Organic Geochemistry Unit, School of Chemistry , University of Bristol , Cantock's Close, Bristol BS8 1TS , U.K.
Bristol Radiocarbon Accelerator Mass Spectrometer , University of Bristol , 43 Woodland Road , Bristol BS8 1UU , U.K.
Anal Chem. 2018 Sep 18;90(18):11025-11032. doi: 10.1021/acs.analchem.8b02713. Epub 2018 Sep 7.
Preparative capillary gas chromatography (pcGC) is widely used for the isolation of single compounds for radiocarbon determinations. While being effective at isolating compounds, there are still genuine concerns relating to contamination associated with the isolation procedure, such as incomplete removal of solvent used to recover isolated compounds from the traps and cross-contamination, which can lead to erroneous C determinations. Herein we describe new approaches to identifying and removing these two sources of contamination. First, we replaced the common "U" trap design, which requires recovery of compounds using organic solvent, with a novel solventless trapping system (STS), consisting of a simple glass tube containing a glass wool plug, allowing condensation of a target compound in the wool and its solventless recovery by pushing the glass wool directly into a foil capsule for graphitization. With the STS trap, an average of 95.7% of the target compound was recovered, and contamination from column bleed was reduced. In addition, comparison of C determinations of fatty acid methyl ester (FAME) standards determined offline to those isolated by pcGC in STS traps showed excellent reproducibility and accuracy compared to those isolated using the commercial "U" traps. Second, "coldspots" were identified in the instrument, i.e., the termini of capillaries in the preparative unit, which can be cleaned of compounds condensed from earlier runs using a heat gun. Our new procedure, incorporating these two modifications, was tested on archeological fat hoards, producing C dates on isolated C and C fatty acids statistically consistent with the bulk dates of the archeological material.
制备性毛细管气相色谱（pcGC）广泛用于分离用于放射性碳测定的单一化合物。虽然在分离化合物方面非常有效，但仍存在与分离过程相关的污染问题，例如从捕集器中回收分离化合物所用溶剂的不完全去除和交叉污染，这可能导致错误的 C 测定。本文描述了鉴定和去除这两种污染源的新方法。首先，我们用一种新型的无溶剂捕集系统（STS）代替了常见的“U”型捕集器设计，该系统需要使用有机溶剂回收化合物，STS 由一个简单的玻璃管组成，其中包含一个玻璃纤维塞，允许目标化合物在羊毛中冷凝，并通过将玻璃纤维直接推入箔胶囊中进行无溶剂回收，用于石墨化。使用 STS 捕集器，目标化合物的平均回收率为 95.7%，同时减少了柱泄漏造成的污染。此外，与使用商业“U”型捕集器分离的方法相比，离线测定的脂肪酸甲酯（FAME）标准品的 C 测定值与在 STS 捕集器中通过 pcGC 分离的 FAME 标准品的 C 测定值之间的比较表明，其重现性和准确性均非常出色。其次，在仪器中鉴定出了“冷点”，即制备单元中毛细管的末端，可以使用热喷枪清除从先前运行中冷凝的化合物。我们的新程序结合了这两个改进，在考古脂肪宝库上进行了测试，所分离的 C 和 C 脂肪酸的 C 年代与考古材料的总年代在统计学上是一致的。
