Book Description

The presence of volatile organic compounds (VOCs) in ground and surface water resources poses a threat to public health. The measurement of these trace level contaminants in water is of significant importance. Conventional methods for analysis of trace volatile organic compounds in water include purge and trap, head space analysis, and solid phase microextraction (SPME). While these are excellent laboratory techniques, none of them can be used for continuous, on-line monitoring of water streams. Membrane separation of organic compounds from water provides an exciting possibility for on-line extraction and analysis. In previous investigations, water continuously flowed on the feed side of the membrane and the analytes were continuously removed by an inert gas stream or a vacuum. The measurement was based on steady state permeation. This approach has several limitations. For example, the steady state can not be reached instantly, resulting in a long analysis time. Another limitation is that this instrument can not be used for analyzing small discrete samples. In this study, a novel approach, referred to as pulse introduction membrane extraction (PIME), is presented. This technique eliminates steady state requirements and can be used for continuous monitoring, as well as for discrete analysis of trace levels of VOCs in water. Water samples are introduced as a pulse into a membrane module. An eluent is used to transport the sample onto the membrane. The permeated organic compounds are extracted by an inert gas, concentrated in a micro-sorbent trap and injected into a GC for analysis. An aqueous boundary layer which forms at the membrane surface due to the poor mixing of water with the membrane appears to be the major resistance to mass transfer for the permeation process. Boundary layer effects were reduced by nitrogen purge of the membrane, and by an alternative membrane module design. A mathematical model which takes into account the aqueous boundary layer effects was developed to describe the nonsteady state, pulse introduction process. A qualitative model of extraction efficiency is also presented here to illustrate the factors that affect analytical sensitivity. The combination of system optimization, nitrogen purge and improved module design results in higher sensitivity and faster response than other methods reported in the technical literature. Detection limits are at ppb levels, precision and extraction efficiency are excellent. As the result of this research, the capability of continuous monitoring of trace levels of organic compounds in water has been demonstrated. The PIME system was compared with previously reported steady state membrane permeation system. The advantages of the PIME system include higher sensitive and faster response and can also be used for discrete sample analysis. Comparison of the PIME with the purge and trap technique, which is currently the most popular method for VOCs analysis, showed that the results are in good agreement. Contaminated ground water samples from the Naval Engineering Research Station were analyzed to demonstrate the practicality of the PIME system. This study was extended to the analysis of sernivolatile organic compounds (SVOCs) in water. Continuous monitoring of SVOCs in water using membrane extraction and on-line HPLC analysis was explored. The system was based on continuous extraction rather than pulse introduction. It demonstrated the capability for enriclunent of SVOCs from water into a solvent. Continuous monitoring of SVOCs was demonstrated at ppb level using HPLC. System parameters which affect the enrichment factors were studied.