Three-dimensional derivative spectrochimatograms in high-performance liquid chromatography and their implications for peak homogeneity validation

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.

Standardization of a multi-wavelength UV detector for liquid chromatography-based toxicological analysis

The performance of a multi-wavelength UV detector for automated drug identification following liquid chromatographic separation was evaluated. The ability of selected wavelength ratios to distinguish two closely related drugs was considered at different concentrations. Calibration of the detector based on wavelength ratios was then utilized to standardize two different detectors and to evaluate instrument-to-instrument variation of a series of detectors. Reproducibility of the second-derivative zero intercept for these drug spectra was also evaluated. Standardization of detector performance by reference to these two parameters permitted the transfer of UV spectral libraries stored on one instrument to another without compromising the reliability of qualitative data.

Determination of norethindrone stability on red delicious apples

A stability-indicating assay procedure for the determination of norethindrone on the surface of Red Delicious apple slices was developed. The apple slice is homogenized in pH 10 borate buffer solution and norethindrone is then partitioned into chloroform using a diffusion chamber that has a membrane that is permeable to hydrophobic compounds separating the buffer and chloroform. An aliquot of the chloroform extract is then evaporated to dryness and reconstituted in tetrahydrofuran. The reconstituted sample is then chromatographed on a C18 reversed-phase column using a mobile phase consisting of water:tetrahydrofuran:methanol (63:26:11, v/v/v). Detection is in the UV range at 254 nm. The chromatographic system is specific for norethindrone in the presence of its autoxidation products. Stability data indicate that norethindrone is stable for up to 6 h on the surface of Red Delicious apples.

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.