758
Anal. Chem. 1984, 56,758-761
Effect of Vial Composition and Diameter on Determination of Efficiency, Background, and Quench Curves in Liquid Scintillation Counting John C. Elliott California State University, Fullerton, California 92634
Efflclencles, backgrounds, and quench curves were determined for varlous llquld sclntlllatlon vials by means of the external standards method. Vlal material, vlal dlameter, and adapter materlak were varied In these determlnatlons. It was found that the 3H quench curves for these vlals are Independent of the vlal or adapter materlal but dependent on the vlal dlameter. Mlnlvlals ( 7 mL) were characterized by a quench curve of higher efflclency at a glven quench number than large (20 mL) vials, regardless of vlal or adapter material. Vlal and adapter materlals slgnlflcantly affected efflclency and background for all vials tested. Speclally prepared mlnlvlals modlfled for use In a large vlal machine wlthout addltlonal adapters showed efflclencles for trltlum comparable to large diameter factory trltlum standards, but wlth lower backgrounds. Figures of merlt (effIclency*/background) of up to 230% greater than factory trltlum standards were obtalned wlth thls system.
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The quest for optimum efficiency a t minimum cost has prompted the development of a wide variety of liquid scintillation counting cocktails, vial materials, and sample volumes. Vial materials commonly used include flint glass, low potassium borosilicate glass, polyethylene vials, plastic film bag systems, and, more rarely, Teflon or quartz vials. The above materials are available for vials which range in volume from minivial categories (approximately 7.0 mL), to conventional vials of approximately 18-22 mL capacity. When minivials are used in liquid scintillation counting apparatus not specifically designed for them, they require adapters, which may also be composed of polyethylene, quartz, flint or borosilicate glass, or polycarbonate plastic. In practice, adapters are not necessarily composed of the same materials as the vials they are carrying. In recognition of the interdependency of counting efficiency, quench, and sample composition, individual sample preparation and counting techniques vary widely. Various efficiency determination techniques include internal standards, sample channels ratio, and external standard efficiency determination. The external standard technqiue involves generation of a quench number via an external radioactive standard (1). The external standard protocol relies on an external source of y radiation (226Ra,Packard; 137Cs,Beckman; 133Ba,Tracor Northern) to produce Compton electrons in the sample vial. The amount of quenching present in the sample will shift the spectrum of the Compton electrons toward lower apparent energies. The degree of spectrum shift is expressed as an index (QIP, Packard; H#, Beckman; ESR, Tracor Northern). This quench number is correlated to a set of quenched standards to provide an efficiency curve. The external standard technique does not preclude simultaneous use of internal standards or sample channels ratio methods of quench determination. The sample channels ratio technique, however, is limited in accuracy to a relatively
narrow quench range, especially in the case of dual labeled samples, and decreases in accuracy with low count rates. Internal standards techniques, on the other hand, require additional sample manipulations and precision addition of an accurately labeled standard of the same material as the sample. Given these considerations, there is little reason for using more than one quench determination procedure. Previous studies have dealt with many of the variables involved with liquid scintillation counting of samples. Muse and Rao have compared counting efficiencies of 12 commercially available cocktails at fixed sample volumes and have briefly examined the problems of decreased counting efficiency with time (2). Joseph and Kramer have examined techniques for the optimization of liquid scintillation sample composition (3). Ring et al. have examined the relative counting efficiency of glass minivials with and without polyethylene adapters in different liquid scintillation counters (4). While noting that vial diameters must be kept constant in samples and standards in order to eliminate a potentially large source of error, they speculated that an observed counting efficiency increase of approximately 3.2 % in small vial and polyethylene adapters as compared to large vials was possibly due to the adapters. They also noted that the difference in color vs. chemical quenching was more exaggerated in large vials. This paper examines the effects of variations in vial diameter, vial material, and adapter material between various samples and the quench standards to which they were compared. This has been accomplished by the generation of quench curves for selected sample vial and vial/adapter combinations, and comparison to the curves generated with commercially available quenched standards. The unquenched efficiency and figure of merit have been studied for each of the systems tested, and variations in the quench curves between large vials and small vials with adapters have been noted.
EXPERIMENTAL SECTION Prior to initiation of the experiment, the reproducibility of the 3H curves constructed by the Packard 300 C/Dliquid scintillation counter from one run to the next for a given quench series was examined. This was done by repeated counting over several days of both commercially prepared (argon purged, flame sealed glass vials, hereinafter referred to as “commercial standards”) and user prepared quenched standards. Their quench vs. efficiency curves were then compared. Three types of large diameter vials, and three types of minivials carried in six kinds of adapters were employed in this study. The large diameter group consisted of polyethylene (PE) vials, low potassium borosilicate glass vials, and commercial quenched standards in glass vials. The minivials group consisted of minipolyethylene, mini-low-potassiumglass, and mini-commercialquench standards. Minivial adapters used were commercially available glass and polyethylene adapters, an uncapped wide mouth polyethylene large diameter vial and a modified thin wall polyethylene vial used as adapters, a clear plastic adapter, and a specially constructed grommeted, wall-less adapter system (Figure 1).
0003-2700/84/0356-0758$01.50/00 1984 Amerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4,APRIL 1984 A
B
759
C
668
Flgure 1. Grommeted (wall-less)adapter system. In order to count minivials (A) in a large vial counter without altering the background and efficiency by using conventional adapters, two caps from large diameter polyethylene vials were modified to serve as grommets (B). These grommets were posltioned on minivials as shown in (C), and secured by friction or by thermal welds at the junction.
Effects of vial material and diameter on unquenched 3Hefficiency were exammined by comparing the unquenched efficiencies for the large vials to those of the various minivial/adapter combinations. Quench curve effects were studied by comparison of large diameter vial quench curves to the quench curves produced by the minivial/adapter sets. For the aforementioned techniques, the liquid scintillation cocktail consisted of spectrophotometric grade toluene, to which PPO and Bis-MSB were added. Fluor proportions consisted of 15 g of fluor per gallon of toluene, in the ratio of 98% PPO, 2% Bis-MSB (New England Nuclear Omnifluor "Galpacks"). Prepared cocktail was added to the scintillation vials in the quantity of 5 mL per minivial and 15 mL per conventional vial. This cocktail was identical with that employed in the commerical standards. To generate quench curves, an aliquot of tritiated toluene (approximately 246500 dpm) was added to a sample vial or vial/adapter combination, and the unquenched vial was counted for 10 min in a Packard 300 C/D liquid scintillation counter. Factory settings for tritium (0-19 keV in channel A, 2-19 keV in channel B), using external standard without automatic efficiency control, were used. After determination of initial activity for each vial, a quantity of the chemical quenching agent, nitmmethane, was added. The vial or vial/adapter was then recounted to determine the next point of the quench curve. This procedure was repeated ten times using 0-78 p L of nitromethane (large vials) or 0-26 p L of nitromethane (minivials),until a total of 144 p L of nitromethane was added to the large vials and 48 pL of nitromethane was added to the minivials. This gave a quench for each vial or vial/adapter combination ranging from unquenched (quench number approximately = 800-900) to nearly totally quenched (quench number approximately = 100) for each sample. This technique also eliminated the possibility of variations in the amount of radioactivelabel between samples for a given group. To further eliminate procedural and environmental variations, all samples for each of the groups of vials were prepared and run on the same day. Quench curves generated by the Packard 300 C/D were used for comparison of vial and adapter performance of each group. Initial unquenched efficiencies were used for comparison of vials and vial adapters in figure of merit determination.
RESULTS AND DISCUSSION Quench vs. efficiency determinations repeated over several days showed that the curves generated by the liquid scintillation counter for disintegrations per minute determination were remarkably consistent for a given series of quench samples. Curves printed out by the counter were essentially superimposable from one day to the next. Besides this degree of reproducibility, it was determined that, for a given vial diameter, vial material had little or no effect on the shape of the quench curve. With large diameter vials as an example, essentially identical quench curves were obtained regardless of whether the vials were polyethylene, glass, or commercial standards (Figure 2). The same holds true for miniglass, minipolyethylene, and minicommercial standards in any adapter, including the grommeted, wall-less
334
Ob
16
32
48
I
64
% EFFICIENCY Figure 2. Vial material independence. Quench curves for commercially available large diameter quenched standards (A)are identical with the curves obtained with user prepared quench sets employing large dlameter polyethylene (0)and glass (0)vials. Minivials also share
common curves, regardless of vial material.
4;
33
0
16
32
48
64 0
% EFFICIENCY Flgure 3 . . Adapter material independence. Quench curves for com-
mercially available ministandards are independent of adapter material, for glass adapters (0),polyethylene adapters (O),wide mouth polyethylene vials used as adapters (A), modiiled thin wall polyethylene vials used as adapters (0),and a walkless, grommeted adapter system (0). adapter system. Only minor differences ( 5 2 % efficiency/ quench no.) were noted in the curves of each vial material, aside from the variations in the end point (maximum efficiency), which was highest for commercial standards in both diameter categories. Since vial material appeared to have no effect of the shape of the quench curves, the most efficient minivials (minicommercial quench standards) were used to test the role of adapter material on quench curve shape (Figure 3). While the maximum efficiency (end point) varied, the general shapes of the quench curves were similar. There was only approximately 2% efficiency per quench number ( E / Q )variation between the curves. Considerable variation in the quench curves occurred as a result of diameter changes, however (Figure 4). Minivials showed a 345% efficiency increase at any given quench number over their large vial counterparts measured a t the same quench number. This efficiency difference between large and small diameter vials was consistent throughout the entire quench curve, with the exception of extremely heavily quenched samples (Q0-250). One must conclude, therefore, that large errors will be introduced if quench curves determined with large diameter standard vials are used to correct experiments run with minivials, and vice versa. The difference in quench curves between minivials and large vials is a diameter-related effect only and is unrelated to the vial materials or to the adapters used with the minivials. This is indicated by the fact that all the minivials/adapters tested
760
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table I. Vial and Adapter Characteristics
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% EFFICIENCY Flgure 4. Diameter dependence. Quench curves for large diameter, commercially avallable standards (0)are distinct from small diameter commerclal standards (A),even when a wall-less adapter system precludes adapter related effects.
had similar quench curves (including the wall-less, grommeted adapter system). Ring et al. also found that quench curves differed between large diameter vials and minivials. Furthermore, they found that this difference was present when different brands and models of liquid scintillation counters were used. This fact supports the conclusion that neither the isotope used as the external standard nor the method of calculating external standard ratio (quench number) are related to this diameter sensitive difference ( 4 ) . A compelling explanation for this improvement in efficiency with decreasing diameter is that counting geometry is maximized as the sample approaches a thin line source configuration (as opposed to a diffuse volume source). Decreased self-absorption due to the shorter photon path length in the narrower vials may augment this improvement. A possible test for diameter effects would be to construct a 1-2 mL capacity, very small diameter vial, and count it in a grommeted, wall-less adapter. This type of vial would only be useful at low energies, since high-energy /3 particles would escape from the vial without completely interacting with the cocktail (5). Self-absorption and energy-related effects are currently being examined for higher energy emitters (14C, 32P). In addition, similar efficiency and quench curve experiments are planned for a (226Ra)and y (lZsI,51Cr)emitters. For the vials and vial/adapter combinations tested, the maximum unquenched efficiencies (Table I) range from 53.2% for a polyethylene minivial in a wide mouth polyethylene vial used as an adapter, to 62.2% for a large diameter, glass commercial standard. Glass ministandards, in both glass adapters and wall-less grommeted adapters, yielded maximum efficiencies nearly equal to the large diameter standard (61.7% and 62.0%, respectively). Consideration of background through figure of merit (E2/B)determinations showed a clear advantage of the wall-less adapter over large diameter glass and polyethylene viak, as well as other minivial/adapter combinations (Table 11). It is apparent from the above, that both vial and adapter material had an important effect on the maximum unquenched efficiency of the sample. For a given material, vials and adapters having thicker and denser walls exhibit higher backgrounds, while vials and adapters made of more opaque materials result in lower efficiency. In conclusion, it is necessary to use considerable care in the distintegrations per minute determination process. Quench curves should be constructed with standards having physical and chemical characteristics as close as possible to those of the samples being examined. Commercial standards, while useful, may not have a response which is representative of the sample, especially if they are of different diameter, if different
vial adapter glass stds NA Iarge glass NA large PE NA miniglass stds clear plastic miniglass stds wide mouth PE vial miniglass stds PE adapter miniglass stds modified PE vial miniglass stds glass adapter miniglass stds grommets mini-PE vials wide mouth PE vial mini-PE vials clear plastic mini-PE vials PE adapter mini-PE vials modified PE vial mini-PE vials glass adapter mini-PE vials grommets a
r
unquenched backefficiency, ground, % CPm 19.3 i 1.8' 62.2 i 1.5 16.0 i 2.2 56.0 i 0.6 10.5 i 1.4 56.5 i 0.5 16.9 i 3.2 55.9 i 1.3 56.1 i 1 . 9 15.7 i 2.4 14.9 i 1.7 13.6 i 1 . 4
57.4 i 1.6 58.5 I0.8
20.4 i 3.6 12.4 i 2.9 10.0 ?r 1.4
61.7 i 1.1 62.0 i 1.0 53.2 t 0.4
8.8 i 1.8 f 2.1
1.6
53.6 2 0.3 54.5 I0.6 55.6 f 0.4
16.3 i 2.6 7.5 i 1.8
59.3 f 0.5 59.3 i 0.4
9.6 9.1
?:
std dev ( n = 6).
Table 11. Vial and Adapter Ranked Figures of Merit % of large
vial
adapter
large std large glass large PE mini std, glass mini std, glass mini std, glass mini std, glass mini std, glass mini std, glass mini PE vials mini PE vials mini PE vials mini PE vials mini PE vials mini PE vials
NA NA NA clear plastic glass adapter wide mouth PE vial PE adapter modified PE vial grommets glass adapter wide mouth PE vial PE adapter clear plastic modified PE vial grommets
E21 standards B E2/B
201 196 304 185 186 201 221 252 310 216 283 310 325 340 469
100
98 151 92 93 100 110
125 154 108 141
154 162 169 233
adapters are used, or if color quenching is a factor. Although commercial standards can yield reasonable accuracy, precision work seems to require construction of quench standards to the same characteristics as the vials being evaluated. If maximum counting efficiency is the primary consideration, it would appear that glass minivials counted in glass adapters are the most efficient commercially available minivial system. For lowest background, a polyethylene minivial in a thin wall polyethylene adapter is the lowest background commercially available system tested at this time. While not yet commercially available, a grommeted or wall-less adapter system offers the best performance for both glass and polyethylene minivials of any adapter system tested. Maximum counting efficiency was approximately equal to large diameter commercial standards, while background was approximately 37-8570 of other vial or vial/adapter systems. In addition to exhibiting the lowest background and highest efficiency, the wall-less system also offers a number of additional advantages. I t requires one less step in the minivial preparation process and also presents one less component to keep clean and static free. Static problems are more easily dealt with than with other adapter systems, since there are no adapter walls to block a particles, static sprays, or ion static
Anal. Chem. 1984, 56,761-763
eliminator beams. By virtue of its larger base, this vial is more stable than standard minivials. And finally, its large diameter cap keeps user’s fingers remote from the vial and the vial/cap junction, decreasing personal exposure from the vial contents, and minimizing personal contamination from leaks around the cap.
ACKNOWLEDGMENT The author is indebted to Robert Koch and Glenn Nagel for constructive advice in preparing this publication. Graphs present in this paper were prepared with the help of Richard Deming. Norman Nitzberg provided invaluable technical assistance in construction of the modified vials.
781
Registry No. Tritium, 10028-17-8;polyethylene, 9002-88-4. LITERATURE CITED (1) Horrocks, Donald L. “Applications of Llquid Scintiilatlon Counting”; Academic Press: New York, 1974; Chapter 10. (2) Muse, L. A.; Rao, V. Health Phys. 1976, 37, 457-459. (3) Joseph, S.; Kramer, G. H. AECL-7611; Chalk River Nuclear Laboratorles: Chalk River, Ontarlo, 1982. (4) Ring, J. G.; Relch, A. R.; Marcec, J. J.; Moore, D. H. “The Performance of Liquid Scintillation Counters Using Large and Small Vials”; Packard Instrument Co.: Downers Grove, IL, 1978; p PR-18. (5) Horrocks, D. L. “Liquid Scintillation Counting-Volume Corrected Results”; Beckman Instrument Co.: Irvine, CA, 1983.
RECEIVED for review August 24, 1983. Accepted December 19, 1983.
Determination of Radium in Water by Liquid Scintillation Counting after Preconcentration with Ion-Exchange Resin Hideo Higuchi,* Masaki Uesugi, Kaneaki Satoh, and Naoyuki Ohashi J a p a n Chemical Analysis Center, 295-3 Sanno-cho, Chiba, J a p a n
Masayasu Noguchi Radioisotope a n d Nuclear Engineering School, J a p a n Atomic Energy Research Institute, 2-28-49 Honkomagome, Bunkyo-ku, Tokyo
A method for the determination of radium in envlronmental water by means of a liquid scintillatlon counting technique has been developed. The radium was collected on cation exchange resin with 100% recovery by the batch method. The other radionuclides except uncompiexed elements such as ceslum can be removed by washing the resin with 0.01 M EDTA solution at pH 8. The counting sample was prepared by immersing the resin into emuislon llquld sclntiiiator in a Teflon vial. The chemical procedure took about 2 h and the detectlon limit was 0.03 pCI of 226Ra/L.
has been applied praktically to various surveys (6) of radon in environmental water, and also determination (7,8) of radium combining with the usual chemical procedure based on coprecipitation with barium sulfate. Recently, Prichard et al. (9) have proposed a new analytical technique for n6Ra in water by means of cation exchange resin and LSC without any special procedure for purification of radium. The present paper describes not only an analytical technique of rapid cation exchange separation of radium from other radioactive nuclides but also some information on the migration of radon from an ion exchange resin to a liquid scintillator.
Determination of radium in environmental water has become a matter of interest in public health because radium is one of the most hazardous elements with respect to internal exposure. Therefore it is desirable to develop a method of high sensitivity to analyze a large number of samples rapidly. Analytical methods of conventional use are mostly based on measurements of a rays from radium coprecipitated with barium sulfate (I,2) or of radon gas emanated from radium (3, 4 ) . The former, usually carried out by using a ZnS(Ag) scintillation counter or a proportional counter, requires the time-consuming operation of chemical separation for preparing a thin sample for a ray counting. The latter, usually carried out with an ionization chamber or a radon counter (e.g., Lucas counter), requires complicated operations for handling radon gas. One of the authors (5) has already reported a measurement method for radon in water using a liquid scintillation counting (LSC) technique. Features of the LSC method are easy and efficient extraction of radon from a large quantity of water and simplified measurement with high counting efficiency by a commercial liquid scintillation counter. The LSC method
EXPERIMENTAL SECTION Reagents and Apparatus. Analytical grade reagents and deionized water were used throughout this work. The radioactive solution used for ion exchange procedures contains S4Mn,6oCo, 85Sr,13’Cs, 144Ce,and 226Ra.The solution of 0.01 M EDTA was prepared by dissolving 3.72 g of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) in 990 mL of water, and 1M NaOH solution was added to adjust the pH to 8.0. The cation exchange resin used in this study was Bio-Rad AG50W-X8 (Na’ form, 50-100 mesh) which was preconditioned with sodium chloride solution. The liquid scintillator was Aquasol-I1 emulsion scintillator prepared by New England Nuclear Co. A low background type liquid scintillation counter (Aloka LB-1, Japan) which incorporates anticoincidence guard counters and a 100-mL counting vial was used to measure 222Rnand four daughters (hereafter, called a usual measurement). In addition, a coincidence module specially designed for a+ coincidence was also used. counting of 214Biand 214P0 Procedures. Based upon preliminary experiments, the following analytical procedures were applied to samples of water with pH 1to 8 and salinity less than 0.5%. Two liters of sample was adjusted to pH 1 or above with sodium hydroxide or hy-
0003-2700/84/0356-076 1$01.50/0
0 1984 American Chemical Society
