Identification and quantitation of sodium-thyroxine and its degradation products by LC using electrochemical and MS detection.

High performance liquid chromatography (HPLC) was used in combination with an amperometric and mass spectrometric detection to elucidate and quantitate the degradation products and contaminants of the photo-sensitive Na-thyroxine. Using HPLC with amperometric detection, seven decomposition compounds were separated. These products, which occur mostly as contaminants, were then identified by a developed liquid chromatography-mass spectrometry technique. The same HPLC method was also employed to analyze Na-thyroxine and its degradation products in three commercially available brands of Na-thyroxine tablets.

Evaluation of amperometric detection for the liquid-chromatographic determination of tetracycline antibiotics and their common contaminants in pharmaceutical formulations.

A sensitive high-performance liquid chromatographic technique with amperometric detection has been developed for the determination of seven commercially used tetracyclines in bulk powders and pharmaceutical preparations. The technique is based on the oxidation of these compounds and their contaminants at the glassy carbon electrode. The extraction procedures are simple and the HPLC conditions separate the tetracyclines from their major degradation products. The chromatography was performed using a commercially available octadecylsilane column, with a mobile phase: KH2 PO4 (pH = 2.5; 0.05 M) - acetonitrile (84:16, v/v) and detection at 1.2 V. The technique permits the simultaneous determination of trace amounts of chlortetracycline, demeclocycline, doxycycline, methacycline, minocycline, oxytetracycline and tetracycline as well as the separation of their common impurities (epi, anhydro and epianhydro contaminants) with detection limits of 0.1-1.0 ng microl(-1) and recoveries of 99.1-100.4%. No interference was observed from the commonly present excipients in pharmaceutical formulations.

Liquid chromatography with amperometric detection for the determination of non-steroidal anti-inflammatory drugs in plasma.

A simple and sensitive liquid chromatographic method with electrochemical detection is described for the quantitative determination of the non-steroidal anti-inflammatory drugs diflunisal, indomethacin, naproxen, piroxicam and sulindac in human plasma. Isolation of the drug from the biological fluid is achieved using a Sep-pak RP18 cartridge. Separation of plasma components occurs on a reversed-phase C18 column with a mobile phase consisting of methanol-water-phosphate buffer. For the amperometric detection the potential of +0.9 V was set on the working electrode. The detection limit of the assay is 10-20 ng/ml. The method showed good concordance for plasma samples containing the drugs (r = 0.999) and can be readily utilized for clinical pharmacokinetic studies.