Colloids and nanoparticles in natural waters
The analysis of natural aquatic colloids and nanoparticles by Field-Flow Fractionation (FFF) is very well-established. During recent years a growing number of articles have been published which show that FFF can also be successfully used for the analysis of engineered nanomaterials in environmental samples. In most cases, FFF is coupled on-line with both Light Scattering and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS), to obtain size-based elemental distributions.
FFF is a family of analytical techniques which are ideally suited for very gentle and high resolution separation and characterization of large molecules and particles, which traditional chromatographic techniques cannot perform. In FFF the separation is achieved through the interaction of the sample with an external physical field instead of a stationary phase. FFF can separate both colloidal and soluble materials and has the advantage of avoiding problems associated with chromatographic techniques (Size Exclusion Chromatography) such as shear degradation, unwanted interaction between sample and stationary phase, and low separation power for large materials. Hence, an outstanding characteristic of FFF is the broad size range, 5 orders of magnitude from 1 nm to > 100 μm. No other technique can cover 5 orders of magnitude of particle size, that is the entire colloidal, polymeric and even most of the microparticle domain.
Environmental samples are often very complex and heterogenous systems which are difficult to characterize. Using several FFF techniques is often a beneficial strategy, due to the more flexible separation ability and the broader and complementary scope of information which is then accessible. Postnova offers “The FFF Platform” which makes it possible to deploy any FFF separation technique and any light scattering and/or concentration detector. The FFF platform can be configurated and later upgraded with all commercially available FFF techniques, with on-line Multi Angle Light Scattering (MALS) and Dynamic Light Scattering (DLS) detectors, and with UV-Vis, RI and fluorescence detectors. The Postnova FFF platform has been widely coupled with most brands of ICP-MS. A fraction collector is also available for the collection of narrow fractions for further off-line characterization (with mass spectrometry, microscopy, etc).
The analysis of natural colloids and engineered nanoparticles in environmental samples is a challenging application where the Postnova FFF Platform concept can deliver its full benefits. For more than two decades, Centrifugal FFF (CF3, also called Sedimentation FFF) coupled on-line with MALS and ICP-MS has been the benchmark in high resolution separation of natural colloids and nanoparticles in aqueous and soil matrices. It should be noticed here that the latest generation of Postnova CF3 machines features a centrifugal separation field up to 2 680 g with a lowest size limit of about 7 nm for dense particles and 10-20 nm for lighter ones. Also Centrifugal FFF has a markedly higher upper size limit (100 to 300 μm) and a higher resolution power for nanoparticles than Flow FFF. To the opposite of Flow FFF, the separation of species with same hydrodynamic radius but different density is possible by Centrifugal FFF.
More recently, both CF3 and Asymmetric Flow FFF (AF4) have been used for the detection and characterization of heavy metals in environmental samples. CF3 and AF4 are quite complementary with regard to their separation range, separation principle, resolution power and the information they provide to the colloid analyst. The Postnova FFF Platform coupled with MALS, DLS and ICP-MS gives particle size (Rgyration, Rhydrodynamic and Rvolume) distributions, particle shape, and the distribution of elements as a function of particle size.