Uncertainty Assessment of Alpha-Spectrometry Analysis of 210Po in Marine
Sediment Sample from Jakarta Bay
Ali Arman Lubis
Kelompok Kelautan dan Lingkungan, Pusat Aplikasi Teknologi Isotop dan Radiasi
Badan Tenaga Nuklir Nasional, BATAN
e-mail: alilubis@batan.go.id
Received 21 March 2007, accepted for publication 12 March 2008
Abstract
The expression of measurement uncertainties in a standardized form is a requirement for result reliability as it imposes
implications to the interpretation of analytical results. In this work, the analytical procedure involves sample
preparation steps for drying, dissolution and further radiochemical separation, self deposition and counting of 210Po of
marine sediment sample are discussed. An isotopic tracer (209Po) is used to determine the overall chemical yield and to
ensure traceability to a national standard. The uncertainty assessment of 210Po is discussed using marine sediment
sample from Jakarta Bay with quality assurance based on Standard Reference Material, International Atomic Energy
Agency, No. 300 (SRM IAEA-300) and SRM IAEA-368 for marine sediments. 210Po is measured by a PIPS detector
coupled to a multichannel spectrometer. The contribution of the sources to the expanded standard uncertainty in the
concentration of 210Po in marine sediment is assessed and the sample activity which contributed 71.10% of total is
identified as the major contribution.
Keywords: Uncertainty, Alpha spectrometry method, 210Po, Environmental radionuclide, Marine sediment
Abstrak
Standarisasi ketidakpastian dalam pengukuran merupakan suatu keharusan dalam menyampaikan hasil pengukuran
karena akan mempengaruhi interpretasi hasil analisis. Dalam penelitian ini dibahas analisis sampel sedimen yang
mencakup tahapan preparasi sampel yaitu pengeringan, pemisahan kimia dan radiokimia, deposisi spontan dan
tahapan pengukuran 210Po. Penelusur isotopik 209Po digunakan untuk analisis kuantitatif dan ketertelusuran pada
standar nasional. Kajian ketidakpastian dalam pengukuran 210Po menggunakan sampel sedimen laut dari Teluk Jakarta
dan kualitas pengukuran ditelusuri dengan membandingkan hasil pengukuran sedimen laut dengan SRM IAEA-300 dan
SRM IAEA-368. 210Po diukur dengan spektrometer multi saluran alpha menggunakan detektor PIPS. Selanjutnya
dibahas kontribusi dari komponen ketidakpastian terhadap total ketidakpastian dalam analisis 210Po dalam sedimen laut
dan pengukuran sampel memberikan kontribusi sebesar 71,10% yang merupakan kontribusi paling besar terhadap total
ketidakpastian.
Kata kunci: Ketidakpastian, Metode spektrometri alpha, 210Po, Radionuklida alam, Sediment laut
radionuclide 210Po in marine sediment core samples in
environmental level. Furthermore, the concentration of
210
Po through sediment core which is assumed to be in
equilibrium with 210Pb is applied to determine the age
and sedimentation rate (Lubis, et al., 2005; 2006).
In this work, the uncertainty sources for the
relative method of alpha spectrometry were applied to
determine 210Po in sediment samples as part of the
Quality Assurance System implementation at PATIR,
BATAN. Various uncertainty sources for the
preparation step and for the alpha spectrometry
measurement were considered. The contributions of the
uncertainty sources to the expanded uncertainty in the
concentration of 210Po were assessed. For the accuracy
and sensitivity measurement, a Standard Reference
Material (SRM) IAEA-300 (Baltic sea sediment) and
IAEA-368 (Pacific ocean sediment) were used. These
SRMs contain 210Pb which was determined by several
1. Introduction
The expression of measurement uncertainties in
a standardized form allows the comparison of results
from different laboratories. It is also important in
reaching decisions about the result in complying with
the regulatory limits, as measurement uncertainties
have implications on the interpretation of analytical
results. Due to the growing demand on the quality
assurance of analytical laboratories, the presentation of
analytical results with their related uncertainties is a
recent requirement in method validation and laboratory
accreditation (EURACHEM/CITAC, 2000); (Moreira,
et al., 2006).
The alpha spectrometry method has been
frequently used at the Marine and Environmental
Laboratory of Center for the Application of isotopes
and Radiation Technology (PATIR), National Nuclear
Energy Agency (BATAN) for analysis of natural
54
Lubis, Uncertainty Assessment of Alpha-Spectrometry Analysis of 210Po in Marine 55
laboratories around the world under Proficiency Test
organized by IAEA. The process of uncertainty
assessment includes the specification of the measurand,
the identification of uncertainty sources, the
quantification of the individual standard uncertainties,
the calculation of the combined standard uncertainty,
and the presentation of the expanded uncertainty.
efficiency. Moreover, these mixed
covered the energy of 209Po and 210Po.
4 g dried
sediment
Filtered
and dried
the filtrate
+ 0.4 mL HCl
+ 10 mL HCl
+ 10 mL HNO3
+ 15 mL H2O
+ 4 drops H2O2
2. Experiment
Sediment sample from Jakarta Bay and IAEA300 and IAEA-368 were analyzed to determine 210Po
activity as well as 210Pb activity. Moreover, these SRMs
were used for quality assurance through the accuracies
and sensitivities of analytical methods used in this
study. Wet and dry masses of samples before and after
drying in calibrated oven at 60 οC were determined
using a calibrated Satorius analytical balance, and dry
bulk density and porosity were calculated (IAEATECDOC 1360, 2003).
2.1 Analytical procedures
210
Po analysis were performed according to the
methodology developed by Sanchez-Cabeza, et al.,
(1993). The procedure is as follows: 4 g of dried
homogenized sediment, IAEA-300 and IAEA-368
marine sediment were placed in different beaker glass.
Each of sample and SRM was spiked with 0.4 ml
(0.169 Bq) of 209Po tracer using a calibrated pipette for
the determination of the chemical yield as well as for
quantitative analysis. This tracer was produced by
NIST (National Institute for Standard and Technology)
USA. A mixture of HCl, HNO3 and H2O2 (all from
Merck) and H2O was used to digest the sample. The
remaining sample was filtered and ascorbic acid was
added to complexify any iron present. 209Po and 210Po
in solution were plated onto a copper disc for 8 hours at
room temperature while stirred to produce a thin film.
Detail of the procedure is depicted in a flowchart in
Figure 1. Polonium isotope was counted for
approximately 8 hours using α-spectrometer equipped
with PIPS (Passiveted Implanted Planar Silicon)
detector (Canberra model A450-20AM) with the
resolution 20 keV and active area 450 mm2. The PIPS
detector is optimized for high resolution (≤ 20 keV),
high sensitivity (≥ 25% efficiency) and low background
(≤ 1 count/hr) of alpha spectroscopy. 209Po and 210Po
were determined in the energy of 4.88 MeV and 5.33
MeV, respectively. 210Po was assumed to be in
radioactive equilibrium with 210Pb in the sediment
samples (Ebaid, et al., 2006); (Guogang, et al., 2003).
The calibration and background of detector are
measured monthly. Calibration is performed using a
mixed radionuclides of alpha emitters (238U, 234U, 239Pu
and 241Am) point source which was traceable to NIST
for qualitative analysis and determination of detector
radionuclides
+ 100 mL HCl
+ 400 mg ascorbic acid
Heated in
Waterbath
For + 10
minutes
Dried in
waterbath
+ 10 mL HCl
+ 40 mL H2O
Heated in
Waterbath
For+10
minutes
Self deposition
For alpha
spectrometer
Figure 1. Flowchart of the analytical procedures of
Po.
210
2.2 Specification of the measurand
The measurand is the concentration of 210Po in
sediment sample from Jakarta Bay. The concentration
of 210Po is determined by means of the following
equation (IAEA-TECDOC 1401, 2004):
CA =
AA
f1 f 2 f 3 f 4
mA .q
(1)
in Bq.kg-1 wet weight at the date of sampling with:
⎡ R − RBA
⎤⎛ p ⎞
AA = cTVT ⎢ GA
− q1 ⎥⎜⎜ αT ⎟⎟
⎣ RGT − RBT
⎦⎝ pαA ⎠
(2)
f1 = e ( + λA .(ts −tE ))
(3)
f2 =
λ AtG
1 − e ( −λ
f 3 = e (−λ
T
f4 =
AtG
)
.( t s −t c ))
λT t G
(4)
(5)
(6)
1 − e (−λ t )
where : AA = Activity of the 210Po on the deposition
copper disk (Bq), mA = mass of dried sample used for
the alpha analysis (kg), q = mass ratio (wet/dry) of
sample, cr = certified concentration of tracer solution
209
Po (Bqml-1), at date of calibration, VT = volume of
tracer (209Po) used (ml), RGA = gross counting rate of
the 210Po in s-1 (with counting time tG), RBA = blank
counting rate of the 210Po in s-1 (counting time tB), RGT
= gross counting rate of 209Po in s-1(counting time tG),
RBT = blank counting rate of 209Po in s-1 (counting time
tB), q1 = isotopic impurity ratio of 210Po in the tracer
T G
56 JURNAL MATEMATIKA DAN SAINS, JUNI 2008, VOL. 13 NO. 2
mentioned previously is treated in the discussion of this
paper.
solution, pαT = sum of alpha emission probabilities for
209
Po region of interest, pαA = sum of alpha emission
probabilities for 210Po region of interest, f1 = correction
for decay of 210Po from sampling to measurement, f2 =
correction for decay of the 210Po during the counting
interval period tG, f3 = correction for decay of 209Po
from its calibration date to measurement, f4 = correction
for decay of 209Po during the counting interval period
tG, λA = decay constant of
210
−1
Po in ( s ), λT = decay
−1
constant of 209Po in ( s ), tE = end of sampling time, ts
= beginning of measurement time, tC = time of
calibration of 209Po solution, tG = counting time interval
of the gross counting in s.
The radiochemical yield η of the 209Po, an
indicator of the performance of the procedure, can be
calculated from the following equations:
( R − RBT )
η = GT
εcTVT
ε=
Figure 2. Cause and effect diagram for alpha
spectrometry of analyzing 210Po in sediment sample
2.4 Traceability links
All the parameters used in the alpha
spectrometry method of analysis 210Po in sediment are
traceable in SI unit. The traceability links were
established by means of oven and balance calibration
by Calibration Laboratory, Directorate of Quality
Control, Ministry of Trade, Indonesia (DEPDAG)
using balance weight and thermocouple, respectively.
The calibrated radioactive sources for qualitative and
efficiency were determined by Canberra, USA. The
certified reference solution (209Po) as a tracer for
quantified determination was from NIST.
(7)
( RGPS − RBPS )
AA, PS Ρ
(8)
with ε the detection efficiency for alpha particles, and
RGPS, RBPS, AA,PS and Ρ are the gross counting rate,
background, activity and probability of each
radionuclide in mixed radionuclides point source,
respectively.
3. Results and Discussion
2.3 Identification of uncertainty sources
The data of efficiency (ε) calculated using
equation (8) based on the available data from
certificated mixed radionuclides and extended
uncertainty with the confidence level 95% (k=2) are
shown in Table 1 and Figure 3. Radiochemical yield
(η) of tracer 209Po as an indicator of the performance of
the method was then determined using equation (7).
The result of radiochemical yield is 79.48%. The
calculation of concentration of 210Po in marine
sediment from Jakarta Bay was done with the equations
(1) and (2). The result was 11.837 Bqkg-1.
The uncertainty sources considered in this work
are shown in the cause and effect diagram in Figure 2
(IAEA-TECDOC 1401, 2004) and (Heydorn, 2004). As
can be seen in Figure 2, the individual uncertainty
consists of dry weight of sample, mass ratio, decay
correction of both radionuclides 209Po and 210Pb,
concentration and volume of tracer 209Po, counting
statistics, isotopic impurities and alpha probabilities.
The assessment of the various uncertainty sources as
Table 1. Data of radionuclides used for calibration detector for determining qualitative analysis and detector efficiency
Radionu
clides
238
Energy
(MeV)
Activity
(Bq)
4.198
1.485
U
4.775
1.478
U
239
5.157
1.508
Pu
241
5.486
1.540
Am
* Uncertainty (k=2), 95% confidence level
234
Half-life
(year)
4.47x1009
2.46x1005
2.41x1004
4.32x1002
Probability
(%)
79
71.38
73.38
84.5
Expanded
uncertainty*
(Bq)
0.0344
0.0333
0.0344
0.0356
Efficiency
detector
0.1806
0.1945
0.1743
0.1622
Expanded
uncertainty of
efficiency*
0.0262
0.0297
0.0224
0.0181
Lubis, Uncertainty Assessment of Alpha-Spectrometry Analysis of 210Po in Marine 57
0.25
Efficiency
0.2
0.15
Eff = -0.0065*(Energy)2 + 0.025*(Energy) + 0.1643
R = 0.914
0.1
0.05
0
4.198
4.775
5.157
5.486
Energy (MeV)
Figure 3. Efficiency curve of detector PIPS using
point source of mixed radionuclides 238U, 234U, 239Pu
and 241Am with the counting time tG = 18000 s.
Table 2 lists the contributions for combined
standard uncertainty based on cause and effect in
determining the uncertainty of 210Po in marine sample
(Figure 2).
The uncertainty of sample mass (mA) was
combined standard uncertainty from repeatability and
calibration. The repeatability contribution, a type A
uncertainty, was taken from a control chart of 0.1 g
measurements, with number of measurements were 10
times. The other contributions were taken from the
balance calibration certificate. Similarly, the combined
uncertainty of the mass ratio (q) of wet and dry sample
was from the sample mass (repeatability and
calibration) of wet and dry sample and the calibrated
oven. Combined uncertainty of sample mass u m was
0.000094 kg and mass ratio u q was 0.064604.
Furthermore, the contribution of mA and q to total
(u C ) 2 are 2.46% and 7.58%, respectively (Table 2 and
Figure 4).
The uncertainty in the concentration of standard
tracer depends on the certified concentration values for
the standard solutions (taken from the solution
certificates) and on the volume of the solution pipetted
onto the samples. The following sources of uncertainty
were considered: volume repeatability, the statements
by the pipettor producers about the imprecision of
pipettors in dispensing liquids and the uncertainty from
volume expansion due to the temperature at the time of
pipettor calibration. Moreover, a variation of ± 30C of
the environment temperature will result in an
uncertainty of 3.2x10-4 ml. The combined standard
uncertainties were 0.009587 ml and 0.005 Bqml-1 for
the volume uVT and the concentration u cT of 209Po,
respectively. The contributions of both parameters to
total (u C ) 2 are 0.65% and 2.67% for the volume and
concentration of tracer 209Po.
Table 2. Individual uncertainty components and contributions to the square of the combined uncertainty
Reference list and
symbol
Dry weight for analysis (mA)
Value of variable
Uncertainty
( uC )
Percent contribution to (u C ) 2
0.004 kg
0.000094 kg
2.46
1.53521
0.064604
7.58
1.558601
1.000369
0.99343
1.000001
0.012051
0.006262
0.008841
0.008984
0.28
0.18
0.37
0.37
0.423 Bq.ml-1
0.005 Bq.ml-1
0.65
0.4 ml
0.009587 ml
2.67
0.003074 s-1
0.000008 s-1
0.010822 s-1
0.000007 s-1
0.000338 s-1
0.000007 s-1
0.000541 s-1
0.000064 s-1
71.10
3.20
10.75
0.17
0.0011
0.00024
0.003
1.000
1.000
0.0024
0.0064
0.03
0.19
Mass ratio
( q)
Decay corrections:
f1
f2
f3
f4
209
Po concentration
( cT )
209
Volume of Po
( V T)
Counting rates:
RGA
RBA
RGT
RBT
Isotopic impurity
( q 1)
Alpha probabilities:
pαT
pαA
Component uncertainty
58 JURNAL MATEMATIKA DAN SAINS, JUNI 2008, VOL. 13 NO. 2
paA
paT
q1
RBT
RGT
RBA
RGA
VT
cT
f4
f3
f2
f1
q
ma
0
5
10
% Contribution to (uc)
70
80
2
Figure 4. Contribution of individual uncertainties to
the total of uncertainties of sediment sample.
The contribution of the decay constants
( f1 , f 2 , f 3 and f 4 ) depend on the uncertainty of the
half-life of the radionuclides (equation 3 to 6). Halflives were converted to seconds and their uncertainties
in decay constant were propagated as exponential
uncertainties. Uncertainties due to the decay time of
samples and standard may be neglected for the
radionuclide under consideration as these uncertainties
contributed only 1.20% of (uC ) 2 .
The counting statistics component to uncertainty
is available from the measurement result as the square
root of the measured activity, as it follows the Poisson
distribution. The contribution of counting statistics of
measurement needs to be considered because this is the
most important contribution to the uncertainty in
nuclear analytical methods. The combined standard
uncertainty originated from the gross counting rate of
analyte and tracer and the gross counting rate of
background at the same energy of analyte and tracer.
The background was the averaged of a series of blank
analyses (n=10). In this case, the contributions from the
peak overlap and covariant were neglected due to the
non-overlapping peak amongst tracer 209Po, sample
210
Po and other isotopes impurities. Accordingly, from
Table 2 and Figure 4, the total contribution of all
counting statistics, namely; gross counting rates of
sample and tracer and gross counting rates of
background sample and tracer, are 85.22% with
71.10%, 10.75%, 3.20% and 0.17% for RGA, RGT, RBA
and RBT, respectively.
From all contributions of uncertainties which
has been mentioned above, the result of the calculation
of analyzing 210Po (equilibration with 210Pb) in the
sample was 11.837 Bqkg-1 ± 3.270 Bqkg-1 with 95%
confidence level.
Table 3 presents the 210Po concentration results
obtained in this work for the SRMs with associated
expanded uncertainties. For comparison, certified
values are also presented. A good agreement is
observed between the obtained results and certified
values with the differences are 2.44% and 5.67% for
SRM IAEA-300 and IAEA-368, respectively. For 210Pb
which is assumed to be in equilibrium with 210Po on the
reported uncertainties are in the same order of
magnitude of the certified values uncertainties, showing
the suitability of the alpha spectrometry method to the
analysis of sediments.
4. Conclusion
This paper shows the various steps involved in
the expanded uncertainty assessment for the
concentration of 210Po in sediment sample by alpha
spectrometry method. Sample activity was the most
important source of uncertainty. The alpha counting
statistics was the strongest contribution to the activity
uncertainty. The percentage of contribution to the total
uncertainty was approximately more than 80%.
Table 3. 210Pb (in equilibrium with 210Po) in SRM IAEA-300 and IAEA-368 in comparing to the certified values
No
SRM
1.
IAEA-300
2.
IAEA-368
Certified value 210Pb (Bq.kg-1)
This work 210Pb (Bq.kg-1)
360
(339 – 395)
23.2
(19.8 – 27.2)
368.77
(311.85 – 397.23)
24.52
(18.42 – 30.62)
♦ Range of certified values and results in parentheses
♦ k=2, level of confidence = 95%.
Difference
(%)
2.44
5.67
Lubis, Uncertainty Assessment of Alpha-Spectrometry Analysis of 210Po in Marine 59
References
Ballestra, S. and T. Hamilton, 1994, Basic Procedures
Manual
Radiochemistry,
IAEA-Marine
Environment Laboratory, Monaco, 75-79.
Ebaid, Y. Y. and A. E. M. Khater, 2006, Determination
of 210Pb in environmental samples, J.
Radioanal Nucl. Ch., 270:3, 609-619.
EURACHEM/CITAC guide, Quantifying Uncertainty
in Analytical Measurement, 2000, 2nd Ed. UK.
Guogang J., M. Belli, M. Blasi, A. Marchetti, S.
Rosamilia,
and
U.
Sansone,
2001,
Determination of 210Pb and 210Po in Mineral
and Biological Environmental Samples, J.
Radioanal Nucl. Ch., 247:3, 491-499.
Guogang J., M. Belli, M. Blasi, A. Marchetti, S.
Rosamilia, and U. Sansone, 2003, 210Pb and
210
Po Concentrations in the Venice Lagoon
Ecosystem (Italy) and the Potential
Radiological Impact to the Local Public and
Environment, J. Radioanal Nucl. Ch., 256:3,
513-528.
Heydorn, K., 2004, Evaluation of the Uncertainty of
Environmental Measurements of Radioactivity, J. Radioanal Nucl. Ch., 262:1, 249253.
IAEA-TECDOC 1401, 2004, Quantifying Uncertainty
in
Nuclear
Analytical
Measurements,
International Atomic Energy Agency, Vienna,
127-148.
IAEA-TECDOC 1360, 2003, Collection and
Preparation of Bottom Sediment Samples for
Analysis of Radionuclides and Trace
Elements, International Atomic Energy
Agency, Vienna, 63-82.
Lubis, A. A. and B. Aliyanta, 2006, Preliminary Study
of Sediment Ages and Accumulation Rates in
Jakarta Bay Derived from Depth Profiles of
Unsupported 210Pb, Indo. J. Chem, 6:3, 256260.
Lubis, A. A., B. Aliyanta, S. Yatim, dan Y. Menry,
2005, Estimasi Laju Akumulasi Sedimen
Daerah Teluk Jakarta dengan Teknik
Radionuklida Alam unsupported 210Pb,
Risalah Seminar Ilmiah Litbang Aplikasi
Isotop dan Radiasi, 105-110.
Moreira, E. G., B. A. Vasconcellos, and M. Saiki, 2006,
Uncertainty Assessment in Instrumental
Neutron Activation Analysis of Biological
Materials, J. Radioanal Nucl. Ch., 269:2, 377382.
Sanchez-Cabeza, J. A., P. Masque, W. R. Schell, A.
Palanques, M. Valiente, C. Palet, R. P. Obiol,
and J. P. Cano, 1993, Record of
Anthropogenic Environmental Impact in the
Continental Shelf North of Barcelona city,
Proceeding of a Symposium, IAEA.