analysis of Volatile Organic Compounds in Water by Purge-And-Trap and Gas Chromatography Techniques: Operational Parameters Optimization of the Purge Step

Abstract

In order to analyse volatile organic compounds (VOCs) in water by the dynamic headspace method, an adapted purge and trap set up was used along with on-column trapping (packing = Tenax GC) and flame ionisation detection.

Optimization of the purge step working conditions is discussed with respect to extraction efficiency for 14 representative VOCs used to spike a 25 ml water sample. With that aim in mind, the effect of the following parameters were studied: purge gas volume (Vg/Vl = 20 and 40), sparger vessel temperature (25, 30, 35 and 40°C) and ionic strength (dissolution of 10 and 20% of NaCl in the studied sample). A fair increase of the purge gas volume enhances the recovery of most of the VOCs examined. However, the extraction is still unsatisfactory for the less volatile compounds. Nevertheless, a better purge efficiency for all the VOCs studied is observed when the temperature is increased, at the expenses of an increase of the RSD of the results (= 14%). Moreover, for all the less volatile and less soluble purgeable compounds, the increase of the extraction efficiency is significant when NaCl is dissolved in the water examined. The selected purge operational conditions are as follows: dissolution of 20% NaCl in a 25 ml sample, purge at 35°C with helium during 10 minutes and a flow rate of 50 ml min⁻¹. When applied to a tap water spiked with 30 VOCs, including the purgeable priority pollutants listed by U.S. EPA, these conditions lead to a better than 90% recovery for most of these compounds (RSD > 10% for triplicate sample analysis).

Key Words:

• Purge and trap GC
• water analysis
• organic volatile pollutants