The Purge Phase

1. The Purge Phase

During the purge phase the volatile organic components are driven out of the matrix. The purge gas (He or N 2 ) is finely divided in the case of liquid samples (drinking water, waste water) by passing through a special frit in the base of a U-tube (frit sparger, Fig. 2.26A). The surface of the liquid can be greatly increased by the presence of very small gas bubbles and the contact between the liquid sample and the purge gas maximised.

Fig. 2.26 Possibilities for sample introduction in purge and trap. (A) U tube with/without frits ( fritless/frit sparger) for water samples. (B) Sample vessel (needle sparger) for water and soil samples (solids). (C) Sample vessel (needle sparger) for foaming samples for determination of the headspace sweep.

The purge gas extracts the analytes from the sample and transports them to the trap. The analytes are retained in this trap and concentrated while the purge gas passes out through the vent. The desorption gas, which comes from the carrier gas provision of the GC, enters this phase via the 6-port switching valve in the gas chromatograph and maintains the con- stant gas flow for the column (Fig. 2.27).

2.1 Sample Preparation 41

Fig. 2.27 Gas flow schematics of purge & trap-GC coupling, switching of phases at the 6-port switching valve. Bold line: Purge and baking out phase; dotted line: Desorption phase.

Solid samples are analysed in special vessels (needle sparger, see Fig. 2.26 B and C) into which a needle with side openings is dipped. For foaming samples the headspace sweep technique can be used.

The total quantity of volatile organic compounds removed from the sample depends on the purge volume. The purge volume is the product of the purge flow rate and the purge time. Many environmental samples are analysed at a purge volume of 440 mL. This value is achieved using a flow rate of 40 mL/min and a purge time of 11 min. A purge flow rate of

40 mL/min gives optimal purge efficiency. Changes in the purge volume should conse- quently only be made after adjusting the purge time. Although a purge volume of 440 mL is optimal in most cases, some samples may require larger purge volumes for adequate sensi- tivity to be reached (Fig. 2.28).

The purge efficiency is defined as that quantity of the analytes which is purged from a sample with a defined quantity of gas. It depends upon various factors. Among them are: purge volume, sample temperature, and nature of the sparger (needle or frit), the nature of the substances to be analysed and that of the matrix. The purge efficiency has a direct effect on the percentage recovery (the quantity of analyte reaching the detector).

Control of Purge Gas Pressure During the Purge Phase The adsorption and chromatographic separation of volatile halogenated hydrocarbons is im- proved significantly by regulating the pressure of the purge gas during the purge phase. By additional back pressure control during this phase, a very sharp adsorption band is formed in the trap, from which, in particular, the highly volatile components profit, since a broader distribution does not occur. The danger of the analytes passing through the trap is almost completely excluded under the given conditions.

During the desorption phase the narrow adsorption band determines the quality of the sample transfer to the capillary column. The result is clearly improved peak symmetry,

42 2 Fundamentals

Fig. 2.28 Glass apparatus for the purge and trap technique (Tekmar). (A) U-tube with/without frit (5 mL and 25 mL sizes). (B) Needle sparger: left: single use vessels, middle: glass needle with frits, right: vessels with foam

retention, 5 mL, 20 mL and 25 mL volumes. (C) Special glass vessels: 25 mL flat-bottomed flask, 40 mL flask with seal, 20 mL glass (two parts) with connector, 40 mL glass with flange, 40 mL screw top glass. (D) U-tube for connection to automatic sample dispensers: left: 25 mL and 5 mL vessel with side inlet, right: 5 mL vessel with upper inlet for sample heating, 25 mL special model with side inlet.

and thus better GC resolution and an improvement in the sensitivity of the whole proce- dure.

The Dry Purge Phase To remove water from a hydrophobic adsorption trap (e. g. with a Tenax filling) a dry purge phase is introduced. During this step most of the water condensed in the trap is blown out by dry carrier gas. Purge times of ca. 6 min are typical.