tion for these acids, their values for A and C are not include’j in the overall averages shown in the table. The latent heats of vaporization calculated for these acids using t h ? average values obtained for A and C differ somewhat from the literature values, indicating that the degrees of assclciation reported ( 1 ) do not apply exactly to our experimental conditions.
LITERATURE CITED
(1) Armitage, J. W., Gray, P., Trans. Faraday SOC.58, 1746 (1962). (2) Dreisbach, R. R., T h y s i p 1 Properties
of Chemical Compounds, 111, No. 29 of Advances in Chemical’Series, A.C.S.,
Washineton. D. C. 11961). (3).DreisGachj ,R. R.‘, “Physi;al Properties of Organic Compounds, The Dow Chemical Co., Midland, Mich. (1953). (4) Hanna, J. G., Siggia, S., ANAL. CHEM.35,911 (1963).
(5) “International Critical Tables,” Vol. V, McGraw-Hill, New York, 1929. (6) Siggia, S., Banna, J. G., Serencha, N. M., ANAL.CHEM.35,365 (1963). J. GORDON HANNA
SIDNEY SIGGIA Olin Research Center Olin Mathieson Chemical Corp. 275 Winchester Ave. New Haven 4, Conn. for review June 18, 1963. AcRECEIVED cepted September 5, 1963. ~~~
~
Modification of Electron Probe to Detect Carbon SIR: The electron probe can analyze the atomic contents of a I-micron cube a t the surface of a solid sample ( 2 ) . This is achieved by focusing a 1micron diameter electron beam on the sample and analyzing the emitted characteristic x-rays m a vacuum x-ray crystal spect-ometer. The crystal spacing, d, of generally available crystals prevents the detection of elements with an atomic rumber less than about 11 (sodium) because of the Bragg law: nX = 2d sin 0. An Applied Reseainch Laboratory electron probe has been modified to detect carbon (Ka wavelength 44.0 A.) by making two chan,ges: A pseudo crystal with a d spacing: of -50 A. has been made of 200 monomolecular layers of barium stearate by the Langmuir-Blodgett technique (3). Monomolecular layers of barium stearate are successively picked up from an aqueous substrate. The layers go on with the barium end of the molwule alternating up and then down. This creates a layered structure consisting of planes of barium atoms separated by a distance roughly equal to twice Ihe length of the aliphatic carbon chain. The soap film crystal was laid down on an ARL Cinch radius crystal backing plate. C. L. Andren s ( 1 ) constructed a similar crystal for x-ray diffraction woi-k in 1940. The C K a pulse-height distribution from the proportional counter for a pure 300
v)
n
z 0 PO0
u
v)
?E f
100
8 0
0
10
30 VOLTS
40
40
Figure 1. Pulse height of K a line distribution
50
60
carbon
with P-10 gas (10% methane, 90% argon) a t atmospheric pressure. The counter operating voltage was -1200 volts. The central wire diameter was 0.0015 inch. The counter window was made of two superimposed layers (to avoid pin holes) of collodion (4) each 2000 A. thick. The double window was supported on a nickel mesh 250 lines per inch, 50% transmission.
IPO0
ACKNOWLEDGMENT
0
‘
1
1 I
450
Figure 2. line
50°
Se
I
I
I
I
hi
550
The help and suggestions of B. L. Henke of the Pomona College physics department are gratefully acknowledged.
,
600
Scan across carbon Ka
carbon target is shown in Figure 1 and the spectral line profile is shown in Figure 2. The peak-to-background ratio is approximately 25 to 1. The ratio of 25 to 1gives an overly optimistic estimate for the detection sensitivity of carbon in steel, for example. To give a more practical measure of carbon sensitivity in a suitation where most of the background would be coming from iron as the major component, the following experiment was done: With the x-ray spectrometer set correctly for carbon, the counting rate decreased by a factor of 12 when the target was shifted from carbon to iron. A sample of cast iron containing flakes of pure carbon was viewed in the scanning mode of operation. Figure 3 is a photograph of the cathode-ray tube which shows bright areas for carbon flakes and dark regions for iron. The snakelike regions of carbon are about 10 microns thick. The operating conditions were: accelerating voltage of electron beam 6 kv. (higher voltage causes excessive white radiation which in turn causes the soap film crystal to emit secondary x-radiation; electron beam current 0.2 pa.; and electron beam diameter -4 microns. The proportional counter was filled
Figure 3. Scan over cast iron sample with carbon K a radiation detection Width of light areas, -1
0 microns
LITERATURE CITED
(1) Andrewe, C. L., Rev. Sci. Znstr. 11, 111 (1940). (2) Birks, L. S., ANAL.CHEM.32, No. 9,
19A (1960). (3) Blodgett, K. B., Langmuir, I., Phys. Rev. 51, 964 (1937). (4) Hall, C. E., “Introduction t o Electron Microscopy,” pp. 312-16, McGrawHill, New York, 1953. JACK MERRIW C. E. MULLER W. M. SAWYER, JR. A. TELFER Shell Development Co. Emeryville 8, Calif. RECEIVEDfor review August 2, 1963. Accepted September 20, 1963. VOL. 35, NO. 13, DECEMBER 1963
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