Field Flow Fractionation FFF Coupled with ICP-MS

Let us now turn our attention to another technique that is being used to characterize nanoparticles—field flow fractionation (FFF) coupled with ICP-MS. ITF has been used for a number years coupled with UV/Vis detection, but is now being used with ICP-MS for elemental specificity. The FFF-ICP-MS technique is relatively mature compared to sp-ICP-MS, and as a result, is probably being used in more-routine type environments. 

FFF is a single-phase chromatographic-type separation technique, where 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 (AF4), where the field is generated by a cross-flow applied perpendicularly to the channel. The flow and sample are confined within a channel, consisting of two plates separated by a spacer, which is typically 100-500 pm thick. The upper channel plate is impermeable, while the bottom channel plate 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 slower closer to the boundary edges than it does at the center 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 20.10 Separation in an asymmetrical 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). (Courtesy of Colorado School of Mines.) 

FIGURE 20.11 Fractogram of the particle size separation of a mixture of gold and silver particles using FFF with UV-absorbance detection. 

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