Determination of Temperature Labile Compounds with the Gas Density Detector Bernard M. Mitzner, George Hild, and J. F. Gates Clarke, Jr.‘ Polak’s Frutal Works, Inc., Middletown, N. Y. 10940
The determination of molecular weights via two independent gas density detectors connected to two separate columns using a split sample has proved to be a useful tool for the analytical chemist. In this system, two carrier gases are used with different molecular weights, and the molecular weights of the samples are calculated from the ratios of the peak heights obtained from each detector. Such a system has proved to be surprisingly accurate when analyzing stable compounds. We recently examined a sample containing linalool, linalyl acetate, and various terpene alcohols, and obtained anomalous results. Molecular weights were obtained that were far less than those expected for these compounds. The molecular weight of linalool was calculated to be 81, although its true molecular weight is 154. The calculated molecular weight of linalyl acetate was 118, although its true molecular weight is 196. The normal gas chromatographic analysis of compounds of this type is difficult due to the tendency of breakdown in the injector ( I , 2). Sample breakdown in the injector is evidenced by symmetrical peaks from terpenes eluting in the early portion of the gas chromatogram. We surmised that decomposition may be taking place in the detectors. If linalool is dehydrated, limonene and other terpenes are formed along with water. Because of the low molecular weight of the water, the appearance of a low molecular weight species is erroneously recorded. One must realize that all the various molecules are in the detector a t the same time, and the response of the water is overriding. In the case of the linalyl acetate (3), limonene and other terpenes are found as well as acetic acid. The significantly lower molecular weight of the acetic acid will have the greatest influence in the detector response, thus indicating that a lower molecular weight species is erroneously present. The detector temperature was reduced from 250 to 225 “C and the anomalies disappeared, i.e., the molecular weight of linalool was calculated a t 154, and that of linalyl acetate, 196. (The analyst must also beware of contaminants such as halogens and acids present on the inner surPresent address, Hercules Research Center, Wilmington, Del.
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face of the detector which behave as catalysts and promote breakdown.) I t is, therefore, imperative that the analyst who is using an instrument equipped with a gas density detector(s), take special care to adjust its temperature and be certain that the detector is free of surface contamination to ensure that there will be no sample breakdown. We suggest that analysts run known samples in the family of compounds they wish to analyze in order to adjust parameters SQ that breakdown can be prevented or significantly reduced. There may be situations where the breakdown of materials may be advantageous. After running a sample a t a relatively moderate detector temperature, the temperature may be significantly increased in a subsequent run. In those cases where peaks have an apparent decreased molecular weight, we can assume that temperature-labile compounds are present, and this information may be of significance to the analyst.
LITERATURE CITED (1) B. M. Mitzner, Anal. Chem., 36, 242 (1964). (2) B. M. Mitzner and G. Hild, Anal. Chem., 46, 1352 (1974). (3) B. M. Mitzner. S.Lemberg, and E. T. Thelmer, Can. J. Chem., 44, 1090 (1966).
RECEIVEDfor review March 26, 1975. Accepted June 2, 1975. Polak’s Frutal Works, Inc. is a wholly owned subsidiary of Hercules Inc.
CORRECTION Determination of Trace Level Quantities of Arsenic via a Novel Kinetic Method
In this note by T. Tarumoto and H. Freiser, Anal. Chem., 47, 180 (1975), in column 1 in the Recommended Procedure, line 4, please add “2 ml of potassium bromate solution”.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
