Rapid determination of the skin irritant p-phenylenediamine (PPD) in henna products using atmospheric solids analysis probe mass spectrometry

Henna (Lawsonia inermis) is applied to hair, skin and fingernails to produce a red-orange or rust colour stain. Producers of henna add para-phenylenediamine (PPD) to create 'black henna' that rapidly stains the skin black when temporary tattoos are applied. However, PPD may cause severe hypersensitivity reactions following skin contact [1]. Since PPD is unstable in solution, laborious, multistep methods of analysis should be avoided. A rapid screening method for the allergen in henna that involves no sample preparation was developed, using an atmospheric pressure solids analysis probe (ASAP) coupled to a quadrupole mass spectrometer (Q-ToF-MS). The PPD standard was repeatedly analysed to optimise the corona current, sample cone voltage, source temperature, and desolvation gas temperature for ASAP-Q-ToF-MS analysis. Samples were analysed by merely touching the sample with the probe capillary before inserting it into the mass spectrometer. Of the 109 samples analysed, 11 tested positive for PPD using ASAP-MS. Ultra-performance liquid chromatography electrospray ionization-mass spectrometry (UPLC-Q-ToF-MS) of the methanol extracts was subsequently used to verify the results from ASAP and to determine the concentrations of PPD in the henna products. The allergen was quantified in the same 11 samples using UPLC-MS analysis and the concentrations ranged from 0.05 – 4.21% (w/w). It is concerning that 10% of the samples, purchased from 16 countries, were positive for PPD. Our results prove that the sensitivity of the ASAP-MS technique is sufficient to allow screening of henna samples for the presence of PPD. This relatively new technique can be applied to commercial products without extraction, sample treatment or chromatographic separation.

Acknowledgements: We are grateful to the Tshwane University of Technology, SAMRC Herbal Drugs Research Unit and the National Research Foundation for financial support.

Keywords: Lawsonia inermis, henna, para-phenylenediamine (PPD), atmospheric pressure solids analysis probe.

Reference:

[1] Sharma S, Dixit V. P-Phenylenediamine and hair dyes. IJPRD 2013; 5: 95 – 102