Analyte ion formation

Two models for analyte ion formation have been proposed. The older model -which had not had a well-defined name before 2013 and is now proclaimed as Coupled Physical and Chemical Dynamics (CPCD) model - assumes neutral analyte molecules in the expanding plume - regardless of whether the analytes were incorporated in the matrix crystals as neutral species or were quantitatively neutralized by their counterions upon cluster dissociation in the case of precharged incorporated analyte molecules. Subsequent to photoionization of the matrix (Eqs 1.3 and 1.4) and secondary intermolecular matrix reactions leading to the generation of protonated as well as deprotonated matrix ions (Eqs 1.5 and 1.6)... 

Fig. 7.8. PTR mass spectrometer. The ion source of the lonicon PTR-TOF 2000 is divided into a compartment for reactant ion generation from H2O by (1) a hollow cathode and (2) a source drift region in front of the sample inlet, and (3) the drift tube for analyte ion formation. The TOF analyzer has its own turbomolecular pump to maintain high vacuum not shown). By courtesy of lonicon Analytik GmbH, Innsbrack, Austria...

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

APCI. The column effluent is nebulised into an atmospheric-pressure ion source. Through a corona discharge, electrons initiate the reactant gas-mediated ionisation of the analytes. Proton transfers are typical reactions generating [M + H]+ or [M — H] ions, although radical ion formation is possible as in high vacuum chemical ionisation (Cl). The ions formed are injected into the high vacuum of the mass spectrometer. APCI typically accepts flow rates of up to 2 mL min-1. 

Analytes of very high polarity are not anymore ionized by field ionization. Here, the prevailing pathways are protonation or cationization, i.e., the attachment of alkali ions to molecules. [78] The subsequent desorption of the ions from the surface is effected by the action of the electric field. As [M-t-Na]" and [M-i-K] quasi-molecular ions are already present in the condensed phase, the field strength required for their desorption is lower than that for field ionization or field-induced [Mh-H]" ion formation. [37,79] The desorption of ions is also effective in case of ionic analytes. 

Schnlten, H.-R. Ion Formation From Organic Solids Analytical Applications of Field Desorption Mass Spectrometry, Springer Series in Chemical Physics [25], Benninghoven, A., editor Springer-Verlag Heidelheig, 1983 pp. 14-29. 

The analyte may be neutral or ionic. Solutions containing metal salts, e.g., from buffers or excess of noncomplexed metals, may cause a confusingly large number of signals due to multiple proton/metal exchange and adduct ion formation. [91] The mass range up to 3000 u is easily covered by FAB, samples reaching up to about twice that mass still may work if sufficient solubility and some ease of ionization are combined. 

The elder model of ion formation, the charged-residue model (CRM), assumes the complete desolvation of ions by successive loss of all solvent molecules from droplets that are sufficiently small to contain just one analyte molecule in the end of a cascade of Coulomb fissions. [9,42,84] The charges (protons) of this ultimate droplet are then transferred onto the molecule. This would allow that even large protein molecules can form singly charged ions, and indeed, CRM is supported by this fact. [23]... 

Cole, R.B. Harrata, A.K. Solvent Effect on Analyte Charge State, Signal Intensity, and Stability in Negative Ion ESI-MS Implications for the Mechanism of Negative Ion Formation. J. Am. Soc. Mass Spectrom. 1993,4,546-556. 

Electroanalytical chemists and others are concerned not only with the application of new and classical techniques to analytical problems, but also with the fundamental theoretical principles upon which these techniques are based. Electroanalytical techniques are proving useful in such diverse fields as electro-organic synthesis, fuel cell studies, and radical ion formation, as well as with such problems as the kinetics and mechanisms of electrode reactions, and the effects of electrode surface phenomena, adsorption, and the electrical double layer on electrode reactions. 

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