Improvements in Isotopic Carbon Assay and Chemical Analysis of Organic Compounds by Dry Combustion DAVID R. CHRISTMAN, NANCY E. DAY, PATRICIA R. HANSELL, and R. CHRISTIAN ANDERSON Chemistry Department, Brookhaven National Laboratory, Upton, Long Island, N . Y. Modifications in the apparatus and combustion procedures used in this laboratorj afford certain advantages over the original design. Iniprovements are described with regard to the combustion tube filling and arrangement, the renioval of nitrogen oxides, the trap and manometer sj stem, and the carbon-14 assay method, using proportional gas counters.
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~ I P O R T A S Tmodifications have been made in the apparatus ( 1 ) by which micro samples of organic compounds are burned t o carbon dioxide and water, the latter being measured in a low pressure gas manometer and then employed directly for isotopic assay. I n the search for improvements in what has proved in 4 years to be a basically sound apparatus, several changes have afforded advantages in rapidity of operation as well as accuracy and reproducibility of results. The alterations are chiefly in the combustion tube and filling, the removal of nitrogen ouides, the trap and manometer system. and the isotopic assay for cnrbon14. COMBUSTION TUBE AND FILLING
The combustion tube, its filling, and operation have been changed as described by Kirsten (8, 9) (Figure 1). This system operates a t 930" to 970" C. with a minimum of tube filling and 011 the principle of thermally cracking the organic compound first and t,hen oxidizing the reactive fragmenk in the flowing oxygen. As a result, combustions are complete in much shorter times, preconditioning time for new tubes is greatly reduced, and the likelihood of radioactive contamination from sample to sample is essentially eliminated. The sample is placed inside a platinum thimble and run directly into the main combustion furnace. The oxygen flow is about 30 cc. per minute, and the tot,al conihustion time is only 30 to 40 minut,es. This filling, which consists
of a nickel fin and sleeve, has been satisfactory with all compounds tried so far and is less tedious to use than the standard copper oxide filling. An automatic advancing mechanism is used to move an electromagnet which pushes the sample into the furnace; it is capable of varying speeds, but normally about 10 minutes is required to move the sample into the furnace. The parts used inside the combustion tube are conveniently prepared by hand from pure 10-mil nickel foil. Just prior to insertion in the quartz tube, these nickel components are heated to about 130" C. for 30 minutes in concentrated sulfuric acid and washed, as recommended by Kirsten, to decrease the time required for obtaining a low blank. The silver, used in the form of balls of fine wire in the tube, must also be pretreated to reduce the blank. The procedure of Niederl and Niederl(l1) is satisfactory and consists of heating the silver components in porcelain boats in a quartz tube to 400" C. for 1 hour with hydrogen and then 1 hour with oxygen. When the parts are treated in this manner, a blank of less than 1 mm. in the carbon dioxide trap is found in 40 miuutes, following a preconditioning time of 1 to 2 days. -4small blank persists throughout the lite of the tube, but this is also observed \Then a Pregl filling is used, and its magnitude is such that it is ignored in actual practice. This blank is not due to Xvater, since if the oxygen purification train is functioning properly no detectable water blank is observed. 4 list of specimen annlpes 11-ith this filling is shon-n in Table I.
Table I. Compound Benzoic acid
Isocaproanilide o-Chlorobenzoic acid Anthracene Acetanilide
Glutaric acid Acridine H1
so4--
TWO-LIO?llD MINOMETER CWSTRXTI3U
d
Bensanilide
Sample, 3Ig. 5.255 8.237 5,901 10.680 6.007 4.724 6.894 6.028 7.702 6.340 9.026 7.519 5.205 6.769 4.450 4.359 4.442 5.950 4.654 7.103
Specimen Analyses ~~
c, %
Theoret. 68.84
75.35 53.04
94.34 71.08
H, 70 Found 68.87 69.13 69.12 68.88 68.44 75.23 75,lG 53.40 53,84 93.50 70.95
Theoret. 4.96
8.95 3.22 5.66 6.71
70,84
45,45 87.12 79.16
71.91 71 .OS 45.57 45.62 87.28 87.91 78.60 78.90
6.10 5.06 6 62
Found 4.83 4,89 4 91 4.93 4.99 9 02 8.96 3.22 3.21 5.69 6.76 6.72 6.76 6.66 6.04 6.07 5,13 5.23 5.69 5.72
PRESSURE REGULATOR FROM PilRlFIING SYSTEM
1
COI TRAP
H,O TRAP MnOt TRAP
Figure 1. Combustion line and gas measurement system, manometer construction, and pressure regulator
If the temperature observed on the thermometers in the wells of the volumes (VI and V2, Figure 1 ) is not reasonably constant (within about 2" C.), the calibration and analysis measurements should all be corrected to a standard reference temperature by a gas law calculation. Certain compounds which contain other elements in addition to carbon, hydrogen, oxygen, and nitrogen often prove intractable or cause undue attrition on the standard combustion tube. Iodoform might be cited, as well as alkali salts. The latter leave a residue which attacks the quartz tube, affects water values, and may form active carbonate which can undergo exchange and 1935
ANALYTICAL CHEMISTRY
1936 MANOMETER
thus introduce cross contamination. Such compounds may be handled by wet combustion, but the following procedure has been found satisfactory:
\
The samde to be burned is weighed by ~ . . ~~~~~
~~~~
tapeper joint. The oompound~is covered and mixed with 200 to 500 mg. of cupric oxide wire. The main tube is 6xed with Apiezon wax t o a 4mm. pressure stopcock with two joints, one for the tube and the other for attachment to the load-
With volitile ~chmpoe,.. ~lI.%lih.vochini. d c t a , 35, 217 (1950).
(10) Neville, 0. K., Nuclear Instrumelit and Chem. Corp., Chicago,
Ill., private communication.
(11) Xiederl, J. B., and Siederl. V.. ”Organic Quantitative Microanalysis,” p. 122, Wiley, New York, 1942. (12) Ibid., p. 135. (13) Shell Development Co., Emeryville, Calif., private communication. (14) Sinex, F. 31.,Plasin, ,J., Clareus, D.. Bernstein. W., Van Slyke, D. D., and Chase, It., J . Biol. Chem., 213, 673 (1955). (15) Van Slyke, D. D., Steele, R., and Plazin, J., Ibzd., 192, 769 (1951). RECEIVED for review M a y 0 , 19.55, Arcepted August 1 5 , 19%. RFsearch carried out under the auspice? of t h e U. 9 . I t o n i i c Energy Coinriiission.
Inherent Errors and lower limit of Activity Detection in Gas-Phase Proportional Counting of Carbon-I 4 DAVID R. CHRISTMAN and ALFRED
P. WOLF
Chemistry Department, 6rookhaven N a t i o n a l Laboratory, Upton, Long Island,
The precision of the method of carbon-14 assay used in this laboratorj-by measwenlent with gas proportional counters-is discussed from the standpoint of the identity and magnitude of the various errors involFed. The lower limit of activity detection in these gas proportional counting tubes has heen investigated,
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N VIEW of the interest which has been manifested in the carbon-14 assay method used in this laboratory (1, 4 ) , a discussion of the errors inherent in the system seems appropriate. The systematic errors affect all analyses equally and are therefore important only from the standpoint of the correctness of the absolute activity values reported. Others do not affect different analyses equally and therefore contribute to the deviation in results betneen samples. I n this article, the errors are discussed approximately in the order in which they are encountered in prartice, folloned by a discussion of the lower limit of activity detection using the present method of analv+. The equation used t o calculate the Ppecific activity of a given sample is: net counts per minute Mpc./mg. C = 2220 X T’e X E X mg. C The constant 2220 is the number of disintegrations per minute in 1 mwc. of activity. Ve is the fraction of the total counting tube volume, V t Jwhich is contained within the silvered cathode volume, V , and is therefore defined by Be = V s / V t( 2 ) . E is the apparent counting efficiency of the tubes within the volume fraction Ve, and is now determined by comparisorl with a standard supplied by the National Bureau of St,andards ( 3 , 7 , 8). The milligrams of carbon present in the counting tube is determined by pressure-volume-temperature measurement by means of a calibrated manometer (I, 4 ) . Each of these factors, except the constant 2220, is subject to one or more errors of various types. I n no case does the original weighing of the combustion sample have any bearing on the isotopic assay, so long as the analytical result indicates reasonably complete combustion of the sample. This holds because the sample is not quantitatively transferred t o counting tubes, but rather the amount which is placed in the counting tubes is measured by the pressure change on a calibrated manometer (1, 4 ) . The manometer on the sample loading line ( 4 ) is calibrated by observing the rise obtained from carbon dioxide evolved from weighed amounts of barium carbonate, b y decomposition with sulfuric acid. The purity of this barium carbonate, in terms of carbon dioxide content, must therefore be accurately known. It is also necessary to ensure quantitative evolution and trapping of the carbon dioxide evolved. As this is done a t a preswre of
N. Y.
about 10-3 mm. of mercury, great care must be taken t o prevent spraying of the barium carbonate powder when it is first attacked by the sulfuric acid. At the end of the reactmion,the acid must be heated until solution of the carbonate occurs, in order to ensure its complete decomposition. ‘The result of such measurements is expressed as centimeters of rise per milligram of carbon present, and the root mean square deviation from the mean of 12 determinations made on the sample loading line is f 0.73%. This error in the standard rise affects all samples equally and so need not be taken into consideration when comparing activities of different samples among themselves. d similar error occurs in the calibration of the manometer on the combustion line itself, 17hei-e the standard rise per milligram of carbon is determined b y averaging t.he rise obtained from a number of standard carbon-hydrogen sample runs and/or from carbon dioxide evolved from barium carbonate. The reproducibility of these determinations is of a similar order t o that obtained ivith barium carbonate on the loading line, so for counting tubes loaded directly from the combustion line and not on the loading line a similar error must) st’illbe considered. Correction should be made on bot.h calibration and analysis measurements t o a standard reference temperature, in order to minimize the effect of hemperatwe on the observed gas pressure. If this is done, the effect of this error is small (probably less than 0.2%). On the two-liquid manometers previously described ( 1 , 4 ) , the reproducibility of a given reading of the manometer is about 0.01 em., with t,wo readings being involved in each tube filling and each calibration determination. The percent’ageeffect of this error decreases as the amount of carbon dioxide being measured increases, but x i t h a filling of 1 mg. of carbon on a system where the standard rise is about 5 cm. per milligram of carbon, it amounts to f 0.2%. ilnother error stems from the calibration of the volume fraction, Ve, wit’hin the cathode volume of each counting tube. It is measured b y filling the tube t o the various levels necessary with redist.illed toluene from a buret, then calculating the fraction from the relative volumes so determined. .4s the use of buret readings taken to 0.05 ml. gives results nearly identical to those obtained by weighing the tube a t each stage, the more rapid buret readings are considered sufficient. However, while the root mean square deviation in results on a given tube, as determined by any one person, averages about i.0.25%, the deviation when the tube is calibrated by several different persons is =!= 0.4%. Results obtained by four different persons on one tube are as follows: 0.838 f 0.002; 0.845 + 0.001; 0.844 f 0.004; 0.847. The over-all value is 0.843 =!= 0.0035 (root mean square deviation cal-
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