I CORRESPONDENCE
I
Gas Chromatographic Determination of Plasticizers in Propellant Compositions SIR: The determination of plasticizers is necessary for production control of propellant casting liquids, freshly manufactured, and stored propellants. Recently it was observed that the ballistic performance of certain propellant compositions changed slowly on storage, and the change was accompanied by the degradation of up to 50z of the triacetin present in the original composition. The products of decomposition were shown to be acetic acid and diacetin, and the object of the work described in this paper was to devise a suitable method for analyzing the latter in the presence of residual triacetin. Recent papers by Trowell and Philpot ( I ) and Macke (2) describe the determination of plasticizers and stabilizers in propellant compositions but neither paper takes account of the presence of diacetin. Under the gas chromatographic conditions described by Trowell and Philpot ( I ) , triacetin and diacetin are not separated, possibly because of incomplete silanization, and since the relative response factors for triacetin and diacetin are very similar, any degradation of triacetin would not be observed. A procedure is described for the determination of triacetin, diacetin, dimethyl phthalate, and dimethyl sebacate using diethyl phthalate as internal standard. EXPERIMENTAL
(1) J. M. Trowell and M. C. Philpot, ANAL. CHEM., 41, 166 (1969). (2) G . F. Macke, J. Chrornatogr., 38, 47 (1968).
Table I.
Gas Chromatographic Conditions 2 m X lis'' stainless steel tubing packed with 5 % Antarox CO-990 [nonyl phenoxy poly (ethyleneoxy) ethanol] on 80-100 mesh AW-DMCS Chromosorb G
Column temperature Injector temperature Detector temperature Carrier gas Sample size
80 70
50 40
30 20
DA
0
Apparatus. A Varian Aerograph 1522/1B dual flame detector gas chromatograph and a 1-mV Speedomax W recorder fitted with a Model 224 Disc Instruments integrator were used. Operating conditions are given in Table I. Detector Calibration. Relative response factors were obtained for each of the plasticizers using diethyl phthalate as internal standard after it had been established that there was
Column
90
185 "C 275 "C 275 "C
Nitrogen at 25 ml/min 0 . 5 p1
2
4
6 8 TIME ( M I N . )
10
12
Figure 1. Chromatogram of aged propellant extract
no interference from stabilizers, their degradation products or from nitroglycerine; and the small proportion of diacetin present in stock triacetin used for manufacture did not change under the chromatographic conditions given in Table I. Procedure. Propellant samples were extracted with methylene dichloride. The extract was concentrated, care being taken to avoid complete removal of the solvent and consequent degradation of components, and after adding the internal standard the solutions were made up to 25 ml with methyl isobutyl ketone. Casting liquids were weighed directly into a 25-ml standard flask and treated as above. A suitable aliquot was chromatographed and plasticizer content was determined by peak area measurement. A typical chromatogram is shown in Figure 1 and relative retention times are given in Table 11. DISCUSSION
Table 11. Relative Retention Data Triacetin (TA) Diacetin (DA) Dimethyl sebacate (DMS) Dimethyl phthalate (DMP) Diethyl phthalate (DEP) ~~
542
~~~
0.46 0.74 0.89
1 .oo
1.28
~
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
Adequate separation of plasticizers had been achieved in the past with a polypropylene sebacate column. The disadvantage of this liquid phase was a high bleed rate at the temperature required to give reasonable retention times for analysis which was only 10 "C below its maximum operating temperature of 200 "C. However, polypropylene sebacate did
completely resolve dimethyl phthalate and dimethyl sebacate, eluted in that order, and could be used for the analysis of compositions containing both these esters, should the need arise. Antarox CO-990 has now replaced polypropylene sebacate because of its lower bleed rate at an operating temperature 40 "C below its maximum of 225 "C, and because of improved separation of triacetin and diacetin. Dimethyl sebacate and dimethyl phthalate, eluted in this order, are not completely resolved by this liquid phase. The results obtained were all
within the precision usually expected for gas chromatographic analysis-Le., rtl % relative.
review September 15, 1969. Accepted December 15, 1969. Reproduced by permission of the Controller of Her Majesty's Stationery Office.
RECEIVED for
F. I. H. TUNSTALL
Explosives Research and Development Establishment Waltham Abbey Essex, England
Effects of Micropores on Peak Shape and Retention Volume in Gas-Solid Chromatography SIR: A recent article under the above title by Oberholtzer and Rogers ( I ) raises the question of how great can the masstransfer resistance in the stationary phase be without making elution chromatography impossible. Obviously, if the processes by which the molecules of the sample move in and out of the stationary phase are too slow, the sample band will pass completely through the column without significant retention. In the case of zeolite adsorbents such as used by Oberholtzer and Rogers, retention is due to adsorption on the internal surface of the micropore structure of the crystallites. Adsorption on the external surface of the crystallites is usually negligible (the external surface area is about 0.5% of the internal surface area). If the diffusivity within the micropores of the zeolite is too low, there will be negligible penetration of the crystallite and the observed peak will correspond to only the dead-space volume. Higher diffusivities will permit internal adsorption and a normal retention volume will be observed. Even in this case the major contribution to the mass-transfer resistance is likely to be the internal diffusivity. Qualitatively, the lower the micropore diffusivity the wider the peak, but provided equilibrium is attained near the position of the peak maximum, the retention volume should not be changed. With decreasing diffusivity one can imagine a transitional region in which the contribution of internal adsorption to the apparent retention volume gradually disappears. This situation has been examined theoretically by Funk and Houghton (2). They treated a conventional gas-liquid system in terms of a lumped-film model and showed that as the mass-transfer resistance was increased, the peak broadened about its normal retention volume; then, relatively abruptly, became extremely diffused about intermediate retention volumes, and finally narrowed about the dead-space volume. We interpret the results of Oberholtzer and Rogers as illustrating the two extremes with perhaps a suggestion of the transitional stage: methane and ethane diffuse relatively easily within the micropores of zeolite 5A and give normal peaks; in zeolite 4A their diffusivities are so low that the observed peaks are due to only the dead space but with a slight tailing because of limited penetration of the micropores; isobutane cannot penetrate the intracrystalline pores of either zeolite so that only dead space and external adsorption contribute to the retention volume for isobutane.
The question we wish to consider in this communication is what is the limiting mass-transfer resistance for any given chromatographic system at which normal chromatography becomes impossible--i.e., what is the minimum internal diffusivity within the stationary phase that will just permit adsorption equilibrium to be achieved at some point within the elution band, We have been concerned with this problem in our attempts to explore the range of applicability of the gas chromatographic method for measuring intracrystalline diffusivities in zeolites (3). We have no rigorous solution at this time; the following treatment is only qualitative but it is simple and can offer some useful approximate answers to this general question. We adopt as a primary assumption the criterion that the maximum possible peak base width (taken as the base-line intercept of the tangents to the inflection points) is equal to twice the retention volume. In terms of the calculated plate height this limit corresponds to
(1) J. E. Oberholtzer and L. B. Rogers, ANALCHEM.,41, 1590 (1969). (2) J. E. Funk and G. Houghton, J. Chromarogr., 6,193 (1961).
(3) W. R. MacDonald, H. L. Meier, and H. W. Habgood, Third Canadian Catalysis Symposium, Edmonton, Alberta, October 20,1969.
H
=
LJ4
(1)
where L is the column length. In other words we assume that if the mass-transfer resistance is so great that it results in a plate height greater than one quarter of the column length, then equilibration is not achieved at the internal surface and a peak with the normal retention volume is impossible. This choice of 2V as the limiting peak width is arbitrary and partly a matter of convenience. The value is suggested because it is the maximum possible width for a symmetrical triangular peak-although, of course, in a 4-plate column symmetrical peaks are not to be expected. The calculations of Funk and Houghton do not cover the transitional region in great detail but they suggest that the maximum possible peak width may be somewhat less than 2V and perhaps be closer to V . We therefore feel that this criterion we have chosen does represent a reasonable upper limit to'the possible peak width and hence to the maximum mass transfer resistance. Furthermore, it is relatively easy to modify our results so that they will apply to any other chosen value of the limiting peak width. We have calculated plate height-velocity curves for a number of situations covering both the work of Oberholtzer and Rogers and our own studies. Consider a molecular sieve
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