液相色谱-同位素比值质谱法研究桔橙汁中柠檬酸碳同位素比值

Study on carbon isotope ratios of citric acid in orange juice by liquid chromatography-isotope ratio mass spectrometry

以国际原子能机构提供的蔗糖(δ13C为-10.449‰)作为溯源标准,建立了液相色谱-同位素比值质谱联用法(LC-IR MS)分析天然柑桔、橙汁中柠檬酸碳同位素比的方法,对不同产地个柑桔、橙子中有机酸碳同位素情况进行了研究。基于建立天然水果的柠檬酸碳同位素δ13C值的数据,提出了柑桔、橙子样品的δ13C值范围。方法将果汁用水稀释后,液相色谱-钙离子交换色谱在线制备柠檬酸,氢型离子交换柱分离柠檬酸后采用液相色谱-稳定同位素比质谱分析,柠檬酸方法检出限为5μg/m L,在2.00~100μg/m L水平时,柠檬酸响应与浓度成线性关系,相关系数为0.9997。方法日内、日间和人员比对结果相对标准偏差小于0.82%。收集不同产地161个橙子、167个柑桔测得天然桔汁中柠檬酸δ13C值在-32.87‰^-27.07‰之间,橙汁柠檬酸δ13C值在-32.73‰~26.01‰之间。采集40个市售柑桔、橙汁样品进行鉴定,检出17个掺有C4植物柠檬酸的的阳性样品,新方法可提高勾兑柠檬酸掺假果汁的鉴别能力。

The IAEA sucrose（ δ13C is- 10. 449 ‰） was used as a traceable standard. A method was established for determining carbon isotope ratio of citric acid in natural orange juice by liquid chromatographyisotope ratio mass spectrometry（ LC-IRMS）. Citric acid δ^13C in oranges from different origins was researched.Based on carbon isotope data obtained by measuring the natural fruits,the range of required δ13C values in oranges was proposed. The juices were diluted with water and citric acid was prepared on a calcium citrate ion exchange chromatographic column. Then the separation was carried out on a hydrogen form ion exchange column and detection by liquid chromatography-stable isotopes mass spectrometry. The detection limit of citric acid was5 g / m L and the good correlation coefficient was 0. 9997 in the range of 2. 00 μg / m L- 100 μg / m L. The relative standard deviations were less than 0. 82% for intra-days,inter-days and replicate experimental results. Citric acid δ13C of the natural fruit juices collected from 161 oranges in different origin areas was between- 32. 87‰and- 27. 07‰,and that collected from 167 citrus fruits was- 32. 73% and- 26. 01%. The method was applied to detection of 40 commercial juice samples,and 17 positive samples with citric acid mixed C4 plants were identified. The new method of LC-IRMS can greatly improve the ability to identify juice adulteration of citric acid.