Document 6533850

THE EFFECT OF ALKALINE SAMPLE SOLUTIONS ON THE STABILITY OF LIQUID
SCINTILLATION COCKTAILS PREPARED WITH COMMERCIALLY AVAILABLE
SCINTILLANTS: ASSAYS OF 188W-188Re
Jeffrey T Cessna1 Brian E Zimmerman
Physics Laboratory, National Institute of Standards and Technology, 100 Bureau Dr., Stop 8462, Gaithersburg, MD 20899,
USA
ABSTRACT. The effect of alkalinity on the efficiency-traced activity determinations of liquid scintillation (LS) cocktails
containing 188W-188Re has been investigated for several commercial scintillator formulations using the CIEMATINIST
method. Four commercial scintillants were used. Packard Ultima Gold AB (UG), Packard Hionic Fluor (HF), Packard InstaGel XF (IG), and Wallac OptiPhase Hi-Safe III (HS). The aqueous fractions of the cocktails were adjusted by the addition of
between 0.02 g and 1 g of 1.0 molLl NaOH to nominally 10 mL of scintillant, with the exception of the IG cocktails, which
contained 7 mL of scintillant and 5 mL of either water or 1 molL NaOH. The quenching ranges in the cocktails were
adjusted by the addition of a 10 percent, by volume, dilution of nitromethane in ethanol. For efficiency tracing, additional
series of samples were prepared with tritiated water in such a way as to be chemically identical with the 188W-'88Re samples.
The results indicate that the Insta-Gel cocktails prepared with NaOH as the primary aqueous component decomposed, resulting in a separated milky phase, but that those prepared with water as the aqueous component were stable. In addition, Ultima
Gold AB was shown to be unstable in the presence of ' 88W_188Re and gave widely varying results, depending upon the aqueous (NaOH) fraction of the cocktail. The Hionic Fluor and Hi-Safe III cocktails were all stable over time and gave good
results, despite the fact there was an observed difference of about 0.4% in the averages of the massic activities determined
with each cocktail. The Insta-Gel cocktails prepared with water gave activity values that were in excellent agreement with
those obtained with Hi-Safe III.
1
INTRODUCTION
One of the most important radionuclides emerging in the field of nuclear medicine is 188Re (IznagaEscobar 1998; Zimmerman et al. 1999; Arteaga de Murphy et al. 2001), which is conveniently
obtained from a generator prepared from the parent nuclide, 188W (Knapp et al. 1997). Because both
88j and 188Re are 3-emitters, the method of choice for determining the amount of radioactivity
contained in an equilibrium solution of these nuclides is liquid scintillation (LS) counting. Normally
observed efficiencies would be over 99% for the 188Re and slightly more than 94% for the 188W. The
chemical properties of tungsten dictate that the pH of a solution intended to be stable over long periods of time be kept alkaline to prevent the tungstate from forming insoluble tungstic acid. Most
commercially available scintillation fluids are developed for neutral or acidic samples with moderate ion concentrations. Unfortunately, information as to the ability of these scintillants to handle
alkaline samples with moderate to high ion concentration is not widely available. This paper presents the results of recent experiments designed to determine the best cocktail composition, including scintillant, to assay solutions of 188W in secular equilibrium with its daughter, 188Re.
l
A variety of cocktail compositions using commercial scintillants were examined. The choice of
scintillant was based on either the manufacturer's recommendation of their ability to handle basic
solutions, where available (Packard Instrument Company 1997), or previous experience with the
scintillant. For the non-gel forming scintillants-UG, HF, and HS-the effect of aqueous fraction
was studied. The aqueous fraction was adjusted using an alkaline solution resembling the sample
composition, based on our routine practice of employing an analogous system when counting low
pH solutions. In the case of the Insta-Gel, we examined whether it was better to add water or an alkaline solution to produce the gel phase. An aim of the experiment was to determine stable cocktail
compositions in more than one scintillant, as it is NIST practice to make massic activity determina-
1Corresponding author. Email; [email protected].
© 2002 by the Arizona Board of Regents on behalf of the University of Arizona
LSC 2001, Advances in Liquid Scintillation Spectrometry
Edited by Siegurd Mobius, John Noakes, Franz Schbnhofer. Pages 159-168.
159
160
JCessna, B Zimmerman
tions by LS counting in more than one scintillant, utilizing more than one spectrometer, with the
intention to identify possible bias from either of these sources.
METHODS
Approximately 2 mL of solution containing 188W-' 88Re in NaOH (in all instances, 1 mol.L-1) with
an activity concentration of nominally 925 MBq.mL-'(according to the manufacturer) was received
from Oak Ridge National Laboratory (ORNL) approximately 4 months subsequent to target irradiation and solution preparation. This solution was received as part of a NIST calibration, details of
which are described in Zimmerman et al. (2001). To this solution was added an additional 30 mL of
NaOH to bring the total volume to about 32 mL. Because the values of the mass of dissolved target
and amount of inactive target were not supplied by ORNL, the exact ion concentration of the stock
solution is unknown. It is assumed, however, that there was an appreciable amount of inactive tungsten in the dissolved target, therefore no further material was added as carrier. A portion of this solution was used in a series of gravimetric dilutions with NaOH to bring the activity concentration to
an appropriate level for LS counting. The total dilution factor between the starting and final solutions was nominally 1071.
Liquid scintillation samples were prepared in two trials, the first being prepared with scintillants UG
and HF. These samples correspond to series A, B, D, E, F, and G, as described in Table 1. The second
sets of LS samples were prepared with the IG and HS scintillants. These are listed in Table 1 as
series H, I, J, and K.
In the first trial, each series contains eight individual cocktails in three sub-series, consisting of three
1887, three tritiated water, and two blank background samples. Series A, B, and D, and similarly
series E, F, and G, were adjusted in aqueous fraction by the addition of zero mL, 0.5 mL, and 1.0 mL
of NaOH, respectively, by motorized pipette. The cocktail quenching was varied over all sub-series
by the addition of nominally 40 µL, 120 .tL, and 240 µL, of a 10%, by volume, dilution of
nitromethane in ethanol, with an aspirating pycnometer. The background sub-series samples
received 40 µL and 240 µL. Cocktail components were added in the following order: 10 mL scintillant; adjustment of aqueous fraction, if any; quench agent; and finally active solution. The mass of
active solution dispensed with aspirating pycnometer was determined gravimetrically. Cocktails
were not mixed until all components had been added. All cocktails were prepared in conventional
20-mL glass vials, so that any visible changes or separation of the cocktail could be monitored.
All cocktails were sequentially counted for nine or ten cycles each, in three spectrometers. The spectrometers, in the order used, were a Wallac Guardian model 1414, a Beckman model 78000, and a
Packard Tri-Garb model A2500. Counting times were either 600 or 900 s, with sample count rates
to 3,500 1.
ranging from 800
s
1
s
After appropriate background subtraction and decay correction, data were first considered as count
rates and then were converted to activity by the CIEMAT/NIST method of tritium efficiency tracing
(Coursey et al. 1986; Zimmerman and Colle 1997). NIST uses a modified version of the program
EFFY4, which is an updated version of EFFY2 (Garcia-Torano and Grau Malonda 1985), to calculate the efficiency versus an independent figure of merit for the tritium standard and the radionuclide
of unknown activity, over a range of quenching. By observing the experimental change in efficiency
versus the change in quench indicating parameter for the tritium standard, this method is used to calculate the expected change in efficiency based on the theoretical spectrum for the tritium and that for
the radionuclide of interest. The input values used for the EFFY4 program are listed in Table 2. The
half live values used for decay corrections were 69.783 days ± 0.048 days for 188W-'88Re (in secular
Table
Description of LS cocktail compositions. Series identifiers are assigned to cocktails of similar aqueous fraction with subscripts
w, T and B referring to 188W-188Re tritium and background, respectivelY. All added components are listed. Aqueous fraction was adjusted
- NaOH or
with either 1 molL1
o
water, where «sample» refers to the amount listed due to added activity. The quenching agent used was a 10/o
by volume, dilution of nitromethane in ethanol and the volumes are nominal values based on an assumed volume of 20
µL per dispensed drop.
Key: Key: UG-Packard Ultima Gold AB; HF-Packard Hionic-Fluor IG-Packard Insta-Gel XF HS-Wallac 0PtiPhase Hi-Safe III.
1
Number
188W
Scintillant
Scintillant
volume
(mL)
3
UG
10
0.03
AT
3
UG
10
BW
3
UG
10
BT
3
UG
10
DW
3
UG
10
DT
3
UG
10
AB
2
UG
10
BB
2
UG
10
DB
2
UG
10
EW
3
HF
10
ET
3
HF
10
FW
3
HF
10
FT
3
HF
10
Series
of
ID
samples
AW
1
(g)
solution
3H
solution
(g)
-
0.02
0.03
0.02
0.03
-
0.02
-
0.03
0.03
0.02
0.02
molL-'
NaOH
(mL)
sample
0.5
0.5
H20
0.5
1.0
1.0
1.0
0.5
0.5
0.002
40,120,240
-
0.002
40,120,240
0.047
40,120,240
sample
0.047
40,120,240
0.088
40,120,240
sample
0.088
40,120,240
-
0.000
40,240
0.045
40,240
0.045
40,240
0.088
40,120,240
0.088
40,120,240
0.047
40,120,240
sample
0.047
40,120,240
sample
1.0
1.0
f
Quenching
agent
(µL)
Aqueous
sample
1 Description of LS cocktail compositions. Series identifiers are assigned to cocktails of similar aqueous
fraction, with subscripts
T
and
B referring to i88W-188Re tritium and background, respectively. All added components are listed. Aqueous fraction was adjusted
W>
o
w either 1 mo1 L1- NaOH or water, where «sample» refers to the amount listed due to added activity. The quenching agent used was a 10/o
with
,
by volume, dilution ofnitromethane in ethanol and the volumes are nominal values based on an assumed volume of 20 µL Per dispensed drop.
Table
Key UG-Packard Ultima Gold AB; HF-Packard Hionic-Fluor IG-Packard Insta-Gel XF; HS-Wallac 0ptiPhase Hi-Safe III. (Continued).
:
Number
Scintillant
volume
Series
ID
of
samples
Scintillant
GW
3
HF
10
GT
3
HF
10
EB
2
HF
10
FB
2
HF
10
GB
2
HF
10
HW
3
IG
7
HT
3
IG
7
IW
3
IG
7
IT
3
IG
7
Ha
2
1G
7
IB
2
IG
7
JW
3
HS
10
JT
3
HS
10
1
solution
g
0.03
-
0.02
-
0.02
0.02
mol L -1
solution
f
g
0.02
-
0.02
0.02
1.0
1.0
0.5
1.0
5.0
-
0.02
sample
-
-
0.088
40,120,240
0.088
40,120,240
0.000
40,240
0.045
40,240
0.086
40,240
0.403
40,120,240
5.0
sample
0.403
40,120,240
sample
5.0
0.403
40,120,240
5.0
0.417
40,120,240
0.403
40,240
0.417
40,240
0.002
40,120,240
0.002
40,120,240
-
5.0
-
-
sample
-
5.0
sample
f
Table 1 Description of LS cocktail compositions. Series identifiers are assigned to cocktails of similar aqueous fraction, with subscripts
w T and B referring to 188 W- 88 Re tritium and background, respectively. Al1 added components are listed. Aqueous fraction was adjusted
with either 1 molLi NaOH or water, where samPle refers to the amount listed due to added activity. The quenching agent used was a 10/o
o
by volume, dilution of nitromethane in ethanol and the volumes are nominal values based on an assumed volume of 20
µL per dispensed drop.
Key: UG-Packard Ultima Gold AB; HF-Packard Hionic-Fluor IG-Packard Insta-Gel XF HS-Wallac 0PtiPhase Hi-Safe III. (Continued).
1
_
Number
'gW solution
3H solution
Scintillant
Scintillant
volume
(mL)
(g)
(g)
3
HS
10
0.02
KT
3
HS
10
JB
2
HS
10
KB
2
HS
10
Series
ID
of
samples
KW
-
-
0.02
1 molL-1
NaOH
(mL)
1.0
1.0
1.0
Aqueous
Quenching
agent
H2O
f
(µL)
-
0.088
40,120,240
sample
0.088
40,120,240
0.000
40,240
0.091
40,240
-
164
J Cessna, B Zimmerman
equilibrium) (Unterweger 2001) and 4500 days ± 8 days for 3H (Lucas and Unterweger 2000). The
solution used for the 3H samples is a dilution of a NIST standard reference material (NIST 1991).
Table 2 Summary of nuclear data input parameters for efficiency calculations with
EFFY4. Data are from the National Nuclear Data Center (ENSDF 2000). All transitions
were considered to be allowed with the exception of that marked with an asterisk, which
was taken to be first forbidden-unique.
188W
188Re
Value
3H
Ei,max/keV
P13l%
ER,max/keV
PR/%
ER,maxlkeV
P131%
2118
70.6
349
99.0
18.591
100
1962
26.0
285*
0.15
1487.2
1.68
58.3
0.83
1033.8
0.64
657.7
0.45
354.9
0.185
179.2
0.104
The samples in the second trial were prepared in the same manner as above, with the deletion of the
intermediate water fraction series. Because the supplier underestimated the stock solution activity,
the amount of added'88W_188Re was reduced. The IG samples were prepared in a similar fashion as
those described above, except that the amount of scintillant was 7 mL and the aqueous fraction
increased to 5 mL of either NaOH or water. This amount was added to ensure that the cocktails were
in the gel phase.
The samples were counted sequentially for ten cycles each, in the Wallac and the Beckman LS
s-1 to
counters. The counting times were 600 seconds, with sample count rates ranging from 1000
s-1,
described
as
processed
data
were
excluding samples which were not stable. The resulting
2000
above.
RESULTS AND DISCUSSION
The first result was the inability of UG to accurately produce stable traced 188W-188Re activity values in cocktails containing either 0.5 mL or 1.0 mL of NaOH. As seen in Figure 1, series B and D
showed a pronounced decrease over time in traced activity values, compared to series A. Series A
was found to be 1.4 percent lower than the average of all cocktails deemed to be stable, and
described below. The average standard deviation of 10 determinations of the 188W-188Re massic
activity values for series A is 0.27% for the Wallac, 0.23% for the Beckman, and 0.21% for the
Packard. These values are more than double typical values.
Also shown in Figure 1 are the tritium efficiencies for the same cocktail composition series. It is
interesting to note that while there is a small decrease in the efficiency for series D, these samples
do not show a trend large enough to account for the change in 188W-188Re efficiency. There would
be only a 0.08% change in computed'88W_188Re efficiency for a 1 percent change in 3H efficiency.
Alkaline Sample Solutions and the Stability ofLiquid Scintillation Cocktails
165
The poor 188W-188Re results can also not be accounted for by the quench indicating parameter. This
remained relatively constant in both the 188W-188Re and tritium samples over the cycles shown in
Figure 1. Because the 3H samples in series A, B, and D were relatively stable over time, it is likely
that the instability in the 188W-188Re samples can be attributed to the 188W-188Re chemical species in
the cocktail and not necessarily the amount of NaOH. More studies would be required to determine
the conditions under which Ultima Gold AB can be used for the assay of this radionuclide.
The samples showed similarly poor results in the Beckman and Packard spectrometers. Although
the samples were agitated when switched from one spectrometer to the next, the samples only experienced a few percent increase in count rate. The majority of the samples returned to the low counting efficiency for'88W_188Re by the second cycle.
It is a routine practice in the application of an efficiency-tracing method, such as CIEMAT/NIST, to
vary the quenching range by the addition of a quenching agent such as nitromethane to a series of
LS cocktails. An unexpected, but explainable, result in this study was the formation of a yellow
solution in cocktails that contained both NaOH and nitromethane. Investigation revealed the likely
cause to be the formation of a stable aci- tautomer of nitromethane, with a characteristic yellow
color, which occurs when nitromethane is in the presence of an alkaline solution, and which is in
equilibrium with the nitro- anion (could 1959). It is not clear what effect, if any, this had on the stability of the UG cocktails, but the color made it possible to determine that this solution eventually
separated to a different phase and settled to the bottom of the scintillation vial. There exists the possibility for color quenching and it is known that the aci- form of nitromethane is characterized by
having a broad absorption with 2max = 280 nm ± 5 nm (Asmus and Taub 1968).
The Insta-Gel cocktails prepared with NaOH as the primary aqueous component, series H, decomposed resulting in a milky solution, which eventually separated to the bottom of the vial. The measured 3H efficiencies of these samples increased over time by between 300/ and 49%, for the 10
cycles of the Wallac spectrometer. When the vials were agitated and moved to the second spectrometer, Beckman, the same behavior was observed. During the same counting intervals, the quench
indicating parameters indicated that the amount of quench was decreasing. These effects might be
expected if 3H remains in the clear layer of the vial as the milky layer settles. The composition of
what the milky substance is not evident, but it is possible that a reduction reaction between the
NaOH and the ethoxy alkylphenol component of the scintillant is forming organic salts. Due to the
instability in both the 3H and the 188W-188Re samples, efficiency-tracing calculations were not carried out for series H.
Cocktails prepared with Insta-Gel and with water as the primary aqueous component, series I, were
stable with regard to both 3H efficiency and quench indicating parameter. The 188W-188Re samples
also remained stable. This is most evident in the average standard deviation of 10 determinations of
the 188W-188Re activities for each sample, in each of two spectrometers, which was found to be
0.09%.
The Hionic Fluor and Hi-Safe III cocktail series were all stable over time and gave good results.
Once again this can be seen in the average standard deviation of 10 determinations of the 188W_ 188Re
activities for each sample, in each of two spectrometers. These are 0.08% for the HF cocktails and
0.09% for the HS cocktails. Overall results from all of the traced cocktails are shown in Figure 2.
Not readily apparent in the plot, due to the large spread of UG results, is that the Hionic Fluor
results, while stable, were 0.4% lower than those found with Hi-Safe III and the stable Insta-Gel
samples. An F-test performed on these two populations found the difference to be significant at a
confidence level of 95%, giving an F-value of 13.0, compared to a critical value of 4.7 for 14
166
JCessna,
B Zimmerman
140000
130000
120000
110000
100000
U
90000
80000
70000
60000
-3.0
-2.0
-1.0
0.0
3.0
2.0
1.0
Tid
0.55
r
-.-AT-1
-- AT-2
AT-3
BT-1
--BT-2
--.--BT-3
--DT-1'
0.25
0.20
--------- ----3.0
-
-------
-2.0
-1.0
0.0
-e-DT-2'i
-
1.0
------ -
--2.0
3.0
T/d
Figure 1 Results from series A, B, and D from the Wallac spectrometer. Figure 1 a shows CA tot, total efficiency-traced 188W-'88Re activity concentrations in Bqg', as a function of measurement time in days. Figure
lb shows the measured tritium efficiency as a function of measurement time, T, in days. All samples contain
10 mL of Ultima Gold AB. Series A contains only NaOH from the active solution. Series B and D contain
0.5 mL and 1 mL of NaOH per sample, respectively. Each data point represents a calculational result from a
single measurement. Lines are included as a guide for the eye. All data are decay corrected to a common reference time of T = 0 days.
degrees of freedom. This cocktail dependence was therefore treated as a component of uncertainty.
The final activity values were comprised of series E, F, G, I, J, and K. Uncertainties evaluated in the
determination of the activity of the solution in these measurements are fully explained in Zimmerman et al. (2001).
Alkaline Sample Solutions and the Stability ofLiquid Scintillation Cocktails
167
140000
Packard
Beckman
Wallac
...
130000
...
110000
90000
I
I
60000
A
B
D
E
F
.
..
IG
HF
UG
70000
..
.
..
120000
50000
.
I
]E
G
A
B
D
E
F
G
A
B
D
E
F
G
J
K
J
K
Series
Figure 2 Overall results from series A, B, D, E, F, G, I, J, and K, from all spectrometers. Values for each
series and spectrometer are the average of CA, total efficiency-traced 188W-ittRe activity concentrations
in Bqg ', over normally 10 cycles for each of 3 samples. Uncertainty bars are one standard deviation of
the mean. Series A B, and C contain 10 mL of Ultima Gold AB (UG) and 0.02 mL, 0.5 mL, and 1 mL
of NaOH, respectively. Series E, F, and G contain 10 mL of Hionic Fluor (HF) and 0.02 mL, 0.5 mL, and
1 mL of NaOH, respectively. Series J and K contain 10 mL of OptiPhase Hi-Safe III (HS) and 0.02 mL
and 1 mL of NaOH, respectively. Series I contains 7 mL of InstaGel XF (IG) and 5 mL of water. Series
H is not plotted, as the cocktails were deemed too unstable for meaningful efficiency tracing.
CONCLUSION
The present results indicate that the Insta-Gel cocktails prepared with NaOH as the primary aqueous
component decomposed, resulting in a white precipitate, but that those prepared with water as the
aqueous component were stable. In addition, Ultima Gold AB was shown to be unstable in the presence of '88W-lt8Re and gave widely varying results, depending upon the magnitude of aqueous
(NaOH) fraction of the cocktail. The Hionic Fluor and Hi-Safe III cocktails were all stable over time
and gave good results, despite the fact there was an observed difference of about 0.4% in the averages of the activities determined for each cocktail. The Insta-Gel cocktails prepared with water gave
activity values that were in excellent agreement with those obtained with Hi-Safe III. Finally,
although there is no direct evidence of a detrimental effect on the LS counting one should be aware
of the formation of aci- tautomers of nitromethane, when used to vary quenching in the presence of
alkaline solutions, recognized by a yellow color.
DISCLAIMER
Certain commercial equipment, instruments, or materials are identified in this report to foster understanding. Such identification does not imply recommendation by the National Institute of Standards
and Technology, nor does it imply that the materials or equipment are necessarily the best available
for the purpose.
ACKNOWLEDGMENTS
The authors would like to thank Dr FF Knapp of Oak Ridge National Laboratory for supplying the
188W-I88Re solution and Dr R Colle for many useful discussions on the topic of LS counting.
168
J Cessna, B Zimmerman
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