Qualitative, quantitative analysis and “standardless” analysis NON DESTRUCTIVE CHEMICAL ANALYSIS Notes by: Dr Ivan Gržetić, professor University of Belgrade – Faculty of Chemistry

Qualitative, quantitative analysis and “standardless” analysis Every element of the periodic system of elements (except H & He) have its characteristic X-ray emission lines. To be sure that we will obtain line of interest the excitation potential (incident radiation) should be at least three times greater than the energy of emission line in question. For example, if one wants to get Cu K α line (8,04keV) than the excitation potential must be 24 kV or greater.

Qualitative, quantitative analysis and “standardless” analysis Every producer of SEM-EDS, SEM-WDS, WD-XRF, ED- XRF and Handheld ED-XRF has their own program packages for qualitative, quantitative analysis and “standardless” analysis. Usually all these packages have similar purposes, but different names. They are always protected with trademarks. But, once you get acquainted with one set of programs produced by one firm, immediately you gain experience for all of them. Why? Theoretical and scientific background is always the same. The differences appear only for specialized requirements set by users like semiconductor industry, metallurgy, environmental science, inspections, etc.

ED XRF Qualitative analysis Very simple procedure: if one already has a sample one selects: appropriate excitation potential (4-50kV), corresponding current (<2mA) (higher currents Si(Li)-detector cannot sustain) and counting time (~100sec).

ED XRF Qualitative analysis Identification of X-ray emission lines nowadays is fully automated, but no one should take program results as absolutely correct. Overlaps, sum peaks and escape peaks are always present. Escape peak (an artefact): With Si(Li) detectors Si escape peaks occurs 1.74 keV below each 'true' peak.

Escape Peak in EDS spectrum

Overlaps in EDS spectrum

Sum peaks

WD XRF Qualitative analysis Procedure that requires knowledge. If one already has a sample one selects: appropriate diffraction crystal, collimator, increment with (a fraction of 2 Θ ), counting time per increment (~1-2 sec), excitation potential (~60kV) and corresponding current (~50mA). The whole qualitative analysis depends on number of selected diffraction crystals, increment with and counting time. An average time for qualitative analysis is usually between min (5-10min per crystal).

WD XRF Qualitative analysis Identification of X-ray emission lines nowadays is fully automated, but no one should take program results as absolutely correct. Overlaps and peaks of the X-ray tube material are usually present in WDS spectrum.

WD XRF Qualitative spectrum

WD XRF Qualitative spectrum light elements

XRF Quantitative analysis X-ray fluorescence analysis belongs to a group of very advanced analytical methods. It requires wide and profound knowledge background in chemistry and physics. Quantitative analysis its theoretical background and corresponding calculation steps are the same for both EDS and WDS analysis. But, there are differences between calculation approaches for corpuscular and photon (X- ray) induced X-ray fluorescence.

XRF Quantitative analysis In both cases, corpuscular and photon (X-ray) induced X-ray fluorescence, there are three important matrix efects that should be taken in account in the case of quantitative analysis: ◦ Effect of the atomic number (Z), ◦ Absorption (A), and ◦ Enhanced Fluorescence (F). These effect are different when, for example, excitation is performed with electrons or with X-rays because electrons and X-rays have different penetration depths in the „body of sample“ and as a result we have different effects of ZAF in these two cases.

XRF Quantitative analysis - Effect of the atomic number (Z) Generally speaking, the matrix effects in X-ray fluorescence analysis result from the influence of the variations of chemical compositions of the sample matrix on the fluorescent intensity of the wanted element. These effects can manifest themselves either via a difference in the absorption of both the primary and fluorescence radiations in samples of different matrix composition (absorption effect) or via an increase of the radiation intensity (enhancement effect) due to the fluorescence radiation of some of the inter elements. Penetration depth is also important, higher penetration depth means more extensive effects of absorption and fluorescence.

XRF Quantitative analysis – Absorption (A) Absorption: Any element can absorb or scatter the fluorescence of the element of interest. This effect occurs when the variations in the matrix chemical composition result in changes of the mean absorption coefficients of both the primary radiation of the source and the fluorescence radiation of the wanted element. The absorption effects occurring in a matrix may either decrease or increase the intensity of the fluorescence radiation of the element under determination, depending on whether the matrix composition changes diminish or augment the mass absorption coefficient. A strong decrease of the fluorescence radiation of the wanted element will be observed if the concentrations of disturbing elements of slightly lower atomic numbers become larger.

XRF Quantitative analysis – Enhanced Florescence (F) Enhancement: Characteristic x-rays of one element excite another element in the sample, enhancing its signal. This effect involves an extra excitation of the atoms of the wanted element by the fluorescence radiation of some of the matrix elements, which in this case become interferents. The mechanism of the enhancement effect involves retransmission of the energy of the primary radiation of the source in the form of secondary (fluorescence) radiation of interelements. The energy of this secondary radiation is just slightly higher than the absorption edge of the wanted element (Figure VI.3), the latter will be excited more efficiency than by the primary radiation of the source, whose energy is higher than that of the secondary radiation and, consequently, further from the absorption edge.

Absorption-Enhancement Affects

QUANTITATIVE ANALYSIS Calibration-Standard Methods. The analyte-line intensity from samples is compared with that from standards having the same form as the samples and, nearly as possible, the same matrix. Internal Standardisation. The calibration-standard method is improved by quantitative addition to all samples of an internal standard element having excitation, absorption and enhancement characteristics similar to those of the analyte in the particular matrix. The calibration function involves measuring the intensity ratio of the analyte and internal standard lines.

QUANTITATIVE ANALYSIS “STANDARDLESS ANALYSIS” Mathematical Corrections. Absorption-enhancement effects are corrected mathematically by the use of influence coefficients for each element present (these are derived experimentally from reference samples). The basic approach is that the XRF intensity at a particular wavelength will in some way be affected by each element in the sample. Standardless analysis – the major advantages of X-ray fluorescence analysis (XRF) – allows fast and easy determination of the chemical composition without performing a calibration. Due to powerful matrix correction based on variable alphas (coefficients for the correction of matrix effects) every kind of sample can be analyzed with optimized measurement parameters for the chemical composition, no matter which kind of sample preparation has been used.

Now it is really THE END