Chemistry Reference
In-Depth Information
The first microfluidic system for solvent extraction was a variation of an H-filter design, fabricated in
quartz and comprising 250
m wide channels. Introduction of Fe(II) in an aqueous stream and
trioctylmethylammonium chloride in an organic (chloroform) stream allowed extraction of the ion-pair
product in the organic phase. Extraction was shown to occur in less than 45 s, representing an order of
magnitude improvement over conventional extraction times in separation funnels [113]. The authors have
also demonstrated the extraction of Ni(II) complexes in microchannels [114], integration of neutral ionophore-
based ion-pair extraction on-chip [115], and sequential ion sensing via 'slug' flow in microchannel
environments [116]. The last one is of particular interest since the approach allows the determination of
multiple ions in a single sample by pumping aqueous and organic phases intermittently through a fluidic
network.
A potential drawback when using microfluidic systems for solvent extraction is the low unit throughput
(normally between 1 and 100
μ
l min
−1
). This problem can be obviated by operating arrays of parallel channels
concurrently. To this end, the fabrication of silicon/glass micro-contactor arrays for the extraction of single
feeds at rates of 250 ml h
−1
have been reported [117]. LLE is achieved by contacting fluidic streams at
constricted openings between distinct channels. The approach is attractive since flows can be separated
naturally as the channels diverge.
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17.3.3.2
Solid phase extraction (SPE)
Solid phase extraction is a broadly used technique in which a target molecule is retained by a chromatographic
stationary phase material and subsequently eluted in an appropriate (and selective) solvent. SPE functions
either as a sample clean-up method and a preconcentration method. This fact is due not only that the target
analyte is retained within the stationary phase and the unwanted components of the sample matrix flow to
waste, but also after elution in the desired volume entails preconcentration of the analyte.
Several and creative approaches for performing solid phase extraction in microfluidics have been proposed.
One of them entails to coat channel walls with a high affinity stationary phase, although this methodology is
less common. The other one involves packing the microchannels with the stationary phase material. The
advantages and disadvantages are clearly defined into miniaturized environments. Coating channel approaches
depend on the available surface area for interaction and, consequently, into micron-size channels the contact
surface is very small. A simpler way to increase the surface area is to pack the microchannels with stationary
phase; however, the packing process is not easy and sometimes this alternative is avoided. A very attractive
possibility, compatible with miniaturized dimensions, is to replace conventional stationary phase materials by
a continuous porous bed in situ formed from polymerization of organic monomers. The process of bed
formation is easy, since a low-viscosity monomer solution can be introduced by vacuum or pressure into the
microfluidic channel prior to initiation.
Selected examples of relevant approaches will be given next in order to provide the grade of implementation
of these approaches on microfluidics.
Octadecyltrimethoxysilane coated walls channels were firstly used for SPE of neutral coumarin dye [118].
A simple fluidic network allowed for both enrichment (80-fold increase in concentration) and elution of the
dye within 4 min. The problem of limited surface areas in open-channel devices can be improved to some
degree by utilizing sophisticated fabrication techniques. Taking in account the idea of increasing surface area
by packing microchannels with stationary phase material, fabrication and testing of a 330 pl chromatographic
bed integrated within an electroosmotically pumped microsystem was carried out [119, 120]. The authors
utilized weirs within a microfabricated channel to trap coated silica beads (1.5-4
m diameter). These are
then used to perform both solid phase extraction and electrochromatography of small molecules. Concentration
enhancements of up to 500 times were demonstrated for two fluorescent dyes. Reversed-phase beads have
been used in the SPE device because of their extensive use for the chromatography of proteins and peptides.
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