Method development for the determination of elements in Hypericum perforatum L. (St John's wort) herb and preparations using inductively coupled plasma-optical emission spectroscopy and microwave digestion

Objectives A method was developed to analyze St John's wort (Hypericum perforatum L.) herb and preparations using inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the quantity of 11 elements (Al, B, Ba, Ca, Cu, Fe, Mg, Mn, Ni, Sr and Zn). Methods This study includes the evaluation of digestion acids and calibration methods, as well as instrumental parameters such as choice of nebulizer and emission wavelength. Key findings Two nebulizers (Conikal and SeaSpray) performed similarly for most elements, and two optimum wavelengths were determined for each element. Five acids were evaluated for the digestion of the Polish Certified Reference Material Tea Leaves (INCT-TL-1), while three were taken forward to use for the different St John's wort formulations (i.e. herb, capsule and tablet). A simple protocol using 5 ml HNO3 was sufficient in most cases; however, variability was observed for elements often bound in silicates (e.g. Al, Fe and Zn). An external weighted calibration was also found to be preferential over unweighted, and the use of standard addition affected some concentration values up to 20%. Conclusions Therefore, this paper presents the development and optimized method parameters to be used with ICP-OES that will allow the analysis of 11 key elements present in St John's wort herb and preparations.