verify this a second monochromator was connected to the exit slit of the monochromator normally used. With the first monochromator set at 253.57 mp the second monochromator was scanned from 200 mp to 700 mp. No scattered light could be detected. Both monochromators were then set at 253.57 mp and the slits were varied to optimize the signal to noise ratio for parathion. No improvement could be realized over the single monochromator arrangement. Optimum sensitivity of the detector was realized with an 85 % He carrier gas. Figure 8 illustrates the superior response obtained for parathion over either pure A carrier gas or pure He carrier gas. An optimum monochromator slit of 6 mp was determined. These two parameters most affected the sensitivity and selectivity of the detector. As has been reported (4) for 100% A, the detector here shows an enhancement in sensitivity for reduced pressure over that obtained at atmospheric pressure. Maximum sensitivity was obtained at the lowest pressure, 25 mm, that was possible with the type of vacuum pump used.
The detector, when evaluated in terms of signal to noise ratio, is relatively insensitive to microwave power and flow rate of carrier gas. Column temperature had no effect o n background radiation or signal to noise ratios. These independencies allow the detector to be operated over a wide range of chromatographic conditions without loss of efficiency. Temperature programming would have no effect on the detector. The A-He microwave emission chromatograph has been used in this laboratory on a routine basis, giving progressively less operating difficulties than those experienced with a variety of electron capture instruments. Studies aimed at extending the usefulness to chlorinated hydrocarbons and other types of compounds are underway.
RECEIVED for review May 12, 1967. Accepted July 19, 1967. This investigation was supported in part by U.S. Public Health Service Grant EF 00203-06 from the Division of Environmental Engineering and Food Protection.
Repetitive Gas Chromatographic Analysis of Thermal Decomposition Products Paul D. Garn and Gaylord D. Anthony The University of Akron, Akron, Ohio 44304 An apparatus for studying the kinetics and mechanisms for thermal decompositions is described. The sample holder is connected to the sample loop of a gas chromatograph by a long narrow diffusion path. This arrangement causes the decomposition to take place in an atmosphere of its own decomposition product gases, while only a negligible amount i s retained within the sample holder and the diffusion path. The decomposition product gases are repetitively sampled into the gas chromatography by means of a solenoidoperated sampling valve controlled by an adjustable timer. The adjustable timer also controls the integration of the chromatographic peaks through a set of time-delay relays. The type of data obtainable is shown for a lanthanum oxalate hydrate.
SEVERAL METHODS, including thermogravimetry and differential thermal analysis, have been used to study thermal decompositions ( I ) . Reactions may also be followed by continuous evolved gas analysis or repetitive time-of-flight mass spectrometry. Differential thermal analysis and thermogravimetry both have limitations as well as capabilities for the study of systems which decompose to yield one or more gaseous products. Differential thermal analysis has an advantage in studying reversible reactions in that pressure change will yield heats of reaction and equilibrium data, at least for rapid reactions. It does not give good numbers, however, except for total heat effects. The hopeful assumption that the instantaneous value of AT measures the rate of reaction at that moment does not make it so. Nor does the arbitrary assignment of temperature homogeneity to a sample being heated from the edge cause the temperature to become uniform. So we must look elsewhere for kinetic data. ~~
Thermogravimetry has been used with an assortment of mathematical treatments to obtain such kinetic data as the kinetic order followed and the activation energy. The treatments are, in general, restricted to some limited portion of the weight loss curve. In most cases they assume uniform temperature even at heating rates common for DTA. With the current fascination for small samples, a substantial fraction of the weight must be lost to permit detection, and this early weight loss is measured as a small difference between two large numbers. For any but simple reactions its use is impaired because the nature of the weight loss must be inferred. This latter difficulty has caused experimenters to send the gaseous products through some other measuring device to determine the products, but if this is done the thermobalance is superfluous. Continuous measurement of evolved gases by most detectors suffers the same flaw, lack of identification. However, simple reactions can be followed well because the sample holder can be designed to assure temperature homogeneity, and the sensitivity can be varied to measure small initial reaction as well as rapid reaction. Repetitive mass spectrometry at high repeat rates can be performed by allowing the evolved gases to enter the ionizing chamber of a time-of-flight mass spectrometer (Z), but this technique suffers three disadvantages : quantitative measurement is poor; atmosphere conditions in the sample holder are limited by the need to make this type of measurement; few experimenters wishing t o study complex thermal decompositions can acquire a time-of-flight mass spectrometer and the equipment would be wasted on most studies wherein numbers are more important than identification.
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(1) Paul D. Garn, “Therrnoanalytical Methods of Investigation,” Academic Press. New York, 1965.
(2) Henry L. Friedman, J . Appl. Polymer Sci., 9, 651-62 (1965). VOL. 3 9 , NO. 12, OCTOBER 1967
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TUBING W A L L
VALVE
k
CHROMATOGRAPHI C COLUMN
Figure 1. Sample holder for thermal decompositions in selfevolved atmospheres Gas formed in upper chamber will tend not to interchange with gas from sample loop because of the small annular path between the thermocouple shield and the tubing. Excess pressure drives a portion down into the loop so when sampling valve operates, gas is analyzed
Continuous mass spectrometry (3, 4) using a conventional mass spectrometer t o monitor a particular mje ratio can be useful in studying the evolution of one gas from systems which evolve more than one gas simultaneously. This method would permit good measurement of the one gas without interference by the other products. The atmosphere conditions in the sample holder will again be limited by the need t o make this type of measurement. Further, in many cases a less expensive measurement can be made. The major utility for this type of measurement will presumably be in the realm of complex organic decompositions in which some of the possible products are similar chemically but differ, for example, in the length of the carbon chain. During an earlier study of reactions by repetitive chromatographic analysis of gases evolved from samples subjected t o differential thermal analysis (5), some deficiencies in the method were noted, one major problem being that the chromatographic identification restricted the use of atmospheres in differential thermal analysis. Furthermore, the thermal behavior was not representative of any probable real system. In earlier work on lanthanum oxalate hydrate (6), differential thermal analysis disclosed a number of reactions in a neutral atmosphere which can be identified more or less clearly by use of carrier gases of differing densities. In an oxidizing atmosphere, the reactions are substantially different but even in another nonoxidizing atmosphere, helium (5), the thermal effects appear washed-out, partly because in helium the specimen shrinks away from the walls, causing
(3) H. G. Langer and R. S. Gohlke, ANAL.CHEM.,35, 1301-2 (1963). (4) H. G. Langer, R. S. Gohlke, and D. H. Smith, Ibid.,37, 433-4 (1965). (5) Paul D. Garn, Taluiifa, 11, 1417-32 (1964). (6) P. D. Garn and J. E. Kessler, ANAL.CHEW,33,952-4 (1961). 1446
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heat transfer problems and high drift, and partly because of the high rate of diffusion of helium. In this experiment, the detection by differential thermal analaysis was not particularly effective compared to the thermal conductivity measurement on the evolved gases, and was less informative than the repetitive analysis. In short, the differential thermal analysis served no useful purpose when the conditions of the experiment had to be modified to accommodate the other measurements. This is often the case with simultaneous imposition of two, three, or even four measurements upon a helpless specimen. Separate measurements will ordinarily provide better data. The present technique comprises the decomposition of the sample at a programmed or controlled temperature in a closed chamber sample holder (Figure 1) and sampling the gases differing from the holder into a gas chromatograph at automatically timed intervals. The sample holder comprises a cylinder into which a tube is inserted. The sample cavity is '/
