Chemiluminescence intensities

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Fig. 1. Time courses of the chemiluminescence intensity from oxalate—hydrogen peroxide systems in ethyl acetate as solvent, 0.7 mM TCPO. The curves correspond to the following concentrations of triethylamine (TEA) catalyst A, 0.05 mM B, 0.10 mM and C, 0.20 mM (70).

Most peroxyoxalate chemiluminescent reactions are catalyzed by bases and the reaction rate, chemiluminescent intensity, and chemiluminescent lifetime can be varied by selection of the base and its concentration. Weak bases such as sodium saUcylate or imidazole are generally preferred (94). 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Addition of fluorescent energy acceptors such as 9,10-dibromoanthracene substantially increases chemiluminescence intensity by transferring excitation energy (132,133), as is the case with dioxetanes. 

The rates and chemiluminescent intensities of atom-transfer reactions are proportional to the concentrations of the reactants, but the intensity is inversely proportional to the concentration of inert gas present. The latter quenches the excited state through coUision with an efficiency dependent on the stmcture of the inert gas. Chemiluminescence Qc increases with temperature, indicating that excitation has a higher activation energy than the ground state... 

Recombination reactions are highly exothermic and are inefficient at low pressures because the molecule, as initially formed, contains all of the vibrational energy required for redissociation. Addition of an inert gas increases chemiluminescence by removing excess vibrational energy by coUision (192,193). Thus in the nitrogen afterglow chemiluminescence efficiency increases proportionally with nitrogen pressure at low pressures up to about 33 Pa (0.25 torr) (194). However, inert gas also quenches the excited product and above about 66 Pa (0.5 torr) the two effects offset each other, so that chemiluminescence intensity becomes independent of pressure (192,195). 

In this work the development of mathematical model is done assuming simplifications of physico-chemical model of peroxide oxidation of the model system with the chemiluminesce intensity as the analytical signal. The mathematical model allows to describe basic stages of chemiluminescence process in vitro, namely spontaneous luminescence, slow and fast flashes due to initiating by chemical substances e.g. Fe +ions, chemiluminescent reaction at different stages of chain reactions evolution. 

This model permits a determination of the rate constants for the rise of the chemiluminescence intensity and its subsequent decay and, more importantly, allows a quantitative assessment of the effects of reaction conditions, such as solvent variation, temperature, or additives, on the rates (r and f), the time required (t... 

The resulting chemiluminescent intensity was about 10 times more sensitive compared with that from the usual reaction between trichlorophenyl oxalate and H202.[4 1... 

The Effect of Temperature on Chemiluminescence Intensity The Mutual Relation of Isothermal and Non-lsothermal Studies 480... 

A direct oxidation of polymer additive, which may occasionally give a much stronger signal than the oxidation of polymer itself. This is very important as it may lead to an erroneous relation between the rate of polymer oxidation and chemiluminescence intensity. 

Qualitatively two different patterns of chemiluminescence intensity with time may be observed for polymer materials, namely,... 

Provided that chemiluminescence intensity Iql is proportional to the rate of peroxyl radicals termination, that is Icl [PO ]2, which is often assumed in the literature, chemiluminescence intensity should achieve some quasi-stationary level when hydroperoxide concentration becomes stationary and its decay should correspond to consumption of oxidizable groups, PH, in a polymer. At the same time, the chemiluminometric curves of type (a), which are typical with an autoaccelerating increase of the light emission (Figure 4) are relevant for... 

Table 2 Summary of temperature coefficients determined from Arrhenius plots of chemiluminescence intensity vs. temperature plots...

Data for temperature coefficients (activation energies) given in Table 2 show one common phenomenon when plotting chemiluminescence intensity vs. temperature in Arrhenius coordinates, namely, that in the higher temperature range the activation energy is usually higher than at lower temperatures, which is... 

In the published literature efforts exist to plot the so-called total chemiluminescence intensity (TLI) (surface below the chemiluminescence curve) vs. amount of oxygen absorbed [15]. In some cases a straight line may be obtained. A more detailed examination of the Equation (6) reveals that rate of oxygen uptake is in fact involved, so that we have... 

In some laboratories chemiluminescence intensity and DSC signals from samples situated in one oven were obtained in parallel. Blakey and George [42], for example, presented the correlation between DSC signal and chemiluminescence (CL) intensity I during oxidation of PP. The quadratic dependence of I on DSC may be well seen in Figure 11. 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

Figure 12 Chemiluminescence intensity and absorption of oxygen [43] scans for oxidation of polyisoprene in oxygen, temperatures 90°C and 100°C.

On well characterised non-stabilized PP samples [48] having molar mass within 45-180 kg/mol with differing tacticity and crystallinity, we can see that the increasing molar mass leads to an increase of induction time and reduction of the maximum chemiluminescence intensity (Figure 14). The polymer with higher average molar mass appears to be more stable than that with lower molar mass. This may be ascribed to the effect of increased concentration of more reactive terminal groups, which promote initiation of thermal oxidation. 

THE EFFECT OF TEMPERATURE ON CHEMILUMINESCENCE INTENSITY THE MUTUAL RELATION OF ISOTHERMAL AND NON-ISOTHERMAL STUDIES... 

Increasing temperature shortens the induction time and increases the maximum chemiluminescence intensity in the case of chemiluminescence of PP powder (type (a), see Figure 15), whereas it increases the initial chemiluminescence intensity in the case of poly(2,6-dimethyl-l,4-phenylene oxide) (type (b), see Figure 5). This is perhaps not surprising as the rate of oxidation reaction increases with temperature as well. 

Figure 16 Relaxation changes of chemiluminescence intensity for oxidized polyisoprene during replacement of oxygen by nitrogen and vice versa.

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

The case of reduction of chemiluminescence intensity for stabilized polymers of the chemiluminescence curves of the type (b) (Figure 5) has not yet been studied.)... 

The chemiluminescence intensity accompanying the oxidation of polymers is usually dependent on the concentration of oxygen in the surrounding atmosphere (Figures 21 and 22). This is confirmed by restricting the diffusion... 

Figure 23 Plot of maximum and equilibrium chemiluminescence intensity and its dependence on concentration of oxygen in a mixture with nitrogen.

The effect of oxygen concentration is thus included in a constant m, which modifies both the resulting maximum of the chemiluminescence intensity and the apparent rate constant k of hydroperoxide decomposition. 

The same may be observed with magnesium carbonate in cellulose [70] (Figure 26). The chemiluminescence intensity at a given temperature increases with pH of the sample almost linearly. As it is evidenced by DSC, the sample with pH 7.2 is the least stable. Figure 26 is also a demonstration of the much higher sensitivity of the chemiluminescence method when compared with DSC. DSC exotherms, which accompany the final stages of the cellulose decomposition... 

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