In this installment of "Sample Prep Perspectives," Norwegian authors from the University of Oslo describe the practical aspects
of hollow fiber liquid-phase microextraction in the three-phase mode (HF
LPME). The guest authors highlight important practical issues related to the supported liquid membrane, the hollow fiber,
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 column installment describes practical aspects of hollow-fiber liquid-phase microextraction in the three-phase mode (HF3LPME). HF3LPME is a microscale sample preparation technique (1) in which 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 fiber. Here, we highlight important
practical issues related to the SLM, the hollow fiber, and the extraction itself, as 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 fiber, 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 fiber
by capillary forces, forming an SLM. A 10–30 ÁL volume of aqueous acceptor solution is then injected into the lumen of the
hollow fiber. For basic analytes, the acceptor solution is acidic, whereas it is alkaline for acidic analytes. The hollow
fiber is finally placed into the sample and the whole assembly (sample vial and hollow fiber) 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 analyzed directly by high performance liquid chromatography
(HPLC), capillary electrophoresis (CE), mass spectrometry (MS), or other related analytical techniques.
Figure 1: Illustration of a typical hollow-fiber liquid-phase microextraction in the three-phase mode (HF3LPME) setup.
The major advantages of HF3LPME can be summarized as follows:
- High enrichment (up to 25,000-fold) (3)
- Excellent sample cleanup
- Direct compatibility with HPLC, CE, and MS
- Low solvent consumption (10–30 ÁL of solvent per 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).