海洋沉积物有机碳分析方法中干燥预处理过程人为误差的发现及其意义

以1个新鲜的半远洋沉积样品(来自南海陆坡)和2个新鲜的近海沉积样品(来自中国青岛胶州湾)为例,针对目前国际通用沉积物有机碳检测方法的样品前处理过程进行了一系列的实验.发现用冷冻真空干燥法替代目前流行使用的热烘干法,所有的3个样品的有机碳含量值都比使用烘干法所得值高出20%以上.通过运用气相色谱-质谱联用仪(GC-MS)对热烘干步骤的蒸发物进行检测,证实了其中有机碳的存在并显示了其组成成份.另一方面,不同干燥时间的冷冻真空干燥对比实验则证实了该方法没有造成有机碳的损失.基于这些结果,认为在目前的国际通用沉积物有机碳检测方法中存在着较大的系统误差(＞20%),这个误差是由样品前处理中的热烘干步骤所造成的,而采用冷冻真空干燥的方法可以有效地消除这一误差.因此,建议修改现有的国际通用测定方法:前处理过程中用冷冻真空干燥法取代热烘干法.同时,鉴于这一误差的重要性,对已发表的科学文献中的有机碳数据,以及基于这些数据而推导的科研结论等都可能有必要进行重新审查或验证.

Freeze-drying pretreatment improves organic carbon determinations of marine sediments

Using one fresh hemipelagic and two fresh coastal marine sediment samples collected on the continental slope of the South China Sea and in Jiaozhou Bay near Qingdao, China, we compared sample pretreatment schemes of organic carbon determination. By replacing the heat-drying step in the sample pretreatment scheme with a freeze-drying-under-vacuum step, the organic carbon content values of all three samples were more than 20% higher. GC-MS examination of the vapor phases collected during the heat-drying step confirmed the presence of organic carbon in these phases and revealed the composition of this organic carbon. When each sample was subjected to freeze-drying for different times, identical results were obtained and no organic carbon loss was observed. Based on these observations, we believe that there is a large systematic error (>20%) caused by the heat-drying step in the common sample pretreatment scheme and that this error could be effectively corrected if a freeze-drying-under-vacuum step is used instead. Considering the significance of this error, we believe that some previously published organic carbon data, as well as the scientific understandings that were based on those data, may need to be re-evaluated.