The Use of Organo-Clays as Gas Chromatographic Stationary Phases J. V. MORTIMER and P. L. GENT The British Petroleum Co. ltd., Chertsey Road, Sunbury-on-Thames,
M iddlesex, England
b The need for a stable stationary phase for the gas chromatographic separation of the C8 alkylbenzenes has led to a re-examination of the use of dimethyldioctadecyl ammonium bentonite. The organo-clay has been successfully modified with a number of conventional liquid stationary phases and dispersed on Celite to give column packings which are particularly suitable for the separation of meta- and paraisomeric aromatics. The effect of varying the operating temperature, and the quantity and nature of the modifying solvent are discussed and the conditions necessary for the separation of Cg to C9 alkylbenzenes, dichlorobenzenes, cresols, and xylenols are described.
The separation factor for m- and pxylenes on dimethyldioctadecyl ammonium bentonite at 70" C. is such that, if symmetrical elution curves could be obtained, a 99.9% separation could be achieved using only 800 theoretical plates. Indeed, the organo-clay compound is so selective in its retention of the meta-isomer that the elution order for the xylenes is para, ortho, meta. The use of organo-montmorillonite as a stationary phase was first described by White in 1957 (9). The cations in the naturally occurring clay had been replaced by alkyl quaternary ammonium ions and the resulting compound exhibited a selective retention of aromatics relative to paraffins and naphthenes. He postulated that separation of the paraffins was due to partition chromatography while the retention of the INCE DESTY,GOLDUP, AND SWANTON aromatics was due, at least partially, to adsorption. I n 1958, White and Cowan (2) described the use of 7,8-benzo(IO) published the results of an investiquinoline for the separation of the xygation into the performance of dimethyllenes in 1959, this stationary phase has dioctadecyl ammonium bentonite for the been widely used. However, its popuchromatography of paraffins and delarity has been mainly due to the lack of scribed the material as having "ideal" a more suitable alternative, for its chromatographic behavior, being caphysical properties are not those of a n pable of producing symmetrical elution ideal stationary phase. I t s relatively curves and constant retention volumes. high vapor pressure at normal operating However, the separation of nontemperatures (about 80" C.) causes i t paraffinic compounds was less satisto bleed continually from the column factory and Hughes, White, and Roberts resulting in noisy detector signals, re( S ) , when describing the separation of stricted outlet tubes, and decreasing the individual xylenes, cresols, and toluiretention volumes. dines, drew attention to the asymWhen the need arose for a particumetry of the elution curves obtained. larly stable column packing capable of The disadvantage of the asymmetrical effecting the complete separation of the elution curves was overcome to a large xylenes i t was decided to reconsider the extent by Cowan and Hartwell ( 1 ) and suse of organo-clay compounds, the again by Van Rysselberge and Van Der possibilities of which can be seen from Stricht ('7) when separating the dichloroTable I.
S
Table 1.
Separation Data for m - and p-Xylenes (Band Impurity = 0.1%) NO.
Stationary phase Squalane Benzyldiphenyl Polyethylene glycol 7,8-Benzoquinoline Dimethyldioctadecyl ammonium bentonite For symmetrical peaks.
Temperature, "C.
Separation factor ( a )
theoretical plates ( N )
78 78 50 78
1.015 1.032 1,035 1.070
100,000 40,000 35,000 8,500
70
1.265
Q
754
ANALYTICAL CHEMISTRY
800"
benzenes and the xylenes, respectively. Both groups of workers used very small sample sizes, about 0.025 pl., together with ionization detectors. While the peak shapes were greatly improved, the base widths, particularly in the case of the xylenes, were large and the total elution times were about 1 hour, A more satisfactory separation of the xylenes has been achieved by using a n organo-clay modified with a liquid solvent of the type used in conventional gas chromatography. The authors (4) described the separation of hydrocarbon mixtures, including the Cs to Cg aromatics, on a Bentone column modified with Silicone Oil %IS 555 and drew attention to the following points. The elution curves were as symmetrical as the curves obtained in conventional gas liquid partition chromatography; the sample volumes used were 3 PI., which enabled commercial thermal conductivity apparatus to be used; the elution order of the xylenes was pura, meta, ortho, [similar to that since reported by other workers (6, S)], in contrast to para, ortho, meta, on an unmodified Bentone-34 column; and n-nonane eluted just before ethylbenzene. They also described separations using columns modified with squalane and polyethylene glycol (4, 5 ) , and, more recently, columns modified with diisodecylphthalate and Apiezon L have been reported by Spencer ( 6 ) and Van Der Stricht and Van Rysselberge ( 8 ) . A detailed investigation of the performance of modified organo-clay columns has now been completed and is described in the present gaper. EXPERIMENTAL
Apparatus. The instruments employed were Perkin-Elmer single-stage chromatographs fitted with thermistor detectors and the columns were mostly of 12-foot copper, 1/4-inch 0.d. and 3/16-inch i.d., the packing being prepared using the procedure described below. One part by weight of Bentone-34 (F. W. I3erk and Co. Ltd., London) and the selected weight of modifying solvent (for example, silicone oil) were dispersed in 30 ml. of benzene. This was added t o a slurry of 10 parts by weight of Celite (60 to 100 mesh) in 200 ml. of benzene. The benzene was
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