GAS-PHASE CHROMATOGRAPHY: A CLASS EXPERIMENT DESMOND BRENNAN and CHARLES KEMBALL The Queen's University of Belfast, Belfast, North Ireland
R E m m N m has been made in the preceding paper (p. 484) to the rapid advances which have been achieved recently in the field of gas-phase chromatography and it is clear that the technique will play an increasingly important role in the work of both the organic and physical chemist. For this reason, it seems desirable that advanced students should acquire first-hand experience of the method, and this they can do with the present apparatus which is simple and easily constructed.
and Martin.' The column is enclosed in a glass tube to screen it from drafts. The thermal conductivity cell will be described in detail because the performance of the apparatus depends on the proper functioning of this unit and the present design was found to be highly satisfactory. The cell, Figure 2a, is made from 5-mm. bore glass tubing; the platinum seals PI and Pz should be as close to the side arms as possible in order to minimize turbulence in these regions. The ends of the thin platinum wire W (48 S.W.G. (0.0016-in. diameter), length 4 APPARATUS inches) are spot-welded to the heavy gage (25 S.W.G. The apparatus is shown schematically in Figure 1. (0.020-in. diameter)) platinum terminations PI and Pp. The nitrogen, kept at constant pressure by the bubbler The wire is then inserted into the tube, which is clamped H, passes through the flow meter F, the U-shaped in a vertical position; a 5-g. weight hanging from PI column C, and then the thermal conductivity cell T. serves to keep the wire taut. The lower seal is made Fluctuations in pressure are removed by the ballast first, then the upper one. The electrical circuit (Figure flasks B, and Bz and the flow rate is controlled by the 3) takes the form of a Wheatstone bridge, one arm of needle valve V. S is the injection site and the mercury which is the wire of the thermal conductivity cell (R,). manometer M , which measures the pressure drop across The disturbing effect of changes in the relatively large the column, is optional. thermoelectric e. m. f's which exist in the bridge can The column is 6 feet in leneth and 5 mm. in bore:. he ~- minimized bv makine the bridge electrically symit is filled with dinonyl phthajate (di-3,5,5-trimethylJAMES,A. T.,AND A. J. P. hexyl phthalate) supported on the kieselguhr, Celite Biochem, (Ladon,, 545, in accordance with the recommendations of James so, 670 (1952).
-
J ,
VOLUME 33, NO. 10, OCTOBER. 1956
491
microamp. and a resistance of about 1000 ohms. R, is a decade box which may be used t o shunt the galvanometer while the zero position is being adjusted. The power for the bridge is obtained from a 6-volt storage battery and the input to the bridge is maintained a t 5 volts by adjustment of the rheostat R. Liquid samples are injected through the self-sealing serum cap S by means of a micrometer syringe, as shown in Figure 4. This part of the apparatus is wound with a heater and lagged; it is maintained a t a temperature of about 100°C. t o obtain rapid volatilization of the sample. When a sample is injected, care should be taken to ensure that the drop from the needle is deposited a t the far side of the tube and that none of the liquid falls into the cup across which the serum cap is stretched. A RECOMMENDED EXPERIMENT
The object of the following analysis is to give the student a clear understanding of the methods which may be used to obtain quantitative data from chromatograms. Three mixtures (A, B, and C) of ethyl ether, methylene chloride, and acetone in differentproportions metrical. To this end, the second arm of the bridge R, are given and the calibrations obtained from the study is made electrically identical to the first by means of a of these mixtures are used to determine the composition dummy cell (Figure 2b) which is filled with dry nitro- of a fourth mixture (X). The components of the mixgen and sealed off. The two cells are strapped to- tures have been chosen to give peaks fairly evenly disgether with copper wire, each turn of which should touch tributed along the time axis which does not extend for the next (this has not been shown in Figure 2a in order more than 30 minutes. If the mixtures have the fol-. t o preserve the clarity of the diagram). I n this way, a lowing composition (by volume), assignment of the radiation shield of fairly uniform temperature encloses peaks to the respective components is readily achieved both cells. The two remaining arms of the Wheat- by inspection of the chromatograms and, further, the stone bridge, R, and Rz,are wound noninductively on calibration range is sufficiently large. Figure 5 is the the limbs of the cell from 34 S.W.G. (0.0092-in. diameter) glass-covered constantan wire. RI and R2 should be placed as far above the cells as possible to avoid disturbance from the heat generated in these resistances. For the same reason, the junction corresponding to the point q in Figure 3 should be in the vicinity of Pz (Figure 2a). The cell assembly is mounted in a silvered Dewar vessel and the electrical connections to the external parts of the circuit are made via stout copper wires passing through the rubber bung. R, and R,, when constructed according to the speci5 10 15 20 25 fications given, have a resistance of 25-30 ohms and the values of Time (minutes) R, and Rz should he approximately the same. R, is a decade box by which the bridge can be A, ether; B, acetone; C, methylene dichloride. balanced to give an arbitrary zero reading on the galvanometer G which should have a chromatogram obtained for the analysis of 10 microrim,la4 sensitivity of about 250 mm./ liters of mixture X a t a flow rate of 40 ml./min. and a
JOURNAL OF CHEMICAL EDUCATION
492
the column for each component of mixture X from the relation :% H.E.T.P.
L 6)'
= -
where
column temperature of 20°C. The student is asked to investigate the relative merits of peak height versus pe& areas for calibration purposes. It is pointed out that ratios of the parameters derived from the analysis of any one mixture are more reliable than absolute values derived from the separate analyses of a number of mixtures. The compositions of the three standard mixtures are such that it will be found most convenient to define ratios with respect to the acetone parameters. The student may also be introduced, a t this stage, t o the concept of height equivalent to a theoretical plate (H.E.T.P.) and be asked t o calculate the efficiency of
L = length of column h = Height of peak a = Area. under peak 1 = Retention dlstance of peak
Other simple exercises can be devised. For example, an unknown mixture is given containing three components; also given are a number of pure liquids included among which are the components of the mixture. The student is asked to identify the components of the mixture and determine the volume ratios in which they are present. 'Lmmwooo, A. B., C. S. G. P n r ~ ~ mAND s , D. T. PxmE, . I . Chem. Soc., 1955, 1480.
