Quantitative analysis spectrophotometry

Spectrophotometric determinations aim at evaluation of actual versus permitted concentrations of synthetic colorants. Quantitative analysis of colorants resulting from these procedures can be performed by various techniques. Spectrophotometry allows individual or simultaneous quantitative analyses of colorant mixtures having similar absorption spectra. " ... 

Because of peak overlappings in the first- and second-derivative spectra, conventional spectrophotometry cannot be applied satisfactorily for quantitative analysis, and the interpretation cannot be resolved by the zero-crossing technique. A chemometric approach improves precision and predictability, e.g., by the application of classical least sqnares (CLS), principal component regression (PCR), partial least squares (PLS), and iterative target transformation factor analysis (ITTFA), appropriate interpretations were found from the direct and first- and second-derivative absorption spectra. When five colorant combinations of sixteen mixtures of colorants from commercial food products were evaluated, the results were compared by the application of different chemometric approaches. The ITTFA analysis offered better precision than CLS, PCR, and PLS, and calibrations based on first-derivative data provided some advantages for all four methods. ... 

It has proved to be very useful, providing both qualitative and quantitative information derived from mathematical processing of UV/VIS spectra. The principles of derivative spectrophotometry were discussed [15,16]. Obviously, derivatisation of spectra does not provide any additional information to that acquired during the measurement, but allows for easier interpretation. In particular, the possibility of resolving overlapping peaks makes derivative spectrophotometry a valuable tool for multicomponent analysis. Typically, derivative spectrophotometry is useful for the simultaneous determination of two additives in polymeric materials with very closely positioned absorption maxima. In quantitative analysis, derivative spectrophotometry leads to an increase in selectivity. 

Table 5.5 shows the main characteristics of UV spectrophotometry as applied to polymer/additive analysis. Growing interest in automatic sample processing looks upon spectrophotometry as a convenient detection technique due to the relatively low cost of the equipment and easy and cheap maintenance. The main advantage of UV/VIS spectroscopy is its extreme sensitivity, which permits typical absorption detection limits in solution of 10-5 M (conventional transmission) to 10 7 M (photoacoustic). The use of low concentrations of substrates gives relatively ideal solutions [20]. As UV/VIS spectra of analytes in solution show little fine structure, the technique is of relatively low diagnostic value on the other hand, it is one of the most widely used for quantitative analysis. Absorption of UV/VIS light is quantitatively highly accurate. The simple linear relationship between... 

As the majority of stabilisers has the structure of aromatics, which are UV-active and show a distinct UV spectrum, UV spectrophotometry is a very efficient analytical method for qualitative and quantitative analysis of stabilisers and similar substances in polymers. For UV absorbers, UV detection (before and after chromatographic separation) is an appropriate analytical tool. Haslam et al. [30] have used UV spectroscopy for the quantitative determination of UVAs (methyl salicylate, phenyl salicylate, DHB, stilbene and resorcinol monobenzoate) and plasticisers (DBP) in PMMA and methyl methacrylate-ethyl acrylate copolymers. From the intensity ratio... 

Y. Nomura and M. Tamura. Quantitative analysis of hemoglobin oxygenation state of rat brain in vivo by picosecond time-resolved spectrophotometry. Journal of Biochemistry, 109 455-461, 1991. 

Quantitative analysis is also possible. The spot representing the component of interest can be cut (in the case of paper chromatography) or scraped from the surface (TLC), dissolved, and quantitated by some other technique, such as spectrophotometry. Alternatively, modern scanning densitometers, which utilize the measurement of the absorbance or reflectance of ultraviolet or visible light at the spot location, may be used to measure quantity. 

Their increased application in light food and drink products has given a new impetus to develop fast and accurate method for their determination. Among computer-controlled instruments multivariate calibration methods and derivative techniques are playing very important role in the multicomponent analysis of mixtures by UV-VIS molecular absorption spectrophotometry [2]. Both approaches ate useful in the resolution of overlapping band in quantitative analysis [3, 4]. 

Absorption spectrometry is a traditional method used for the measurement of various chemical substances and makes it possible to carry out visual colorimetry allowing easy measurements. Conventional absorption spectrophotometry is the measurement of numerical values such as that of absorbance to carry out qualitative and quantitative analysis. In such cases, if the spectra obtained are complicated, the determination often becomes difficult. However, even if the spectral changes are quite complicated, our eyes recognize them simply as color changes. Determination utilizing the colors themselves is a perceptual method instead of simple absorption spectrophotometry. 

In the case of non-ideal systems, where overlapping of bands is combined with more pronounced deviations from the Lambert-Beer law, the procedures of quantitative analysis are considerably more complex. An introduction into these problems has been provided by Baumann (1962), Perkampus (1992) and Massart et al. (1988). Nonlinear multi-component analysis by IR spectrophotometry has been described by Maris and Brown (1983). 

Covello, M., Ciampa, G. Quantitative analysis of lithium, potassium and caesium chloride by paper-chromatography and flame spectrophotometry. J. Chromatogr. 20, 201 (1965)... 

Another quantitative analysis of three-acid powder by dual wavelength linear-regression spectrophotometry in pharmaceutical preparations has been reported (25). 

This equation forms the basis of quantitative analysis by absorption photometry. When is 1 cm and c is expressed in moles per liter, the symbol e is substituted for the constant a. The value for c is a constant for a given compound at a given wavelength under prescribed conditions of solvent, temperature, pH, etc., and is called the molar absorptivity. The nomenclature of spectrophotometry is summarized in Table 3-2. Values for e are useful to characterize compounds, establish their purity, and compare sensitivities of measurements obtained on derivatives. Pure bilirubin, for example, when dissolved in chloroform at 25 °C, has a molar absorptivity of 60,700+1600 at 453 nm. The molecular weight of bilirubin is 584. Hence a solution containing 5mg/L (0.005 g/L) should have an absorbance of... 

After LLE into ethanolic KOH, the antioxidant BHT (32a) used in aircraft fuel was determined in the presence of Cu(n) ions, by UVV spectrophotometry at 368 nm. Linearity was observed in the 0 to 30 ppm range, RSD <2% . A UVV spectrophotometric method for determination of -cyclodextrin (126) is based on the formation of a complex with phenolphthalein (19, Section Vin.A.3). Both the intensity and hnear range are affected by the pH and the concentration of 19 in solution. The method was considered to be inadequate for precise determinations of 126 of purity higher than 98% see also application of a-cyclodextrin (85) for analysis of phenolics in Section IV.B.4 . Phenol in the presence of sodium nitroprusside, Na2[Fe(CN)5NO], and hydroxylamine, at pH 10.26-11.46, developed a blue coloration that could be applied for quantitative analysis (kmax 700 nm, e 1.68 x 10" LmoU cm , Sandell sensitivity 0.0052 p,g phenol cm ). Beer s law was found to be valid from 0.1 to 6.5 ppm". ... 

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. 

The absorption of electromagnetic radiation of wavelengths between 200 and 800 nm by molecules which have n electrons or atoms possessing unshared electron pairs can be employed for both qualitative and quantitative analysis as such, it is known as spectrophotometry. As a wide variety of pharmaceutical substances absorb radiation in the near-ultraviolet (200-380 nm) and visible (380-800 nm) regions of the electromagnetic spectrum, the technique is widely employed in pharmaceutical analysis. 

Ultraviolet-visible spectrophotometry is perhaps the most widely used spectrophotometric technique for the quantitative analysis of chemical substances as pure materials and as components of dosage forms. It has found increasing usefulness as a means of assaying pharmaceutical substances described in the pharmacopeias. 

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