Chemical conversion methods, measurement

The discussion that follows is divided into two sections. The Spectroscopic Methods section includes those measurement techniques that involve the interaction of a photon with a peroxy radical. The Chemical Conversion Methods section describes the measurement of another molecule or radical to which a peroxy radical has been converted. 

Chemical Conversion Methods. Laser-Induced and Resonance Fluorescence of HO. Considerable effort has been applied to the measurement of HO in the stratosphere and troposphere. Ultraviolet fluorescence techniques based on lasers or resonance lamps have received a great deal of attention and study. Because HO concentrations are typically factors of one-tenth to one-hundredth those of H02 in the atmosphere, the difficulties associated with making HO measurements by using fluorescence [low signal-to-noise ratio, laser-generated HO, background fluorescence, etc. see the... 

H02 and R02 There are three approaches that are used to measure H02 and/or R02 (1) conversion of H02 and/or R02 to OH and measurement of the latter using techniques already described, (2) a chemical amplifier method, and (3) matrix isolation ESR. 

For example, Cantrell and co-workers (1993) estimate the efficiency of conversion of simple alkyl peroxy radicals to vary from 0.93 for CH3CH202 to 0.47 for (CH3)2C02, and it may be even less for larger alkyl peroxy radicals. This may be the reason that in some intercomparison studies, the matrix isolation-ESR technique (vide infra), which measures the sum of ROz, gives some higher concentrations for some individual measurements than the chemical amplifier method (e.g., Zenker et al., 1998). 

Reiner, T., M. Hanke, and F. Arnold, Atmospheric Peroxy Radical Measurements by Ion Molecule Reaction-Mass Spectrometry A Novel Analytical Method Using Amplifying Chemical Conversion to Sulfuric Acid, J. Geophys. Res., 102, 1311-1326 (1997). 

Chemical Amplification. The measurement of a small electrical signal is often accomplished by amplification to a larger, more easily measured one. This technique of amplification can also be applied to chemical systems. For peroxy radicals, Cantrell and Stedman (117) proposed, as a possible technique, the chemical conversion of peroxy radicals to N02 with amplification (i.e., more than one N02 per peroxy radical). This method has also been used for laboratory studies of H02 reactions on aqueous aerosols (21). The following chemical scheme was proposed as the basis of the instrument ... 

A last possibility, which has not been reported so far, is a method in which one measures the heat of reaction, which is released when drops containing component 1 coalesce with drops containing component 2. This method is only suitable in continuous operation, as otherwise the temperature rise that would occur would affect both the interaction rate and the chemical conversion rate. All other methods mentioned so far are suitable both for batch operation and for continuous operation, with a slight preference for the latter since steady-state operation probably will give more reproducible results. A limitation of all the above methods is that only the interaction rate of an aqueous dispersed phase can be measured, because of the requirement that the chemical reaction be nearly instantaneous. A further disadvantage is that the dispersed phase itself is not of uniform composition, so that the interfacial tension may not be the same for all drops, and therefore the drop size may depend on the amount and type of reactants which the drops contain. 

Rapidly solidifying compositions used for reactive injection molding place some restrictions on measurement. In the time required to prepare the reaction mixture, place a sample in the measuring cell of an instrument, and achieve a steady state in the sample and the measuring system at a preset temperature, chemical conversion of the material may advance considerably, making viscosity measurements meaningless. The volume of lost information depends on the ratio of the transient time necessary to achieve a steady state in the sample and the characteristic time of the chemical reaction. The sensitivity of the reaction rate to temperature is also important. In order to avoid the necessity to maintain isothermal conditions for the measurements, a non-isothermal scanning method for viscosity measurements was proposed.156... 

The direct accurate measurement of local OH concentrations has been one of the major technical challenges in atmospheric chemistry since the early 1980s. This goal was first achieved in the stratosphere (e.g., Stimpfle and Anderson, 1988), but the troposphere proved more difficult (Crosley, 1995). Nevertheless, early long-baseline absorption methods for OH were adequate to test some basic theory (e.g., Poppe et al., 1994). Current successful direct methods include differential optical absorption near-UV spectroscopy with long baselines (e.g., Mount, 1992 Dorn et al, 1995 Brandenburger et al., 1998), laser-induced fluorescence after expansion of air samples (e.g., Hard et al., 1984, >1995 Holland et al., 1995), and a variety of chemical conversion techniques (Felton et al, 1990 Chen and Mopper, 2000 Tanner et al., 1997). 

The instability and chemical conversion of some OPA derivatives imply that a denvatized compound may, in fact, result in one fluorescent and two radioactive peaks (Simson and Johnson, 1976, Fig. 1). The chemical rearrangement of the derivatives may, however, be a minor factor with respect to retention and the fluorescent and nonfluorescent derivatives may coelute. The use of more chemically stable amino acid derivatives, i.e. those formed by reaction with FMOC chloride, eliminates this problem. When the radioactivity of an amino acid is measured, it is often desirable and necessary to inject larger concentrations of amino acids than in a routine expenment. With the OPA method it is then critical to (a) make sure that OPA is present in the required molar excess (Lindroth and Mopper, 1979), (b) lower the pH of the reagent mixture to spare the top of the column, and (c) use the same or lower proportion of organic solvent in the sample as in the beginning of the gradient in order to obtain a concentration of the derivatives on the column top. 

It has been shown that in many cases for thermoset systems, there is a quantitative relationship between the chemical conversion of the thermoset and its Tg value, independent of the time-temperature cure history (111). This is very convenient from an applications standpoint because measurement of Tg is equivalent to a direct measurement of conversion. It implies that either the molecular structure at a given degree of conversion is the same, regardless of the reaction path, or that differences in the structures produced for different reaction paths do not affect Tg. While a Tg-degree of cure relationship has been found valid for many epoxy-amine and other thermoset materials, it has been observed not to hold in some cases, which include some epoxy-DICY, cyanate-ester, and phenolic systems. It has also been found not to hold for a given resin system cured by two different methods such as thermal and microwave (112) or thermal and electron beam cures (113). 

Other integral methods include in situ spectroscopic techniques, such as time-resolved infrared spectroscopy, which measures concentration or conversion as well as provides information on chemical identities of components in the reactor. In contrast to the chemical sampling method, the infi ared (IR) spectrum of the reacting system may be collected at a much faster pace than that normally possible for the chemical sampling method. Consequently, rate data may be derived with good accuracy by differentiating the IR intensity data with respect to time. 

Two such processes are the excitation of either thermal crystal lattice oscillations (spin-lattice relaxation) and/or excitement of oscillation in the nuclear magnetic system (spin-spin relaxation). Both processes are characterized by constants 7T and T2 71 is the spin-lattice relaxation time and T2 is the spin-spin relaxation time. Valnes of 71 and T2 render an essential influence upon the form of resonance curves. On the other hand, the relaxation processes depend on the mobility of one atomic group or another in the substance. So, experimental measurement of the relaxation processes serves as a method of measurement of the dynamics of chemical conversions depending on time, temperature, chemical conversion particularity and other factors. 

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