for the blank, required between the first and second end points.
2-iodobenzoic acid. Coniplete combustion was not achieved n-ith sodium oxalate.
RESULTS AND DISCUSSIONS
ACKNOWLEDGMENT
The results obtained for a variety of compounds are shon n in Table I. A mean absolute deviation of 0.3y0from the true result n-as found. I n general. organic compounds containing nitrogen, sulfur, boron. and alkali metals are readily analyzed. ~ ~ results were obtained for halogen compounds such as 2-chlorol~enzoicacid and
The authors thank Josef IYemeth for providing most of the pure samples used in this work.
-
LITERATURE CITED
FIT.J., Pvleloche, I-.\V.,~“Ele~ ~ Analysis,” mentary Quantitative p. 358, Row, Peterson &- Co., Evanston, Ill., 1957.
(1) Blaedel,
( 2 ) Gotte, H., Kretz, R., Baddenhausen H., AnQeW. CheVL. 6 9 , 561 (1957).
(3) Hempel, W., Ibid., 5 , 393 $11892). (4) Niederl, J. B., Niederl, V., Quantita-
tive Organic Analysis,” 2nd ed., p. 101, Wiley, New York, 1952. ( 5 ) Schoniger, JV., Alikrochim. Acta 1955, 123; 1956, 869. R . S. JUVET JENCHIIT
Noyes Chemical Laboratory University of Illinois Urbana, 111.
i ~ f RECEIVED for review September 25, 1959 Accepted October 29, 1959.
Gas-Proportional Counting of Carbon-14 and Tritium, and the Dry Combustion of Organic Compounds SIR: I n a previous report, the lower limit of activit’y detection and the precision with which it could be accomplished by gas-proportional counting were discussed ( 5 ) . Since that time, a n anticoincidence counter of conventional design, made to operate with a ring of proportional counters and n-ith two proportional sample counting channels, has been installed in this laboratory. By this means, both the precision and the lon-cr limit of activity detection have been significantly improved. The unit is siniilar to those used for carbon-I4 dating by gas-proport ional counting ( 6 ) , t’sc(’Ljt that no intwury shield is employed. Thc ordinary P y r m $740 or Kinible KG-33 glass 100-cc. counters ( 1 ) have shown backgrounds generally between 60 and $0 counts per minute inside a 2inch lcad shicld. The same counters in the anticoincmidrnce circuit have backgrounds betn-een about 16 and 25 c.p.ni., with a spread over several weeks’ time of about f1 c.p.ni. About 10 t o 15 c.p.m. of this background appears to be due, to radioactivity in the glass itself, as metal and quartz counters of similar size have backgrounds n-hich are lower by about that amount. However, the backgrounds of tubes made from recently purcliasetl samples of t’hese glasses have brcn consistently verj- much higher than those made before 1958, and they have bern very erratic. Different picxes of glass from the same shipment have recently yielded counters with widely different backgrounds, some having run as high as 200 c.p.m. Because the counters have been made othrrwise identical t o those used preriously, one can only conc.lude that the amount of radioactivity in these borosilicate glasses has rccently increased appreciably, and that the amount of activity now present can vary considerably from batch to
T. STOPCOCK
O 2 IASCARITE-ANHYDROhE SUPPLY
ABSORBER
COPPER T U B E W I T H B R A S S B A L L JOINTS
Figure 1. Top view of oxygeninlet system
batch. Glass intended for use in counters should now be checked for activity level before large numbers of counters are made from it, and older stocks of tubing should be used n h e n possible. Assuming that this effect is due to contamination by fallout of the alkali used in making borosilicate glasses, it might also prove fruitful to investigate glasses made in the southern hemisphere, where the amount of fallout is presumably less. Metal counters of a design similar to the ordinary glass counters can be made easily, either with glass ends sealed to Kovar or with soldered metal bottoms and Teflon connectors a t the upper end. An example of this type of counter has been described recently (IO). While Kovar itself is noticeably better than even the older borosilicate glass tubes with regard t o background, it is not as good as steel or nickel (and perhaps other metals) in this r e s p c t . Counters whose active volume is made of optical grade quartz, with the usual silver coating, also have escellent background characteristics. These have been made with a standard-taper ground on each end of the center section, with borosilicate ends sealed on by means of Apiezon n ax; the general construction is othern ise similar to the ordinary
Bernstein-Ballentine tubes ( 1 ) . These counters, when made with a total volume of about 100 cc., show backgrounds in a 2-inch lead shield of about 45 t o 50 c.p.m., and with the anticoincidence circuit about 5 to 7 c.p.m. I n the latter case, the background is seldom seen to vary more than about i 0 . 5 c.p.m. on a single filling, so long as the electronic circuits are norking properly. These tubes must be calibrated b j counting known amounts of activity several times in each tube, because the usual mcthod of filling n i t h toluene from a buret (8)is inapplicable. K i t h this counting system, assuming a background of 5 c.p.m. to be counted for 400 minutes, the standard deviation of the background is calculated to be AO.11 c.p.m. (as against 1 0 . 4 3 c.1i.m. for a similar tube in a 2-inch lead pig) ( 5 ) . For a sample having 10 c.p.m. (about 12 disintegrations per minute), counted for 180 minutes, the standard deviation is 1 0 . 3 0 c.p.m. and the net count is 10.0 10.32 c.p.m. Under the previous conditions, the deviation was calculated as 1 0 . 8 0 c.p.m. ( 5 ) . For a sample having 2.0 c.p.m., similar calculations indicate a net count of 2.0 1 0.23 (as against a standard deviation of 10.80 without the anticoincidence circuitry) ( 5 ) . Khereas about 2 c.p.m. was previously considered the 1013-er detection limit, it is obvious that 1 c.p.m. can be detected with certainty in a reasonable time when simple anticoincidence circuitry is used, and t h a t even 0.5 c.p.m. should be detectable if somewhat longer counting times are used. With gas-proportional counting, the lower detection limit and the precision with n hich tritium can be counted are the same as for carbon-14, because the counting efficiency is essentially the same for the tn o isotopes (6). Suitable VOL. 32, NO. 1, JANUARY 1960
131
~
tritium samples for this purpose are easily prepared by the method of Wilzbach (2, 9).
part of it was introduced from the end of the combustion tube in a manner somewhat similar t o that used with many Dumas nitrogen combustion lines ( 7 ) .
DRY COMBUSTION OF ORGANIC COMPOUNDS
A T-stopcock is now provided a t the exit end of the absorption tube of the oxygen purification system, with ball joints on each of the exit arms of the stopcock (Figure 1). A 1l4-inch copper tube with brass ball joints is used to connect one of these exit arms to the side arm of the combustion tube, and a similar copper tube is led to the end of the combustion tube. Instead of a cap, the combustion tube is provided with a n adapter to permit it to be joined to the second copper tube. After introduction of the sample, oxygen is allowed to back-flush through the adapter for about 30 seconds; then the ball joint on the adapter is joined to that on the copper tube (which moves easily on the ball joint at the T-stopcock). The flow of oxygen is immediately started by adjustment of the stopcock at the exit end of the combustion tube, and the movement of the sample into the main combustion furnace is started. During the entire combustion, oxygen flows both through the side arm and down the entire length of the tube, thus eliminating any possibility of vapor’s being trapped in a closed pocket of the combustion tube. KOsamples have exploded since this system n a s put into use.
While the combustion system used in this laboratory for isotopically labeled organic compounds has proved generally satisfactory (3, 4),a problem arose during the combustion of very volatile liquids. Specifically, during the combustion of several samples of pentane and one of benzene, the sample exploded with sufficient violence t o break the combustion tube after the closing of the cap a t the end of the tube, and before the start of the oxygen flow through the trapping system. Cooling of the portion of the tube around the sample with dry ice did not solve the problem. I t was concluded that this effect rvas probably due t o slight volatilization of the sample into the closed pocket between the end cap and the entrance of the side arm carrying oxygen into the tube, and that in this static system the leakage of some vapor from the sample toward the heated zone of the main combustion furnace allowed a flash back to occur. This was solved by splitting the incoming ouygen stream so that
ACKNOWLEDGMENT
The authors acknolwedge the help of Karl Walther and Casimir Kawrocki in the design and construction of the quartz and metal counters described. LITERATURE CITED
(1) Bernstein, W., Ballentine, R., Rev.Sci.
Instr. 21, 158 (1950).
(2) Christman, D. R., Chemist Analyst
46, 5 (1957). (3) Christman, D. R., Day, X. E., Hansell, P. R., Anderson, R. C., ANAL. CHEM.27, 1935 (1955). (4) Christman, D. R., Stuber, J. E., Bothner-By, A. A , , Ibid., 28, 1345 (1956). (5) Christman, D. R., Wolf, A. P., Ibid., 27, 1939 (1955). (6) Fergusson, G. I., lVucleonics 13, Xo. 1, 18 (1955). (7) Kirsten, W.,ANAL. CHEAI.22, 358 11950): 25. 74 11953). (Si Van’Slyke, 6. D.,’Steele, R., Plazin, J., J . Bid. Chem. 192,769 (1951). (9) Wilzbach, K. E., Kaplan, L., Brown, W. G., Science 118, 522 (1953). (10) Wolfgang, R., Mackay, C. F., Nucleonics 16, N o . 10, 69 (1958). DAVIDR. CHRISTMAN CATHERINE 31. PAUL Chemistry Department Brookhaven Kational Laboratory Upton, L. I., N.Y. RESEARCH performed under the auspices of the U. S. rltomic Energy Commission.
Stable Diazo Salts for Chromatographic Spray Reagents SIR: For a number of years investigators in the fields of mood and plant chemistry and related fields have employed diazotized amine spray reagents for locating phenolic materials on paper chromatograms. Commonly used spray reagents include diazo reagents prepared from sulfanilic acid ( I ) , benzidine (S),and p-nitroaniline (2). I n all cases, diazo spray reagents, because of their instability, were prepared immediately prior to use by mixing a n acid solution of the aromatic amine with a solution of sodium nitrite. Even stock solutions of these amines are somewhat unstable, and fresh solutions must be prepared from time to time. I n the course of our work on the paper chromatography of wood lignins and extractives and their oxidation products we learned of the commercial production of stabilized diazo salts of many aromatic amines ( 4 ) . These dyestuff intermediates are water-soluble powders which are stable a t room temperature, even for years. One of the available salts was Fast Red Salt GG, a stabilized diazo salt of p-nitroaniline, one of the aromatic amines we employed
132
ANALYTICAL CHEMISTRY
in our chromatographic studies. Accordingly, chromatograms of 20 phenolic compounds related to wood chemistry were sprayed with a 0.057, solution of Fast Red Salt GG, and the colored spots produced upon spraying with sodium carbonate solution were compared with those produced from the same compounds sprayed with diazotized p-nitroaniline solution prepared in the usual manner ( 2 ) . Colors nere identical in all instances, but the normal background color (probably because of coupling of the diazo salt n ith some unchanged p-nitroaniline) obtained by the older method was absent. Thus, commercially produced stable diazo salts of a number of aromatic amines provide readily available spray reagents for locating and identifying phenolic compounds (and aromatic amines) on paper chromatograms. Furthermore, they obviate the necessity for mixing stock solutions immediately prior to spraying and for periodic preparation of fresh stock solutions. A complete paper with a table of colors obtained with 20 phenolic compounds
and 30 different commercial stabilized diazo salts together with spectrographic studies on the effect of concentration on the nature of the colored spots produced is in preparation and will be published shortly. ACKNOWLEDGMENT
The authors are indebted t o Antara Chemicals Division, General -4niline & Film Corp., for the sample of Fast Red Salt GG. LITERATURE CITED
(1) Bsldridge, R. L., Lewis, H. B., J . B i d . Chem. 202,169 (1953). ( 2 ) Brav. H. G., Thoroe. K.V.# Khite, ’ K., B”zochem.J . 46, $71 (1950). (3) Koch, J. E., Krieg, W.,Chem. Ztp. 62, 140 (1938). (4) Laughlin, E. R., Institute of Paper Chemistry, Appleton, K i s . , private communication. IRNIX -4.PURL PATRICI.~ F. McCoy
The Institute of Paper Chemistry Appleton, T’vis.
