by direct measurements, the Columbia chemist points out. This achievement was made possible by the use of a mica track technique developed at General Electric research and development center, Schenectady, N.Y., by Dr. R. L. Fleischer, Dr. P. B. Price, Dr. R. M. Walker (now at Washington University), and Dr. E. L. Hubbard (now at Lawrence Radiation Laboratory). The technique consists of using mica crystals in a way similar to the use of nuclear emulsions. According to Dr. Jodogne, the advantages of using mica in this study are: • It is almost totally insensitive to the incidence of neon-20 ions, as well as the recoiling products from nonfusion reactions. •Recoiling products from the total fusion of neon-20 and aluminum-27 are expected to be registered with 100% efficiency. These advantages stem from observations made by the GE group that for each solid, there exists a critical rate of ionization. Particles more heavily ionizing than this value produce continuous tracks with unit efficiency, while less heavily ionizing particles produce no tracks [Phys. Rev., 156, 353 (1967)]. The cross section data were derived by counting the numbers of tracks registered in mica which was previously exposed to the irradiation with heavy ions while in contact with a thin aluminum foil. The tracks—average length 10 microns—were revealed by etching the mica with hydrofluoric acid. The actual counting was done with the aid of an optical microscope. Theoretically, in the aluminum-27neon-20 system, total fusion should lead to the formation of vanadium-47 compound nuclei, which decay to residual products. These products would be isotopes of sulfur, chlorine, argon, potassium, calcium, scandium, and titanium, Dr. Jodogne says. Using the mica technique, he adds, it is not necessary to separate these isotopes and count the activity to determine the cross section data. Such separation and counting would be a difficult task because many of these isotopes are either stable to radioactive decay or have a short half-life. Although the results of this study have been enlightening, the Columbia scientist points out, it is possible that total fusion events followed by rare fission or multiple alpha particle emission could escape observation with mica detectors. Bearing this in mind, the Columbia group is using more sensitive detectors and other systems to collect more definitive data on the total fusion phenomenon.
Discharge lamp aids arsenic determination 1554TH
ACS NATIONAL MEETING Analytical Chemistry
An electrodeless discharge lamp in an Evenson cavity has been used by Dr. Oscar Menis and T. C. Rains at National Bureau of Standards (Washington, D.C.) as a primary light source in the determination of arsenic by atomic absorption spectrophotometry. The lamp provides high-intensity, far-ultraviolet, arsenic line radiation. In addition, Dr. Menis and Mr. Rains developed a new separation and concentration technique and a new fuel system, making possible a 0.1-p.p.m. detection limit for arsenic. Elements such as arsenic and selenium, whose ground-state resonance lines are in the far UV (below 2000 A.), have been difficult to determine by atomic absorption spectrophotometry because the far-UV radiation is absorbed appreciably in air. Dr. Menis and Mr. Rains found that an electrodeless discharge lamp, developed 17 years ago but seldom used for atomic absorption, provides radiation intense enough to overcome this difficulty. The lamp is powered by a radiofrequency generator which supplies 100 watts at 2450 MHz. The cavity which transfers this power from the microwave source to the lamp is of Evenson design. It eliminates objectionable and hazardous radiation to the body and to the electronic components of the instrument. This radiation was caused by the output from the hemispheric antenna first used to transmit power to the lamp. Using a new fuel system, argon-hydrogen, and a Zeiss burner, the NBS workers obtained detection limits for arsenic of 0.1 p.p.m. for the 1890-A. and 1937-A. lines and 0.2 p.p.m. for the 1972-A. line. The argon-hydrogen fuel gave lower background absorption and better detection limits for arsenic than the oxyhydrogen flame. The separation of arsenic from the selenium begins with the precipitation of selenium from a 2N hydrochloric acid solution with sulfurous acid. Since arsenic (III) can be lost from hydrochloric acid as arsenic trichloride or arsine during evaporation, the arsenic was concentrated by extraction with diethylammonium diethyldithiocarbamate (DEDC) in chloroform. To make the arsenic extraction quantitative, the arsenic was reduced to arsenic (III) with potassium iodide and sodium metabisulfite. Arsenic is then stripped from the
D E D C by an exchange reaction with cupric ion. Dr. Menis and Mr. Rains found that the separation of arsenic from DEDC and chloroform is necessary because organic materials interfere with the final determination. Carbon radicals in the flame produce a high absorption blank at all three resonance lines of arsenic. To separate and preconcentrate the arsenic in cast iron, the NBS workers dissolve the sample in nitric acid and evaporate it to near dryness. After the sample is taken up in water, the iron and copper are extracted with 2thenoyltrifluoroacetone in carbon tetrachloride, with the pH controlled at 2.5 by the addition of pH 4.6 acetate buffer. To further concentrate the arsenic and to remove acetate ion, the solution is made 3N in hydrochloric acid and the arsenic extracted with DEDC as with the selenium samples. The NBS workers verified the recovery of arsenic from the samples by following the dissolution, separation, and preconcentration steps with arsenic-77 and by using the method of standard additions. The tracer work showed that many previous analyses gave low results because of the loss of arsenic (III) from hydrochloric acid solutions.
Pyrolysis unit ties gas chromatographs 154 TH
ACS NATIONAL MEETING Analytical Chemistry
A tandem gas chromatograph—two gas chromatographs in series with a hightemperature pyrolysis unit between them—is a relatively inexpensive, rugged analog of the gas chromatograph-mass spectrometer combination. In the tandem system, developed for extraterrestrial research, as explained by Dr. Clarence J. Wolf and John Q. Walker of McDonnell-Douglas Corp., St. Louis, Mo., compounds are separated by the first chromatograph, the separation chromatograph. They then pass into the pyrolysis unit, where the temperature is between 500° and 850° C , and are degraded into simpler molecules. The decomposition products are analyzed by the second chromatograph, the analysis chromatograph, yielding pyrograms of the compounds in the sample. The apparatus consists of an F&M Model 5750 as separation chromatograph, a coiled gold tube 36 inches long and 0.04 inch in inner diameter as pyrolyzer, and a Perkin-Elmer Model 11 as analysis chromatograph. The first chromatograph is temperature-programed. The pyrolyzer SEPT. 25, 1967 C&EN
47
Tandem gas chromatogram combines separation and analysis chromatograms
usually operates at 650° or 700° C , and peaks pass through the pyrolyzer in 10 to 20 seconds. Using the tandem gas chromatograph as an analytical system requires the pyrolysis to be reproducible and the second chromatograph to be fast enough that the pyrograms of the com pounds separated by the first chroma tograph do not overrun each other. Many workers have used a gas chro matograph to analyze the fragments emerging from a pyrolysis oven and have found the pyrolysis reproducible. Dr. Wolf and Mr. Walker have cut the analysis time of the second chro matograph to four or five minutes for each compound by programing the column flow rate. In many cases, how ever, it is necessary to use stop-start (interrupted) elution, in which the analysis of the first chromatograph is stopped when necessary by depressurizing the helium carrier gas in the column while the analysis of the py rolysis product continues. The col umn can be shut down for as long as three hours without affecting the sepa ration, the McDonnell-Douglas work ers say. They are working on an auto mated stop-start procedure. The information obtained from the tandem gas chromatograph is similar to that from the gas chromatographmass spectrometer combination. The second chromatography pyrogram is analogous to, but less complex than, the mass spectrometer's fragmentation patterns. Following mass spectrom etry practice, the peak areas of the analysis chromatographs (the pyro grams) are normalized with respect to the largest single peak in the spectra. Since the molecular fragments are stable, unlike the mass spectrometer's ion fragments, they can be collected and analyzed by other techniques, if desired. 48 C&EN SEPT. 25, 1967
When Dr. Wolf and Mr. Walker analyzed n-decane in the tandem gas chromatograph, they found that 5 to 10% of the compound is degraded by the pyrolysis. The analysis chromat ograph shows 10 peaks, most of them alpha-olefins, with a nearly equal dis tribution of fragments up to the C 9 olefin. Pyrolysis of an aliphatic C 1 0 alcohol and the methyl ester of a C 9 acid yields 11 peaks, but the ratios be tween the peaks are different. A mix ture of a C 9 acid methyl ester and a C 1 0 alcohol may not be separated by the first chromatograph, but the pres ence of the two compounds can be de duced from the pyrogram. Dr. Wolf and Mr. Walker have used the tandem gas chromatograph on paraffins, ole fins, alcohols, and methyl esters, and are now studying amino acids. The system can analyze compounds with as many as 20 carbon atoms.
Pendent hydrogen atoms degrade polymer «ι Γ Λ ΙΟτΤΗ
ACS NATIONAL MEETING Polymer Chemistry
Although ordinarily quite unreactive, aromatic hydrogen atoms can be sur prisingly labile and may participate in polymer degradation even in inert atmospheres. If oxygen is present, C-H bonds are even more susceptible to attack. In designing thermally stable and oxidation-resistant poly mers, it follows that it would be de sirable to minimize the number of hydrogen atoms and other pendent groups that do not directly contribute to the strength of the polymer chain. To test this idea, Dr. Stephen S. Hirsch of Chemstrand Research Cen
ter prepared an aromatic polymer totally devoid of pendent groups and compared its thermo-oxidative stability with a corresponding material contain ing pendent hydrogen. It was desired to eliminate the available valency en tirely, and not merely replaco pendent hydrogen with another group. For example, = C H groups can be replaced with nitrogen atoms, and methylene groups with either oxygen or sulfur. Such modifications lead to a hetero cyclic series being partially organic and partially inorganic and having no bonds other than those necessary to propagate the chain. The result is a polymer with no "handles" of any kind that enter into degradative reactions at low temperatures. The composition chosen for investi gation was the product resulting from the reaction of pyrazinetetracarboxylic dianhydride(PTDA) with 2,5-diamino-l,3,4-thiadiazole(DATD). The new polyimide has been designated PTDA-DATD PL
A polyimide was chosen because it represents a class of materials already possessing excellent thermal proper ties. In addition, aromatic dianhydrides, with four points of attachment to the center ring, have few remaining positions to be deprived of hydrogen, and synthesis is not too hard. The standard material for compari son was the corresponding polymer from pyromellitic dianhydride con taining two pendent hydrogen atoms per repeating group in the chain. Tests on other materials comparable with PTDA-DATD PI were not re ported. Films of the standard material and PTDA-DATD PI were maintained in air at 400° C. for 25 hours and in spected periodically. PTDA-DATD PI appeared to undergo no change other than a slow decrease in size while the standard suffered charring, bubbling, wrinkling, embrittlement, and cracking. To determine whether the high-tem perature-air resistance was equivalent to that in nitrogen, thermogravimetric analyses (DTA) and zero strength measurements were made on PTDADATD PI in both media. Up to 600° C , the TGA curves were nearly superimposable. The zero strength curves showed no difference due to air oxidation. In contrast, the hydro gen-containing analog charred exten sively above 320° C. in either air or
