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The development of four novel atom cells for the determination of volatile organometallic compounds by coupled gas chromatographyflame atomicabsorption spectrometry is described. Tetraalkyllead compounds provided a model system in the optimisation of the four atom cells by the variable stepsize simplex method. The effects of the various parameters on analytical performance are discussed. In the most sensitive system presented the effluent from the chromatograph was fed to a small hydrogen diffusion flame and the atoms from this flame were swept into a flame-heated ceramic tube. This simple, readily demountable arrangement enjoys the advantages of continuous operation associated with flames but, because of the relatively long atomic residence times, gave detection limits of 17 pg for both tetraethyl-and tetramethyllead. These limits are superior to any previously reported for a gas chromatographicatomic-absorption spectrometric technique, including those employing electrothermal atomisation.

A detailed study of product formation in the initial stages of pyrolysis of propane has been carried out with a static system at pressures between 26 and 416 mmHg at temperatures between 441 and 503°C. Under all conditions employed the stoicheiometry of the reaction may be represented as (1 + α ) C 3 H 8 = H 2 + C 3 H 6 + α (CH 4 + C 2 H 4 ) with α = 10 0.59 ± 0.15 exp – [(2580 ± 540)/ RT ] + 10 -2.39 ± 0.46 exp [(24000 ± 1600)/ RT ] [C 3 H 8 ], where [C 3 H 8 ] (mole ml. -1 ) is the initial propane concentration. At any given temperature and pressure α is almost independent of extent of reaction and is identical in vessels with surfaces of Pyrex, potassium chloride on Pyrex, and quartz with surface to volume ratios in the range 0.9 to 6.3 cm -1 . The individual rates of product formation are also unaffected by the nature of the surface at 503°C. In contrast, at 456°C an increased surface to volume ratio leads to markedly reduced rates. Furthermore, there is no marked self-inhibition at temperatures below 460°C, in contrast to findings at temperatures of 500°C and above. It is shown that the pressure dependence of α results from competition between reactions (13) and (6 s ) CH 3 CHCH 3 + C 3 H 8 → CH 3 CH 2 CH . 2 + C 3 H 8 , (13) CH 3 CHCH 3 → H . + 3 H 6 (6 s ) at the ‘low’ temperatures of this study. The experimental results yield a value for k 13 of 10 12.1 exp( —17000/ RT ) ml. mole -1 s -1 if the value of k 6s is 10 15.0 exp( —42000/ RT ) s -1 , which appears to be the best currently available estimate. Although reaction (13) can provide a partial explanation for the finding of diminished self­ inhibition at low temperatures, it is shown that reaction (14) H → wall (14) is mainly responsible for this effect. Moreover, reaction (14) accounts for the observed dependence of rate on surface to volume ratio at low temperatures, and allows a semi-quantitative understanding of the temperature and pressure dependence of rates of product formation which cannot be explained in terms of purely homogeneous termination reactions.

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