COMMUSICATIONS TO THE EDITOR
Feb., 1963
make secondary black membranes from the total chloroform-methanol extract (supernatant of step 2) but the thick regions tend t o solidify before the membranes become fully black and, after a few minutes, the gradually increasing resistance falls and the membranes then sometimes break. The addition of liquid additives such as tetradecane, mineral oil, squalene, caprylic acid, or a-tocopherol to the chloroform-methanol lipid solution prevents the solidification and allows the formation of secondary black to go to completion with high final resistance. For most purposes the best of these additives is atocopherol (fresh, non-oxidized and light amber in color) which, in amounts above 20% of lipids, makes for extremely rapid formation of complete secondary black in 10-30 sec. Squalene or caprylic acid can be used as single solvents without chloroform-methanol or other additives. The addition of cholesterol (2%) increases the dielectric strength of the excitable membrane, and one standard solution in use a t this time has the com2% cholesterol in position: 2% H20 washed lipids 1.0:2.2:2.8 a-tocopherol: CHC13: CH30H solvent, but wide variations are possible and the actual bilayer composition is probably not simply related to the bulk solution composition. This solution deterioratee up )n repeated use after several days to a week because of solvent evaporation or hygroscopicity. Techniques of Membrane Formation.-For the study of electrical properties the membrane solution is spread with a trimmed no. 3 sable hair paint brush on a 1.0-mm. diam. hole in the side of a 6 cc. polyethylene Beckman pH cup resting in a 60 X 20 mm. glass petri dish. A heated needle held in a drill chuck is used to melt a polished hole after pressing the mall of the pH cup to 0.2 mm. thickness with heated pliers acting over aluminum foil. One compartment usually is filled with 0.1 M KC1 a t pH 6.7, the other with 0.1 M S a C l at pH 7.3, and both solutions are buffered with 5"OmM cysteine or histidine. D.c. pulses or a.c. are applied through known resistors and the potential acroes the membrane is recorded through a cathode follower with an oscilloscope or recorder. Interference colors and black formation are observed under reflected light at 10-16 X magnification. The incidence of success in making full black membranes of high resistance within a few minutes is practically 10070. The membranes persist for a t least 24 hr. and probably are indefinitely stable. Aqueous solutions of test materials can be added several drops at a time to either compartment followed by withdrawing an equal volume and then stirred with a segment of rubber band. This results in only occasional breakage. For optical studies a metal, polyethylene, or hair loop of 1-2 mm. diameter is dipped into the lipid solution and then transferred through air into a beaker containing 0.1 M S a C l at 30-47'. Alternatively, the loop is put into the saline and the lipid solution applied to it with the brush. Electron micrographs are obtained by transferring a membrane formed on a hair loop into a small cup containing buffered Os04 for 10-20 min. and from there into another cup filled with 3% agar solution, the entire process carried out under saline a t 40". The surrounding saline then is drained, the agar hardened by lowering
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the temperature, and finally dehydrated by alcohol, embedded in Vestopal, and sectioned in the standard way . A discussion of the physico-chemical aspects of these methods and of the molecular structure of the bilayer will appear el~ewhere.~ (7) P. Mueller, D. 0. Rudin, H. Ti Tien, and W.C. Wescott, "Progress i n Surface Science,' J. F. Danielli and .4. C. Riddiford, Ed., in press.
DEPARTMENT OF BASICRESEARCH PAUL MCELLER 0. R U D ~ K EASTERN PESNSYLVAXA DOXALD PSYCHIATRIC INSTITUTE H. TI TIEK PHILADELPHIA, PENNSYLVAKIA WILIJAMC. WESCOTT RECEIVED NOVEMBER 24, 1962
SEPARATION AND DETECTION OF HYDROGES, TRITIUM HYDRIDE, AND TRITIUM AT LOW-LEVEL TRITIUM ACTIVITIES BY GAS CHROMATOGRAPHY
Sir: In a recent publication' the resolution of hydrogen, tritium hydride, arid tritium by gas chromatography at tracer levels (2.21 x 10-I mc. per standard ml.; NTI G 10-4) of tritium activity was reported. Approximately 3 standard ml. of a gaseous mixture of hydrogen, tritium hydride, and tritium containing total (lO-*Otritium in the mole fraction range to 10-17 total moles) and having an activity range of api,o mc./standard ml. has now proximately been resolved and analyzed. Resolution of the isotopes was effected 011 two 8-ft. 20% ferric oxide on alumina columns2 in series with helium carrier at 77°K. Preliminary deactivation and reactivation of the columns was accomplished by filling with distilled water follopved by heating at 145" for a period of 8-12 hr. under a low helium flow rate. Hydrogen elution was detected by thermal conductivityl and the tritium activity was adequately detected in an etha,nol-quenched G-M flow counter with helium as the carrier gas ai, atmospheric pressure as shown in Fig. 1. The counter was used in conjunction with a Suclear-Chicago Model 182A Scaler equipped with an impedance-matched pulse integrator attached to the scope output in such a manner that the count-rate increase upon elution of tritium hydride and tritium through the G-M tube could be recorded as a function of time on a 0-100 mv. (or 0-5 mv.) Brown electronic recorder. The cylindrical flow counter was constructed from Pyrex glass eo that the volume was about 45 ml. An end-to-end, shielded 0.05 mm. stainless steel wire probe and cylindrical copper-gauze cathode were employed in the detector. An inverted ethanol bubbler was employed in series with the counter to add quench gas to the helium carrier after elutioii from the chromatographic column. The helium flow rate through the detector was variable from 80-150 ml./min. A plateau for the counter a t about 2150 v. was observed with a, starting potential around 2000 v. dependent, of course, upon the amount of ethanol vapor added to the gas stream. Similar flow counters of different geometry3 and employing different quenching agents4 have been (1) H. A. Smith and E. €1. Carter, Jr., "Proceedings of the International Atomic Energy Agency Symposium on the Use of Tritium i n the Physical
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U
2 1.6 : 1.4 ';j 1.2 -
E 1.0 +
I
w
2
I
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25
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36 40 45 50 55 60 Time after sample introduction, min.
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Fig. 1.-Typical synchronized katharometer and pulse-inteT grator recorder traces for Hz-HT-Tz sample (3.1 std. ml., N ~ = 6.85 X 10-13, AET = 7.5 X 1 O - I o mc. per std. ml., N T ~= 8.44 X AT^ = 9.2 X lo-" mc./std. ml.) with helium carrier at a flow rate of 90 ml./min. through two 8-ft. ferric oxide on alumina columna in series a t 77°K.
described previously but were not used for direct hydrogen-tritium gas mixture counting in conjunction with prior chromatographic resolution of the isotopic components. No shielding other than from photo effects was employed and a residual background count rate of 150200 c.p.m. was observed. The scaler-pulse integratorrecorder system mas sufficiently sensitive to count adequately, amplify, integrate, and record total changes in the background of 30 c.p.m. over a several minute interval; hence no shielding was deemed necessary for the tritium detection. This system for detecting low-level tritium activity combined with the gas chromatographic method of concentration of the radioactive gas should make possible the determination of the tritium content of natural water with a minimum of electrolytic concentration and a relatively lorn cost for detection equipment. This application is under investigation. I n addition, deuterium content can be determined simultaneously. 6 Acknowledgment.-The authors are grateful to the United States Atomic Energy Commission for support of this research. (6) H. A. Smith and P. P. Hunt, J . P h y 8 . Chem., 64, 383 (1960); P. P. H u n t and H. A. Smith, ibid.. 66, 87 (1961); E. H. Carter, Jr., and H. A. Smith, in preparation.
E. H. CARTER,JR. CHEMISTRY DEPARTMENT HILTONA. SMITH THEUNIVERSITY OF TENXESSEE TENNESSEE KNOXVILLE, RECEIVED DECEMBER 17, 1962
A SECOND CRYSTALLINE PHASE OF XEXON TETRAFLUORIDE Sir: From an X-ray diffraction study of the new compound xenon tetrafluoridell a second monoclinic phase has been
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found in addition to that described by Siegel and Gebert.2 The modification of XeF4 described here is apparently the less stable form a t room temperature since it is produced somewhat less readily, although both condense from the vapor near room temperature. The greater density of this second phase probably indicates that it is a lower-temperature polymorph. Crystals of this form of XeF4 are visually distinguishable, by their pyramidal habit, from the platelets of the low-density form. The XeF4 was prepared by reaction of the elements,' identified, and its purity verified by infrared analysis in the apparatus described by D. F. Smith3; then it was distilled into evacuated thin-walled quartz tubing, where crystals of both phases were grown simultaneously and studied with a microscope and by X-ray diffraction. Single-crystal precession photographs, taken with hfoK X-rays, provided the data for determination of the unit cell and space group. The unitcell dimensions are: a = 6.64 + 0.01, b = 7.33 & 0.01, c = 6.40 0.01, and p = 92'40' i 5'; the probable space group is P21/c. Four XeF4 molecules per unit cell give a calculated density of 4.42 g./cm.a. This value is remarkably higher than the value of 4.07 for the other phase. The general hlcl reflections are strong only when h,lc,l are all odd or all even, implying a face-centered arrangement of the heavy atoms. Accordingly, the Xe atoms are placed in the following special positions of P2dC
and the fluorines must occupy the general positions
with four crystallographic kinds of fluorine atoms. The location of Xe atoms at centers of symmetry4 requires the XeF4 molecules to be centrosymmetric and planar, as was previously shown for the other phase (2). Further work on the structure is in progress. Acknowledgments.-The cooperation of Dr. D. F. Smith, Mr. P. A. Agron, and Mr. J. E. Eorgan in preparation and analysis of samples and helpful discussions with Dr. H. A. Levy and Dr. J. A. Ibers are gratefully acknowledged. (1) H. H. Claassen, H. Selig, and J. G. Malm, J. Am. Chem. Soc., 84, 3593 (1962); C. L. Cherniok. et al., Sczence, 1S8, 136 (1962). (2) S. Siegel and E. Gebert, J. A m . Chem. Soc.. 86, 240 (1963). (3) D. F. Smith, J . Chem. Phys., in press. (4) The other possibility, not requiring molecular centrosymmetry, is for X e t o OCOUPY 4(e) wlth z = 0.25, y = 0, z = 0.25. (5) Operated for the U. S. Atomic Energy Commission by Union Carbide Corporation.
JOHN H. BURNS REACTOR CHEMISTRY DIVISION OAKRIDGENATIONAL LABORATORY OAKRIDGE,TENNESSEE RECEIVED DECEMBER 17, 1962
