Abstract：
The determination of urea in pet feed at contaminant levels using the spectrophotometric method described in Commission Regulation (EC) No 152/2009 has been reported by several EU laboratories to lack the required selectivity. Whilst urea is not authorised as an additive in pet feed, the control of urea in pet feed is of economic importance, because the addition of urea may unlawfully increase the apparent protein content. To investigate the capabilities of different analytical techniques, a proficiency test was organised where the participants (EU official control laboratories, laboratories from the academia and private laboratories) were free to use their method of choice for analysing three dog feed test materials, two samples of which were spiked with urea. Twenty-one laboratories submitted results using the following techniques: spectrophotometry (Implementing Regulation (EC) No 152/2009), LC-MS/MS, HPLC-UV, enzymatic-colorimetry, gravimetry and an \'in-house photometric\' method. Only two laboratories that used LC-MS/MS were able to quantify urea accurately in the test material containing a mass fraction of 18.9 mg kg-1 whereas satisfactory results at the level of 258.9 mg kg-1 were obtained by one participant that used an \'in-house photometric method\' and one that used the enzymatic method, in addition to the five participants using LC-MS/MS. The technique that provided the highest success rate across the three test materials was LC-MS/MS, whereas spectrophotometry, the enzymatic-based and HPLC-UV methods led to overestimated results in addition to a dispersion of results not suitable for compliance analysis. To address the determination of urea in pet feed at low levels, a better performing method than the one described in the legislation is required.
摘要：
几个欧盟实验室报告了使用委员会法规（EC）第152/2009号中描述的分光光度法测定宠物饲料中的尿素含量，缺乏所需的选择性。虽然尿素不被授权作为宠物饲料中的添加剂，控制宠物饲料中的尿素具有经济重要性，因为添加尿素可能会非法增加表观蛋白质含量。为了研究不同分析技术的能力，组织了能力测试，参与者(欧盟官方控制实验室，来自学术界和私人实验室的实验室)可以自由地使用他们选择的方法来分析三种狗饲料测试材料，其中两个样品掺加了尿素。21个实验室使用以下技术提交了结果：分光光度法（实施法规（EC）No152/2009），LC-MS/MS,HPLC-UV,酶比色法，重力法和“内部光度”方法。只有两个使用LC-MS/MS的实验室能够准确量化质量分数为18.9mgkg-1的测试材料中的尿素，而一个使用“内部光度法”的参与者和一个使用酶法的参与者在258.9mgkg-1的水平上获得了令人满意的结果。除了使用LC-MS/MS的五名参与者在三种测试材料中提供最高成功率的技术是LC-MS/MS，而分光光度法，基于酶的方法和HPLC-UV方法除了导致结果分散而不适用于合规性分析外，还导致结果被高估。为了解决宠物饲料中低水平尿素的测定，需要一种比立法中描述的方法更好的方法。
