Experimental study on the retention of silica particles in gravitational field-flow fractionation effects of the mobile phase composition

Effects of mobile phase composition can play an effective role in modulating the retention of particles in gravitational field-flow fractionation (GFFF), the simplest and cheapest among field-flow fractionation (FFF) techniques. In the framework of an optimized procedure for the GFFF characterization of particulate systems, an experimental approach to the effects of the mobile phase composition on the retention of silica particles retention is presented. The role of the ionic strength and the presence of surfactant are emphasized, with special regards to the shape of the particles. Moreover, the first experimental evidence of potential-barrier GFFF is reported.     

Field-flow fractionation in bioanalysis: A review of recent trends

Field-flow fractionation (FFF) is a mature technique in bioanalysis, and the number of applications to proteins and protein complexes, viruses, derivatized nano- and micronsized beads, sub-cellular units, and whole cell separation is constantly increasing. This can be ascribed to the non-invasivity of FFF when directly applied to biosamples. FFF is carried out in an open-channel structure by a flow stream of a mobile phase of any composition, and it is solely based on the interaction of the analytes with a perpendicularly applied field. For these reasons, fractionation is developed without surface interaction of the analyte with packing or gel media and without using degrading mobile phases. The fractionation device can be also easily sterilized, and analytes can be maintained under a bio-friendly environment. This allows to maintain native conditions of the sample in solution.In this review, FFF principles are briefly described, and some pioneering developments and applications in the bioanalytical field are tabled before detailed report of most recent FFF applications obtained also with the hyphenation of FFF with highly specific, sensitive characterization methods. Special focus is finally given to the emerging use of FFF as a pre-analytical step for mass-based identification and characterization of proteins and protein complexes in proteomics.     

On the retention mechanisms and secondary effects in microthermal field-flow fractionation of particles

The behavior of nanometer or micrometer-sized particles, dispersed in liquid phase and exposed to temperature gradient, is a complex and not yet well understood phenomenon. Thermal field-flow fractionation (TFFF), using conventional-size channels, played an important role in the studies of this phenomenon. In addition to thermal diffusion (thermophoresis) and molecular diffusion or Brownian movement, several secondary effects such as particle–particle and/or particle–wall interactions, chemical equilibria with the components of the carrier liquid, buoyant and lift forces, etc., may contribute to the retention and complicate the understanding of the relations between the thermal diffusion and the characteristics of the retained particles. Microthermal FFF is a new high-performance technique allowing much easier manipulation and control of the operational parameters within an extended range of experimental conditions in comparison with conventional TFFF. Consequently, in combination with various other methods, it is well suited for a detailed investigation of the mentioned effects. In this work, some contradictory published results concerning the thermal diffusion of the colloidal particles, studied by TFFF but also by other methods, are analyzed and compared with our experimental findings.     

Newly designed flow field-flow fractionation channel for macromolecules and particles in the submicrometer and micrometer range

The flow field-flow fractionation (FIFFF) technique is a promising method for separating and analysing particles and large size macromolecules from a few nanometers to approximately 50 μm. A new fractionation channel is described featuring well defined flow conditions even for low channel heights with convenient assembling and operations features. The application of the new flow field-flow fractionation channel is proved by the analysis of pigments and other small particles of technical interest in the submicrometer range. The experimental results including multimodal size distributions are presented and discussed.     

Potential barrier field-flow fractionation for the separation and characterization of colloidal particles

Summary The reversibility of adsorption of colloidal particles on the channel wall in Sedimentation Field-Flow Fractionation (SFFF), which is based on the variation of the ionic strength of the carrier solution, suggests a new method, for the separation and characterization of colloidal materials. This new method has been called Potential Barrier Field Flow Fractionation (PBFFF).     

Newly designed flow field-flow fractionation channel for macromolecules and particles in the submicrometer and micrometer range

The flow field-flow fractionation (FIFFF) technique is a promising method for separating and analysing particles and large size macromolecules from a few nanometers to approximately 50 μm. A new fractionation channel is described featuring well defined flow conditions even for low channel heights with convenient assembling and operations features. The application of the new flow field-flow fractionation channel is proved by the analysis of pigments and other small particles of technical interest in the submicrometer range. The experimental results including multimodal size distributions are presented and discussed.     

Separation and characterization of nanoparticles in complex food and environmental samples by field-flow fractionation

The thorough analysis of natural nanoparticles (NPs) and engineered NPs involves the sequence of detection, identification, quantification and, if possible, detailed characterization. In a complex or heterogeneous sample, each step of this sequence is an individual challenge, and, given suitable sample preparation, field-flow fractionation (FFF) is one of the most promising techniques to achieve relevant characterization.The objective of this review is to present the current status of FFF as an analytical separation technique for the study of NPs in complex food and environmental samples. FFF has been applied for separation of various types of NP (e.g., organic macromolecules, and carbonaceous or inorganic NPs) in different types of media (e.g., natural waters, soil extracts or food samples).FFF can be coupled to different types of detectors that offer additional information and specificity, and the determination of size-dependent properties typically inaccessible to other techniques. The separation conditions need to be carefully adapted to account for specific particle properties, so quantitative analysis of heterogeneous or complex samples is difficult as soon as matrix constituents in the samples require contradictory separation conditions. The potential of FFF analysis should always be evaluated bearing in mind the impact of the necessary sample preparation, the information that can be retrieved from the chosen detection systems and the influence of the chosen separation conditions on all types of NP in the sample. A holistic methodological approach is preferable to a technique-focused one.     

Gravitational field-flow fractionation in combination with flow injection analysis or electrothermal AAS for size based iron speciation of particles

A simple gravitational field-flow fractionation (GrFFF) system was used for size separation of micron sized silica particles coated with hydrous iron oxide (geothite). The amount of iron on the particles was monitored either on-line by reverse-flow injection analysis (r-FIA) with chemiluminescence detection using luminol or off-line by electrothermal atomic absorption spectrophotometry (ETAAS). The combination of GrFFF with reverse FIA or with ETAAS has been demonstrated to be a cost-effective tool for size based iron speciation of particles.     

Combination of gravitational SPLITT fractionation and field-flow fractionation for size-sorting and characterization of sea sediment

A combination of gravitational split-flow thin (SPLITT) fractionation and sedimentation/steric field-flow fractionation (Sd/StFFF) has been used for continuous size-sorting of a sediment sample and for size analysis of the collected fractions. An IAEA (International Atomic Energy Agency) sediment material was separated into four size fractions (with theoretical size ranges <1.0, 1.0–3.0, 3.0–5.0, and >5.0 m in diameter) by means of a three-step gravitational SPLITT fractionation (GSF) for which the same GSF channel was used throughout. The GSF fractions were collected and examined by optical microscopy (OM) and by Sd/St FFF. The mean diameters of the GSF fractions measured by OM were within the size interval predicted by GSF theory, despite the theory assuming that all particles are spherical, which is not true for the sediment particles. The Sd/St FFF results showed that retention shifted toward shorter elution time (or larger size) than expected, probably because of the shape effect. The results from GSF, OM, and Sd/StFFF are discussed in detail.     

Field-flow fractionation: An efficient approach for matrix removal of soil extract for inductively coupled plasma optical emission spectrometry

An on-line coupling between a continuous-flow sequential extraction (CFSE) unit and flow field-flow fractionation with cross flow matrix removal (FlFFF/CFM) with ICP-OES detection was developed for determination of metal leachability from soil. The use of high concentration of Mg(NO₃)₂ in exchangeable phase can cause undesirable matrix effects by shifting ionization equilibrium in the plasma, etc., resulting in a clear need for matrix removal. Therefore, the capability of FlFFF/CFM to remove Mg matrix ion from soil extract was evaluated. Poly(ethylene imine) (PEI) having molecular weight of 25,000 Da was added to form complexes with analyte elements (Cu, Mn, Pb, and Zn) but not the matrix element (Mg). The free Mg matrix ions were then removed by filtering off through the ultrafiltration membrane, having a 1000-Da molecular weight cut-off, inside the FlFFF channel. With the use of FlFFF/CFM, matrix removal efficiency was approximately 83.5%, which was equivalent to approximately 6-fold dilution of the matrix ion. The proposed hyphenated system of CFSE and FlFFF/CFM with ICP-OES detection was examined for its reliability by checking with SRM 2710 (a highly contaminated soil from Montana). The metal contents determined by the proposed method were not significantly different (at 95% confidence) from the certified values.     

Different methods for the fractionation of magnetic fluids

 Magnetic fluids are used in many fields of application, such as material separation and biomedicine. Magnetic fluids consist of magnetic nanoparticles, which commonly display a broad distribution of magnetic and nonmagnetic parameters. Therefore, upon application only a small number of particles contribute to the desired magnetic effect. In order to optimize magnetic fluids for applications preference is given to methods that separate magnetic nanoparticles according to their magnetic properties. Hence, a magnetic method was developed for the fractionation of magnetic fluids. Familiar size-exclusion chromatography of two different magnetic fluids was carried out for comparison. The fractions obtained and the original samples were also magnetically characterized by magnetic resonance and magnetorelaxometry, two biomedical applications. The size-exclusion fractions are similar to those of magnetic fractionation, despite the different separation mechanisms. In this respect, magnetic fractionation has several advantages in practical use over size-exclusion chromatography: the magnetic method is faster and has a higher capacity. The fractions obtained by both methods show distinctly different magnetic properties compared to the original samples and are therefore especially suited for applications such as magnetorelaxometry. Received: 12 July 1999/Accepted in revised form: 9 November 1999     

Improved theory of cyclical electrical field flow fractionation

Previously reported theories for cyclical electrical field flow fractionation (CyElFFF) are severely limited in that they do not account for diffusion, steric, or electric double layer effects. Experiments have shown that these theories overpredict the retention of particles in CyElFFF. In this work, we present a model for prediction of steric, diffusion, and electrical effects. The electrical double layer effects are treated using a lumped electrical circuit model that accounts for the field shielding by the electrical double layer formed at the electrode-carrier interface. The electrical effects are shown to dominate retention times and outweigh the contributions of diffusion and particle size. Detailed results from the simulations are presented in this work, and a comparison between the theoretical and experimental results obtained from the retentions of polystyrene particle standards is presented in this paper. The models are shown to correctly predict the retention of the polystyrene standards in CyElFFF with a reasonable error, while existing models are shown to have significant failings.     

Hollow-fiber flow/hyperlayer field-flow fractionation for the size characterization of airborne particle fractions obtained by SPLITT fractionation

Hollow-fiber flow field-flow fractionation (HF FlFFF) was applied for the separation and size characterization of airborne particles which were collected in a municipal area and prefractionated into four different-diameter intervals >5.0, 2.5-5.0, 1.5-2.5, <1.5 microm) by continuous split-flow thin (SPLIIT) fractionation. Experiments demonstrated the possibility of utilizing a hollow-fiber module for the high-performance separation of supramicron-sized airborne particles at steric/hyperlayer operating mode of HF FlFFF. Eluting particles during HF FlFFF separation were collected at short time intervals (approximately 10 s) for the microscopic examination. It showed that particle size and size distributions of all SPLITT fractions of airborne particles can be readily obtained using a calibration and that HF FlFFF can be utilized for the size confirmation of the sorted particle fraction during SPLITT fractionation.     

A quantitative approach to the analysis of supermicron dispersions by field-flow fractionation with UV-vis detectors. The application of an absolute method

Summary Quantitative analysis in field-flow fractionation is becoming a necessary requirement for routine applications, instrumental optimization and scale-up to preparative separations. The use of detection systems which show complex dependence on sample characteristics (i.e. UV spectrometry) has hindered the application of quantitative methods of analysis in field-flow fractionation. A standardless model, shown valid in flow-through, homogeneous systems, is applied here to a heterogeneous system (dispersed supermicron particles) in field-flow fractionation by single peak area measurements. Absolute analysis in the fractionation of spherical silica particles for high-performance liquid chromatography column packing by gravitational field-flow fractionation with UV-Vis detectors is presented. It has been shown that for such samples extinction coefficients are independent of sample concentration and are determined by the size and density of the particles. The accuracy of such an approach to absolute analysis is discussed. In memory of J. C. Giddings Presented at FFF'95-Fifth International Symposium on Field-Flow Fractionation, Park City, UT, USA, July 10–12 1995.     

Magnetic field effect on chemical compositions of spherical Fe/Co fine particles synthesized from a gaseous mixture of iron pentacarbonyl and cobalt tricarbonyl nitrosyl

Under UV light irradiation on a gaseous mixture of Fe(CO)₅ and Co(CO)₃NO, both the crystalline deposits with sizes of 5 and 18 μm and the spherical particles with a mean diameter of 0.3 μm were produced. From FT-IR spectra and SEM–EDS analysis, it was suggested that the chemical structure of the crystalline deposits was the one of Fe₂(CO)₉ being modified by involving Fe(CO)Co bond. By decreasing a partial pressure of Fe(CO)₅ to 0.5 Torr in the gaseous mixture, only the spherical aerosol particles could be produced. Chemical composition of the particles was rich in Co species. From the disappearance of bridging CO band in the FT-IR spectra of the particles and the appearance of CO bands coordinated to a metal atom, Fe atom in Fe(CO)₄ was suggested to be coordinated by the O atom in bridging CO bond in Co(CO)Co structure and/or in α-diketone structure which was formed from two CO groups in dicobalt species. Chemical compositions of the crystalline deposits and the spherical particles were influenced differently by the application of a magnetic field. Atomic ratio of Fe to Co atom decreased in the crystalline deposits whereas it increased in the spherical particles with increasing magnetic field up to 5 T. Linearly aggregated particles (i.e., particle wires) as long as 30 μm were produced on the front side of a glass plate placed at the bottom of the irradiation cell.     

Evaluation of a combined fractionation and speciation approach for study of size-based distribution of organotin species on environmental colloids

Results relating to the first original application of an analytical approach combining asymmetric flow field-flow fractionation (As-Fl-FFF) with multi-detection and chemical speciation for determination of organotins in a landfill leachate sample are presented. The speciation analysis involved off-line head-space solid-phase microextraction (HS-SPME)–gas chromatography with pulsed-flame photometric detection (GC–PFPD) performed after three consecutive collections of five different fractions of interest from the As-Fl-FFF system and cross-flow part (assumed to be representative of the <10 kDa phase). After 0.45 μm filtration and without preconcentration before fractionation and speciation analysis, limits of detection (LOD) were 4–45 ng (Sn) L⁻¹ in the sample, with relative standard deviations (RSD) of 3–23%. The As-Fl-FFF fractionation of this sample enables characterization of two distinct populations—organic-rich and inorganic colloids with gyration radius up to 120 nm. Total Sn and mono and dibutyltins (MBT and DBT) appear to be distributed over the whole colloidal phase. Tributyl, monomethyl, monooctyl, and diphenyltins (TBT, MMT, MOcT, and DPhT) were also detected. Quantitative speciation analysis performed on the two colloidal populations and in the <10 kDa phase revealed concentrations from 130 ± 10 (MMT) to 560 ± 50 ng (Sn) L⁻¹ (DPhT).     

A one-dimensional transient model of electrical field flow fractionation

A well-developed classical theory is available for constant-voltage electrical field flow fractionation (EFFF). Recent experimental research, however, has demonstrated that pulsed fields may enhance retention in some cases. A generalized mathematical approach is presented for the prediction of retention ratios for any field type, pulsed or constant. The methodology is applied and demonstrated for a square wave protocol. Complex concentration profiles arise wherein particles are focused either towards the walls or into the channel center. The computational results indicate that pulsation can either increase retention time or decrease retention time by manipulating the effective electric field and suggest that separation resolution may also be improved.     

Preserving the activity of cellulase in a batch foam fractionation process

Foam fractionation isone of the low operating-cost techniques for removing proteins from a dilute solution. The initial bulk solution pH and air superficial velocity play an importantrole in the foam-fractionation process. Denaturation of proteins (enzymes) can occur, however, during the foamfractionation process from the shear forces resulting from bursting air bubbles. At the extreme bulk solution pHs (lower than 3.0 and higher than 10.0), the en zymatic activity of cellulase in the foamate phase drops significantly. Within these two pH boundsan increase in the air superficial velocity, V_o, and a decrease in the bulk solution pH leads to a decrease in the separation ratio (SR), defined as theratio of the protein concentration in the foamate to the protein concentration in the residue. On the other hand, an increase in V_o provides a higher foamate-protein recovery. The process efficiency is defined as the product of foamate-protein recovery times the SR times the cellulase activity. The optimal operating condition of the cellulase foamfractionation process is taken into account at the maximum value of the processefficiency. In this study, that optimal condition is atan air superficial velocity of 32 cm/min and a bulk-solution pH of 10.0. At this condition, the recovered foamate is about 80% of the original protein mass, the SR is about 12, and the en zymatic activity is about 60% of the original cellulase activity.     

The induced magnetic field in cyclic molecules

The response of a molecule to an applied external magnetic field can be evaluated by a graphical representation of the induced magnetic field. We have applied this technique to four representative, cyclic organic molecules, that is, to aromatic (C(6)H(6), D(6h)), anti-aromatic (C(4)H(4), D(2h)) and non-aromatic (C(4)H(8), D(4h), and C(6)H(12), D(3d)) molecules. The results show that molecules that contain a pi system possess a long-range magnetic response, while the induced magnetic field is short-range for molecules without pi systems. The induced magnetic field of aromatic molecules shields the external field. In contrast, the anti-aromatic molecules increase the applied field inside the ring. Aromatic, anti-aromatic, and non-aromatic molecules can be characterized by the appearance of the magnetic response. We also show that the magnetic response is directly connected to nucleus-independent chemical shifts (NICS).     

Theoretical study of magnetic properties of ammonia molecule in nonuniform magnetic field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electromagnetic field has been used to define magnetic multipole moment operators and operators for the magnetic field of electrons acting on the nuclei of a molecule in the presence of nonhomogeneous external magnetic field. Perturbation theory has been applied to evaluate the induced electronic moments and magnetic field at the nuclei. Multipole magnetic susceptibility and nuclear magnetic shielding tensors have been introduced to describe the contributions arising in nonuniform fields, and their origin dependence has been analyzed. Extended numerical tests on the ammonia molecule in a static, nonuniform magnetic field have been carried out, using the random-phase approximation within the framework of accurate Hartree-Fock zero-order wavefunctions, and allowing for both angular momentum and torque formalisms in the calculation of paramagnetic contributions.