Multielement analysis of tea leaves by inductively coupled plasma optical emission spectrometry using slurry nebulisation

Abstract

In order to evaluate the slurry nebulization method as an alternative method for the analysis of tea, the metal ion content of tea samples of different origins was determined. The concentrations of six elements, Al, Ba, Ca, Mg, Mn and Zn, were determined by introducing tea as a slurry into an inductively coupled plasma optical emission spectrometer (ICP-OES). Calibration was performed by using either aqueous standard solutions or by using the simplified generalized standard additions method (GSAM). The simplified GSAM is an expansion of the conventional standard additions method, based on the principle of varying both the sample mass and the amount of standard solution added. Initially the tea was ground for 3 h prior to slurry preparation. A carbonization procedure was included before the sample was ground. The grinding time was reduced to only 30 min. For comparison, tea was also digested by boiling in acid and being introduced into the plasma as an aqueous solution. The results obtained using slurry analysis were much higher than the results obtained from digested tea using aqueous standards for calibration. The metal content of the tea samples was found to be suitable for differentiating amongst the tea samples studied. The accuracy of the method was checked by analyzing a tea certified reference material. The Student's t-test showed that values obtained using slurry nebulization were close to the certified values at a 95% confidence level. The simplified GSAM was applied to further confirm the accuracy of the slurry technique, and the values obtained using this method were comparable with those obtained using the slurry method with aqueous calibration. The slurry method can, therefore, be successfully applied to the analysis of tea leaves without the need to pre-dissolve them. This could avoid the use of hazardous chemicals, incomplete dissolution and loss of volatile analytes.