Introduction to XRF

Introduction to X-Ray fluorescence

Analysis Dr. Aseel B. AL-

Zubaydi

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Introduction to XRF

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.

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Introduction to XRF

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.

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Introduction to XRF

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.

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Introduction to XRF

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Introduction to XRF

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.

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Introduction to XRF

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.

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Introduction to XRF

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.

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Introduction to XRF

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.

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Introduction to XRF

“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. The second ejected electron is called an Auger electron

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Introduction to XRF

X-ray Spectra

X-rays are generated and caught by detectors

Introduction to 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

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Introduction to XRF

• 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).

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Introduction to XRF

Advantages of X-ray Fluorescent Analysis

1. Rapid analysis 2. Nondestructive analysis 3. No spectrum is affected by chemical bonding 4. Easily analysis of the element among the

same family elements 5. High accurate analysis (5B to 92U can be

analysis) 6. Easy qualitative analysis 7. Easy sample preparation

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Introduction to XRF

• Can analyzed oxygen but 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

= .

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Introduction to XRF

• Schematic figure of an x-ray fluorescence spectrophotometer

θλ sin.2dn =BASIC PRINCIPLE: ۱٦

Introduction to XRF

X-ray generator

Sample chamber

collimator

Analyzing crystal

collimator

To counting and recording part

To spectrometer part

X-ray generator part

Spectrometer Part

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Introduction to XRF

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.

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Introduction to XRF

There are two types of x-ray tubes:

1.End Window Type X-ray Tube 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.

2.Side Window Type X-ray Tube

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Introduction to XRF

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Introduction to XRF

• 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.)

Analyzing crystal

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Introduction to XRF

• 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.

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Introduction to XRF

Example of a qualitative measurement result.

Fluorescent spectrum recording of a stainless steel ۲۳

Introduction to XRF

Sample Preparation

1.Powders: Grinding (<400 mesh if possible) can minimise scatter affects due to particle size. Additionally, grinding insures that the measurement is more representation of the entire sample, vs. the surface of the sample.

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Introduction to XRF

2.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.

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Introduction to XRF

3. Liquids: Samples should be fresh when analysed and

analysed with short analysis time - if sample is evaporative.

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Introduction to XRF

XRF Application 1. Ecology and environmental measurement of

heavy metals in soils. 2. Geology and mineralogy: Metallurgy and

chemical industry: quality control of raw materials.

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Introduction to XRF

XRF Application 6. Jewelry: measurement of precious metals

concentrations 7. Fuel industry: monitoring the amount of

contaminants in fuels 8. Food chemistry: determination of toxic metals in

foodstuffs 9. gardening: trace metals analysis in soils and

agricultural products 10. Archaeology and archaeometry Art Sciences:

study of paintings, sculptures etc.

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Introduction to XRF

Introduction to XRF

Introduction to XRF

Introduction to XRF

Introduction to XRF QUIZ :What is the difference between xrd

& xrf? 1. XRF and XRD measure different things, each giving different information about the same

sample. 2. XRF, or X-Ray Fluorescence analysis, measures the intensity of x-rays flouresced by

individual elements in a sample, irrespective of the different compounds present that may contain those elements. eg. in cement the XRF analysed Ca percentage is the total Ca contributed by all calcium compounds in the cement.

3. XRD, or X-Ray Diffraction analysis, measures the intensity of crystal diffraction peaks due to the individual chemical compounds in the sample. ie CaCO3, CaO, CaSO4 etc. The result is estimated percentages for each compound of interest.

4. One method is not necessarily better than the other, they are simply complementary techniques which, when combined, give the total picture.

5. XRD is becoming more popular because it can estimate the quantity of clinker minerals more accurately than the traditional Bogue equations. which use the XRF chemical results. And XRD can also quickly analyse important compounds such as Free lime which are time consuming to analyse by any other method. With advances in computer power and programming sofware the complex calculations required to estimate clinker phases have become faster and more accurate, enabling this method to be used in on-line analysers for both clinker and cement.

6. Lastly, the formulae for calculating clinker phases can allow for free lime by simply subtracting the free lime from the XRF CaO value and using that value in the equation.