Concentration field flow fractionation

CONCENTRATION FFF Concentration field flow fractionation (concentration FFF) is the only FFF technique that could make use of a concentration gradient of a mixed solvent across the channel in order to induce effective chemical forces or chemical field. 

By coupling flow field-flow fractionation (flow FFF) to ICP-MS it is possible to investigate trace metals bound to various size fractions of colloidal and particulate materials.55 This technique is employed for environmental applications,55-57 for example to study trace metals associated with sediments. FFF-ICP-MS is an ideal technique for obtaining information on particle size distribution and depth profiles in sediment cores in addition to the metal concentrations (e.g., of Cu, Fe, Mn, Pb, Sr, Ti and Zn with core depths ranging from 0-40 cm).55 Contaminated river sediments at various depths have been investigated by a combination of selective extraction and FFF-ICP-MS as described by Siripinyanond et al,55... 

FIG. 2.1 Sedimentation field flow fractionation (SdFFF) (a) an illustration of the concentration profile and elutant velocity profile in an FFF chamber and (b) a schematic representation of an SdFFF apparatus and of the separation of particles in the flow channel. A typical fractionation obtained through SdFFF using a polydispersed suspension of polystyrene latex spheres is also shown. (Adapted from Giddings 1991.)... 

In principle, all powerful element-specific methods that are able to monitor continuously the effluents of separation processes commonly in the range of a few mimin-1 and in element concentrations of some Klpg liter-1. A well-suited method is based on modern element-specific quadrupole mass spectrometry (MS) with an inductively coupled plasma (ICP) interface to the separation unit [e.g., liquid chromatography (LC) or field-flow fractionation (FFF)].Tlie ICP-MS detection can also be used for continuously characterizing the effluent of any kind of packed column (Metreveli and Frimmel, 2007). By this, the transport and elution properties of... 

Because the chapter is about DOM, detailed information about the role of colloids and the analytical techniques are given elsewhere (e.g., Buffle and Leppard, 1995 Kretzschmar et al., 1999 Frimmel et al., 2007). Different separation techniques, like ultrafiltration, size exclusion chromatography, and flow field-flow fractionation can be coupled with UV-vis absorption and ICP-MS to show the interaction of metals and colloids. Elements like Ni, Cu, Cr, and Co are associated mainly with smaller-size DOM fractions whereas Al, Fe, lanthanides, Sn, and Th are associated with larger-size DOM fractions (Bolea et al., 2006). The laser-induced breakdown detection (LIBD) is a new, sensitive method for the quantification of aquatic colloids of lower-range nanometer size in very low concentration, which cannot be... 

Lee, H., Williams, S.K.R. and Giddings, J.C. (1998) Particle size analysis of dilute environmental colloids by flow field-flow fractionation using an opposed flow sample concentration technique. Anal. Chem., 70, 2495-2503. 

In many separation processes (chromatography, countercurrent distribution, field-flow fractionation, extraction, etc.), the transport of components, in one dimension at least, occurs almost to the point of reaching equilibrium. Thus equilibrium concentrations often constitute a good approximation to the actual distribution of components found within such systems. Equilibrium concepts are especially crucial in these cases in predicting separation behavior and efficacy. 

In the filtration-type methods (the first three techniques listed above), components accumulate as a steady-state (polarization) layer at a barrier or membrane [4] this occurs in much the same way as in field-flow fractionation or equilibrium sedimentation. However, there are several complications. First, fresh solute is constantly brought into the layer by the flow of liquid toward and through the filter. This steady influx of solute components can be described by a finite flux density term J0. Second, components can be removed from the outer reaches of the layer by stirring. Third, the membrane or barrier may be leaky and thus allow the transmission of a portion of the solute, profoundly affecting the attempted separation. In fact, one reason for our interest in layer structure is that leakiness depends on the magnitude of the solute buildup at the membrane surface. As solute concentration at the surface increases, more solute partitions into the membrane and is carried on through by flow. 

We note that the distribution is a simple exponential superimposed on the constant background concentration of the solute, J0/v (see Figure 6.2). The effective thickness of the exponential component is seen to be identical in form (but with DT replacing D) to that found for field-flow fractionation, Eq. 6.20 f = DTI u. ... 

The first volume concentrates on separation techniques. H. Pasch summarizes the recent successes of multi-dimensional chromatography in the characterization of copolymers. Both, chain length distribution and the compositional heterogeneity of copolymers are accessible. Capillary electrophoresis is widely and successfully utilized for the characterization of biopolymers, particular of DNA. It is only recently that the technique has been applied to the characterization of water soluble synthetic macromolecules. This contribution of Grosche and Engelhardt focuses on the analysis of polyelectrolytes by capillary electophore-sis. The last contribution of the first volume by Coelfen and Antonietti summarizes the achievements and pitfalls of field flow fractionation techniques. The major drawbacks in the instrumentation have been overcome in recentyears and the triple F techniques are currently advancing to a powerful competitor to size exclusion chromatography. 

The separation and characterization of submicron-sized particles in water is difficult, in particular because of artifacts from sampling and concentration techniques. Lead et al. (1997) have presented a critical review of the different techniques for separation and analysis of colloids (filtration, dialysis, centrifugation, but also volta-metry, gels (DET/DGT), field-flow fractionation, SPLITT). Ultrafiltration membranes have been developed with nominal cutoff sizes ranging from the thousands of daltons (Da) to hundreds of thousands of daltons, which have been used to separate the colloidal pool into several fractions. 

Concentration of Dilute Colloidal Samples by Field-Flow Fractionation... 

The understanding of the effects of sample concentration (sample mass) in field-flow fractionation (FFF) has being obtained gradually with the improvement of the sensitivity (detection limit) of high-performance liquid chromatography (HPLC) detectors. Overloading, which was used in earlier publications, emphasizes that there is an upper limit of sample amount (or concentration) below which sample retention will not be dependent on sample mass injected into the FFF channels [1]. Recent studies show that such limits may not exist for thermal FFF (may be true for all the FFF techniques in polymer separation), although some of the most sensitive detectors on the market were used [2]. 

Field-flow fractionation is, in principle, based on the coupled action of a nonuniform flow velocity profile of a carrier liquid with a nonuniform transverse concentration profile of the analyte caused by an external field applied perpendicularly to the direction of the flow. Based on the magnitude of the acting field, on the properties of the analyte, and, in some cases, on the flow rate of the carrier liquid, different elution modes are observed. They basically differ in the type of the concentration profiles of the analyte. Three types of the concentration profile can be derived by the same procedure from the general transport equation. The differences among them arise from the course and magnitude of the resulting force acting on the analyte (in comparison to the effect of diffusion of the analyte). Based on these concentration profiles, three elution modes are described. 

Field-flow fractionation (FFF) represents a family of versatile elution techniques suited for the separation and characterization of macromolecules and particles. Separation results from the combination of a nonuniform flow velocity profile of a carrier liquid and a nonuniform transverse concentration profile of an analyte caused by the action of a force field. The field, oriented perpendicularly to the direction of the flow, forms a specific concentration distribution of the analyte inside the channel. Because of the flow velocity profile, different analytes are displaced along the channel with different mean velocities, and, thus, their separation is achieved. 

Field-flow fractionation experiments are mainly performed in a thin ribbonlike channel with tapered inlet and outlet ends (see Fig. 1). This simple geometry is advantageous for the exact and simple calculation of separation characteristics in FFF Theories of infinite parallel plates are often used to describe the behavior of analytes because the cross-sectional aspect ratio of the channel is usually large and, thus, the end effects can be neglected. This means that the flow velocity and concentration profiles are not dependent on the coordinate y. It has been shown that, under suitable conditions, the analytes move along the channel as steady-state zones. Then, equilibrium concentration profiles of analytes can be easily calculated. 

Another separation technique of particular application for proteins, high-molar-mass molecules, and particles is the general class known as field-flow fractionation (FFF) in its various forms (cross-flow, sedimentation, thermal, and electrical). Once again, MALS detection permits mass and size determinations in an absolute sense without calibration. For homogeneous particles of relatively simple structure, a concentration detector is not required to calculate size and differential size and mass fraction distributions. Capillary hydrodynamic fractionation (CHDF) is another particle separation technique that may be used successfully with MALS detection. 

The association of pollutants such as trace metals, nutrients, and toxic organic molecules to colloids is intimately connected to the health of natural waters. Colloids, with their large specific surface area, play a dominant role in the transportation and eventual deposition of these pollutants. Of particular interest is the size speciation data. It is important to know not only the total amount of pollutant present but also where it is distributed. It has been inherently difficult to study pollutant-colloid interactions because of the lack of methods for particle size determination and fractionation as well as the low concentrations of pollutants present in many systems. This entry outlines a new approach using field-flow fractionation (FFF). 

Field-flow fractionation (FFF) presents a unique method where particles move in a liquid flow, maintaining a quasi-equilibrium Boltzmann transverse concentration distribution in an FFF channel [1]. It allows one to obtain, from experiments, the transverse Peclet number Pe defining the thickness of the layer, where particles are accumulated, and the retention of the FFF process Ret ... 

---