Peak homogeneity determination for the validation of high-performance liquid chromatographic assay methods

The determination of 3-nitrophenol and some other aromatic impurities in 4-nitrophenol by reversed phase HPLC with peak suppression diode array detection

In this work the peak suppression technique is used for the determination of 3-nitrophenol and some other aromatic impurities in 4-nitrophenol by reversed phase HPLC with diode array detection. Taking into account the differences between the absorption spectra of the two compounds, two wavelengths were selected in order to obtain the maximum difference between the spectral contribution for 3-nitrophenol and to maintain a small, similar spectral contribution for 4-nitrophenol (the main compound). Then we used the wavelength corresponding to a small spectral contribution of 3-nitrophenol as the reference wavelength. It was shown that taking lambda(an) = 266 nm and lambda(ref) = 364 nm, a broad elution peak of 4-nitrophenol was suppressed deconvoluting the peak of 3-nitrophenol. Moreover, quantitation of 3-nitrophenol was achieved without chemometric tools. Under the proposed conditions the detection limits for 3-nitrophenol and other common impurities of 4-nitrophenol used in the pharmaceutical industry (4-chlorophenol, 4-nitrophenol, 1-chloro-2-nitrobenzene, 1-chloro-4-nitrobenzene, 4,4'-bisfenilether, and 4,4'-dichloroazobenzene) were not significantly affected as compared with respective detection limits evaluated in the absence of 4-nitrophenol and using standard detection conditions (lambda(an) = 280 nm and lambda(ref) = 420 nm).

Strategies for peak-purity assessment in liquid chromatography

One of the key requirements for the validation of chromatographic methods is to determine the purity of a chromatographic peak. Statistical modelling of the chromatographic process suggests that overlapping components are highly probable in a chromatogram. Hence extensive efforts have been directed at the development of sensitive, reliable and robust methods to assess peak purity. This is especially the case in the pharmaceutical industry, where liquid chromatography (LC) is widely utilized and the demands on method validation are justifiably high. On-line multiwavelength absorptiometric detection is often used to generate the additional data required to facilitate peak-purity assessment in LC. This approach, using photodiode-array technology, is directly compatible with the aqueous-based reversed-phase LC solvents used extensively in drug analysis. Consequently, this work highlights many of the peak-purity algorithms, which may be applied using LC with diode-array detection. The relative merits of the individual techniques are discussed, and a rationale is developed for their application.

Evaluation of a photodiode array detector for the verification of peak-homogeneity in high-performance liquid chromatography

Photodiode array liquid chromatography detectors are claimed to have the ability of evaluating the homogeneity of chromatographic peaks and this could provide a very powerful tool in support of method development. However, in pharmaceutical analysis, for this to be of practical value it must be capable of detecting inhomogeneities at low levels. In this paper, a test has been devised to challenge the sensitivity of instruments to this application. The test makes use of mixtures of the similar benzodiazepines temazepam and lormetazepam in a chromatographic system which does not separate them. One instrument has demonstrated the ability to detect levels of just 0.5% w/w of one benzodiazepine in the other. Statistical F-tests and t-tests have been used to demonstrate that non-homogeneities have been detected with a high level of confidence. It is concluded that photodiode array detectors have the potential to evaluate the homogeneity of chromatographic peaks with a high degree of sensitivity. However, most instruments do not realize this potential because their software does not make proper use of all the data available.

In vitro recovery of carbamazepine from ensure

The in vitro recovery of three different dosage forms of carbamazepine (CBZ) when dispersed in gastric or intestinal fluids, in the presence or absence of Ensure was determined. An equivalent of 1 mg of pure CBZ from Tegretol 200 mg of conventional tablets, chewtablets of Tegretol 200-mg and Apo-carbamazepine (200-mg tablets) were dispersed in five dissolution mediums (0.5 ml of Ensure; 0.5 ml of Ensure and 1.0 ml of gastric fluid; 1.0 ml of gastric fluid; 0.5 ml of Ensure and 1.0 ml of intestinal fluid; and 1.0 ml intestinal fluid) and mixed for 1 hr and filtered. The filtrates were then assayed for CBZ using a UV spectrophotometer. The mean recoveries of CBZ for all dosage forms in the various dissolution mediums were: Ensure/gastric fluid, 85%; gastric fluid, 75%, Ensure/intestinal fluid, 59%; intestinal fluid, 79%; and Ensure, 58%. The differences in CBZ recovery from gastric or intestinal fluid, in the presence or absence of Ensure were found to be statistically significant (p less than 0.05). The difference in dosage forms were statistically not discernible. The significant differences observed in recoveries of CBZ due to Ensure warrants an in vivo study to realize the clinical implication of administering CBZ with Ensure.

Selection of high-performance liquid chromatographic methods in pharmaceutical analysis. III. Method validation

The most important steps in the validation of high-performance liquid chromatographic (HPLC) methods are discussed. The establishment of system suitability data and the assessment of peak purity are demonstrated on the example of bisquaternary amino steroids. For the recognition of incomplete resolution of adjacent peak pairs, the absorbance-ratio method in which the ratio of absorbances at two preselected wavelengths are plotted as a function of time in combination with the separation of sample components subjected to various chemical and physico-chemical treatments (stress conditions) is applied. The separation power and performance of the HPLC systems are characterized by the system resolution (SR) and system selectivity (SS). The special demands of stability-indicating methods are summarized.

Bioanalytic considerations for pharmacokinetic and biopharmaceutic studies

The correct evaluation of pharmacokinetic and biopharmaceutic data can only be achieved if accurate analytic data are obtained. The accuracy of analytic data depends on the criteria used to validate the method. Consequently, careful scrutiny of drug stability, assay sensitivity, selectivity, recovery, linearity, precision, and accuracy is necessary for the proper interpretation of data. The importance of method validation and its influence on pharmacokinetic and biopharmaceutic data evaluation and interpretation will be discussed.

Computer-aided strategies for archive retrieval and sensitivity enhancement in the identification of drugs by photodiode array detection in high-performance liquid chromatography

An archive retrieval algorithm for high-performance liquid chromatography with UV detection (HPLC-UV) has been developed for the first time for the rapid identification of spectra acquired by rapid-scanning photodiode array detection in HPLC. The algorithm is based on a database of spectra (normalised with respect to area), inverse files of key spectral features, a selective search window, with parabolic weighting factors and least-squares comparison of test and retrieved spectra. The performance of the library search system is demonstrated with respect to a small library of solutes, including: cortisone acetate, ethynyl estradiol, ethisterone, progesterone; caffeine, theobromine, theophylline and 8-chlorotheophylline; morphine and diamorphine; and cycloserine. Since photodiode array detection operates in the domains of both wavelength and time, to generate a matrix of (A, lambda, t) data, the optimum conditions for sensitivity enhancement by ensemble averaging in these domains have been examined. At a given observation wavelength, increase in the detector bandwidth (or "diode bunching") yields a value of delta lambda which gives optimum sensitivity; this value is systematically related to the spectral bandwidth of the analyte. Sensitivity can also be optimised by varying the integration period in the time domain. Sensitivity can be further increased by combination of these instrumental optima, for which there is evidence of dependence on the particular instrument design. Response was found to be linear for detector bandwidths up to twice the optimum value. The comparative sensitivity of some commercially-available photodiode array detectors has been assessed relative to that of conventional detectors under strictly controlled conditions.

Analysis and characterisation of aromatic amino acids, metabolites and peptides by rapid-scanning photodiode array detection in high-performance liquid chromatography

The role of rapid-scanning UV detectors based on the linear photodiode array has been examined in studies on the chromatography of aromatic amino acids, some metabolites and peptides. Multichannel detectors can generate characteristic qualitative information presented as individual spectra, as three-dimensional spectrochromatograms or as contour maps in the (lambda, t) plane. The use of such contour plots for mapping amino acids and their metabolites, and for peptide mapping Tyr-Gly, Gly-Tyr, Phe-Gly, Tyr-Tyr and Phe-Val, is examined. The application of computer-aided methods for validating peak homogeneity by spectral suppression, second derivative in the time domain and by other methods is discussed with respect to Tyr and its metabolite dopamine, which are not resolved by the system described, and which have closely similar spectral characteristics. The use of colour graphics with the IBM Personal Computer is demonstrated for simplifying the presentation of contour maps of amino acids, metabolites and dipeptides.

Applications of rapid-scanning multichannel detectors in chromatography. Plenary lecture

A review of progress in the field of multichannel detection in column and thin-layer chromatography is presented, together with some novel applications of a computer-based, linear photodiode array UV--visible spectrophotometer for detection in high-performance liquid chromatography (HPLC). Computer-aided methods for simultaneous monitoring of the elution profile at three wavelengths with automatic peak detection and capture of UV spectra are described. The continuous calculation of absorbance ratios during elution is discussed as an index of peak homogeneity. A novel technique for the enhancement of qualitative identification in HPLC, based on transformation of captured spectra to the second derivative or to the decadic logarithm, is proposed. These developments are exemplified by a model system of diacetylmorphine and its principal metabolites and degradation products, morphine and 6-acetylmorphine. The potential utility of three-dimensional projections of (A, lambda, t) data is discussed in the context of pharmaceutical, bioanalytical and forensic applications.

The microcomputer revolution

The role of microcomputers in the analytical laboratory is growing rapidly as applications software becomes more generally available. In this survey, the impact of microcomputers on the practice of analysis in pharmaceutical and biomedical areas is examined in the context of available hardware. The advent of 16-bit microcomputers and the implications of computer access to networked data bases are also discussed.

Some strategies for improving specificity and sensitivity in the analysis of anti-cancer drugs

Some approaches are discussed for introducing specificity and sensitivity into analytical methods for anti-tumour agents which include a liquid chromatographic step. Various modes of HPLC have been exploited to monitor these drugs specifically and at therapeutically low levels. The use of column switching technology and chemical derivatization techniques to enhance both specificity and sensitivity are discussed. Multiple columns (linked through switching valves) containing packings exhibiting different affinities for the analytes cisplatin and riboxamide provide (a) a high degree of selectivity with convenient analysis times, (b) the opportunity for preconcentration of analytes, (c) improved longevity of analytical columns, (d) a solution to the 'general elution problem', and (e) allow direct application of biological fluid to the HPLC system. The use of chemical derivatization techniques (pre- and post-column) to achieve improved sensitivity and altered chromatographic and chemical properties of these and other anti-tumour agents (galactitol, tamoxifen, emetine) is also described. The high chemical reactivity of many anti-tumour agents often requires their rapid derivatization after a biological sample is drawn to prevent chemical degradation in the sample vial. The use of chemical and photochemical derivatization techniques combined with spectrophotometric, fluorometric and voltammetric detectors illustrates the power and utility of derivatization technology in trace drug analysis.

Novel applications of multichannel spectroscopy in pharmaceutical analysis

Multichannel detectors based on the linear photodiode array, the silicon vidicon tube and charge coupled (or injection) devices are reviewed with reference to their applications in analytical spectroscopy and high-performance liquid chromatography (HPLC). The use of computer-aided techniques, including second derivative spectroscopy and spectral deconvolution methods, for the multichannel spectroscopic analysis of pharmaceuticals is discussed, and applications are reviewed in clinical chemistry, enzyme analysis and studies on aromatic amino acids in proteins. In HPLC, the principles and applications of digital algorithms for validation of peak homogeneity are considered, with reference to spectral suppression, spectral deconvolution, absorbance ratio and the second derivative of the elution profile. The isometric projections of (A, lambda, t) data, and their cartographically equivalent contour plots, are discussed and the impact of multicolour graphics is assessed. The implications of two-dimensional multichannel detectors for detection of luminescence radiation are considered in the context of analytical spectroscopy, and detection in both HPLC and thin-layer chromatography. The potential contribution of charge coupled and charge injection devices as the next generation of multichannel detectors is considered.