Determination of Oxygen, Hydrogen, Nitrogen, Argon, and Carbon in Thin Films by Flash Discharge Lamp SIR: There is a current interest and need for a method suitable for analysis of thin films. Such an analysis presents a challenge to the chemist for several reasons: the amount of material for analysis is extremely small, the film is normally deposited on a glass or ceramic substrate, and the metals used generally form very stable compounds-Le., the refractory metals. The analytical methods normally used for such measurements become borderline when applied to thin film analysis. Also, the recovery of nitrogen from metals like tantalum, zirconium, etc., has never been feasible by vacuum fusion, ?et nitrogen is known to be present 111 considerable amounts. The principle of this new method is based on the application of a xenon flash discharge lamp capable of inducing a film temperature on the order of 5000' to 8000' C. When the metal a m is flashed in vacuum, this extreme temperature is sufficient to evaporate the film directly from a glass substrate and to dissociate such stable compounds as
tantalum oxide and nitride. The gases evolved are collected by a high speed mercury diffusion pump and later transferred with a Toepler pump to a measuring pipet. The gases are compressed to a known volume, and the pressure is measured by means of a cathetometer. By means of low pressure techniques, a sample is transferred to a calibrated sample loop. The gas mixture within the loop is swept out by means of the helium carrier gas and into a gas chromatograph. The gases are separated with a 2-meter 5A molecular sieve refrigerated with dry ice in series with a similar sieve a t room temperature and detected with a Gow-Mac thermistor. The chromatograms are obtained in about 30 minutes. A standard curve is readily obtained by admitting micro quantities of oxygen, nitrogen, argon, and carbon monoxide to the chromatograph to determine the area under the peak per microgram of gas. The hydrogen value is obtained best by difference since the thermal conduc-
tivities of the hydrogen and the helium are so near alike that the sensitivity is not adequate. The presence of oxygen in fair amounts has made it possible to measure the carbon as carbon monoxide, part of this peak is oxygen and the remainder the carbon. The detection level with the current apparatus, which is not designed for optimum sensitivity, is of the order of 0.1 atom yo. Since the films may contain as much as a hundred times this sensitivity level, the method is very suitable for this application. This method offers a unique and rapid approach to the simultaneous analysis of thin films for oxygen, hydrogen, nitrogen, argon, and carbon, and is being applied to film studies to be described in detail later.
W.G. GULDNER Bell Telephone Laboratories, Inc.
Murray Hill, N. J.
RECEIVEDfor review June 6, 1963. Accepted August 7, 1963.
Use of Molten Salts as a Solvent in Thin Layer Chromatography SIR: Only a few applications of molten salts as solvents for chromatography have been reported although the high ionic character of these liquids should be useful in the separation of inorganic and polar organic substances. The higher temperatures available with these liquids could make changes in equilibrium in reactions an advantage. Juvet and Wachi (6) and Hanneman, Spencer, and Johnson (4) have reported the use of a molten salt substrate in gas chromatography to separate volatile inorganic halides and organic substances, respectively. Gruen et al. (8) investigated the use of LiNOrKNOs eutectic with added amounts of chloride to separate several transition metal ions on an alumina column. More recently, Alberti, Grassini, and Trucco (1, 9) have used the LiNOs-KNOa eutectic as a solvent on glass fiber paper and in electrophoretic separation. 1744
ANALYTICAL CHEMISTRY
This communication reports preliminary experiments on the applicability of molten salts to thin layer chromatography. EXPERIMENTAL
Glass plates, 1 inch X 12 inches, were coated with silica gel from which cation impurities had been removed by washing with 1 to 1 HC1 to water and distilled water. The plates were activated by drying a t 110' C. for a minimum of one hour. The chromatographic chamber consisted of a long borosilicate glass tube containing 1 to 2 inches of the LiiYOs-KNOs eutectic (43 mole % LiN03). To maintain isothermal conditions, this tube was immersed in a large molten salt p t h of LiNOa-KN03 eutectic a t 270 =k 2" C. Applied t o the plate were 0.002 ml. of 0.1.44 aqueous solutions of the ions as nitrates and the chromatograms were run in the usual
manner. Because of the low volatility of the molten salts, and the large bath used, no cover or thermal shields were necessary. Approximately one hour was required for the solvent to travel 10 cm. The plates were then removed and cooled to room temperature. The excess silica gel above the solvent front could be rubbed off leaving the solvent front well defined. Location of the ions used in these preliminary experiments was accomplished by spraying the plates with dilute aqueous )4"(
2s.
RESULTS
In Ihc trial experiments, the various ions of the first group in the qualitative a n a h i s scheme were tried using the simple LiXOSE(V0, eutectic. The values of R, are listed in Table I. In a mixture of ions it has been possible to separate silver, lead, and mercury(1) using this method. Other than the
