Water / Wastewater 63
Figure 3: Chromatogram obtained with auto-TD-GC/FID coupled with Purge and Trap sampling system for the analysis of 502.2 standard solution.
injected inside this glass purging device. An inert gas (nitrogen) is used to purge the water sparger with bubbles of less than 3 mm of diameter at the origin of the frit. The gas is then injected into the gas chromatograph for pre-concentration and analysis.
For each analysis, 385 ml of gaseous sample is drawn into the auto-TD-GC-FID at a fl ow rate of 35 ml/min (sample is integrated over 11 min). The VOCs were pre-concentrated at room temperature in a trap fi lled with Carbopack. The pre- concentrated sample was thermally desorbed at 380°C for 4 minutes and directly injected in a 30 m MXT 624 column (0.53 mm ID, 3.0 um dF) located inside the heated oven of the GC. VOCs were then detected by the FID and the detection limit has been determined to be less than 5 ng/L.
The liquid samples were prepared using 502.2 standard solutions with a concentration of 2000 µg/ml diluted in water to reach 4 µg/L. The chromatogram obtained can be seen in Figure 3. The column gradient has been optimized to achieve a very good separation of the VOCs included in the 502.2 standard solutions. The Purge and Trap system allows the evaluation of the effi ciency of the GC and the liquid sampling system by performing twice the analysis of the same sample and also by comparing the theoretical injected mass and with the experimental measurements. The result of this experiment shows that 100 % of the dissolved VOCs in the liquid were transferred from the Purge and Trap system to the auto-TD-GC- FID and were analyzed and quantifi ed.
Analysis of ppm to % VOC concentrations in water using airmoVOC WMS
The 502.2 method has been developed for the analysis of trace- level VOCs in water (ppt and ppb level). However, water used for industrial processes (refi nery, wastewater treatment plants) may contain very high VOC concentrations (mg/L). Therefore, a specifi c sampling system has been added to the Purge and Trap system for the analysis for VOCs in ppm range.
Before injection into the sparger, 50 µl of the sample is withdrawn into a 1 ml automated syringe. Then 950 µl of pure deionized water is withdrawn and injected into the sparger. After that, 4 ml of water will be added to reach 5 ml of liquid
in the sparger. The dilution before analysis decreases the concentration by a factor of 100 and reproduces the conditions described in the 502.2 method. The sample is then analyzed following the procedure described in the previous section.
The analytical system allows very good separation and quantifi cation of the dissolved BTEX. The same sample was twice analyzed to check that the purge was 100% effi cient. The linearity has been studied and a correlation coeffi cient superior to 0.99 was obtained for all BTEX.
Analysis of VOCs using a headspace sampling system
Industrial processes often used more complex matrices than just water. In such a case, the Purge and Trap system cannot be used: if the matrix components are extracted from the liquid, they could saturate the trap. Chromatotec has designed a specifi c liquid sampling system comprised of a pre-concentration module (AirmoCAL) containing a dynamic headspace module.
An automatic gas chromatograph (chromaFID) equipped with a fl ame ionization detector (FID) has been used for the monitoring of linear alkanes containing from 6 to 11 carbon atoms in a water/ triethylene glycol mixture. The Purge and Trap system cannot be used with triethylene glycol because the trap would be saturated.
The liquid sample fl ows inside the stainless-steel headspace vial. Then, the carrier gas goes into the vial and collects the analytes. The gaseous sample is sent into a 100 µl sampling loop and then injected in the GC system. This system was designed in a specifi c backfl ush mode, called CP backfl ush, to prevent potential matrix impurities to go into the analytical column. The CP backfl ush includes a 2 m MXT30 CE pre-column (0.28 mm ID, 1.0 µm dF) and 28 m MXT30 CE column (0.28 mm ID, 1.0 µm dF) located inside the heated oven of the GC. Alkanes from n-hexane to n-undecane were detected and quantifi ed by a FID from ppm to % levels.
Conclusion
In this paper, three different analytical methods for continuous and automatic measurement of VOCs in liquids were presented. First, the airmoVOC WMS is designed for the quantifi cation of trace-level VOCs in water. For higher VOCs concentrations, a specifi c dilution module was added. For samples composed mainly of organic molecules (alcohol, oil), the dynamic headspace module is better suited. For aqueous samples, the Purge and Trap system is preferred (with or without the dilution module) for two reasons: fi rst, the analytical performances (linearity and stability) are better for purge systems than headspace systems. Then, the headspace can be very sensitive to matrix effects because the solubility of the VOCs will be affected.
All three systems can be fully autonomous thanks to our gas generators: nitrogen, hydrogen and zero air. An internal calibration system can be added to validate the results. The integrated multiplexing system allows the analysis of unknown gaseous samples, reference cylinders, embedded calibration (permeation tubes) and samples from the Purge and Trap system. The instrument is enclosed in a robust housing and can work for long periods of time without maintenance. The main goal is to improve process effi ciency and control using automatic on-line continuous monitoring with little, if any, human interaction. These characteristics make these systems perfectly suitable for measurements in industrial contexts.
References
Huybrechts, T., Dewulf, J., and Langenhove, H.V. (2003). State- of-the-art of gas chromatography-based methods for analysis of anthropogenic volatile organic compounds in estuarine waters, illustrated with the river Scheldt as an example. Journal of Chromatography A 1000, 283–297.
https://doi.org/10.1016/ S0021-9673(03)00585-5.
Chary, N.S., Fernandez-Alba, A.R. (2012). Determination of volatile organic compounds in drinking and environmental waters. Trends in Analytical Chemistry 32, 60–75. doi:10.1016/j. trac.2011.08.011.
Author Contact Details
Damien Bazin and Jean-Philippe Amiet, Chromatotec • 15 Rue d’Artiguelongue - Saint-Antoine 33240 VAL DE VIRVEE – France • Tel: +33 557 940 626 • Email:
info@chromatotec.com • Web:
www.chromatotec.com
Jean-Philippe Amiet Damien Bazin
WWW.ENVIROTECH-ONLINE.COM
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