The mercury concentration in the SRM Orchard Leaves also was determined using flameless atomic absorption spectrometry and isotope dilution spark source mass spectrometry. The results obtained by these techniques were 0.160 + 0.012 and 0.145 i 0.022 ppm of mercury, respectively. A second new Standard Reference Material, Beef Liver, with a considerably lower mercury content was also analyzed by this technique. Nine samples were analyzed using the bromine separation. The samples contained 0.0145 0.0034 ppm of mercury. The individual results are given in Table IV. The mercury concentration in the SRM Beef Liver was determined using flameless atomic absorption spectrometry. The results obtained by this technique were 0.0160 0.0012 ppm of mercury.
*
*
ACKNOWLEDGMENT
The cooperation of T. C. Rains, P. J. Paulsen, and Robert Alvarez is gratefully acknowledged. Their investigations and analyses gave special credence to this work. The Authors would also like to thank H. P. Yule for his help in the data reduction. RECEIVED for review October 26, 1971. Accepted February 7, 1972. In order to specify procedures adequately, it has been necessary to identify commercial materials and equipment in this report. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Preparation of Gas Samples for Liquid Scintillation Counting of Carbon-14 R. E. Bosshart and R. K. Young Bituminous Coal Research, Inc., 350 Hochberg Road, Monroeuille, Pa. 15146
A scintillation procedure for determining carbon-14 in gases is described. Samples are collected in evacuated cylinders and passed through a catalytic oxidation unit. The carbon dioxide formed is trapped in a pre-weighed amount of ethanolamine contained in a counting vial. Methanol and scintillator solution are then added to form the scintillator cocktail. The radioactive concentration of the gas is determined by multiplying the ratio of the known volume of carbon dioxide trapped to the mole fraction of the carbon components, determined by gas chromatography, by the disintegrations per minute counted in the ethanolamine-carbonate. The counting efficiency is a constant value of 72.0%. The method is precise and requires a minimum of operator attention. The calculated detection limit is 0.04 dpm/ml (STP). MANYMETHODS have been reported for the determination of carbon-14 by incorporating carbon dioxide into a liquid scintillator solution (1-4). The majority of these involve complete oxidation of the carbonaceous material (e.g., biological samples or highly quenched liquids) to carbon dioxide, which is then trapped in an appropriate base. However, few liquid scintillation procedures have been developed to determine the radioactive concentration in a gas containing carbon-14 in other forms such as carbon monoxide or hydrocarbons. This paper presents a method in which the radioactive concentration of a gas is determined by catalytic oxidation of all carbon components to carbon dioxide. The carbon dioxide is trapped in a preweighed amount of ethanolamine contained in a counting vial. Methanol and scintillator solution are added, and the mixture is shaken until homogeneous. (1) D. L. Horrocks, Znt. J. Appl. Radiar. Isotopes, 19, 859 (1968). (2) E. Rapkin, “Measurement of C1402 by Scintillation Techniques,’’ Tech. Bull. No. 7, Packard Instrument Co., Inc., La Grange, Ill., 1962, 8 pp. (3) R. W. Smith and S. H. Phillips, Znt. J. Appl. Radiat. Isotopes, 20, 553 (1969). (4) J. C. Turner, “Sample Preparation for Liquid Scintillation
Counting,” Radiochemical Centre, Amersham, Bucks, England, 1967, 30 pp.
The sample is then cooled to 5 “C and counted in a liquid scintillation counter. EXPERIMENTAL
Apparatus. The composition of the gas samples was determined by using an F & M Model 700-231 gas chromatograph (Hewlett-Packard Co., Avondale, Pa.). The carbon-14 samples were counted in a Mark I liquid scintillation computer Model 6860 (Nuclear-Chicago Corp., Des Plaines, Ill.). All samples were counted in Spectravial I1 glass counting vials (Amersham/Searle Corp., Arlington Heights, Ill.). The procedure was standardized by combusting tagged, liquid samples in a Model No. 1101, double-valve, selfsealing, 360-ml oxygen combustion bomb (Parr Instrument Co., Moline,Ill.). The sampling system used is shown in Figure 1. The manifold was a l/r-inch piece of stainless steel tubing to which short sections of l/S-inch stainless steel tubing were attached with silver solder. The sample cylinders (Model A-4, A. C. Tank Co., Burlington, Wis.) were fitted with regulating valves (No. 1RM4-S4-316, Whitey Research Tool Co., Emeryville, Calif.) and connected to the ‘/s-inch tubing by means of Swagelok fittings for easy removal. The pressure regulator (Model 11-14) was purchased from Matheson Co, (East Rutherford, N.J.) and the control valves were purchased from Whitey Research Tool Co. The flow meter was corrosionresistant and had a range of 0 to 100 ml/min. (Any flow meter or flow indicator could be used since it is not necessary to know the precise flow rate.) The oxidation train was constructed as shown in Figure 2. One-eighth inch stainless steel tubing was used between the sample cylinder connection and the flow meter. Thereafter, glass tubing with ball and socket joints was used. A Matheson low pressure regulator (Model 71) and a Matheson microflow valve (Model 151) were used in conjunction with a Brooks flow meter to control the flow rate. Hydrogen sulfide was removed by bubbling the gas stream through a 6Z solution of cadmium sulfate saturated with sodium sulfate. Water was removed by passing the gas through absorption bulbs filled with indicating Drierite. The oxidation equipment was purchased from Burrell Corp. (Pittsburgh, Pa.) ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1117
flow
Gauge
st
\
J
v
Gas Sampling Cylindors
Figure 1. Stainless steel sampling system Flow Motor
Constant
n
m td-7.3I r‘! I n I
Trap
t
Micro rroi Cant--'
valve
”-
+J
I
:.+
-
H ~ O
co2+c14o2
Trap
Trap
Exhaust
Oxidation Unit
W N2 Tank
Figure 2. Flow diagram for catalytic oxidation of radioactive gas samples
and consisted of a catalyst tube (No. 40-337) plus heater (No. 40-378) and a copper oxide tube (No. 40-333) plus heater (No. 40-372). A gas mixing stand was used to pressurize the sample cylinders with helium and oxygen. This was constructed of a 1/4-inchstainless steel manifold connected to a three-way valve which led to the atmosphere, a manometer, and a vacuum pump. One-eighth inch stainless steel tubing was used to connect the manifold to the sample cylinders and to high purity gases. The cylinders were pressurized to the appropriate level by observing the partial pressure on the manometer. An infrared heat lamp was used to mix the gaseous components after additions of helium or oxygen. The cylinders were heated to 50-60 “C and then cooled to room temperature before analysis. Reagents. All chemicals were ACS reagent grade unless otherwise indicated. The ethanolamine was purchased from Packard Instrument Co. (Downers Grove, Ill.). The scintillator solution was composed of 9.0 grams of scintillation grade PPO (2,5-diphenyloxazole, Packard Instrument Co.) dissolved in 1 liter of xylene. This solution was stored under refrigeration in low actinic glass bottles. Sampling. The gas samples were collected in 1800-ml evacuated, stainless steel cylinders using the system shown in Figure 1. For high pressure samples, both control valves were opened and the pressure was adjusted to obtain an adequate flow for purging the manifold. Then the valve on the appropriate cylinder was opened (while watching the flow meter to ensure there was a positive flow from the sample inlet). When the cylinder pressure equaled the manifold pressure, the outlet control valve was closed and the cylinder was pressurized to 3 atm (gauge). This pressure gave more than an adequate volume for gas chromatographic and carbon- 14 analyses. 1118
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
Samples were also collected at atmospheric pressure by purging the manifold, closing the outlet control valve, and very slowly opening the cylinder valve. Samples collected in this manner were later pressurized with helium to 3 atm (gauge). Gas Chromatographic Analysis. The samples were analyzed by gas chromatographic methods using helium as the carrier gas as described by Chang and Glenn (5). The raw area counts were converted to percentages and normalized to 100.00%. The individual carbon component percentages were recorded and used in later calculations. Catalytic Oxidation Procedure, Oxygen was added to a cylinder in a sufficient quantity to ensure complete oxidation of all combustible components (6). The cylinder was then heated to thoroughly mix the gases, cooled to room temperature, and attached to the oxidation train shown in Figure 2. The cylinder valve was opened and the flow rate was adjusted to approximately 30 ml/min. The COz 14C02 were trapped in the apparatus shown in Figure 3. After 30 minutes, the cylinder valve was closed and the oxidation train was purged with nitrogen for an additional 30 minutes. This served the dual purpose of sweeping residual carbon dioxide from the train as well as any oxygen which may have been dissolved in the ethanolamine. The gas absorption apparatus was removed and weighed to determine the gain of COz l4cO2. Ten milliliters each of methanol and scintillator solution (in that order) were
+
+
(5) T.L.Chang and R. A. Glenn, “Gas Chromatographic Methods
Developed on the BCR Two-Stage Super-pressure Gasification Pronram.” Amer. Chem. SOC..Diu. Fuel Chem. Preprints, 12(3),
l i l g (1968). ( 6 ) “Burrell Manual for Gas Analysts,” 7th ed., Burrell Corporation, Pittsburgh, Pa.. 1951, p 30.
added using volumetric dispensers. The mixture was shaken until homogeneous and placed in the liquid scintillation counter overnight before counting. Gas Absorption Apparatus. Approximately 1.5 grams of ethanolamine was added to a tared counting vial. The rubber stopper holding the two pieces of glass tubing (see Figure 3) was inserted and the apparatus was weighed. The trapping apparatus was then connected to the oxidation train by a short piece of Tygon tubing. The reaction between the carbon dioxide and the ethanolamine produced a viscous mixture which adhered to the sides of the glass tube. For this reason, the difference between the initial and final weights of the gas absorption apparatus was not the weight of the COZ lCOz ultimately counted. This was determined by removing the rubber stopper, weighing the counting vial, and applying the following equation :
1
Figure 3. Apparatus for trapping COZ W O a
+
1. Gasinlet 2.
+
W=
(D - 0 ( E - A) (D - c>
(B - A )
+
2
5
Gasoutlet
3. Glass tubing, 5 mm 0.d. 4. Rubber stopper, No. 1,two-hole 5. Counting vial, 22-ml volume 6. Ethanolamine, approximately 1.5 grams
(1)
where
+
W = Weight of the COz l4COZtrapped and counted A = Tare weight of the counting vial B = ( A ) plus approximately 1.5 grams of ethanolamine C = Weight of the trapping apparatus plus the ethanolamine D = (C) plus the COz 14C02weight gain E = ( A ) plus the ethanolamine-carbonate mixture
+
Standardization. The procedure was standardized by combusting benzene-14C, of known radioactive concentration, in a Parr bomb. Between 0.5 and 1 ml of benzene-IC was placed in the sample holder. (It is not necessary to know the exact volume as will be shown in a later calculation.) The fuse wire was attached, the bomb assembled, and oxygen was added to a pressure of 24 atm (gauge) as prescribed in the Parr manual (7). After combustion and a temperature equilibrium period of about 10 minutes, a piece of pressure tubing was attached between the bomb relief valve and the inlet of the oxidation train. The relief valve was opened and the COZ 14C02were trapped as previously described. The disintegrations per minute in the counting vial were calculated by the following equation:
+
where V, = Milliliters at STP = Weight of COz 14COzfrom Equation 1 W 22,414 = Milliliters (STP) per mole = Molecular weight of carbon dioxide in g/mole Mc The mole fraction of the carbon components in the sample, adjusted to a carbon dioxide equivalent basis, was calculated as follows:
+
where
X, = Total mole fraction of carbon components on a n
N x
= = =
carbon dioxide basis Number of carbon components in the sample Number of carbon atoms in the component Mole fraction of the carbon component
Combining Equations 3 and 4 with the disintegrations per minute from the trapped COz l4COZgives the radioactive concentration of the sample.
+
where
R Mb
W 6 Mc Db
Radioactive concentration in the benzene in dpm/ml = Molecular weight of the benzene in g/mole = Weight of the COz l4CO2trapped and counted from Equation 1 = Number of moles of carbon per mole of benzene = Molecular weight of carbon dioxide in g/mole = Density of benzene in g/ml at room temperature =
+
Calculations. Since the initial sample volume was unknown, the radioactive concentration was determined by taking a ratio of the known volume of COZ l4CO2trapped in the ethanolamine to the mole fraction of the carbon components, adjusted for the number of carbon atoms, and multiplying by the disintegrations per minute counted in the ethanolamine-carbonate mixture. The volume of COr ICOZtrapped in the counting vial was calculated by :
+
+
(3) (7) “Oxygen Bomb Calorimetry and Combustion Methods,” Tech. Manual No. 130, Parr Instrument Co.,Moline, Ill., 1960, 56 PP.
_dpm _ _ -(dpm) (Xd ml(STP) VZ
(5)
However, since the counting efficiency was a constant value, Equation 5 can be expressed as:
_dpm _ _ - (cpm) (XA ml (STP)
(6)
- (0.720) ( V , )
RESULTS AND DISCUSSION
Counting EWciency. The counting efficiency was determined by combusting benzene- 1YS, of known radioactive concentrations, in a Parr bomb. This technique was quite simple and was similar to the one used by Sheppard and Rodegker (8). Twenty-two samples were analyzed using three different concentrations of tagged benzene and the results are shown in Table I. The average counting efficiency was 72.0 f 0 . 6 z with a range of 70.5 to 73.2 The weight of ethanolamine used varied from 1.4207 to 1.8105 grams and the weight gain of carbon dioxide ranged from 0.3406 to 0.4795 gram. Neither the weight of the
z.
(8) H. Sheppard and W. Rodegker. Anal, Biochem., 4, 246 (1962). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
0
1119
Table I. Determination of Counting Emciency by Oxidizing Standardized Benzene-1V: in a Parr Bomb No. of Dpm/ml (STP) Average dpm trapped Average observed Average counting samples counted in the product gas in the ethanolamine counting rate, cpm" efficiency, 15 4 3
(1
6.119 14.02 27.74 All samples were counted for 100 minutes and corrected for
1,283 2,495 4,920
925.9 1,785 3,541 a background count rate of 33.6 cpm
ethanolamine used nor the weight gain of the carbon dioxide could be correlated with the counting efficiency. Therefore, it was not necessary to derive a quench correction curve, and the average counting efficiency was used in all dpm calculations. Precision. The precision of the method was determined by analyzing 77 duplicate samples which ranged from 4.07 to 13.78 dpm/ml (STP). The standard deviation, as calculated from Equation 7 (9),was 0.07 dpm/ml (STP). The average difference between duplicates was 0.08 dpm/ml (STP).
(7) where s = Standard deviation of a single measurement d = Difference between duplicates n = Number of duplicates analyzed
Catalytic Oxidation Unit. The catalytic oxidation unit was tested by passing a gas mixture containing carbon monoxide, methane, and hydrogen (each over 10% concentration by volume) through the unit at flow rates up to 50 ml/min. The product gas was channeled into a sampling loop of known volume and injected into a gas chromatograph. No peaks were observed for the three original components. Effect of Weight Loss in the Ethanolamine. Since the critical parameter in the method is the weight gain of the trapped COz lCOz, the possibility of a weight loss in the ethanolamine due to the flow of oxygen and nitrogen was investigated. A mixture of oxygen and nitrogen was bubbled at a flow rate of 30 ml/min for 30 minutes through a preweighed trapping apparatus containing ethanolamine. Nitrogen was then bubbled through the ethanolamine for 30 minutes and the trapping apparatus was re-weighed. The weight loss was less than 1 mg. The same conditions were employed using various mixtures of ethanolamine-carbonate. The weight losses varied from 0 to 1 mg. Therefore, it was concluded that if the flow rates and purge times were kept reasonably constant, any error from these weight losses would be negligible. Scintillator Cocktail Composition. Ethylene glycol monomethyl ether was initially used in the scintillator cocktail in the same ratio as described by Jeffay and Alvarez (IO). However, after inconsistent and, at times, quite high background count rates were experienced, methanol was substituted for the ether. A mixture of ethanolamine, methanol, and xylene in a ratio of 1.5 :10: 10 by volume resulted in reproducible background count rates.
+
(9) W. J. Youden, "Statistical Methods for Chemists," John Wiley and Sons, New York, N.Y., 1951, p 16. (10) H. Jeffay and J. Alvarez, ANAL.CHEM., 33,612 (1961). 1120
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
72.16 71.59 71.96
Optimum Amount of Ethanolamine. The amount of ethanolamine which can be dissolved in a 22-ml counting vial depends upon the solvent used, the freezer temperature, and the amount of carbon dioxide trappedin the ethanolamine. To determine the maximum solubility of ethanolamine, the volumes of methanol and scintillator solution were held constant at 10 ml, the freezer temperature at 5 "C, and varying amounts of ethanolamine, saturated with carbon dioxide, were prepared and stored overnight. It was found that 1.5 grams of ethanolamine could be completely saturated with carbon dioxide without forming an emulsion. However, since the ethanolamine is rarely saturated, an amount from 1.4 to 1.7 grams can be used, which means that the ethanolamine can be added to the counting vial by an automatic dispenser. As a general rule, if the total weight of ethanolamine-carbonate is less than 2.2 grams, the scintillation cocktail will remain homogeneous at 5 "C. With a flow rate of 30 ml/min for 30 minutes, approximately 0.42 gram (9.5 moles) of carbon dioxide can be trapped in 1.5 grams of ethanolamine. Detection Limit. The limit of detection of the method, using the equation defined by Moghissi et ai. (11) was calculated as follows: ,-
where pCi/g = Picocuries per gram of carbon counted B = Background in cpm E = Efficiency in cpm/dpm M = Weight of the sample in the scintillator cocktail in grams Assuming 0.4 gram of carbon dioxide (0.1092 gram of carbon) is trapped, counted with an efficiency of 0.720, and the background is 33.6 cpm, the limit of detection is 33.2 pCi/g(carbon). Expressed in terms of volume of carbon dioxide, the limit is 0.04 dpm/ml (STP).
CONCLUSIONS
Using the basic procedure described, it is possible to determine the radioactive concentration of the individual components in the sample. For example, if the radioactive concentration of the hydrocarbon components is desired, the sample is first passed through the catalytic oxidation unit and the dpm/ml (STP) are determined. Next the catalyst tube is replaced by a copper oxide tube which will oxidize only carbon (11) A. A. Moghissi, H. L. Kelley, J. E. Regnier, and M. W. Carter, Int. J. Appl. Radiat. Isotopes, 20, 149 (1969).
monoxide and another portion of the sample is passed through the oxidation train. The procedure is basically the same as for catalytic oxidation except the flow rate is decreased to 20 ml/min and the oxidation time increased to 40 minutes. Also, the computation of the mole fraction (Equation 4) must reflect the fact that only carbon monoxide and carbon dioxide are involved in the calculation. The radioactive concentration of the hydrocarbon
components is then determined by taking the difference between the catalytic and copper oxide results.
RECEIVED for review September 30,1971. Accepted February 16, 1972. This paper is based on work conducted by Bituminous Coal Research, Inc., for the Office of Coal Research, U.S. Department of the Interior, under Contract No. 1401-0001-324.
SemiintegraI EIectroanaIysis: Theory and Verification Morten Grenness’ and Keith B. Oldham Trent University, Peterborough, Canada
Theory is presented which describes the properties of m, the semiintegral of‘ the faradaic current which flows when complete diffusion control follows a preexisting equilibrium. According to the predictions of the theory, m will be proportional to the concentration of the electroactive species, to the square root of its diffusion coefficient, to the number of electrons involved, and to the area of the electrode. The theory has been verified using the reduction of Cd2+ and O2 at a mercury electrode. These studies demonstrate that the semiintegral electroanalysis method i s valid for the measurement of concentrations and diffusion coefficients, and possesses a number of novel and potentially attractive features.
A PRELIMINARY ANNOUNCEMENT of the new analytical method “semiintegral electroanalysis” has already been made (I); the present article presents the theory in some detail together with an experimental verification. THEORY
Consider an electroreducible species Ox to be dissolved in solution at a concentration C. Two or three electrodes are immersed in the solution, one of these being the working electrode at which the electrode reaction Ox
+ ne-
-+
Rd
(1)
is possible. Initially, however, this reaction does not occur, either because the working electrode is polarized at a potential sufficiently positive to inhibit reaction 1 or simply because the circuit is open. Commencing at time t = 0, a signal is applied to the working electrode as a result (though not necessarily as an immediate result) of which reaction 1 occurs. If transport of Ox to the working electrode is solely as a result of semiinfinite linear diffusion with D being the diffusion coefficient, then the equations
C(co ,t>
=
c
(3)
Present address, School of Mathematics and Physics Sciences, The University of Sussex, Falmer, Brighton, England.
(1) K. B. Oldham, ANAL.CHEM., 44, 196 (1972).
and C(r,O) =
C
(4)
apply where C(r,t) denotes the concentration of Ox at distance r from the electrode surface at time t . Moreover, the flux of Ox at the surface of the working electrode is given by
D
a
-
br
C(0,t) = -
nAF
where A is the electrode area, F is Faraday’s constant and i(t) denotes the cathodic faradaic current. Now, it has been demonstrated rigorously (2)that the relationship
is a direct consequence of Equations 2 through 5. Here m(t) denotes the semiintegral (7) of the faradaic current i(t) with respect to time. The operation of semiintegration has been explained in the electrochemical literature (2, 3) ; the result of this operation applied to i(t) is to produce a quantity m(t) intermediate between the current i(t) and the integral
of the current, i.e., the charge passed. As an alternative to regarding m(t) as the semiintegral of the current, as defined in Equation 7, it may, with equal validity, be regarded as the semiderivative ( 2 ) of the charge,
To exemplify the operations of semiintegration and semidifferentiation, Table I lists some particularly simple instances of i(t), together with the corresponding m(t) and q(t) functions. The interrelationship of these three quantities is summarized in the diagram (2) K. B. Oldham and J. Spanier, J. Electroanal. Chem. Interfacial Electrochem., 26, 331 (1970). (3) K. B. Oldham, ANAL, CHEM., 41, 1904, (1969). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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