e
I
e
t
i
m a
b
n
m C
Figure 3. Disk details (top view): (a, b, c) disks; (e)M2 brass screw; (j) M4 Nylon screw; (k) 1.6 mm diameter hole; (m) 0.5 mm diameter hole; (n) 1.4 mm diameter hole
OPERATION Figure 2 shows the disk position for sample filing or syringe cleaning. This position is achieved automatically after an injection or manually by pressing disk c upward toward the syringe barrel. After sample filling and needle wiping pulling disk c downward will realign the disks along the needle a t equal distances for optimum support, prior to injection as in Figure 1. For GC application disk c is made from glass filled poly(tetrafluoroethy1ene). This device can also be used with syringes other than the one described. Generally the center hole in disk a should have a diameter 0.1 mm larger than the syringe barrel diameter and the center holes in disks b and c should have a diameter 0.05 mm larger than the needle diameter.
I Figure 2. Sample loading and post injection position: (a, b, c) disks
RECEIVED for review December 20,1976. Accepted February 3, 1977.
Determination of Americium and Curium in Biological Samples by Extraction and Liquid Scintillation Counting G. J. Ham,* G.
N. Stradling, and S. E. Breadmore
National Radiological Protection Board, Harwell, Didcot, Oxon, UK
Metabolic studies of americium and curium in rats often require the analysis of a large number of tissue and excreta samples. One method previously used involves the production of carrier free actinide by an ion-exchange separation process ( I ) . Alternative methods involving direct scintillation counting are more rapid but lack the sensitivity required (2-4). Extraction-scintillation methods of analysis are becoming of increasing importance for the determination of the higher actinides (5-7). They do not involve extensive chemical manipulations, reducing the risk of cross contamination, and have a limit of detection of about 0.05 disintegration min-' g-l (5). We are unaware of any suitable extraction-scintillation methods for the determination of americium and curium in biological material. An extraction system based on monoisooctyl phosphate appeared to be the most promising since the reagent, when present only as an impurity in a solution of di-2-ethylhexyl phosphate in isooctane, will extract am1268
*
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
ericium from dilute nitric acid (8). Due, however, t o the commercial unavailability of monoisooctyl phosphate the studies reported here involve the appraisal of a homologue mono-3,5,5-trimethylhexyl phosphate (nonylphosphate). The method described utilizes the direct extraction of americium and curium from an acidic solution of sample ash into a solution of nonyl phosphate and scintillants in toluene. Liquid scintillation counting is done with both phases present in the vial.
EXPERIMENTAL Apparatus. All the samples were prepared in standard 22 cm3 low potassium glass scintillation vials. They were capped using polythene inserts and shaken in a box shaker, controlled by a timer programmed for a l-h shake, 1/2-hrest cycle. Liquid scintillation counting was done using a Beckman LS 250 (Beckman RIIC L a . , Glenrothes, Scotland). Reagents. The americium-241 and the curium-244 solutions were prepared from standardized solutions in 0.5 M "OB (The
Table I. Determination of Americium and Curium in Biological Materials
Sample SDleenb Lungsb
I
i
Gdneysb Small organs Thymusb Ovariesb Blood (4 mL) Liverb GIT~ Carcass (8% w/w) Urine (1day 10% w/w) Urine (14 day 10% w/w) Feces (1day 1 0 % w/w) Feces (14 day 1 0 % w/w) a N = Number of samples assayed. Ramsden ( 1 2 ) .
Background [ X -i. SD(N)O] (counts/min) 23.4
i
29.5 t 35.6 f 33.8 i 32.5 i
1.1 (36)
1.4 ( 6 )
1.9 ( 6 ) 1.4 ( 6 ) 1.9 ( 6 ) 48.0 t 2.3 (10) 29.9 f 1.1( 6 ) 32.9 i 2.1 (10) Total organ assayed.
Radiochemical Centre, Amersham, England) with a radionuclidic purity of 99.9% (by CY activity); there were no other contributing activities to liquid scintillation counting. Commercial grade monononyl phosphate as an approximately 50x50 mixture with dinonyl phosphate was obtained from Albright & Wilson Ltd. (Oldbury, West Midlands, England) and was used as supplied. The extraction-scintillation solution was made by dissolving 435 cm3 of nonyl phosphate in 565 cm3 of sulfur-free toluene containing 5 g of p-terphenyl and 0.05 g of 1,4-bis[2-(5phenyloxazolyl)benzene](POPOP). After stirring overnight any undissolved scintillant was filtered off. This solution was stable for several months. Samples. The samples used in these experiments were obtained from female rats of an inbred strain supplied by the Medical Research Council's Radiobiological Unit (Harwell, Didcot, England). The animals were about 10 weeks old and weighed from 150 to 250 g. The lungs, kidneys, spleen, thymus, ovaries, and blood (4 mL) were transferred directly into scintillation vials. Liver, gastrointestinal tract, and the remaining carcass (100 g) were placed in beakers. Urine and fecal samples were collected in beakers for periods up to 14 days. Procedure. All samples were dried under heat lamps and ashed at 500 "C for 16 h after gradually raising the temperature over 10 h. They were then alternately digested in hot 8 M HN03 and ashed at 500 "C until a carbon-free ash was produced. The vials were neither damaged nor distorted in this process. Liver, gastrointestinal tract, carcass, urine, and fecal samples were then Gastrointestinal tract and liver samples dissolved in 8 M "OB. were transferred into scintillation vials with 8 M HN03 and evaporated to dryness. Carcass samples were made up to 250 cm3 and excreta samples to 100 cm3 with 8 M HNO,; 8% and 10% (by volume) respectively, were transferred to scintillation vials and evaporated to dryness. All samples were then moistened with sufficient 6 M HN03/6 M HF solution to soak the ash and dried under heat lamps. The residue was covered with 8 M HN03 and evaporated t o dryness. The samples were dissolved in 10 cm3of 0.5 M HN03 and 5 cm3 of the extraction-scintillation solution was added. The phases were then equilibriated. With most samples this was achieved by shaking overnight; liver samples required 2-3 days. Before counting, the samples were placed in a water bath at 50 "C for a few minutes to separate the phases fully. RESULTS AND DISCUSSION Initial Investigations. Preliminary screening of extraction conditions was done using 10 cm3 of a carrier-free nitric acid aqueous phase and 5 cm3 of the extraction-scintillation solution. Nitric acid was chosen for the aqueous phase since it is the commonly-used ashing agent. The overall recoveries of americium-241 and curium-244 using two different batches of extractant are shown in Figure 1. There is an obvious batch variation which could be due to the different mono/di-nonyl
% recovery [ X
Americium 96.9 i 1.9 (36)
* SD(N)a] Curium 96.7
i
Limit of detectionC (total sample) (dpm)
1.7 (48)
96.2 i 1.6 ( 6 ) 97.5 t 1.4 ( 8 ) 93.3 i 3.7 ( 6 ) 96.0 t 2 . 4 ( 8 ) 89.5 * 5.1 (48) 92.6 i 4 . 2 ( 8 ) 96.5 t 2.8 (12) 96.2 i 2.5 ( 8 ) 95.7 i 2.6 (36) 96.1 t 2.2 (8) 94.2 f 2.7 (12) 95.1 i 1.8 (8) 91.5 i 5.7 (36) 88.2 + 4.9 (8) Calculated for a 20-min count using the method
2.2
2.5 2.9 36 26 32 25 29 of Watt and
phosphate ratio. Typically the extractant contains 40-50% monononyl phosphate (9). The highest recoveries are obtained from a 0.5 M H N 0 3 aqueous phase and at this concentration the effect of both batch variation and concentration of extractant is minimized. Subsequently, all further experiments were carried out with 0.5 M HNOB and 1.5 M nonyl phosphate of Batch B. Spectra of both quenched and unquenched samples were determined and a window setting of 2-10 with a gain of 2.7 was found to give the optimum counting conditions. The spectra showed that the method will not differentiate americium and curium. The effect of some interfering materials was investigated. These were fluoride, as the ashing method requires the use of HF; calcium phosphate, as it is one of the main constituents of bone ash; and iron, as it is known to cause quenching in liquid scintillation counting. The amounts of these materials per vial which reduced actinide recovery to 95% are: fluoride (as NaF) 7.5 mg; calcium phosphate 1000 mg; and iron (as FeCL3) 3 mg. These values are compatible with the traces of soluble fluoride left in the vials; 8% of the carcass sample from a 400-g rat (IO); and the iron content of 6 mL of blood which contains 0.5 mg cm-3 (11). Therefore the materials would not be expected to cause lowered recoveries in rodent samples using the procedure described. The recovery of plutonium by this method was found to be 98.7 f 1.8%. If a sample was known to contain plutonium as well as americium and curium, their activities could be determined separately by first extracting and counting the plutonium only, using the di-2-ethylhexyl phosphate method. Under those conditions Keough and Powers state 0.01% of the americium is extracted (5). After removal of the organic layer and evaporating the aqueous layer to dryness, the americium and curium can be determined using the method described. Analysis of Biological Samples. The samples were all treated by the procedure described previously except that, prior to the fuming with HF, all samples were dissolved in 8 M HN03 and spiked with about 20 000 dpm of the americium-241 or curium-244. The recovery, background rate, and limit of detection are summarized in Table I. A high, reproducible recovery of both actinides was obtained from all the organs, blood, and urine. Carcass and large fecal samples, however, have lowered recoveries and higher variability. I t is therefore recommended that duplicate aliquots are taken from these samples, one of which is spiked with sufficient activity t o allow a recovery ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1269
lower than those reported for direct scintillation methods ( 3 , 4).
lOOr
LITERATURE CITED
t
40
L
1
0
1
2
3
4 5 M HN03
6
7
8
Flgure 1. Factors influencing the extraction of americium and curium using nonyl phosphate. (0)0.5 M nonyl batch A Am. (A)1.5 M nonyl batch A Am. (0)1.5 M nonyl batch B Am. (A)1.5 M nonyl batch B
Cm
determination to be made. This can then be used to correct for losses in the sample. The background count rate is dependent upon the sample, as =e the limits of detection, which are approximately 20 times
(1) F. E. H. Crawley and Elizabeth Goddard, Health Phys.,30, 191-197 (1976). (2) A. Lindenbaum and M. A. Smyth, in “Organic Scintillants and Liquid Scintillation Counting”, D. L. Horrocks and L. T. Peng, Ed., Academic Press, New York, 1971, pp 951-958. (3) T. Mo and JaW. Prim, “Inhalation Toxicology Research Institute, Loveiace Foundation, Annual Report 1974-75”, 1975, pp 108-11 1. (4) S.J. Powers, BNWL-1950, Pt. 1, 66-67 (1975). (5) R. F. Keough and S.J. Powers, Anal. Chem., 42, 419-421 (1970). (6) W. J. McDowell, in “Organic Scintillants and Liquid Scintillation Counting”, D. L. Horrocks and L. T. Peng, Ed., Academic Press, New York, 1971, pp 937-950. (7) W. J. McDoweil and C. F. Caleman, in ‘‘Promedings of International Solvent Extraction Conference”, Vol. 3, pp 2123-2133, Lyon, September 1974, SOC.of Chem. Ind., London. (8) E. S. Gureev, V. N. Kosynkov, and S. N. Yokovlev, Radiokhimiya, 6, 655-665 (1964). (9) Aibr!ght and Wilson Ltd., Product Technical Data Sheet, Industrial Chemisby Division, Oldbury, Warley, W. Midlands. (10) F. E. H. Crawley, E. R. Humphreys, and J. W.Stather, Health Phys., 30, 491-493 (1976). (11) P. L. Altman and D. S.Dlttmer, “Blood and Other Body Fluids”, Biological Handbook Series, Federation of American Societies for Experimental Biology, 196 1. (12) D. E. Watt and D. Ramsden, “High Sensitivity Counting Techniques”, Pergamon Press, Oxford, 1964.
RECEIVED for review February 14, 1977. Accepted April 11, 1977.
Digital Device for Precise Determination of Drop Times at Dropping Mercury Electrodes Pamela J. Peerce and Fred C. Anson” Arthur A. Noyes Laboratory, California Institute of Technology, Pasadena, California 9 1 125
Thermodynamic information on the adsorption of ions and molecules a t mercury electrodes is often obtained from electrocapillary curves (1). The necessary values of the interfacial tension can be determined directly by means of a capillary electrometer or evaluated from measurements of the natural drop time of a dropping mercury electrode ( 2 ) . The tedious experimental aspects of the former approach are well known. The latter procedure is more attractive because it is experimentally simpler and readily automated (2-6). Several methods for detecting drop fall have been proposed previously (5). The two most common employ photoelectric or impedance measurements to detect the end of drop life. Corbusier and Gierst (2) were among the first to suggest the photoelectric method, which requires that the capillary be positioned very precisely in the light beam. However, their device and several of those based on impedance measurements ( 3 , 4 ) do not allow the lifetimes of consecutive drops to be measured. In this note, a simple, digital drop timer is described which provides a rapid, convenient means for determining the lifetimes of up to 1024 consecutive drops. The potential of the electrode can be controlled by a standard, commercial polarograph. (A Princeton Applied Research model 174 Polarographic Analyzer was utilized, but any polarograph having a current measuring circuit with an output which can be adjusted for dc level and polarity would be acceptable.) 1270
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
The experimental arrangement for measuring drop times is the same as that for recording a polarogram. The current output of the polarograph is connected directly to the input terminal of the drop timer. Since the device can be set to time up to 1024 consecutive drops at each potential, drop lifetimes can be determined very accurately. Once started, the timer runs continuously without operator intervention and displays the number of clock pulses elapsed during the last run while performing the next one. This permits the operator to record the result of each run while the next run is in progress. The digital output of the timer is well suited for interfacing to a computer if even greater automation is desired.
EXPERIMENTAL Description of the Instrument. The drop timer is shown schematically in Figure 1. The abrupt change in current which accompanies the detachment of each drop from the capillary is used to sense the end of drop life. The input signal to the timer is sharpened by a Schmitt trigger, IC1, which is set to fire when the negative edge of the signal at its input crosses the threshold level of +2.6 V. (Since the timer accepts only positive input voltages, it is necessary to reverse the polarity of the current output when the current changes sign.) The clock frequency is 120 Hz. The unrectified, 60-Hz signals from the transformer secondary are differentiated and fed to separate inputs of the Schmitt trigger, IC2. Normally, as one input is high while the other is low, the exclusive OR output is high. However, just before and after the voltage reverses direction (twice
