Chemical Sciences Division, Materials and Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8393, United States.
Anal Chem. 2013 May 7;85(9):4675-85. doi: 10.1021/ac400324v. Epub 2013 Apr 24.
Studies of climate change increasingly recognize the diverse influences exerted by monoterpenes in the atmosphere, including roles in particulates, ozone formation, and oxidizing potential. Measurements of key monoterpenes suggest atmospheric mole fractions ranging from low pmol/mol (parts-per-trillion; ppt) to nmol/mol (parts-per-billion; ppb), depending on location and compound. To accurately establish the mole fraction trends, assess the role of monoterpenes in atmospheric chemistry, and relate measurement records from many laboratories and researchers, it is essential to have good calibration standards. The feasibility of preparing well-characterized, stable gas cylinder standards for monoterpenes at the nmol/mol level was previously tested using treated (Aculife IV) aluminum gas cylinders at NIST. Results for 4 of the 11 monoterpenes, monitored versus an internal standard of benzene, indicated stability in these treated aluminum gas cylinders for over 6 months and projected long-term (years) stability. However, the mole fraction of the key monoterpene β-pinene decreased, while the mole fractions of α-pinene, d-limonene (R-(+)-limonene), p-cymene, and camphene (a terpene not present in the initial gas mixture) increased, indicating a chemical transformation of β-pinene to these species. A similar pattern of decreasing mole fraction was observed in α-pinene where growth of d-limonene, p-cymene, and camphene has been observed in treated gas cylinders prepared with a mixture of just α-pinene and benzene as the internal standard. The current research discusses the testing of other cylinders and treatments for the potential of long-term stability of monoterpenes in a gas mixture. In this current study, a similar pattern of decreasing mole fraction, although somewhat improved short-term stability, was observed for β-pinene and α-pinene, with growth of d-limonene, p-cymene, and camphene, in nickel-plated carbon steel cylinders. β-Pinene and α-pinene showed excellent stability at over 6 months in aluminum cylinders treated with a different process (Experis) than used in the original study.
气候变化研究越来越认识到单萜在大气中发挥的多种影响，包括在颗粒物、臭氧形成和氧化潜能方面的作用。关键单萜的测量结果表明，大气摩尔分数范围从低 pmol/mol（万亿分之一；ppt）到 nmol/mol（十亿分之一；ppb），具体取决于地点和化合物。为了准确确定摩尔分数趋势、评估单萜在大气化学中的作用以及将来自许多实验室和研究人员的测量记录联系起来，拥有良好的校准标准是至关重要的。以前在 NIST 用经过处理的（Aculife IV）铝气瓶测试了在 nmol/mol 水平下制备具有良好特征、稳定的单萜气体钢瓶标准的可行性。对 11 种单萜中的 4 种进行了监测，与苯的内部标准进行了比较，结果表明这些经过处理的铝气瓶在超过 6 个月的时间里稳定，并且可以长期（数年）稳定。然而，关键单萜 β-蒎烯的摩尔分数下降，而 α-蒎烯、d-柠檬烯（R-(+)-柠檬烯）、对伞花烃和莰烯（初始气体混合物中不存在的萜烯）的摩尔分数增加，表明β-蒎烯向这些物质发生了化学转化。在α-蒎烯中也观察到类似的摩尔分数下降模式，其中在仅用α-蒎烯和苯作为内部标准制备的处理过的气瓶中观察到了 d-柠檬烯、对伞花烃和莰烯的生长。目前的研究讨论了测试其他气瓶和处理方法，以确定单萜在混合气中的长期稳定性潜力。在当前的研究中，β-蒎烯和α-蒎烯的摩尔分数也观察到了类似的下降模式，尽管短期稳定性有所提高，同时 d-柠檬烯、对伞花烃和莰烯的生长也有所增加，在镀镍碳钢气瓶中。β-蒎烯和α-蒎烯在经过与原始研究中使用的不同工艺（Experis）处理的铝气瓶中，超过 6 个月时表现出极佳的稳定性。
