Impurity determination methods

For concentration impurity determination methods, linearity of the main component of interest should be demonstrated, not of all known impurities. For volatile component determinations (solvent exchanges, residual solvents, GTIs), only the linearity of the volatile component(s) being measured should be demonstrated. For completion of reaction methods (RAP), linearity of the reaction product, starting material, and any stable impurity used in the COR calculation should be demonstrated. 

The spectra of these ion pairs are largely different, which forms the basis of the determination method for the spectrophotometric determination of acidic impurities. 

Crowther, J. B., Salomons, P. and Callaghan, C., Analytical Method Development for Assay and Impurity Determination in Drug Substances and Drug Products, Chapter 12. In Analytical Chemistry in a GMP Environment, Miller, J. M. and Crowther, J. B., Eds., John Wiley Sons, Inc., New York, 2000. 

Erythromycin, a macrolide antibiotic, lacks a significant chromophore. Detection sensitivity was enhanced by using a wavelength of 200 nm and selecting an injection solvent of lower conductivity than the BGE. In order to facilitate the separation of erythromycin and its related substances, 35% (v/v) ethanol was incorporated into a 150 mM phosphate buffer pH 7.5. Resolution of all of the compounds was achieved in approximately 45 min. The method was employed as an assay method for erythromycin and for impurity determination. Peptide antibiotics, such as colistin and polymyxin, are mixtures of many closely related compounds. A validated CZE method for impurity analysis of polymyxin B was described, employing 130 mM triethanolamine-phosphate buffer at pH 2.5 to reduce the adsorption of analyte onto the capillary wall. Methyl-/l-cyclodextrin (M-/1-CD) and 2-propanol were found to be necessary for selectivity enhancement. Using similar buffer additives, the same group developed and validated a method for colistin analysis. ... 

For many years, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) methods have been used as an essential tool to determine the hydrodynamic size, monitor product purity, detect minor product or process-related impurities, and confirm batch-to-batch consistency of protein and antibody products. ITowever, gel-based techniques have several limitations, such as lack of automation, varying reproducibility, and a limited linear range. SDS-PAGE is also labor-intensive and generates large volume of toxic waste. Most importantly, the technique does not provide quantitative results for purity and impurity determination of proteins and antibodies. 

The first work on pKa determination by zone electrophoresis using paper strips was described by Waldron-Edward in 1965 (15). Also, Kiso et al. in 1968 showed the relationship between pH, mobility, and p/C, using a hyperbolic tangent function (16). Unfortunately, these methods had not been widely accepted because of the manual operation and lower reproducibility of the paper electrophoresis format. The automated capillary electrophoresis (CE) instrument allows rapid and accurate pKa determination. Beckers et al. showed that thermodynamic pATt, (pATf) and absolute ionic mobility values of several monovalent weak acids were determined accurately using effective mobility and activity at two pH points (17). Cai et al. reported pKa values of two monovalent weak bases and p-aminobenzoic acid (18). Cleveland et al. established the thermodynamic pKa determination method using nonlinear regression analysis for monovalent compounds (19). We derived the general equation and applied it to multivalent compounds (20). Until then, there were many reports on pKa determination by CE for cephalosporins (21), sulfonated azo-dyes (22), ropinirole and its impurities (23), cyto-kinins (24), and so on. 

A. Not more than 0.1% of any single impurity determined by the following method. Carry out the method for liquid chromatography, Appendix III D, using the following solutions in the mobile phase. Solution (1)... 

The diastereomeric conjugates were well separated by RP LC, and the procedure could be used to determine the enantiomeric purity of the drug enantiomers in the range of 99.84-99.98%, illustrating the utility of this approach. Furthermore, this work is another exception to the generalization that trace-enantiomeric-impurity determinations cannot generally be performed reliably with the indirect method. 

Determine origin of impurities and methods for elimination or reduction Understand degradation pathways and methods to minimize degradation... 

When determining trace elements in steel, removal of the Fe by extraction with diethylether from a strong aqueous acid solution is very appropriate. A preconcentration of trace impurities as dithiocarbamates, and eventual adsorption onto activated carbon has been proposed for trace determinations in aluminum subsequent to sample dissolution (see e.g. Ref. [317]). However, multitrace determination methods such as plasma atomic spectrometry are a better alternative to AAS methods in this instance. 

Release testing of raw materials or drug substances must be performed before they can be used for manufacturing a batch of the product. Assay and ordinary impurity determinations are usually performed by HPLC methods. However, when a reference standard for the active ingredient is not available, assay by titration is the method of choice. 

In the HPLC drug substance methodology, it is advantageous to have a different sample concentration for the assay and in-process impurity determinations. The impurity method should consist of sufficiently high concentration of the test sample such that the impurities at 0.05% levels will have sufficient... 

Aluminum and titanium are the common impurities, determined by colorimetric methods. The USP limit of Al3+ is 10 pg/ml of the solution... 

The fundamental possibilities for impurity determination are also offered by other isolation methods. For instance, a-ketoglutaric acid [30], oxalic acid [31] and phthalic acid [32] have been used for decomposing 2,4-phenylhydrazones. Semicarbazones may yield suitable compounds on reaction with phosphoric acid [33]. 

Table 10.1. Methods of Impurity Determination of Chemical Compounds (I)...

This chapter will look at the use of CE for pharmaceutical analysis and will include descriptions of the various modes of CE and their suitability for quantitative and qualitative analysis of pharmaceutical compounds. Practical applications of CE for the analysis of pharmaceuticals will be covered, these applications include drug assay, impurity determination, physicochemical measurements, chiral separations, and the analysis of small molecules. A section covering the approach to CE method development for pharmaeeutical analysis will include guidelines to selecting the best mode of CE for an intended separation. Extensive data will be provided on successful pharmaceutical separations with references to extra source material for the interested reader. This chapter will provide a comprehensive and up to date view of the role and importance of CE for the analysis of pharmaceuticals and will provide the reader with practical information and real data that will help them to decide if CE is suitable for an intended separation. 

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