Determination of shelf life of Chelidonium majus, Sambucus nigra, Thymus vulgaris and Thymus serpyllum herbal tinctures by various stability-indicating tests

Stability testing of herbal preparations has recently been recognized as essential for quality control to support their shelf life. Various stability-indicating tests were assessed for their predictive power in herbal tincture stability testing and compared to reference quantitative determination of marker compounds. Herbs from Western herbal medicine with different active constituents were selected: Chelidonium majus, Sambucus nigra flowers, Thymus serpyllum and Thymus vulgaris. Their freshly prepared and commercially available tinctures were tested for stability under normal and accelerated conditions. Quantitative chromatographic assays were developed for chelidonine in Chelidonium tincture, for isoquercitrin, rutin and quercetin in Sambucus tinctures, and for thymol in Thymus tinctures. Additional procedures were assessed for their predictive power on tincture stability: chromatographic profiling, spectrophotometric evaluation of tincture colour, DPPH antioxidant assay. With the exception of the DPPH assay in Sambucus tincture, none of the assessed stability-indicating tests was satisfactory in comparison with reference determination. Sambucus tincture was stable for more than six months, while Chelidonium and Thymus tinctures were stable for less than 1.5 months with the decrease in marker compound >10%. Compound borneol is proposed as a marker for the deterioration of Thymus tinctures. Chelidonium tincture was additionally tested for its stability on UV and visible light. Chelidonine degrades in the tincture under the visible light. The results of the present study implicate that the only assays suitable for stability testing of different tinctures are those that determine concentration of the active compounds or some active compound-linked property of the tincture.