两种离子化技术气相色谱-质谱法测定橄榄油中甾醇烯类物质的比较

分别采用电子轰击(EI)和正化学电离(PCI)两种离子源技术建立了气相色谱-质谱联用法(GC-MS)同时测定橄榄油中3,5-菜甾二烯、3,5,22-豆甾三烯和3,5-豆甾二烯3种甾醇烯含量的方法,并对这两种方法进行了比较。样品经石油醚溶解,硅胶柱净化后,分别采用GC-EI/MS和GC-PCI/MS分时段选择离子监测模式进行测定,以3,5-胆甾二烯为内标进行定量。结果表明,两种方法的线性、准确度、精密度、灵敏度均较好。3,5-菜甾二烯、3,5,22-豆甾三烯和3,5-豆甾二烯分别在0.024~0.48、0.02~0.50和0.03~0.75 mg/L浓度范围内线性关系良好(r>0.999)。在3个加标水平下,GC-EI/MS和GC-PCI/MS的平均回收率分别为88.7%~99.5%、87.1%~109.2%,两种检测方法的相对标准偏差(RSD,n=6)均不超过8.3%。定量限(S/N=10)分别为0.03 mg/kg(EI),0.03~0.10 mg/kg(PCI)。通过对两种方法的比较研究发现,EI能提供更多碎片离子和结构信息,而PCI中则主要为准分子离子及反应气加合离子。应用于样品中甾醇烯的测定时,PCI的选择性和抗干扰能力明显优于EI。两种方法可相互补充和替代应用于日常检测中。

A comparative study of two ionization techniques in gas chromatography-mass spectrometric determination of sterenes in olive oil

The gas chromatography-mass spectrometric (GC-MS) method with two different ionization techniques, electron impact (EI) and positive chemical ionization (PCI), have been developed for the simultaneous determination of three sterenes in olive oil. The olive oil sample was dissolved in petroleum ether, followed by cleanup on a silica gel column and finally analyzed by GC-EI/MS and GC-PCI/MS in selected ion monitoring mode. The quantitation was performed by internal standard calibration with cholesta-3, 5-diene as internal standard. These two methods were compared in terms of linearity, accuracy, precision and sensitivity. Good linearities were obtained with correlation coefficients (r) higher than 0.999 for campesta-3, 5-diene in the range of 0.024 to 0.48 mg/L, stigmasta-3, 5, 22-triene in the range of 0.02 to 0.50 mg/L and stigmasta-3, 5-diene in the range of 0.03 to 0.75 mg/L. The accuracy test was carried out at three spiked levels in extra virgin olive oil. The average recoveries ranged from 88.7% to 99.5% (EI) and 87.1% to 109.2% (PCI), with relative standard deviations (RSDs, n=6) not higher than 8.3%. Limits of quantification (LOQs, S/N=10) were 0.03 mg/kg (EI) and 0.03-0.10 mg/kg (PCI). Comparative analysis showed a large number of fragment ions could be obtained in EI mode while quasi-molecular ions or other adduct molecular ions were the most abundant ion formed in the PCI mode. Besides, PCI has a better selectivity than EI when applied to real samples. In conclusion, these two ionization techniques are complementary and alternative in the routine analysis of sterenes in olive oil.