Analytical and Advisory Services Division, Government Laboratory, 88 Chung Hau Street, Homantin Government Offices, Hong Kong.
J Chromatogr A. 2011 Sep 28;1218(39):6907-13. doi: 10.1016/j.chroma.2011.08.021. Epub 2011 Aug 16.
A new certified reference material (CRM) of melamine in milk GLHK-11-02 was developed aiming to address the great demand from the testing community after the melamine crises. The material was prepared by adding an appropriate quantity of melamine into the skimmed milk samples and the final product was in the form of fine lyophilized powder. Characterization of the material relied on two newly developed gravimetric isotope dilution mass spectrometry (IDMS) methods, one using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and another gas chromatography-mass spectrometry (GC-MS). Experimental parameters with crucial effects on the performance of the two IDMS methods were thoroughly investigated. These included purity of standard used, equilibration time of isotopes, efficiency of extraction methods as well as possible interferences from the matrix and melamine analogues. Precision was found to be excellent with a coefficient of variation of 2.5% for the LC-IDMS/MS (n=46) and 1.9% for the GC-IDMS (n=30) respectively. Using one-tail Student's t-test at 95% confidence interval, analytical data sets generated from the two methods were found to exhibit no significant difference. Measurement accuracy of the methods was further verified through an Asia Pacific Metrology Program (APMP) pilot study. Analytical results of the present LC-IDMS/MS for the two milk test samples at the concentration level of about 0.45 and 3.5 mg kg(-1) were proven to be very good. There were excellent overlaps between our results and the assigned reference values, and the absolute deviation was less than 3.2%. Both the LC-IDMS/MS and GC-IDMS methods were shown to be sufficiently reliable and accurate for certification of the melamine CRM. Certified value of melamine in dry mass fraction in GLHK-11-02 was 1.14 mg kg(-1). Expanded uncertainty due to sample inhomogeneity, long term and short term stability and variability in the characterization procedure was at 7.1% or 0.08 mg kg(-1). The CRM is primarily used to provide a complete method validation for and to improve the technical competence of melamine analysis to food and chemical testing laboratories.
针对三聚氰胺危机后检测界对三聚氰胺标准物质的大量需求，我们研制了一种新的牛奶中三聚氰胺认证参考物质（CRM）GLHK-11-02。该物质是通过将适量的三聚氰胺添加到脱脂乳样品中制备而成的，最终产品为精细的冻干粉末。该物质的特性依赖于两种新开发的重量法同位素稀释质谱（IDMS）方法，一种使用液相色谱-串联质谱（LC-MS/MS），另一种使用气相色谱-质谱（GC-MS）。对两种 IDMS 方法中对性能有重要影响的实验参数进行了深入研究。这些参数包括标准物质的纯度、同位素平衡时间、提取方法的效率以及基质和三聚氰胺类似物可能产生的干扰。结果表明，两种方法的精密度均非常好，LC-IDMS/MS（n=46）的变异系数为 2.5%，GC-IDMS（n=30）的变异系数为 1.9%。在 95%置信区间下，使用单尾学生 t 检验，发现两种方法生成的分析数据无显著差异。通过亚太计量规划组织（APMP）试点研究进一步验证了方法的测量准确性。本研究中，LC-IDMS/MS 对两种牛奶测试样品（浓度水平约为 0.45 和 3.5 mg/kg）的分析结果非常好。我们的结果与赋值参考值之间有极好的重叠，绝对偏差小于 3.2%。LC-IDMS/MS 和 GC-IDMS 两种方法均被证明对三聚氰胺 CRM 的认证具有足够的可靠性和准确性。GLHK-11-02 中三聚氰胺干物质分数的认证值为 1.14 mg/kg。由于样品不均匀性、长期和短期稳定性以及特性化程序中的变异性而导致的扩展不确定度为 7.1%或 0.08 mg/kg。该 CRM 主要用于为食品和化学检测实验室提供三聚氰胺分析的完整方法验证，并提高其技术能力。
