impurity center

例句：

1. 1.The purity center of the sample is located at the bottom of the container.
   样品的纯净中心位于容器底部。
2. 2.The impurity center in the soil is mainly due to the presence of heavy metals.
   土壤中的杂质中心主要是由于重金属的存在。
3. 3.The purity center of the sample is determined by measuring the concentration of the desired substance.
   样品的纯净中心是通过测量所需物质的浓度来确定的。