Combination of continuous distillation and head space analysis

3 Combination of continuous distillation and head space analysis 

To avoid the influence of mechanical contaminations of waste water on the membrane a head space variant can be used. In this variant the covered IRE is not in direct contact with the liquid water but with the vapor above this water. To improve the detection limit the waste water can be heated and the IRE can be cooled (see Fig. 6.5-6). 

The vapor phase from heated waste water often enriches the organic compound. This enriched vapor partly condenses at the membrane on the IRE. Additionally, the enrichment factor increases because of the lower temperature of the membrane. Therefore the head space method has mainly two advantages first, protecting the IRE from contamination and second, improving the detection limit. 

For instance, the large solubility of dichloromethane in water causes a relatively low enrichment factor of ca. 80 (PDMS, 30 °C). As a result the detection limit for direct measurements of aqueous solutions with membrane covered IRE s is ca. 1.9 -10 mol/L. Using the head space variant with heated aqueous sample (90 °C) and cooled IRE (5 °C) the detection limit is improved by a factor of 30 to 6.2 10 mol/L. 

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