page 1eichrom users’ group meeting determination of 226 ra in environmental and personal...

EIChroM Users’ Group Meeting

Determination of 226Ra in Environmental and Personal Monitoring Samples

Billy Lawrie

Geoffrey Schofield Laboratories


Overview

Introduction

– Why measure 226Ra?

– Properties of Ra

– Methods available

– Difficulties

Experimental

Results and Discussion

Conclusions


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)


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


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


Methods of Determining Radium

Detection following BaSO4 co-precipitation

Radon emanation

Gamma ray spectrometry

Thermal ionisation mass spectrometry (TIMS)

Alpha spectrometry


Barium Sulphate Co-precipitation

Ba(Ra)SO4

Tedious

Slow

Analyst dependent


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


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.


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


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


Difficulties

Adsorption

Dissolution

Tracer

Electrodeposition

Interferences

– chemical

– spectral


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


Dissolution

Total dissolution required

Must be free from suspended particles

– eliminates loss of radium

Microwave digestion

Fusion


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)


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


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


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


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


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


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


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


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 )


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


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


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


Acknowledgements

Thanks are expressed to:

– Jim Desmond

– Debbie Spence

– Scott Anderson

– Clare Edmondson

page 1eichrom users’ group meeting determination of 226 ra in environmental and personal...

Documents