The concept of membrane-controlled processes is widespread in nature. Nearly all biological mechanisms concerning mass transport and exchange are regulated by membrane barriers and a variety of technical and biotechnological applications have been devised based on this mechanism. Membrane applications in analytical chemistry are geared towards the enrichment of target substances from an aqueous solution or the separation of compounds from a complex matrix. This article describes membrane-assisted extraction processes to separate traces of polar pharmaceutical substances the so called emerging micropollutants from aqueous samples. Basic prospects and examples of membrane-supported extractions are presented.

Since the introduction of dialysis 30 years ago, membranes have proven to be useful tools for the separation of polar and ionic compounds in a variety of applications, including drugs from biofluids and pollutants from aqueous environmental samples.

Nano- to micromolar concentrations of drugs present in plasma or urine can be determined, even when a complex biological matrix is present. When drugs have been administered, about 10–30% of the initially applied amount is excreted unchanged. Together with conjugated and metabolized drug portions they enter the environment via wastewater because conventional wastewater treatment cannot completely eliminate these mostly polar and persistent substances.

Let's assume that these compounds are suspected to present certain risks related to reproduction cycles and the health of organisms. Permanent monitoring is recommended but the low concentration of drugs within complex matrices makes their determination challenging. Simple and economic protocols particularly for preconcentration and matrix separation have to be established.

Can membrane-assisted extraction be a possible alternative to solid-phase extraction — the current standard procedure for the trace analysis of bioactive compounds?

In recent times, membrane-assisted microextraction techniques are among the most promising developments in sample enrichment and separation. The membrane probe design is flexible and easy to adapt to quite different analytical instruments. The high potential for automation makes membrane-assisted microextraction interesting for high-throughput analysis.

Membrane-assisted extraction processes become apparent as an attractive alternative to time- and solvent-consuming liquid–liquid extraction processes. Particularly, the opportunity to miniaturize a membrane-assisted extraction process popularized this sample preparation technique in the last decade.

In addition to the demands in environmental monitoring, drug design and admission procedures, medical therapies require analytical methods that are able to manage little sample volumes to detect traces (μg/L and below) of polar pharmaceutical compounds in an aqueous, complex matrix that includes proteins, carbohydrates, fatty acids etc.

Despite high sensitive instruments such as LC–MS–MS an enrichment of drugs prior to analysis is absolutely necessary. Current procedures in environmental drug analysis use solid-phase extraction (SPE) of high sample volumes (1 L and more), solvent amounts between 5 and 12 mL for elution, and further portions for subsequent clean-up are required mostly when wastewater has to be analysed.

Figure 1

Among the well-established solid-phase microextraction techniques such as SPME,² in-tube extractions³ and stir bar sorptive extraction (SBSE),⁴ membrane-assisted extractions are included increasingly in drug analysis within pharmaceutical,^5,6 medical and environmental studies.^7,8 Because of its compact and small design, particularly, micro hollow-fibre membranes can clearly save time and solvent for sample preparation when compared with standard SPE (Figure 1).