Absorbing species

The iimnodified temi absorbance usually means this quantity, though some authors use the Napierian absorbance B = -hiT. The absorbance is so iisefiil because it nomially increases linearly with path length, /, tlirough the sample and with the concentration, c, of the absorbing species within the sample. The relationship is usually called Beer s law ... 

The relationship between a sample s absorbance and the concentration of the absorbing species (A = zbC). 

Chemical Limitations to Beer s Law Chemical deviations from Beer s law can occur when the absorbing species is involved in an equilibrium reaction. Consider, as an example, an analysis for the weak acid, HA. To construct a Beer s law calibration curve, several standards containing known total concentrations of HA, Cmt, are prepared and the absorbance of each is measured at the same wavelength. Since HA is a weak acid, it exists in equilibrium with its conjugate weak base, A ... 

Quantitative Analysis of Mixtures The analysis of two or more components in the same sample is straightforward if there are regions in the sample s spectrum in which each component is the only absorbing species. In this case each component can be analyzed as if it were the only species in solution. Unfortunately, UV/Vis absorption bands are so broad that it frequently is impossible to find appropriate wavelengths at which each component of a mixture absorbs separately. Earlier we learned that Beer s law is additive (equation 10.6) thus, for a two-component mixture of X and Y, the mixture s absorbance, A, is... 

Accuracy Under normal conditions relative errors of 1-5% are easily obtained with UV/Vis absorption. Accuracy is usually limited by the quality of the blank. Examples of the type of problems that may be encountered include the presence of particulates in a sample that scatter radiation and interferents that react with analytical reagents. In the latter case the interferant may react to form an absorbing species, giving rise to a positive determinate error. Interferents also may prevent the analyte from reacting, leading to a negative determinate error. With care, it maybe possible to improve the accuracy of an analysis by as much as an order of magnitude. 

A UV/Vis absorbance detector can also be used if the solute ions absorb ultraviolet or visible radiation. Alternatively, solutions that do not absorb in the UV/Vis range can be detected indirectly if the mobile phase contains a UV/Vis-absorbing species. In this case, when a solute band passes through the detector, a decrease in absorbance is measured at the detector. 

The intensity of light transmitted (subscript t) through a sample Ij depends on the intensity of the incident (subscript 0) light Iq, the thickness 1 of the sample, and the concentration [c] of the absorbing species. 

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

Conversion Processes. Most of the adsorption and absorption processes remove hydrogen sulfide from sour gas streams thus producing both a sweetened product stream and an enriched hydrogen sulfide stream. In addition to the hydrogen sulfide, this latter stream can contain other co-absorbed species, potentially including carbon dioxide, hydrocarbons, and other sulfur compounds. Conversion processes treat the hydrogen sulfide stream to recover the sulfur as a salable product. 

Generally, photooxidation has an even stronger negative effect on lasing and stimulated emission in conjugated polymers than it has on the EL-performance. It not only reduces the number of excited 5j states but additionally creates charged absorbing species that partly compensate the stimulated emission due to the neutral excited states. 

In this expression the only variable is N0 and it is this which governs the extent of absorption. Thus it follows that the integrated absorption coefficient is directly proportional to the concentration of the absorbing species. 

First, consider a blank sample in which the concentration of both components is equal to zero. With no absorbing species in the sample, there would be no absorbance at any of the wavelengths, and the spectrum would be plotted at [0, 0, 0], the origin of this absorbance data space. Now, consider the spectrum of a sample that contains 1 concentration unit of Component 1 and none of Component 2. This spectrum will have some absorbance at each of the... 

Number of absorbing species per unit volume, Number of nuclei over... 

Beer s law The absorbance of electromagnetic radiation by a sample is proportional to the molar concentration of the absorbing species and the length of the sample through which the radiation passes, beta (P) decay Nuclear decay due to fi-particle emission, beta (P) particle A fast electron emitted from a nucleus in a radioactive decay. 

The most valuable bromine compound probably is silver bromide, which is the light-absorbing species on which most film photography is based (see Box 20.2, Chemistry and Technology, on page 895). Bromine compounds are also used extensively as fire retardants, particularly in carpets, rugs, and clothing... 

It was also reported that microcrystallites of layered semiconductor Pblj were prepared in colloidal form. The spectrum of such a colloidal solution consisted of three absorption bands in the UV which were considerably bu hifted from the absorption threshold of macrocrystalline Pblj. These results were explained by carrier confinement in three differently sized crystallites, each a single layer ( 7 A) thick. However, complexes of Pbl2 with iodide have similar absorption bands, and it seems at the present time that additional experiments have to be carried out to ascertain the colloidal nature of the absorbing species. Size quantization was also reported for colloids of red Hgl2 in acetonitrile... 

It can be shown [4] that the innovations of a correct filter model applied on data with Gaussian noise follows a Gaussian distribution with a mean value equal to zero and a standard deviation equal to the experimental error. A model error means that the design vector h in the measurement equation is not adequate. If, for instance, in the calibration example the model was quadratic, should be [1 c(j) c(j) ] instead of [1 c(j)]. In the MCA example h (/) is wrong if the absorptivities of some absorbing species are not included. Any error in the design vector appears by a non-zero mean for the innovation [4]. One also expects the sequence of the innovation to be random and uncorrelated. This can be checked by an investigation of the autocorrelation function (see Section 20.3) of the innovation. 

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