绿茶中28种元素含量测定的消解条件比较

Effects of Digestion Conditions on Measuring Contents of 28 Elements in a Green Tea Standard

前处理条件对准确测定样品中多矿质元素含量至关重要,本研究设置4种酸消解体系(HNO3、HNO3-H2O2、HNO3-HF、HNO3-HF-H2O2)及5组消解温度(100~120℃、100~120~140℃、100~130~150℃、100~130~160℃、100~120~140~160~180℃),探究不同消解条件对绿茶标准物质GBW10052中28种多元素浓度的影响。结果表明,不同酸消解体系及消解温度下的各元素浓度均有差异,相比Ca、K、Fe、Mg、Na等主量元素,As、Cr、Cs、Cd、Li、Ni、Se、Rb、Tb、Th、Tl、U、V、Y等微量及痕量元素浓度变化更显著。HNO3中各元素浓度最低,14个元素的相对误差(Relative error,RE)大于25%;混合酸消解效果较好,相比于HNO3,混合酸中大部分元素浓度更接近参考值。HNO3-H2O2中,除Mo、U、Se、V、Si、Pb外,其余元素浓度增大,RE减小;HNO3-HF中U、V、Si、Li、Zn、Sr、Cr等浓度继续增大,RE持续减小,As、Pb、Cd等浓度减小,RE增大;HNO3-HF-H2O2中As、Pb、Cd等持续更减小,RE增大,其余元素浓度更接近参考值,与HNO3-HF相比,HNO3-HF-H2O2中各元素RE减小,但变化不显著。升温能促进溶液中部分多元素消解,以HNO3-HF-H2O2为例,消解温度为100~140℃的体系对获取主量元素更有利;高温消解对微量及痕量元素影响较大,尤其是As、Pb、Cd、Cr等易挥发元素,当温度超过140℃时,As、Pb、Cd、Cr亏损严重,RE均高于25%。因此,应该根据目标元素选择相应的消解条件,从而提高分析结果的准确性。

Pretreatment conditions are of great importance to measure contents of mineral elements in samples precisely.This study employed four acid digestion systems of different acid reagents(HNO3,HNO3-H2O2,HNO3-HF and HNO3-HF-H2O2)and five digestion temperature setups(100-120℃,100-120-140℃,100-130-150℃,100-130-160℃and 100-120-140-160-180℃)to explore effects of different digestion conditions on test results of contents of 28 elements in a standard green tea substance(GBW10052).The results showed that measured contents of elements in the standard sample varied notably under different digestion systems or/and digestion temperatures,variations of minor elements,such as As,Cr,Cs,Cd,Li,Ni,Se,Rb,Tb,Th,Tl,U,V and Y,were more significant than those of major elements,such as Ca,K,Fe,Mg and Na.Briefly,measured concentrations for all elements were the lowest and relative errors of 14 elements were higher than 25%in the pure HNO3 digestion system while measured element concentrations by mixed acid digestion systems were more closely equal to the reference values,implying mixed acid digestion systems provided better results than the single acid digestion system did.It was found that,compared to the single acid digestion system,the HNO3-H2O2 mixed acid digestion system reduced relative errors of all elements except Mo,U,Se,V,Si and Pb;measured concentrations of U,V,Si,Li,Zn,Sr and Cr increased while corresponding relative errors decreased in the HNO3-HF digestion system;the digestion system measured concentrations more closer to their reference values for all elements except volatile elements such as As,Pb and Cd.Rising digestion temperature promoted the digestion efficiency of target elements.Take the HNO3-HF-H2O2 digestion system as the example,the digestion temperature range of 100-140℃was optimum for major elements,when the temperature was set above 140℃,it impaired the results of minor and trace elements,especially the volatile elements such as As,Pb,Cd and Cr.In summary,we suggest that the appropriate digestion condition should be selected on comprehensively considerations of object elements to improve the analytical precision.