自发单层分散理论用于固体进样检测食品中痕量重金属

In China, there was urgent need for quick determination of heavy metal in foods. Our previous work coupling on-line matrix separation apparatus with solid sampling atomic fluorescence spectrometry (SS-AFS) had nearly gotten the target, except a little limitation come from some vaporization interferences caused by complex matrix. The spontaneous monolayer-dispersion (SMD) theory pointed out that many salts and oxides could spontaneously disperse on supports’ surface under certain conditions to form monolayer or sub monolayer. According to the SMD theory, the diverse food complex matrix could be converted to simple inorganic matrix; then, the interferent would be out of touch with the analyte, so the interference caused by matrix vaporization would probably be removed. Basing on our previous work that cadmium less than 10-6g/g could be completely dispersed on ZrO2 in several minutes, this subject will characterize the ashed mixture of supports and foods to find each component’s distribution of crystal phase and surface phase, using XRD and XPS; then, the dispersion status for each component will be deduced; furthermore, the applicable range of SMD theory (Which had been in widespread use for catalyst preparing field with high concentration, simple component and long dispersion time.) will be studied on elemental analysis for food (Which deal with low concentration, complex matrix and short preparing time system.); and the optimum dispersion condition for matrix replacement will be attained. Moreover, the SS-AFS determination results for dispersed products will be compared with the results using standard method to evaluate the elimination of vaporization interference; meanwhile, the mechanism for matrix interference removed will be studied by on-line ICP-MS. Finally, the quick determination methods for trace heavy metal such as Cd, Pb and As in foods will be established.

当前急需食品中重金属准确快速检测方法，课题前期将在线基体分离用于固体进样原子荧光（SS-AFS）已非常接近解决该问题，但还受限于基体蒸发干扰。自发单层分散（SMD）理论指出，盐类和氧化物在一定条件下可在载体表面自发单层分散，本项目拟利用该理论将多样的食品复杂基体置换为单一高熔点无机基体，使待测元素与干扰物脱离接触，从而有效消除蒸发干扰。拟在前期Cd可快速分散的基础上，利用X射线衍射和X射线光电子能谱，表征食品与适宜无机载体混匀后一定温度下的快速灰化产物，获取晶相和表面相分布，评价各组分的分散状态，研究催化领域（高浓度、简单组分）广泛应用的SMD理论在食品元素分析领域（低浓度、多组分）的适用范围，摸索最佳分散条件以实现基体置换。以SS-AFS检测基体置换产物，考察该方法的基体干扰消除能力，并通过原位电感耦合等离子体质谱技术研究干扰消除机理。最终建立食品中痕量Cd、Pb、As的快速分析方法。

当前急需食品中重金属准确快速检测方法，课题前期将在线基体分离用于固体进样原子荧光（SS-AFS）已非常接近解决该问题，但还受限于基体蒸发干扰。自发单层分散（SMD）理论指出，盐类和氧化物在一定条件下可在载体表面自发单层分散，本项目拟利用该理论将多样的食品复杂基体置换为单一高熔点无机基体，使待测元素与干扰物脱离接触，从而有效消除蒸发干扰。经实验表征后，课题确定采用如下步骤实现待测元素的分散：首先将样品与高熔点载体α-Al2O3混合后300oC左右碳化；之后加入适量乙醇后分散处理，待乙醇蒸干后将样品于不同温度，不同条件下处理，使得待测元素在载体表面分散。课题在尝试利用文献中常用的X射线衍射和X射线光电子能谱法测量载体上待测物元素的残余晶相和表面含量失败后，采用二次离子质谱得到了α-Al2O3表面Cd/Al的显著升高，并且这一结果分散后样品中Cd多次测量的相对标准偏差减少相对应，说明可以通过多次测量的相对标准偏差变化来表征分散。利用这一结果，课题先后考察了不同温度下Cd、As、Pb三种元素在α-Al2O3表面的分散情况，发现Cd、Pb两种元素则在600～700oC半小时左右可以实现良好的分散，对应的测量RSD可以从不经加热处理的20%左右降低到5%左右；而As则因为其氧化物熔点较低，直接处理时尚未分散就出现了较大损失，但在初始阶段加入少量MgO则可以避免损失，且可在600～700oC半小时左右实现良好的分散。由于分散后元素不仅含量降低，而且均匀性也有所改善，非常类似于溶液中的稀释行为，故可以采用该方法实现样品的固相稀释，对高含量样品的固体进样检测有一定意义；另外，由于分散使得待测元素和基体脱离了接触，基体干扰也明显降低，可直接使用液体标准曲线进行测量。总的来说，通过对这些元素的分散基本实现了课题预设的目标——实现了固相稀释并且降低了基体干扰。同时项目还对固体进样中涉及的仪器，和核心装置捕获器做了相关研究，获得了较好的结果。