Methods for the determination of purity

Methods for determining the purity of povidone are described in detail in the European, U.S. and Japanese pharmacopoeias. They cover all the parameters listed in Table 32. 

Some of the former pharmacopoeial methods are not always entirely relevant. This applies particularly to the titration tests of vinylpyrrolidone or aldehydes, as the methods are not specific and inaccurate, and therefore no longer do justice to the purity of povidone availabe today. For this reason, the harmonized monograph Povidone introduced an HPLC method for vinylpyrrolidone and an enzymatic test for acetaldehyde. 

HPLC method for the determination of free N-vinylpyrrolidone, 2-pyrrolidone and vinyl acetate in povidone and copovidone 

A sample of povidone or copovidone is analyzed after dissolution in a water/ methanol mixture using HPLC reverse phase chromatography (gradient run). The polymeric material is kept away from the chromatographic system using a precolumn and switching technique. 

The determination limits are 2 mg/kg N-vinylpyrrolidone, 200 mg/kg 2-pyrro-lidone and 10 mg/kg vinyl acetate. 

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Palermo E, Chiu J (1976) Critical Re view of Methods for the Determination of Purity by Differential Scanning Calorimetry. Thermochim Acta 14 1-12. 

The basis of all methods for the determination of purity by calorimetry is the observation that tv o or more components can interact vith each other and influence their melting behavior. A particularly simple description can be derived for systems in vhich the components are completely miscible in the liquid state and completely immiscible in the solid state, that is, eutectic systems. The coexistence between the solution and the solid impurity is described by... 

Nonaqueous Titration Method for the Determination of the Purity of Ammonium Picrate ,... 

LaClair (44) and Soloway et al. (SO) have modified the dehydrohalogenation reaction to permit the determination of p,p -DDT in dusts and oil solutions containing technical DDT. The reaction between the base and the halide is carried out at 20° to 30° C., as Cristol (16) has found that under proper conditions at this temperature the p,p -DDT reacts completely, whereas the o,p -DDT and most of the impurities react only slightly. The Association of Official Agricultural Chemists (3) tentative method for the determination of the purity of p,p -DDT, which employs the dehydrohalogenation procedure, has been modified by Fleck (30) so that the reaction is carried out at 25° C. instead of under reflux conditions. 

Raban, M., Modern Methods for the Determination of Optical Purity, 2, 199 See also Mislow, K.,... 

Since the separated enantiomers of a dissymmetric compound must crystallize in a different space group than does the racemic mixture, it should not be unanticipated that quantitative XRPD would be useful in the determination of enantiomeric composition. For instance, the differing XRPD characteristics of (S)-(+)-ibuprofen relative to the (-RS)-racemate have been exploited to develop a sound method for the determination of the enantiomeric purity of ibuprofen samples [53]. 

The product of this preparation is the most enantioselective catalyst developed to date for asymmetric epoxidation of a broad range of unfunctionalized olefins.6 The procedure includes a highly efficient resolution of trans-1,2-diaminocyclohexane as well as a convenient analytical method for the determination of its enantiomeric purity. This method is general for the analysis of chiral 1,2-diamines. The Duff formylation described in Step B is a highly effective method for the preparation of 3,5-di-tert-... 

Although gradient elution is generally required for RPLC separations of proteins, isocratic elution can be successful in some instances. For example, isocratic elution has been used for the determination of purity of production batches of biosynthetic human growth hormone (HGH).42 The method was used to... 

Size-based analysis of SDS-protein complexes in polyacrylamide gels (SDS-PAGE) is the most common type of slab gel electrophoresis for the characterization of polypeptides, and SDS-PAGE is one of the most commonly used methods for the determination of protein molecular masses.117 The uses for size-based techniques include purity determination, molecular size estimation, and identification of posttranslational modifications.118119 Some native protein studies also benefit from size-based separation, e.g., detection of physically interacting oligomers. 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In the... 

Optical Purity, Modem Methods for the Determination of Raban and... 

Thin-layer chromatography (TLC) is also a simple and rapid method for the determination of enantiomeric purity, especially for nonvolatile substances. Suitable plates (Chi-raplate) are commercially available273. 

Therefore, direct Group I methods for the determination of enantiomeric purity are generally preferred. A most important merit of the direct methods is the fact that the chiral auxiliary compounds, e.g., the chromatographic stationary phase, need not be enantiomerically pure. 

Agbaba et al. [56] developed an HPTLC method for the determination of omeprazole, pantoprazole, and their impurities omeprazole sulfone and N-methylpantoprazole in pharmaceutical. The mobile phase chloroform-2-propanol 25% ammonia-acetonitrile (10.8 1.2 0.3 4), enables good resolution of large excesses of the drugs from the possible impurities. Regression coefficients (r > 0.998), recovery (90.7-120.0%), and detection limit (0.025-0.05%) were validated and found to be satisfactory. The method is convenient for quantitative analysis and purity control of the compounds. 

The development of accurate methods for the determination of enantiomeric purity, which began in the late 1960 s, has been critical for the assessment of enantioselective synthesis. Thus a prerequisite in the enzyme-catalyzed kinetic resolution of racemates is a precise and reliable assessment of the degree of enantioselectivity (E), enantiomeric excess (ee) and conversion (c). Among these methods are 1) polarimetric methods, 2) gas chromatographic methods, 3) liquid chromatographic methods and 4) NMR spectroscopy. The most convenient and sensitive methods used are chiral GC and HPLC. 

Radhakrishna et al. developed a gradient liquid chromatographic method for the determination and purity evaluation of benazepril hydrochloride in bulk and pharmaceutical dosage forms [31]. The method is simple,... 

The two forms of capillary array electrophoresis are emerging as powerful methods for the determination of enantiomeric purity of chiral compounds in a truly high-through-put manner. Various modifications are possible, for example, detection systems based on UV/Vis, MS, or electrical conductivity. Moreover, chiral selectors in the CE electrolyte are not even necessary if the mixture of enantiomers is first converted into diastereo-mers, for example, using chiral fluorescent-active derivatization agents [51,57]. 

These methods are typically qualitative rather than quantitative (see Section 1.3.3), and therefore their use for the determination of purity and yield in SPS has not been... 

Fifty-three peptide samples were submitted by 48 laboratories. Previous studies by this committee have demonstrated the need for multiple analytical methods for the assessment of purity. Therefore the peptides in this study were analyzed by AAA, HPLC, ESI-MS and MALDI-MS to determine purity (Table I). Only two peptide samples had less than 50% of the desired product, and three other samples had less than 70% of the desired product, as judged by their mass spectra, amino acid composition and HPLC retention time. Overall, the peptides were of excellent quality. 

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