Instrumentation for Headspace Sampling

All the devices that are commonly used for gas sampling may be applied to headspace analysis, including gas-tight syringes and gas-sampling valves. A particular problem in HS-GC is the internal pressure in the headspace vial that is generated during thermostatting at elevated temperature, represented by the sum of the partial vapour pressures of all the volatile compounds present, including water (since most samples contain some water). The vial must therefore be closed by a septum (preferably lined with polytetra-fluoroethylene) (PTFE) and crimp-capped pressure-tight by an aluminium cap. This internal pressure may cause problems with sample loss during sample transfer with a gas syringe if it is not equipped with a pressure-tight valve. So that they can operate independently of the internal vial pressure, most automated headspace samplers surmount this problem by pressurizing the vial up to a certain pressure level above the original pressure prior to sample transfer. Although it is not possible here to describe the various commercially available instruments in detail, the common principle is shown schematically in Figure 2.

Inert carrier gas enters the gas chromatograph through valve V and branches before the column. Part of the gas is directed to the sampling needle N. If this needle penetrates the septum, carrier gas flows into the vial and pressurizes it, usually up to the column head pressure, but any other pressure may be applied. Sample transfer is subsequently performed by closing valve V for a short time (usually few seconds), thus disconnecting the gas supply. The pressurized head-space gas in the vial expands either through the sample loop of a gas-sampling valve to atmosphere or directly onto the column. This on-column headspace sampling (also known as balanced pressure sampling) has the advantage that no headspace gas is wasted by unnecessary expansion to atmosphere, allowing the application of cryofocusing enrichment techniques. The actual volume of the headspace gas and the amount of analyte in it can be calculated from the transfer time (seconds) during which valve V is closed and from the volume flow rate (mL s "1) at the column head.

The carrier gas flow rate in a capillary column is much lower than in a packed column, and therefore a much smaller volume of the headspace gas is introduced during the same sampling time. The resulting lower sensitivity can be circumvented by an increased sampling time, provided the accompanying band-broadening is suppressed by the technique of cryofocusing (also called cryogenic trapping or cold trapping). The normal admissible sample volume in a capillary column is about 50-200 |L, which is only 1% of the usually available volume of 5-20 mL head-space gas in the vial. With this technique of splitless on-column headspace sampling it is possible to

Figure 2 Principle of headspace sampling by either direct on-column sampling or by pressure/loop-filling with previous pressurization of the headspace vial.

Carrier gas

Carrier gas

Cooling gas Fused silica Fused silica separation cryocapillary column capillary column

Figure 3 Principle of cryogenic headspace trapping with splitless on-column headspace sampling.

Cooling gas Fused silica Fused silica separation cryocapillary column capillary column

Figure 3 Principle of cryogenic headspace trapping with splitless on-column headspace sampling.

extend the sample transfer time from a few seconds up to several minutes with an accompanying increase in the headspace gas volume and sensitivity.

In the automated headspace sampler shown in Figure 3 the cryotrap is placed in the oven of the gas chromatograph. The cryotrap is essentially a short piece of fused silica capillary column, either the first coil of the separation capillary or a corresponding short piece of a different capillary column. The latter, called here the cryocapillary column, is coated, preferably with dimethylsilicone, a substance with a glass transition temperature of — 114°C. Dimethyl-silicone works as a stationary phase even at that low temperature, dissolving the compound in the liquid phase rather than just trapping by condensation. This cryocapillary column may then be connected to any other type of a capillary column by a butt connector. The cryocapillary column is jacketed by a 0.5 m PTFE tube, through which cold nitrogen gas flows outside the capillary column but in the opposite direction to the flow of warm carrier gas inside. The volatile analytes are trapped along the resulting strong temperature gradient in the capillary column. When sample transfer is interrupted by opening valve V, the flow of cooling gas is also stopped. A very rapid desorption is then achieved with a sharp starting band profile, since the warm carrier gas inside the capillary now heats the low mass fused silica capillary rapidly up to the oven temperature. The nitrogen gas used for cooling is produced outside the gas chromatograph by passing the nitrogen through a metal coil in a cooling bath, for example through liquid nitrogen. The sampling time and thus the head-space gas volume is restricted to only a few minutes before ice forms from the water sample, causing a blockage. However, a remarkable improvement of the sensitivity compared to the usual injection times of a few seconds is obtained. Injection times of up to 10 min can be obtained by placing in the sample transfer line a small trap containing lithium chloride on a solid support in a small tube. This water trap (optional and not shown in Figure 3) is regenerated after each analysis by heating to 200°C and backflushing the released water. The chromatogram in Figure 4 gives an example of the headspace analysis of low ppb concentrations of volatile aromatic hydrocarbons (BTEX) in a water sample by cryo-HS-GC.

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