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Anchor 1

Reference Materials 

I decided to use custom reference materials for creating the calibration curves for three reasons:

 

1. Commerically available soil, clay and ceramic (brick) reference materials had Ca content much lower than the concentration that I epxected in my unknown samples. Therefore they would be inappropriate for EDXRF calibration since essentially they represent a different chemical matrix. The only reference materials I could find that had relatively suitable Ca content (SO-3/Canada Center for Mineral and Soil Technology and NCSDC73326/TechLab) were unavailable for purchase.

 

2. Commercially availble materials tend to be rather expensive. For the same amount of money that I would spend for purchasing two commercial SRMs I paid for the analysis of 23 custom ones. 

 

3. My strategy involved measuring fired briquettes as well. Therefore, I needed a sufficient quantity of material. Commercially available materials are sold in very small quantities to ensure availability (usually 50 - 100 gr.). 

 

The samples were sent to an external ISO-Certified laboratory (ActLabs Canada) and analysed using their Lithogeochemistry protocol. This ensured that the resulting values are of high accuracy and suitable to be used as reference values. The samples were prepared as molten beads using lithium metaborate/tetraborate fusion. The beads were rapidly digested in a weak nitric acid solution. The fusion ensures that the entire sample is dissolved. This attack ensures that major oxides including SiO2, refractory minerals (i.e. zircon, sphene, monazite, chromite, gahnite, etc.), REE and other high field strength elements are put into solution. Analysis was done by ICP-OES and ICP-MS.

 

Below you can find a table with the description and reference concentrations of the reference materials used in this work. Only the concentrations of elements for which calibration curvers were created are reported.

 

 

* RM_1 - RM_7 and RM_20 - RM_23 were collected by Virginie Renson as part of her PhD work at Vrije Universiteit Brussel and are part of the refence collection of V.U.B. 
** RM_8 - RM_12 were collected by Maria Dikomitou - Eliadou with the help of Dr. Costas Xenophontos in the framework of her research in the site of Marki, Cyprus.
*** The fired briquettes for RM_15 - RM_19 were prepared by Maria Dikomitou - Eliadou and Noemi Muller at NCRS - Dimokritos. 

**** RM_13 and RM_14 are ceramic sherds from the site of Alassa - Pano Mandilaris provided by Ariane Jacobs with permission from Sophocls Hadjisavvas. 

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Calibration curves

The most robust way of calculating XRF calibration curves as proposed by Rousseau (Rousseau 1984, 1996, 2006) is by using the following equation to relate measured intensities of an analyte to known concentration values:

(1)

This approach however requires the calculation of the matrix effects coefficients manually. Instead we will be using the estimation of the instrument software which is using a default Fundamental Parameters (FP) method to provide elemental concentrations. In any FP algorithm the calculated intensity is given by the equation:

(2)

where MiFP  represents the matrix correction coefficients that the software has calculated. If we multiply each member of equation (1) by MiFP we get:

(3)

and substituting from (2):

This means that calculating calibration curves between the software FP estimations and the reference concentrations is as valid as calculating the curves using measured net intensities. 

 

The range, Instrumental Limit of Detection (ILD) and Limit of Quantification (LOQ) for each type of calibration curve are presented in the table below.

R.M. Rousseau, Fundamental Algorithm between concentration and intensity in XRF analysis, X-Ray Spectrometry, Vol 13, No3 (1984)

 

R.M. Rousseau, J.P. Wills, A.P. Duncan, Practical XRF calibration procedures for major and trace elements, X-Ray Spectrometry, Vol 25, 179-189 (1996)

 

R.M. Rousseau, Corrections for matrix effects in XRF analysis - a tutorial, Spectrochimica Acta Part B, 61, 759 - 777 (2006)

 

 

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