Standardless Analysis by XRF but I don t know what s in my sample!! Dr Colin Slater Applications Scientist, XRF Bruker UK Limited

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1 by XRF but I don t know what s in my sample!! Dr Colin Slater Applications Scientist, XRF Bruker UK Limited

2 XRF Standardless Analysis In this talk What is meant by standardless analysis? Fundamental Parameters How can I improve my results from FP? Something a bit different analysis of multi-layered samples

3 XRF in every day life Can you just XRF this for me?? Routine analysis of samples using a matrix matched calibration Increasing demand for what s in my sample? type analysis. - Unknown materials from an industrial process - Coatings or scrapings from pipes - Filtrates from engines - etc No standards available what do you do? STANDARDLESS ANALYSIS

4 XRF is a comparative technique As a primary X-Ray photon enters a sample, it is absorbed. When a fluorescence photon is emitted, it too is affected by absorbance as it escapes the sample matrix. Other fluorescence photons being produced may have either an absorbing or enhancing effect. These effects are known collectively as MATRIX EFFECTS

5 Fundamental Parameters A theoretical approach to quantitative X-Ray spectroscopy The basis of all quantitative X-Ray spectroscopy methods is an assertion by von Hamos in the 1940s: the intensity of fluorescence radiation from an element in a material should be proportional to the concentration of that element in that material. R i = C i K i R i is the ratio of the measured intensity of element i in a sample, compared to the intensity of a sample of pure element i. C i is the concentration of element i in the sample K i a constant is a function of the sample composition, the mass absorbance coefficients of the sample constituents and the measurement geometry.

6 Fundamental Parameters A theoretical approach to quantitative X-Ray spectroscopy Sherman (1950s) derived an equation to better understand absorbance / enhancement effects in multi-element samples. After re-arrangement to account for polychromatic incident radiation, and the inclusion of alpha coefficients, this was later adopted as the fundamental parameters method. Using this method, it is theoretically possible to measure a single standard, provided the surface reflectance is similar to the unknown samples.

7 Fundamental Parameters A theoretical approach to quantitative X-Ray spectroscopy The fundamental parameters are: - Total mass absorbance coefficients For the sample - Mass photoabsorbance coefficients - Tube spectrum profile and intensity - Shell fluorescence yields - Line transition probabilities For each element in the sample - Line Energy - Concentration of element - Geometric considerations (instrument parameters) Any FP based method is only as accurate as the information it is given about the sample.

8 Sample Na 86 (AlO 2 ) 86 (SiO 2 ) 106.xH 2 O (Aldrich, Molecular Sieve 13X) Expected wt% Na Si Al Expected Si/Al Ratio 1.28 Sample Prep 5g of dried material mixed with 3g of WAX binder, pressed into a 40mm pellet.

9 Lots of O-Atoms Lots of Water Has an absorbing effect on the fluorescence signal from lighter elements. Variable water content gives rise to variable absorbance. Exchangeable Species Variable Densities

10 Element Concentration Al Ca 0.02 Cl 0.22 Fe 0.02 Ga 0.01 K 0.25 Na 7.00 P 0.03 S 0.02 Si Sample Na 86 (AlO 2 ) 86 (SiO 2 ) 106.xH 2 O Expected wt% Na Si Al Expected Si/Al Ratio 1.28 Measured Si/Al Ratio 1.41

11 Lots of O-Atoms Light elements difficult to see by XRF. Invisible elements give rise to errors when computing background correction factors. Lots of Water Chemical bonding (and mineralogy) can also affect the result. Exchangeable Species Variable Densities

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13 Element Concentration Al Ca 0.01 Cl 0.18 Fe 0.01 K Na P 0.02 S 0.02 Si Sample Na 86 (AlO 2 ) 86 (SiO 2 ) 106.xH 2 O Expected wt% Na Si Al Expected Si/Al Ratio 1.28 Measured Si/Al Ratio 1.29

14 Lots of O-Atoms Lots of Water Variable densities give rise to variable mass attenuation coefficients for materials of similar chemical composition Exchangeable Species Variable Densities

15 Penetration Depth Information Region (Critical Depth) X-Ray Tube Detector

16 MATRIX Element Line Energy (KeV) Graphite Glass Iron Lead Cd KA Mo KA Cu KA Ni KA Fe KA * Cr KA S KA Mg KA Ni LA Fe LA * C KA * B KA = cm = mm = µm

17 Rh Kα Compton Intensity of compton peak is very sensitive to the average atomic number. As this decreases, measured intensity of compton peak (I M ) increases. Theoretical compton intensity can be calculated from sample composition (I T ) I T / I M = 1 Numbers less than 1 indicate XRF invisible species present. Compton intensity can be used to correct for very low density materials.

18 Element Concentration Al Ca 0.01 Cl 0.21 Fe 0.01 K Na P 0.02 S 0.02 Si Sample Na 86 (AlO 2 ) 86 (SiO 2 ) 106.xH 2 O Expected wt% Na Si Al Expected Si/Al Ratio 1.28 Measured Si/Al Ratio 1.29

19 Something a bit different Layer thickness analysis The ratio of α:β line intensities is well known (line transition probabilities). Variations in absorbance due to layers in a sample can be calculated by measuring the change to this expected ratio for a given element. Done using Fundamental Parameters: Need to know Which elements are in the layer / substrate What order the layers are in (which elements give rise to signal attenuation)

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