Introduction to X-Ray Fluorescence Analysis

When an element is placed in a beam of x-rays, the x-rays are absorbed When an element is placed in a beam of x-rays, the x-rays are absorbed. The absorbing atoms become ionized (e.g. due to the x-ray beam ejects the electron in the inner shell). An electron from higher energy shell (e.g., the L shell) then fall into the position vacated by dislodged inner electron and emit x-rays or characteristic wavelength. This process is called x-ray fluorescence.

The wavelength of fluorescence is characteristic of the element being excited, measurement of this wavelength enable us to identify the fluorescing element. The intensity of the fluorescence depends on how much of that element is in x-ray beam. Hence measurement of the fluorescence intensity makes possible the quantitative determination of an element.

The process of detecting and analyzing the emitted x-rays is called “X-ray Fluorescence Analysis.” In most cases the innermost K and L shells are involved in XRF detection. A typical x-ray spectrum from an irradiated sample will display multiple peaks of different intensities.

The characteristic x-rays are labeled as K, L, M or N to denote the shells they originated from. Another designation alpha (α), beta (β) or gamma (γ) is made to mark the x-rays that originated from the transitions of electrons from higher shells. Hence, a Kα x-ray is produced from a transition of an electron from the L to the K shell, and a Kβ x-ray is produced from a transition of an electron from the M to a K shell, etc. Since within the shells there are multiple orbits of higher and lower binding energy electrons, a further designation is made as α1, α2 or β1, β2, etc. to denote transitions of electrons from these orbits into the same lower shell.

The X-Ray Fluorescence Process Example: Titanium Atom (Ti = 22) 1) An electron in the K shell is ejected from the atom by an external primary excitation x-ray, creating a vacancy.

2) An electron from the L or M shell “jumps in” to fill the vacancy 2) An electron from the L or M shell “jumps in” to fill the vacancy. In the process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the L or M shell.

3) When a vacancy is created in the L shell by either the primary excitation x-ray or by the previous event, an electron from the M or N shell “jumps in” to occupy the vacancy. In this process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the M or N shell.

“Auger” Electron The excitation energy from the inner atom is transferred to one of the outer electrons causing it to be ejected from the atom. This process is a competing process to the XRF.

X-ray fluorescence's spectroscopy provides a means of identification of an element, by measurement of its characteristic X-remission length or energy The method allows the quantification of a given element by first measuring the emitted characteristic line intensity and then relating this intensity to elemental concentration

The energy of the peaks leads to the identification of the elements present in the sample (qualitative analysis), while the peak intensity provides the relevant or absolute elemental concentration (semi-quantitative or quantitative analysis).

Advantages of X-ray Fluorescent Analysis Rapid analysis Nondestructive analysis No spectrum is affected by chemical bonding Easily analysis of the element among the same family elements High accurate analysis (5B to 92U can be analysis) Easy qualitative analysis Easy sample preparation

Elemental oxygen can be analyzed but oxides content is estimated by assuming the sample contains certain oxides. Consequently oxides content is estimated result because XRF can only determine elements. Elemental carbon and sulfur can also be analyzed but not CO3=, SO4=, SO3= .

Schematic figure of an x-ray fluorescence spectrophotometer BASIC PRINCIPLE:

X-ray generator part Spectrometer Part X-ray generator Sample chamber collimator Analyzing crystal To counting and recording part To spectrometer part X-ray generator part Spectrometer Part

X-RAY GENERATOR X-ray tube for XRF spectrometer is a diode (vacuum tube) consist of the filament generating thermo- electron and the anode (target) generating x-rays. Near the target, there is a window to pass x-rays through to the outside tube. The window material, Beryllium, is employed because of its nature for having the excellent transmission (penetration) of x-rays.

There two types of x-ray tubes: - Side Window Type X-ray Tube - End Window Type X-ray Tube

Comparison on the grounding method and window thickness of x-ray tubes Type Target Material Ground-ing Window Thickness Max Load Cooling Water Side window W, Cr, Mo, Au, etc Anode 1000μm, 300μm (Cr) 3 kW City water End window Rh, etc Cathode 127μm 3kW Deionized water* * Less than 2 μS/cm (higher than 50x104Ωcm

Rh target end-window type x-ray tube has the features that since it is effectively sensitive to the element less than the atomic number 16 (S) and it can also obtain relatively the good sensitivity to the heavy elements. It can make the measurements from heavy elements to light elements without exchanging X-tube. The frequency of using this Rh target X-ray tube is high

In case of side window type x-ray tube, the W and Mo target x-ray tubes will be applied for the analysis of heavy elements and the Cr target x-ray tube will be applied for the analysis of light elements. Especially , the Mo target x-ray tube will be frequently applied for the analysis of environmental pollution such as Hg, Pb, As and so on.

Analyzing crystal The diffraction phenomenon of x-ray through the single crystal is utilized for the dispersion of x-rays. This crystal is called the analyzing crystal. In accordance with the wavelength region of the fluorescence x-rays, the optimum analyzing crystal is employed respectively.

Analyzing crystals such as TAP, RAP, ADP, EDDT, PET, NaCl etc Analyzing crystals such as TAP, RAP, ADP, EDDT, PET, NaCl etc. are sensitive to humidity. If they are left in the air, their surface will deliquesce, leading to lower x-ray reflection intensities and deteriorated resolution. It is therefore necessary to keep the interior of the spectroscopic chamber in a vacuum even during the time that the x-ray spectrometer is not used. Samples which are highly acid or alkaline, or which will sublime at low temperatures will deteriorate analyzing crystal leading to lower reflection intensities.

The fluorescence x-ray excited from the specimen are diffracted through the analyzing crystal by scanning goniometer. The diffracted x-rays are detected by the counter and through the electronic circuit, the intensities are recorded automatically on the charge recorder. The characteristic wavelength (λ) emitted by each element in the sample is analyzed by applying a diffracting (an analyzing) crystal which has a certain d value.

Diffracting angles (θ) are measured and λ of each element is determined using Bragg’s law. By determining the elemental spectra recorded on a chart, we can learn the name of elements containing in the specimen.

Example of a qualitative measurement result. Fluorescent spectrum recording of a stainless steel

The primary radiation was supplied by tungsten-target tube operated at 50 kV, and the sample was stainless steel containing 18%Cr and 8% Ni. The K lines of all the major constituents (Fe, Cr and Ni) and of some of the minor constituents (Mn and Co) are apparent. In addition tungsten L lines can be seen; these always be present when a tungsten tube is used. The copper K lines are due to copper existing as an impurity in the tungsten target

Consider the quantitative analysis result of a dry limestone sample which is obtained from XRF analysis ; Mg and Ca are found as carbonates, while Fe and Si as oxides. Convert the data to mol % of the actual substances (CaCO3, MgCO3, Fe2O3 and SiO2) present in the limestone. Mol weight : Ca = 40.1; Mg = 24.3; C = 12.0; O = 16.0 and Fe = 55.8, Si=28.1. Spectra of oxygen and carbon are not considered. ELEMENT SPECTRUM WT% Mg Mg Kα 1.2 Ca Ca Kα 90.0 Fe Fe Kα 2.3 Si Si Kα 6.5

Quantitative Analysis XRF is a reference method, standards are required for quantitative results. Standards are analysed, intensities obtained, and a calibration plot is generated (intensities vs. concentration). XRF instruments compare the spectral intensities of unknown samples to those of known standards. Concentration Intensity

Sample Preparation Powders: Grinding (<400 mesh if possible) can minimise scatter affects due to particle size. Additionally, grinding insures that the measurement is more representative of the entire sample, vs. the surface of the sample. Pressing (hydraulically or manually) compacts more of the sample into the analysis area, and ensures uniform density and better reproducibility..

Solids: Orient surface patterns in same manner so as minimise scatter affects. Polishing surfaces will also minimise scatter affects. Flat samples are optimal for quantitative results.

Liquids: Samples should be fresh when analysed and analysed with short analysis time - if sample is evaporative. Sample should not stratify during analysis. Sample should not contain precipitants/solids, analysis could show settling trends with time.

XRF Application During the last two decades, the development in X-ray detectors has established the XRF method as a powerful technique in a number application fields, including: Ecology and environmental management: measurement of heavy metals in soils, sediments, water and aerosols Geology and mineralogy: qualitative and quantitative analysis of soils, minerals, rocks etc. Metallurgy and chemical industry: quality control of raw materials, production processes and final products Paint industry: analysis of lead-based paints

XRF Application Jewelry: measurement of precious metals concentrations Fuel industry: monitoring the amount of contaminants in fuels Food chemistry: determination of toxic metals in foodstuffs Agriculture: trace metals analysis in soils and agricultural products Archaeology and archaeometry Art Sciences: study of paintings, sculptures etc.