Abstract: Inductively coupled plasma-mass spectrometry (ICP-MS) was used to measure the aluminum content in the quality control sample of milk powder, and an X-control chart was established for runaway judgement and handling during the operation process. The precision and median line of the control chart for new data points obtained one year later were evaluated using Minitab software with test for equal variance and analysis of variance. The standard deviation for the previous 25 data points was 0.186 mg/kg compared to 0.126 mg/kg for the 35 new ones, while the median values of the two groups did not differ from each other. More reliable control limit was calculated based on all the data points, and the new median value was 1.334 mg/kg with a standard deviation of 0.154 mg/kg. The decrease in standard deviation suggested that the precision was improved with smaller data fluctuations. In conclusion, calculation of new and more reliable control limit from more data is useful to find obvious systematic errors, abnormal data and slow changing trend in a timely and effective way and can provide evidence to decide whether data should be retained or removed so as to directly reflect the stability and tendency of the detection.

Key words: inductively coupled plasma-mass spectrometry; milk powder; aluminum content; control chart; assessment

摘要： 采用电感耦合等离子体-质谱（inductively coupled plasma-mass spectrometry，ICP-MS）法测定乳粉质控样中铝含量，建立乳粉基质质控样铝含量的X-控制图；将控制图应用于运行过程中的失控判断及处理，利用Minitab软件的等方差检验和方差分析对1 年后产生新数据点的控制图精密度和中位线进行评估。结果表明：新增35 个数据的标准偏差由0.186 mg/kg变为0.126 mg/kg，中位值没有明显变化，以1 年后新增后的所有数据点计算新的、更为可靠的控制限，新的中位值为1.334 mg/kg，新的标准偏差为0.154 mg/kg；标准偏差变化是基于方法和仪器等的成熟稳定条件下，新的标准偏差较初始标准偏差有所减小，说明整体精密度变好，数据波动性减小，因此利用更多数据重新计算可得到新的、更为可靠的控制限。此方法可以及时、有效地发现明显的系统误差、异常现象和缓慢变化趋势，同时也是决定结果数据取舍的依据之一，从而可以直观地反映检测的稳定性和趋向性。

关键词: 电感耦合等离子体-质谱法；乳粉；铝含量；控制图；评估