Development of micro-scale and automated membrane extraction systems for water analysis
Department of Chemistry and Environmental Science
Doctor of Philosophy
Farinas, Edgardo Tabion
Monitoring of trace level contaminants in water involves the extraction and enrichment of analytes. Over the years several techniques have been developed for this purpose. Currently there is an urgent need for the development of micro-scale extraction techniques that can lead to simple, fast trace analysis. Automated instruments that can be used for continuous, on-line analysis are also important. Membrane extraction can address both these needs, because it can be carried out in a continuous fashion and can also be miniaturized. This research addresses these two aspects of membrane extraction for the next generation of sample preparation.
Micro-Scale Membrane Extraction:
Micro-scale hollow fiber membrane extraction was studied for extraction and preconcentration of non-polar analytes as well as haoacetic acids in water. They represent two important classes of pollutants. The goal was to optimize solvent systems to provide high enrichment and reproducible results. The concept of using a barrier film as a means of providing a diffusional barrier on the membrane fiber was developed. It was found to play an important role by protecting the organic acceptor from migration, resulting in much less acceptor loss. The analytes were extracted from water sample without any pretreatment into an organic acceptor via an organic layer on the membrane (referred to as barrier film). The concentrated extract was analyzed by HPLC and the separation was achieved within 12 minutes. Enrichment factors as high as 4555 were obtained in a 60-minute extraction. This represented a 27 times enhancement over that has been preiously published in the literature. Detection limits were at low to ppt level with RSDs between 1.60 and 7.65%. Large linear dynamic ranges with good linearity (R2 between 0.9870 and 0.9997) were achieved. The implementation of a barrier film represents a major development in the area of membrane based microextraction because it allows a wide variety of solvents to be used and also the use of rigorous extraction conditions.
Micro-fluidic devices were designed and fabricated for analyte enrichment. The analytical approach is based on supported liquid membrane extraction (SLME) followed by direct HPLC-UV detection without any derivatization. Channel dimensions and the flow rates affected enrichment factors and extraction efficiencies. Enrichment factors (EF) as high as 54 were obtained on a 2x2 cm extraction module. Large linearity ranges with good linearity (R2 between 0.9895 and 0.9996), high precisions (RSD between 3.56 and 8.54%) and detection limit as low as 2 nglmL were obtained.
Automated, On-line Membrane Extraction:
On-line extraction involves the continuous introduction of water on the feed side of the membrane and continuous enrichment of analytes on the permeate side. In fact membranes represent the only extraction media where such a continuous process is possible. Two different extraction approaches were investigated. The first was for realtime monitoring of haloacetic acids in water, and the second was for on-line liquid-liquid membrane extraction followed by membrane concentration.
Hollow fiber liquid-liquid membrane extraction (LLME) and SLME followed by on-line HPLC-UV detection were developed for continuous monitoring of the nine HAAs. With continuous LLME, seven halo-acetic acids could be analyzed and EF was around 50. All the nine acids could be extracted and quantified by continuous SLME. Experiments with laboratory standards demonstrated that EF and extraction efficiency could be as high as 500 and 54%, respectively. Relative standard deviations based on seven replicates were between 3.3 and 10.3 %, and the MDLs were at sub-ppb levels.
A total analytical system (TAS) was developed by interfacing continuous membrane extraction, pervaporation and on-line HPLC-UV detection. Organics from a water sample were extracted into an organic solvent, and then concentrated via pervaporation prior to HPLC-UV detection. Factors affecting the system performance were studied. With optimized experimental parameters enrichment factors as high as 192 were obtained, the method detection limits were at low ng/mL levels, and the precisions were better than 5%.
njit-etd2005-137 (134 pages ~ 10,888 KB pdf)
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Created September 8, 2008