ICP-MS测定淀粉铝含量的不确定度评定及改进方法

Improving Analytical Methods by Uncertainty Evaluation with the Case of Determination of Aluminum in Starch Products by ICP-MS

测量不确定度,表征合理地赋予被测量之值的分散性。应用ISO/IEC Guide 98:1993Guide to the expression of uncertainty in measurement(简称GUM方法)的自下而上的策略进行测量不确定度的评定,剖析完整的分析测量过程,可以找出影响测量结果准确性的主要因素;采取针对性的措施,设法消除或降低这些因素的影响,可以改进测量方法。以电感耦合等离子体质谱法测定淀粉和面包糠中铝含量为例,其不确定度来源于测量重复性、最小二乘法拟合工作曲线的过程、标准储备液及其分取稀释过程的不确定度、溶液体积及样品称量过程的不确定度。经过各不确定度分量的评定,得知主要分量是由最小二乘法拟合工作曲线引入的相对标准不确定度u_(rel)(cAl)_1,其次是由配制标准溶液系列的稀释过程引入的相对标准不确定度u_(rel)(cAl)_3、由测量结果的重复性引入的相对标准不确定度urel(rep)。因此,采取较高灵敏度的质谱工作模式、增加测定次数、合理选取校准曲线的系列浓度点数值、选用相对误差较小的量器等措施进行方法改进。改进之后,三个主要分量u_(rel)(cAl)_1,u_(rel)(cAl)_3,u_(rel)(rep)分别从(0.035 8,0.013 2,0.008 5)降为(0.006 0,0.010 5和0.003 3),铝量的合成相对标准不确定度从0.039降为0.013,扩展不确定度从1.8mg·kg^(-1)降为0.4mg·kg^(-1)(k=2),效果显著。

The measurement uncertainty is a non-negative parameter associated with the result of a measurement that characterizes the dispersion of the quantity values that could reasonably be attributed to the measurand.In the present study measurement uncertainty is estimated using the GUM（ISO/IEC Guide 98：1993Guide to the expression of uncertainty in measurement）bottom-up approach.The steps were followed：specifying the measurand;identifying all the associated sources of uncertainty;quantifying the uncertainty components;combining the uncertainty components;determining the extended combined standard uncertainty;reviewing the estimates and reporting the measurement uncertainty.In this process,the major uncertainty components with greater impact were identified;try to eliminate or to reduce the impact of these components can improve measurement methods.Examples were the determination of aluminum in starch and bread crumbs by inductively coupled plasma-mass spectrometry（ICP-MS）.The uncertainties of aluminum contents were from measurement repeatability,variability of calibration curve,standard stock solution,dilution,solution volume and sample weighing.The data indicated that the major contributions to the uncertainty budget originating from u_（rel）（cAl）_1（the relative standard uncertainty of aluminum content derived from linear least squares calibration）,u_（rel）（cAl）_3（the relative standard uncertainty of aluminum content derived from the dilution of the standard stock solutions）and u_（rel）（rep）（the relative standard uncertainty derived from the repeatability）.Based on the analysis of the main individual contributions of each uncertainty source to the total uncertainty value,several modifications were proposed.Firstly helium collision mode was replaced by no gas mode to improve the sensitivity of mass spectrometry.Secondly the number of measurements was increased.Thirdly let the mean of data points in the calibration closer the measurand.Finally the relative error smaller gauges were used.After these modifications,u_（rel）（cAl）_1,u_（rel）（cAl）_3and u_（rel）（rep）were from（0.035 8,0.013 2,0.008 5）down to（0.006 0,0.010 5,0.003 3）,respectively;the combined relative standard uncertainty of aluminum was from0.039 down to 0.013;the expanded uncertainty from 1.8down to 0.4 mg·kg^（-1）（coverage factor k=2）.The improvement effect was significant.