Laser-induced breakdown detection combined with asymmetrical flow field-flow fractionation: application to iron oxi/hydroxide colloid characterization

The combination of asymmetrical flow field-flow fractionation (AsFlFFF) with the laser-induced breakdown detection (LIBD) is presented as a powerful tool for the determination of colloid size distribution at trace particle concentrations. Detection limits (D1) of 1, 4, and 20 microg/L have been determined for a mixture of polystyrene reference particles with 20, 50, and 100 nm in size, respectively. This corresponds to injected masses of 1, 4, and 20 pg, which is lower than found in a previous study with the symmetrical FlFFF (SyFlFFF). The improvement is mainly due to the lower colloid background discharged from the AsFlFFF channel. The combined method of AsFlFFF-LIBD is then applied to the analysis of iron oxi/hydroxide colloids being considered as potential carriers for the radionuclide migration from a nuclear waste repository. Our LIBD arrangement is less sensitive for iron colloid detection as compared to reference polystyrene particles which results in a detection limit of approximately 240 microg/L FeOOH for the AsFlFFF-LIBD analysis. This is superior to the detection via UV-Vis absorbance and comparable to ICP-MS detection. Size information (mean size 11-18 nm) for different iron oxi/hydroxide colloids supplied by the present method is comparable to that obtained by sequential ultrafiltration and dynamic light scattering. A combined on-line ICP-MS detection is used to gain insight into the colloid-borne main and trace elements.     

Coupling gravitational and flow field-flow fractionation, and size-distribution analysis of whole yeast cells

This work continues the project on field-flow fractionation characterisation of whole wine-making yeast cells reported in previous papers. When yeast cells are fractionated by gravitational field-flow fractionation and cell sizing of the collected fractions is achieved by the electrosensing zone technique (Coulter counter), it is shown that yeast cell retention depends on differences between physical indexes of yeast cells other than size. Scanning electron microscopy on collected fractions actually shows co-elution of yeast cells of different size and shape. Otherwise, the observed agreement between the particle size distribution analysis obtained by means of the Coulter counter and by flow field-flow fractionation, which employs a second mobile phase flow as applied field instead of Earths gravity, indicates that yeast cell density can play a major role in the gravitational field-flow fractionation retention mechanism of yeast cells, in which flow field-flow fractionation retention is independent of particle density. Flow field-flow fractionation is then coupled off-line to gravitational field-flow fractionation for more accurate characterisation of the doubly-fractionated cells. Coupling gravitational and flow field-flow fractionation eventually furnishes more information on the multipolydispersity indexes of yeast cells, in particular on their shape and density polydispersity.     

Potential barrier gravitational field-flow fractionation based on the variation of the pH solution for the analysis of colloidal materials

Summary Potential barrier gravitational field-flow fractionation (PBGFFF) is a new technique for the separation and characterization of colloidal materials. It consists in changing the potential energy of interaction between the colloidal particles and the channel wall by varying the solution ionic strength or the Hamaker constant and the surface potential of the particles. In this work the PBGFFF technique based on the particles' surface potential variation, by varying the pH, is presented. Polydisperse colloidal particles of the sulphide CuZnS (with molar ratio Cu/Zn-10/90) are used as a model sample. Comparison of the results obtained by PBGFFF with those given by conventional gravitational fieldflow fractionation and laser counter measurements, shows that one could use PBGFFF not only for the separation and characterization of colloidal materials, but also for the investigation of the interactions between colloids and solid surfaces.     

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.     

Flow field-flow fractionation with off-line electrothermal atomic absorption spectrometry for size characterization of silver nanoparticles

Flow field-flow fractionation (Fl-FFF) with off-line electrothermal atomic absorption spectrometry (ETAAS) detection was developed and employed for particle size characterization of Ag NPs stabilized by citrate, pectin, and alginate. Citrate stabilized-Ag NPs were prepared from sodium borohydride reduction of silver nitrate. Sodium citrate was used as the capping agent to stabilize Ag NPs and prevent the aggregation process. Pectin stabilized- and alginate stabilized-Ag NPs were prepared from ascorbic acid reduction of silver nitrate. Pectin and alginate were used as the capping agent for pectin stabilized- and alginate stabilized-Ag NPs, respectively. Three types of Ag NPs were characterized by using FlFFF, zeta potentiometer, and TEM technique. The mean particle sizes of Ag NPs as characterized by FlFFF were 9 nm, 19 nm, and 45 nm for citrate stabilized-, pectin stabilized-, and alginate stabilized-Ag NPs, respectively, in deionized water. Further, FlFFF with ETAAS detection was employed to observe the aggregation of Ag NPs of various types in environmental water in the absence and presence of humic acid. Citrate stabilized-Ag NPs underwent aggregation more rapid than the pectin stabilized- and alginate stabilized-Ag NPs as the latter two types were sterically stabilized. Further, humic acid could prolong the stability of Ag NPs in the environment.     

Hybrid gravitational field-flow fractionation using immunofunctionalized walls for integrated bioanalytical devices

In this work, the biospecific recognition antigen–antibody reaction was implemented in gravitational field-flow fractionation (GrFFF), a flow-assisted separation technique for micron-sized particles, in order to realize a hybrid immunomodulated GrFFF system in which two different principles are combined to achieve highly versatile fractionation. Micron-sized polystyrene beads coated with horseradish peroxidase (HRP) were used as a model sample, and anti-HRP antibodies were immobilized on the accumulation wall of the GrFFF channel. Ultrasensitive chemiluminescence imaging was employed to visualize the beads during elution and to optimize experimental conditions. The same principle was then applied to real biological samples composed by Yersinia enterocolitica and Escherichia coli cells. Results show the possibility to modify the elution of selected sample components and even to retain them into the channel. The hybrid immunomodulated GrFFF system is a step towards the development of a module that could be integrated in a lab-on-a-chip-based point-of-care testing device which includes sample pre-analytical cleanup and analysis.     

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.     

Separation of carbon nanotubes by frit inlet asymmetrical flow field-flow fractionation

Flow field-flow fractionation (flow FFF), a separation technique for particles and macromolecules, has been used to separate carbon nanotubes (CNT). The carbon nanotube ropes that were purified from a raw carbon nanotube mixture by acidic reflux followed by cross-flow filtration using a hollow fiber module were cut into shorter lengths by sonication under a concentrated acid mixture. The cut carbon nanotubes were separated by using a modified flow FFF channel system, frit inlet asymmetrical flow FFF (FI AFIFFF) channel, which was useful in the continuous flow operation during injection and separation. Carbon nanotubes, before and after the cutting process, were clearly distinguished by their retention profiles. The narrow volume fractions of CNT collected during flow FFF runs were confirmed by field emission scanning electron microscopy and Raman spectroscopy. Experimentally, it was found that retention of carbon nanotubes in flow FFF was dependent on the use of surfactant for CNT dispersion and for the carrier solution in flow FFF. In this work, the use of flow FFF for the size differentiation of carbon nanotubes in the process of preparation or purification was demonstrated.     

Optimisation of ambient and high temperature asymmetric flow field-flow fractionation with dual/multi-angle light scattering and infrared/refractive index detection

Asymmetric flow field-flow fractionation (AF4) enables to analyse polymers with very high molar masses under mild conditions in comparison to size exclusion chromatography (SEC). Conventionally, membranes for AF4 are made from cellulose. Recently, a novel ceramic membrane has been developed which can withstand high temperatures above 130 °C and chlorinated organic solvents, thus making it possible to characterise semicrystalline polyolefins by HT-AF4. Two ceramic membranes and one cellulose membrane were compared with regard to their quality of molar mass separation and the loss of the polymer material through the pores. Separating polystyrene standards as model compounds at different cross-flow gradients the complex relationship between cross-flow velocity, separation efficiency, the molar mass and peak broadening could be elucidated in detail. Moreover, the dependence of signal quality and reproducibility on sample concentration and mass loading was investigated because the evaluation of the obtained fractograms substantially depends on the signal intensities. Finally, the performance of the whole system was tested at high temperature by separating PE reference materials of high molar mass.     

On the no-field method for void time determination in flow field-flow fractionation

Elution time measurements of colloidal particles injected in a symmetrical flow field-flow fractionation (flow FFF) system when the inlet and outlet cross-flow connections are closed have been performed. This no-field method has been proposed earlier for void time (and void volume) determination in flow FFF Giddings et al. (1977). The elution times observed were much larger than expected on the basis of the channel geometrical volume and the flow rate. In order to explain these discrepancies, a flow model allowing the carrier liquid to flow through the porous walls toward the reservoirs located behind the porous elements and along these reservoirs was developed. The ratio between the observed elution time and expected one is found to depend only on a parameter which is a function of the effective permeability and thickness of the porous elements and of the channel thickness and length. The permeabilities of the frits used in the system were measured. Their values lead to predicted elution times in reasonable agreement with experimental ones, taking into account likely membrane protrusion inside the channel on system assembly. They comfort the basic feature of the flow model, in the no-field case. The carrier liquid mostly bypasses the channel to flow along the system mainly in the reservoir. It flows through the porous walls toward the reservoirs near channel inlet and again through the porous walls from the reservoirs to the channel near channel outlet before exiting the system. In order to estimate the extent of this bypassing process, it is desirable that the hydrodynamic characteristics of the permeable elements (permeability and thickness) are provided by flow FFF manufacturers. The model applies to symmetrical as well as asymmetrical flow FFF systems.     

Determination of thickness, aspect ratio and size distributions for platey particles using sedimentation field-flow fractionation and electron microscopy

Sedimentation field-flow fractionation (SdFFF) can be used to prepare fractions of very narrow mass range for electron microscopic (EM) analysis. Assuming the particle density is the same for all particles within that fraction the equivalent spherical diameter for the particles can be calculated from SdFFF theory. Integration of the micrograph image of each particle yields an area measurement which, when used in conjunction with the equivalent spherical particle diameter (from SdFFF), provides information about the particle thickness and aspect ratio. Thus SdFFF-SEM can be used to provide detailed information about clay morphology across the particle size distribution of the sample. Three clay minerals have been studied using the methodologies outlined in this paper. The aspect ratio for the Purvis School Mine kaolinite ranged from 2.8–5.9, for RM30 illite from 11.3–24.3, and for Muloorina illite from 3.1–4.3.     

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.     

Temporal variability of colloidal material in agricultural storm runoff from managed grassland using flow field-flow fractionation

This paper reports the use of flow field-flow fractionation (FlFFF) to determine the temporal variability of colloidal (<1 μm) particle size distributions in agricultural runoff waters in a small managed catchment in SW England during storm events. Three storm events of varying intensity were captured and the colloidal material in the runoff analysed by FlFFF. The technique had sufficient sensitivity to determine directly the changing colloidal profile over the 0.08–1.0 μm size range in the runoff waters during these storm events. Rainfall, total phosphorus and suspended solids in the bulk runoff samples were also determined throughout one storm and showed significant correlation (P < 0.01) with the amount of colloidal material. Whilst there are some uncertainties in the resolution and absolute calibration of the FlFFF profiles, the technique has considerable potential for the quantification of colloidal material in storm runoff waters.     

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.     

非对称场流分离系统的构建及其在淀粉颗粒粒径表征中的应用

淀粉颗粒粒径与分子尺寸分别在1～100μm和20～250 nm之间,是影响淀粉功能特性的重要因素之一.非对称场流分离(AF4)是一种基于样品与外力场相互作用机制的分离技术,已应用于表征淀粉分子尺寸分布.商品化的AF4系统的粒径检测范围为1 nm～10μm,对于淀粉颗粒粒径表征具有一定的局限性.该文研制了AF4分离系统;...     

Using flow field-flow fractionation (Fl-FFF) for observation of salinity effect on the size distribution of humic acid aggregates

Flow field-flow fractionation (Fl-FFF) was used to investigate the effect of salinity on the size distribution of humic acid (HA) aggregates in estuarine water. In water with high salinity as estuarine water, size distributions were slightly broadened with increasing contact time between HA and estuarine water. At the longest contact times (89 days) and highest salinity value (28 psu, g kg^?1), the peak maxima were observed at 1.7 and 8.6 nm when detected at 254 nm, and at 1.9 and 9.1 nm when detected at 400 nm. In addition, Fl-FFF with an inductively coupled plasma mass spectrometry was applied to examine the effect of salinity on the size distribution of Cd, Ce, Cu, Mn and Pb-binding HA aggregates in estuarine water with different salinity values. At 1 day contact time, the peak maxima of Cd, Ce, Cu, Mn and Pb-binding HA aggregates in water with increased salinity values were increased and gave the larger breadth of size distribution. The larger size fraction of HA aggregates showed more affinity for Pb, Cd, Ce and Mn than Cu whereas the smaller size fraction of humic aggregates showed preferential binding towards Cu.     

Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 °C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R(g)) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC, AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination.     

Micro-thermal field-flow fractionation: new high-performance method for particle size distribution analysis

Micro-thermal field-flow fractionation (mu-TFFF) was applied to the separation of polystyrene latices. This new high-resolution technique allows determination of the particle size distribution (PSD) if carried out under optimized experimental conditions. The optimum temperature of the accumulation wall, which influences the relaxation processes and, consequently, the zone broadening, was chosen on the basis of our prior work. The flow rate was chosen as a compromise between the theoretical optimum value, which is very low because the diffusion coefficients of the colloidal particles are very small, and a value allowing performance of the PSD analysis in a reasonable time. These experimental conditions can be manipulated easily due to the high versatility of mu-TFFF, which follows from a large decrease of the heat energy flux across the channel with its reduced dimensions in comparison with standard TFFF. The PSDs obtained from mu-TFFF data are compared with results from quasi-elastic laser light scattering (QELS) and transmission electron microscopy (TEM). It has been found that a baseline resolution of a model mixture of two samples of close average particle diameters can be achieved by an appropriate choice of the temperature drop in mu-TFFF, whereas only a broad, unresolved PSD of the mixed sample was obtained from the QELS measurement. The TEM of the mixed sample revealed the presence of two particle size populations. However, the number of particles which are practically counted on a TEM picture is several orders of magnitude lower than the number of particles taken into account in mu-TFFF or QELS. Consequently, the PSD obtained from the TEM did not represent the whole sample. Comparison of mu-TFFF with modern hydrodynamic chromatography (HC) has shown that the methods exhibit roughly the same resolution and time of analysis. Nevertheless, mu-TFFF is a more universal technique because the separation of the colloidal particles or of the macromolecules within a broad range of molar masses is carried out on the same channel, as demonstrated previously.     

Asymmetrical flow field-flow fractionation with multi-angle light scattering detection for the analysis of structured nanoparticles

Synthesis and applications of new functional nanoparticles are topics of increasing interest in many fields of nanotechnology. Chemical modifications of inorganic nanoparticles are often necessary to improve their features as spectroscopic tracers or chemical sensors, and to increase water solubility and biocompatibility for applications in nano-biotechnology. Analysis and characterization of structured nanoparticles are then key steps for their synthesis optimization and final quality control. Many properties of structured nanoparticles are size-dependent. Particle size distribution analysis then provides fundamental analytical information. Asymmetrical flow field-flow fractionation (AF4) with multi-angle light scattering (MALS) detection is able to size-separate and to characterize nanosized analytes in dispersion. In this work we focus on the central role of AF4-MALS to analyze and characterize different types of structured nanoparticles that are finding increasing applications in nano-biotechnology and nanomedicine: polymer-coated gold nanoparticles, fluorescent silica nanoparticles, and quantum dots. AF4 not only size-fractionated these nanoparticles and measured their hydrodynamic radius (r_h) distribution but it also separated them from the unbound, relatively low-M_r components of the nanoparticle structures which were still present in the sample solution. On-line MALS detection on real-time gave the gyration radius (r_g) distribution of the fractionated nanoparticles. Additional information on nanoparticle morphology was then obtained from the r_h/r_g index. Stability of the nanoparticle dispersions was finally investigated. Aggregation of the fluorescent silica nanoparticles was found to depend on the concentration at which they were dispersed. Partial release of the polymeric coating from water-soluble QDs was found when shear stress was induced by increasing flowrates during fractionation.     

Improvement of lipoprotein separation with a guard channel prior to asymmetrical flow field-flow fractionation using fluorescence detection

In this article, a simple experimental approach to improve lipoprotein separation and detection in flow field-flow fractionation (FlFFF) is detailed. Lipoproteins are globular particles composed of lipids and proteins in blood serum and their roles include transferring fats and cholesterols through blood vessels throughout the body. Especially, presence of small, dense low-density lipoproteins (LDL) is associated with cardiovascular risk. Two experimental approaches were explored in this study: an increase in the reproducibility of LDL particle separation by implementing a guard channel prior to an asymmetrical FlFFF (AFlFFF) channel in order to deplete small molecular weight serum proteins and reducing the required injection volume of a serum sample by implementing fluorescence detection. The guard channel was made of a simple hollow fiber module so that the serum sample can be washed with the help of radial flow prior to injection into the AFlFFF channel. The channel was tested with protein standards and serum samples to ensure precision of the retention time and the protein recovery rate. A fluorescent phospholipid dye was utilized to label lipoprotein particles before separation for fluorescence detection, which resulted in a reduction of the required injection volume of serum.