GC analyzes alloys, inorganics Gas chromatography can be readily adapted to the quantitative analysis of many inorganic compounds and alloys, according to chemists at the University of Illinois (Urbana). Dr. Richard S. Juvet, Jr., and his coworker, Dr. Richard L. Fisher (now at the Du Pont Experimental Station, Wilmington, Del.), have found that certain pure metals, alloys, carbides, oxides, sulfides, and metal salts can be fluorinated with elemental fluorine and quantitatively determined in a rapid GC procedure. The work on the analysis of inorganic compounds and alloys is part of an extensive research program carried out by Dr. Juvet and his Illinois coworkers on the separation of metal halides by GC. The program covers the analysis and ultrapurification of metal halides by GC as well as the relevant thermodynamics and mechanisms of solution. The Illinois team's approach to GC metal analysis through conversion to metal fluorides effectively complements the analytical schemes that involve metal chelates. Earlier work involved Dr. Juvet at Illinois, Dr. Robert E. Sievers, Dr. Ross W. Moshier, and their coworkers at Wright-Patterson Air Force Base, Ohio, William D. Ross at Monsanto Research Corp. (Dayton, Ohio), and others. They have shown that many metal chelates, including those of transition metals and rare earths, can be used in metal analysis by GC (C&EN, July 1, 1963, page 4 1 ; Nov. 22, 1965, page 39). Using GC, Dr. Juvet and Dr. Fisher have studied the reaction and elution properties of uranium, sulfur, selenium, tellurium, tungsten, molybdenum, rhenium, silicon, bismuth, osmium, vanadium, iridium, and platinum in various chemical forms. With their fluorination technique they have developed quantitative GC methods for U, S, Se, Te, W, Mo, and Re [Anal Chem., 38, 1860 (1966)]. Dr. Juvet and Dr. Fisher use a research gas chromatograph modified to include a special stainless steel reactor-injector system and a stainless steel thermal-conductivity cell. The cell is equipped with nickel filaments that resist corrosion by metal fluorides. The polytetrafluoroethylene column used is 22 ft. long and 1 / 4 inch in diameter. It is packed with 15% w / w Kel-F oil No. 10 (polytrifluoromonochloroethylene) on 40-60 mesh Chromosorb T (polytetrafluoroethylene). Chromosorb T does not react with most metal fluorides. It's the only solid support found useful so far for the quantitative determination of these reactive materials. The weighed sample, on a nickel 32 C&EN DEC. 12, 1966
heating element, is inserted into the reaction cavity. The cavity is evacuated to remove air, fluorine gas is admitted, and the sample is heated electrically at red heat for one minute. In finely divided form, metals often adsorb oxygen or form an oxide. With certain metals it is necessary to reduce adsorbed oxygen or oxides by heating at elevated temperatures in a hydrogen atmosphere before fluorination to eliminate oxyfluoride formation. After heating, the reaction products are immediately swept into the GC column through the gas sampling valve. If the reaction cavity's temperature
Juvet, Zado, Shaw Flame photometric detector important
does not exceed 150° C , all samples except vanadium carbide, Pt, and Ir react quantitatively with the fluorine. Normally the cavity is held at 75° C , well above the boiling point of the least volatile material studied (UF G , which boils at 56.5° C.). Using their fluorination-GC technique, the Illinois chemists have made rapid, accurate analyses of two tungsten-molybdenum alloys and samples of tungsten carbide and molybdenum carbide. They have also used the technique to determine the uranium in uranyl nitrate hexahydrate, uranyl acetate dihydrate, and uranium carbide and oxide. Dr. Juvet and Dr. Fisher have obtained a chromatogram showing the elution and separation of SF G , SeF G , and TeF 0 at dry ice-acetone temperatures (about - 7 2 ° C.). Since Kel-F oil No. 10 begins to solidify at - 6 ° C , separation of these three hexafluorides seems to be caused by gas-solid adsorption rather than partitioning, they say. Dr. Juvet and his coworkers are now using GC to study the interactions and solution mechanisms of metal chlorides in liquid phases made up of
mixtures of nonvolatile inorganic fused salts. These are the only liquid phases that can withstand the temperatures (up to 800° C.) required to elute some of the metal chlorides. These studies can yield valuable information on solute-solvent interactions such as complex formation. GC retention is a function of the stability of the complexes formed, Dr. Juvet explains. Besides providing fundamental information, these studies on metal chlorides should have practical uses in the ultrapurification of inorganic compounds, particularly semiconductor elements. The metal chlorides can be reduced easily to the pure metals with ultrapure hydrogen, he says. As one practical outgrowth of the metal chloride studies, Dr. Juvet and Dr. Franjo M. Zado (a postdoctorate fellow on leave from the Institute Ruder Boskovic, Zagreb, Yugoslavia) have found that molybdenum pentachloride, niobium pentachloride, and tantalum pentachloride are completely separated on a 2-inch-long GC column containing an InCl 3 -TlCl eutectic mixture as the liquid phase. Separation occurs at a column temperature of about 330° C. Because of complex formation between the volatile solute and the inorganic liquid phases, it's often possible to use very short chromatographic columns such as in the molybdenum pentachloride-niobium pentachloridetantalum pentachloride separation. Some separations have been made on columns only 1 / 4 inch long, Dr. Juvet points out. A vital aid to the metal chlorides work is a special flame photometric detector developed by Dr. Juvet, Dr. Zado, and Dr. Ronald P. Durbin (now at Hercules, Wilmington, Del.). This detector is extremely sensitive and essentially free from corrosion even when it is used with the highly reactive metal chlorides. Now that most of the experimental difficulties have been overcome, Dr. Juvet expects that chemists will show increased interest in using gas chromatography to analyze and separate transition metal compounds. One of his graduate students, Vernon Shaw, is currently studying the complexes formed by zirconium and hafnium chlorides with various inorganic fused salt liquid phases. Since zirconium and hafnium are perhaps the most difficult elements in the periodic table to separate by conventional methods, one aim of this work is to develop a GC method for separating zirconium chloride and hafnium chloride. For this separation the neodymium chloridesodium chloride and dysprosium chloride-sodium chloride eutectic mixtures look especially promising for use as the fused salt liquid phase.
