对于土壤中重金属元素的检测, 传统的化学分析方法测试周期长, 前处理复杂, 使用的强酸还会对环境造成二次污染, 能量色散X射线荧光光谱法具有无损、 快速、 前处理简单、 仪器轻便等优点, 特别适合现场快速检测, 但用该方法对镉元素进行检测时, 通常用到的K系特征线能量较大, 普通能谱的检出限较高。 基于能量色散X射线荧光光谱技术, 研究了土壤中痕量重金属镉的快速检测方法, 通过选择合适的仪器部件, 搭建仪器测试系统, 综合考虑待测元素的谱峰强度和相对强度, 对仪器结构和测试条件进行优化。 研究表明, 峰强度随着管电流的升高基本上是线性增加的, 而相对强度则没有明显的变化, 对于镉的测试, 在光管条件允许的情况下选择尽可能大的管电流进行测试, 之后, 综合考虑镉的峰强度和相对强度随滤光片厚度以及管电压的变化情况, 使用理论标准偏差确定测试最优条件为: 管电压为55 kV, 管电流为48 μA, 滤光片为1.25 mm钼片; 测试时间会影响测试结果的相对标准偏差, 在测试时间小于500 s时, 峰强度的相对标准偏差随着测试时间的增加是降低的, 在测试时间大于500 s时, 峰强度的相对标准偏不再有明显的变化趋势, 由于测试结果的相对标准偏差越小, 测试结果的短期精密度越好, 仪器的重现性就越好, 因此测试时间最终选择为500 s; 样品测试条件也会对测试结果产生影响: 随着测试薄膜厚度的增加, 镉的峰强度和相对强度均是降低的, 根据实验结果, 最终选择厚度为12.5 μm的聚酯膜进行测试; 镉的峰强度和相对强度随着样品质量的增加而增加, 在样品质量大于3 g时, 镉的峰强度随样品质量的增加变化比较缓慢, 相对强度则没有明显的变化, 选择样品的质量大于3 g进行测试; 随着含水量的增加, 镉的峰强度和相对强度均会略有降低, 因此水分会对测试结果产生影响, 对土壤样品的测试应该风干或烘干。 使用以上经过优化的测试条件, 用国家标准样品和电感耦合等离子体质谱法定值的样品绘制工作曲线, 线性相关系数可达0.993; 使用镉含量为1.12 mg·kg^-1的国家标准样品GSD-10测试11次, 测试的结果的标准偏差为0.09, 相对标准偏差为8.22%; 用高纯二氧化硅测试方法检出限, 可达0.16 mg·kg^-1, 小于国标一级土壤的限值; 测试实际样品, 并与电感耦合等离子体质谱法测试的结果进行对比, 测试结果的一致性较好。 通过对仪器结构和样品测试条件进行优化, 基于能量色散X射线荧光光谱法对土壤中痕量重金属镉的检出限有了很大的降低, 对污染地区土壤镉的快速筛查及大面积测定意义重大。

For the detection of heavy metals in soil, conventional methods of chemical analysis are of many deficiencies, such as long time consuming, complex pretreatment process, and the strong acid used will cause secondary pollution to environment. However,energy dispersive X-Ray fluorescence spectrometry have many advantages, such as non-destruction, fast test, simple pretreatment, easy to carry etc., which make it especially suitable on rapid on-site monitoring. When using this method to detect cadmium, the energy of K series characteristic line is larger, making the limit of the detection relatively high for general EDXRF. Based on the energy dispersive X-Ray fluorescence spectroscopy, the rapid detection of trace cadmium in soil was studied. Appropriate devices were chosen to constructing the instrument platform, and the instrument structures and test conditions were optimized considering peak intensity and relative intensity of determined element.Research showed that the peak intensity increased linearly with the increase of tube current, while relative intensity had no obvious change. Hence, for the detection of cadmium, tube current chosen were as large as possible with the permission of X-Ray tube. Afterwards, considering peak intensity and relative intensity of cadmium with different thickness of filter and tube voltage, the optimized conditions could be ascertained by using theoretical relative deviation. And the optimal conditions were as follows: the tube voltage was 55 kV, tube current was 48 μA, the filter was molybdenum with the thickness of 1.25 mm. The measurement time had an influence on relative standard deviation of results. When measuring times were less than 500 seconds, the relative standard deviation of peak intensity decreased with time increasing, while having no more obvious change after measuring times longer than 500 seconds. And the relative standard deviation was smaller, the short-term precision of results better, which made repetition appropriate for the detection. So 500 seconds was chosen as measuring time. At the same time, test conditions of samples also had an influence on the results. Since both the peak intensity and relative intensity decreased with increasing thickness of film, the thickness of 12.5 μm of polyester film was chosen to be used. And both peak intensity and relative intensity of cadmium increased with increasing weight of samples at first, but when the weight was more than 3g, as the weight increased, the peak intensity of cadmium changed slowly and relative intensity had no obvious change. So the weight of samples should be more than 3 g. The peak intensity and relative intensity decreased as water content increased in soil, which meant that soil moisture would affect test results, and soil samples should be air drying or oven drying. Adopting the optimized conditions above, the linear correlation coefficient was 0.993 of samples when using national standard samples or samples determined by inductively coupled plasma mass spectrometry. A national standard sample was tested 11 times in which the concentration was 1.12 mg·kg^-1, and the standard deviation of the results was 0.09, and the relative standard deviation was 8.22%. The limit of detection of this method was 0.16 mg·kg^-1 detected by high purity silicon dioxide for 11 times, which was below the limit of the first grade soil. Compared with inductively coupled plasma mass spectrometry, the test results had significant consistency. After optimizing instrument structure and test conditions of samples, the limit of detection was lower for trace cadmium in soil based on energy dispersive fluorescence spectrometry, which will play an important part in the rapidly screening of pollution area and the measurement in large area of cadmium.