Quantitative emission measurements

For many years, flames have been used to excite emission spectra for various elements, and most modern atomic absorption spectrometers are readily adapted for flame emission measurements. Flames are not widely, used for this purpose, however, because for most ingle-element determinations, absorption methods provide as good or better accuracy, convenience, and detection limits. For multielement analyses, plasma sources are far superior to flames in most regards. For these reasons flame emission spectrometry is little used except for the determination of the alkali metals and occasionally calcium. These elements are excited at the relatively low temperatures of flames to give spectra that are remarkably simple and free of interference from other metallic species. Alkali-metal spectra generally consist of a relatively few intense lines, many of which are in the visible region and are well suited to quantitative emission measurements. 

In situ quantitation Fluorimetric measurements were made by reciting at = 365 nm and measuring the fluorescence emission at Xf] >560 nm (cut off filter FI 56). 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

The authors made no attempt at quantitative emission-intensity measurements. They did make the qualitative observation, however, that the rare-earth-ion emissions from the D20 solvated crystals were much more intense than the emissions from the H20 solvated ones. 

The relaxation processes from the MLCT state can be evaluated quantitatively by measuring the temperature dependence of the emission yield and lifetime because the MLCT state is both the lowest and emissive excited state. Higher temperature caused larger 0. - However, 0g and Xe became smaller and shorter at higher temperature, respectively. These experimental results could be well simulated by assuming that the photochemical reactions occur via the thermally accessible higher excited... 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

Single-stranded DNA containing deoxyguanosine residues can be quantitated in the presence of terbium(III). Ten micromolar Tb3+ in a pH 6 cacodylate buffer shows no detectable fluorescence, with excitation at 290 nm and emission measured at 488 nm. In the presence of single-stranded DNA, Tb3+ coordinates with deoxy-guanosine-5-phosphate nucleotides, and a linear dependence of fluorescence on concentration has been observed over the 1-10-pg/mL range of thermally denatured rat liver DNA.19... 

In addition to incomplete radiation measurements, the strong dependence of the results on the condition of the surface is a further difficulty. Impurities also play a role, alongside roughness. Even a very thin film of water or an oxide layer can completely change the radiation behaviour compared to the base material alone. It is therefore no surprise that the emissivities measured by various researchers often differ significantly. Unfortunately, in the description of the experiments the surface properties were inexactly or incompletely characterised, which frequently occurs due to the lack of quantitative measures for surface properties. The emissivities presented in Tables B12 and B13 of the Appendix must therefore be taken to be relatively uncertain. 

The yield of excited states in organic liquids, and in particular in benzene solutions, is critically compared using results from light emission measurements, light absorption in pulse radiolysis studies, and total chemical yields, in particular as determined by isomerization methods. It is shown that the total yield of excited states originating from solvent excited states and from solute excited states formed without previous solvent excitation is not less than G = 5. Quantitative data are obtained for the yields of the different precursors including singlet and triplet states. 

As in fluorescence, phosphorescence signals depend upon both structural and environmental factors. Under appropriate experimental conditions, the intensity of the phosphorescent emission measured is linearly related to the concentration of the phosphor. This relationship, which constitutes the theoretical basis of quantitative phosphorescence analysis, is very similar to that obtained for fluorescence, except that the efficiency of intersystem crossing , which populates the triplet, should also be taken into account. Therefore, at very low concentrations of the analyte (trace analysis), the basic equation is... 

At Erebus, Kilauea, and Stromboli, VLP events are temporally linked to visual observatiOTis of explosions the correlation is cmisistent with models for the VLP that involve ascent of a gas slug through the conduit to the surface. In a few cases, VLP events have been recorded coincident with high-resolution gas emission measurements. These cases provide a means for quantitative estimates of the relationship between outgassing and VLP events. At Asama (Kazahaya et al. 2011) and for certain types of VLP events at Fuego (Waite et al. 2013), the VLP event amplitudes correlate with the mass of SO2 measured for each discrete gas emission (Fig. 5). The correlation is less clear at Etna but is present under certain conditions (Zuccarello et al. 2013). These correlations demonstrate the influence of unsteady gas flow on deformation in the shallow conduit. They can be used to infer the role of gases for VLP events even when quantitative measurements are not available. 

The methods for detection and quantitation of radiolabeled tracers are deterrnined by the type of emission, ie, y-, or x-rays, the tracer affords the energy of the emission and the efficiency of the system by which it is measured. Detection of radioactivity can be achieved in all cases using the Geiger counter. However, in the case of the radionucHdes that emit low energy betas such as H, large amounts of isotopes are required for detection and accurate quantitation of a signal. This is in most cases undesirable and impractical. Thus, more sensitive and reproducible methods of detection and quantitation have been developed. 

Liquid scintillation counting is by far the most common method of detection and quantitation of -emission (12). This technique involves the conversion of the emitted P-radiation into light by a solution of a mixture of fluorescent materials or fluors, called the Hquid scintillation cocktail. The sensitive detection of this light is affected by a pair of matched photomultiplier tubes (see Photodetectors) in the dark chamber. This signal is amplified, measured, and recorded by the Hquid scintillation counter. Efficiencies of detection are typically 25—60% for tritium >90% for and P and... 

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

Several instmmental methods are available for quantitative estimation of from moderate to trace amounts of cerium in other materials. X-ray fluorescence is widely available, versatile, and suitable for deterrninations of Ce, and any other Ln, at percent levels and lower in minerals and purer materials. The uv-excited visible luminescence of cerium is characteristic and can be used to estimate Ce content, at ppm levels, in a nonluminescing host. X-ray excited optical luminescence (15), a technique especially appropriate for Ln elements including cerium, rehes on emissions in the visible, and also measures ppm values. Atomic emission spectrometry is appHcable to most lanthanides, including Ce (16). The precise lines used for quantitative measurement must be chosen with care, but once set-up the technique is suitable for routine analyses. 

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