Ionization methods, mass cluster ions

There are several methods of producing gas-phase inorganic ions, the starting materials in mass spectrometric studies. The properties of the source of the ions required for study are important in the choice of ionization method. The production of bare metal ions from an involatile nonmolecular source requires a large amount of energy deposited on the surface of the material. The processes that occur after the initial ionization process may also affect the ions finally observed (e.g., clustering). At the other end of the ionization energy spectrum, gas-phase ions of a complexity similar to those observed in the condensed phases require a soft ionization process. A brief description of some of the ionization methods follows. 

Besides some rare experimental setups the mass analyzer of any mass spectrometer can only handle charged species, i.e., ions that have been created from atoms or molecules, more seldom from radicals, zwitterions or clusters. It is the task of the ion source to perform this crucial step and there is a wide range of ionization methods in use to achieve this goal for a wide variety of analytes. 

Derivatization of samples with an equimolar mixture of labeled and unlabeled reagents to produce ion clusters has been applied recently to the characterization of the bleomycins by FD and FAB mass spectrometry. A variation of this technique utilizes a mixture of labeled and unlabeled reagent gases pyridine/[ Hs]pyridine, trimethylchlorosilane (TMCS)/ [ HsJTMCS, tetramethylsilane (TMS)/[ HizITMS," and NHa/N Ha in conjunction with Cl mass spectral analyses. The isotope cluster technique also may be exploited with the newer ionization methods of SIMS and FAB through the addition of appropriate counter-ions (for example, Ag " ). 

The use of laser ionization methods offered a more subtle and gentle way to ionize neutral clusters, which could then be mass-analyzed using time-of-flight (TOP) methods. Castleman first introduced these methods to clusters (named ion-dip spectroscopy) and applied to it electronic transitions. The method is depicted in Fig. 4. 

All three leading soft ionization techniques, FAB, ESI, and MALDI, are used for generating gas-phase ions of CyD inclusion complexes. However, there is still an open question about how accurately mass spectra reflect the solution-phase chemistry. There is no general answer to this question and each system has to be treated separately. A few helpful hints, however, have to be borne in mind. Polar interactions, such as hydrogen bonds and electrostatic attraction, are usually stronger in the gas phase than in solution, especially in polar solvents. Therefore, it is possible to observe some cluster ions in the gas phase which cannot be detected in solution. On the other hand, nonpolar interactions are usually weakened in the gas phase. Thus, some complexes that do exist in solution, as confirmed by spectral methods such as NMR spectroscopy (presented in Chapter 9), cannot be transferred to the gas phase without decomposition. Applying these rules to complexes of CyDs, the conclusion can be drawn that only complexes with relatively polar compounds can be observed in the gas phase [58, 60]. 

Polyoxometalate speciation in nonaqueous solvents has been investigated by electrospray ionization mass spectrometry (ESMS). This low energy ionization method minimizes fragmentation, e.g., (Bu°4N)2[Mo60i9] gives ion clusters with m/z values centered at 440 and 1,123 due to [MogOig] " and (Bu"4N)[Mo60i9] respectively. 

Figure 3.11 summarizes such key experimental points. As a first point, we have to choose the appropriate ionization method for the detection of small metabolites, we have alternative choices other than MALDI, such as secondary ion mass spectrometry (SIMS) [15], nanostructure-initiator mass spectrometry (NIMS) [20,21], desorption/ionization on silicon (DIOS) [22], nanoparticle-assisted laser desorptiopn/ ionization (nano-PALDI) [23], and even laser desorption/ionization (LDI) [24,25]. We consider that MALDI is stiU the most versatile method, particularly due to the soft ionization capability of intact analyte. However, other methods each have unique advantages for example, SIMS and nano-PALDI have achieved higher spatial resolution than conventional MALDI-IMS, and above aU, these mentioned alternative methods are all matrix-free methods, and thus can exclude the interruption of the matrix cluster ion. Next, if MALDI is chosen, experimenters should choose a suitable matrix compound, solvent composition, and further matrix application method for their target analyte. All these factors are critical to obtain sufficient sensitivity because they affect efficiency of analyte extraction, condition of cocrystallization, and, above all, analyte-ionization efficiency. In addition, based on the charge state of the analyte molecule, suitable MS polarity (i.e., positive/ negative ion detection mode) should be used in MS measurement. Below, we shall describe the key experimental points for MALDI-IMS applications of representative metabolites. 

The other chief comptment of a mass spectrometer, the ion source, determines the types of ions that can be examined when starting fi om a specific sample (Gross and Caprioli, 2007). Laser-based methods, variably dubbed ablation, ionization, and desorption/ionization supposedly depending on the involved laser power, have played an important role fi om initial studies of bare metal ions to the widespread sought-after production of cluster ions for these latter species, the development of the so-called cluster sources was key to progress (Duncan, 2012). In more recent years, electrospray ionization (ESI) has played a central role due to its capacity to transfer/produce ions from solutions under mild conditions. Besides yielding new types of ions for chemical probing, ESI became a method of choice for the identification of solution species (speciation) and for direct observation of reaction... 

Which ionization method should one choose for a particular compound Both APCI and ESI are considered soft ionization techniques, although ESI is the softer method. They are often implemented in the same ion source, with very simple conversion procedures to go from one to the other. APCI will also occasionally produce fragments that are not mass spectral fragments, but rather ones caused by the high operating temperatures typical of the APCI probe. ESI, being the softer method, is more prone to cluster formation. Both situations can produce misleading results. 

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