Analytical procedures to be used in pharmaceutical quality control have to be validated strictly in accordance with highly formalized standards and guidance. Manuscripts submitted for publication presenting new procedures must therefore describe the corresponding validation characteristics and the validation approach and procedure accordingly. Interestingly, many of these manuscripts and — what is worse — already published (and therefore accepted) articles in various periodicals are characterized by several typical methodological failures and misconceptions — even though authors have regularly been informed of such mistakes in the hope they can be avoided. The corresponding ICH guideline seems to be well-known, and is frequently cited in these manuscripts, but to some extent is not fully understood and is even partially neglected. In this paper the authors present a collection of typical failures, misconceptions, and misleading data captured as reviewers and readers of published articles over the last two years, thus giving a survey of the most frequent problems of TLC/HPTLC validation. Moreover, the authors provide a list of references describing optimum approaches to validation of specific TLC/HPTLC procedures. We suggest these are followed in addition to the general concepts of the ICH guideline.
The acidic visualization methods most often used for steroids have been optimized and compared. Sulfuric acid at three different concentrations, phosphomolybdic acid, and phosphoric acid, with different heating temperatures for different times, have been examined as reagents for visualization of seven steroids separated by OPLC on HPTLC silica gel as adsorbent. For sulfuric and phosphoric acids the chromatograms were evaluated by inspection under long-wavelength UV light (366 nm). For phosphomolybdic acid the chromatograms were evaluated in daylight. It was found that visualization at higher temperatures for shorter periods usually results in greater sensitivity, although heating for longer periods at lower temperatures results in a more stable, robust visualization.
An OPLC purity test suitable for routine analysis of norethisterone bulk drug substance and in tablets has been developed. Separation was performed on a fine-particle silica gel adsorbent layer by continuous development with
-hexane and butyl acetate-chloroform, 85 + 15 (
), as mobile phases. Visualization of the chromatogram by use of sulfuric acid enables detection of less than 0.05% of possible impurities by use of long-wave UV illumination. The selectivity and efficiency of this OPLC method were compared with those of the pharmacopeial TLC methods and those of HPLC method.
A semiquantitative OPLC purity test has been developed for inprocess control of nandrolone and compared with other chromatographic methods. TLC was not sufficiently selective, a key impurity with low UV absorption could not be detected by HPLC, and nandrolone was slightly degraded during gas chromatography. OPLC proved to be a suitable means of testing for all potential impurities in nandrolone. The separation was performed by multiple development on fine-particle silica gel with cyclohexane-ethyl acetate-chloroform, 50 + 25 + 25 (
), as mobile phase. After spraying the chromatograms with sulfuric acid, then heating, the impurities could be sensitively detected by visual inspection in long-wave UV light. Detection limits were ≤0.01 μg.