基于冷冻熔炼的液-液萃取/气相色谱法测定啤酒中的N-二甲基亚硝胺

建立了一种新型的冷冻熔炼液-液萃取法直接提取啤酒中的N-二甲基亚硝胺（N-nitrosodimethylamine，NDMA），萃取液经GC-FID分析，双柱法定性，外标法定量。优化的前处理条件为：以二氯甲烷为萃取溶剂，啤酒样品和萃取溶剂的体积比为9：1，-19 ℃下冷冻16 h。结果表明：NDMA在5~200 mg/L范围内具有良好的线性关系（r²=0.9996）；在5、10和20 mg/L 3个加标水平下的回收率分别为84.94%、83.24%、85.14%，平行7次测定的相对标准偏差（RSD）分别为3.06%、3.19%、2.63%。与常规液-液萃取方法（需要经过蒸馏、萃取、旋蒸、氮吹4个步骤）相比，本方法稳定、可靠，且操作简单，有机溶剂消耗量少。

Determination of N-nitrosodimethylamine in beer by frozen zone melting liquid-liquid extraction/gas chromatography

A simple and effective sample enrichment method of frozen zone melting liquid-liquid extraction was optimized and validated for the analysis of trace N-nitrosodimethylamine (NDMA) in beer samples. The method was based on high pressure liquid-liquid extraction with a low temperature frozen step. The 90 mL beer was placed in a container with 10 mL dichloromethane. After agitation, the sample was kept in a freezer for 16 h at -19 ℃. The organic extract was analyzed by gas chromatography with a flame ionization detector (GC-FID). The accuracy, precision, detection and quantification limits and linearity of the method were evaluated. The results showed that the calibration curve of NDMA was linear in the range of 5-200 mg/L with a good correlation coefficient (r²) of 0.9996. The recoveries at the spiked levels of 5, 10 and 20 mg/L were 84.94%, 83.24%, 85.14% with the relative standard deviations (n=7)of 3.06%, 3.19%, 2.63%, respectively. The ordinary extraction method of N-nitrosodimethylamine in beer includes the four steps of low-temperature distillation, liquid-liquid extraction, rotary evaporation and nitrogen blowing concentration. With the extremely low volume of solvent used, the proposed extraction method proved to be easy and simple, and adequate for high-throughput analysis at low cost.