Metal Determination in Tea, Wheat, and Wheat Flour Using Diluted Nitric Acid, High-Efficiency Nebulizer, and Axially Viewed ICP OES

Abstract

This study deals with the determination of Cu, Al, Cd, Mo, Pb, V, Sr, Ni, Cr, Co, Zn, Mn, and Fe in tea, wheat grain, and wheat flour by inductively coupled plasma optical emission spectrometry (ICP OES) with axially viewed plasma. Ultrasonic nebulization (USN) or pneumatic nebulization with aerosol desolvation (PN/DES) is used to introduce the sample solution into the ICP. Solutions of nitric acid (HNO₃) with different concentrations of the acid (4.6 to 9.1 mol L⁻¹) and hydrogen peroxide (H₂O₂) were investigated for sample decomposition in closed vessel (made of TFM-polytetrafluoroethylene (PTFE)) under microwave irradiation. Certified reference materials of tea, wheat flour, or bush branches and leaves were analyzed (0.5 g of sample in the presence of 2 mL H₂O₂ + 3 mL HNO₃ + 2 mL H₂O). Element concentrations found were in accordance with those certified in most cases, excepting for Al and V. Plasma-related matrix effects were not observed as denoted by the signal of the Ar 420.069-nm emission line. The analyte recoveries in spiked chamomile tea and wheat flour samples ranged from 87 to 107 %. By using USN, the limits of quantification (LOQs) of Cu, Al, Cd, Mo, Pb, V, Sr, Ni, Cr, Co, Zn, Mn, and Fe were 0.033, 0.006, 0.023, 0.33, 2.0, 0.33, 0.001, 0.33, 0.07, 0.033, 0.07, 0.001, and 0.02 μg g⁻¹, respectively. Quite similar LOQs were obtained for both nebulizers employed, with the exception of Pb; the LOQ of Pb was three times lower by using PN/DES. Commercialized tea, wheat grain, and wheat flour were analyzed, and the concentrations found were reported.