Analysis by spectrophotometry

Fortunately, automated fiber-optic probe-based dissolution systems have begun to appear for these solid dosage-form applications. One such system uses dip-type UV transflectance fiber-optic probes, each coupled to a miniature photodiode array (PDA) spectrophotometer to measure drug release in real time. This fiber-optic dissolution system can analyze immediate- and controlled-release formulations. The system is more accurate and precise than conventional dissolution test systems, and it is easier to set up than conventional manual sampling or automated sipper-sampling systems with analysis by spectrophotometry or HPLC. 

A technique that involves combustion of the polymer under controlled conditions in a platinum crucible, followed by dissolution of the residual ash in a suitable aqueous reagent prior to final analysis by spectrophotometry is of limited value. A quite complicated and lengthy ashing programme is necessary in this technique to avoid losses of alkali metal during ignition 0-1 hour from start heat to 200 °C 1-2 hours from start hold at 200 °C 3-5 hours from start heat to 450 °C 5-8 hours from start hold at 450 °C. 

When preparing for a quantitative analysis by spectrophotometry, one should ask himself several questions ... 

It is possible to determine the metacide content with the use of ionic associates of metacide with BKM, BPR, CPR polyguanidine with azodyes SB and MG by spectrophotometry. The monomers, from which one synthesizes of metacide and polyguanidine, and which are present in actual objects of the analysis, do not react with dyes. 0,01-0,20 mg metacide at use BKM (0,01-0,10 mg at use CPR) is determined in 25 ml of solution. It s possible to determine 9-16 mg/1 of polyguanidine (pH 4-5) and 35 -400 mg/1 (pH 11-12) using magneson. 

First, let us consider batch mixing processes, as exemplified by ordinaiy laboratory practice in solution kinetics. A portion of one solution (say, of the substrate) is added by pipet to a second solution (containing the reagent) in a flask, the flask is shaken to achieve homogeneity, and then samples are withdrawn at known times for analysis, or the solution is subjected to continuous observation as a function of time, for example, by spectrophotometry. For reactions on a time scale (measured by the half-life) of hours or even several minutes, the time consumed in these operations is a negligible portion of the reaction time, but as the half-life of the reaction decreases, it becomes necessary to consider these preliminary steps. Let us distinguish three stages ... 

Maris, M.A., C.W. Brown, GJ. Kavamos, "Nonlinear Multicomponent Analysis by Infrared Spectrophotometry", Anal. Chem. 1983 (55) 1694-1703. 

The scope of UV analysis of dissolved polymer/additive matrices is thus quite restricted and mainly limited to special cases in which the additive package is known, e.g. the determination of Irganox 1098 in GFR-PA4.6 after dissolution in H2SO4/HNO3. Fibre-optic dissolution analysis by means of a UV diode array spectrometer is well known. In comparison to IR spectroscopy, UV spectrophotometry is better equipped to provide quantitative data. 

An alternative to AAS for the end analysis of stannane generated by hydrogenation, could be collection in permanganate solution and spectrophotometric determination with phenylfluorone (3). This was applied to submicrogram Sn/L concentrations in fresh and marine waters276. Determination by hydride generation-AAS was found to be about 20 times more sensitive than by spectrophotometry of the phenylfluorone (3) complex28. 

Urine, feces and food were analyzed for calcium content by atomic absorption spectrophotometry. Data were subjected to statistical analysis by analysis of variance and Duncan s Multiple Range Test. 

Europeum metal may be analyzed by AA, ICP and X-ray methods. The metal or its salts must be digested with nitric acid and brought into aqueous solution prior to analysis by flame or furnace AA or ICP spectrophotometry. 

Pharmacopoeial methods rely heavily on simple analysis by UV/visible spectrophotometry to determine active ingredients in formulations. These methods are usually based on the use of a standard A (1 %, 1 cm) value for the active ingredient being assayed and this relies on the UV spectrophotometer being accurately calibrated as described earlier in the chapter. Such methods also presume that there is no interference from excipients (preservatives, colourants, etc.) present in formulations and that the sample is free of suspended matter, which would cause light scattering. 

An oral suspension of chlorpromazine has the following composition and must be extracted so that excipients are removed prior to analysis by UV spectrophotometry Chlorpromazine 0.025% w/v, parahydroxybenzoic acid methyl ester 0.1 % w/v, neutral water soluble dye, lipophilic flavouring agent, sodium lactate buffer. 

For a compound to be analyzed by spectrophotometry, it must absorb light, and this absorption should be distinguishable from that due to other substances in the sample. Because most compounds absorb ultraviolet radiation, measurements in this region of the spectrum tend to be inconclusive, and analysis is usually restricted to the visible spectrum. If there are no interfering species, however, ultraviolet absorbance is satisfactory. Proteins are normally assayed in the ultraviolet region at 280 nm because the aromatic groups present in virtually every protein have an absorbance maximum at 280 nm. 

A soln of reduced glutathione (H-yGlu-Cys-Gly-OH 5.2 mg, 0.017mmol) and 2 (6.9mg, 0.029 mmol) in 50% aq TFA (1 mL) was kept at rt for 24 h. The mixture was concentrated and the residue dissolved in 10% aq AcOH (1 mL). The soln was then applied to a Sephadex G-25 SF column (3 x 54 cm), equilibrated, and eluted with 10% aq AcOH. The product was located in the effluent by spectrophotometry, the UV absorption being similar to that of tryptathionine. The relevant fractions were combined and lyophilized. The product was further purified by a second gel filtration yield 5 mg (58%). Amino acid analysis of an acid hydrolyzate with TosOH 10 gave Glu 1.00, Gly 0.98, Cys 0.70, oxindolylalanine 1.08, and traces of cystine. 

PHOTOMETRIC ANALYSIS. Chemical analysis by means of absorption or emission of radiation, primarily in the near UV, visible, and infrared portions of the electromagnetic spectrum. It includes such techniques as spectrophotometry, spectrochemical analysis, Raman spectroscopy, colorimetry, and fluorescence measurements. 

The quantitation of substances separated by TLC may be carried out in several ways. The most common method is to remove the spot from the plate, elute the compound from the adsorbent and measure the concentration of the compound in solution by spectrophotometry, fluorimetry, etc. The elution process has been significantly improved and facilitated with the Eluchrom instrument developed by Sandoz and marketed by Camag (see Fig.3.6). This instrument permits direct elution from the plates via small PTFE cups in a continuous flow-through mode without the necessity of removal of the adsorbent and with the minimum requirement of solvent (usually less than 1 ml). The measuring instruments used are those available for classical solution analysis. A discussion of these instruments is beyond the scope of this book. 

West, T. S. Analysis by Atomic-Absorption Spectrophotometry. Chem. Ind. (London) 1967, 452. 

Radi [41] used an anodic voltammetric assay method for the analysis of omeprazole and lansoprazole on a carbon paste electrode. The electrochemical oxidations of the drugs have been studied at a carbon paste electrode by cyclic and differential-pulse voltammetry in Britton-Robin-son buffer solutions (0.04 M, pH 6-10). The drug produced a single oxidation step. By differential-pulse voltammetry, a linear response was obtained in Britton-Robinson buffer pH 6 in a concentration range from 2 x 10-7to 5 x 10 5 M for lansoprazole or omeprazole. The detection limits were 1 x 10 8 and 2.5 x 10 8 M for lansoprazole and omeprazole, respectively. The method was applied for the analysis of omeprazole in capsules. The results were comparable to those obtained by spectrophotometry. 

---