Photochemical properties of purpurin and its implications for capillary electrophoresis with laser induced fluorescence detection

• Purpurin is structurally similar to the dye alizarin and is found in madder root.
• Purpurin is a photo-chemically sensitive compound especially at high pH.
• Separation of dyes using capillary electrophoresis is affected by pH.
• 1st order rates constants for the photo-decomposition of purpurin were determined.
• The most important photodecomposition product of purpurin was phthalate ion.

Purpurin is an anthraquinone dye that is found in plant material such as Madder root. The separation of purpurin using capillary electrophoresis under alkaline condition can be complicated by reproducibility in peak height and peak shape. In this study the poor peak reproducibility of capillary electrophoresis separations was attributed to photochemical properties of purpurin. Experiments were carried out using UV–visible and fluorescence spectrophotometry to measure the 1st order rate constant for the photochemical fading of purpurin. At pH = 9.24 the rate constant was 4.5 × 10−3 s−1 and at pH = 7.0 the rate constant was determined to be 5.0 × 10−5 s−1. Proton NMR, thin layer chromatography, gas chromatography and UV–visible spectrophotometry techniques were also employed to identify the photo-degraded products of purpurin.

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