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					 Method Development: Rapid
 Screening of Alpha-emitting
Radionuclides in Food Matrices
            Stephanie L. Healey1,2
                Zhichao Lin1
        1Winchester   Engineering and Analytical Center
                   Food and Drug Administration

  2University   of Massachusetts Lowell, Department of Physics
       Radiological Emergency
• Alpha emitting radioisotopes are the most dangerous to
  ingest
• Food contamination may result from all types of nuclear
  events involving alpha emitters, posing risk of internal
  exposure
• Current gross alpha methods are unsuitable for food
  analysis
• a method for screening alpha activity in food is needed in
  order to efficiently respond to a radiological emergency
• Liquid scintillation counting determines the activities of
  several alpha emitters with counting efficiencies
  approaching 100%
                Purpose
• Develop a method for the rapid detection
  of alpha activity in food matrices in the
  event of a radiological emergency using
  Liquid Scintillation Counting
 Liquid Scintillation Counting (LSC)
• Advantages
   – Allows for the accurate measurement of very small amounts of a
     wide range of radioactive materials
• Principle
   – Energy emitted during radionuclide decay is absorbed by a
     fluorescent material (scintillator) and re-emitted as light. The
     light is then detected by a photomultiplier tube and converted to
     electrical energy for analysis
   – Liquid scintillation involves bringing the radioactive material in
     close proximity to the scintillator by dissolving both in a suitable
     solvent
   – This is particularly beneficial for the quantitative measurement of
     radiation with limited penetrating power like alpha radiation

                            decay                     Light emitted
 α emitter
 Solvent                                                              Photomultiplier
 Scintillator       α emitter       Energy absorbed
                                    by Scintillator
                        Challenges to LSC
• Quenching
       – Chemical quench: absorption of energy by an interfering material
       – Color Quench: interference with the transmission of light from the
         scintillator to the photomultiplier tube by a colored material
                             Chemical quench            Light emitted
α emitter
Solvent                           decay                                    Photomultiplier
Scintillator          α emitter           Energy absorbed
                                          by Scintillator
                                                            Color quench
    Sample                                          Liquid Scintillation Vial


                 Protein                                    •15 mL scintillation cocktail
                 Fat                                        •5 mL of clear sample:
                 Carbohydrate                                dilute acid and radionuclide
       Milk      Phosphate
                 Vitamins
                 Minerals
                 enzymes
                Approach
• Use an extraction chromatographic resin
  to remove Actinides from milk sample
• DGA resin: 50 μm size
• Milk samples tested:
  – Skim milk, 1%, 2%, whole, and baby formula
  – Both blank milk samples and spiked (~1Bq)
    were analyzed for each type
                                DGA Resin
  I. Add           II. Digest     III. Pour sample                    IV. Add resin to
Radionuclide     sample for 1/2   into separatory                    sample and stir for
                      hour        funnel, separate                        30 mins.
                                  fat
  Am-241
                      100 mL
  Pu-239
                       Milk +                                              Sample+
                       100 mL     fat                                        DGA
  100 mL               Conc.                                                Resin
    Milk               HNO3
                                          sample

                        Heat
 V. Pour sample through            VI. Strip radionuclide off resin and into 50
  separation column to                      mL beaker with dilute acid
       isolate resin

                                        0.1M HCl +0.1M Oxalic acid
               Resin w/                                                     resin
               radionuclide

                                         50 mL beaker: 30 mL
                                          stripping solution +
        sample                                radionuclides
                                             DGA Resin
   VII. Evaporate acid                  VIII. Burn off                IX. Rinse beaker
                                        oxalic crystals
                                                                   2 mL
                                                                   concentrated
                                                                   HNO3


                                  Place beaker in oven for 15
                                  minutes at 400oC              Then evaporate acid
                                                                                      Heat
               Heat

                                                                VII. Count Samples on a
      X. Transfer                    XI. Add 15 mL Ultima
                                                                Liquid Scintillation Counter
      sample to vial                        Gold AB
Rinse beaker     Transfer                cocktail to vial.


 5ml
 0.5M
 HCl
        50 ml                0.5M HCl and
                      vial   radionuclides
        beaker
                                       Results

                                                     Estimated
                                          blank      total dpm      Percent        Counting
                               total    corrected    from NIST    recovery of     error (cpm)
   Sample       Radionuclide   cpm        (cpm)       standard   radionuclides        2s
Blank          none             10.5           0.0         0.0   N/A                       0.4
skim milk      none             10.8           0.3         0.0   N/A                       0.4
1% milk        none             10.9           0.3         0.0   N/A                       0.4
2% milk        none             11.1           0.6         0.0   N/A                       0.4
whole milk     none             10.4          -0.2         0.0   N/A                       0.3
baby formula   none             10.6           0.0         0.0   N/A                       0.4
skim milk      Am-241&Pu-239    66.0          55.5        59.1             93.9            3.4
1% milk        Am-241&Pu-239    59.8          49.3        59.1             83.4            3.2
2% milk        Am-241&Pu-239    64.5          53.9        59.1             91.2            3.3
whole milk     Am-241&Pu-239    68.8          58.3        59.2             98.5            3.5
baby formula   Am-241&Pu-239    67.4          56.9        59.2             96.2            3.4


MDC: 0.35 Bq/ kg                       Recovery of method: 92.6% + 11.6%
Analyst time (5 samples): 6 hours                                Counting Time: 6 hours
                  Conclusions
• This method utilizing an extraction chromatographic
  resin can extract and measure Americium and Plutonium
  isotopes from a variety of milk samples with 92.6%
  recovery + 11.6%
• Milk samples show no significant natural activity
• The minimum detectable concentration (0.35 Bq/ kg) is
  well below the derived intervention levels for gross alpha
  activity (2-5 Bq/ kg)
• This method can be used to screen milk samples for
  Americium and Plutonium isotopes with a rapid turnover
  time (~12 hours).
                    Empire ‘09
• This was a multi-agency exercise meant to test
  laboratory management and capabilities
  following a radiological emergency
     • A radiological dispersion device releases Am-241
• 5 food samples: 3 milk, strawberries, animal
  feed
  – This procedure used for milk
  – Modified for dilute acid rinse of the vegetation
    samples
• Results took 18 hours
             Current Status
• We are currently working to expand this
  method to include procedures for bottled
  water, rinses of raw food product, and
  surface swipes of food packaging as well
  as to address complications introduced by:
  – Natural Uranium and Thorium Activity?
  – Beta activity (spillover)?

              Future Work
• Matrix extension work

				
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posted:12/2/2011
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