Positive Ionization

Mostly protonated molecules, MH , are formed, while the fragment ions formed are small in number. In some cases, adduct formation will occur, for example, the formation of (M + when ammonia is used as a reagent gas. 

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In practice, the valne depends on the type of analysis being attempted. Structural studies may require the extent of fragmentation to be maximized, while quantification may require the opposite, i.e. the efficient prodnction of a small number of ions of different m/z ratios, in order to maximize sensitivity. Selectivity may be obtained simply by monitoring of the molecular ions formed. On occasions, when significant background is observed, this may not be adequate and fragmentation of the molecnlar species may be necessary to provide a number of ions to be monitored. Some componnds may be ionized very effectively under positive-ionization conditions, while others may require the formation of negative ions to allow analysis. 

Holz, M Lucas, O Muller, C, NMR in the Presence of an Electric Current, Simultaneous Measurements of Ionic Mobilities, Transference Numbers, and Self-Diffusion Coefficients Using an NMR Pulsed-Gradient Experiment, Journal of Magnetic Resonance 58, 294, 1984. Hooper, HH Baker, JP Blanch, HW Prausnitz, JM, Swelling Equilibria for Positively Ionized Polyacrylamide Hydrogels, Macromolecules 23, 1096, 1990. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

HH Hooper, JP Baker, HW Blanch, JM Prausnitz. Swelling equilibria for positively ionized polyacrylamide hydrogels. Macromolecules 23 1096-1104, 1990. 

The lamp is filled under reduced pressure with an inert gas such as argon or neon. The open end of the cathode faces the window of the lamp, which is constructed of borosilicate glass or quartz the latter window must be used if ultraviolet radiation is measured. A sufficient potential is applied across the electrodes to cause a current of from 1 to 50 mamp to flow. The inert gas is positively ionized at the... 

Compounds were optimized in positive ionization mode and in negative mode if necessary. Automaton can also perform automatic MS method development from solutions containing multiple compounds to increase throughput. When mixture solutions are used, Automaton injects a mixture once to determine all precursor ions and DP values and then injects once per compound to determine product ion and CE value. This approach allows automatic and unattended optimization of MS parameters for hundreds of compounds. The optimized parameters are stored in a compound database that permits fast and efficient retrieval of information about a specific compound and allows a compound to be used in multiple assays, eliminating the need to re-optimize the LC/MS/MS conditions. 

A Waters Micromass triple quadrupole mass spectrometer was used with an electrospray ionization interface in positive ionization mode desolvation gas (400), cone gas (70), collision gas (2.74 x 10 3 mbar), capillary (3.0 kV), cone (14 (kV), source temperature (105°C), and desolvation temperature (300°C). The detection and quantitation of amlodipine and nimodipine were performed... 

Sample preparation requires only dissolution of the sample to a suitable concentration in a mixture of water and organic solvent, commonly methanol, isopropanol, or acetonitrile. A trace of formic acid or acetic acid is often added to aid protonation of the analyte molecules in the positive ionization mode. In negative ionization mode ammonia solution or a volatile amine is added to aid deprotonation of the analyte molecules. 

K. H. Kingdon. A Method for the Neutralization of Electron Space Charge by Positive Ionization at Very Low Gas Pressures. Phys. Rev., 21(1923) 408-418. 

It is now well established that when a surface presents electron donor or electron acceptor sites, it is possible to ionize molecules of relatively high electron affinity (> 2 eV) or low ionization potential values, resulting in paramagnetic radical ions. For instance anthracene and perylene are easily positively ionized on alumina (7 ) (IP = 7.2 and 6.8 eV respectively). The adsorption at room temperature of benzenic solution of perylene, anthracene and napthalene on H-ZSM-5 and H-ZSM-11 samples heated up to 800°C prior to adsorption did not give rise to the formation of the corresponding radical cation. For samples outgassed at high... 

Pharmacophoric features are red for positively ionizable, green for hydrogen bond acceptors, light blue for hydrophobic and orange for ring aromatic. Shape restraints are shown in light... 

A from the center of a positive ionizable group was identified. However, its predictive performance on a test set consisted of eight structurally similar compounds was relatively poor. To achieve a computational model with greater predictability, a descriptor-based QSPR model was also developed. Descriptors related to molecular hydrophobicity as well as hydrogen bond donor, shape and charge features contributed to explain hOCTl inhibitor properties of the analyzed compounds. 

In this section, we shaU outline a many-electron treatment of charge transfer, similar in spirit to that of Tully, which enables different charge-exchange mechanisms to be incorporated in the formalism simultaneously. Although we shall concentrate on the TDAN model of resonant neutralization and negative ionization, we shall indicate how other neutralization processes can be included, and the approach for the reverse process of positive ionization will be fairly apparent. 

Protonated and cationized species are the most commonly detected ions using the ES process if a positive ionization mode is selected. Protonation is a result of the addition of proton(s) to a neutral molecule for every proton added, a net charge of +1 will result. Similarly, canonization is due to the addition of cation(s) to a neutral molecule. Detection of cationized ions can be useful in the determination of molecular mass of unknown analytes.If ESI is operated under negative ion mode, deprotonated ions will usually be the most dominant ions. Eor either operation mode, solvent adducts of the protonated/deprotonated ions are frequently detected in ESI/MS mass spectra. 

Fig. 2.6 Titration assay of BSA and methyl orange (MO, MW 305 Da, Kj = 450 pM) by GPC spin column/ESI-MS methodology as a function of [MO]/[BSA] molar ratios in the positive ionization mode (A) and the negative ionization mode (B). The mass spectra in the top five panels are exploded...

Fig. 2.7 ESI mass spectra of ligands present in CPC spin column eluates have a linear response with increasing concentration ESI (positive ionization mode) mass spectral analysis of the eluate from the CPC spin column titration ofWY252 (MW 457 Da) with MM P-1 where the molar ratios of MM P-1/ WY252 are constant at 1 5 while their individual concentrations linearly increase. The volume injected for each sample was...

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