. 165,166 In comparison, the quasidirect plasmas yield an abundance of higher-energy charge-transfer species, including N2 + , O2 + , and NO3 . 137,138 The higher energy FAPA discharge is capable of producing a significant amount of NO + . 149 The direct plasma methods can be capable of providing even higher energy reagent species, to the point where EI-like fragmentation spectra can

Surface effects and electrochemical cell capacitance in desorption electrospray ionization

Time resolved measurements show that during a desorption electrospray ionization (DESI) experiment, the current initially rises sharply, followed by an exponential decrease to a relatively steady current. When the high voltage on the spray emitter is switched off, the current drops to negative values, suggesting that the direction of current flow in the equivalent DESI circuit is reversed. These data demonstrate that the DESI source behaves as a dc capacitor and that the addition of a surface between the sprayer and the counter electrode in DESI introduces a new electrically active element into the system. The charging and discharging behavior was observed using different surfaces and it could be seen both by making current measurements on a plate at the entrance to the mass spectrometer as well as by measuring ion current in the linear ion trap within the vacuum system of the mass spectrometer. The magnitude of the steady state current obtained without analyte present on the surface is different for different surface materials, and different capacitor time constants of the equivalent RC circuits were calculated for different DESI surfaces. The PTFE surface has by far the greatest time constant and is also able to produce the highest DESI currents. Surface properties play a crucial role in charge transfer during DESI in addition to the effects of the chemical properties of the analyte. It is suggested that surface energy (wettability) is an important factor controlling droplet behavior on the surface. The experimental data are correlated with critical surface tension values of different materials. It is proposed, based on the results presented, that super-hydrophobic materials with extremely high contact angles have the potential to be excellent DESI substrates. It is also demonstrated, using the example of the neurotransmitter dopamine, that the surface charge that develops during a DESI-MS experiment can cause electrochemical oxidation of the analyte.

Surface Sampling by Spray-Desorption Followed by Collection for Chemical Analysis

Desorption electrospray ionization (DESI) directly analyzes soluble chemical components present on surfaces when a pneumatically assisted electrospray is directed at the sample. Here we demonstrate that the same spray desorption mechanism that operates in DESI can be used as a general technique to collect soluble materials present on surfaces. After desorption analytes are collected on a suitable collection surface, large areas can be scanned and collected onto a small collected area, which allows for preconcentration of low abundance material before analysis. This collection surface can then subsequently be analyzed by DESI but also by many other techniques such as gas chromatography-mass spectrometry or UV-vis spectroscopy. In addition this technique can be used to study desorption mechanisms in DESI independently from ionization mechanisms. Preliminary results indicate that the optimized conditions in DESI are a compromise between those conditions that are optimum for desorption and conditions that lead to efficient ionization.

Chapter 8:Spray Desorption Collection and DESI Mechanisms

The real-time in-line microlocalized-desorption sample processing that takes place during ambient ionization are of general analytical use, in addition to their utility during in direct analysis mass spectrometry. By decoupling the microlocalized-desorption sample-processing steps from direct analysis many benefits are realized, such as separate optimization capabilities for desorption and ionization. By using these novel sample-processing steps benefits are also realized over traditional sample-preparation procedures, such as solvent extraction, or swabbing for surface collection. The chapter illustrates how the decoupled desorption procedure can be used to obtain detailed information about the overall mechanism of the ambient ionization methods, illustrated by application to desorption electrospray ionization (DESI), as an example. It is shown that the desorption and ionization aspects of DESI respond differently to changes in operational conditions. This information will help practitioners of ambient ionization to select appropriate conditions for their analyses. In addition, applications of the spray desorption collection (SDC) technique are shown for analyses other than direct mass spectrometry.