Post on 19-Dec-2015
Page 1EIChroM Users’ Group Meeting
Determination of 226Ra in Environmental and Personal Monitoring Samples
Billy Lawrie
Geoffrey Schofield Laboratories
Page 2EIChroM Users’ Group Meeting
Overview
Introduction
– Why measure 226Ra?
– Properties of Ra
– Methods available
– Difficulties
Experimental
Results and Discussion
Conclusions
Page 3EIChroM Users’ Group Meeting
Why Measure 226Ra?
There are 4 naturally occurring isotopes of Ra:
232Th decay series224Ra ( emitter, t½ 3.66 days)228Ra (- emitter, t½ 5.75 years)
235U decay series223Ra ( emitter, t½ 11.4 days)
238U decay series226Ra ( emitter, t½ 1600 years)
Page 4EIChroM Users’ Group Meeting
Why Measure 226Ra? (2)
226Ra causes the most concern
– long half life
– radiological effects
Toxic
Widespread
Concentrates in bones
– increases internal radiation dose of individuals
Page 5EIChroM Users’ Group Meeting
Properties of Radium
Alkaline earth metal
Only one oxidation state (+2)
Not easily complexed
– majority of compounds are simple ionic salts
Ca, Sr, Ba & Ra form a series of closely related elements
– problems in chemically isolating Ra
Few complexes that pass into the organic phase
– solvent extraction not suitable
Page 6EIChroM Users’ Group Meeting
Methods of Determining Radium
Detection following BaSO4 co-precipitation
Radon emanation
Gamma ray spectrometry
Thermal ionisation mass spectrometry (TIMS)
Alpha spectrometry
Page 7EIChroM Users’ Group Meeting
Barium Sulphate Co-precipitation
Ba(Ra)SO4
Tedious
Slow
Analyst dependent
Page 8EIChroM Users’ Group Meeting
Radon Emanation
Can only measure 226Ra
Indirect
Time consuming
Slow (20 days to achieve full equilibrium)
222Rn is a gas
– potential problems during sample handling
Low levels of detection require large sample volumes
Page 9EIChroM Users’ Group Meeting
Gamma Ray Spectrometry
Limited to the analysis of 226Ra
Direct determination (186keV)
– low -emission probability
– 235U interference
Determination via 214Bi
– time consuming equilibration between 226Ra and 214Bi
Distribution of 214Bi must be homogeneous throughout sample
Standard must have the same configuration & density as sample
– neither of these two options are easy to achieve.
Page 10EIChroM Users’ Group Meeting
TIMS
Shorter analytical time
Improvement in analytical precision
Reduction of the Ra sample size
Requires Ra load to be extremely pure
– in particular it has to be free from Ba
Page 11EIChroM Users’ Group Meeting
Alpha Spectrometry
Can determine all -emitting Ra nuclides directly
No consideration of equilibria loss
High resolution surface barrier detectors
– determination virtually specific
– very few spectral interferences
Can be performed in a timely manner
– 2 days source prep. + counting time
Electrodeposition gives robust source
Page 12EIChroM Users’ Group Meeting
Difficulties
Adsorption
Dissolution
Tracer
Electrodeposition
Interferences
– chemical
– spectral
Page 13EIChroM Users’ Group Meeting
Adsorption
Ra adsorbs onto suspended particles, colloids & container walls
Precautions to avoid loss of radium
– sampling
– analysis
Water samples acidified
Collected in acid washed plastic containers
Sample vigorously shaken before sub-sampling
– re-suspend any adsorbed radium
Page 14EIChroM Users’ Group Meeting
Dissolution
Total dissolution required
Must be free from suspended particles
– eliminates loss of radium
Microwave digestion
Fusion
Page 15EIChroM Users’ Group Meeting
Suitable Tracer
Tracer v’s parallel standards
224Ra
– occurs naturally
– short t½ (3.66 days)
225Ra
– not naturally occurring
– also has a short t½ (14.8 days)
Page 16EIChroM Users’ Group Meeting
Suitable Tracer (2)
133Ba
– acceptably long t½ (10.66 years)
– non-isotopic
– susceptible to error regardless of chemical similarity1
226Ra parallel standard
– parallel sample spiked with 226Ra
– best option for a batch laboratory
1. Sill, C.W. Nucl. Chem. Waste Manage. 1987, 7, 239
Page 17EIChroM Users’ Group Meeting
Chemical Interferences
Major interferent is Ba
A little makes a significant difference
– 10g can result in a 50% reduction in recovery of Ra1
Can be minimised by:
– limiting sample size to ~ 0.1g
– washing column with 1.5M HCl1
– adding (COONH4)2 to the ED solution2
1. Alvarado et al J. Radioanl. Nucl. Chem. 1995, 1, 163
2. Orlandini et alRadiochim. Acta 1991, 55, 205
Page 18EIChroM Users’ Group Meeting
Spectral Interferences
226Ra produces a doublet
– 4.602MeV (5.55%) & 4.785MeV (94.45%)
226Ra daughters 222Rn, 218Po & 214Po are present
Main possible interferent is 234U (4.773MeV)
234U in equilibrium with it’s parent 238U (4.194MeV)
– if no 238U then 234U won’t be contributing to 226Ra peak
Shouldn’t pose a significant problem
– U should be removed at the separation stage
Page 19EIChroM Users’ Group Meeting
Electrodeposition
One of the most awkward species to ED
Partial deposition observed during deposition of 228Th1
Method for deposition at pH 8-9 developed by Roman2
Orlandini et al showed advantage of using Pt and (COO NH4)2
Add g amounts of Pt3
– ‘Pt black’ film produced
– Ra quantitatively plated
– robust source (COO NH4)2 increases efficiency in presence of 5-10g Ba3
1. Sill, C.W. et al Anal. Chem. 1974, 46, 17252. Roman, D. Int. J. Appl. Radiat. Isot., 1984, 35,
990 3. Orlandini, K.A. et al Radiochim Acta 1991, 55,
205
Page 20EIChroM Users’ Group Meeting
Experimental - Overview
Microwave digestion
– double digestion procedure
– HCl, HNO3 & HF followed by EDTA & H3BO3
Separation (BioRad AG-50W-X8 resin)1
Ba elimination (EIChroM Sr.spec resin)2
Electrodeposition [400g Pt, 0.17M (COO NH4)2, 0.14M HCl]
Counting (-spectrometry)
1. Alvarado et al J. Radioanal. Nucl. Chem., 1995, 1, 163
2. Chabaux et al Chem. Geol., 1994, 114, 191
Page 21EIChroM Users’ Group Meeting
Experimental
dissolve
evaporate
sample (1)1M HCl
(2) 1.5M HCl(3) 6M HCl
wasteevaporate
sample (1)
(1) & (2)
(3)
3M HNO3
waste
(2)
(1) evaporat
eelectrodeposit
-spec
(2) 3M HNO3
Pt
ED sol.
BioRad
AG-50W-X8
EIChroM
Sr.spec
Page 22EIChroM Users’ Group Meeting
Intercomparison Results
Sample % recovery226Ra
Reported value/ mBqL-1
True value / mBqL-1
CNS 1/ 1 92.3 ± 2 .5
CNS 1 / 2 9 9 .1 ± 2 .4< 1 0
CNS 2 / 1 9 7 .1 ± 2 .3
CNS 2 / 2 1 0 0 .6 ± 2 .4 2 8 .5 ± 8 .5 2 9 .7
CNS 3 / 1 1 0 0 .2 ± 2 .4
CNS 3 / 2 9 9 .4 ± 2 .5 1 4 2 ± 2 0 1 4 8 .6
Page 23EIChroM Users’ Group Meeting
Recoveries from Marine Sediment Reference Sample IAEA-135
Sample 226Ra content / Bqkg-1 dry weight
IAEA-135 / 1 25.9 ± 3 .1
IAEA-1 3 5 / 2 2 3 .4 ± 2 .9
IAEA-1 3 5 / 3 2 0 .6 ± 3 .0
IAEA-1 3 5 / 4 2 4 .4 ± 3 .4
mean 2 3 .6 ± 3 .6
recommended value 2 3 .9 (conf. int. 2 0 .6 – 2 5 .0 )
Page 24EIChroM Users’ Group Meeting
Results from Environmental & Personal Monitoring Samples
sample mean recovery / % approx. level ofactivity in sample
Ground water 83.4 ± 4 .3 8 0mBqL-1
Filter thimble 6 7 .7 ± 5 .4 1 5 0mBqg-1
Silica gel 9 2 .3 ± 4 .4 <2 0mBqg-1
Carbon /Activated carbon
7 9 .7 ± 9 .4 3 0mBqg-1
Ash 7 9 .5 ± 6 .2 1 0 0mBqg-1
Fly ash 7 0 .8 ± 6 .0 1 0 0mBqg-1
Coal / coal ash 6 9 .9 ± 3 .9 1 0 0mBqg-1
Page 25EIChroM Users’ Group Meeting
Results Summary
Intercomparison results compare favourably with actual levels
Marine sediment reference material IAEA-135
– mean result obtained from intercomparison exercise
– 66 labs participated
– 64 used -ray spectrometry; 2 used Rn emanation
– 11 outliers
– recommended value 23.9Bqkg-1
– confidence interval 20.6 - 25.0Bqkg-1 @ 95% confidence level
– range of accepted results 13.6 - 36.0Bqkg-1
–from replicate parallel analysis obtained 23.6 3.6Bqkg-1
Good recoveries achievable on a variety of problematic matrices
Page 26EIChroM Users’ Group Meeting
Conclusions
Robust method
Capable of analysing wide variety of matrices
Good LOD’s achievable
– 20mBqL-1 for water samples (100ml)
– 20mBqg-1 for solid samples (0.1g)
Future work will focus on 228Ra
– -spectroscopy
– electrodeposition disc used for 226Ra
Page 27EIChroM Users’ Group Meeting
Acknowledgements
Thanks are expressed to:
– Jim Desmond
– Debbie Spence
– Scott Anderson
– Clare Edmondson