In this instalment of "Sample Preparation Perspectives," Norwegian authors from the University of Oslo describe the practical
aspects of hollow fibre liquid-phase microextraction in the three-phase mode (HF3
LPME). The guest authors highlight important practical issues related to the supported liquid membrane, the hollow fibre and
the extraction itself. They also discuss practical work with electromembrane extraction (EME), which is related to HF
LPME but uses an electrical potential as the driving force for the extraction.
This month's instalment describes practical aspects of hollow-fibre liquidphase microextraction in the three-phase mode (HF3LPME). HF3 LPME is a microscale sample preparation technique (1) where target analytes are extracted from an aqueous sample through
a supported liquid membrane (SLM) that is immobilized in the pores of a porous polymeric material and into a volume of acceptor
solution (typically, 10–30 ÁL). In this context, the porous polymeric material is a hollow fibre. Here, we highlight important
practical issues related to the SLM, the hollow fibre and the extraction itself because these issues are important for successful
HF3LPME. We also discuss practical work with electromembrane extraction (EME), which is related to the HF3LPME device but uses an electrical potential as the driving force for the extraction (2).
How Does HF
HF3LPME can be used for extraction of basic or acidic analytes from aqueous samples. Figure 1 illustrates a setup for HF3LPME. The sample is contained in a sample vial and the pH is adjusted in the sample before extraction to keep the analytes
in their uncharged state. For basic analytes, the sample is made alkaline, and for acidic analytes, the sample is acidified.
A small piece of a porous hollow fibre, typically made of polypropylene, is closed in one end and dipped in an organic solvent
immiscible with water. In a few seconds, this organic solvent is immobilized in the pores in the wall of the hollow fibre
by capillary forces, forming an SLM.
Figure 1: Illustration of a typical hollow-fibre liquid-phase microextraction in the three-phase mode (HF3LPME) setup.
A 10–30 ÁL volume of aqueous acceptor solution is then injected into the lumen of the hollow fibre. For basic analytes, the
acceptor solution is acidic, whereas it is alkaline for acidic analytes. The hollow fibre is finally placed into the sample
and the whole assembly (sample vial and hollow fibre) is agitated for typically 15–45 min. During this time, analyte molecules
are extracted in their uncharged state from the sample into the SLM, and further into the acceptor solution. In the acceptor
solution, the analyte molecules become ionized, which prevents them from re-entering the SLM. After extraction, the acceptor
solution is collected and analysed directly by high performance liquid chromatography (HPLC), capillary electrophoresis (CE),
mass spectrometry (MS) or other related analytical techniques.
The major advantages of HF3LPME can be summarized as follows:
- High enrichment (up to 25,000-fold) (3)
- Excellent sample clean-up
- Direct compatibility with HPLC, CE and MS
- Low solvent consumption (10–30 ÁL of solvent for each extraction).
Advantages, as well as limitations, of HF3LPME have been discussed substantially in the literature and several reviews discussed a broad range of applications (4–11).