Applied Radiation and Isotopes 90 (2014) 170–176
Contents lists available at ScienceDirect
Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Evaluation of plating conditions for the recovery of
on a Ag planchet
210
Po
Hyun Mi Lee a, Gi Hoon Hong a,n, Mark Baskaran b, Suk Hyun Kim a, Young ILL Kim a
a
b
Korea Institute of Ocean Science and Technology, Ansan, Kyonggi 426-744, South Korea
Department of Geology, Wayne State University, Detroit, MI 48202, USA
H I G H L I G H T S
Various methods of extraction of 210Po from seawater samples and subsequent plating onto silver planchet were compared.
The 210Po preconcentration using manganese rather than iron was found to yield sharper peak resolution in alpha spectrum.
The maximum plating efficiency was found to be achievable at 15 hours of constant stirring at room temperature.
art ic l e i nf o
a b s t r a c t
Article history:
Received 14 August 2012
Received in revised form
26 February 2014
Accepted 26 March 2014
Available online 13 April 2014
The polonium-210 in the sea and its radiological consequences have been widely studied. Current
processes for 210Po recovery from seawater vary significantly. We compared selected processes to
determine optimal conditions for recovery in modestly equipped laboratories. Plating 210Po onto a Ag
planchet with constant stirring for 15 h at room temperature after preconcentration from seawater
samples with Mn was preferred, achieving more than 96% recovery with 3% or less precision. Possible
contaminants were masked only by ascorbic acid treatment.
& 2014 Elsevier Ltd. All rights reserved.
Keywords:
Po-210
Plating procedure
Optimization
Silver planchet
Seawater
1. Introduction
In contrast to earlier claims of the full recovery of 210Po from
a plating solution (e.g., Heussner et al., 1990), the quantitative
recovery of 210Po by its spontaneous plating onto silver planchets
has proven difficult under modestly equipped marine laboratory
conditions. The quantitative removal of polonium, though not
essential for the determination of 210Po backed by a yield tracer
(209Po or 208Po), is essential for the subsequent determination of
210
Pb in the same sample, as residual Po (both 210Po and tracer) could
affect the determination of in situ 210Pb via the determination of
ingrown 210Po. Therefore, the solution remaining after 210Po plating
must be subjected to anion exchange resin treatment to remove the
remaining in situ 210Po and added tracer (Baskaran et al., 2009).
In general, if the 210Po concentration is high ( 100 Bq), it can be
counted easily using a liquid scintillation counter (LSC). LSC is usually
applied in laboratory simulations of 210Po uptake by bacteria, plants,
n
Corresponding author.
E-mail address: [email protected] (G.H. Hong).
http://dx.doi.org/10.1016/j.apradiso.2014.03.025
0969-8043/& 2014 Elsevier Ltd. All rights reserved.
and animals (Moroshima et al., 2001; Stewart and Fisher, 2003a,
2003b; Wallner, 1997) for rapid analysis (on the order of minutes).
However, when the total activity of 210Po is 2 mBq or less, as is
commonly found in natural environmental samples (e.g., a relatively
small volume of seawater), alpha particle spectrometry appears to be
the only option to accurately determine the activity. Samples must be
counted in an alpha spectrometer for a few to several days using lowbackground surface barrier detectors in order to accumulate enough
counts for statistically meaningful data.
When polonium is in contact with a less noble metal surface
(e.g., nickel, copper, iron, or silver) in aqueous solution, it is
reduced (E0 ¼ þ0.76 V for Po/Po4 þ ) and deposited onto the metal
surface with a standard reduction potential that is higher than
polonium, such as silver (E0 ¼ 0.7996 V for Ag þ /Ag) (Vanýsek,
2005; Johansson, 2008). Spontaneous plating has been used
widely to isolate 210Po from environmental matrices. Since the
late 1960s, radio-Po has been plated spontaneously onto polished
silver (Flynn, 1968), nickel (Figgins, 1961; Lepore et al., 2009), or
copper planchets (Bonotto et al., 2009). It also has been electrodeposited onto a stainless steel planchet by applying external
electricity (Uesugi et al., 2010), or by using a membrane filter
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
impregnated with Ag (Moskvin et al., 1993). Between Ni and Ag,
the polished Ag planchet was found to yield a higher recovery
(Johansson, 2008), and it has been overwhelmingly used by the
marine science community (Baskaran et al., 2009). 210Po that has
been plated onto a metal surface is reported to evaporate under
vacuum, where solid-state silicon detectors accumulate signals
from alpha particles emanating from 210Po. However, 210Po plated
onto a Ag planchet was reported to evaporate the least under
vacuum and was less likely to contaminate the solid-state silicon
detectors compared to Ni and Cu (Henricsson et al., 2011).
For plating onto a Ag planchet, it is surprising to note that the
plating conditions vary widely, e.g., the planchet size, the plating
solution pH, plating temperature and duration, presence or absence
of stirring, and the use of chelating agents or resin to remove the
potentially interfering ionic contaminants in the aqueous solution.
However, systematic comparisons of these plating variables are rare
for marine environmental samples, including seawater, although
recently, comprehensive reviews of analytical methodologies for
determining 210Po in environmental freshwater materials were
published by Benedik and Vrecek (2001) and Matthews et al.
(2007). In this study, we attempted to evaluate previously reported
conditions for plating 210Po onto a Ag planchet to determine the
most suitable conditions for a modestly equipped ordinary laboratory setting for various marine scientific research purposes.
2. Materials and procedures
We prepared a 210Po stock solution by dissolving a uranium
reference material (RGU-1) from the International Atomic Energy
Agency (IAEA). RGU-1 was originally prepared by dilution of the
uranium reference ore BL-5 with a silica matrix of negligible U and
Th content to give a final U concentration of 400 72.1 μg U g 1.
BL-5 was certified to be in secular equilibrium with 226Ra and 210Pb
(IAEA, 1987). 210Po was calculated to be 4.89 70.11 Bq g 1 and
assumed to be in secular equilibrium with 238U. 226Ra and 210Pb
were determined using a low-background high-purity germanium
(HPGe) detector (Canberra GCW2523, 25% relative efficiency) and
found to be 4.83 70.05 Bq g 1 and 4.99 70.21 Bq g 1, respectively. This 210Po stock solution was used as tracer for evaluating
various plating variables throughout these experiments. A known
amount of 209Po was used to convert counts per second (CPS) to
disintegrations per second (Bq).
Silver planchets (99.9% Ag, Aldrichs) were used as substrates
for the spontaneous plating of 210Po. The planchets were prepared
by polishing one side with commonly available silver polish,
washing with acetone, and drying in air. A thin layer of enamel
paint (or commercial nail polish) was applied with a small brush
to the unpolished side of the planchet, cleaned with distilled
water, and dried in air.
Tests of various plating conditions were carried out by spiking
the RGU-1 stock solution (1 mL) into one of three water samples
(1 L): seawater (salinity of 34.6) collected from the Korea Strait;
well water drawn from 25 m below the ground in the middle of
the Korean Peninsula; and deionized water. The aqueous samples
were magnetically stirred for 30 min after addition of the 209Po
tracer (ca. 65 mBq) to ensure homogeneous mixing with the 210Po
in the sample.
Plating was accomplished by suspending the Ag planchet in the
upper part of the plating solution contained in a beaker (glass
or Teflons) with constant gentle agitation via magnetic stirring.
The beaker was covered with a lid. A magnetic stirrer/heater with
a temperature sensor (IKAs RCT basic safety control) was used to
control the plating solution temperature in the beaker with a
precision of 71 1C at 90 1C. After the designated time (1–144 h),
the planchet was retrieved, rinsed gently with distilled water, and
171
dried in air for 3 h. The 210Po activity was assayed with an Alpha
spectrometer (Canberra series 35 MCA, equipped with an A45018AM PIPS detector with an active surface area of 450 mm2).
The background activity for each detector was carefully monitored
and subtracted from the total counts for each region of interest.
The evaluation processes were iterated based on the previously
established optimum conditions for each variable until the best
recovery was obtained. Therefore, the data presented here represent the best available comparison of each plating variable when
the other variables were fixed at the optimum conditions.
3. Results and discussion
The conditions used for plating 210Po onto the Ag planchets in
this study are summarized in Table 1.
3.1. Silver planchet size
The diameters of the Ag planchets used in prior research
differed from 10 mm (Murray et al., 2005) to 50 mm (SanchezCabeza et al., 1998). To evaluate the effect of planchet size on the
recovery of 210Po from the plating solution, five planchets with
diameters ranging from 13 mm (surface area: 133 mm2) to 50 mm
(surface area: 1963 mm2) were compared for 210Po recovery with
three replicates (Fig. 1). The 24 mm planchet yielded the highest
recovery (97 72%). Uncertainties were estimated by the propagation of counting and pipetting errors. Due to differences in the
solid angle subtended by the Ag planchets, the alpha detector
efficiency was lower for areas that were smaller or larger than the
size of the active area of the detector. The detection efficiencies
for a 13 mm (133 mm2) and 24 mm diameter (452 mm2) 0.01 μCi
241
Am source (Eckert Ziegler Analytics) were 0.27–0.34 and 0.22–
0.25, respectively, for three different detectors of the same size
(A450-18AM PIPS) coupled to a Canberra Series 35 multichannel
analyzer, due to the lower alpha detector efficiency of the larger
surface area planchet. However, the overall yield was highest for
the 24 mm diameter planchet. Therefore, we recommend using Ag
planchets sized identically to the alpha detector for better yields.
3.2. Acidity of the plating solution
Because Ag is soluble in nitric acid (HNO3), any nitric acid used
prior to plating, such as for the acid digestion of solid marine
samples, should be exchanged for hydrochloric acid. The molar
concentrations of HCl in the plating solutions in prior reports
varied from 0.01 (pH 2) (Flynn, 1968) to 1 M HCl (pH 0) (Desideri
et al., 2009). We assessed seven solutions with pH values ranging
from 0.3 to 2 and obtained the maximum recovery at pH 0.5 (ca.
0.3 M HCl). Lower recoveries resulted for pH values both higher
and lower than pH 0.5 (Fig. 2). Therefore, the recommended
acidity of the plating solution is pH 0.5.
3.3. Heating and plating time
The volatility of polonium during sample treatment depends
upon factors such as its oxidation state, chemical species, temperature, and the duration of the particular chemical and heat
treatments. 210Po and its tracer evaporation during digestion
and plating has always been of concern, particularly during the
acid digestion of solid samples. Polonium begins to evaporate
at temperatures above 100 1C. Organic complexes and halides of
polonium are particularly volatile even below 200 1C (Matthews et
al., 2007). 210Po losses due to evaporation may remain well below
1% in the temperature range 150–250 1C, and rise to 14% to 90% at
300 1C to 600 1C (Pankratov et al., 2003; Jia and Torri, 2007).
172
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
Table 1
Conditions for plating
210
Po onto silver planchets.
Deposition conditions
Yield (%)
pH
Fe: ascorbic acid (w/w)
Silver planchet size (diameter)
Spontaneous deposition onto the silver planchet
50 mm
30 mm
24 mm
18 mm
13 mm
90 7 5 1C
Active area (mm2)
907 1 1C
Room temperature
Teflons beaker
Glass beaker
Preconcentration and purification
Fe-Sr-resin-EC-AAa
Fe-DDTC-SE-AAb
Fe-AAc
Mn-AAd
2.0
1.0
0.75
0.5
0.3
0.0
0.3
0.05
0.1
0.2
0.3
0.35
0.4
1963.50
706.86
452.39
254.47
132.73
1h
2h
3h
4h
6h
8h
1h
2h
3h
5h
1h
3h
6h
8h
15 h
24 h
48 h
72 h
96 h
144 h
6h
15 h
23 h
72 h
96 h
144 h
Seawater
Groundwater
Seawater
Groundwater
Seawater
Groundwater
Distilled water
Seawater
Distilled water
86.17 2.4
87.4 7 2.6
91.17 2.9
93.17 2.9
92.3 7 2.6
82.8 7 2.6
76.9 7 2.5
2.6 7 0.5
7.6 7 0.5
86.0 7 6.3
93.17 5.2
97.6 7 2.3
94.6 7 5.2
65.247 3.36
85.89 7 3.77
98.96 7 4.16
87.29 7 3.58
86.95 7 3.73
43.5 7 1.4
76.9 7 2.8
86.2 7 3.0
88.6 7 2.7
89.6 7 2.8
91.6 7 2.6
50.17 1.5
69.8 7 1.9
87.3 7 2.6
87.2 7 2.5
17.17 1.5
44.9 7 2.5
77.9 7 4.3
84.5 7 4.1
96.0 7 5.1
96.2 7 5.1
97.7 7 4.4
97.7 7 4.6
96.2 7 4.3
93.9 7 4.4
69.8 7 1.7
84.3 7 3.9
89.5 7 4.0
94.4 7 4.1
96.4 7 2.5
95.3 7 2.0
68.9 7 14.4
59.5 7 13.7
73.4 7 9.1
60.17 11.6
92.3 7 1.6
83.5 7 6.7
94.6 7 0.1
96.2 7 4.0
96.7 7 2.8
a
Iron coprecipitation followed by strontium resin extraction chromatography and ascorbic acid masking.
Iron coprecipitation followed by DDTC solvent extraction and ascorbic acid masking.
c
Iron coprecipitation followed by ascorbic acid.
d
Manganese coprecipitation followed by ascorbic acid masking.
b
The deposition temperatures used in prior research differed
widely from room temperature (Benoit and Hemond, 1988) to 85–
90 1C (Flynn, 1968) or 97 1C (Figgins, 1961). The duration of plating
also varied significantly from 75 min (Flynn, 1968), 3 h (Church et
al., 1994), 6–7 h (Casacuberta et al., 2009), 15 h (Heussner et al.,
1990) to more than 6 d (Benoit and Hemond, 1988). In most cases,
the solutions were usually stirred either continuously using a
magnetic bar placed at the bottom of the beaker or through
intermittent bubbling with an inert gas when the beaker was
placed in a water bath. Under ordinary marine laboratory conditions, maintaining a near-constant temperature in an aqueous
solution on a hot plate for more than 6 h or in a heated water bath
( 90 1C) would seriously limit the number of samples that could
be processed without temperature-controlled devices. Usually,
marine science studies require a few tens or more of samples to
be processed within a relatively short period of time (preferably
within a week or less after sampling) due to in-growth of 210Po
from in situ 210Pb via 210Bi (t1/2 ¼5.01 d).
Therefore, we compared the efficiency of the two most widely
used plating methods: hot plating at 80–90 1C for less than 8 h and
cold plating at room temperature for a longer time. As 210Po is
reported to be strongly adsorbed on glass (Flynn, 1968), glass and
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
higher temperature depending upon the longevity of the plating
experiment. The 210Po activity calculations were based on the midplating date in order to account for the decay of 210Po over the
plating period. The results can be summarized as follows:
Recovery (%)
100
90
80
70
60
50
0
500
1000
1500
2000
Silver planchet active area (mm2)
Fig. 1. Size of Ag planchets versus polonium recovery (%). The active surface area of
the PIPS detector was 450 mm2.
100
Recovery (%)
90
80
70
60
50
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Acidity of plating solution (pH)
Fig. 2. Acidity of the plating solution versus polonium recovery (%).
100
80
Recovery (%)
173
60
Room temperature (Teflon® beaker)
Room temperature (glass beaker)
90±5 °C (Teflon® beaker)
90±1 °C (Teflon® beaker)
40
(1) At 90 71 1C, evaporation occurred progressively with time
(fraction of remaining water¼ 0.995–0.0188 time (h), R2 ¼
0.994); from a 100 mL sample, as much as 8 mL was lost after
3 h, and 13 mL after 7 h. The solution pH decreased slightly,
from pH 0.3 to ca. pH 0.25 after 8 h. The recovery of 210Po
increases with the time of plating, from 43% after 1 h to 92%
after 8 h. The heating experiment was terminated after 8 h, in
consideration of the normal working hours of an ordinary
laboratory and safety concerns surrounding an unattended hot
plate not equipped with an automatic temperature controller,
stable electricity, or other safety features.
(2) At room temperature, evaporation also occurred progressively
with time, but with a much reduced loss of water (fraction
of remaining water¼ 1.000–0.0003 time (h), R2 ¼1.000): as
much as 0.4 mL from an initial 100 mL sample after 15 h.
The pH of the plating solution remained invariant over 15 h
at room temperature. The polonium recovery was found to
be 96 70.1% after 15 h, and increased slightly after 48 h
(98 70.4%).
(3) Among the three methods, hot plating at 90 1C in the Teflons
beaker yielded consistently higher recoveries of polonium on
the Ag planchets than cold plating at room temperature in the
Teflons beaker for 8 h. However, cold plating at room temperature in the Teflons beaker recovered more polonium than
hot plating in the Teflons beaker beyond 10 h. Cold plating at
room temperature in the Teflons beaker recovered more
polonium on the planchet than the glass (Pyrexs) beaker until
the 96th hour, and less at the 144th hour. In the glass beaker
at room temperature, the polonium recovery progressively
increased from 6 h (69.8 71.7%) to 96 h (96.4 72.5%). Considering that continuous heat control for more than 8 h is not
readily available in most marine laboratories, cold plating
at room temperature for 15 h using a Teflon beakers is
recommended for the spontaneous plating of 210Po onto a Ag
planchet.
3.4. Preconcentration of 210Po from seawater samples and masking
foreign ion interference during plating
20
0
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140 150
Plating duration (h)
Fig. 3. Polonium plating efficiencies on Ag planchets: in a Teflons beaker at room
temperature; in a Teflons beaker at 907 1 1C (temperature-controlled heater); and
in a glass beaker at room temperature. The plating efficiencies in a Teflons beaker
at 907 5 1C without automatic temperature controller were also included here for
comparison.
Teflons beakers were also compared for their impact on the
recovery of 210Po on a Ag planchet (Fig. 3). Plating in the Teflons
beaker was compared at 90 71 1C using a hot plate with a
temperature controller (or 907 5 1C using a hot plate without
temperature controller) and at room temperature (ca. 14–30 1C).
The evaporation of the plating solution was suppressed simply by
covering the beaker with a Teflons watch glass. We frequently
checked both the solution temperature and pH (pH test paper,
Toyo Roshi Kaisha Ltd.) during the course of the experiment. The
partial pressure of HCl over aqueous HCl solution increases with
temperature; although the amount is small, it is nonetheless
greater than three orders of magnitude at 90 1C than at 20 1C
(Evans, 1993). Therefore, the pH was expected to vary with time at
Polonium is found in the II, þII, þIV, and þVI oxidation
states in aqueous solution. Of these, þII and þ IV states of
polonium are insoluble and soluble in aqueous solutions, respectively (Johansson, 2008). Polonium easily forms complexes and
adsorbs onto different surfaces in the environment (Figgins, 1961).
In strongly acidic conditions, polonium occurs as Po4 þ and may
form complexes with oxalic and phosphorus acids. In weakly
acidic and neutral groundwater, polonium appears to exist as
PoO(OH) þ , PoO(OH)2, and PoO2, whereas in alkaline conditions, its
predominant form is PoO23 . In the hydroxylated form, (PoO(OH)2),
210
Po is easily adsorbed onto inorganic and organic colloids of
different sizes (Vesterbacka, 2005; Kim and Kim, 2012). Therefore,
prior to the plating step, samples are typically treated with
concentrated HNO3 and HCl along with HF in order to recover
all the 210Po present in the environmental samples, especially
organic-rich lacustrine, estuarine, and coastal marine sediments
(El-Daoushy et al., 1991). Harada et al. (1989) observed that a
tracer did not reach isotopic equilibrium with the natural 210Po in
the groundwater in uranium-enriched phosphate strata, even
though the samples were stored in an acidified state for more
than 24 h after adding the polonium yield tracer. They also found
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
3.5. Ascorbic acid
As described above, iron is present in most environmental
samples and is frequently used to coprecipitate polonium from
seawater as a convenient means to reduce volume. However,
it is well known that iron interferes with the auto-deposition
of 210Po onto Ag planchets (Figgins, 1961; Benoit and Hemond,
1988). Therefore, various iron reducing or complexing agents
(ascorbic acid, sulfite, fluoride, hydrazine hydrochloride, citrate,
100
90
80
70
seawater
60
groundwater
deionized water
50
40
Fig. 4. Polonium recovery (%) versus method of masking interference by foreign
ions during polonium plating on silver planchets. Fe-Sr-EC-AA: iron coprecipitation
followed by strontium resin extraction chromatography, and then ascorbic acid
masking. Fe-DDTC-SE-AA: iron coprecipitation followed by DDTC solvent extraction, and then ascorbic acid masking. Fe-AA: iron coprecipitation followed by
ascorbic acid masking. Mn-AA: manganese coprecipitation followed by ascorbic
acid masking.
100
80
Recovery (%)
that much of the natural polonium failed to co-precipitate with
iron. Therefore, the fraction that co-precipitated with Fe3 þ was
designated as “Fe-scavengeable polonium.”
Seawater generally contains a number of metal ions, including
other natural alpha emitters. Earlier investigators preconcentrated the polonium via ammonium pyrrolidinedithiocarbamate
(APDC) co-precipitation with cobalt (Fleer and Bacon, 1984) or lead
(Kadko, 1993), recovering the resulting precipitates onto Millipores filters or extracting into methyl isobutyl ketone (MIBK) in a
separatory funnel (Radakovitch et al., 1998). Diethyldithiocarbamate (DDTC)-toluene extraction has also been used to isolate 210Po
and 210Bi from Pb2 þ , Fe3 þ , UO2þ , Th þ , and Ra2 þ ions in aqueous
solutions (Uesugi et al., 2010). The DDTC solvent extraction (DDTCSE) method and extraction chromatography using Sr-Resin (Sr-EC)
are still preferred for freshwater samples (Kim et al., 2009).
Iron( þIII) is widely used by the marine scientific community to
preconcentrate 210Po in seawater (e.g., Baskaran et al., 2009). FeCl3
is added to an acidified water sample with a polonium yield tracer
and allowed to equilibrate for several hours. After equilibration,
the solution is brought to pH 7 by adding NH4(OH) to coprecipitate Po with Fe(OH)3. The precipitate can be separated from the
solution by a combination of either decantation–centrifugation or
decantation–filtration though filter paper. The precipitate is then
dissolved in mild HCl solution. The resulting solution is subjected
to ascorbic acid treatment before plating according to methods as
described by the IAEA (2009). The ascorbic acid treatment refers to
the continuous addition of the reductant ascorbic acid until the
yellow color of Fe3 þ disappears after 10 min constant stirring
(Jia and Torri, 2007). This readies the solution for plating.
Manganese has also been used to extract polonium from
seawater samples by adding 0.4 M KMnO4 and MnCl2 4H2O to
the acidified seawater sample with a polonium yield tracer. After
equilibration for several hours, the solution is brought to pH
9.3 with NH4(OH), and precipitates are isolated by centrifugation
or filtration. The precipitate is dissolved in mild HCl solution and a
sufficient amount of ascorbic acid is added to render the solution
colorless. This solution, too, is ready for plating.
Four treatment methods were used for the water samples
to preconcentrate Po and mask the interference of other ions.
We compared (1) Fe-coprecipitation with subsequent ascorbic
acid treatment (Fe-AA method); (2) a combination of Fecoprecipitation with subsequent DDTC-SE and ascorbic acid
treatment (Fe-DDTC-SE-AA method); (3) a combination of Fecoprecipitation with subsequent Sr-resin and ascorbic acid treatment (Fe-Sr-EC-AA method); and (4) Mn-coprecipitation with
subsequent ascorbic acid treatment (Mn-AA method).
The tracer recoveries for the four methods are shown in Fig. 4.
For seawater, the Fe-AA treatment yielded 92 72% tracer recovery,
whereas the Fe-DDTC-SE-AA method produced only 73 79% tracer
recovery. The third method, Fe-DDTC-SE-AA, resulted in 69 714%
tracer recovery. The Mn-AA treatment yielded 967 4% recovery of
the tracer. The ground water samples yielded much lower tracer
recoveries in most cases. Therefore, we conclude that the Mncoprecipitation and ascorbic acid treatment is the preferred
method for isolating polonium from seawater samples, based on
its simplicity and high yield.
Recovery (%)
174
60
40
20
0
0
1
2
3
4
5
Ascorbic acid/Fe (g/g)
Fig. 5. Amount of ascorbic acid relative to added iron (ascorbic acid: Fe, g/g) versus
polonium recovery (%).
and hydroxylamine hydrochloride) have been used to circumvent
this interference. The most commonly used additive is ascorbic
acid (e.g., Flynn, 1968; Thomson and Turekian, 1976). Ascorbic acid
was found to be more effective than hydroxylamine hydrochloride
(Smith and Hamilton, 1984) or a combination of sodium citrate
and hydroxylamine hydrochloride (e.g., Benoit and Hemond, 1988;
Church et al., 1994; Desideri et al., 2009). Flynn (1968) noted that
the amount of reducing reagent needed was dependent upon the
nature and concentrations of the metal species present and the pH
of the plating solution. The mole ratio between iron(III þ) and
ascorbic acid was reported to be 0.5 at pH 0.6 in an iron solution
(Sahasrabudhey et al., 1999). Therefore, we evaluated the effect on
the recovery of 210Po against 100 mg of Fe3 þ (as used generally for
seawater samples) for various amounts of ascorbic acid. The best
yield was observed when the ascorbic acid content amounted to
3.5 times that of the added Fe (w/w) (Fig. 5). Therefore, iron
appeared to form six-coordinate complexes with ascorbic acid in
the plating solution at pH 0.5.
We additionally performed the plating exercise with and without ascorbic acid for the Fe-free solution to see if ascorbic acid
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
enhances spontaneous electrodeposition of polonium onto the Ag
planchet in mild HCl aqueous solution, as mentioned in Johansson
(2008). However, we failed to observe any difference in polonium
recovery with ascorbic acid (95.9 7 2.5%) or without (96.2 7 2.5%).
Millard Jr. (1962) reported that the effect of hydrazine on the
deposition of 210Po was 96% compared to 95% when hydrazine was
absent. Therefore, we may conclude that direct contribution of
ascorbic acid to spontaneous deposition may be negligible at most
under modestly equipped laboratory conditions.
One can generally assume that the polonium alpha peak becomes
better resolved as the interfering ions are increasingly removed
from the plating solution. We determined the full widths at half
maximum (FWHMs) for polonium under the four experimental
1.0
Fe-DDTC-SE-AA
0.8
Fe-Sr-EC-AA
Counts per minute
0.7
preconcentration and masking conditions described in Section 3.4.
The resulting FWHMs of the alpha spectra for the various 210Po
extraction methods from seawater were 18.5 keV, 19.1 keV, 19.3 keV,
and 19.6 keV for the Fe-Sr-EC, Mn-AA, Fe-DDTC-SE, and Fe-Sr-EC-AA
methods, respectively (Fig. 6). Although the Fe-Sr-EC method
afforded a sharper peak than the Mn-AA method, the latter is
preferred for seawater considering its higher tracer yield and greater
economy than the former method.
3.7. Stability of covering material on the back of silver planchet
during plating
3.6. Alpha peak resolution
0.9
175
Fe-AA
0.6
Mn-AA
0.5
0.4
To avoid counting both sides of the planchet (Figgins, 1961),
various agents have been applied to cover one side of the Ag
planchet while exposing the remaining side to the plating solution. These agents include glyptal (Millard Jr., 1962), silicon grease
(Benoit and Hemond, 1988), and Scotchs tape. Another approach
is to mount the planchet on rods suspended in the plating solution
(e.g., Flynn, 1968; Jia and Torri, 2007). However, enamel paint and
Scotchs tape are used most widely in marine laboratories. In our
tests, Scotch tapes and painted enamel coverings were found to
be deformed upon inspection after the first hour of plating at
90 1C. However, the surface with enamel paint was intact after
15 h at room temperature (Fig. 7). The Scotchs tape was detached
after 15 h at room temperature. Therefore, enamel paint is recommended for covering one side of Ag planchet.
3.8.
210
Po contents in stable lead yield tracers
0.3
0.2
0.1
0.0
5.20
5.22
5.24
5.26
5.28
5.30
5.32
Eα (MeV)
Fig. 6. Full widths at half maximum (FWHM) of alpha spectra for various methods
of polonium extraction from seawater. Fe-Sr-EC-AA, Fe-DDTC-SE-AA, Fe-AA, and
Mn-AA are as defined in Fig. 4.
About 10 mg Pb is usually added prior to the digestion of solid
marine samples or is co-precipitated with iron or manganese in
seawater samples to determine the chemical yield for 210Pb after
210
Po analysis. The 210Po contents were determined in currently
available Pb(NO3)2 (Sigma-Aldrich, Batch # 02218HJ) and PbS
(galena, Sigma-Aldrich, Lot # MKBG98009V) and were found to
contain 65.8 7 24.9 and 4.17 2.2 mBq 210Po per gram Pb, respectively. Therefore, galena is the preferred choice and the 210Po as
well as 210Pb content in the stable Pb reagent must be measured in
every batch of 210Po determination experiments.
Fig. 7. Stability of covering materials blocking polonium plating on the silver planchet.
176
H.M. Lee et al. / Applied Radiation and Isotopes 90 (2014) 170–176
4. Conclusions
On the basis of our results, the following practical procedure
for the plating of 210Po onto a Ag planchet in a modestly equipped
marine laboratory is recommended. First, 210Po that has been
coprecipitated from seawater with manganese does not require
further isolation of 210Po prior to plating with an organic chelating
agent or extraction chromatography. However, a small amount of
ascorbic acid may be required to prevent the interference of iron
with 210Po plating onto the Ag planchet. Second, the aqueous
plating solution should be placed in a Teflons or other plastic
beaker with a magnetic bar on a stirrer, and the acidity of the
plating solution should be adjusted to pH 0.5. Third, the Ag
planchet, polished on one side and blocked on the other with
enamel paint should be suspended in the middle of the solution.
Finally, the plating solution should be gently agitated overnight
(4 15 h) at room temperature. Following these recommendations
will result in 96% or better 210Po plating efficiency. Furthermore,
tens of samples could be easily processed in one batch during
a 24 h period immediately after sampling to meet most marine
scientific research needs.
Acknowledgements
The thorough and insightful comments of three anonymous
reviewers are gratefully acknowledged. This study was supported
in part by grants from the Korea Institute of Ocean Science
and Technology (E98564, E98743) to GHH and the U.S. NSF
(OCE-0851032) to MB.
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