Chemistry Reference
In-Depth Information
25
l HPLC
syringe
μ
Aluminium
foil
150
μ
l organic
phase
Teflon
ring
2
μ
l aqueous
(a2) drop
(pH 2.1)
2.0 ml
aqueous
water sample
(pH 13)
al
Figure 17.2
Basic configuration for LLLME [11].
derivatization (to reduce its polarity or increase its volatility) are common practices that, in general, contribute
to increase the extraction efficiency and reduce the analysis time. The simplicity of the analytical procedure,
the possibility to perform it manually or (semi-)automatic using an autosampler, and the feasibility of
obtaining ready-to-analyse extracts have probably been additional factors contributing to the rapid
development and acceptation of this environmental-friendly technique in different research fields.
Direct-immersion SDME has demonstrated to be useful for the extraction of relatively non-polar and
semivolatile analytes from water samples that contain little or no particulate or dissolved matter. However,
the analysis of more complex matrices, such as urine, requires a previous filtration of the sample [8]. Due to
its characteristics, the technique is particularly suited for the treatment of size-limited samples, as recently
demonstrated by Wu
et al
. [9], who used it for the simple, fast and efficient extraction of drugs from relatively
complex samples (i.e. biological fluids). The analysis required only 10
l of organic
extractant, and was named drop-to-drop microextraction (DDME). Despite the relatively high limits of
detection (LODs), the technique allowed proper detection of the investigated drugs in blood, serum and urine.
Application of SDME to the analysis of polar compounds required a modification that resulted in a three-
phase SDME system named liquid-liquid-liquid microextraction (LLLME) [10]. In this approach, the
deionized polar analytes were preconcentrated from the aqueous sample in a few microlitres of organic phase
placed in a PTFE ring and subsequently back-extracted in an aqueous micro-drop that acted as receiving
phase (Figure 17.2). Next, this micro-drop was withdrawn into the syringe and directly subjected to liquid
chromatography (LC) or capillary electrophoresis (CE) analysis. Therefore, the organic phase acts as an
organic liquid membrane allowing the simultaneous enrichment and purification of the analytes. Its higher
stability as compared to the organic drop of the two-phase SDME format allows higher stirring rates,
something that combined with the small volume of receiving organic phase, resulted in fast extraction
processes (ca. 15 min) with higher enrichment factors (in the 200-500 range). The complete renewal of the
phases in between extractions also contributes to reduce the risk of cross-contamination. Although the
μ
l of sample and 0.5-1.0
μ
