Isothiazolinones Quantification in Shampoo Matrices: A Matter of Method Optimization or Stability Driven by Interactions?

Methylisothiazolinone (MI) is one of the most used preservatives in shampoos and also one of the most effective. A preservative mixture known as Kathon (TM) CG is commercially available. It contains 5-chloro-2-methyl-4-isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) (3:1) and stabilizers. The aim of this study is to evaluate the influence of formulation factors in the quantification and stability of isothiazolinones in shampoos. Two shampoo bases containing Kathon (TM) CG as a preservative were prepared. Some ingredients that are at risk of interfering with the preservative stability were added to these formulations. The preservative was quantified by HPLC-DAD (High-performance liquid chromatography with a diode-array detector) after preparation of the formulation and after storage at room temperature and at 40 degrees C. The addition of magnesium silicate proved to be essential for the breakdown of the interaction between the matrix and the analytes in the extraction procedure. The content of CMI/MI decreased right after preparation indicating that immediate interactions between CMI/MI and the ingredients may have occurred after preparation resulting in a decrease in the preservative concentration. Detrimental interactions between the ingredients, regarding the stability of the isothiazolinones were detected immediately after preparation and over time resulting in the reduction of CMI/MI concentration in these cosmetic shampoos.