Novel applications of multichannel spectroscopy in pharmaceutical analysis

The efficient use of microcolumns in chromatographic systems

Microcolumn techniques involving pre-column concentration and post-column derivatization make good use of micro-scale high-performance liquid chromatography. The efficient use of microcolumns is illustrated by some applications to the analysis of amino acids, bile acids and catecholamines. The potential of new detector techniques based on the multichannel photodiode array detector, and on immobilized enzyme post-column reaction systems with fluorimetric detection, is also discussed.

Evaluation of multiwavelength chromatograms for the quantification of mixtures of pesticides by high-performance liquid chromatography-diode array detection with multivariate calibration

Three multivariate calibration methods, partial least squares (PLS-1 and PLS-2) and principal component regression, were applied to the simultaneous determination of the five pesticides iprodione, procymidone, chlorothalonil, folpet and triazophos by high-performance liquid chromatography with diode array detection. Such detection gives multiwavelength chromatograms from a single analysis of one sample. In this paper, calibration models at two different wavelengths were developed to resolve mixtures of five pesticides with overlapping chromatographic peaks. The first model, carried out at 220 nm as detector compromise wavelength, yielded satisfactory sensitivity for accurate estimation of the concentration of iprodione, procymidone, chlorothalonil and folpet and the second model, at 200 nm, was used for accurate estimation of triazophos. Both calibration models were evaluated using the chromatograms and first-derivative (1D) chromatograms by predicting the concentrations of independent test set samples. Finally, the proposed 1D calibration models were successfully applied to the determination of these pesticides in groundwater and soil samples. In all cases, the PLS-1 calibration method showed superior quantitative prediction ability than the PLS-2 or principal component regression methods.

Estimation of impurity profiles of drugs and related materials. Part 14: the role of HPLC/diode-array UV spectroscopy in the identification of minor components (impurities, degradation products, metabolites) in various matrices

The possibility of the rapid identification of drug related minor components by HPLC/diode-array UV spectroscopy is demonstrated by three examples. Hydroxylated impurities (degradation products) of norgestrel (6 alpha and beta, 10 beta-hydroxy derivatives) were identified on the basis of their UV spectra and retention matching with the synthesized impurities. The position of the phenolic hydroxyl groups in the mono- and dihydroxylated metabolites of bisaramil was established by UV spectroscopy and retention matching with the synthesized metabolites. The discrimination between the isomeric 4-ene-3-ketone and 1-ene-3-ketone components in crude 19-nortestosterone, product of the Birch reduction of 3-methoxy-1,3,5(10)-oestratriene-17 beta-ol, was also based on the diode-array UV spectra.

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.

Digital techniques for luminescence detection in liquid chromatography with an intensified linear photodiode array

The use of a rapid-scanning fluorescence detector in liquid chromatography is examined in the context of its potential contribution in pharmaceutical and biomedical analysis. The data generated by an intensified linear photodiode array detector is presented as an isometric plot of (I(em),lambda(em),t) at a defined excitation wavelength. Techniques examined for peak homogeneity assessment include: emission spectral normalisation at points through the chromatographic peak profile, second order differentiation (d(2)I/dt(2)) and the fluorescence emission ratio chromatogram, generated by calculating the ratio of emission intensifies at two defined emission wavelengths, at all points in the time domain elution profile. These techniques are illustrated with reference to some polynuclear aromatic hydrocarbons and to the beta-blocker drug atenolol and its related impurities. Future developments of this new detector technology in LC are also considered.

Analysis of steroids. XXXVIII. The use of high-performance liquid chromatography with diode-array UV detection for estimating impurity profiles of steroid drugs

High-performance liquid chromatography (HPLC) diode-array UV-spectrophotometric detection is used for estimating impurity profiles of steroid drugs. It is shown to be a very useful first screening method for the identification of UV-active impurities and degradation products, giving a rapid answer to many questions or at least providing important initial information to complement the results obtained by other spectroscopic techniques. In this paper the estimation of the impurity profiles of ethynyloestradiol, norgestrel, and norethisterone, and of the degradation product of RGH-1113 (3-chloro-, 6,9-difluoro-11 beta,16 alpha,17 alpha,21-tetrahydroxy-1,3,5-pregnatrien-20-one 16,17-acetonide 21-acetate) will be discussed.

Rapid spectrophotometric pH measurements based on the dual-wavelength technique

A spectrophotometric method for the measurement of pH in solutions of lidocaine hydrochloride (Xylocaine) for injection is presented. 4-Nitrophenol is used as an indicator for determinations of pH in the range 6.5-7.0. The method was found to be faster than the conventional potentiometric method, mainly due to the utilization of a computer-controlled photodiode array spectrophotometer for the measurements. A further advantage with the spectrophotometric method is that errors arising from varying liquid junction potentials are avoided.

Determination of coumermycin A1 in plasma by reversed-phase high-performance liquid chromatographic analysis

Coumermycin A1 is an antibiotic isolated from Streptomyces hazeliensis var. hazeliensis nov. sp. as a sodium salt which exhibits antistaphylococcal activity. A sensitive and selective high-performance liquid chromatographic method was developed for the determination of the compound and three known homologues which are extracted from plasma buffered to pH 6.5 into methyl-tert.-butyl ether-2-propanol (97.5:2.5), the residue of which is dissolved in the mobile phase and analyzed by automated reversed-phase high-performance liquid chromatography using UV detection at 330 nm for quantitation. Novobiocin is used as the internal standard. The method was used to determine the plasma concentration--time profile of coumermycin A1 in the dog following a single intravenous administration of a 12 mg/kg dose of a solubilized dosage form of the bulk drug substance.

Analytical strategies for therapeutic monitoring of drugs in biological fluids

Therapeutic drug monitoring can involve quantitation in either microgram, nanogram or picogram concentrations present in a complex biological matrix (whole blood, urine or tissue). The chemical structure of a compound influences not only the analytical method best suited to its quantitation, but also its acid/base character (pKa) and its extractability. The dose administered, the bioavailability of the dosage form, and the pharmacokinetic profile of the drug govern the circulating concentrations of either the parent drug and/or its metabolites present in vivo, and dictate the ultimate sensitivity and specificity required of the analytical method. The degree of sample preparation required is dependent on the analytical method used (gas--liquid chromatography, thin-layer chromatography, high-performance liquid chromatography) and on the tolerance of the specific type of detection system to contamination. Factors leading to compound losses during sample preparation (adsorption, stability) are critical at low concentrations and can adversely affect the reliability of an assay, therefore maximizing the overall recovery of the assay is essential not only for high sensitivity but also for good precision and accuracy. Therefore, the criteria to be used in sample preparation should aim to optimize all of the above factors in the overall development of a reliable and validated method for the compound suitable for use in clinical therapeutic monitoring.

Determination of furazolidone residues in eggs by HPLC followed by confirmation with a diode-array UV/Vis detector

A sensitive HPLC method for the determination of furazolidone residues in eggs (10-1,000 micrograms/kg) is described. Recovery is about 86%. With the aid of a UV/Vis Diode-Array detector confirmation up to the 15-ppb level was possible. In order to test this method with "real" samples, three laying hens received 30 mg each of furazolidone in feed (single dose). The eggs were collected for five days. After five days traces of furazolidone (5 micrograms/kg) could still be detected.

Flow injection analysis: A new approach to pharmaceutical determinations

A review is presented in which the fundamental principles, instrumentation and practical aspects of flow injection analysis (FIA) are discussed. A particular focus is put on applications in pharmaceutical chemistry, which offers a wide scope to the new technique.

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.