the efficiency of separation; without stirring impurity concentration was only slightly enhanced. Slower lowering speeds were more efficient than faster. Variations of 10" in the cooling temperature had little effect on the efficiency of concentration. Removing relatively small samples (1/40 to 1/60 of charge) obtained higher impurity concentrations in the samples removed. This takes advantage of the impurity distribution in a liquid R-hich is progressively frozen. This process, however, is best suited for low initial concentrations of impurities, as efficiency decreases as impurity concentration increases. The primary purpose of these studies is the concentration of impurities for identification. I n the identification of impurities in a sample of reagent grade benzene, benzene containing 0.25 weight yo impurities mas placed in a tube 28 mm. in inside diameter and stirred while being lon-ered a t 5 cm. per hour into the freezing bath a t -25" C. As a preliminary concentration step, a large (20%) fraction Tyas removed from the first cycle of the benzene charge. Analysis showed that it contained 65% ?f the total impurities in the charge. Thls fraction was then recycled in a tube 22 mm. in inside diameter and about 1 gram was removed and analyzed. This small fraction contained 44% of the impurities present at the start of the cycle and the concentration of impurities had been raised to 5.14 weight %, equal to a 20-fold increase over the original concentration of impurities, Analysis of the initial benzene charge revealed no detectable quantities of C7 paraffins or C8 and Cg aromatics (Table I). The small sample removed from the second cycle, hon-ever, reveals that all these were present in the original charge, although a t undetectable concentrations. As the material balance for the over-all experiment wis
Table
1.
Concentration of Impurities in Reagent G r a d e Benzene
Weight, g. Impurities, n-t. yG Impurity composition, wt. % Toluene Minimum Ce cyclics and/or mono-olefins Minimum C, crclics and /or mono-olefins" Minimum Ce paraffins Minimum C, paraffins Ca aromatics Cg aromatics
First Cycle Initial 1st charge fraction 73.3315 14 3033 0.25 0.81
Second Cycle Charge 13 6777 0 81
1st
fraction 0 9487
Calcd. Initial Conrn.. Wt. %' ~
~
~~~~
5 14
0.05
0.22
1.25
0 06
0.06
0 27
1.46
0.07
0.03 0.11
0.13 0.19
0.66
0.03 0.02
0.00 0.00 0.00
98.8%, i t is possible to calculate the concentration for these groups in the original charge stock. It is assumed that in the concentration procedure all impurities are affected equally-i.e., the relative composition of the mixed impurities remains the same and no impurity is concentrated preferentially. On this basis, the calculated concentrations for Cs and Cg aromatics in the initial charge are about 0.0005 and 0.001 weight %, respectively. This explains \Thy these groups were not detected in the initial charge by mass spectrometry. The C7 paraffins are calculated to be a t an initial concentration of 0.06 weight yo, but they are not detected in the initial charge, because the parent mass spectral peak for the C7 paraffins ( m / e = 100) is masked by the mercury peak (m/e = 100) when the C7 paraffin concentrations are low. (Mercury is present, because of its use in the sample introduction system of the spectrometer.) However, progressive freezing raised the concentration of the C7 paraffins to a level rrhere the peak was distinct and easily measured. The parent peak for the C6 paraffins \\-as contributed to by the undetected C7 paraffins; this explains lvhy the calculated value
... . .. ... ... ...
0.00 0.00 0.00
0.45 1.2'3
001 0.02
0.06 0.0005
0.001
for the Csparaffins is much lower than what was found. The total of the impurity concentrations calculated to be initially present in the charge (excluding the very low values for Ce and CS aromatics) is 0.24%. while that actually found in the initial charge is 0.25%. ACKNOWLEDGMENT
The authors thank the Geophysical Development Division, Gulf Research Bt Development Co., for the design and construction of the lon-ering mechanism. LITERATURE CITED
(1) Dickinson, J. I]., Eaborn, C., Chem. & Znd. (London) 1956, 959. (2) Glasgow, A. R., Ross, G., J . Research A7at2. Bur. Standards 57, 137 (1956). (3) Goodman, C. H. L., Research 7, 177
(1954). (4) Handley, R., Herington, E. F. G., Chem. & Znd. (London) 1956, 304. (5) Herington, E. F. G., Handley, R., Cook, A. J., Ibid., 1956, 292. (6) Rock, H., Natur~cissenschaften 43, 81 (1956). (7) Sc'hildknecht, H., AIannl, A., Angew. Chem. 69, 634 (1957). (8) Schumacher, E. E., J . Metals 5, 1428 (1953). (9) Schwab, F. W., Kchere, E., J . Research Natl. BUT.Standards 25. 747 (1940). (10) Zbid., 32, 253 (1944).
System for Sampling the Gases in Processed Transistors C. Gordon Peattie, G. R. Kornfuehrer, R. 6000 Lemmon Ave., Dallas 9,Tex. of a study of the processes occurring within encapsulated and canned transistors during aging tests, the problem arose of analyzing the gas in the transistor can. Because this gas ll-as believed to be present in small amounts and was of unknolyn composition, mass spectrometry was chosen. the problem One Of sampling the gas comprising the atmosphere within a transistor can, a system was devised t o permit crushing of the transistor can in an evacuated system ASPART
E. Trueb, and J.
R. Moreland, Texas Instruments Inc.,
and subsequent transfer of this gas to a mass spectrometer. The cEc h ~ 21-401 ~ d ~spec-~ trometer was used in the analytical, rather than the isotope, mode. T o avoid exposing the sample gas to selective absorption by the stopcock grease used in the inlet system of the spectrometer, the gas was admitted directly intotheionsource, Inamassspectrometer having a grease-free inlet system, this would have been unnecessary. The sample cylinder was a stainless steel tube 6 inches long and 0.460
inch in inner diameter with a mall thickness of 0.020 inch, closed a t one end with a helium-shielded arc weld. to a The Other end was flange having a 1.25inch face* Each cy1inder was hydrogen-fired and of the The glass Portion before use. inlet system also had a stainless steel flange sealed to it. With a Teflon 0ring and a n interthreaded coupling, a n easily demountable, vacuum-tight joint mas obtained. Three transistors, fastened to a strip VOL. 31, NO. 6, JUNE 1959
1125
of 12-mesh stainless steel gauze a t measured intervals, were inserted into the sample cylinder, which was then coupled tightly to the flange. The replacement inlet system was evacuated through the conventional mass spectrometer inlet system, then isolated by closing both Hoke vacuum valves. The transistors, whose locations within the sample cylinder were known from their positions on the wire gauze, were crushed bv squeezing the steel cylinder against the wooden block on which it rested, with a C-clamp. The strain produced by this action was insulated from the rest of the system by two retaining brackets and the flexible glass bellows. I n use this system proved most satisfactory, permitting rapid interchange of samples, positive crushing of the samples, and freedom from leaks. After completion of the work, it was found that Norton [Rev. Sci. Instr. 26, 238 (1955)l had anticipated this design in a system used for sampling the gas in
fi
T R 4,N S I S T O R
A S S BELLOWS
.INDER
vacuum tubes. However, the system described here is superior to Norton’s in three ways. Most important, use of a stopcock and consequent selective adsorption by its grease are avoided. Second, this system permits simpler,
more rapid interchange of samples. Third, samples can be crushed and analyzed immediately, while Xorton’s required removal of the sample tube to a vise for crushing of the electron tubes contained n ithin the sample tube.
Modified Molecular Pot Still H. E. Ungnade, University of California, Los Alamos Scientific Laboratory, Los Alamos, N. M.
of the coniinercially molecular pot still designed by Hickman [Hickman, K. C. D., Sandford, C. R., J . Phys. Chem. 34, 637 (1930); Morton, A. A,, “Laboratory Technique in Organic Chemistry,” p. 120, NcGrarv-Hill, Xew York, 19381 has been used for some time in these laboratories. It has advantages over other modifications [Riegel, B., Beiswanger, J., Lanzl, G.. IKD.EKG.CHEW, AKAL.ED. 15, 417 (1943); Groves, K., Legault, R. R., AKAL. CHEM. 29, 1724 (1957)l. MODIFICATION
A available
cleaning of the equipment after distillation. Standard-taper ground joints are satisfactory for stills up to 70 mm. in diameter; spherical joints or flanges are preferred for larger stills. [A 3-inch demountable pot still with flanges is available from Distillation Products, Inc. (“Information on High Vacuum Distillation,” p. 5, Type MS4-5, KO. 8203).] The design of the vacuum line prevents the loss of distillate observed r i t h some commercial models. The smallest still (Figure 1) is used
3
The three most useful models of the molecular pot still vary in capacity from 0.1 to 30.0 grams, and in outside diameter from 20 to 70 mm. A larger and more uniform distilling surface is provided by use of a flat bottom (a flat disk is sealed into the bottom), controlled heating by a Dural block equipped with cartridge heater (J/sinch diameter Chromalox C-414A cartridge heater, E. L. Wiegand Co., 7500 Thomas Blvd., Pittsburgh 8, Pa.) for the small stills, and a Glas-Col mantle (Glas-Col Bulletin 4, p. 22, No. 0-592. 150 W-110 V) or a Dural block for t h e 70-mm. still. The disadvantages of the common commercial still (Fisher Scientific Co., catalog No. 111, p. 366, No. 9-124) have been overcome by use of a ground-joint cooling finger and a sharply bent vacuum line. Cooling surfaces which may be rotated and removed permit the return of distillate during the degassing phase, and 1 126
ANALYTICAL CHEMISTRY
k
69 mm
primarily for distillation of analytical samples. It is equipped with a Dural heating block 29 mm. in outside diameter and a receiver holding a screw-cap vial. The distillation temperature is estimated by a direct-reading tliermocouple which fits into the l/s-inch hole in the block just below the heating surface. The cooling finger has a clearance of preferably 1.5 to 2 mm. from the malls of the still, and is cooled by running water, ice, or dry ice. A further modification has been suggested (K.C.
--
~-
Figure 1.
Semimicro molecular pot still
Figure 2. Molecular pot still 36 mm. in outside diameter and larger
