Physical & chemical sedimentology

The Itrax XRF Core Scanner is the latest development in non-destructive analytical instrumentation aimed at paleoclimate research. Scanning of a core produces an optical RBG and a micro-radiographic image as well as micro-X-ray fluorescence spectrometry (XRF) elemental profiles, all at high resolution. The XRF data are produced as peak areas and is a relative concentration measurement only. The images are produced as tiff files while the elemental data is produced as a text file that can be opened with Excel. The generated digital images can be overlaid and compared to changes in chemical composition using special post-processing software. We suggest using ItraxPLOT, which has been developed by BOSCORF at the National Oceanography Centre in Southampton, UK.

The Itrax can accommodate a variety of sample types including sediment and rock cores as well as drop stones, corals and wood (providing they are the right shape). The instrument can analyse split cores, slabs or U-channel samples up to 1800 mm in length with a diameters up to 120 mm and with a minimum thickness of 1 cm. XRF core scanning differs from conventional XRF in that water and organic matter are not removed from the matrix and the grain size is not homogenized. This must be kept in mind when interpreting the data after analysis so good characterization of your sample matrix is key.

Measurement Procedure

Samples are first scraped to obtain a flat and fresh surface. Cores are then loaded into the instrument and fed into the central tower where the cameras and XRF instrumentation sits. A surface scan is initiated which records the surface topography. This ensures that the XRF detector does not collide with the sample on subsequent scans and that the sample-detector distance is constant, which is important for reliable XRF analysis. During this scan the optical image is acquired. The user must then set the radiographic parameters including the tube voltage and current and the exposure time. The aim here is to generate a radiograph with suitable contrast and brightness. The next step is to set the XRF parameters. The user selects elements of interest likely to be found in the sample and optimises the peak fitting parameters. If some elements are missed in this initial selection, these can be recovered later by reprocessing the spectral data generated during the scan. A primary signal is then recorded; this is akin to a ‘blank’ reading. The final step for the user is to specify the core name and file destination and the XRF step-size and exposure time. It is possible at this stage to set-up multiple analyses of the same core. Once all this information is entered the Itrax starts scanning for first the radiograph, and then the XRF elemental profiles.

Things to Consider

When designing the measurement protocol several decisions must be made. Firstly, it must be decided which tube will be used. At the SLAM Lab we are able to offer analyses using both Mo and Cr tubes. The Mo tube is suitable for both transition and heavy elements while the Cr tube is appropriate for lighter elements, i.e., Al, Si, P, S, K and Ca. XRF is not a suitable method for all elements and which elements are acquired during an analysis depends on the actual concentration of that element in the sample and how long each step is analysed for. Essentially, by doubling your measurement time you will double the response of an element. The matrix of the sample also impacts the analysis where the response from organic-rich sediments is lower than that from mineral-rich sediments. Finally, the step size for analysis must be decided. Our instrument can measure from 200 µm upwards. If you have very fine laminations this might be an appropriate step size but a homogenous sample can be analysed at larger step sizes. In April 2014 a new XRF detector was installed that allows for much higher count rates. As a result we are able to cut analysis time significantly. The analysis of typical 1 m sediment at 1 mm resolution for both radiograph and XRF takes 3–4 hours.

Cost for Analysis

Itrax XRF core analysis costs 3375 SEK/day. When you fist arrive we will have a consultation to set up your analysis. Thereafter you will be trained on the instrument and can run and change samples yourself. Alternatively, we can run the samples but you will be charged for personnel time. All personnel time, including the consultation, costs 900 SEK/hour. These prices have been updated January 2017 and are subject to change at anytime.

For more information please contact: Malin Kylander, malin.kylander@geo.su.se