Dowsett Mark G, Adriaens Annemie, Jones Gareth K C, Poolton Nigel, Fiddy Steven, Nikitenko Sergé
Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
Anal Chem. 2008 Nov 15;80(22):8717-24. doi: 10.1021/ac800895n. Epub 2008 Oct 15.
XANES and EXAFS information is conventionally measured in transmission through the energy-dependent absorption of X-rays or by observing X-ray fluorescence, but secondary fluorescence processes, such as the emission of electrons and optical photons (e.g., 200-1000 nm), can also be used as a carrier of the XAS signatures, providing complementary information such as improved surface specificity. Where the near-visible photons have a shorter range in a material, the data will be more surface specific. Moreover, optical radiation may escape more readily than X-rays through liquid in an environmental cell. Here, we describe a first test of optically detected X-ray absorption spectroscopy (ODXAS) for monitoring electrochemical treatments on copper-based alloys, for example, heritage metals. Artificially made corrosion products deposited on a copper substrate were analyzed in air and in a 1% (w/v) sodium sesquicarbonate solution to simulate typical conservation methods for copper-based objects recovered from marine environments. The measurements were made on stations 7.1 and 9.2 MF (SRS Daresbury, UK) using the mobile luminescence end station (MoLES), supplemented by XAS measurements taken on DUBBLE (BM26 A) at the ESRF. The ODXAS spectra usually contain fine structure similar to that of XAS spectra measured in X-ray fluorescence. Importantly, for the compounds examined, the ODXAS is significantly more surface specific, and >98% characteristic of thin surface layers of 0.5-1.5-microm thickness in cases where X-ray measurements are dominated by the substrate. However, EXAFS and XANES from broadband optical measurements are superimposed on a high background due to other optical emission modes. This produces statistical fluctuations up to double what would be expected from normal counting statistics because the data retain the absolute statistical fluctuation in the original raw count, while losing up to 70% of their magnitude when background is removed. The problem may be solved in future through optical filtering to isolate the information-containing band, combined with the use of higher input X-ray fluxes available on third-generation light sources.
X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）信息传统上是通过X射线的能量依赖吸收的透射测量或通过观察X射线荧光来获取的，但二次荧光过程，如电子和光学光子（例如200 - 1000纳米）的发射，也可以用作X射线吸收光谱（XAS）特征的载体，提供诸如提高表面特异性等补充信息。在近可见光子在材料中的穿透深度较短的情况下，数据将更具表面特异性。此外，在环境池中，光辐射可能比X射线更容易透过液体逸出。在此，我们描述了用于监测铜基合金（例如文物金属）上的电化学处理的光学检测X射线吸收光谱（ODXAS）的首次测试。对沉积在铜基底上的人工制造的腐蚀产物在空气中以及在1%（w/v）倍半碳酸钠溶液中进行了分析，以模拟从海洋环境中回收的铜基物品的典型保护方法。测量是在英国达累斯伯里的SRS的7.1和9.2 MF站使用移动发光终端站（MoLES）进行的，并辅以在欧洲同步辐射装置（ESRF）的DUBBLE（BM26 A）上进行的XAS测量。ODXAS光谱通常包含与在X射线荧光中测量的XAS光谱类似的精细结构。重要的是，对于所研究的化合物，ODXAS具有显著更高的表面特异性，并且在X射线测量受基底主导的情况下，对于厚度为0.5 - 1.5微米的薄表面层，其特征大于98%。然而，宽带光学测量得到的EXAFS和XANES叠加在由于其他光学发射模式导致的高背景上。这产生的统计波动高达正常计数统计预期值的两倍，因为数据保留了原始原始计数中的绝对统计波动，而在去除背景时损失高达70%的幅度。未来，通过光学滤波来分离含信息波段，并结合使用第三代光源可获得的更高输入X射线通量，这个问题可能会得到解决。
