From Lab to Field: Why Teaching XRF Matters

Transcription

1 From Lab to Field: Why Teaching XRF Matters

2 Welcome John Patterson, Ph.D. Director of Marketing and Product Management Bruker Elemental Dr. Peter Palmer Professor, Analytical- Environmental Chemistry San Francisco State University Alexander Seyfarth Product Manager, XRF Bruker AXS Inc

3 Today s Agenda Introduction Dr. Peter Palmer will describe his approach to teaching XRF and give you examples of applications he has used in his curriculum Alexander Seyfarth will describe several different types of XRF equipment and the appropriate application of each Summary Question and answer session 3

4 Audience Poll Please use your mouse to answer the question to the right of your screen: What elemental analysis techniques do your students gain experience with as part of your undergraduate laboratory curriculum? (Check all that apply.) Gravimetric methods (ex. Cl via precipitation with AgNO 3 ) Titrimetric methods (ex. Ca via EDTA titration) UV-Visible Spectrophotometry (ex. Pb via chelation with dithizone and UV/Vis) Atomic Spectrophotometry (ex. Pb via Flame Atomic Absorption Spectrophotometry) X-ray Fluorescence Spectrometry Other 4

5 From Lab to Field: Why Teaching XRF Matters Introduction: XRF theory is well understood and easy to teach Technique provides qualitative, semi-quantitative and quantitative analysis Provides opportunity to teach not just the technique but also scientific thought The equipment is relatively inexpensive Handheld equipment is portable enough to take it almost anywhere from the Laboratory to the Field Poll Results 5

6 Teaching XRF Dr. Peter Palmer 6

7 7

8 FIT OF XRF IN THE SCIENCES Although XRF is a physical phenomena involving the interaction of X-rays with matter, the applications of XRF are in areas predominantly outside of physics (geology, anthropology, environmental sciences, etc.) Although XRF, like any other spectroscopic technique, requires specialized knowledge in chemistry (spectral interpretation, calibration, sample prep, etc.), it is not even mentioned in 99% of undergraduate chemistry programs in the U.S. With continuing advances in technology and ever growing number of XRF applications, it is critically important to bring more attention to XRF and related techniques within the U.S. academic system SF State University is one of only a very few universities to integrate XRF into its undergraduate chemistry curriculum Quantitative Chemical Analysis (lecture and lab) Instrumental Analysis (lecture and lab) Independent research projects 8

9 ELECTROMAGNETIC SPECTRUM 9

10 ATOMIC SPECTROMETRY FUNDAMENTALS Atomic Absorption Light of specific wavelength from Hollow Cathode Lamp promotes electron to higher energy level (excitation) Atomic Emission Heat energy from high intensity source (flame or plasma) promotes electron to higher energy level (excitation) Excited State Ground State Δ Selectivity based on use of element-specific light source (Hollow Cathode Lamp) Light absorption is proportional to concentration: A = -log(p/p 0 ) = εbc Selectivity based on emission of light at characteristic λ for element Light emission is proportional to elemental concentration: I = kc Mass Spectrometry Heat energy from high intensity source (plasma) separates electron from atom (ionization) Δ - - Selectivity based on use of measurement of characteristic mass of elemental ion Ion intensity is proportional to elemental concentration: I = kc X-Ray Fluorescence Energy from X-Rays (high energy) separates electron from atom (ionization), then inner shell electron fills vacant hole by emitting light Selectivity based on use of emission of light at characteristic λ for element (X-Ray) Light intensity is proportional to elemental concentration: I = kc

11 XRF PHYSICAL PROCESS 1. When an X-ray photon of sufficient energy strikes an atom, it dislodges an electron from one of its inner orbitals (recall that X-rays are ionizing radiation) 2. To regain stability, the atom fills the vacancy with an electron from an outer orbital 3. As electron drops to lower energy state, excess energy is released as an X-ray Step 2: Step 1: Step 3: 11

12 XRF PHYSICAL PROCESS Pb N 4s 2 p 3 d 10 f 14 Pb M >16 kev L L β kev 3s 2 p 3 d 10 L α kev 2s 2 p 6 K 1s 2 Quantum states for each element are different, and hence the characteristic energy of X-ray fluorescence can be correlated to a specific element(s) The XRF spectrum of an element is independent of its chemical form (recall that we re removing inner shell electrons, not bonding electrons)

13 SIMPLE XRF SPECTRUM ~10% Pb in imported tableware Pb L α line kev Pb L β line kev Intensity (cps) Energy (kev) Presence of Pb confirmed through observation of two peaks centered at energies close to their tabulated (reference) line energies 13

14 XRF SPECTRA Consecutive elements in periodic table Zn Ga Ge As Se Intensity (cps) Energy (kev) 14 Energy (kev) proportional to atomic number Z 2 (Moseley s law)

15 BOX DIAGRAM OF XRF INSTRUMENT X-ray Source Detector Digital Pulse Processor XRF Spectrum (cps vs kev) software Results (elements and conc s) Sample X-ray source X-ray tube (range of energies) Detector and digital pulse processor Energy-dispersive (no monochromator required) Peltier-cooled solid state detector (Li-doped Si or newer Si drift detector) This is a multi-channel analyzer that measures the energy of photon in kev - related to type of element emission rate in counts-per-second (cps) related to concentration of element Analyzer software converts spectral data to direct readout of results Concentration of an element determined from factory calibration data, sample thickness as estimated from source backscatter, and other parameters While this software is powerful, it can still yield false positives, false negatives, and erroneous concentration data 15

16 BRUKER TRACER III-SD XRF SYSTEM 16

17 SAFETY CONSIDERATIONS Scenario/situation exposure units Exposures from background radiation sources Chest X-ray 100 mrem/x-ray Grand Central Station 120 mrem/year Airline worker 1000 mrem/year Exposure limits set by regulatory agencies Max Permissible Limit during pregnancy 500 mrem/9 months Max Permissible Limit for entire body 5000 mrem/year Max Permissible Limit for an extremity (i.e., finger) 50,000 mrem Exposures from normal operation of XRF analyzer in sampling stand Left/right/behind analyzer << 0.1 mrem/hour Exposures from unacceptable use of XRF analyzer outside sampling stand 4 feet directly in front of analyzer window 14 mrem/hour 1 foot in front of analyzer window 186 mrem/hour Directly in front of analyzer window 20,000 mrem/hour Any device which generates X-rays must be operated in a manner which ensures that human exposure to these X-rays is as low as reasonably achievable (ALARA) When the XRF analyzer is used properly (using sampling stand), radiation exposure will be non-detectable 17

18 QUALITATIVE ANALYSIS Issues to consider WHAT ELEMENTS do we want to look for (toxic elements such as As, Cd, Hg, Pb, and Se, nutrient elements such as Fe, etc.)? Define the problem what are element(s) of interest Are there any SPECTRAL OVERLAPS with other elements in sample? Determine potential overlapping species (i.e., Pb and As) If we detect a toxic element, do we know for certain that it is in the product and NOT in the background materials used to hold the sample or the product packaging? Measure what you want to measure How do we know that the analyzer is not generating an ERRONEOUS identification (false positives or false negatives)? Users must evaluate the spectrum to verify the reported results positive identification of an element requires observation of two peaks at energies close to their tabulated values 18

19 5 SPECTRAL OVERLAPS Basket Sample ppm Hg and As standard 3 Zn K α As K α Overlapping lines As K β and Hg L β Intensity (cps) 2 Hg L α Pb L α Br K α Pb L β 1 Zn K β Br K β Energy (kev) 19 Neither Hg nor As is present in this basket sample

20 QUANTITATIVE ANALYSIS Issues to consider Are the element concentrations within detection range of XRF (% to ppm)? Define the problem research sample composition or take a measurement Can the samples be analyzed as is or is some sample prep required? Define the problem is sample homogeneous, can sample be ground up, what sort of accuracy and precision are required What is the most appropriate calibration model to use? Fundamental Parameters (used for metallic samples) Compton Normalization (used for soil samples) Thin Film mode (used for filter samples) Empirical Calibration (are appropriate standards available?) Standard Additions (used to correct for matrix effects) 20

21 CALIBRATION CURVES As, Hg, and Pb in cellulose Intensity (cps) As Hg Pb As y = x R 2 = Pb y = x R 2 = Hg y = 0.345x R 2 = Concentration (ppm) Linear cal curves indicate acceptable method for preparation of authentic standards (dispersing metal salt into cellulose using powder or cryogenic mill) LODs in single ppm range (surprisingly just as sensitive as lab-grade XRF instrument) 21

22 Audience Poll Please use your mouse to answer the question to the right of your screen: What are the barriers to teaching XRF in your curriculum? (Check all that apply.) Expertise or knowledge of technique Availability of teaching materials (lecture notes, lab experiments) Access to an XRF instrument (lack of funding, cost of equipment) Effort involved in changing the curriculum Safety concerns Other 22

23 STUDENTS IMPACTED Undergraduate and Graduate Classes 22 students from Quantitative Analysis in Spring students from Quantitative Analysis in Fall students from Quantitative Analysis in Spring students in Instrumental Analysis in Spring students in grad level X-Ray Techniques class in Spring attendees for Dr. Bruce Kaiser s XRF Workshop Independent Research Matt Sanchez Patience Adagba Charlie Bupp Heather Gregory Pete Baker Rene Johnson David Luong Anthony Trinh Kara Cross Brian Rebold Chris Alleyne-Chin Kensuke Yamamoto Siri Webber Yeon Kyoung Hwang Kelly Ferguson Fe in beer Cl in PVC identifying fake drugs identifying fake drugs ppb levels of As in drinking water ppb levels of As in drinking water Pb in tableware leachate Pb in tableware As and Hg in basket collections toxic elements in soil paint analysis toxic elements in supplements toxic elements in supplements toxic elements in food toxic elements in food 23 Poll Results

24 XRF LAB MODULE 20 students in each Quantitative Chemical Analysis lab section 4 groups comprised of 5 students, each working on different XRF application 1 or 2 XRF analzyers (Niton XLt, Innov-X α, Bruker Tracer III-V) DAY 1: Lecture (theory, qual and quant analysis) Demo (instrument operation, acquiring spectra, analyzing data) Define application (target element, lines of interest, expected conc s) DAY 2: Prep standards and samples (homogenization) Develop method (excitation conditions, measurement time) Acquire data and download spectra to Excel (for subsequent plotting of spectra and generation of cal curves) DAY 3: Evaluate results, compute figures of merit (linearity, accuracy, precision), and give brief presentation to entire lab Group discussion on XRF versus other analytical methods 24

25 XRF EXPERIMENTS IN QUANT Hazmat analysis Students analyzed several samples and identified U, lead arsenate, Th, and V Software enables verification of presence of an element from its reference line energies Results clearly showed utility of XRF as powerful screening tool 25

26 XRF EXPERIMENTS IN QUANT (Cont.) Hazmat analysis Students analyzed several samples and identified U, lead arsenate, Th, and V Software enables verification of presence of an element from its reference line energies Results clearly showed utility of XRF as powerful screening tool 26

27 XRF EXPERIMENTS IN QUANT Determination of Ca in powdered milk Sample contained 1% Ca Students often failed to correctly prepare several standards of CaCO 3 in cellulose or use actual concentrations in their cal curves (teaching moment) Use of external standards (CaCO 3 in cellulose) gave erroneously low results due to attenuation of Ca fluorescence by K in milk Use of standard additions (CaCO 3 in powdered milk) corrected for this matrix effect (<1% error) Peak Area Ca in Milk via Standard Additions y = x R 2 = % Ca Compared to classical EDTA titration method, XRF simpler, faster, gives [Ca] 27

28 XRF EXPERIMENTS IN QUANT Determination of Pb in SRM LINEAR FIT for Pb in Cab-O-Sil via XRF SRM contained 2480 ppm Pb Students prepared standards of Pb salt in Cab-O-Sil matrix in attempt to emulate composition of SRM (mostly Si) Intensity (cps) y = x R 2 = Linear cal curve showed evidence of selfabsorption (fall off in intensity at higher concentrations) Nonlinear cal curve gave better fit of standards and blank and more accurate quantitation of Pb in SRM (< 3% error) XRF results obtained much faster than FAAS method 28 Intensity (cps) ppm Pb 2nd ORDER FIT for Pb in Cab-O-Sil via XRF y = -5.79E-05x E+00x E+04 R 2 = 9.94E ppm Pb

29 XRF EXPERIMENTS IN QUANT Determination of Pb in tableware Students prepared aqueous standards of Pb and leachate extracts from 13 different pieces of imported Mexican tableware XRF results showed Pb ranging from ND to over 4000 ppm, with most samples giving levels well above the FDA limit of 1 ppm Comparison of XRF vs FAAS results showed results were not statistically different at 95% confidence level Compared to FAAS method, XRF results obtained much faster, gave cal curve linear from ppm, and readily identified presence of Cu in new glaze Pb Response / Compton Response y = x R 2 = ppm Pb 29

30 COMPARISON OF ELEMENTAL ANALYSIS METHODS Criteria FAAS GFAAS ICP-AES ICP-MS XRF Sample Prep [significant - homogenization, digestion, filtration] minimal Detection Limits ppm-ppb ppb-ppt ppb ppt ppm Dynamic Range Interferences Moderate Moderate Significant Few Moderate Speed Moderate Slow Moderate Moderate Fast Elemental Coverage Moderate Moderate Moderate Excellent Moderate Multi-element No No Yes Yes Yes Sample Size ml μl ml ml ml/g Capital Cost $ $$ $$ $$$ $ Operating Cost $ $$ $$ $$$ $ 30

31 TRENDS IN ELEMENTAL ANALYSIS METHODS * Denotes multi-element analysis technique REQUIRED DETECTION LIMITS low high FAAS GFAAS XRF* ICP-AES* ICP-MS* low NUMBER OF SAMPLES high 31

32 FINAL THOUGHTS ON ACADEMIC USE OF HANDHELD XRF ANALYZERS What can we do to further this discipline? Promote inclusion of XRF into analytical chemistry textbooks Promote integration of XRF into undergraduate lecture and lab classes (quant analysis, instrumental analysis, environmental chem, material science, special topics course on X-ray techniques) FAST measurement times suitable for large lab sections INEXPENSIVE handheld instrument, can be rented or purchased COMPLEMENT/REPLACE classical gravimetric and titrimetric experiments ENGAGE students with state of the art instrument and relevant applications Provides many opportunities to TEACH relevant analytical chemistry topics (sample homogeneity, preparing standards, calibration models, instrument optimization, data analysis) Focus students on interpreting and understanding SPECTRA (not list of elements and concentrations) Get students to THINK about the measurement process Develop more active XRF research programs Case studies and field work 32

33 XRF Instrumentation Alexander Seyfarth 33

34 How characteristic X-rays are generated in an atom 34

35 XRF: X-ray Fluorescence Analysis Sample Energy of X-ray photons Which element Qualitative analysis Number of X-ray photons at a given energy Concentration Quantitative analysis 35

36 X-ray Fluorescence Analysis: Energy-Dispersive XRF (EDX or EDXRF) Sample The detector is used to record both: the energy E and the number N of X-ray photons 36

37 Wavelength-Dispersive XRF (WDX or WDXRF) Sample An analyzer crystal separates the various wavelengths, λ (energies) The detector records only the number, N, of X-ray photons at a given wavelength (energy) 37

38 Primary Signal: ED and WD counts / Δ E counts / Δ 2θ For both methods, the primary signal is the number of detected X-ray photons ("counts") Counts are collected: as a function of the energy (EDX) as a function of the reflection angle 2θ (WDX) E 2θ 38

39 Primary Signal: ED and WD counts / Δ E counts / Δ 2θ Resolution (separation of peaks) Count rate (number of detected photons) Background (scatter from sample) E 2θ 39

40 X-ray Fluorescence Analysis: Energy-Dispersive XRF (EDX or EDXRF) Sample The detector is used to record both: the energy E and the number N of X-ray photons 40

41 Traditional EDXRF 41

42 EDXRF Can detect multiple elements simultaneously Excitation can be controlled by selecting the current (number of photons) as well as high voltage (excitation energy) Beam filters allow selective absorption of the primary beam to optimize certain regions of the spectra Using Silicon Drift (SDD) detectors allows for quality resolution of < 150 ev while capable of high total spectra count rate of > counts per second NO liquid nitrogen! NO carrier gasses! Instrument can operate under air, vacuum or helium Power level 50 Watts; large irradiated bulk sample spots of 30-mm diameter 42

43 Excitation of light elements 43

44 Excitation of heavy elements 44

45 Benchtop EDXRF e.g. University Western Ontario Murray State Univ 45

46 PXRF/ HH-XRF EDXRF in a small package Tube (Rh, Ag) Si-Pin Detector <170 ev (40 000cps) IR SENSOR FILTER /TARGET EXIT WINDOW 46

47 Point and Shoot vs. TRACER DEVELOPED WITH NASA e.g. Metals SORTER Point and shoot Factory calibrations PDA Multi-calibration Multi-filter and Vacuum Factory AND user calibrations PC software AND PDA 47

48 TRACER III-SD with X-Flash SDD Detector 48

49 Mobile use, stationary or even point and shoot 49

50 The usage difference TRACER III will let you adjust: Voltage Current Filter Secondary target Variations of the above 4 Bruker Software (distributed with equipment) Subtraction Normalization Analysis locations Overlays and deconvolution Perform the sample analysis from back to front vice versa Transmission test or calculations Analyzed layer 50

51 X-ray Fluorescence Analysis Energy-Dispersive TXRF Detector E, N The detector is used to record both: the energy E and the number N of X-ray photons Sample Monochromatic X-rays at low incidence angle to skim the very thin sample 51

52 TXRF low background = lowest LLD s PICOFOX does PICOgrams Sample:1 ng Ni Time: 100 s Sensit.:37 cps/ng LLD: 0.95 pg 52

53 TXRF Applications: Chemotherapy Sample preparation and analysis Pt was analyzed in serum only centrifugation and separation of serum for accurate analysis an external calibration, applying spiked samples and Compton-normalization measurements: 50 kv/750 µa, Mo-excitation, 100 s Data kindly supplied by E. Greaves, Universidad Simón Bolívar, Apartado 89000, Caracas 1080A, Venezuela. E- mail: greaves@usb.ve DL: 67 µg/l (100s) 53

54 Benchtop TXRF e.g. Loyola University (filters), Lawrence Berkeley National Lab, British Columbia Institute of Technology See our extensive TXRF webinars posted on 54

55 Wavelength-Dispersive XRF (WDXRF) Sample An analyzer crystal separates the various wavelengths, λ (energies) The detector records only the number, N, of X-ray photons at a given wavelength (energy) 55

56 EDXRF versus WDXRF 56

57 Sample Preparation Pressed Pellet Black Shale XRF for YOU Seminar Nashville, TN H. Rowe At that time U. Kentucky 57

58 Black Shale Analysis Pressed Pellet 58

59 Calibration Not so many standards for Black Shales! Need for mixing and matching (similar matrices) Need for developing standards XRF for YOU Seminar Nashville, TN H. Rowe At that time U. Kentucky 59

60 Floor standing units (WDXRF) e.g. University of Kentucky, Michigan State University, University of Wisconsin Milwaukee Geoscience applications 60

61 Summary John Patterson 61

62 From Lab to Field Why Teaching XRF Matters Conclusion: XRF is an excellent technique for teaching atomic spectroscopy and analytical techniques XRF provides an opportunity to engage the student and teach scientific reasoning XRF is suitable for use in almost any location so take the analysis to the samples rather than the reverse XRF is an important job skill industry is always looking for trained operators; this will expand the resume of a recent graduate 62

63 Advantages of XRF for Teaching XRF can be completely nondestructive leading to use of very interesting and unusual samples There is little or no sample prep No dangerous chemicals to dispose of (acids) No expensive Argon or explosive gasses needed/no LN Low operating costs Direct analysis of solids, liquids and inert gasses (TRACER only) Multiple techniques to select from based on applications / funding PXRF/HH-XRF EDX TXRF WDXRF 63

64 We want to make it easy to adapt Educational package PPT and exercises (and solutions) Materials for calibration and applications Software package (SPECTRA/SpectraEDX) which can be freely distributed to students, faculty to use Networked units allow multiple users to use the unit, then crunch data and evaluate them offline Train the trainer by our experienced applications staff Priority applications support 64

65 We want to make it easy to adapt Alexander Seyfarth can provide all the Bruker AXS material and support ( to Dr. Palmer s XRF lecture materials and laboratory experiments are available on request ( to palmer@sfsu.edu) 65

66 Any Questions? Please type any questions you may have in the Q&A panel and then click Send. 66

67 Where can you learn more about XRF? University of Western Ontario XRF2010 Short Course 2 weeks: May 31 June 11, 2010 o 1 st week is basics o 2 nd week is quantification 2 topical workshops: Sample Prep Workshop June 4, th Anniversary Symposium: Practical Applications of XRF in Industries Charles Wu at ctwu@uwo.ca 67

68 Where can you learn more about XRF? ICDD XRF Clinic - Practical X-ray Fluorescence 5 days: May 3-7, 2010 Newtown Square, Pennsylvania Taught by a dedicated and experienced group of XRF specialists Live instrumentation: WDXRF, EDXRF and more 68

69 Where can you learn more about XRF? DXC 2010 Workshops Monday & Tuesday, August 2 & 3, 2010 Denver Marriott Tech Center Hotel, Denver, CO XRF Workshops: Basic XRF Trace Analysis XRF Specimen Preparation Quantitative Analysis Standards and Advanced Sample Preparation for XRF Analysis 69

70 Where can you learn more about XRF? Bruker Training Central (BTC) Online Training Courses 2-hour web-based training courses delivered through your browser Include slides, audio, video and participant Q&A Upcoming live: April Ultra-Low Sulfur Determination by XRF July XRF Standardless Analysis September 9-10 Getting the Most From Your WDXRF System On-demand: XRF Basics I: From Theory to Intensity XRF Basics II: From Intensities to Concentrations XRF Sample Preparation 70

71 Thank You for Attending! John Patterson, Ph.D. Director of Marketing and Product Management Bruker Elemental Dr. Peter Palmer Professor, Analytical- Environmental Chemistry San Francisco State University Alexander Seyfarth Product Manager, XRF Bruker AXS Inc When you exit the webinar, please complete our brief survey. Your feedback is very important to us. 71

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