Quasi-species, molecular

Semi-permeable membranes are quasi-two-dimensional barriers which, given a fluid mixture of two or more species of particles (usually two different molecular species, mixed or in solution) on one side, allow the passage either way through the membrane of one or more, but not all, of the molecular species, in either direction. This gives the possibility of separating, at... 

HPLC ESI MS is also a useful tool in the analysis of non-anthraquinone red dyestuffs. The use of this technique allows the identification of carthamin as the main colour component of safflower.[34] Ten species of the genus Alkanna are extracted with hexane, and dissolved in water-methanol solution after evaporation. [47] Ammonium formate buffer (pH 3.0) was used as the mobile phase modifier. In the preparations, alkannin and many hydroxynaphthoquinones (alkannin derivatives) were identified by comparison of retention times, as mass spectra (in the NI mode) for all compounds consisted only of quasi-molecular peaks. 

Here we are considering the dynamic equilibrium between molecular species in the gas phase and the adsorbed gas species on a surface. Let us consider the following quasi-chemical equilibrium between the species B in the gas, Bg, and the available sites at the surface of the adsorbate ... 

Protonated Molecule An ion formed by interaction of a molecule with a proton, and represented by the symbol [M+H]+. The term protonated molecular ion is deprecated this would correspond to a species carrying two charges. The terms pseudo-molecular ion and quasi-molecular ion are deprecated a specific term such as protonated molecule, or a chemical description such as [M+Na]+, [M-HJ, etc., should be used [1],... 

Besides, the structure, nature and reactivity of the chemisorbed molecule could not be unambiguously identified because the physical tools used could not lead easily to a complete understanding of the quasi molecular character of surface chemisorbed species and move precisely to the definition of the elementary steps occurring during the molecular transformations taking place on the surfaces. 

Fujii developed a method for detecting radical species in the gas phase with the use of lithium ion attachment to chemical species. Li ions have been chosen as reactant ions, because the affinity of the species is highest among all the alkah metal ions. The author also explored some of the unique properties of Li ion attachment in mass spectrometry. This technique provides mass spectra of quasi-molecular [R + Li]+ ions formed by lithium-ion attachment to the radical species under high pressure . ... 

Currently it appears that there are no difficulties in saturating atomic species, while molecular species may be saturated with sufficient laser power. There are some difficulties associated with saturation. Because of chemistry, the quasi-equilibrium population of a species may change substantially when excited. See, for example, Daily and Chan (7), and Muller, et al. (17). 

Various molecular and quasi-molecular ions can be formed under MALDI conditions. The formation of protonated analyte (A) molecules, [A -F H]+, is generally most important at least for samples containing slightly basic centres, such as the peptides and proteins, MALDI mass spectrometry of which is known to be most facile and reproducible. Therefore, proton transfer from the electronically excited, neutral or ionized, or protonated matrix species is considered to be crucial in the overall MALDI process . Notably, proton transfer can occur already in the condensed phase, followed by desorption of the preformed ions . However, the generation of the [A -F H]+ ions is believed to take place preferably in the so-called plume , that is, in the energized, short-hved and relatively dense vapour phase generated above the solid matrix upon excitation by the laser pulse. The actual proton donor species (be it one or several) in a given case is still a matter of... 

More importantly, a molecular species A can exist in many quantum states in fact the very nature of the required activation energy implies that several excited nuclear states participate. It is intuitively expected that individual vibrational states of the reactant will correspond to different reaction rates, so the appearance of a single macroscopic rate coefficient is not obvious. If such a constant rate is observed experimentally, it may mean that the process is dominated by just one nuclear state, or, more likely, that the observed macroscopic rate coefficient is an average over many microscopic rates. In the latter case k = Piki, where ki are rates associated with individual states and Pi are the corresponding probabilities to be in these states. The rate coefficient k is therefore time-independent provided that the probabilities Pi remain constant during the process. The situation in which the relative populations of individual molecular states remains constant even if the overall population declines is sometimes referred to as a quasi steady state. This can happen when the relaxation process that maintains thermal equilibrium between molecular states is fast relative to the chemical process studied. In this case Pi remain thermal (Boltzmann) probabilities at all times. We have made such assumptions in earlier chapters see Sections 10.3.2 and 12.4.2. We will see below that this is one of the conditions for the validity of the so-called transition state theory of chemical rates. We also show below that this can sometime happen also under conditions where the time-independent probabilities Pi do not correspond to a Boltzmann distribution. 

The species of the different quasi-molecular states in form (46) and (47). The point group C2. and the g and u in brackets are only defined for homonuclear molecules. The underlined species in the lowest row refer to one surface the not-underlined species to the other. The 2n species refers to both. 

At any conversion, p, of functional group, a mixture of the distribution of the Pj species (containing i monomer units) with the modifier (M) constitutes a multicomponent system. In this case, a simplified description may be made by using a quasi-temary system consisting of an average molecular species Pi, the remaining monomer. Pi, and the modifier M. 

There are two important distinctions between the quasi-molecular ion and a molecular ion formed by conventional electron. First, the energy imparted to the sample molecule during the collision-induced ionization is relatively low (<20 eV for methane as the reactant gas). Second, the quasi-molecular ion is an even-electron species and therefore more stable than the odd-electron molecular ion. Quasi-molecular ions are therefore almost always seen in Cl mass spectra. 

Levsen K, Schiebel H-M, Terlouw JK et al (2007) Even-electron ions a systematic study of the neutral species lost in the dissociation of quasi-molecular ions. J Mass Spectrom 42 1024—1044... 

The approach proposed in the previous section makes it possible to develop the most rigorous model of reacting gas mixtures, since it takes into account the detailed state-to-state vibrational and chemical kinetics in a flow. However, practical implementation of this method leads to serious difficulties. The first important problem encountered in the realization of the state-to-state model is its computational cost. Indeed, the solution of the fluid dynamics equations coupled to the equations of the state-to-state vibrational and chemical kinetics requires numerical simulation of a great number of equations for the vibrational level populations of all molecular species. The second fundamental problem is that experimental and theoretical data on the state-specific rate coefficients and espiecially on the cross sections of inelastic processes are rather scanty. Due to the above problems, simpler models based on quasi-stationary vibrational distributions are rather attractive for practical applications. In quasi-stationary approaches, the vibrational level populations are expressed in terms of a... 

Amphoteric polysaccharides are, perhaps, the most chemically sophisticated polysaccharides, which may explain the reluctance of formulators to explore them. Depending on the pH of a cosmetic formulation, the amphoteric polysaccharide can be cationic, anionic, or both i.e., it can be a quasi-nonionic species or zwitterionic (Fig. 40). A zwitterion is a molecular species wherein a proton from the acidic portion of the amphoteric resides on the basic. The resulting molecule has positive and negative charges in close proximity, effectively canceling one another and leaving the polysaccharide nonionic overall. 

As observed earlier (9, 10), most peptides show up as proto-nated species (see example in Fig. 25.5), whereas the peptides Pea-CAH-I and II predominantly yield Na - and -adducts. Images of the respective m/z values of these two Pea-CAH-related quasi-molecular ion species confirm that they originate from identical sites in the tissue. Their images nicely reflect previous immunohistochemical work confirming the ccg (glandular lobe of the cc) as the site of synthesis (11, 12). 

In 1989, the Fuji group developed the method for detection of chemical species in the gas phase with use of Li ion attachment to chemical species [11-13]. The principle is based upon a phenomenon that LL ions get attached to chemical species (M) by means of intermolecular association reactions to produce (M + Li)+ adduct ions. Then they are transferred to a mass spectrometer for mass analysis. This approach is exactly the same to cationization for detection of molecular species. Since the potential of Li ion attachment in MS has not yet been reahzed, they attempted to reveal and explore some of the unique properties of Li I AMS. Li ions have been chosen as reactant ions, because the affinity of the species is the highest among all the alkali metal ions. This technique provides mass spectra of quasi-molecular ions formed by lithium ion attachment to the chemical species (M) under high pressure. Results are obtained in the form of trace of LL adduct ions (M + Li)+ (also referred as cationized molecules). The newly developed lAMS [10], manufactured by instmmental maker (Canon Anelva Corp., Japan), exhibits several advantages over conventional mass spectrometers. Currently, ion association MS is available commercially in a various form. Recently, some reviews have been published on the principles, instmmental techniques, unique characteristics, and applications of I AMS [15-20]. 

This section describes a new method for detection and identification of intermediate radical species in the gas phase. This method is based upon the establishment of an alkah ion attachment to radicals through termolecular reaction. It provides mass spectra of quasi-molecular ions formed by lithium ion attachment to the radical species (R) under high pressure. Results are obtained in the form of a mass spec-trometric trace of LT adduct radicals. The advantages of the present method are (i) a measme of mass is a guide to radical identity, (ii) adaptability to a condition of higher pressures, and (hi) direct continuous measurements of any species in dynamic systems can be made. The method is apphed to the study of the microwave (MW) discharge in CH, C, CHy02, or CH4/N2 and was successfully... 

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