Preformed ions

If the liquid that is being bombarded contains ions, then some of these will be ejected from the liquid and can be measured by the mass spectrometer. This is an important but not the only means by which ions appear in a FAB or LSIMS spectrum. Momentum transfer of preformed ions in solution can be used to enhance ion yield, as by addition of acid to an amine to give an ammonium species (Figure 4.3). 

When buffer is present in the mobile phase and the analytes are largely ionic in nature or give preformed ions in solution, the ion-evaporation mechanism is... 

FAB-MS has been used for the analysis of lubricant additives, thermally labile or involatile organic compounds, such as macromolecules and dyes, and inorganic compounds. Cationic dyes and dye intermediates, which are typically acid salts, readily yield preformed ions in the FAB matrix solution. They are also very difficult to address by other MS ionisation methods due to their involatility. Lay and Chang [85] used positive ion FAB to characterise a mixture of amine and ketimine cross-linking agents for polymer coatings. Bentz et al. 

Fig. 11.5. Diagram illustrating the components of an ESI source. A solution from a pump or the eluent from an HPLC is introduced through a narrow gage needle (approximately 150 pm i.d.). The voltage differential (4-5 kV) between the needle and the counter electrode causes the solution to form a fine spray of small charged droplets. At elevated flow rates (greater than a few pl/min up to 1 ml/min), the formation of droplets is assisted by a high velocity flow of N2 (pneumatically assisted ESI). Once formed, the droplets diminish in size due to evaporative processes and droplet fission resulting from coulombic repulsion (the so-called coulombic explosions ). The preformed ions in the droplets remain after complete evaporation of the solvent or are ejected from the droplet surface (ion evaporation) by the same forces of coulombic repulsion that cause droplet fission. The ions are transformed into the vacuum envelope of the instrument and to the mass analyzer(s) through the heated transfer tube, one or more skimmers and a series of lenses.

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

Provided the salt is sufficiently soluble in the matrix, the signals normally exhibit high intensity as compared to those of the matrix. This result is consistent with the model of preformed ions in solution that only need to be desorbed into the gas phase. 

Musselman, B. Watson, J.T. Chang, C.K. Direct Evidence for Preformed Ions of Porphyrins in die Solvent Matrix for FAB-MS. Org. Mass Spectrom. 1986, 27, 215-219. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

More recent work revealed the importance of gas phase proton transfer reactions. [91-94] This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas phase ion-ion reactions (Chap. 11.4.4). The proton exchange is driven by the difference in proton affinities (PA, Chap. 2.11) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

The DLI interface became the second commercially available LC/MS interface in 1981. The liquid eluent is introduced into the ion source through a capillary or a pinhole diaphragm. As the name implies, the analyte in DLI LC/MS is introduced from solution into the MS ion source.When sufficient energy is given to the solution, the preformed ions in solution such as protonated molecules, deprotonated molecules, cationized molecules and solvated ions can be desorbed into a mass spectrometer while the bulk solvent is vaporized and eliminated by the vacuum system. 

ESI has become the most commonly used interface for LC/MS. It was recognized by John Fenn and co-workers as an important interface for LC/MS immediately after they developed it as an ionization technique for MS. ESI transforms ions in solution to ions in the gas phase and may be used to analyze any polar molecule that makes a preformed ion in solution. The technique has facilitated the ionization of heat-labile compounds and high-molecular-weight molecules such as proteins and peptides. ESI is a continuous ionization method that is particularly suitable for use as an interface with FiPLC. It is the most widely accepted soft-ionization technique for the determination of molecular weights of a wide variety of analytes and, has made a significant impact on drug discovery and development since the late 1980s. 

Peroxodisulfate oxidation of 3,14-dimethyl-4,7,10,13-tetraazadeca-3,13-diene-2,5-dionedioxima-toiron(II) proceeds by electron transfer within a preformed ion pair. " The electrochemistry of [Fe (dmgH)2(imH)2], with the kinetics of its autooxidation (catalyzed by Cu but inhibited by... 

Figure 12-15 Schematic drawing of the active site of a cysteine protease of the papain family with a partial structure of an acyl-enzyme intermediate in green. The thiolate-imidazolium pair of Cys 25 His 159 lies deep in the substrate-binding cleft and bridges an interface between two major structural domains, just as the Ser His pair does in serine proteases (Fig. 12-10). This may facilitate small conformational changes during the catalytic cycle. Asn 175 provides a polarizable acceptor for positive charge, helping to stabilize the preformed ion pair, and allows easy transfer of an imidazolium proton to the product of substrate cleavage. The peptide NH of Cys 25 and the side chain of Gin 19 form an oxyanion hole.

From the XPS data it is not clear if the protonated amine is due to a bicarbonate salt (reaction with moist ambient C02), as suggested by some authors [26, 27], or perhaps due to protonation achieved via surface hydroxyl-groups (Fig. 1(e)). A bicarbonate, if present, represents a rather favorable preformed ion and should lead to a sizable CO, or HCOf ion signal in the negative SSIMS spectrum [25]. While this peak would interfere with the Si02 ion at mlz = 60, or the Si02H ion at mlz = 61, the lack of any sensitivity to surface temperature for these peaks for any of the samples studied, as well as excellent exact mass fits for both peaks, appears to indicate that a bicarbonate is not present. 

Experimentally, the compound consisting of the preformed ion and its counter ion (such as barium perrhenate for perrhenate emission) does not produce ions when heated—instead, only neutral species sublime. It must be embedded in a suitable matrix and then heated. The limited experimental evidence collected to date indicates that the ion to be emitted must have significantly greater mobility and/or vapor pressure in this matrix than its counter ion, allowing diffusion of the ion of interest. When the temperature gets sufficiently hot the ion migrates to the... 

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. 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

At a certain droplet-srze/charge ratio, field-indnced ion evaporation [18] of preformed ions in solntion may take place. The resnlting free ions in the gas phase are amenable to mass analysis. 

With ionic analytes and preformed ions in solution, ion evaporation is most important gas-phase ion-molecule reactions may lead to reneutralization reactions. Buffer composition and concentration must be optimized in order to promote ion evaporation and to reduce gas-phase reactions. In most cases, low ammonium acetate concentrations must be used. 

Zolotai et al. [9-10] performed similar experiments using glycerol or water as solvent and they introduced the term field evaporation of ions from solution. Direct emission of preformed ions in solution is assumed to occur. 

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