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Petrologie und Geochemie
* µXRF –Röntgenfluoreszenzspektrometer

Prinzip/Principle

XRF spectrometry is one of the most widely used and versatile analytical technique. An XRF spectrometer uses primary radiation from an X-ray tube to excite secondary X-ray emission from a sample. The radiation emerging from the sample includes the characteristic X-ray peaks of major and trace elements present in the sample. Dispersion of these secondary X-ray into a spectrum, usually by X-ray diffraction, allows identification of these elements present in the sample. The height of each characteristic X-ray peaks relates to the concentration of the corresponding element in the sample, allowing quantitative analysis of the sample for most elements in the concentration range of 1ppm to 100wt%.

X-ray are electromagnetic radiation with wavelengths in the range of 0,003 to 3 nm. They are produced when electrons jump between the K, L, M shells, the most tightly bound electron energy levels in atoms.

The sample is usually in the form of a solid disk and is irradiated by an intense beam of primary X-rays and finally emits secondary (fluorescent) X-rays. A small proportion of these secondary X-rays are collimated into a parallel beam and disperse into a spectrum through diffraction by a synthetic analysing crystal, using a primary collimator. The diffracted X-ray by the crystal are further collimated and collected in an X-ray detector, using a secondary collimator. The analysing crystal can rotate about an axis on its surface, and the detector and secondary collimator are linked so that they moved in an arc around the rotation axis of the crystal (goniometer).

The XRF spectrometer produces a spectrum in which the intensity of the diffracted beam is related to the angle of reflection. The angle teta (θ) is related to the wavelength () through the Bragg equation:

n  =2dsin (θ)

where n is an integer and d is the lattice spacing of the analysing crystal

X-ray tube

The principal component of the X-ray tube is the W filament and metal anode contained within a glass tube under a high vacuum. The filament is heated by means of an electric current and produces a cloud of electrons which are accelerated away from the filament (100kV voltage applied). The electrons strike the anode and the anode atoms releases energy as X-rays with a distribution of energy forming a continuum. The impact of high energy electrons can also ionised the anode atoms by ejecting electrons from their inner shells. The subsequent replacement of the ejected electrons by electrons from the outer shells generate X-rays with wavelengths characteristic of the anode material. The X-ray spectrum generated thus consists of continuous and discontinuous components. the conversion of electrical energy into X-rays is very inefficient since only 1% of the total applied power is released as X-ray. Most of the rest is released as heat which has to be dissipated by cooling the anode.

Sample

In XRF analysis, primary radiation from the X-ray tube is used to irradiate a sample. This has the effect of ionising the sample by displacing electrons from its component atoms, since X-ray photons have energies comparable with the energies binding electrons from the inner shells of the atom concerned. This process can only occur with photons more energetic than any given binding energy, but is more efficient for photons with energies only a little higher. X-ray tubes emit radiation with a range of wavelength and energy encompassing the K- or L-shell binding energies of all but the lightest atoms. An electron vacancy is rapidly filled by an electron transferring from an outer shell. The energy released, equal to the difference in binding energy of electrons in the two levels, is emitted as a secondary, or X-ray fluorescent, X-ray photon with a discrete energy and wavelength, characteristic of the individual element. This photon will either escape from the atom or be absorbed. Absorption of the photon within the atom (the Auger effect) is most pronounced for smaller atoms, and severely limits the yield of secondary X-rays from the lighter elements.

Electrons jumping from the outer shells into the K and L shells give rise to K and L lines respectively. Figure 2 shows the principle K and L lines, with their names and the electron transitions involved. The conventional naming of lines is far from systematic with respect to electron transition, but instead reflects the relative intensity. Thus alpha lines are more intense than beta lines which are more intense than gamma lines, and K lines are generally more intense then the corresponding L lines. The sample is placed close to the tube window in order to irradiate it with an intense X-ray beam. The flat surface is irradiate over an area of about 3cm. Samples can be prepared by pressing powder into a die under pressure or by casting a molten mixture of sample and lithium borate flux into a suitable mould.

Collimators

The radiation entering the spectrometer consists of characteristic fluorescent radiation from the sample, superimposed on primary tube radiation scattered by the sample. In order for radiation from the sample to be dispersed into a spectrum it must be formed into a parallel beam which is done by a collimator made of parallel metallic plate which either let pass or absorb the X-rays. The X-tray beam is then scattered from an analysing crystal and a secondary collimator selects a parallel beam of scattered X-rays to pass to the detectors. The purpose of the secondary collimator is to ensure that only those X-rays scattered by the crystal at an angle (teta) equal the angle of incidence are allowed to reach the detectors.

Detectors

The diffracted X-ray beam is directed, via the secondary collimator into one or more X-ray detector. Most spectrometer use two types of detectors, a gas flow detector (wavelength longer than 0,15 nm) and a scintillation detector (short wavelength)

Gas proportional detectors are filled with gas (Ar-CH4). The X-ray pass through a small and thin window and collide and ionise numerous gas molecules. Electron liberates during collision are accelerate toward the central wire. Those electrons collide and ionise more gas molecule on its way to the wire, causing an avalanche of electron to converge to the anode wire. The charge collected on the anode is converted into a voltage pulse, amplified and recorded by the computer.

Scintillation detector are made of three elements: A scintillation crystal that respond to ionising radiation and converts X-ray photon into light pulses; a photocathode of SbCs alloy that reacts to the light pulses by emitting photoelectrons and an electron multiplier which convert the photoelectrons to an electrical charge collected on an anode after signal amplification using secondary emission at a series of dynodes.

Quantification

Theory:

The output from the spectrometer consists of peak and background count rates for an X-ray emission line of each of the elements whose concentration is to be determined. The choice of suitable emission line is governed by several factors and the most important is the freedom of interferences:

Where possible K lines are preferred to L lines and alpha lines are preferred to beta lines. The intensity of primary radiation falls sharply at short wavelengths with a consequent reduction in the efficiency of secondary K-lines excitation for heavy elements. For most application iodine is the heaviest element for which K a line can be used in XRF spectrometry. L lines must be used for heavier elements. The lines intensity should therefore be linearly correlated with the concentration of the element of interest in the sample, following this equation:

I = k*C, where k is a constant determined by relating intensity to concentration for one or more reference standards. However this simple relationship rarely applies in practice. Several source of error inherent to XRF exists:

Dead Time

Error introduce by the non linearity of the X-ray detector. Once the detector has responded to a photon, a certain dead time must elapse until the detector can respond to another photon. This dead time is therefor important for high count rate and is calculated, corrected and measured in nanosecond.

Line overlap

Overlap factors have to be calculated from synthetic standard and corrections has to be applied. These factors are expressed in apparent concentration per unit concentration of the interfering element.

Absorption and enhancement effects

Secondary X-ray photons emitted by a sample must be able to escape from the sample before they can be dispersed and counted. Some photons however will absorbed by atoms in the sample itself and stimulate the emission of tertiary fluorescent-rays. The proportion of photons absorbed will depend on their wavelength and the composition of the sample.

Particle-size effects

The depth of penetration of the radiation will depend on the wavelength of the radiation and will be in the order of a few micrometers of hundreds of micrometer. Grinding technique produce heterogeneous particles with a mean size of a few tens of micrometers, therefore the powder will be heterogeneous on the scale of penetration of long-wavelength radiation. The sample has to be fused to make it completely homogeneous. A powdered sample is mixed with a flux and the mixture fused. Dilution of the sample with light mass flux raises X-ray background levels by increasing the scattering efficiency and reduces the net intensity of fluorescent radiation.

Measurements on Press Pellets

Item/oxide
Measuring range: ppm/Wt. %
Precision *: %

SiO2
0,2 - 90
0.3

TiO2
0,1 - 4
0.5

Al2O3
0,01 - 60
0.3

Fe2O3
0,01 - 30
0.3

MnO
0,002 - 1
2.1

MgO
0,01 - 50
0.6

CaO
0,04 - 55
0.3

Na2O
0,01 - 11
2.1

Na2O
0,01 - 16
0.4

P2O5
0,01 - 1.4
0.4

 

S
100 - 18000
5.5

V
5 - 6501
1.7

CR
6 - 25000
1.3

Ni
3 - 2500
1.1

Cu
5 - 7000
3.4

Zn
5 - 1500
0.7

Rb
3 - 4000
0.7

Sr
3 - 4600
0.5

Y
2 - 800
1.1

Zr
10 - 11100
0.5

Nb
2 - 1000
1.0

Ba
10 - 4000
0.5

Ce
5 - 2300
3.2

Pb
3 - 300
3.5

Th
2 - 1000
5.8

U
1 - 700
21.4

 

 

 

 

 

Measurements on Fused Disk

Item/oxide
Measuring range: ppm/Gew. %
Precision *:   %

SiO2
0,1 - 93

TiO2
0,1 - 4

Al2O3
0,1 - 60

Fe2O3
0,02 - 60

MnO
0,002 - 3

MgO
0,01 - 50

CaO
0,03 - 56

Na2O
0,01 - 11

Na2O
0,01 - 16

P2O5
0,01 - 1.5

 

V
5 - 100

Cr
10 - 25000

Ni
5 - 11500

Rb
3 - 4000

Sr
3 - 5200

Y
2 - 800

Zr
10 - 11100

Nb
2 - 1000

Ba
30 - 69000

La
10 - 1400

Ce
15 - 2300

Th
2 - 1000

 

 

 

 


Eagle II

Spektrometer-Konfigurationen/Spectrometer configurations

Spektrometer: Röntgenanalytik Meßtechnik Eagle II μ-XRF
Generator: max. Spannung 50 kV, max. Strom 40 W
Röhre: Seitliches Fenster 125
μm Be, Rh-Anode
Kapillare: Länge 160 mm, Punktgröße < 50
μm, Incident-Winkel 65°
Detektor: Empfindlicher Bereich 30 mm2, Resolution < 140 keV (for 30 mm2), Take off-Winkel 60°, Dewar 5 l, LN2-Verbrauch: approx. 1 l/d
Shaper: Zeitkonstanten 2,5, 6, 10, 17, 35, 50, 100
μs
Probentisch: max. Probengröße 250x200x120 mm3, max. Gewicht 5 kg.

-------------------------

Spectrometer: Röntgenanalytik Meßtechnik Eagle II μ-XRF analyser
Generator: max.
Voltage 50kV, max. Power 40 W
Tube: Side window 125 μm Be, Rh anode
Capillary: Length 160 mm, spot size < 50 μm, incident angle 65°
Detector: Sensitive area 30 mm2, Resolution < 140 keV (for 30 mm2), Take off angle 60°, Dewar 5 l, LN2-consumption: approx. 1 l/d
Shaper: time constants 2,5, 6, 10, 17, 35, 50, 100 μs
Stage: max. Sample size 250x200x120 mm3, max. Load 5 kg.


Analysen/Analyses

  • Energiedispersive (EDS) Analyse mit Eagle II: schnelle qualitative und quantitative Analytik
  • Analyse der Haupt- und Nebenelemente (EDS)
  • Nachweisgrenzen: bei WDS ca. 1 ppm; bei EDS ca. 0,5 Gew. %

------------------------------

  • Energy dispersive (EDS) analysis with Eagle II: fast qualitative and quantitative analysis technique.
  • Analysis of major elements (EDS)
  • Detection limits: WDS approx. 1 ppm; EDS approx. 0,5 weigth%.

Meßdauer/Measuring time

Für EDS-Analysen mit dem Eagle II können bis zu 100 Punktpositionen für Punkt-, Linien- oder Matrixanalysen vorab in einer „stage location“ Tabelle eingespeichert werden. So ist die Messung von bis zu 15 000 Messpunkten mit der Auto run-Option möglich.

-----------------------------

With Eagle II, up to 100 stage positions for point, line or matrix analysis can be saved in a stage location table. In this way, the analysis of up to 15 000 measurement points by auto run option is possible.


Messzeit/Measurement appointments

Hinsichtlich Fragen zu Messzeit und deren Kosten wenden Sie sich bitte an die untenstehenden Kontaktpersonen.

------------------------------

For questions or measurement appointments please contact the below persons.


Selbständiges Messen/Analysis without assistance

Voraussetzung für selbständiges Messen ist eine Einweisung in die Bedienung des Gerätes. Ohne Einweisung muss für die Messungen ein begleitender HiWi (EUR 50/Tag) angestellt werden.

-----------------------------

If you prefer to carry out the measurements on our devices by your own, a referring introduction through our staff is crucial. If you do not like to have the introduction to the device, we need to give you technical assistance by hand during the analysis. This will cause additional charges.


Service-Analysen/Analysis with assistance

Service-Analysen inkl. Präparation und Auswertung werden nach Absprache ebenfalls von uns durchgeführt.

-----------------------------

If you prefer to have the analysis done by our staff, we will be happy to help you.


Kontakt/Contact

PD Dr. Sabine Klein
Telefon: ++49(0)69-798-40135
eMail klein@kristall.uni-frankfurt.de

Prof. Gerhard Brey
Telefon: ++49(0)69-798-40123
eMail brey@em.uni-frankfurt.de

Fax: ++49(0)69-798-40121


 

 

geändert am 09. April 2009  E-Mail: Webmasterkautz@kristall.uni-frankfurt.de

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Druckversion: 09. April 2009, 10:30
http://www.uni-frankfurt.de/fb/fb11/ifg/mineralogie/petrologie-geochemie/xrf/index.html