Simultaneous determination of l-thyroxine (l-T(4)), d-thyroxine (d-T(4)), and l-triiodothyronine (l-T(3)) using a sensors/sequential injection analysis system.

Requirements of high purity and enantiopurity for the raw materials of active substances used for the pharmaceutical formulations involved utilization of high reliable analytical techniques for the analysis of the active compound. Sequential injection analysis system with electrochemical sensors as detectors proved to be a very good alternative for the chromatographic methods, as it is more reliable, not expensive, and faster. Drugs containing only l-thyroxine (l-T(4)) or both l-T(4) and l-triiodothyronine (l-T(3)) are formulated for the dysfunctions of thyroid. A sequential injection analysis system that can use two amperometric immunosensors (for the assay of l-T(3) and l-T(4)) and an amperometric biosensor (for the assay of d-thyroxine, d-T(4)) as detectors is proposed for the purity and enantiopurity tests of the raw materials used for the formulation of the drugs for thyroid. The system proved to be very reliable. The three compounds can be determined on-line in synthesis process control with a frequency of 20 samples per hour.