Mass spectrometry in
radionuclide analyses
P E Warwick, I W Croudace & T Warneke
Southampton Oceanography Centre
Range of mass spectrometric
techniques
•
•
•
•
•
TIMS
SIMS
RIMS
AMS
ICPMS
- quadrupole ICPMS
- HR-ICPMS
- MC-ICPMS
Advantages of mass spectrometry
• Often rapid analyses
• Improved data quality
(e.g. 238U:235U ratios)
• Permits analyses that are not possible
radiometrically
(e.g. 239Pu and 240Pu separately)
• Improved sensitivity for long lived nuclides
Comparison of mass
spectrometric techniques
Method
U
required
for
detection
U required
for isotope
ratios
Isotopes reported
Typical
accuracy
Typical
precision
(2σ)
HRGS
10 µg
1 mg
235
U, 238U
10%
10%
Alpha spec
10 ng
10 µg
234
U, 238U
10%
5%
Quad ICPMS
5 pg
1µg
235
U, 236U, 238U
2%
5%
HRICPMS
50 fg
5µg
234
U, 235U, 236U, 238U
1 – 8%
0.1 – 1%
TIMS
1 fg
1 ng
234
U, 235U, 236U, 238U
0.1 – 2%
0.1 – 0.2%
SIMS
5 pg
5 ng
235
U, 238U
1 – 5%
10%
MCICPMS
5-50 fg
1 pg
234
U, 235U, 236U, 238U
0.1 – 0.2%
0.1 – 0.2%
Modified from Toole et al, 1997
Sensitivity of ICPMS versus alpha
spectrometry
210
10000
1000
228
Po
Th
208
Po
232
Minimum activity (Bq)
100
10
U
209
238
241
Pu
Am
226
1
0.1
243
Am/240Pu
230
Ra
239
0.01
Po
Th
233
U
242
Pu
234
0.001
U
237
Pu
Np
236
0.0001
235
0.00001
0.000001
0.0000001
1E-01
U
1E+01
1E+03
U
2 x 106 years
238
1E+05
1E+09
1E+07
Half life (years)
U
232
Th
1E+11
Assuming 10ppt limit
5ml solution
Sensitivity of MC-ICPMS versus alpha
spectrometry
210
1
0.1
228
Minimum activity (Bq)
0.01
0.001
Th
Po
208
Po
232
U
209
238
Pu
241
Po
Am
243
230
226
Ra
0.0001
1E-05
239
1E-06
Am/ 240Pu
Th
233
U
242
Pu
Pu
234
1E-07
U
237
Np
236
1E-08
235
1E-09
1E-10
1E-11
1E-01
U
ca . 300 years
1E+01
U
238
U
1E+03
1E+05
1E+07
Half life (years)
1E+09
232
Th
1E+11
Assuming 5ppq limit
1ml solution
Challenges
•
•
•
•
•
•
•
Isobaric interferences (e.g. 99Ru on 99Tc)
Polyatomic interferences (e.g. 197Au40Ar on 237Np)
Peak tailing
Isotopic fractionation
Beam instability
Matrix effects
Method blanks
High Precision
Pu Isotope Ratio
Measurements
Ian W. Croudace
Thorsten Warneke
Phillip E. Warwick
Rex N. Taylor
J. Andy Milton
Geosciences Advisory Unit
Southampton Oceanography Centre
University of Southampton, UK.
www. gau.org.uk
240
239
240
Analytical
techniques
used
for
Pu/
Analytical techniques used for Pu/239Pu
Pu
METHOD
ADVANTAGE
DISADVANTAGE
NOMINAL
PRECISION
2 sigma
~50 fg
Alpha
Spectrometry
Not suitable because alpha energies interfere
AMS
Can measure ~50fg
High potential cost
~ 18%
ICPMS Quad
High ionisation
efficiency
Ion beam instability
> 30%
ICPMS Sector
High ionisation
efficiency
Better stability than
ICPMS Quad
~ 3%
TIMS
Stable ion beam
•
Low ionisation
efficiency
~ 10%
•
No internal
interelement
fractionation
correction
MC-ICPMS
•
Unstable ion beam
but multicollection
negates this effect
•
Interelement mass
fractionation
correction
capability
•
Can measure 5 fg
High ionisation
efficiency
~ 1%
Some applications for 240Pu/239Pu ratios
Source characterisation
Analogous to using 238Pu/239,240Pu
but is a clearer discriminator
e.g. the 240Pu/239Pu in weapon’s
testing depend on the parameters of
each individual test . Therefore the
240Pu/239Pu in the fallout varies with
time.
Dating
using impulse and
continuous events
Similar to using 239,240Pu, 241Am or
137Cs
The 240Pu/239Pu versus time has
significant features that can be
attributed to certain years.
Range
Range of
of Pu
Pu isotope
isotope ratios
ratios
Gas
Cooled
Reactor
Presurised Advanced Pressure Tube
Boiling
Gas Cooled
Water
Water
reactor
Reactor
Reactor
Boiling
Water
Reactor
Sellafield discharge
1950s
Power reactors
Recent
Chernobyl accident
Average weapon test
Weapon grade
Weapon (modern - Los Alamos National Laboratories)
Weapon (pre-1960 Los Alamos National Laboratories)
0
0.2
0.4
240
Pu/239Pu
0.6
0.8
Stage 1 piggy-back columns
1.
Load sample in 10 ml 8M HNO3 with 1drop of concentrated HCl
2.
Elute 20 ml 8M HNO3 followed by 30ml of 3M HNO3
6 x 0.7 cm i.d.
Eichrom 1-X8 ANION RESIN
Pu
2 x 0.7 cm i.d.
Eichrom UTEVA resin
U
STAGE 2
Separate the columns
30ml 3M HNO3
25ml 9M HCl to remove Th
Pu eluted with 50 ml of
fresh 1.2M HCl/H2O2
(50:1)
U eluted with 10ml 0.02M HCl
Anion
UTEVA
Pu
U
Pu
Plus use a small 2
column
nd
U
anion clean-up
238
Removal of any U remaining because U
239
hydride interferes with Pu measurement.
Plutonium
Plutonium isotope
isotope ratio
ratio measurement
measurement
Objective:
To measure 240Pu/239Pu with a reproducibility and
accuracy <5% 2sd on samples containing <50fg Pu
(<150 µBq), to enable the analysis of low-level
environmental samples.
Method:
Multicollector ICP-MS (Micromass IsoProbe) using :
Peak jump ion counting through a Daly detector with
inter-peak normalisation to 236U.
Corrections required:
1. On-peak blank
subtraction including
detector zero
2. Tail from 238U at +1
a.m.u. (200 ppb)
3.
238UH+
interference at
m/z 239 (5.5 ppm)
4. Pu addition from
236+233U spike (2 ppm)
5. Mass bias of U-Pu
(0.6% amu-1)
counts.
-1
sec
300
200
239Pu
238U
tail
238UH+
100
0
Acid
blank
238.5
239.0
239.5
m/z 239 composition using:
25 ppq Pu
30 ppb U
no 233+236U
Sequence
1
Sequence
2
Sequence
3
Axial Daly
240
239
242
Low 1 Faraday
(237)
236
(239)
Low 2 Faraday
236
(235)
238
Low 3 Faraday
(234)
233
236
Detector and peak-jump array for Pu isotope
ratios using U-double spike
Taylor R.N., Warneke T., Milton J.A., Croudace I.W., Warwick P.E. and Nesbitt R.W. (2001) Plutonium
isotope ratio analysis at fg to ng levels by multicollector ICP-MS. J. Anal. At. Spectrom., 16, 279-284.
236U-233U
(1:1) added to separated Pu solutions to correct for
mass bias and instrument drift between Daly peak jumps
-0.2%
-0.3%
242
-0.4%
Pu/239Pu
mass bias
amu-1
-0.5%
-0.6%
-0.7%
-0.7% -0.6% -0.5% -0.4% -0.3% -0.2%
236
U/233U mass bias amu-1
1ng 239Pu = 2.3 Bq
10
pg ml-1
I
1
pg ml-1
I
100
pg ml-1
I
0.230
0.225
240
Pu/
239
Pu
0.235
100
fg ml-1
I
Therefore only ~1g of a typical UK soil
with fallout Pu of 0.3 Bq/Kg is needed
for a precise analysis of the Pu isotope ratio
0.220
Rothamsted
grass
MC-ICP-MS Site 112
1 gram
UK soil
0.215
0.00001
0.0001
0.001
239
Pu Volts
0.01
0.1
0.26
RIMS
this study
0.24
MC-ICP PJD
0.22
240
Pu
0.20
239
Pu
ICP
AMS
TIMS
0.18
ICP
TIMS
0.16
0.14
1
10
100
1000
femtograms (g.10-15) of Pu analysed
10000
240
Accuracy for
Certified
240
Pu/ Pu atom ratios
239
Pu/
NBL 126
NBL 128
Measured
Pu
Atom ratio
NBL122
239
0.1320
0.0209
0.0007
240
239
Pu/
Pu
0.5 ng/ml
5 ng/ml
0.1318 ± 0.001
0.1321 ± 0.0001
(n=4)
(n=3)
0.0211
0.0204
(n=1)
(n=1)
-
0.0007
(n=1)
UK-Pu-5
0.9662 ± 0.0011
-
0.9645 ± 0.0013
(n=7)
NBL –US New Brunswick National Laboratory; UK-Pu-5 - AEA Technology
An application of the developed method
Establishing a northern latitude fallout record
1 Rothamsted Grass Archive
(IACR Rothamsted, Harpenden)
Unique collection of
herbage and soil since
1843. Samples collected
and stored annually or biannually
Given permission to take
50 grams of dried grass
from 1945 until 1990
2 Alpine ice core
116 m ice core, Mont Blanc
Warneke et al.
Literature data
Warneke et al.
France &
PRC
ITB Treaty
US &
USSR
Mike + NTS
NTS June 1952
Warneke, Croudace, Warwick & Taylor (2002) Earth Planetary Science Letters, 203, 1047-57
Warneke, Croudace, Warwick & Taylor (2002) EPSL, 203, 1047-57
Uranium isotope - 238U/235U fallout record - Europe
2sd error
2sd error
-52
1990
Alpine Ice Core
-57
-62
1980
-67
-72
year
1975
-77
1970
-82
1965
-87
-92
1960
-97
1955
-102
-107
1950
1945
137
-112
137.5
138
238
U/235U
138.5
125
130
135
238
U/235U
-117
140
depth (m)
1985
Rothamsted
Grass (UK)
Summary
MC-ICP-MS is a highly effective method to measure
240Pu/239Pu in environmental and other samples.
Precise measurements are possible at <10 fg Pu (<30 µBq).
Used to investigate fallout history, global and local nuclear
events, sediment ages in estuarine environments, source of
plutonium contamination.
Other Possible Future Application
Has great potential in plutonium and uranium bioassay
Precise U isotopic analysis using only 50ml of sample
Typically 1-2 litres urine (bulked monthly) are analysed
using alpha spectrometry
Clearly MC-ICPMS can greatly enhance these data quality