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土壤有机碳同位素研究已在古植被、古气候重建工作中得到了广泛应用,其分析方法传统上采用“密封石英管法”.随着技术的发展,元素分析仪与同位素质谱仪联机在线测试方法(“在线法”)得到推广.但是,对同一种样品而言,这两种方法的测试结果有多大差别尚不明确,而这一点对不同实验室数据交叉检验和地质记录对比十分重要.本文分别采用“密封石英管法”和“在线法”对宝鸡黄土剖面有机碳同位素样品进行了测定.结果表明.同传统的“封管法”相比较,“在线法”测得的δ^13C值要偏正.比如,“在线法”获得的全新世古土壤S0和末次冰期黄土L1的有机质δ^13C平均值比“封管法”的数值分别偏正1.32‰和0.63‰.通过分析样品制备过程中可能影响实验结果的因素,我们推测,黄土中有机质含量很低,由于“在线法”燃烧时间(2min)太短,有机质中以有机无机复合体形式存在的低δ^13C值的胡敏酸未完全分解,从而导致“在线法”测定结果相对偏正.因此,对于低有机碳含量的地质样品,当以有机无机复合体形式存在的胡敏酸相对比例较高时.采用“在线法”测定有机碳同位素组成时需谨慎. “,”The carbon isotope composition of soil organic matter has been widely used to reconstruct paleovegetation and paleoclimate with the traditional technique of sealed-tube combustion. Recently, an online measurement technique with Elemental Analyzer and Isotope Mass Spectrometer has been increasingly employed for carbon isotope analysis. However, difference in δ^13C values of bulk organic matter obtained using the two analytical methods remains unclear, which is very important for data comparison between studies and across different laboratories. In this study, the carbon isotopic composition of bulk organic matter from the Baoji loess section was measured for the Holocene and the last glacial period. Results show that the online measurement technique yields more positive δ^13C values than the sealed-tube combustion method. For example, for the Holocene soil (S0 ) and the last glacial loess unit (L1), the averaged δ^13C value obtained by the online measurement technique is 1.3‰ and 0.63‰ more positive than that obtained by the sealed-tube combustion method, respectively. By evaluating several potential factors influencing the analytical results, we suggest that the short combustion time (2 min ) in the online measurement method may not completely release the humic acid, which is characterized by low δ^13C values and occurs possibly in the form of an organic-mineral complex, thus leading to more positive δ^13C values. Therefore, great caution should be taken in the application of the online measurement method to geological samples with low content of organic matter.