融化-再凝结法对冰芯气泡中甲烷的提取分析

Extraction and Analysis of the Methane Trapped in Ice Core by Melting-Refreezing Method

用融化 -再凝结方法对达索普冰芯气泡中的CH4 进行了提取分析 .大约 80g的冰芯样品在真空容器中融化 ,通过特殊方式 ,使融水自容器底部向上结冰 ,并经气相色谱仪对释放出气体中的CH4含量进行测试 .空白测试结果表明 ,该方法仅产生 15± 4nmol·mol-1的污染 ;对每个样品 3次重复测试结果的标准偏差上限为± 2 5nmol·mol-1.

Air was extracted from the bubbles trapped in the Dasuopu ice core by melting-refreezing method, of which the methane was measured. A core sample of a certain depth was cut to four pillars vertically. One pillar, 15 cm in length, was separated into three specimens for repeated measurement. The outer surface of each specimen was removed to eliminate any external contamination. Ice specimen, about 80 g in weight, was melted in an evacuated stainless-steel container sealed with tinfoil, and then a piece of bronze was fitted to the base of the container and soaked into a -60 ℃ alcohol bath. The melted specimen was thus slowly refrozen from the bottom and completely expels the dissolved air. The expelled gas was then expanded into an evacuated sample loop and injected into a Gas chromatograph equipped with a flame ionization detector through a six-port valve. Thus methane was extracted and its concentration can be measured. Blank tests were performed by adding 776 nmul.mol\+\{-1\} standard gas to artificial bubble-free ice. The contamination caused by melting appears to be proportional to melting time. However, the contamination is less than 15 nmol×mol\+\{-1\} when melting time is within 10 min. In the measurement melting time was controlled within approximately 5～7 min. Therefore, the contamination during the testing was estimated to be 15±4 nmol×mol\+\{-1\}. The scattering of methane concentrations for three or four injections was ±25 nmol×mol\+\{-1\}. The agreement between two measurements taken in China (LICCRE) and France (LGGE) was examined at two levels. First, a glass flask filled with LGGE standard gas was measured in LICCRE. Second, an ice sample from the Antarctic D47 ice core, which contains gas since the 1700s and is free of any melt feature, was cut lengthways and analyzed several times in both laboratories. As the preindustrial CH\-4 trend was already determined from Greenland and Antarctic ice cores in LGGE using the same technique as for the Dasuopu samples. It is confident that the results presented here can be directly compared to the polar records for determining the difference in CH\-4 mixing ratio between sub-tropical and polar latitudes.