Summer Students lectures

Transcription

1 Summer Students lectures XRF: X-ray fluorescence spectrometry Matthias Alfeld XRF: X-ray fluorescence spectrometry Hamburg,

2 > What is XRF? X-Ray Fluorescence spectrometry > What can it do? Detect elements heavier than Na (Z=11) with limits of detection of a few parts per million (ppm) and less. Determine the distribution of elements in a sample. Quantify the elemental composition of a sample. Operate under ambient conditions. It is (largely) non-destructive. > What can it not do? Directly investigate the chemical state of an element. Determine isotopic ratios Investigate organic compounds (Exception: Metals in proteins). Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 2

3 > XRF spectrometer Source - Radioactive sources - X-ray tubes - Synchrotron radiation sources Detector - Energy dispersive detectors - Wavelength dispersive detectors Sample - Biological samples - Geological samples - Cultural heritage samples - Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 3

4 > Like any other forms of spectrometric imaging or spectrometry several characteristics are desirable for XRF: High sensitivity Low limits of detection Long linear range (or methods to achieve it in data processing) High lateral resolution Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 4

5 > Lateral resolution Is dependent on the size of the beam employed for scanning and the step size. The size and intensity of a beam are dependent on the optics used (see lecture: Optics for Synchrotron Radiation Experiments by H. Schulte-Schrepping) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 5

6 > Like any other forms of spectroscopic imaging or spectroscopy several characteristics are desirable for XRF. High sensitivity Low limits of detection Long linear range (or methods to achieve it in data processing) High lateral resolution m i N t k m i mass of element i in the beam [g] N Number of photons detected for element i I Intensity of fluorescence of element i [photons/s] ( N ) N N I t m i k t I m i 4 p ( E ij ) E E max edge ij I 0 ( E) ( E) ij A ( E) de Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 6

7 Primary radiation Vacancy K L M N Photo electron Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 7

8 α K L M N Fluorescence radiation Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 8

9 Auger electron K L M N Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 9

10 Auger electron K L M N Fluorescen ce Yield Number of emitted photons Number of vacancies Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 10

11 Fe 100% Fe I i m i I 0 k I i Intensity of fluorescence of element i [photons/s] m i mass of element i in the beam [g] I i Intensity of primary beam [photons/s] ω fluorescence yield k constant m i d w i B sin d sample thickness [cm] ρ sample density [g/cm 3 ] w i weight fraction of element i in sample B area excited by beam α excitation angle 60% Fe 40% Ni Fe Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 11 Fe:Ni = 60:40 Ni

12 L-Lines β α α β K-Lines Fe Ni K L M N I m i I 0 p k i element j shell k transition p transition probability ω fluorescence yield ij Fe Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 12 Ni

13 > Moseley s Law E a jk ( i bjk 2 ) Z Element Energie 21 Sc-Kα 4.1 kev 41 Nb-Kα 16.6 kev 61 Pm-Kα 38.5 kev 81 Tl-Kα 72.2 kev Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 13

14 > Moseley s Law E a jk ( i bjk 2 ) Z Element Energie 21 Sc-Kα 4.1 kev 41 Nb-Kα 16.6 kev 61 Pm-Kα 38.5 kev 81 Tl-Kα 72.2 kev Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 14

15 > Photoelectric cross section L-edges Pb M-edges K-edge Pb L-edges Pb N-edges I m I p ( E) k i 0 ij ij τ photoelectric cross-section [cm 2 /g] Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 15

16 Pb-L lines Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 16

17 Pb-L lines Pb-Lα Pb-Lβ Pb-Lγ Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 17

18 > There are two notations for XRF lines: > IUPAC notation: Describes transitions Is systematic Complex > Siegbahn notation: Describes observed peaks, which are combinations of several transitions Is less systematic Easy to use IUPAC: Fe-KL 2 + Fe-KL 3 is identical to Siegbahn: Fe-K a Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 18

19 Energy dispersive (ED-) XRF Wavelength dispersive (WD-) XRF source semiconductor detector source diffracting crystal sample sample detector Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 19

20 ED-XRF WD-XRF Energy resolution (Mn-K a =5.9 kev) ~150 ev ~40 ev Acquisition Simultaneous Sequential Typical maximum count rate Speed ~0.5 Mcps (counts per second) Seconds or fractions of seconds Price Moderate High Solid angle Ω large small Several Mcps Seconds to Minutes Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 20

21 ED-XRF WD-XRF Rigaku Energy resolution (Mn-K a =5.9 kev) ~150 ev ~40 ev Acquisition Simultaneous Sequential I Maximum count rate Speed ~0.5 Mcps (counts per second) Seconds or fractions of seconds Price Moderate High mi I0 p ( E ) k 4 Solid angle Ω ij ij large small E Energy of fluorescence line ε Quantum efficiency of the detector Ω Solid angle covered by the detector Several Mcps Seconds to Minutes Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 21

22 > Quantum efficiency Itrans( E) T( E) exp( ( E) d) I ( E) 0 T Transmission I trans intensity of transmitted radiation I 0 intensity of primary radiation μ mass absorption coefficient [cm 2 /g] ρ density of absorber [g/cm 3 ] d thickness of absorber [cm] sample absorbers detector window detector crystal ( E) T ( E) T ( E) (1 Tdet ( E)) window absorber ector T window Transmission of detector window T detector Transmission of detector crystal T absorber Transmission of any additional absorber (e.g. air) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 22

23 > Solid angle I I recorded emitted A 4r detector 2 A detector active area of detector r distance of sample to detector The solid angle Ω is expressed in steradians. (1 steradians = 1 r 2 ) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 23

24 > Like any other form of imaging or spectroscopic several characteristics are desirable for XRF. High sensitivity Low limits of detection Long linear range (or methods to achieve it in data processing) High lateral resolution I mi I0 p ij ij ( E ) k 4 less variable experimental parameter experimental parameters fundamental parameters Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 24

25 > More photons are not always better. Dead time X busy/dead detector N I Number of photons reaching the detector N o Number of photons detected Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 25

26 > More photons are not always better. Dead time Pile-up peaks Pb-L/Hg-L Ca Fe Mo Pd X Cd X Sn X + = Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 26

27 > Other artifacts: Escape peaks Fe-K a - Si-K a = 6.40 kev kev = 4.66 kev Ca Fe Ar K 4.66 kev? Fe-K a escape Mn Ti Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 27

28 > Self-absorption source Itrans( E) T( E) exp( ( E) d) I ( E) 0 Integrated over d: A ( E0, E ) ( E0 1 exp( d ( E d ( E, E 1 ) ( E sin( ) 0 0, E ) ) )) 1 sin( ) detector α sample β d Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 28

29 > Small ρd: (infinite thin) 1exp( d ( E, E )) d ( E0, E > Large ρd: (infinite thick) > Intermediate ρd: I ~ m i 0 1exp( d ( E0, E )) 1 I ~ wi / ( E0, E ) Easy quantification. ) With: m If Χ is known: Easy quantification. i d w i B sin A wi ( E, E 0 ) (1 exp( d ( E 0, E ))) Quantification is challenging. Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 29

30 > Like any other forms of spectroscopic imaging or spectroscopy several characteristics are desirable for XRF. High sensitivity Low limits of detection Long linear range can be achieved in data processing High lateral resolution I m i 4 p ( E ij ) E E max edge ij I 0 ( E) ( E) ij A ( E) de Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 30

31 > Secondary Fluorescence Is difficult to model by the fundamental parameter approach. (see R.M. Rousseau, J.A. Boivin, The fundamental algorithm: a natural extension of the Sherman equation part I: theory, The Rigaku Journal, 15 (1998) ) A different approach is Monte Carlo simulation in that the interaction of a huge number of photons with the sample is simulated (see T. Schoonjans, V.A. Sole, L. Vincze, M. S. del Rio, K. Appel, C. Ferrero, "A general Monte Carlo simulation of energy-dispersive X-ray fluorescence spectrometers - Part 6. Quantification through iterative simulations, Spectrochimica Acta B, 82 (2013) ) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 31

32 > Limits of detection Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 32

33 > Limits of detection Are determined by the Signal to Background ratio. A peak is considered detectable if its intensity is 3 times that of the standard deviation of the background. A higher sensitivity improves the limits of detection. A better energy resolution improves the limits of detection. A lower background intensity improves the limits of detection. c LOD Nback, i 3 ci ( t ) N signal Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 33

34 > The background results from: Scattered primary radiation Incomplete charge collection in the detector Fluorescence from the sample matrix Blind contributions from the instrument > The background can be corrected for in data processing. Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 34

35 > Moseley s Law E a jk ( i bjk 2 ) Z Element Energie 82 Pb-Kα 75.0 kev 82 Pb-Lα 10.5 kev 82 Pb-Lβ 12.6 kev 33 As-Kα 10.5 kev 36 Kr-Kα 12.6 kev Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 35

36 > Data treatment by least squares (χ) fitting 2 n n0 w n ( S n F n ( a 0, a 1, a 2,... b 0, b, b,...)) F B i j k i1 j1 k 1 a G ( E, b0, b1, b2,...) background: filter or polynomial linear intensity factor Gauss shape dependent on non-linear parameters. (e.g. energy calibration and detector settings) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 36

37 > Data treatment by least squares (χ) fitting 2 n n0 w n ( S n F n ( a 0, a 1, a 2,... b 0, b, b,...)) F B i j k i1 j1 k 1 a G ( E, b0, b1, b2,...) background: filter or polynomial linear intensity factor Gauss function dependent on non-linear parameters. (e.g. energy calibration and detector settings) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 37

38 > Data treatment by least squares (χ) fitting 2 n n0 w n ( S n F n ( a 0, a 1, a 2,... b 0, b, b,...)) F B i j k i1 j1 k 1 a G ( E, b0, b1, b2,...) background: filter or polynomial linear intensity factor Gauss function dependent on non-linear parameters. (e.g. energy calibration and detector settings) Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 38

39 > Radioactive sources Independent of power supply Inflexible primary energy Subject to restrictive legislation *After K. Janssens, "X-ray based methods of analysis" in "Non-destructive Microanalysis of Cultural Heritage Materials", Koen H.A Janssens. R.E. Van Grieken (Eds.), Elsevier, Amsterdam, The Netherlands, 2004 Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 39

40 > Radioactive sources Independent of power supply Inflexible primary energy Subject to restrictive legislation > X-ray tubes Mobile Of limited brilliance Primary radiation either polychromatic or fixed to one energy Divergent, incoherent radiation *After K. Janssens, "X-ray based methods of analysis" in "Non-destructive Microanalysis of Cultural Heritage Materials", Koen H.A Janssens. R.E. Van Grieken (Eds.), Elsevier, Amsterdam, The Netherlands, Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 40

41 > Radioactive sources Independent of power supply Inflexible primary energy Subject to restrictive legislation > X-ray tubes Mobile Of limited brilliance Primary radiation either polychromatic or fixed 30 kv to one energy 20 kv Divergent, incoherent radiation Rh-anode X-ray tube Characteristic radiation Bremsstrahlung 45 kv *After K. Janssens, "X-ray based methods of analysis" in "Non-destructive Microanalysis of Cultural Heritage Materials", Koen H.A Janssens. R.E. Van Grieken (Eds.), Elsevier, Amsterdam, The Netherlands, Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 41

42 > Radioactive sources Independent of power supply Inflexible primary energy Subject to restrictive legislation > X-ray tubes Mobile Of limited brilliance Primary radiation either polychromatic or fixed to one energy Divergent, incoherent radiation > Synchrotron radiation sources Stationary Brilliant sources Parallel, monochromatic primary radiation of flexible energy Synchrotron radiation is polarized Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 42

43 > Monochromatic radiation is desirable for: I Reduced spectral background from scattered radiation Easier quantification Necessary for many X-ray optics Allows to vary sensitivity for selected elements m i 4 max p ij ( E ) X I0( E) ij ( E) A ( E) de X edge E ij > Polarized primary radiation E Reduces the intensity of the scattered radiation, if the detector is placed in 90 to the incident radiation. Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 43

44 > Monochromatic radiation is desirable for: I Reduced spectral background from scattered radiation Easier quantification Necessary for many X-ray optics Allows to vary sensitivity for selected elements m i 4 max p ij ( E ) X I0( E) ij ( E) A ( E) de X edge E ij > Polarized primary radiation E Reduces the intensity of the scattered radiation, if the detector is placed in 90 to the incident radiation. Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 44

45 > Variants of XRF Bulk XRF Monitoring of production processes In-situ spot analysis Commonly not done at synchrotrons Total Reflection XRF Scanning XRF imaging Full Field XRF imaging Confocal XRF Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 45

46 > Variants of XRF Bulk XRF Total Reflection XRF The sample is infinite thin (easy quantification) The contribution of scattered radiation to the spectral background is reduced -> Improved Limits of detection. The sample is efficiently excited (high sensitivity). Mainly done with X-ray tube based instruments. Scanning XRF imaging Full Field XRF imaging Confocal XRF detector sample carrier Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 46

47 > XRF imaging: Elemental distribution images Pb-L Lead white: Expensive pigment Zn Zinc white: Cheap pigment Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 47

48 > XRF imaging Scanning XRF imaging detector Full-Field XRF imaging camera micro/nano-beam broad-beam Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 48

49 > XRF imaging Scanning XRF imaging detector Full-Field XRF imaging camera micro/nano-beam broad-beam Sequential Established Lower Radiation Dose Limiting factor: Motor control Simultaneous Experimental Higher Radiation Dose Limiting factor: Imaging optic Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 49

50 > For FF-XRF see: O. Scharf, et al., Anal. Chem. (2011) 83: For an application example: I. Reiche, K. Mu ller, M. Albe ric, O. Scharf, A. Wa hning, A. Bjeoumikhov, M. Radtke, R. Simon, Anal. Chem. (2013) 85: Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 50

51 > Confocal XRF Allows for separation of surface and bulk Allows for depth profiling Second optic acts as a filter Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 51

52 > Medical samples: Caries H.H. Harris, S. Vogt, H. Eastgate P.A. Lay, J Biol Inorg Chem (2008) 13: Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 52

53 > Medical samples: Breast cancer M. Ando, K. Yamasaki, C. Ohbayashi, H. Esumi, K. Hyodo, H. Sugiyama, G. Li, A. Maksimenko, T. Kawai, Jpn. J. Appl. Phys. (2005), 44:L998-L1001 Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 53

54 > Samples: Paleontology N.P. Edwards, R.A.Wogelius, U. Bergmann, P. Larson, W.I. Sellers, P.L. Manning, Appl. Phys. A (2013) 111: Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 54

55 > Biological samples: Cucumber leafs R. Terzano, M. Alfeld, K. Janssens, B. Vekemans, T. Schoonjans, L. Vincze, N. Tomasi, R. Pinton, S. Cesco, Anal. Bioanal. Chem. (2013) 405: Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 55

56 > Software For the evaluation of XRF data: PyMCA ( X-ray data base: xraylib ( Graphs were made with the Interactive Data Language ( Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 56

57 > Conclusions XRF is non destructive and provides qualitative and quantitative information on the sample. XRF is a versatile technique applied to a broad range of samples in diverse fields. It is easy to apply, as XRF investigations can be performed under ambient conditions (no need for vacuum). XRF provides complimentary information to other, slower X-ray techniques, such as XANES or XRD. Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 57

58 > Questions? Matthias Alfeld XRF: X-ray fluorescence spectrometry Page 58