Species selective ionization

Tam et al. [37-47] developed an impressive generalized method for the determination of ionization constants and molar absorptivity curves of individual species, using diode-array UV spectrophotometry, coupled to an automated pH titrator. Species selection was effected by target factor analysis. Multiprotic compounds with overlapping pK s have been investigated binary mixtures of ionizable compounds have been considered assessment of inicroconstants have been reported. 

In summary, the distinctiveness of this soft ionization technique is that source optimization can be achieved through an independent adaptation of the ionization region, the electrical field gradient, and the gas flow. It is well suited for aromatic hydrocarbons that are selectively ionized, including nonpolar species, such as PAH, yielding to the lowest detection limits by orders of magnitude, as compared to all other API methods. 

These and many other variations (see below) make it possible to find a chromatographic system suitable for application to most complex mixtures. The species to be separated may be large or small, polar or nonpolar, isomeric or homologous, molecular or ionic, volatile or nonvolatile, and, of course, colored and thus visible (as with Tswett s work) or, more commonly, invisible, requiring a sensitive detector based on UV adsorption, selective ionization, and so on. 

From the analysis of the specifications in Table 12.2, IPC - MS hyphenation requires (1) specific optimization of mobile phase composition, selecting only volatile components, (2) devising strategies to minimize eluent fiow, and (3) an interface to make IPC effluent amenable to MS detection. The goals of the interface are (1) separation of the analyte from the bulk eluent, (2) ion evaporation for ionic species or ionization of non-ionic solutes, and (3) fragmentation and quantitative transfer of analyte fragments to the mass analyzer. 

A resonance ionization mass spectrometer (RIMS) uses a tunable, narrow bandwidth laser to excite an atom or molecule to a selected energy level that is then analyzed by MS. The selective ionization often is accomplished by absorption of more photons from the exciting laser, but can also be effected by a second laser or a broadband photon source. Multiple photon absorption can result in direct ionization or in production of excited species that can then be ionized with a low-energy photon source (IR laser) or by a strong electric field. Resonance ionization methods have been applied to nearly all elements in the periodic table and to many radionuclides, including Cs (Pibida et al., 2001), Th (Fearey et al., 1992), U (Herrmann et al., 1991), Np (Riegel et al., 1993), Pu (Smith, 2000 Trautmann et al., 2004 Wendt et al., 2000), radioxenon and radiokrypton (Watanabe et al., 2001 Wendt et al., 2000), and 41Ca (Wendt et al., 1999). 

The range of concentrations (dynamic range) over which ESI can be used is only three to four orders of magnitude, after which the ionization process saturates. Therefore, ESI can be thought of as a method in which the amount of ion current that can be produced is limited irrespective of how much material is introduced. The lack of any separation in the flow injection mode makes the problem of selective ionization particularly problematic, as the most polar species will sequester the ionization capacity when mixtures are analyzed. The limited capacity for ionization is less of a problem when LC is used to separate compounds with different polarities, as each compound is ionized individually upon elution from the column. 

ESI intrasource factors that influence selective ionization of lipid classes have been examined [26]. For example, GPC molecular species can be ionized selectively from other classes of lipids in the positive-ion mode after cationization with Li+ ions. In contrast, GPE molecular species can be ionized selectively in the negative-ion mode after similar cationization with Li+ ions. 

The reaction products from the furnace are swept into a gas-liquid contactor where they are mixed with an appropriate solvent (Figure 7). The support solvent is selected to promote ionization of the reaction species over ionization of interfering compounds. This solvent is usually circulated through a closed system containing beds of ion exchange resins to purify and condition the solvent for reuse. From the gas-liquid contactor the support solvent flows to the conductivity cell, where detection takes place, either after separation of the liquid from insoluble... 

Metal-containing ions are useful reactants for the identification of organic compounds by mass spectrometry. The formation of metal adducts is especially advantageous when traditional methods of ionization (El, Cl) do not result in stable molecular ions or protonated species. Chemical ionization with metals and metal-containing ions provides high selectivity and sensitivity to specific types of analytes (unsaturated and functionalized hydrocarbons, peptides, crown ethers,... 

The combination of resonant laser excitation to an intermediate level and the subsequent mass analysis of the ion also makes the technique species selective. Therefore, one can distinguish REMPI spectra of systems that have the same mass, e.g. phenol-H2 and phenol-CO. On the other hand, care needs to be taken not to approach the REMPI experiment in a brute-force approach, as one may be tempted to increase weak signals by increasing the laser pulse energies. Non-resonant multiphoton ionization (MPI) processes may destroy the carefully adjusted species selectivity, as will be shown in the example of the REMPI investigation of CaH/CaD reaction products see Figure 9.5 below. 

The chapter IR Spectroscopic Techniques to Study Isolated Biomolecules gives an overview of some of the most common experimental practices currently in use to characterize the strucmre of isolated biomolecules by infrared spectroscopy. We address especially two main categories of experimental approaches conformation-selective infrared spectroscopy of jet-cooled neutral species and infrared (multiple-photon) dissociation spectroscopy of mass-selected ionized biomolecules. Molecular beam laser spectroscopy methods form the experimental basis for the topics covered in the sixth to eighth chapters. Mass spectrometry-based ion spectroscopy provided the experimental data for the studies reviewed in fourth and fifth chapters (and seventh inpart). 

Its ion source could serve as a separation device to selectively ionize a class or a certain category of lipid species based on the charge propensity of lipid classes, thereby leading to the analysis of different lipid classes and individual molecular species with high efficiency and low ion suppression without prior LC separation. 

When the GPL mixture was analyzed in the positive-ion mode, the mass spectrum showed that the substantive ion peaks were comprised of the various adducts of PC species (Figure 4.3b), indicating the selective ionization of PC species from other classes of lipids. These mass spectra also showed that the disappearance ofprotonated ion peaks is due to the neutralization of protons by LiOH, and the appearance of lithiated adducts is due to the increased availability of lithium ions in the infusion solutions after increasing addition of LiOH. The persistent presence of ion peaks... 

---