Inductively Coupled Plasma Atomic Emission Spectrometry with Internal Standards for Water-Soluble Silicon Determination in White Tea

Objective A method for determining the content of water-soluble silicon in white teas using inductively coupled plasma atomic emission spectrometry (ICP-AES) with internal standards was developed.

Method The ICP-AES procedures were first optimized using the internal standards of iridium, indium, germanium, and silicon with 1.00, 2.00, 3.00, 4.00, 5.00, and 6.00 g sample weights in a boiling water bath for a 20, 40, 60, 90, or 120 m extraction. White tea samples were ground, filtered through a 5.0 μm sieve, and oven-dried at 80oC for more than 4 h as a pretreatment prior to spiking the filtrate with the internal standards and making up to volume. Water-soluble silicon was determined by the optimized ICP-AES, and the results correlated to the standard curve.

Result The linearity of the silicon measurement curve against iridium standard showed the highest correlation coefficient of greater than 0.9999 with a detection limit of 7.5 µg·L⁻¹, a relative standard deviation (RSD) of 1.53%, and a recovery rate on spiked standards of 99.4%-104.8%. On the 10 white tea specimens, including 4 Baimudan and 4 Shoumei stored for one, 3, 7, and 10 years, and 2 Paohuang Baimudan (Baimudan tea made of leaves from bushes not plucked for years) stored for one and 3 years, the content of water-soluble silicon in Shoumei increased with increasing storage age, while Baimudan of different ages remained relatively constant; and that in Paohuang Baimudan was significantly lower than those in the others.

Conclusion The newly developed ICP-AES assay was precise and reliable in detecting water-soluble silicon in white teas. It was considered adequate for the determination. By unveiling the differences in the content, in-depth studies on the health effect of varied white teas and quality improvement for new products were significantly facilitaed.