Micro-scale membrane extraction for monitoring organic pollutants at ultra-trace level

Abstract

Hollow-fiber based liquid phase micro-extraction (LPME) is a relatively new extraction technique that combines the miniature of the solid-phase micro-extraction to the principle of liquid-liquid extraction (LLE). Fundamental study comparing the extraction efficiency of LPME and LLE using a simple mathematic model suggested that the analyte partition coefficient controls the extraction in LPME. The data also pointed to other parameters that govern the partition process such as molecular shape, size, and functional groups that control the molecular hydrophobicity. The simple mathematic model is limited to the thermodynamics of ideal partition and not includes kinetics parameters. Acid–base characteristics of the analyte strongly influence the extraction. These characters can be predicted, controlled, and manipulated with good accuracy to maximize the enrichment by simple chemistry concepts. pH has no influence on neutral compounds. Parameters affecting the enrichment were determined to be choices of the extracting solvent, choices of the acceptor solution, extraction time, sample volume, ionic properties of both donor and acceptor solutions, as well as other parameters that can affect the mass transfer rate such as solvent viscosity, temperature, and carrier concentration. The fundamentals were applied to some current pollutants. Three extraction methods were developed for high water soluble pesticides (glyphosate and its metabolite, quaternary nitrogen herbicides) and drinking water disinfection by-products. All methods were tested for their analytical performances and demonstrated excellent capability surpassing many published standard methods. LPME principle employs microliters of extracting solvent to concentrate the analytes without additional solvent evaporation steps. Extractant can be analyzed directly by simple analytical instrument. Three developed LPME methods coupled to analytical procedures for three groups of current organic pollutants surpassed strict validation protocol of trace analysis and deem fit for routine environmental analysis.