Gas chromatographic separation of isomeric hexanes at very low


Publication Date: September 1975. ACS Legacy Archive. Cite this:Anal. Chem. 47, 11, 1874-1874. Note: In lieu of an abstract, this is the article's fir...
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I CORRESPONDENCE Gas Chromatographic Separation of Isomeric Hexanes at Very Low Levels in Diluent Sir: Alkane pyrolyses proceed through chain reactions and, in most conditions, the chains are very long. Thus, the products of chain termination usually occur a t remarkably high dilution in unreacted alkane and major products. Marshall and Purnell ( I ) and their collaborators have, in the recent past, shown how valuable kinetic and thermochemical information relating to individual chain steps can be derived if reliable analysis for chain termination products can be carried out. The gas chromatographic problem is not simple since termination product yields are often in the region of 10-4-10-3 Torr absolute pressure. In the pyrolysis of propane, in appropriate conditions, three isomeric hexanes may be present at this level as chain termination products. This note describes a solution to the resulting analytical problem. The literature contains three accounts of the separation of the three hexane isomers of interest. The only real problem is that of separating 2,3-dimethyl butane from 2-methyl pentane which are both readily resolved from n-hexane. Waksmundzkai (2) used a pair of series columns at 30 "C, each containing a binary mixture (2 m of quinoline/brucine 1 m of tetradecanol/quinoline). Such a system has two defects. First, it does not allow separation of all the other products of interest and, second, the extremely low level of the hexanes eluting very late demands maximum FID sensitivity which becomes unattainable because of column bleed of these relatively volatile solvents. Desty and Swanton ( 3 ) have shown the most impressive separations of the compounds of interest using various quinolines and naphthalenes at 20 "C. Again, although a-values as high as 1.15 for 2,3-dimethyl butane and %methyl pentane can be achieved with isoquinoline, volatility problems and interference from C5 products prevent use of such solvents in this particular case. Finally, Vollert and Mautsch ( 4 ) show a partial separation of the three isomers with a column of N,N-dimethyl-1-napthylamine at 20-30 OC. We have been unable to reproduce this. It must be emphasized that the above described separation procedures all related to isomer mixtures alone, in substantial amount, and in roughly equal proportions. In our

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"-Hexane

I-Methyl Pentane

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Tlme(rn,ns)

Figure 1. Hexanes (5 X Torr) section of chromatogram of unreacted propane and its pyrolysis products (200 Torr)

experimental situation, there is very substantial band broadening of earlier large peaks, and separations of earlier eluents are required. Thus, a much more effective separation is, in any case, demanded. From a number of systems studied, a resolution of the problem was found in the use of a 5-ft X 0.25-in. column of 10% w/w NaOH/alumina column, the alumina having also been coated with 0.5% w/w of PEG-400. The NaOH was deposited from aqueous solution and the PEG-400, subsequently, from methanol. The packing was dried at 100 "C. The column was operated at 56 "C in nitrogen and the optimum flow rate to allow separation of all lower products as well led to an overall analysis time of about 75 min. Figure 1 illustrates the hexane section of a complete analysis of 200 Torr of sample containing about 4 Torr of products of which the hexanes comprise only about 5 X Torr.

LITERATURE CITED (1)D. G. Hughes, R. M. Marshall, and J. H. Purnell, J. Chem. SOC.,Faraday Trans. 1, 70,594-600 (1974). (2)A. Waksmundzkai, Z. Suprynowicz, and T. Pietrusinska, Chem. Anal. (Warsaw), 10, 367-375 (1965). (3)D. H. Desty and W. T. Swanton, J. Phys. Chem., 766 (1961). (4) U. Vollert and M. Mautsch, Chem. Tech. (Leipzig),24, 100 (1972).

Eileen Smith Department of Chemistry University College of Swansea Swansea SA2 8PP, U.K. RECEIVEDfor review February 10, 1975. Accepted June 2, 1975. I thank the S.R.C. for the award of a scholarship.

ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975