5. Conclusions
- Page ID
- 74238
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Advantages of XRF
Selectivity: |
True multi-element analysis (from S to U, ~80 different elements) Measures total element concentration (independent of chemical form) |
LODs: |
1 to 10 ppm at best (depends on source, element, matrix, etc.) |
Linearity: |
Linear response over 3 orders of magnitude (1-1000 ppm) |
Accuracy: |
Relative errors ~ 50% with factory calibrated instrument Relative errors < 10% using authentic standards for calibration |
Precision: |
RSDs < 5% (must have homogeneous sample) |
Speed: |
Minimal sample prep (analyze “as is” or homogenize and transfer to cup) Fast analysis times (typically seconds to minutes) |
Cost: |
$25,000-$50,000 for field portable instrument Far less expensive per sample than FAAS, GFAAS, ICP-AES, and ICP-MS |
Miscellaneous: |
Simple (can be used by non-experts in the field) Nondestructive (sample can be preserved for follow up analysis) Field-portable instruments can operate under battery power for several hours |
Limitations of XRF
Selectivity: |
Interferences between some elements (high levels of one element may give a false positive for another due to overlapping emission lines and limited resolution of ~0.2 keV FWHM) No info on chemical form of element (alternate technique required for speciation) |
Detection Limits: |
Must use alternate technique to measure sub-ppm levels (TXRF, GFAAS, ICP-AES, ICP-MS) |
Accuracy: |
XRF is predominantly a surface analysis technique (X-rays penetrate few mm into sample) To get more accurate results, one must homogenize the samples and calibrate instrument response using authentic standards |
Trends in Elemental Analysis Techniques
XRF and ICP-MS are complementary
These techniques are replacing conventional atomic spectroscopy techniques such as FAAS and GFAAS
Technique | XRF | ICP-MS |
Elements | Na-U | Li-U |
Interferences | spectral overlaps, limited resolution | isobaric ions |
Detection Limit | ~1-10 ppm |
~10 ppt (liquids) ~10 ppb (solid-0.1 g into 100 mL) |
Sample prep | minimal (homogenization) | significant (digestion/filtration) |
Field work | yes | not possible |
Capital cost | $25-50K | $170-250K |
Safety Considerations
- XRF X-ray tube sources are far less intense than medical and dental X-ray devices
- When an XRF analyzer is used properly, users will be exposed to nondetectable levels of radiation
Scenario/situation | exposure | units |
---|---|---|
Exposures from normal operation of XRF analyzer in sampling stand | ||
Left/right/behind analyzer | << 0.1 | mREM/hour |
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 unauthorized and 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 |
References and Additional Reading
Good non-commercial website with XRF info
www.learnxrf.com
Excellent reference text on the subject matter
R. Grieken, A. Markowicz, Handbook of X-Ray Spectrometry, 2nd Ed., CRC Press, Boca Raton, FL, 2002.
Feature/Perspectives article on FDA applications of XRF
P.T. Palmer, R. Jacobs, P.E. Baker, K. Ferguson, S. Webber, “On the Use of Field Portable XRF Analyzers for Rapid Screening of Toxic Elements in FDA-Regulated Products”, Journal of Agricultural and Food Chemistry, vol. 57, 2009, pp. 2605-2613.
EPA method based on XRF for soil analysis
EPA Method 6200 – Field Portable XRF for the Determination of Toxic Elements in Soils and Sediments (find at www.epa.gov)