Ionizable Selectivity

T. Watanabe, A. Namera, M. Yashiki, Y. Iwasaki, T. Kojima, Simple analysis of local anaesthetic in human blood using headspace SPME and GC MS electron impact ionization selected ion monitoring, J. Chromatogr. B, 709, 225 232 (1998). 

Using two pulsed tunable dye lasers, Na atoms in a beam are excited to an optically accessible ns or ml state as they pass between two parallel plates. Subsequent to laser excitation the atoms are exposed to millimeter wave radiation from a backward wave oscillator for 2-5 [is, after which a high voltage ramp is applied to the lower plate to ionize selectively the initial and final states of the microwave transition. For example, if state A is optically excited and the microwaves induce the transition to the higher lying state B, atoms in B will ionize earlier in the field ramp, as shown in Fig. 16.5. The A-B resonance is observed by monitoring the field ionization signal from state B at fB of Fig. 16.5 as the microwave frequency is swept. 

One of the remarkable uses for high-density charge clusters (HDCCs) is to ionize selected gases, combine the positive ions with the highly negative HDCC, and accelerate the combined cluster to fractional light velocities. The following description is adapted from a paper by Jin and Fox [18]. 

A potentially more efficient method of enrichment would involve the use of sophisticated tunable lasers to ionize selectively and not fU. The ionized could then be made to react with negative ions to form another compound, easily separated from the mixture. For this method to work, we must construct lasers capable of producing radiation monochromatic enough to excite one isotope and not the other—a difficult challenge. 

Hj-Lorazepam, 3-hydroxy-1,4-benzodiazepine, 45, a sedative hypnotic and antianxiety agent, has been synthesized in a seven-step procedure presented in equation 12 for use as an internal standard in GC-MS-NICI-SIM (NICl-SIM = negative ion chemical ionization-selective ion monitoring) quantitative analysis of this drug in complex matrices encountered in forensic work and for study of its complex kinetics. The procedure involved selective trideuteriation, protection of the amino group and... 

Two general methods have been proposed for separating uranium isotopes. In the photoionization method to be discussed in Sec. 9.2, in uranium metal vapor is ionized selectively and then separated from unionized by deflection in electric or magnetic fields. In the photochemical method, to be described in Sec. 9.3, UFj in UFj vapor is excited selectively and caused to react chemically to produce a solid lower fluoride, which is then separated from unreacted UF5 vapor. 

Fagerquist CK, Lightfleld AR, Confirmatory analysis of beta-lactam antibiotics in kidney tissue by liquid chromatog-raphy/electrospray ionization selective reaction monitoring ion trap tandem mass spectrometry. Rapid Commun. Mass Spectrom. 2003 17 660-671. 

UV, laser Surface ionization Selective (N, P, As, S) High Medium Complex... 

Techniques GC-MS, negative ion chemical ionization, selected ion monitoring. Reference D. L. Swackhamer, M. J. Charles, and R. A. Hites, Quantitation of toxaphene in environmental samples using negative ion chemical ionization mass spectrometry. Anal. Chem. 59 913-917 (1987). 

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. 

Gas chromatogram with electron-impact ionization selected ion monitoring for BPA, BPS, and BP extracted from liquid in a can of peas and carrots. [From P. Vihas, N. Campillo,... 

The a-hydroxyl allyl compounds presented in Scheme 30 are easily ionized selectively by (CF3S02)20 at -78 °C in anhydrous ether to generate s-trans-cis-diene cationic precipitates via an intramolecular S 2 mechanism. The resulting s-rran -cw-1,3-diene cationic precipitates are thermally unstable as temperatures increase to 25 °C and decompose liberating free dienes. We utilized this method for chemoselectively synthesizing cis-1,3-dienes and cis-l,3,5-hexatrienes ° with 50-70%... 

A Chemical Ionization Selected Ion Monitoring Assay for Methylphenidate and Ritalinic Acid... 

Detector MS, Hewlett-Packard 5989B, electrospray ionization, selected ion monitoring, m/z 506.6... 

Since multiphoton excitation in mass spectrometry takes place in the more or less tight laser focus, which can easily be shifted in space and time or be subject to other variations, it can be combined with different ion optical or mechanical arrangements (e.g. sources of neutral molecular systems) without the need for much additional hardware. Thus, by combination with chromatography (particularly gas chromatography), species selection has successfully been realized. Another very promising combination, which has frequently been applied in the recent past for the study of involatile molecules (e.g. polycyclic aromatics, biomolecules), is that of laser desorption of neutral molecules and resonance enhanced multiphoton ionization. All the benefits of multiphoton mass spectrometry, such as soft ionization, selective ionization, controllable fragmentation or secondary excitation for tandem mass spectrometry, may be used in this field. 

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