Field-Flow Fractionation
FFF is a single-phase chromatography technique in which separation is achieved within a very thin channel, against which a
perpendicular force field is applied. One of the most common forms of FFF is asymmetrical-flow FFF, in which the field is
generated by a cross-flow applied perpendicular to the channel. The flow and sample are confined within a channel that consists
of two plates separated by a spacer that is typically 100–500 µm thick. The upper channel plate is impermeable and the bottom
channel is made of a permeable porous frit material. A membrane covers the bottom plate to prevent the sample from penetrating
the channel.
Within the flow channel, a parabolic flow profile is created because of the laminar flow of the liquid. As a result, the stream
moves slowly closer to the boundary edges than it does at the centre of the channel flow. When a perpendicular force field,
in this case fluid cross-flow, is applied to the flowing, laminar stream, the analytes are driven toward the boundary layer
of the channel.
Figure 1: Separation in an asymmetrical field-flow fractionataion (FFF) channel is a result of the imposition of a parabolic
channel flow velocity profile on analytes that are positioned on the channel at heights that arise from the balance of the
applied field (U) and the back diffusion (D).
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Diffusion, associated with Brownian motion, creates a counteracting motion. Smaller particles that have higher diffusion rates
will reach an equilibrium position higher up in the channel where the longitudinal flow is faster. The smaller particles are
transported much more rapidly along the channel than the larger particles, which results in the smaller particles being eluted
before the larger ones.
Figure 2: A fractogram of the particle size separation of a mixture of gold (Au) and silver (Ag) particles using field flow
fractionation (FFF) with UV absorbance detection. Adapted from reference 7.
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After a sample is injected through the inlet port of the FFF system, the flows are manipulated in such a way as to concentrate
the particles into a narrow band. Following this period, the channel and cross-flows are applied to create the separation.
The separated particles exit the outlet port, move into the detection system and are displayed as a temporal signal called
a fractogram (similar to a chromatogram in chromatographic separation techniques). The fundamental principles of this separation
process are shown in Figure 1. A typical fractogram of a mixture of silver and gold particles using UV-absorbance detection
is shown in Figure 2 (7).
FFF Coupled with ICP-MS
ICP-MS has become the dominant technique for ultratracelevel quantitation of metals in environmental matrices, with multielement
capabilities similar to ICPoptical emission spectroscopy (ICP-OES), and detection limits an order of magnitude lower than
graphite furnace atomic absorption spectroscopy (GFAAS). This makes the technique ideal for detecting, quantifying and characterizing
metal nanoparticles with extremely high sensitivity and selectivity, as well as avoiding many of the known interferences associated
with complex environmental samples (8). The added benefit of using ICP-MS is that it is a rapid multielement technique, so
it can be applied to the analysis of metal salt semiconductors such as cadmium selenide quantum dots. Dissolution of the cadmium
selenide core is inhibited by the addition of an outer shell of zinc sulphide or similar material. ICP-MS can detect all these
metals, therefore allowing the study of size-dependent dissolution or aggregation of these kinds of multielement-containing
nanoparticles.
Figure 3: A typical instrumental set-up for coupling an FFF system to an ICP-MS system. The nondestructive detectors can include
light-scattering, UV–vis, refractive index or fluorescence techniques.
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ICP-MS is also relatively straightforward to couple to FFF because the sample flow rate of the ICP-MS sample introduction
system is similar to the outlet flow rate of the FFF system (~0.5–2.0 mL/min) (9). However, some challenges still have to
be overcome to quantify metal concentrations in fractionated samples, as some nanoparticles tend to stick to the internal
membrane of the FFF system. Figure 3 shows a typical instrumental setup for coupling an FFF system to an ICP-MS system. (Note:
All the ICP-MS data published in this study were generated on an ELAN DRC II ICP-MS system from PerkinElmer, Inc.)
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