原文：

铅镉等重金属，作为用途广泛的工业化原料，随着环境污染的加剧和金属容器具的大范围使用，逐渐混入到食品中，对人类健康造成了巨大危害。铅镉元素进入人体以后，轻者会损害人体一些正常的生理机能，重者会致癌，最终危及生命。监控食品中的重金属含量已经到了刻不容缓的地步。研究基于中国食品企业亟需较为低廉检测方法的现状，应用酶法检测食品中的铅镉重金属，在传统的检测基础上又增加一种新的检测方法。

脲酶对重金属有高度的敏感性，其活性不易受大多数农药和有机化合物的影响，因此常被用来评价环境样品中重金属的毒性。研究运用脲酶比色法直接检测食品样品预处理液中的铅镉重金属，得到了良好的效果。

采用“抑制率”的概念来表征高浓度的铅镉重金属溶液(以120µg/mL为例)对脲酶的抑制作用大小，应用单因素试验和正交试验，建立并优化了检测条件。

对于铅离子来说，正交试验表明，高浓度的铅离子溶液抑制脲酶活性的{zj0}反应参数为：脲酶用量2.0mL、反应温度30℃、反应时间20min、显色时间30min，此时铅离子溶液对脲酶活性的抑制率{zg}，可以达到87.59%。试验证明在整个反应体系中，使用0.5mL 3.5g/L的邻菲啰啉溶液、1.0mL 0.5g/L的酒石酸钾钠溶液和1.25mL 20g/L硫氰酸铵溶液做联合掩蔽剂，可以有效掩蔽住其他离子对铅试验结果的干扰。在此基础上进行吸光度对铅离子溶液的线性回归时发现，铅溶液浓度为40µg/mL是其抑制与xx脲酶活性的分界点：当铅浓度＞40µg/mL时，铅离子会抑制脲酶的活性，当铅浓度＜40µg/mL时，铅离子会xx脲酶的活性；考虑到食品中铅含量较小，配制了0µg/mL~1.0µg/mL范围内的标准梯度溶液，做出的标准曲线方程为A=0.6153C+0.7232，线性相关系数R²为0.9993，线性相关性较好。试验证明在此范围内，可以满足铅离子的定量检出要求。针对硝酸铅纯溶液，方法的加标回收率为86.3%~104%，精密度＜3(n=5)，检测限为0.192µg。

对于镉离子来说，正交试验表明，高浓度的镉离子溶液抑制脲酶活性的{zj0}反应参数为：脲酶用量0.5mL、反应温度50℃、反应时间30 min、显色时间30 min，此时镉离子溶液对脲酶活性的抑制率{zg}，可以达到85.53%。在检测镉离子的试验体系中，1.0mL 100g/L的酒石酸钾钠溶液、0.25mL 50g/L的氢氧化钠溶液和0.75mL 62.5g/L的柠檬酸钠溶液做联合掩蔽剂，可以有效掩蔽食品样品预处理液中的其他离子对镉检测试验结果的干扰；在此基础上做出的吸光度对镉离子溶液的标准曲线方程为y=-0.3715x+2.8895，线性相关系数R²为0.9994，并经试验证明在0µg/mL~30µg/mL的范围内，可以满足镉离子的定量检出要求。针对氯化镉纯溶液，方法的加标回收率为91.8%~109.4%，精密度＜4(n=5)，检测限为0.376µg。

国家标准中规定食品中铅含量一般≤0.2mg/kg，镉含量一般≤0.03mg/kg，验证后发现，该方法可以满足食品限量标准中对重金属含量的{zd1}检测要求。用该法与石墨炉原子吸收法分别检测番茄酱和松花蛋预处理液中的铅镉元素，比较后发现两者无显著性差异(p=0.95)。针对番茄酱和松花蛋的预处理液，方法的铅镉加标回收率为83.2%~99.7%，可靠性良好。

研究表明，该方法可作为检测食品中铅镉重金属含量的一种方法，有望在实际检测中应用。

译文：

Owing to the increasing environmental pollution and the wide use of metal containers, heavy metals such as lead (Pb) and cadmium (Cd), versatile industrial raw materials, are gradually mixed into food, causing great harm to human health. After Pb and Cd are absorbed into human body, they can damage some of the body’s normal physiological functions, and in serious cases they can cause cancer and ultimately threaten the life. Therefore, to monitor the heavy metals in food is of great urgency. Based on the condition that Chinese food companies need much cheaper detection method, the study used urease to detect Pb and Cd in food, adding a new method to the traditional detection. 

Urease is highly sensitive to heavy metals and its activity is hardly affected by most pesticides and organic compounds. Thus, it is often used to evaluate the toxicity of heavy metals in environmental samples. The study directly used urease to detect lead and cadmium concentrations in pretreatment solutions of food samples by colorimetric method and got good results.

We used “inhibition rate” to characterize the effect of heavy metals solution with high levels of lead and cadmium (120 ug/ml) to urease. Single-factor experiments and orthogonal experiments were conducted to establish and optimize detecting conditions.

The results showed that, the optimal reaction parameters of high levels of lead ion solution to inhibit urease activity are as follows: the amount of urease was 2.0 ml, the reaction temperature was 30℃, the reaction time was 20 min, the coloring time was 30 min. The inhibition rate was the highest and reached 87.59%. Throughout the reaction system, using 0.5 ml, 3.5g/L o-phenanthroline solution, 1.0 ml, 0.5 g/L potassium sodium tartrate solution and 1.25 ml, 20 g/L ammonium thiocyanate solution as joint masking agent can effectively mask other ions’ interference in test results. On this basis, through the linear regression of lead ion solution , we found that the concentration of 40 ug/ml was the cut-off point: the lead ions can inhibit the urease activity when its concentration is higher than 40 ug/ml and activate the activity when lower. Considering that the amount of lead in food is quite small, we prepared standard gradient solution (0 ug/ml—1.0 ug/ml), and the standard curve equation was A=0.6153c+0.7232, the coefficient (R²) of the linear correlation was 0.9993. The test showed that the requirement of quantitative detection of lead ion could be met in this range. For pure lead nitrate solution, the recovery rate was between 86.3% and 104%, with the precision lower than 3 (n=5), the detection limit was 0.192ug.

Through the test, we found that the optimal reaction parameters of high levels of cadmium ion solution to inhibit urease are as follows: the amount of urease was 2.5 ml, the reaction temperature was 50℃, the reaction time was 30 min, the coloring time was 30 min. Under such conditions, the inhibition rate was the highest and reached 85.53%. In the test of detecting cadmium ion, using 1.0 ml, 100 g/L potassium sodium tartrate solution, 0.25 ml, 50 g/L sodium hydroxide solution and 0.75 ml, 62.5 g/L sodium citrate as joint masking agent can effectively mask other ions’ interference in test results. On this basis, the standard curve equation of the absorbance to cadmium ion solution was y= -0.3715x + 2.8895, the coefficient (R²) of the linear correlation was 0.9994. The test proved that it could meet the quantitative detection of cadmium ion in the 0 ug/ml—30 ug/ml range. For pure cadmium chloride solution, the recovery rate was between 91.8% and 109.4%, with the precision lower than 4 (n=5), the detection limit was 0.376ug.

According to national standard, the lead content in food shall not be higher than 0.2 mg/kg and the cadmium content not higher than 0.03 mg/kg. Through the test and verification we found that this method could meet minimum testing requirements on heavy metals content in accordance with the limit standard in food. Compared with the results of graphite furnace atomic absorption spectrometry, there was no significant difference (p=0.95) between the tests of lead and cadmium respectively in pretreatment solutions of tomato sauce and preserved eggs. As for these two pretreatment solutions, the recovery rate of lead and cadmium of this method was between 83.2% and 99.7%, which was much reliable.

The study showed that the method could be used to detect the content of lead and cadmium in food, and it was expected to be put into practice.

                                                                                                                     （2010年4月）