The tendency of a molecule to attach itself to a finely divided solid (adsorption).

3* The tendency for a molecule to enter the vapour state or evaporate (volatility).

Mixtures of substances to be separated by chromatography are placed in a dynamic experimental situation where they can exhibit two of the above properties. In GLC a volatile

substance is distributed between a moving gas phase and a stationary liquid phase on an inert support. The support and liquid film is contained in a column and the moving phase termed carrier gas, passed through the column.

If a single component sample is considered, then on introduction to the system it will vaporise and be swept onto the column by the carrier gas. On reaching the stationary phase the major part of the sample will be adsorbed and an equilibrium established between adsorbed vapour and the amount remaining in the carrier gas. The carrier gas will move forward and the small

amount of unadsorbed vapour still in the gas will again

achieve equilibrium with the stationary phase. Simultaneously, pure carrier gas will come into contact with the portion of sample originally adsorbed by the liquid. Some adsorbed vapour will then re-enter the gas to re-establish equilibrium. The movement of vapour to and from the carrier gas is the

fundamental mechanism of GLC.

Applying the above to a two component sample, the properties of the liquid film are chosen such that the components differ in the extent adsorbed by the liquid and gas. This difference results in separation of the two phases because of different retention times in the column. The use of a detector and recorder will represent the sample components as two separate peaks on a graph.

(b) The Gas Liquid Chromatograph

A Pye 105 Chromatograph was used consisting of three distinct units - the analyser, programmer controller and ionisation amplifier.

The analyser consisted of four sub-units of oven, electrical control unit, gas connection bulkhead and flame ionisation detector head. The oven, powered by the electrical control unit accommodated the chromatographic column suspended from the flame ionisation detector head and injection head. The injection head was heated in order to instantly vaporise

injection port, Hydrogen and air were supplied to the flame ionisation detector and ignited by a removable glow plug ignitor. Nitrogen carrier gas was supplied to the column via the injector head. Figure 36 shows a plan view of the

analyser unit whilst plate 8 shows the GLC equipment used.

The analyser oven temperature was controlled by a programmable controller. Temperature programming facilities provided a linear temperature increase with time with variable periods of initial and final isothermal operation. This mode of operation is employed for samples having components of widely different boiling points. The aim is to allow the resolution of low boiling point fractions at low temperature whilst gradual temperature increase permits resolution of high boiling point components.

The detector used on the 105 chromatograph was a flame ionisation detector (FID). The principle of operation relies on hydrogen being mixed with effluent gases from the column and burnt in a stream of air. Ions produced in the flame conduct a current from the flame jet, which serves as one electrode, to a second electrode above or around the flame. The elution of carrier gas only results in a background current. Any organic compounds eluted b u m to form carbon dioxide (and water) and subsequent ionisation produces a greater current. A flame ionisation amplifier connected to the FID amplified the

current and transmitted it to a chart recorder. The amplifier allowed correction of any base-line drift and an attenuation

control was employed during analysis to ensure all peaks fitted on the chart.

An FID is classed as a mass detector whereby the signal generated is proportional to the mass of component reaching the

1 n)

detector in unit time . For mass detectors the area under a peak, A, is proportional to the mass, m, of the component.

A = fm

The proportionality factor, f, is called the detector response factor. The factor not only depends upon the type of detector but the chemical character of the component. Thus, different peak areas will be generated for equal amounts of different components. For closely related compounds an approximation may be made in assuming identical response factors.

(c) Operating Conditions

The Pye 10£ Chromatograph was used with a stainless steel column 5 Pt long, \ inch I.D., packed with 10% 0V1 on acid washed Chromosorb W, 80-100 mesh. (Phase Separations Ltd.)

Nitrogen and hydrogen flow rates were J4O cm /min. each whilst the air supply registered a pressure of 10 p.s.i.

The temperature programme used was a temperature increase from 170°C to 320°C at 8°C per minute commencing from time of injection. The injection port was heated to +f>0°C of the initial column temperature.

A sample size of 1 pi was obtained by a microsyringe (Terumo Ltd* or Hamilton Ltd*)* The time of injection was marked on the recorder chart which moved at 1+0 inches per hour. During analysis the attenuation control was adjusted accordingly to fit peaks on the chart.