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The aim of the current study was to analyze the stability of rosmarinic acid in ethanolic tinctures of lemon balm ( Melissa officinalis  L.), oregano ( Origanum vulgare  L.), peppermint ( Mentha x piperita ), rosemary ( Rosmarinus officinalis  L.), sage ( Salvia officinalis  L.), and thyme ( Thymus vulgaris  L.). High-performance liquid chromatography with diode-array detection (HPLC–DAD) was employed to monitor the concentration of the marker compound over a six month period. Furthermore, the tinctures were also evaluated for caffeic acid, total phenolic content, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity. We observed that the concentration of rosmarinic acid in tincture stored in closed amber glasses at ambient temperature decreased significantly during 6-month storage. Furthermore, our study squarely confirms the fact that a part of rosmarinic acid is converted to caffeic acid. The tested tinctures can be listed in the following order according to the greatest stability of the marker compound: rosemary &gt; peppermint &gt; oregano &gt; lemon balm &gt; thyme &gt; sage. The results of the study indicated a linear relationship between DPPH values and total phenolic ( R  2  = 0.92) or rosmarinic acid ( R  2  = 0.85) contents.

Investigation of the long-term stability of various tinctures belonging to the lamiaceae family by HPLC and spectrophotometry method