| Zinc(Zn) and cadmium(Cd) are typical heavy metal elements for agri-food safety. Between them, Zn is an essential element required as part of a healthy diet, especially for infant and baby growth, while it is harmful at an excessive level; Cd is responsible for the Itai-Itai Disease and harmful to the human’s kidney, bone and respiratory system. At present, the spectrometric instruments with a conventional pneumatic nebulizer were usually employed to detect Zn and Cd in agri-food. Among them, inductively coupled plasma mass spectrometry(ICP-MS) is widely applied because of its high analytical sensitivity, wide dynamic linear range, strong anti-interference capacity and multi-elements analysis. However, the requisite acid digestion could cause a great decrease of analysis speed and an increasing possibility of pollution and analytes loss, which would not be competent to the rapid detection. In comparison, the solid sampling(SS) technique without the digestion pretreatment could be simpler, faster and of higher introduction efficiency for rapid detection of Zn and Cd in fresh agri-food samples. In this work, on the basis of our previous studies, a novel gas line system was designed and optimized to couple the solid sampling electrothermal vaporization(SS-ETV) apparatus with the ICP-MS instrument and then a sensitive, stable, quick, simply and environmental-friendly method was established to detect Zn and Cd in agri-foods simultaneously.1. Assembling the ETV-SS-ICP-MS instrument. To achieve the instrumental stability, a “double gas lines” design consisting of an extra and carrier gas lines was employed, meanwhile matched with a Teflon interface between the solid sampling apparatus and ICP-MS. For gas lines, Ar and H2 mixture(v: v = 96:4) was used. The solid sampling apparatus was composed by an on-line ashing furnace, porous carbon vaporizer, and sampling boat and TC, same as that in our previous study.2. Trapping and releasing Zn by TC was found for the first time. On the basis of multi-element analysis by ICP-MS, Zn and Cd were found to be simultaneously trapped and released by TC. The linear regression coefficient(R2) of Zn calibration was ≥ 0.996 and the relative standard deviation(RSD) of 11 repeated measurements was 3.1%. Besides, the vaporized, trapped and released Zn species were proven to be almost Zn0 by AFS and XPS, which was consistent with Cd in the previous study.3. Optimizing the parameters of ETV-SS-ICP-MS instrument. The extra gas flow rate, carrier gas flow rate, ETV power and TC releasing time were investigated in this work. The optimized conditions were as follows: 0.3 L/min for extra gas, 0.7 L/min for carrier gas, 80 W(20 s) for ETV and 30 W(0.6 s) for TC releasing procedure, respectively. Under the optimized condition, the limit of detection were 1 pg for Zn and 0.1 pg for Cd. When using the mixed standards of Zn and Cd, the linear ranges were 1 pg- 2.5 μg for Zn and 0.1 pg- 50 ng for Cd, R2>0.99. The RSD of 11 repeated measurements were 3.1% for Zn and 1.4% for Cd. The results mentioned above indicated a well sensitivity and precision for the Zn and Cd measurement by ETV-SS-ICP-MS.4. Establishing and applying the analytical method to detect Zn and Cd in agri-food. Using the matrix-matching calibration, the ETV-SS-ICP-MS method for Zn and Cd in grain, spinach, tea, pork liver and seafood samples was established here. The method limit of quantification were 0.3 μg/kg for Zn and 0.03 μg/kg for Cd(sample size was 10 mg). The found Zn and Cd concentrations in CRMs by ETV-SS-ICP-MS were in the range of the certified values and the spiked recoveries were 88%- 116%, which indicated that the established ETV-SS-ICP-MS method could meet the need of the Zn and Cd determination in agri-foods. Furthermore, the recommended method was also employed to detect Zn and Cn in 14 grain samples, whose results were not significantly different(p > 0.05) from that by microwave digestion ICP-MS method, and RSDs were all less than 10%(n = 3). |
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