X-ray Absorption Spectroscopy: Design and Construction of a Low-Temperature Cell
Background and Objectives
The design and construction of a low-temperature cell to facilitate XAS data collection was the main objective of this study. Metal species in natural samples can undergo undesired redox transformations in the high-energy synchrotron x-ray beam [Image of arsenic patterns below] (Foster et al., 1999), and standard methods to eliminate these effects (use of an LHE-cooled cryostat) increase the time needed for sample loading, alignment, and analysis considerably [Table 1]. Access to synchrotron radiation facilities (where XAS analyses are performed) is limited; therefore, efficiency in data collection is paramount. After examination of various cooling options and selection of the most practical design, we planned to test the cell in the laboratory using thermocouples to monitor temperature in the sample.
Highlights and Key Findings
It was determined that indirect cooling using dry ice [CO2(s)] or liquid nitrogen was probably not feasible because of the water ice buildup that accompanies this type of cooling. Ice buildup has the potential to damage: (1) sensitive electronic equipment that operates in close vicinity to the sample and (2) relatively inexpensive but tedious-to-replace aluminized mylar windows that are used in some detectors. Although ice buildup could be removed via a heating apparatus, the complexity of the cell design would increase as well. Anecdotal information from XAS experts suggests that cooling to 4 °C using a very small peltier device should be sufficient to slow rates of the photocatalyzed and/or radical-catalyzed redox reactions. Such devices are readily available. A technician at SSRL (Joe Reynolds) designed a simple cell using a commercially available “micro Peltier” device whose design we planned to copy and have fabricated. However, since this device is now available to users, there was no need to construct one. However, since it has not been tested on a wide variety of samples, we continue to use the liquid helium cryostat for our XAS analyses.
|Ambient Temperature||Cryogenic Temperature|
|Maximum Sample Volume||0.18 cm3 (~ 0.5 g )||0.09 cm3 (~ 0.2 g )|
|Detector available||Single element Lytle, multi-element Ge||Multi-element Ge|
|Alignment of sample in beam||Easy (visual cues)||Tricky (can't see sample)|
|Alignment of sample relative to detector||Easy||Tricky; scatter off of Aluminum holder can create high fluorescence mistaken for desired K line|
|Time to change sample||< 5 minutes||30 minutes|
Journal Articles, Review Volumes, and Theses
Foster, A.L., Brown, G.E., Jr., Parks, G.A., 1998, X-ray absorption fine-structure spectroscopy study of photocatalyzed, heterogeneous As(III) oxidation on kaolin and anatase: Environmental Science and Technology 32, 1444-1452.
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