Study On Determination Of Cadmium And Lead Ions In Food By Using Enzyme

Lead (Pb) and cadmium (Cd) are versatile industrial raw materials. However, owing to the increasing environmental pollution and the wide use of metal containers, Pb and Cd have caused 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, monitoring the heavy metals in food is of great urgency. Chinese food companies need much cheaper detection method of heavy metals, using urease to detect Pb and Cd in food was studied, which was 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. In this work, urease was used to detect lead and cadmium concentrations in pretreatment solutions of food samples by colorimetric method and got good results.Inhibition rate was used to characterize the effect of heavy metals solution with high levels of lead and cadmium (120μg/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 were as follows: the amount of urease was 12.7U, the reaction temperature was 30℃, the reaction time was 20 min, the coloring time was 30 min. The inhibition rate in this condition was the highest and reached 87.59%. Throughout the reaction system, using 0.5 mL, 3.5 g/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 40μg/mL was the cut-off point: the lead ions can inhibit the urease activity when its concentration is higher than 40μg/mL and activate the activity when lower. Considering that the amount of lead in food is quite small, we prepared standard gradient solution (0μg/mL~1.0μg/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 0.175μg.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 3.175U, 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 this method 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 0.376μg.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 it was found 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 test results 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.