Radicals mass spectrometric sampling

The understanding of the reactions in the plasma [64] is largely impeded due to the complex nature of plasmas. The most difficult task is to determine the relative importance of ions and neutral species in the reactions of plasma. To explain the process many different models have been postulated from the analysis of chemical products observed in the plasma. Models for plasma reactions have invoked ion-molecule reactions, radical-radical reactions, and radical-molecule reaction. However, the extent of their participation is still unclear. The mechanism of polymer production in gas discharge has also been the subject of some controversy [65]. Many reports of mass spectrometric sampling of electric discharges have appeared. Few attempts [57,66] have been made to obtain correlations between the ions and neutral species. 

J.K. Prasain, C.H. Wang and S. Barnes, Mass spectrometric methods for the determination of flavonoids in biological samples, Free Radic. Biol. Med., 37, 1324 1350 (2004). 

The simplest type of study is the mass spectrometric measurement at a fixed station of relatively high concentrations of stable reactants and/or products of atom reactions, when the time of reaction in the flow tube is varied. In this case, available signal-to-noise ratio is adequate for simple techniques to be used, and no sampling problem arises. Much useful work has been carried out in this way, especially when the method is combined with independent measurement of atom and radical concentrations in the same system, e.g. using e.p.r. the kinetics of reaction of 0( P) with NH3 were investigated in this way. ... 

Figure 13 Improved version of collision-free sampling system for mass-spectrometric detection of radicals...

Ionization of the analyte is the first crucial and challenging step in the analysis of any class of compounds by mass spectrometry. The key to a successful mass spectrometric experiment lies to a large extent in the approach to converting a neutral compound to a gas-phase ionic species. A wide variety of ionization techniques have become available over the years, but none has universal appeal. In some techniques, ionization is performed by ejection or capture of an electron by an analyte to produce a radical cation [M+ ] or anion [M ], respectively. In others, a proton is added or subtracted to yield [M - - H]+ or [M — H] ions, respectively. The adduction with alkali metal cations (e.g., Na+ and K+) and anions (e.g., Cl ) is also observed in some methods. The choice of a particular method is dictated largely by the nature of the sample under investigation and the type of information desired. Table 2.1 lists some of the methods currently in vogue. Some methods are applicable to the atomic species, whereas others are suitable for molecular species. Also, some methods require sample molecules to be present in the ion source as gas-phase species, whereas others can accommodate condensed-phase samples. The methods that are applicable to molecular species are the subject of the present chapter those applicable to atomic species are described in Chapter 7. 

Therefore, to determine if oxidized lipids were formed by enzymic processes or by free radical autoxidation, a first step is to visualize the distribution of products. This step requires previous knowledge of the maximum number of oxidized products, their chromatographic behavior and ions associated with mass spectrometric detection of each product. Quantitative analyses almost always require the use of appropriate, pure standards. For samples from more complex sources where the lipids of interest are present at low concentration there may be many interfering ions. In these instances, tandem mass spectrometry can be used to select pairs of precursor ions and product ions formed by collision-induced dissociation in a procedure called selected reaction monitoring (SRM). This type of analysis usually provides a significant improvement in signal to noise so that the product can be accurately quantified. With modem instruments many, up to hundreds, of these transitions can be measured in a single analysis. In conjunction with retention time... 

In principle, mass spectrometry provides a specific and highly sensitive method for the detection of gaseous free radicals. Foner (41 ) has reviewed the techniques available for sampling free radicals, and for their mass spectrometric detection. Unless the radical source is at pressures comparable to atmospheric... 

Along with optical methods, mass spectrometric methods are successfully used for analysis of stable and short-lived products in studying elementary reactions. These methods are combined with a jet reactor for studying reactions involving radicals. The diffusional variant of a jet reactor developed by A.M. Dodonov and V.A. Talroze is most successful. This method allows the detection of both radicals and excited species. The simplified scheme of the method is shown in Fig. 3.6. A sample is taken from a definite point along the axis of the diffusion cloud the removal of a capillary filled with the second reactant allows one to change the reaction time. To... 

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