- •1.1 Introduction
- •1.2 Sample preparation and clean-up procedures
- •1.2.1 Liquid-liquid extraction
- •1.2.2 Solid phase extraction
- •1.2.3 Purge and trap
- •1.2.5 Derivatization
- •1.2.6 Clean-up procedures
- •1.3 Instrumentation
- •1.3.1 Gas chromatography
- •1.3.1.1 Capillary columns
- •1.3.1.2 Sample introduction systems
- •1.3.2 High performance liquid chromatography
- •1.3.2.1 Hplc columns
- •1.3.2.2 Hplc detectors
- •Volatile organic compounds
- •2.1 Introduction
- •2.2 Compounds
- •2.3 General Procedure
- •2.4 Sensitivity
- •3.1 Introduction
- •3.2 Compounds
- •3.3 General Procedure
- •3.4 Sensitivity
- •3.5.1 Procedure 1: Solid phase extraction/cgc/ms
- •4.1 Introduction
- •4.2 Compounds
- •4.3 General Procedure
- •4.4 Sensitivity
- •4.5 Detailed Procedures
- •4.5.1 Procedure 1: pah analysis using hplc (epa 550.0)
- •5.1 Introduction
- •5.2 Compounds
- •5.3 General Procedure
- •5.4 Sensitivity
- •6.1 Introduction
- •7.1 Introduction
- •7.2 Compounds
- •7.3 General Procedure
- •9.1 Introduction
- •9.3 General Procedure
- •9.4 Sensitivity
- •9.5 Detailed Procedures
- •9.5.1 Procedure 1: Tropolone extraction/cgc/aed or cgc/ms
- •10.1 Introduction
- •10.2 Compounds
- •10.3 General Procedure
- •10.4 Sensitivity
- •10.5 Detailed Procedures
- •11.1 Introduction
- •12.1 Introduction
- •12.2 Compounds
- •13.1 Introduction
- •100-90-80-70-60 50 40 30 20-10-0-
- •20 30 40
- •Iceland
- •Ireland
1.2.3 Purge and trap
This particular technique is generally used for the extraction of non-polar volatile organic compounds prior to gas chromatography. An inert gas is bubbled through the water sample, causing the purgeable organics to move from the aqueous to the vapour phase. The volatile compounds are then trapped on an adsorbent such as Tenax or activated charcoal and/or condensed by cryo-coo-ling. The trap containing the sorbent material is generally built in a desorption chamber equipped with a powerful heating mechanism which when activated, permits the desorption of the trapped compounds. A schematic drawing of a modern purge and trap system is shown in Figure 1.2.
This technique has the distinct merit of providing a clean sample, free from its often very dirty matrix. A purge and trap device can easily be mounted on a gas chromatograph equipped with in-series ECD-FID, PID-ELCD or MS. This technique is most appropriate for ppb level analysis of low molecular weight, slightly water-soluble volatile organics with a boiling point below 200° С A variation of purge and trap is closed-loop stripping analysis. Very low levels of organic pollutants in drinking water (ng/1 or ppt) can be analysed with the system shown in Figure 1.3.
The gas in the system is circulated for 90 minutes through the water sample (typically heated to 60° 0) and through a trap containing a very small amount of activated charcoal. The volatiles trapped on the charcoal are micro-extracted with 50 to 100 ul of carbon disulphide or dichloromethane by forcing the solvent backwards and forwards through the charcoal filter.
1.2.4 Static and dynamic headspace
Headspace analysis is used to analyse volatiles in samples whose matrix is of no interest, for example water, soil, polymers etc. The various commercially available headspace autosamplers are based on the principle of static or dynamic headspace, the different modes are depicted in Figure 1.4.
In static headspace, a water sample is transferred to a headspace vial, sealed and placed in a thermostat to drive the volatile components into the headspace for sampling. An aliquot of the vapour phase is introduced via a gas-tight syringe or the sample loop of a gas sampling valve into a GC, equipped with a packed or capillary column. Due to the lower sample loading capacity of a capillary column, split injection is generally used. Static headspace implies that the sample is taken from one phase equilibrium. In order to increase sensitivity, dynamic headspace has been developed whereby the phase equilibrium is continuously displaced by driving the headspace out of the vial with an inert gas. The solutes are collected on an adsorbent such as Tenax or are cold trapped and after enrichment introduced into the GC by heating the trap. Whereas static headspace is applied to analyse ppm amounts (with FID detection), dynamic headspace allows determination of ppb amounts of volatiles. Sample pretreat-ment can often help increase sensitivity and enhance reproducibility, the best known methods include salting-out water samples with sodium sulphate or adjusting the sample's pH, driving organic acids or bases into the headspace.