Matrix plume, MALDI ionization

MALDI ionization is the least understood of the three largely becanse the ionization mechanism is a combination of both a surface ionization process and gas-phase reactions in the ablation plume. Components of the sample that inhibit the formation of crystals decrease the energy transfer at the surface and reduce the ionization efficiency. Reactions of ions and neutrals in the gas phase will also determine the final ion population, similar to APCI. At this point, it is important to distinguish endogenous sample matrix from MALDI ionization matrix. Ionization matrix is the material used to form the crystals that absorb and transfer the photon energy. Endogenous sample matrix is the biological material in a sample. 

How Analyte Ions Are Formed Several diverse views have been expressed on this topic [33-38]. No single model provides a complete picture of MALDI ionization. The widely accepted current view is that the analyte ionization is a two-step process a primary ionization event, followed by in-plume secondary ion-molecule reactions [34-36]. In the first step, reactive matrix ionic species are generated. The analyte ions are produced in the expanding gas plume (an area just above the matrix surface) via extensive secondary ion-molecule charge-transfer reactions between the primary matrix ions and neutral analyte molecules ... 

Knochenmuss, R. and Zhigilei, LV. (2010) Molecular dynamics simulations of MALDI laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization. /. Mass Spectrom., 45, 333-345. 

A number of detailed models for the mechanisms for MALDI ionization have been described. The homogeneous bottleneck mechanism attempts to explain the vibrational mismatch between the matrix and analyte that leads to relatively cool (low internal energy) analyte molecules, while the cool plume or hydrodynamic modeF focuses on the actual expansion into the gas phase. Experimentally, Cotter and Spengler determined the initial kinetic energy distributions of molecular and fragment... 

Various reviews and summary articles have addressed MALDI ionization. Some relevant concepts pre-date the advent of modem MALDI see, for example, Hillenkamp s discussion of LDI in 1983. Significant early MALDI reviews addressed a variety of possible mechanisms. Ehring, Karas, and HUlenkamp elaborated three excitation schemes leading to seven ion types. The key intermediate step for all involved highly excited matrix. Liao and Allison contributed an extensive discussion of protonated and sodiated adduct ions. This work emphasized the central role of ion-molecule reactions in the MALDI desorption plume. The consequent relevance of gas-phase thermodynamics to observed mass spectra has remained a central theme ever since. 

It is important to examine the energetics of MALDI ion generation. Breaking a covalent C-H or 0-H bond to yield and H" requires about 14 eV or 1350kJ/mol. An example is the 0-H bond energy in phenol 14.65 eV. At 355 nm (tripled Nd YAG laser), this would be a 4.2-photon process and hence is not very probable. However, MALDI ionization occurs in the condensed phase or dense plume. Neutral matrix is present in abundance, allowing protonated matrix to be readily formed in the above phenol example (m = neutral matrix). The total ionization process then requires only around 5eV or 480kJ/mol ... 

Perhaps unfortunately, ESPT does not seem to be active with the common matrices. This is because ESPT is highly dependent on efficient charge stabilization by the local environment, even if less so than ground-state ionization. Well-investigated ESPT systems tend to be active only in water or amine environments. Again the matrix plume is not likely to be sufficiently favorable. ESPT activity has also not been independently demonstrated for any MALDI matrix in any environment. For example, indicators of ESPT activity such as characteristic fluorescence shifts were not found for DHB in either solution or clusters. But it need not be the matrix that is ESPT active in MALDI. Other, more strongly active substances can be added to the sample, but this approach has so far been unsuccessful. ... 

To couple a low-pressure MALDI source to a QIT, LQIT, or orbitrap mass analyzer, the primary concern other than having an efficient MALDI source is to be able to trap the gas-phase ions efficiently inside the analyzer. In the low-pressure MALDI experiment, ions can be formed inside the ion trap or they can be generated outside the device and injected into the analyzer. Ions are lifted by the matrix plume at a velocity of 500 m/s with several electron volts of energy and may undergo additional reactions in the plume. Once in the gas phase, the ions are accelerated as a result of either (a) an RF field in the case of internal ionization or (b) an external lens system for the external ion formation/injection scheme. Ions can be trapped efficiently in a QIT or LQIT if they have low kinetic energies (typically <20 eV), but the trap must be set to the appropriate RF trapping potentials and filled with helium buffer gas. In two similar configurations from 1993, the QIT was... 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Various molecular and quasi-molecular ions can be formed under MALDI conditions. The formation of protonated analyte (A) molecules, [A -F H]+, is generally most important at least for samples containing slightly basic centres, such as the peptides and proteins, MALDI mass spectrometry of which is known to be most facile and reproducible. Therefore, proton transfer from the electronically excited, neutral or ionized, or protonated matrix species is considered to be crucial in the overall MALDI process . Notably, proton transfer can occur already in the condensed phase, followed by desorption of the preformed ions . However, the generation of the [A -F H]+ ions is believed to take place preferably in the so-called plume , that is, in the energized, short-hved and relatively dense vapour phase generated above the solid matrix upon excitation by the laser pulse. The actual proton donor species (be it one or several) in a given case is still a matter of... 

From the point of view of the present book, some understanding of the mechanism(s) underlying MALDI is important because of effects of ionization suppression (and possibly enhancement, though the latter has not been widely observed in MALDI) that must be taken into account if MALDI is to be used for quantitation an extended discussion of suppression and enhancement effects is given in Section 5.3.6a for the case of electrospray ionization. MALDI suppression effects have been extensively investigated (Knochenmuss 1996,1998, 2000, 2003) and correlated with the detailed theory of in-plume reactions controlled largely by thermochemical considerations. Examples of the suppression of matrix ions by relatively large amounts of analyte are shown in... 

Although the mechanism of the formation of the MALDI ion plume is not completely understood, it is thought to involve absorption of the laser beam by the matrix, followed by transfer of the energy from the matrix to the analyte. Desorption of the analyte and the matrix then occuns. The analyte is thought to desorb as neutral molecules and then to be ionized by proton-transfer reactions with protonated matrix ions in a dense phase over the surface containing the ma-lri,x. A series of photochemical reactions may produce the protonated matrix ions. 

MALDI is another soft ionization technique used in MS. MALDI involves a two-step process. First, the firing of an ultraviolet (UV) laser beam induces desorption. In this process, matrix material absorbs the UV laser energy in firing, leading to the ablation of the upper layer ( 1 pm) of the matrix material. The hot plume produced during ablation contains many species neutral or ionized matrix molecules, protonated or deprotonated matrix molecules, and matrix clusters (even nanodroplets). In the... 

MALDESI Matrix-assisted laser desorption electrospray ionization Uptake of AP-MALDI plume by ESI spray and transport into API interface [51]... 

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