Elemental and structural analysis of silicon forms in herbal drugs using silicon-29 MAS NMR and WD-XRF spectroscopic methods

The objective of this work was to study concentration of silicon and its structural forms present in herbal drugs. Equisetum arvense and Urtica dioica L. from teapot bags, dietary supplements (tablets and capsules) containing those herbs, dry extract obtained from a teapot bag of E. arvense, and samples of the latter herb harvested in wild habitat over four months were studied using wavelength dispersive X-ray spectroscopy (WD-XRF) and high-resolution solid-state 29Si NMR. The highest concentration of Si, ca. 27 mg/g, was found in the herbal material from the teapot bags containing E. arvense. The Si content in natural E. arvense (whole plants) increased from May to August by ca. 7 mg/g, reaching value 26 mg/g. Three different silicon forms were detected in the studied herbal samples: Si(OSi)4 (Q4), Si(OH)(OSi)3 (Q3) and Si(OH)2(OSi)2 (Q2). Those sites were populated in E. arvense in the following order: Q4 ≫ Q3 > Q2. A dramatic, ca. 50-fold decrease of the Si concentration during the infusion process was observed. The infusion process and the subsequent drying procedure augmented population of the Q4 sites at the cost of the Q2 sites. The WD-XRF and 29Si NMR methods occurred useful and complementary in the study of herbal materials.