Neutral analytes

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

Donovan and Pescatore described another fast-gradient approach with very short columns (20x4.6 mm internal diameter) packed with a porous polymer (known as ODP columns) [38]. This chromatographic support presents a high chemical stability and can be used at pH 2, 10 or 13 to analyze neutral analytes. This procedure allowed a relatively high flow rate (2mLmin ) and a gradient from 10 to 100% methanol in only 7 min. The mathematical treatment was simplified and based on the direct transformation of retention time to log P. For this purpose, two standards (toluene and triphenylene) were used to minimize retention time variations from run-to-run and instrument-to-instrument, and to facilitate the... 

Razak, J. L., Cutak, B. J., Larive, C. K., Lunte, C. E. Correlation of the capadty factor in vesicular electrokinetic chromatography with the octanobwater partition coefficient for charged and neutral analytes. Pharm. Res. 2001, 18, 104-111. 

The majority of optical sensors for neutral (uncharged) species is based on indicators which selectively interact with the analyte and additionally give optical signal changes. Since the diffusion of neutral analytes is not as strongly affected by the polymer matrix as it is in the case of ions, all types of polymer matrices can be used. 

Mohr G.J., Chromo- and fluororeactands Indicators for detection of neutral analytes using reversible covalent bond chemistry, Chemistry, A European Journal 2004 10 1083-1090.. 

Figure 2.9. Schematic of a matrix-assisted laser desorption/ionization (MALDI) event. The SEM micrograph depicts sinapinic acid-equine myoglobin crystal from a sample prepared according to the dried drop sample preparation method. In the desorption event neutral matrix molecules (M), positive matrix ions (M+), negative matrix ions (M-), neutral analyte molecules (N), positive analyte ions (+), and negative analyte ions (-) are created and/or transferred to the gas phase. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

Fig. 11.2. Diagram of the components of an El source. Gaseous samples (gases or vaporized liquids or solids) are introduced into the ionization chamber using a reservoir with molecular leak, direct insertion probe or as an eluent from a GC column. The collimated stream of 70 eV electrons interacts with the neutral analyte molecules to generate stable radical cations (M+ ) and unstable radical cations (M+ ) that undergo dissociation reactions to form the characteristic fragment ions observed in many El mass spectra.

RP-HPLC has been employed for the determination of flavonoids and other phenolic compounds in cranberry juice. The neutral and acidic analytes were preconcentrated octadecyl silica SPE cartridges conditioned with distilled water (neutral analytes) or with 0.01 M HC1 (acidic compounds). Hydrolysis of samples was carried out in aqueous methanol solution acidified with 6 M HC1 at 35°C for 16h. Chromatographic separation was performed in an ODS column (150 X 4.6mm i.d. particle size 5/.an). Solvents A and B were water-acetic acid (97 3, v/v) and methanol, respectively. The gradient started with 0 per cent B (flow rate, 0.9 ml/min), reached 10 per cent B in lQmin (flowrate, 1.0 ml/min) and increased to 70 per cent B in 40min (flowrate, 1.0 ml/min). Analytes were detected at 280 and 360 nm. Some typical chromatograms are presented in Fig. 2.71. The concentrations of flavonoids and phenolic acids are compiled in Table 2.69. It was stated that the SPE-HPLC procedure makes possible the simultaneous determination of phenolic compounds and flavonoids, therefore, it can be employed for the measurement of these classes of analytes in other fruit juices [188],... 

Fluorescent Chemosensors for Cations, Anions, and Neutral Analytes... 

Not all ionization methods rely on such strictly unimolecular conditions as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

In chemical ionization new ionized species are formed when gaseous molecules interact with ions. Chemical ionization may involve the transfer of an electron, proton, or other charged species between the reactants. [8] These reactants are i) the neutral analyte M and ii) ions from a reagent gas. 

In case of an intended protonation under the conditions of Cl one has to compare the PAs of the neutral analyte M with that of the complementary base B of the proton-donating reactant ion [BH]" (Brpnsted acid). Protonation will occur as long as the process is exothermic, i.e., if PA(b) < PA(m). The heat of reaction has basically to be distributed among the degrees of freedom of the [Mh-H]" analyte ion. [12,21] This means in turn, that the minimum internal energy of the [Mh-H]" ions is determined by ... 

Metal ions, in particular singly charged ions such as Na", K", Cs", and Ag are sometimes added to the matrix-analyte solution to effect cationization of the neutral analyte. [98] This is advantageous when the analyte has a high affinity to a certain metal ion, e.g., towards alkali ions in case of oligosaccharides. [6] Addition of a certain cation can also help to concentrate the ions in one species, e.g., to promote [Mh-K]" ions in favor of all other alkali ion adducts upon addition of a potassium salt. 

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