培训X射线荧光分析导论

1、Introduction to XRFLearnXRF.com.Introduction to XRFLearnXRF.com.电子波谱1Hz - 1kHz 1kHz - 1014Hz 1014Hz - 1015Hz 1015Hz - 1021Hz 超低频率超低频率电磁波电磁波无线电波无线电波 微波微波红外线红外线可见光可见光 伽马射线伽马射线 紫外线紫外线Low energyHigh energyX射线射线 Introduction to XRFLearnXRF.com.Theory入射入射X射线轰击原子的内层电子，如射线轰击原子的内层电子，如果能量大于它的吸收边，该内层电子果能量大于它的吸

2、收边，该内层电子被驱逐出整个原子（整个原子处于高被驱逐出整个原子（整个原子处于高能态，即激发态）。能态，即激发态）。较高能级的电子跃迁、补充空穴，整较高能级的电子跃迁、补充空穴，整个原子处于低能态，即基态。个原子处于低能态，即基态。由高能态转化为低能态，释放能量。由高能态转化为低能态，释放能量。 E=Eh-El. 能量将以能量将以X射线的释放，产生射线的释放，产生X射线荧射线荧光光。Introduction to XRFLearnXRF.com.The HardwareIntroduction to XRFLearnXRF.com.SourcesEnd Window X-Ray TubesSi

3、de Window X-Ray TubesRadioisotopesOther SourcesScanning Electron MicroscopesSynchrotronsPositron and other particle beams Introduction to XRFLearnXRF.com.End Window X-Ray Tube X-ray Tubes Voltage determines which elements can be excited. More power = lower detection limits Anode selection determines

4、 optimal source excitation (application specific).Introduction to XRFLearnXRF.com.Side Window X-Ray TubeBe WindowSilicone InsulationGlass EnvelopeFilamentElectron beamTarget (Ti, Ag,Rh, etc.)Copper AnodeHV LeadIntroduction to XRFLearnXRF.com.RadioisotopesIsotopeFe-55Cm-244Cd-109Am-241Co-57Energy (ke

5、V)5.914.3, 18.322, 8859.5122Elements (K-lines)Al V Ti-BrFe-MoRu-ErBa - UElements (L-lines)Br-II- PbYb-PuNonenone While isotopes have fallen out of favor they are still useful for many gauging applications.Introduction to XRFLearnXRF.com.Other SourcesSeveral other radiation sources are capable of exc

6、iting material to produce x-ray fluorescence suitable for material analysis.Scanning Electron Microscopes (SEM) Electron beams excite the sample and produce x-rays. Many SEMs are equipped with an EDX detector for performing elemental analysisSynchotrons - These bright light sources are suitable for

7、research and very sophisticated XRF analysis. Positrons and other Particle Beams All high energy particles beams ionize materials such that they give off x-rays. PIXE is the most common particle beam technique after SEM. Introduction to XRFLearnXRF.com.Source ModifiersSeveral Devices are used to mod

8、ify the shape or intensity of the source spectrum or the beam shape Source Filters Secondary Targets Polarizing Targets Collimators Focusing OpticsIntroduction to XRFLearnXRF.com.Source FiltersFilters perform one of two functionsBackground ReductionImproved FluorescenceIntroduction to XRFLearnXRF.co

9、m.Filter Transmission Curve%TRANSMITTEDENERGYLow energy x-rays are absorbedAbsorptionEdgeX-rays above the absorption edge energy are absorbedVery high energy x-rays are transmitted Ti CrTitanium Filter transmission curve The transmission curve shows the parts of the source spectrum are transmitted a

10、nd those that are absorbedIntroduction to XRFLearnXRF.com.Filter Fluorescence MethodENERGY (keV)Target peakWith Zn Source filterFeRegionContinuum RadiationThe filter fluorescence method decreases the background and improves the fluorescence yield without requiring huge amounts of extra power.Introdu

11、ction to XRFLearnXRF.com.Filter Absorption MethodENERGY (keV)Target peakWith Ti Source filterFeRegionContinuum RadiationThe filter absorption Method decreases the background while maintaining similar excitation efficiency.Introduction to XRFLearnXRF.com.Secondary TargetsImproved Fluorescence and low

12、er background The characteristic fluorescence of the custom line source is used to excite the sample, with the lowest possible background intensity. It requires almost 100 x the flux of filter methods but gives superior results.Introduction to XRFLearnXRF.com.Secondary TargetsSampleX-Ray TubeDetecto

13、rSecondary TargetThe x-ray tube excites the secondary targetThe Secondary target fluoresces and excites the sampleThe detector detects x-rays from the sampleIntroduction to XRFLearnXRF.com.Secondary Target MethodENERGY (keV)Tube Target peakWith Zn Secondary TargetFeRegionContinuum RadiationSecondary

14、 Targets produce a more monochromatic source peak with lower background than with filtersIntroduction to XRFLearnXRF.com.Secondary Target Vs FilterComparison of optimized direct-filtered excitation with secondary target excitation for minor elements in Ni-200Introduction to XRFLearnXRF.com.Polarizin

15、g Target TheoryX-ray are partially polarized whenever they scatter off a surfaceIf the sample and polarizer are oriented perpendicular to each other and the x-ray tube is not perpendicular to the target, x-rays from the tube will not reach the detector.There are three type of Polarization Targets:Ba

16、rkla Scattering Targets - They scatter all source energies to reduce background at the detector.Secondary Targets - They fluoresce while scattering the source x-rays and perform similarly to other secondary targets.Diffractive Targets - They are designed to scatter specific energies more efficiently

17、 in order to produce a stronger peak at that energy. Introduction to XRFLearnXRF.com.CollimatorsCollimators are usually circular or a slit and restrict the size or shape of the source beam for exciting small areas in either EDXRF or uXRF instruments. They may rely on internal Bragg reflection for im

18、proved efficiency. SampleTubeCollimator sizes range from 12 microns to several mmIntroduction to XRFLearnXRF.com.Focusing OpticsBecause simple collimation blocks unwanted x-rays it is a highly inefficient method. Focusing optics like polycapillary devices and other Kumakhov lens devices were develop

19、ed so that the beam could be redirected and focused on a small spot. Less than 75 um spot sizes are regularly achieved. SourceDetectorBragg reflection inside a CapillaryIntroduction to XRFLearnXRF.com.Detectors Si(Li) PIN Diode Silicon Drift Detectors Proportional Counters Scintillation DetectorsInt

20、roduction to XRFLearnXRF.com.Detector Principles2Enen= num ber of electron-hole pairs producedE= X-ray photon energye= 3.8ev for Si at LN tem perw here :atures A detector is composed of a non-conducting or semi-conducting material between two charged electrodes.X-ray radiation ionizes the detector m

21、aterial causing it to become conductive, momentarily.The newly freed electrons are accelerated toward the detector anode to produce an output pulse. In ionized semiconductor produces electron-hole pairs, the number of pairs produced is proportional to the X-ray photon energyIntroduction to XRFLearnX

22、RF.com.Si(Li) DetectorWindowSi(Li) crystalDewarfilled withLN2Super-Cooled CryostatCooling: LN2 or Peltier Window: Beryllium or PolymerCounts Rates: 3,000 50,000 cps Resolution: 120-170 eV at Mn K-alphaFETPre-AmplifierIntroduction to XRFLearnXRF.com.Si(Li) Cross Section Introduction to XRFLearnXRF.co

23、m.PIN Diode DetectorCooling: Thermoelectrically cooled (Peltier)Window: BerylliumCount Rates: 3,000 20,000 cpsResolution: 170-240 eV at Mn k-alphaIntroduction to XRFLearnXRF.com.Silicon Drift Detector- SDDPackaging: Similar to PIN DetectorCooling: PeltierCount Rates; 10,000 300,000 cpsResolution: 14

24、0-180 eV at Mn K-alphaIntroduction to XRFLearnXRF.com.Proportional CounterAnode FilamentFill Gases: Neon, Argon, Xenon, KryptonPressure: 0.5- 2 ATMWindows: Be or PolymerSealed or Gas Flow VersionsCount Rates EDX: 10,000-40,000 cps WDX: 1,000,000+Resolution: 500-1000+ eVWindowIntroduction to XRFLearn

25、XRF.com.Scintillation DetectorPMT (Photo-multiplier tube)Sodium Iodide DiskElectronicsConnectorWindow: Be or AlCount Rates: 10,000 to 1,000,000+ cpsResolution: 1000 eVIntroduction to XRFLearnXRF.com.Spectral Comparison - AuSi(Li) Detector10 vs. 14 KaratSi PIN Diode Detector10 vs. 14 KaratIntroductio

26、n to XRFLearnXRF.com. Optional thin polymer windows compared to a standard beryllium windows Affords 10 x improvement in the MDL for sodium (Na)Introduction to XRFLearnXRF.com.Detector FiltersFilters are positioned between the sample and detector in some EDXRF and NDXRF systems to filter out unwante

27、d x-ray peaks.Introduction to XRFLearnXRF.com.Detector Filter Transmission%TRANSMITTEDENERGYLow energy x-rays are absorbedEOI is transmittedAbsorptionEdgeX-rays above the absorption edge energy are absorbedVery high energy x-rays are transmitted S ClA niobium filter absorbs Cl and other higher energ

28、y source x-rays while letting S x-rays pass. A detector filter can significantly improve detection limits. Niobium Filter Transmission and AbsorptionIntroduction to XRFLearnXRF.com.Filter Vs. No FilterUnfiltered Tube target, Cl, and Ar Interference PeakDetector filters can dramatically improve the e

29、lement of interest intensity, while decreasing the background, but requires 4-10 times more source flux. They are best used with large area detectors that normally do not require much power.Introduction to XRFLearnXRF.com.Ross Vs. Hull FiltersqThe previous slide was an example of the Hull or simple

30、filter method. qThe Ross method illustrated here for Cl analysis uses intensities through two filters, one transmitting, one absorbing, and the difference is correlated to concentration. This is an NDXRF method since detector resolution is not important.Introduction to XRFLearnXRF.com.Wavelength Dis

31、persive XRFWavelength Dispersive XRF relies on a diffractive device such as crystal or multilayer to isolate a peak, since the diffracted wavelength is much more intense than other wavelengths that scatter of the device. DetectorX-Ray SourceDiffraction DeviceCollimatorsIntroduction to XRFLearnXRF.co

32、m.DiffractionThe two most common diffraction devices used in WDX instruments are the crystal and multilayer. Both work according to the following formula.nl l = 2d sinq qn = integerd = crystal lattice or multilayer spacingq q = The incident anglel wavelengthAtomsIntroduction to XRFLearnXRF.com.Multi

33、layersWhile the crystal spacing is based on the natural atomic spacing at a given orientation the multilayer uses a series of thin film layers of dissimilar elements to do the same thing. Modern multilayers are more efficient than crystals and can be optimized for specific elements. Often used for l

34、ow Z elements.Introduction to XRFLearnXRF.com.Soller CollimatorsSoller and similar types of collimators are used to prevent beam divergence. The are used in WDXRF to restrict the angles that are allowed to strike the diffraction device, thus improving the effective resolution.SampleCrystalIntroducti

35、on to XRFLearnXRF.com.Cooling and Temperature ControlThe diffraction technique is relatively inefficient and WDX detectors can operate at much higher count rates, so WDX Instruments are typically operated at much higher power than direct excitation EDXRF systems. Diffraction devices are also tempera

36、ture sensitive. Many WDXRF Instruments use:X-Ray Tube Coolers, and Thermostatically controlled instrument coolersIntroduction to XRFLearnXRF.com.Chamber AtmosphereSample and hardware chambers of any XRF instrument may be filled with air, but because air absorbs low energy x-rays from elements partic

37、ularly below Ca, Z=20, and Argon sometimes interferes with measurements purges are often used. The two most common purge methods are:Vacuum - For use with solids or pressed pelletsHelium - For use with liquids or powdered materialsIntroduction to XRFLearnXRF.com.Changers and SpinnersOther commonly a

38、vailable sample handling features are sample changers or spinners.Automatic sample changers are usually of the circular or XYZ stage variety and may have hold 6 to 100+ samplesSample Spinners are used to average out surface features and particle size affects possibly over a larger total surface area

39、. Introduction to XRFLearnXRF.com.This configuration is most commonly used in higher end benchtop EDXRF Instruments.Introduction to XRFLearnXRF.com.This has been historically the most common laboratory grade EDXRF configuration.Introduction to XRFLearnXRF.com.Energy Dispersive ElectronicsFluorescenc

40、e generates a current in the detector. In a detector intended for energy dispersive XRF, the height of the pulse produced is proportional to the energy of the respective incoming X-ray.DETECTORSignal to ElectronicsElement AElement CElement BElement DIntroduction to XRFLearnXRF.com.Multi-Channel Anal

41、yserDetector current pulses are translated into counts (counts per second, “CPS”).Pulses are segregated into channels according to energy via the MCA (Multi-Channel Analyser).Signal from DetectorChannels, EnergyIntensity(# of CPS per Channel)Introduction to XRFLearnXRF.com.WDXRF Pulse ProcessingvThe

42、 WDX method uses the diffraction device and collimators to obtain good resolution, so The detector does not need to be capable of energy discrimination. This simplifies the pulse processing.vIt also means that spectral processing is simplified since intensity subtraction is fundamentally an exercise

43、 in background subtraction.Note: Some energy discrimination is useful since it allows for rejection of low energy noise and pulses from unwanted higher energy x-rays. Introduction to XRFLearnXRF.com.Evaluating Spectra K & L Spectral Peaks Rayleigh Scatter Peaks Compton Scatter Peaks Escape Peaks

44、 Sum Peaks BremstrahlungIn addition to elemental peaks, other peaks appear in the spectra:Introduction to XRFLearnXRF.com.K & L Spectral Linesv K - alpha lines: L shell e- transition to fill vacancy in K shell. Most frequent transition, hence most intense peak.v K - beta lines: M shell e- transi

45、tions to fill vacancy in K shell.L ShellK Shellv L - alpha lines: M shell e- transition to fill vacancy in L shell.v L - beta lines: N shell e- transition to fill vacancy in L shell.K alphaK betaM ShellL alphaN ShellL betaIntroduction to XRFLearnXRF.com.K & L Spectral PeaksRh X-ray TubeL-linesK-

46、LinesIntroduction to XRFLearnXRF.com.ScatterSome of the source X-rays strike the sample and are scattered back at the detector. Sometimes called “backscatter”SampleSourceDetectorIntroduction to XRFLearnXRF.com.Rayleigh ScatterX-rays from the X-ray tube or target strike atom without promoting fluores

47、cence.Energy is not lost in collision. (EI = EO) They appear as a source peak in spectra.AKA - “Elastic” ScatterEIEORh X-ray TubeIntroduction to XRFLearnXRF.com.Compton ScatterX-rays from the X-ray tube or target strike atom without promoting fluorescence.Energy is lost in collision. (EI EO)Compton

48、scatter appears as a source peak in spectra, slightly less in energy than Rayleigh Scatter.AKA - “Inelastic” ScatterEIEORh X-ray TubeIntroduction to XRFLearnXRF.com.Sum Peaks 2 photons strike the detector at the same time. The fluorescence is captured by the detector, recognized as 1 photon twice it

49、s normal energy. A peak appears in spectra, at: 2 X (Element keV).Introduction to XRFLearnXRF.com.Escape PeaksX-rays strike the sample and promote elemental fluorescence. Some Si fluorescence at the surface of the detector escapes, and is not collected by the detector.The result is a peak that appea

50、rs in spectrum, at: Element keV - Si keV (1.74 keV).Rh X-ray Tube1.74 keVIntroduction to XRFLearnXRF.com.BrehmstrahlungBrehmstrahlung (or Continuum) Radiation: German for “breaking radiation”, noise that appears in the spectra due to deceleration of electrons as they strike the anode of the X-ray tu

51、be.Introduction to XRFLearnXRF.com.InterferencesvSpectral InterferencesvEnvironmental InterferencesvMatrix InterferencesIntroduction to XRFLearnXRF.com.Spectral InterferencesSpectral interferences are peaks in the spectrum that overlap the spectral peak (region of interest) of the element to be anal

52、yzed.Examples: K & L line Overlap - S & Mo, Cl & Rh, As & PbAdjacent Element Overlap - Al & Si, S & Cl, K & Ca.Resolution of detector determines extent of overlap.220 eV Resolution140 eV ResolutionAdjacent Element OverlapIntroduction to XRFLearnXRF.com.Environmental Inter

53、ferencesLight elements (Na - Cl) emit weak X-rays, easily attenuated by air.Solution:Purge instrument with He (less dense than air = less attenuation).Evacuate air from analysis chamber via a vacuum pump.Either of these solutions also eliminate interference from Ar (spectral overlap to Cl). Argon (A

54、r) is a component of air.Air EnvironmentHe EnvironmentAl Analyzed with Si TargetIntroduction to XRFLearnXRF.com.Matrix Interferences Absorption: Any element can absorb or scatter the fluorescence of the element of interest. Enhancement: Characteristic x-rays of one element excite another element in

55、the sample, enhancing its signal.Influence Coefficients, sometimes called alpha corrections are used to mathematically correct for Matrix InterferencesAbsorption/Enhancement EffectsIntroduction to XRFLearnXRF.com.Absorption-Enhancement AffectsIncoming source X-ray fluoresces Fe.Fe fluorescence is su

56、fficient in energy to fluoresce Ca.Ca is detected, Fe is not. Response is proportional to concentrations of each element.Red = Fe, absorbedBlue = Ca, enhancedSource X-rayX-Ray Captured by the detector.SampleIntroduction to XRFLearnXRF.com.Software Qualitative Analysis Semi-Quantitative Analysis (SLF

57、P, NBS-GSC.) Quantitative Analysis (Multiple intensity Extraction and Regression methods)Introduction to XRFLearnXRF.com.Qualitative Scan Peak ID This spectrum also contrasts the resolution of a PIN diode detector with a proportional counter to illustrate the importance of detector resolution with r

58、egard to qualitative analysis. Automated Peak identification programs are a useful qualitative examination tool Element TagsIntroduction to XRFLearnXRF.com.Semi-Quantitative AnalysisThe algorithm computes both the intensity to concentration relationship and the absorption affectsResults are typically within 10 - 20 % of actual values.SLFP Standardless Fundamental ParametersFP (with Stand