元素分析-同位素比值质谱法同时测定蔬菜中碳、氮同位素的前处理方法优化

为探究过筛目数和热风干燥对蔬菜中碳、氮同位素的影响规律,本试验在验证了基于CO₂稀释的元素分析-同位素比值质谱(EA-IRMS)法可靠性的基础上,研究了过筛目数(20、40、60、80、100、200目)、杀青(105℃,30 min)与未杀青处理、热风干燥温度(45、60、75℃)对6种蔬菜(菠菜、豇豆、韭菜、莲藕、茄子、马铃薯)δ¹³C、δ¹⁵N值的影响。结果表明,以89%CO₂稀释比例下EA-IRMS同时测定δ¹³C、δ¹⁵N值的标准偏差满足检测精度要求;当过筛目数超过80目时,6种蔬菜样品的δ¹³C、δ¹⁵N值均趋于稳定且达到测定精度要求;以真空冷冻干燥处理所测的δ¹³C、δ¹⁵N值为参照,采用不同热风干燥温度(45、60和75℃)处理的茄子、菠菜、莲藕和杀青后韭菜中δ¹³C、δ¹⁵N值均无显著差异,但未杀青的韭菜在45℃热风干燥后其δ¹³C、δ¹⁵N值与参照值之间有显著差异(P<0.05),且热风干燥处理的豇豆(δ¹³C和δ¹⁵N值)和马铃薯(δ¹³C值)与参照值相比也表现出不同程度的差异。因此,采用89%CO₂稀释比例下EA-IRMS法同时测定蔬菜中δ¹³C、δ¹⁵N值是可行的;蔬菜样品在进行碳、氮同位素测定前应以不低于80目的过筛处理。茄子、菠菜、莲藕和韭菜蔬菜样品在制备过程可采用热风干燥方式,其中韭菜需进行杀青处理,而豇豆、马铃薯则建议采用真空冷冻干燥方式。本研究为蔬菜样品中碳、氮同位素同时测定及样品制备提供了一定的参考。

Optimization for the Simultaneous Determination of Carbon and Nitrogen Stable Isotope in Vegetables by EA-IRMS

In order to explore the influence of sieving meshes and hot air drying on the determination of carbon and nitrogen isotope ratio in vegetables, and the effects of sieving meshes (20, 40, 60, 80, 100 and 200 mesh), green killing (105℃, 30 min) and no-green killing treatment, drying temperature (45, 60 and 75℃) on the reliability of element analysis isotope ratio mass spectrometry (EA-IRMS) based on CO₂ dilution were studied. The results of δ¹³C and δ¹⁵N in six vegetables (spinach, cowpea, leek, lotus root, eggplant and potato) were compared. The results showed that the standard deviation of δ¹³C and δ¹⁵N values measured by EA-IRMS with 89% CO₂ dilution ratio could meet the requirements of detection accuracy; when the number of sieving mesh exceeded 80 mesh, the determination results of δ¹³C and δ¹⁵N in the six types of vegetable samples tended to be stable and reached the requirements of determination accuracy. The δ¹³C and δ¹⁵N values measured by vacuum freeze-drying process were used as reference. Treatment using different hot air drying temperatures (45, 60 and 75℃) showed no significant change for the δ¹³C and δ¹⁵N values of eggplant, spinach, lotus root and Chinese chive. However, the δ¹³C and δ¹⁵N values of Chinese chive after 45℃ hot air drying were significantly different from the reference values, and the cowpea (δ¹³C and δ¹⁵N values) and potato (δ¹³C values) after hot air drying were also different from the reference values. Conclusion, it is feasible to simultaneously determine δ¹³C and δ¹⁵N values in vegetables by EA-IRMS with 89% CO₂ dilution ratio. Vegetable samples should be sieved with no less than 80 mesh before carbon and nitrogen isotope determination. The samples of eggplant, spinach, lotus root, and leek can be prepared by hot air drying and Chinese chive shall to be treated with green killing first. Cowpea and potato are recommended to be vacuum freeze-dried. This study provides a reference for the sample preparation of the simultaneous determination of carbon and nitrogen isotopes in vegetable samples and.