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1、X 射 线 荧 光 分 析 导 论,电子波谱,1Hz - 1kHz,1kHz - 1014Hz,1014Hz - 1015Hz,1015Hz - 1021Hz,超低频率 电磁波,无线电波,微波,红外线 可见光,伽马射线,紫外线,Low energy,High energy,X射线,Theory,入射X射线轰击原子的内层电子，如果能量大于它的吸收边，该内层电子被驱逐出整个原子（整个原子处于高能态，即激发态）。 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。 由高能态转化为低能态，释放能量。 E=Eh-El . 能量将以X射线的释放，产生X射线荧光。,The Hardware,Sour
2、ces Optics Filters 10,000 300,000 cpsResolution: 140-180 eV at Mn K-alpha,Proportional Counter,Anode Filament,Fill Gases: Neon, Argon, Xenon, Krypton Pressure: 0.5- 2 ATM Windows: Be or Polymer Sealed or Gas Flow Versions Count Rates EDX: 10,000-40,000 cps WDX: 1,000,000+ Resolution: 500-1000+ eV,Wi
3、ndow,Scintillation Detector,PMT (Photo-multiplier tube),Sodium Iodide Disk,Electronics,Connector,Window: Be or Al Count Rates: 10,000 to 1,000,000+ cps Resolution: 1000 eV,Spectral Comparison - Au,Si(Li) Detector 10 vs. 14 Karat,Si PIN Diode Detector 10 vs. 14 Karat,Polymer Detector Windows,Optional
4、 thin polymer windows compared to a standard beryllium windows Affords 10 x improvement in the MDL for sodium (Na),Detector Filters,Filters are positioned between the sample and detector in some EDXRF and NDXRF systems to filter out unwanted x-ray peaks.,Sample,Detector,X-Ray Source,Detector Filter,
5、Detector Filter Transmission,% T R A N S M I T T E D,ENERGY,Low energy x-rays are absorbed,EOI is transmitted,Absorption Edge,X-rays above the absorption edge energy are absorbed,Very high energy x-rays are transmitted,S Cl,A niobium filter absorbs Cl and other higher energy source x-rays while lett
6、ing S x-rays pass. A detector filter can significantly improve detection limits.,Niobium Filter Transmission and Absorption,Filter Vs. No Filter,Unfiltered Tube target, Cl, and Ar Interference Peak,Detector filters can dramatically improve the element of interest intensity, while decreasing the back
7、ground, but requires 4-10 times more source flux. They are best used with large area detectors that normally do not require much power.,Ross Vs. Hull Filters,The previous slide was an example of the Hull or simple filter method. The Ross method illustrated here for Cl analysis uses intensities throu
8、gh two filters, one transmitting, one absorbing, and the difference is correlated to concentration. This is an NDXRF method since detector resolution is not important.,Wavelength Dispersive XRF,Wavelength Dispersive XRF relies on a diffractive device such as crystal or multilayer to isolate a peak,
9、since the diffracted wavelength is much more intense than other wavelengths that scatter of the device.,Sample,Detector,X-Ray Source,Diffraction Device,Collimators,Diffraction,The two most common diffraction devices used in WDX instruments are the crystal and multilayer. Both work according to the f
10、ollowing formula.,nl = 2d sinq,n = integer d = crystal lattice or multilayer spacing q = The incident angle = wavelength,Atoms,Multilayers,While 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 t
11、o do the same thing.,Modern multilayers are more efficient than crystals and can be optimized for specific elements. Often used for low Z elements.,Soller Collimators,Soller and similar types of collimators are used to prevent beam divergence. The are used in WDXRF to restrict the angles that are al
12、lowed to strike the diffraction device, thus improving the effective resolution.,Sample,Crystal,Cooling and Temperature Control,The 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
13、 than direct excitation EDXRF systems. Diffraction devices are also temperature sensitive.,Many WDXRF Instruments use: X-Ray Tube Coolers, and Thermostatically controlled instrument coolers,Chamber Atmosphere,Sample and hardware chambers of any XRF instrument may be filled with air, but because air
14、absorbs low energy x-rays from elements particularly 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 pellets Helium - For use with liquids or powdered materials,Changers and Spinner
15、s,Other commonly available 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+ samples Sample Spinners are used to average out surface features and particle size affects possibly over a large
16、r total surface area.,Typical PIN Detector Instrument,This configuration is most commonly used in higher end benchtop EDXRF Instruments.,Typical Si(Li) Detector Instrument,This has been historically the most common laboratory grade EDXRF configuration.,Energy Dispersive Electronics,Fluorescence gene
17、rates 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.,DETECTOR,Signal to Electronics,Element A,Element C,Element B,Element D,Multi-Channel Analyser,Detector current pulses ar
18、e translated into counts (counts per second, “CPS”). Pulses are segregated into channels according to energy via the MCA (Multi-Channel Analyser).,Signal from Detector,Channels, Energy,Intensity (# of CPS per Channel),WDXRF Pulse Processing,The WDX method uses the diffraction device and collimators
19、to obtain good resolution, so The detector does not need to be capable of energy discrimination. This simplifies the pulse processing. It also means that spectral processing is simplified since intensity subtraction is fundamentally an exercise in background subtraction. Note: Some energy discrimina
20、tion is useful since it allows for rejection of low energy noise and pulses from unwanted higher energy x-rays.,Evaluating Spectra,K & L Spectral Peaks Rayleigh Scatter Peaks Compton Scatter Peaks Escape Peaks Sum Peaks Bremstrahlung,In addition to elemental peaks, other peaks appear in the spectra:
21、,K & L Spectral Lines,K - alpha lines: L shell e- transition to fill vacancy in K shell. Most frequent transition, hence most intense peak.,K - beta lines: M shell e- transitions to fill vacancy in K shell.,L Shell,K Shell,L - alpha lines: M shell e- transition to fill vacancy in L shell.,L - beta l
22、ines: N shell e- transition to fill vacancy in L shell.,K alpha,K beta,M Shell,L alpha,N Shell,L beta,K & L Spectral Peaks,Rh X-ray Tube,L-lines,K-Lines,Scatter,Some of the source X-rays strike the sample and are scattered back at the detector. Sometimes called “backscatter”,Sample,Source,Detector,R
23、ayleigh Scatter,X-rays from the X-ray tube or target strike atom without promoting fluorescence. Energy is not lost in collision. (EI = EO) They appear as a source peak in spectra. AKA - “Elastic” Scatter,EI,EO,Rh X-ray Tube,Compton Scatter,X-rays from the X-ray tube or target strike atom without pr
24、omoting fluorescence. Energy is lost in collision. (EI EO) Compton scatter appears as a source peak in spectra, slightly less in energy than Rayleigh Scatter. AKA - “Inelastic” Scatter,EI,EO,Rh X-ray Tube,Sum Peaks,2 photons strike the detector at the same time. The fluorescence is captured by the d
25、etector, recognized as 1 photon twice its normal energy. A peak appears in spectra, at: 2 X (Element keV).,Escape Peaks,X-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
26、 that appears in spectrum, at: Element keV - Si keV (1.74 keV).,Rh X-ray Tube,1.74 keV,Brehmstrahlung,Brehmstrahlung (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 tube.,Interferences
27、,Spectral Interferences Environmental Interferences Matrix Interferences,Spectral Interferences,Spectral interferences are peaks in the spectrum that overlap the spectral peak (region of interest) of the element to be analyzed. Examples: K & L line Overlap - S & Mo, Cl & Rh, As & Pb Adjacent Element
28、 Overlap - Al & Si, S & Cl, K & Ca. Resolution of detector determines extent of overlap.,220 eV Resolution,140 eV Resolution,Adjacent Element Overlap,Environmental Interferences,Light elements (Na - Cl) emit weak X-rays, easily attenuated by air. Solution: Purge instrument with He (less dense than a
29、ir = 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 (Ar) is a component of air.,Air Environment,He Environment,Al Analyzed with Si Target,Matrix Interferences,Absorption: Any eleme
30、nt can absorb or scatter the fluorescence of the element of interest. Enhancement: Characteristic x-rays of one element excite another element in the sample, enhancing its signal.,Influence Coefficients, sometimes called alpha corrections are used to mathematically correct for Matrix Interferences,A
31、bsorption/Enhancement Effects,Absorption-Enhancement Affects,Incoming source X-ray fluoresces Fe. Fe fluorescence is sufficient in energy to fluoresce Ca. Ca is detected, Fe is not. Response is proportional to concentrations of each element.,Red = Fe, absorbed Blue = Ca, enhanced,Source X-ray,X-Ray
32、Captured by the detector.,Sample,Software,Qualitative Analysis Semi-Quantitative Analysis (SLFP, NBS-GSC.) Quantitative Analysis (Multiple intensity Extraction and Regression methods),Qualitative Scan Peak ID,This spectrum also contrasts the resolution of a PIN diode detector with a proportional cou
33、nter to illustrate the importance of detector resolution with regard to qualitative analysis.,Automated Peak identification programs are a useful qualitative examination tool,Element Tags,Semi-Quantitative Analysis,The algorithm computes both the intensity to concentration relationship and the absor
34、ption affects Results are typically within 10 - 20 % of actual values.,SLFP Standardless Fundamental Parameters,FP (with Standards) NBS-GSC, NRLXRF, Uni-Quant, TurboQuant, etc,The concentration to intensity relationship is determined with standards, while the FP handles the absorption affects. Resul
35、ts are usually within 5 - 10 % of actual values,Quantitative Analysis,Concentration,Intensity,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
36、 compare the spectral intensities of unknown samples to those of known standards.,Standards,Standards (such as certified reference materials) are required for Quantitative Analysis. Standard concentrations should be known to a better degree of precision and accuracy than is required for the analysis
37、. Standards should be of the same matrix as samples to be analyzed. Number of standards required for a purely empirical method, N=(E+1)2, N=# of standards, E=# of Elements. Standards should vary independently in concentration when empirical absorption corrections are used.,Sample Preparation,Powders
38、: 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
39、, 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.,