egrup dem nettav i COV va sylanA kinket part dna - Chalmers ...

Analysis of VOC in water with purge and trap
Martin Jerksjö
Department of Civil and Environmental Engineering
Chalmers University of Technology
ABSTRACT
In year 2000 the EU submitted the so called Water directive. The purpose of the
directive is to achieve “good status” in all waters of Europe and also to achieve a
sustainable usage of the water. To reach the first of these objectives methods for
analysis of pollutions in low concentrations is needed.
This master thesis appeared as a consequence of the Water directive. A purge and trap
method for analysis of the volatile organic compounds (VOC) benzene, toluene,
ethylbenzene and xylene (BTEX) in water was developed. The goal was to replace
liquid-liquid extraction, which is used today at IVL, and to reach a limit of detection
in ng l -1 level. The separation and detection was made with a gas chromatograph (GC)
and a flame ionization detector (FID). In the background study a research of the
methods today available for analysis of VOC in water was done.
Today almost all analysis of VOC in water is performed with purge and trap
technique. The method is well established and has the capacity to reach low detection
levels with low standard deviation. A newer and promising method is solid phase
microextraction (SPME). This method is at present time not capable of reaching as
low limits of detection and low standard deviation as purge and trap, but is more user
friendly, easier, quicker and more cost-efficient. SPME is a method which today
constantly improves and may perhaps one day replace purge and trap as a standard
method for the analysis of several VOC.
During the development of the purge and trap method it was investigated how
parameters such as temperature, gas flow and sample volume affects the recovery.
For all the analytes an recovery of about 80 % was achieved when extraction was
performed at room temperature and at 40 °C. When the temperature was increased to
60 °C the recovery increased to 90 % for all the analytes except benzene. When
higher temperatures was used the time of extraction decreased but at the same time
the uncertainty of the measurements of benzene increased. o-Xylene showed to be the
hardest of the analytes to extract from the water an was completely extracted (90 %)
only at 60 °C. Of the various gas flows used, a flow of 300 ml min -1 showed to give
the best results by resulting in relatively small bubbles without creating an extensive
back pressure in the bottle.
When temperatures of 40 °C and 60 °C was used problems with the chromatography
appeared. This problem was a consequence of water contaminating the adsorbent
material and was solved by using a water trap containing Soda lime.
The used adsorbent material Tenax-TA worked out well for all the BTEX-compounds
except benzene which at some occasions was shown to break through the adsorbent
material.
At a temperature of 60 °C, a gas flow of 300 ml min -1 and an extraction time of
8 minutes surface water from Järnbrott (Göteborg) and groundwater from
Rövarekulan (Skåne) and Gallaredkällan (Halmstad) was analysed. The samples was
analysed with the standard addition method as well as a method without standard
addition. Most of the BTEX concentrations measured was between 2 ng l -1 and
100 ng l -1 . The developed purge and trap method has theoretical limits of detection for
BTEX between 2 ng l -1 and 15 ng l -1 .
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