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5. Conclusions

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)