ther to first weigh the sample in a platinum boat which was then placed in the sample holder, or to weigh the sample directly in the holder. For a given gas flow rate (typically 400 ml/min), a primary limitation on the weight of solid samples to be run is related to the volume of oxygen available for combustion. When the sample weight approaches a limit proportional to gas flow rate, smoke from the sample begins to be observed. Small increases in sample weight then lead to a large increase in smoke, which eventually attenuates the light beam beyond the capability of the IZAA automatic gain control to function correctly. In practice, we limited sample weight so that no more than 10% of the incident light was obscured by smoke. As Equation 2 shows, increasing the gas flow rate decreases the dwell time of the atoms in the absorption tube and, hence, the signal. Consequently, increases in sensitivity cannot be achieved by increasing both sample weight and gas flow. However, we have found that relatively large sample weights can usually be handled, since smoke is not always a limitation. Sample weights in the 20- to 30-mg range or higher are routinely run. This relatively high sample weight permits precise control of sample size, and makes it possible to extend measurements to lower concentrations of mercury. Results of measurements on calibrated solid samples are presented in Table I. Measurements with standard solutions were first carried out over a range of sample volumes in 5-pl steps a t a given mercury concentration. Strict linearity to a t least 20111 was observed. Next, a plot of signal us. concentration with a given (5 pl) volume sample showed linearity for mercury sample weights of 0.1 ng to more than 50 ng. To test for systematic effects, the following measurement parameters were varied: (1) CuO was introduced with the liquid standard to provide excess oxygen; ( 2 ) the oxygen carrier gas was replaced with argon to produce an oxygen deficiency; (3)
the pH of the standard solution was varied over an order of magnitude from 0.03 to 0.3N; (4)the anion of the standard solution was changed from C1- to S042-; ( 5 ) organic mercury solutions and inorganic mercury standards were separately measured, and then separately added to the sample holder and the results checked for additivity; (6) additivity was checked for mixed organic and inorganic mercury solutions; and ( 7 ) the oven temperature was varied over the range 850 to 1010 "C. The measurement results showed additivity for inorganic and organic samples under all conditions, and no dependence on the chemical parameters varied. Results of selected measurements are presented in Table 11.
SUMMARY The two-chamber furnace offers a useful and accurate means for reproducibly vaporizing, combusting, and dissociating samples in solid, liquid, or gaseous form containing volatile elements. An oxidizing carrier gas serves both as an aid to combustion of the sample and as a means for reproducibly carrying the vaporized sample from the combustion chamber to the absorption tube. Consequently. no prior chemical sample preparation is required for solid samples, and drying and ashing cycles are not necessary. Sample measurement times are reduced to about one minute. Solid sample weights in the tens of milligrams can be handled. When employed with the IZAA spectrometer, the automatic background correction permits the measurement of samples in the presence of residual interferences and smoke. Received for review December 17, 1973. Accepted March 21, 1974. Research supported by NSF RAXK Grant AG396 and in part by the U S . Atomic Energy Commission.
Improved Liquid Scintillation Technique for Environmental Monitoring of Iron-55
A. A. Moghissi,' E. L. Whittaker, D. N. McNelis, and R . Lieberman National Environmental Research Center. Las Vegas, Nev. 89 1 7 4
Iron-55 is a radionuclide of great biological significance produced by nuclear weapons and nuclear reactors. Among the several methods for low-level counting of iron55, proportional counting ( I , 2), X-ray spectrum counting ( 2 , 3 ) , and liquid scintillation counting (2, 4, 5 ) are being routinely used. Liquid scintillation is preferred over the other two counting methods because of ease of sample preparation and automatic operation. P r e s e n t l y V i s i t i n g Professor, O f f i c e of I n t e r d i s c i p l i n a r y P r o grams, Georgia I n s t i t u t e o f Technology, A t l a n t a , Ga. (1) "Radiochemical Determination of Iron-55," H A S L - 3 0 0 , Procedures Manual, 1972 edition. (2) F J. Cosoltto, N. Cohen. and H. G. Petrow, "Simultaneous Determination of Iron-55 and Stable Iron by Liquid Scintillation Counting." Anal Chem.. 40,213-15 (1968) ( 3 ) J. Lekven. Nature (London). 235, 284-286 (1972) (4) A. A Moghissi, "The Current Status of Liquid Scintillation Counting.' E. D . Bransome. Ed., Grune and Stratton. New York, N Y , 1970, pp 86-94. ( 5 ) J. D. Eakins and D A. Brown. lnt. .IAppl. Radrat lsotopes 17, 391 (19 6 6 ) .
Investigators that used proportional counting in their method electrodeposited the separated sample iron on a copper disk ( I , 3 ) . As much as 80 mg of iron can be electrodeposited on a 2-inch disk for proportional counting without exceeding the half-value layer (7.5 mg/cm2) for the 5.9 KeV X-ray of iron-55. Other investigators have reported on the measurement of iron-55 by liquid scintillation (2, 5 ) . However, in those reports, the amount of sample or sample plus carrier iron that can be accommodated in a 25-ml counting vial is limited to 20-30 mg without serious loss in counting efficiency (20 mg, E = 0.161; 30 mg, E = 0.108, 40 mg, E = 0.067; 50 mg, E = 0.032) (2). Recently an emulsion technique (41, capable of incorporating u p to 250 mg of iron and thus sufficiently sensitive to measure environmental levels ( 3 ) of 10-100 nCi/g Fe, was proposed. This method was based on solubilization of aqueous iron fluoride in a scintillation liquid-detergent mixture. Upon further investigation, it was found that in certain cases the emulsion was too sensitive to the pres-
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 9, A U G U S T 1974
* 1355
Table I. Relationship between Counting Efficiency i n cpm/dpm and the Volume of Added 49% HF i n the Presence of No Carrier and 200 m g Fe HF volume, ml
0.5 0.7 0.9 1.1
1.3 1.5 1.7
E (cpwdpm)
Efficiency
Fe, mg
TNSX7, 200 mg Fe
TNSX7, no Fe
TNSXS, 200 mg Fe
TNSXS, no Fe
0.184 0.179
0.241 0.221 0.217 0.208 0.225 0.195 0.188
0.196 0.186 0.164 0.156 0.135 ... ...
0.251 0.260 0.222 0.237 0.215 0.209 0,203
0,151
0,149 0.129
... ...
ence of HF and, thus, its routine application would become too elaborate. This paper describes a modified procedure for environmental monitoring of 55Fe. This procedure has been routinely applied and its reproducibility evaluated. EXPERIMENTAL Apparatus. A Beckman LS-200 liquid scintillation spectrometer was used for all counting. Reagents. All chemicals were reagent-grade and were used according to standard techniques. Standard iron-55 was obtained from National Bureau of Standards. Triton N-101 was obtained from Rohm and Haas, Philadelphia, Pa.; sodium xylene sulfonate from Pilot Chemicals, S a n t a Fe Springs, Calif.; and plastic scintillation vials from Kuclear of Chicago, Des Plaines, Ill. The scintillation liquid was prepared from p-xylene containing appropriate quantities of 2,5-diphenyloxazole ( P P O ) and p-bis(omethylstyryljbenzene (bis-MSBj. The detergent mixture was prepared by dissolving 50 g of sodium xylene sulfonate in 100 ml of water and mixing this solution with 1 liter of Triton 5-101. This mixture is henceforth abbreviated as T N S . The following three basic mixtures were used: 1) Xylene containing i g P P O and 1.5 g bis-MSB/I. and Triton 5 - 1 0 1 in a volume ratio of 2 : l . This mixture is henceforth abbreviated as T N X . 2) Xylene containing 7 g P P O and 1.5 g bis-NSB/l. and T N S in a volume ration of 2 : l . l . This mixture is abbreviated as TNSX7. 3 ) Xylene containing 9 g P P O and 1.8 g bis-MSB/l. and TNS in a volume ration of 2 : l . l . This mixture is abbreviated as TNSX9. Procedure. Iron is separated from environmental samples after complete oxidation of sample by wet ashing or dry ashing a t not more than 500 "C. Sample size containing as much as 250 mg of iron can be analyzed by this procedure. Since liquid scintillation is not considered to be very selective in counting for one radionuclide in t h e presence of other radionuclides, it is advisable-if not essential-to separate t h e sample iron from sample residue so as to achieve good decontamination from other radionuclides. However, selective spectrum counting may still need t o be done to account for iron-59 in t h e sample. Adequate methods of separating iron from sample residue t o give good decontamination from other radionuclides are given in t h e literature cited ( I , 3 ) . Included in those methods are procedures for separation by ion exchange, precipitation with cupferron in the presence of cobalt, manganese, and zinc hold-back carriers, and precipitation as ferric hydroxide with ammonia. Subsequent to separation from sample residue, the iron fraction or aliquot containing as much as 250 mg of iron was transferred to a 50-ml polypropylene centrifuge tube provided with a tight sealing cap. The iron was then precipitated as ferric hydroxide with ammonia. Since ammonia is a quenching agent in liquid scintillation systems, it is imperative t h a t t h e iron precipitate be washed thoroughly t o remove all ammonia. T h e precipitate was washed with various mixtures t o remove ammonia. T h e selected washing procedure consisted of three washings with 30 ml each of 50% aqueous ethanol and one washing with 30 ml of water, shaking vigorously in each wash. Subsequently, t h e precipitate was dissolved in an appropriate amount of 49% H F (0.6 ml for 250 mg of iron). The solution was then diluted to 10 ml with distilled water
1356
Table 11. Relationship between Counting Efficiency and the Quantity of Carrier Iron Using 0.6 m l of HF
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O .
0 50
100 150 200 250
EM
TNSX7
TNSXS
TNSX7
TNSXY
0.. 203
0.215 0.206 0.199 0.197 0.194 0.187
...
... 10.3 19.9 29.6 38.8 46.8
0.162 0.166 0.166 0.164 0.159
8.1 16.6 24.9 32.8 39.8
and transferred to a polyethylene scintillation vial. (An aliquot can be taken a t this point for iron recovery determination.) The centrifuge tube was washed three times with 5-ml portions of the scintillation liquid, and the rinse portions were transferred to the scintillation vial. The vial was shaken vigorously for approximately 1 minute and counted after storing overnight in the dark. Since the liquid scintillator solution is a p-xylene solution and pxylene will evaporate slowly through the polyethylene vial wall, samples should be counted within 72 hours and preferably within 48 hours from t h e time of preparation. Since the amounts of iron and H F in the counting vial both affect the counting efficiency, it is recommended t h a t the internal standard method be used for determining t h e counting efficiency. The sample is counted first and then recounted after the addition of iron-55 standard solution of not more than 0.1 ml (in dilute HF).
RESULTS AND DISCUSSION
Table I shows the relationship between the counting efficiency and the volume of added HF for the mixtures TNSX7 and TNSX9. It can be seen that HF reduces the counting efficiency for both mixtures. As a minimum amount of H F was necessary, 0.6 ml was subsequently used to conveniently dissolve the iron precipitate. Table I1 shows the effect of the presence of carrier iron on the counting efficiency. As a matter of convenience the product of E (cpm/dpm) and M (mg of Fe) is also included. It can be seen that EM increases with increasing quantity of Fe although the counting efficiency decreases. Unfortunately, quantities much above 250 mg of iron led to instability of the emulsion; thus, 250 mg was established as the maximum quantity of iron which can reproducibly be used. Experiments with T N X indicated a similar sensitivity to the presence of H F as previously reported ( 4 ) and thus, T N X cannot be advantageously used for a routine operation because of the possibility of a phase separation. Based on these experiments, the mixture TNSX9 with 0.6 ml of HF is recommended A maximum of 250 mg of iron can conveniently be used with this mixture. It has become customary to report the performance of low-level counting systems as Y-value ( 4 ) . This value is defined as the minimum limit of detection a t 1-minute counting time and 1-sigma counting error as follows:
where B is the background in cpm. With a background of 9 cpm, the Y value of the proposed method is 29 pCi/g Fe. Received for review November 16, 1973. Accepted February 25, 1974.
9 , A U G U S T 1974
