Developments in hollow fibre-
based, liquid-phase microextraction
Knut Einar Rasmussen, Stig Pedersen-Bjergaard
The demand for automation in analytical liquid-liquid extraction (LLE)
combined with organic solvent reduction or elimination has led to the
recent development of liquid-phase microextraction (LPME) based on dis-
posable hollow fibres. In this concept, analytes of interest are extracted
from aqueous samples, through a thin layer of organic solvent immobi-
lized within the pores of a porous hollow fibre, and into an acceptor
solution inside the lumen of the hollow fibre. Subsequently, the acceptor
solution is directly subjected to a final analysis by capillary gas chroma-
tography (CGC), high-performance liquid chromatography (HPLC), capil-
lary electrophoresis (CE), or mass spectrometry (MS) without any further
effort. Hollow fibre-based LPME may provide high analyte pre-concen-
tration and excellent sample clean-up, and it has a broad application
potential within areas such as druganalysis and environmental monitor-
ing. This review focuses on basic extraction principles, technical set-up,
recovery, enrichment, extraction speed, selectivity, applications, and
future trends in hollow fibre-based LPME.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Drug analysis; Environmental analysis; Extraction techniques; Hollow
fibres; Liquid-phase microextraction
1. Introduction
There have been several publications on
miniaturising of LLE in analytical chem-
istry. The major ideas behind this were to
facilitate automation and e¡ectively to
reduce the consumption of organic sol-
vents. Miniaturised LLE, or LPME, was
introduced in 1996, and involved the
used of a droplet of organic solvent hang-
ing at the end of a micro-syringe needle
[1^4]. This organic micro-droplet was
placed in an aqueous sample, and the
analytes present in the aqueous sample
were extracted into the organic micro-
droplet (micro-extract). Subsequently,
the organic micro-droplet was with-
drawn into the syringe, the syringe was
transferred to a CGC, and the micro-
extract was injected into the CGC for the
¢nal analysis [1^4].
Alternatively, LPME was performed in
a three-phase system in which analytes in
their neutral form were extracted from
aqueous samples, through a thin layer
of an organic solvent on the top of the
sample, and into an aqueous micro-
droplet (micro-extract) placed at the tip of
a micro-syringe [5,6]. In the latter, the
analytes were transferred to their ionized
state (di¡erent pH from the sample) to
promote trapping within the micro-
extract. Subsequently, the aqueous
micro-extract was transferred to a HPLC
for the ¢nal analysis.
Based on hanging droplets, two-phase
and three-phase LPME were recently
reviewed in this journal [7].Withboth,
high pre-concentration may be achieved
for analytes with high partition coe⁄-
cients because they are transferred by dif-
fusion from a relatively large sample
volume (1^5 ml) and into a micro-extract
of typically 5^50 mL. In addition, the con-
sumption of organic solvent is low, and
the sample clean-up advantages of LLE
are preserved; particularly in the three-
phase mode when both extraction and
back-extraction are included, excellent
clean-up has been observed, even from
complicated biological samples. How-
ever, LPME based on hanging droplets is
not very robust [8],andthedropletsmay
be lost from the needle tip of the syringe
during extraction. This is especially the
case when samples are stirred vigorously
to speed up the extraction process. In
addition, biological samples, such as
plasma, may emulsify substantial
amounts of organic solvents, and this
mayalsoa¡ectthestabilityofhanging
droplets during extraction.
Based on the promising pre-concen-
tration, sample clean-up, and solvent
saving aspects of LPME with hanging
Knut Einar Rasmussen,
StigPedersen-Bjergaard*
School of Pharmacy,
University of Oslo, P.O. Box
1068 Blindern, N-0316 Oslo,
Norway
*Corresponding author.
Tel.: +47 22856576;
Fax: +47 22854402;
E-mail: stig.pedersen-
bjergaard@farmasi.uio.no
Trends in Analytical Chemistry, Vol. 23, No. 1, 2004 Trends
0165-9936/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0165-9936(04)00105-0
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