5A分子筛吸附混合溶剂洗脱-气相色谱-同位素质谱分析土壤中正构烷烃单体碳同位素

利用5A分子筛吸附,环己烷-正戊烷混合溶剂洗脱分离富集正构烷烃,用气相色谱法测定正构烷烃含量,气相色谱-气体同位素质谱(GC-C-IRMS)测定土壤样品中正构烷烃单体碳同位素。实验优化了5A分子筛用量和洗脱剂的比例,需要络合的正构烷烃的量与分子筛加入量呈线性关系,络合x mg的正构烷烃,需加入2.75x g分子筛,络合环己烷-正戊烷最佳比例为9∶91。探讨了络合过程中5A分子筛对不同链长正构烷烃的络合规律,短链正构烷烃被5A分子筛优先吸附,长链正构烷烃的络合相对滞后。正构烷烃的络合洗脱回收率为44%～72%,精密度(RSD,n=6)为4%～8%;正构烷烃单体碳同位素分析精度为0.04‰～0.38‰(1σ)。采用5A分子筛净化混合溶剂洗脱方法,分析加油站附近的实际土壤样品,未分峰基本消除,获得良好的净化效果,满足正构烷烃单体碳同位素分析的要求。     

柱色谱分离-分子筛络合洗脱过程中正构烷烃单体碳同位素分馏研究

应用气相色谱-气体同位素质谱（GC-C-IRMS）分析正构烷烃单体碳同位素之前,需要对饱和烃样品中正构烷烃和异构烷烃进行预分离、富集,在预分离和富集过程中正构烷烃单体碳同位素是否发生分馏是高精度分析正构烷烃单体碳同位素比值（δ13C）的关键。本文以正构烷烃混合溶液为对象,利用柱色谱、5Å分子筛络合、环己烷-正戊烷混合溶剂两次洗脱,GC-C-IRMS分析正构烷烃单体碳同位素,研究前处理过程中正构烷烃单体碳同位素是否发生分馏。结果表明：使用柱色谱分离前后,多数正构烷烃单体碳同位素比值相差-0.2‰~0.2‰;当5Å分子筛不完全络合时,未络合的正构烷烃单体碳同位素比值偏重约0.7‰,可能发生了微弱的碳同位素分馏,但并未影响洗脱后的正构烷烃单体碳同位素比值;使用环己烷-正戊烷混合溶剂洗脱前后,碳同位素比值相差-0.2‰~0.5‰,以同样方式洗脱第二次,获得的正构烷烃单体碳同位素比值与模拟样品相差-0.3‰~0.2‰。分析不同回收率（>20%）正构烷烃的单体碳同位素比值,处理前后的差值基本在0.3‰以内,可见当正构烷烃回收率低至20%左右时,其单体碳同位素仍未发生明显分馏。柱色谱分离-5Å分子筛络合-混合溶剂洗脱方法适用于回收率大于20%的正构烷烃单体碳同位素分析。     

程序升温-气相色谱-同位素比值质谱法分析多环芳烃单体碳同位素组成

环境样品中PAHs的单体碳同位素比值在迁移转化过程中能基本保持稳定,是重要的溯源指标,可通过气相色谱-同位素比值质谱(GC-IRMS)分析获得。对于低PAHs含量的样品,满足GC-IRMS检出限是高精度、准确分析单体碳同位素比值的前提。本文优化了一种程序升温汽化进样(PTV)方法,通过对PTV进样模式及进样口参数进行优化调整,提高目标物谱峰强度,进而提高GC-IRMS碳同位素分析的灵敏度。实验对比研究了恒温不分流、PTV不分流和溶剂分流进样模式,并对PTV进样口参数包括进样口压力梯度、传输温度和时间、蒸发温度和时间、进样口不分流时间进行了对比优化,以选出最优的PAHs单体碳同位素分析条件。结果表明：在PTV不分流进样、进样口压力40psi—60psi—70psi梯度升高、传输温度320℃、传输时间1.0min、蒸发温度55℃、蒸发时间2.5min、不分流时间1.5min条件下,PAHs的单体碳同位素结果最优。增加预柱可以提高峰强,尤其5环PAHs的峰强度提高达50%~100%。单体碳同位素分析精度(1σ)在0.5‰以内,系统性碳同位素分馏可以采用双标法校正。优化后的PTV-GC-IRMS方法可以实现低含量PAHs单体碳同位素的高精度、准确分析,扩大了同位素溯源在环境研究中的适用性。     

新型有机-无机杂化涂层-气相色谱/质谱联用测定正构烷烃性能研究

水体中正构烷烃的含量反映了研究区域有机物污染水平.痕量正构烷烃的测定常常需要结合分离富集方法才能保证灵敏度和选择性,液液萃取等常规分离富集方法存在耗时长、有机溶剂使用量大等缺点,而新型固相微萃取(SPME)分离富集技术相比而言具有高效简便、富集效率高的特点.本文采用溶胶-凝胶法自制稳定性和耐基质性能更优的有机-无机杂化SPME涂层,结合气相色谱-质谱(GC - MS)检测技术,对水样中26种正构烷烃进行同时测定.在优化的SPME - GC/MS条件下,采用浸入式萃取方式,萃取温度为25℃,萃取时间为30 min,26种正构烷烃的分析信号与浓度之间有良好的线性关系(线性相关系数为0.9935 ～0.9999),方法检出限为0.011 ～0.085μg/L(k=3),相对标准偏差(RSD,n=5)为3.9％ ～10.0％.新型有机-无机杂化涂层与传统的分离富集技术相比,减少了有机溶剂的使用,提高了工作效率;与商用PDMS涂层相比,增加了正构烷烃同时检测的种类,提高了灵敏度,检出限低2～10倍.自制的有机-无机杂化SPME涂层体现出优异的分析性能和应用潜力.     

东海泥质区单体正构烷烃的碳同位素组成及物源分析 *

利用气相色谱(GC)和气相色谱/同位素比值质谱(GC/IRMS)对东海近岸泥质区、济州岛西南泥质区和冲绳海槽北部表层沉积物中正构烷烃的单体碳同位素组成及分布进行了分析。结果显示东海不同泥质区典型海洋藻类源正构烷烃C₁₉同位素组成基本相似,在-27.4 ‰ ~-28.0 ‰ 之间,平均为-27.7 ‰ 。典型海洋水生植物源C₂₃同位素组成在-28.5 ‰ ~-31.6 ‰ 之间,平均为-30.5 ‰ ,碳同位素组成从近岸泥质区到冲绳海槽北部逐渐变重,表明海槽区与陆架区海洋水生植物种类有所不同。陆架区长链正构烷烃(C₂₅~C₃₁)部分随着碳数的增加,其同位素组成逐渐变轻,但海槽区这一变化不大,显示陆架区的陆源高等植物蜡具有相似的物源,而冲绳海槽北部由于黑潮主干区和黑潮分支(对马暖流)对陆架沉积物进入深海的控制性阻隔作用,其物源与陆架区区别较大。现代输入东海的陆源植物以C₃植物为显著优势,C₃植物对近岸泥质区北部、近岸泥质区南部、远端济州岛西南泥质区和冲绳海槽北部陆源植物的贡献分别为83 % ,95 ％,75 % 和70 % 。     

末次盛冰期以来泸沽湖沉积记录的正构烷烃分布特征和单体碳同位素组成及其古植被意义

泸沽湖是云贵高原上典型西南季风区的一半封闭湖泊, 本文通过研究水深69.3m处长度为18.3m岩芯中15个样品的陆源高等植物正构烷烃分布特征及碳同位素组成, 揭示了湖区木本/草本植被和C3/C4植被的变化历史, 并试图探讨C3/C4植被变化的可能影响机制。在末次盛冰期至全新世早期, 正构烷烃含量及(C27+C29)/2C31比值逐渐增加, 正构烷烃(碳数>C25)的平均碳链长度(ACL)值逐渐减少, 指示木本植物比例相比草本植物较多且呈逐渐增加态势, 表明气候逐渐向暖湿方向发展; 而同一时期, 陆源高等植物正构烷烃(C27、C29和C31)δ 13 C值均逐渐偏正, 无法用气候变化来解释, 应该反映了C3/C4植被变化, 由此, 通过二元模式计算得出的C4植物比例从19.6 % 逐渐增加至31.9 %, 上述结果表明该时期温度升高对C4植物增多起了主要作用。到全新世中期, 正构烷烃分布特征表明木本植物比例依然较高, 表明此时气候温暖湿润, 而δ 13 C值则呈偏负的趋势, 我们认为这是降雨增加和C4植物减少协同导致的。在全新世晚期, 正构烷烃分布特征指出草本植物比例相对增加, 而该时期的δ 13 C值则稍微偏正, 这可能是因为气候变干所导致的。陆源高等植物正构烷烃分布特征所揭示的植被变化可以与研究区域孢粉记录进行较好对比。研究进一步明确了温度是C4植物出现的主控因素, 而在温度满足要求时降雨的增多会降低C4对C3植物的竞争优势。     

地质样品正构烷烃组分分离纯化的部分问题探究

用实验室自配的标准正构烷烃样品分别经硅胶或氧化铝柱层析后发现,等量溶剂洗脱的情况下硅胶对长链正构烷烃无吸附而氧化铝具有一定吸附性。进一步通过用植物样品测试4种层析柱填充方法,发现不论是只用氧化铝填充还是上部硅胶下部氧化铝和上部氧化铝下部硅胶,都会对长链正构烷烃产生一定的吸附,且这种吸附效果随着碳链的增长而增强。在实验条件下,当碳链加长到C36时,用硅胶加氧化铝填充层析柱的吸附量已达到20%左右,而只用氧化铝填充层析柱的吸附量则高达50%。故建议对研究高碳数正构烷烃的地质样品组分提取时用单一的硅胶柱层析方法。同时,实验显示对于一些杂质多的正构烷烃样品经过尿素络合后比络合前"干净"得多。18个黄土-古土壤和植物样品平均回收率为50%左右,经过尿素络合后的样品正构烷烃各组分相对含量基本不会发生改变,也不会产生明显的同位素分馏效应。因此在进行非正构组分干扰较大的正构烷烃各组分相对含量或同位素分析时,可以选择尿素络合的方法来将其纯化。     

Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

A dry ashing method is commonly used to remove organic material from samples prior to geochemical analysis. In the course of this study, the Cd isotope ratios of a series of soil and plant reference materials and samples were studied to evaluate the effect of the dry ashing method on measurement results of Cd isotope ratios. The samples were pre‐treated using the dry ashing method and high‐pressure bomb for comparison. The results show that the digestion using high‐pressure bombs did not lead to Cd loss, but using the dry ashing method would cause different proportions of Cd loss. The whole range of Cd isotope difference between two methods was from ?0.07‰ to 3.01‰. There was also an obvious difference in measured Cd isotope ratios from the same leaf sample pre‐treated independently by the dry ashing method, indicating that the amount of Cd loss and the effect on Cd isotope measurement during dry ashing is related to the properties of the samples. Therefore, dry ashing may not be appropriate for the removal of organic material in Cd isotope ratio measurement, especially for samples with high organic contents. The δ^114/110Cd values of reference materials NIST SRM 1573a and GSD‐30 are reported for the first time in this study.     

The variations of stable carbon isotope ratio of land plant-derived n-alkanes in deep-sea sediments from the Bering Sea and the North Pacific Ocean during the last 250,000 years

Two piston cores, one located far from the continents (The North Pacific Ocean: ES core), and another located comparatively closer to the continents (The Bering Sea: BOW-8a core) were investigated to reconstruct environmental changes on source land areas. The results show significant contribution of terrestrial organic matter to sediments in both cores. The δ¹³C values of n-C₂₇, n-C₂₉, and n-C₃₁ alkanes in sediments from the North Pacific ES core show significant glacial to interglacial variation whereas those from the Bering Sea core do not. Variations of δ¹³C values of land plant n-alkanes are related to the environmental or vegetational changes in the source land areas. Environmental changes, especially, aridity, rainfall, and pCO₂ during glacial/interglacial transitional periods can affect vegetation, and therefore C₃ / C₄ plant ratios, resulting in δ¹³C changes in the preserved land plant biomarkers. Maximum values of δ¹³C as well as maximum average chain length values of long chain n-alkanes in the ES core occur mostly at the interglacial to glacial transition zones reflecting a time lag related to incorporation of living organic matter into soil and transportation into ocean basins via wind and/or ability of C₄ plants to adapt for a longer period before being replaced by C₃ plants when subjected to gradual climatic changes. Irregular variations with no clear glacial to interglacial trends in the BOW-8a core may result from complex mixture of aerosols from westerly winds and riverine organic matter from the Bering Sea catchments. In addition, terrestrial organic matter entering the Bering Sea could originate from multiple pathways including eolian, riverine, and ice rafted debris, and possibly be disturbed by turbidity and other local currents which can induce re-suspension and re-sedimentation causing an obliterated time relation in the Bering Sea biomarker records.