Spectrophotometry difference

The binding of chromophoric 1-thioglycosides from lactose or cellobiose to CBH I (Tr. r.) was followed by difference spectrophotometry of the ligand, or by the diafiltration technique (Fig. 4). Alternatively, perturbation of the protein spectrum could be used in the case of non-chromophoric ligands (Fig. 5). 

Analysis was carried out by difference spectrophotometry. A one-point standard calibration for the determination of aspirin in dextropropoxyphene compound capsules was prepared by adding a known amount of aspirin to the sample from a standard stock solution. Stated content in the capsules ... 

Wahbi et al. [32] used a spectrophotometric method for the determination of omeprazole in pharmaceutical formulations. The compensation method and other chemometric methods (derivative, orthogonal function, and difference spectrophotometry) have been applied to the direct determination of omeprazole in its pharmaceutical preparations. The method has been validated the limits of detection was 3.3 x 10 2 /ig/ml. The repeatability of the method was found to be 0.3-0.5%. The linearity range is 0.5-3.5 /ig/ml. The method has been applied to the determination of omeprazole in its gastro-resistant formulation. The difference spectrophotometric (AA) method is unaffected by the presence of acid induced degradation products, and can be used as a stability-indicating assay method. 

W2. Waite, J. H., Determination of (catecholato)borate complexes using difference spectrophotometry. Anal. Chem 56, 1935-1939 (1984). 

Stray-light errors are more likely to be observed near the wavelength limits of an instrument, where the radiation intensity of the source and the efficiency of the optical system are reduced, especially below 220 nm and at the crossover point between the ultraviolet and the visible lamps (about 320 to 400 nm). Errors may become serious where the solvent absorbs strongly or where a strongly-absorbing sample is measured by difference spectrophotometry. 

Pharmacopoeial applications include assays for single drugs and mixtures of drugs, analyses involving colour reactions (colorimetric methods), tests for tablet dissolution, limit tests for impurities, and assays of bulk drugs or an extract thereof. Further applications are for physicochemical measurements, such as pK or velocity constants in enzymatic reactions. The scope of such applications has been significantly extended by methods which can confer additional specificity, namely difference spectrophotometry and derivative spectrophotometry. 

The classic example of a pharmacopoeial assay based on the Morton-Stubbs correction is that for vitamin A alcohol and the ester. Other techniques proposed for the correction of non-specific absorption include difference spectrophotometry, second derivative spectrophotometry, the use of orthogonal polynomials, and chemical or physical transformation of the drug to give absorption at a longer wavelength. 

The technique of difference spectrophotometry as usually practiced in protein work, consists of photometrically measuring the absorbance of a... 

In turbid or cloudy solutions, in which absorbance is affected by scattering of the sample, two absorbance measurements are made one at and the other at a different wavelength where the analyte does not absorb. Then, the difference being proportional to concentration of the analyte, the analyte concentration can be calculated. Difference spectrophotometry is used in the determination of constituents in tablets, complex pharmaceutical preparations, plant extracts, syrups, biological matrices such as blood and serum, injectable oil preparations, and the like. For this technique, in addition to other general requirements such as well-matched cells that are positioned accurately, the solution should be made homogeneous, and the instrumental stray radiation at the wavelength of interest should be extremely small. 

Dual wavelength ultraviolet spectrophotometric methods have been described for the determination of triamterene in combination with hydrochlorothiazide, 08 and with benzthiazide3-0 in tablets. Difference spectrophotometry, a technique in which no prior separation of the drug from other materials is required and which provides for automatic cancellation of interferences while retaining the inherent convenience of direct reading, has been described.110... 

Difference spectrophotometry involves the exploitation of the ability to chemically modify the spectrophotometric profile of the analyte alone in the presence of other possible interferents. The analyte may be modified by alteration of pH or through chemical reaction in either the reference or sample cell. pH-induced difference spectrophotometry is most commonly employed, due to its simplicity, though reagents to covalently modify the analyte have also found use. The selective modification of the analyte alone in the presence of interferents permits quantitation on the basis of spectral differences between the otherwise identical refer-... 

Spectroscopic and crystallographic studies of platinum-base complexes give some insight into the reactivity of the platinum compounds and their possible binding sites on DNA. The reactions of the different nucleosides or nucleotides with the chloro and aquo derivatives of O Cs- and trans-DDP have been studied by UV spectroscopy (36), raman difference spectrophotometry (37) and high pressure liquid chromatography (38). For both chloro isomers, the rates of the reactions with various nucleic acid monomers show the following trend GMP > AMP > CMP and dG > dA > dC T, The dichloro and diaquo derivatives react slowly with thymidine and UMP (37) or not at all (38, 39). 

The accessibility of the tryptophyl residues was further investigated by using difference spectrophotometry (c.f. above) and fluorescence studies (49). For a discussion of the interesting conclusions that can be drawn from studies of the tryptophyl residues in the P. notatum cellulase —c.f., Reference 37. 

Khrapunov, S. N. Protas, A. F. Sivolob, A. V. Dragan, A. I. Berdyshev, G. D. Intrinsic fluorescence, difference spectrophotometry and theoretical-studies on tertiary structure of calf thymus histone-Hl. Int. J. Biochem., 1985, 77(2), 217-222. 

Spectrophotometric monitoring with the aid of chemometrics has also been applied to more complex mixtures. To solve the mixtures of corticosteroid de-xamethasone sodium phosphate and vitamins Bg and Bi2, the method involves multivariate calibration with the aid of partial least-squares regression. The model is evaluated by cross-validation on a number of synthetic mixtures. The compensation method and orthogonal function and difference spectrophotometry are applied to the direct determination of omeprazole, lansoprazole, and pantoprazole in grastroresistant formulations. Inverse least squares and PCA techniques are proposed for the spectrophotometric analyses of metamizol, acetaminophen, and caffeine, without prior separation. Ternary and quaternary mixtures have also been solved using iterative algorithms. 

Recent work in the short wavelength region indicates that there may be a potential application of ultraviolet difference spectrophotometry at short wavelengths to studies of helix-random coil transitions (Imahori and Tanaka, 1959 Gla-zer and Smith, 1960). 

Bastian, 1949 Hakala and Schwert, 1955 Schwert and Takenaka, 1955 Laskowski el al., 1956b, 1960a Leach and Scheraga, 1960a). Applications of the technique of ultraviolet difference spectrophotometry to specific proteins will be discussed in Chapter VII. In this chapter we shall explore the basis for the existence of such difference spectra. 

Figure 6.4 displays the denaturation transition curve of chymotrypsinogen at 293 nm in urea. Variations of the extinction coefficient with urea concentration has also been observed for the native and for the unfolded states. These properties have been used to estimate the number of exposed or buried chromophores either by difference spectrophotometry or by the method of solvent perturbation (Herskovits and Laskowski, 1962, 1968 Herskovits, 1965,1967). 

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