针式微萃取结合GC-IRMS测定水溶液中甲酸、乙酸的稳定碳同位素组成

Measurements of stable carbon isotopic compositions of formic and acetic acids in aqueous solution by needle trap coupled with GC-IRMS

甲酸和乙酸稳定碳同位素组成(δ13C)的分析对环境、食品、制药和自然产品等的研究具有重要的应用价值。但目前尚缺乏有效的测定方法。本研究利用最近出现的针式固相微萃取技术(NeedlEX),以吹扫-捕集方式对水溶液中的有机酸进行了萃取,然后利用气相色谱-同位素比值质谱联用仪(GC-IRMS)对所萃取的有机酸分子进行了δ13C的测定。结果显示,质谱计的信号强度与水溶液中有机酸的浓度存在显著的线性相关关系(R2>0.99,P<0.05),表明NeedlEX对水溶液中有机酸具有稳定的萃取能力。在甲酸与乙酸含量分别不低于300μg/mL与200μg/mL的水溶液中,1000mL的吹扫体积可以使两者δ13C多次分析结果的相对误差分别保持在3%和1%左右,且整个实验流程没有造成可检测的碳同位素分馏作用。低于这两个浓度界线,则分析误差随浓度的降低迅速增加。本研究虽然是针对水溶液中有机酸δ13C的测定,其萃取方法对其他水溶性挥发和半挥发有机物δ13C的分析也同样具有应用价值。

Stable carbon isotopic compositions （δ^13C） of formic and acetic acids are important to studies on the environment, food, pharmacy and the natural products. The analytical technique still remains to be established, however. Using the newly invented solid phase microextraction （SPME） technology, the needle trap （NeedlEX）, we developed a purge and trap experimental procedure to extract/preconcentrate formic and acetic acids from water solutions. The extractants were introduced to a GC-IRMS for subsequent isotopic composition measurements. The signal intensity in the mass detector significantly correlates with the concentration of the analytes （ R^2 〉 0. 99, P 〈 0. 05）, indicating that the needle trap is stable during the extraction. At 1000 mL purge volume and sample concentrations of no less than 300 μg/mL for formic and 200 μg/mL for acetic acids, multi-measurements were achieved with the relative error of about 3% and 1% for formic and acetic, respectively, and no detectable carbon isotope fractionation was found through the whole procedure. Below the concentration limits, however, the errors increase dramatically. The results not only provided solution to the measurement of δ^13C in the low molecular weight carboxylic acids, but also to the study of other organic molecules with similar properties in aqueous solutions.