Continuous particle separation through deterministic lateral displacement

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40 seconds with a resolution of approximately 10 nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of approximately 12%.

Microfluidic sorting in an optical lattice

The response of a microscopic dielectric object to an applied light field can profoundly affect its kinetic motion. A classic example of this is an optical trap, which can hold a particle in a tightly focused light beam. Optical fields can also be used to arrange, guide or deflect particles in appropriate light-field geometries. Here we demonstrate an optical sorter for microscopic particles that exploits the interaction of particles-biological or otherwise-with an extended, interlinked, dynamically reconfigurable, three-dimensional optical lattice. The strength of this interaction with the lattice sites depends on the optical polarizability of the particles, giving tunable selection criteria. We demonstrate both sorting by size (of protein microcapsule drug delivery agents) and sorting by refractive index (of other colloidal particle streams). The sorting efficiency of this method approaches 100%, with values of 96% or more observed even for concentrated solutions with throughputs exceeding those reported for fluorescence-activated cell sorting. This powerful, non-invasive technique is suited to sorting and fractionation within integrated ('lab-on-a-chip') microfluidic systems, and can be applied in colloidal, molecular and biological research.

Free Flow Acoustophoresis: Microfluidic-Based Mode of Particle and Cell Separation

A novel method, free flow acoustophoresis (FFA), capable of continuous separation of mixed particle suspensions into multiple outlet fractions is presented. Acoustic forces are utilized to separate particles based on their size and density. The method is shown to be suitable for both biological and nonbiological suspended particles. The microfluidic separation chips were fabricated using conventional microfabrication methods. Particle separation was accomplished by combining laminar flow with the axial acoustic primary radiation force in an ultrasonic standing wave field. Dissimilar suspended particles flowing through the 350-microm-wide channel were thereby laterally translated to different regions of the laminar flow profile, which was split into multiple outlets for continuous fraction collection. Using four outlets, a mixture of 2-, 5-, 8-, and 10-microm polystyrene particles was separated with between 62 and 94% of each particle size ending up in separate fractions. Using three outlets and three particle sizes (3, 7, and 10 microm) the corresponding results ranged between 76 and 96%. It was also proven possible to separate normally acoustically inseparable particle types by manipulating the density of the suspending medium with cesium chloride. The medium manipulation, in combination with FFA, was further used to enable the fractionation of red cells, platelets, and leukocytes. The results show that free flow acoustophoresis can be used to perform complex separation tasks, thereby offering an alternative to expensive and time-consuming methods currently in use.

Continuous flow separations in microfluidic devices

Biochemical sample mixtures are commonly separated in batch processes, such as filtration, centrifugation, chromatography or electrophoresis. In recent years, however, many research groups have demonstrated continuous flow separation methods in microfluidic devices. Such separation methods are characterised by continuous injection, real-time monitoring, as well as continuous collection, which makes them ideal for combination with upstream and downstream applications. Importantly, in continuous flow separation the sample components are deflected from the main direction of flow, either by means of a force field (electric, magnetic, acoustic, optical etc.), or by intelligent positioning of obstacles in combination with laminar flow profiles. Sample components susceptible to deflection can be spatially separated. A large variety of methods has been reported, some of these are miniaturised versions of larger scale methods, others are only possible in microfluidic regimes. Researchers now have a diverse toolbox to choose from and it is likely that continuous flow methods will play an important role in future point-of-care or in-the-field analysis devices.

Characterization of titanium dioxide nanoparticles in food products: analytical methods to define nanoparticles

Titanium dioxide (TiO2) is a common food additive used to enhance the white color, brightness, and sometimes flavor of a variety of food products. In this study 7 food grade TiO2 materials (E171), 24 food products, and 3 personal care products were investigated for their TiO2 content and the number-based size distribution of TiO2 particles present in these products. Three principally different methods have been used to determine the number-based size distribution of TiO2 particles: electron microscopy, asymmetric flow field-flow fractionation combined with inductively coupled mass spectrometry, and single-particle inductively coupled mass spectrometry. The results show that all E171 materials have similar size distributions with primary particle sizes in the range of 60-300 nm. Depending on the analytical method used, 10-15% of the particles in these materials had sizes below 100 nm. In 24 of the 27 foods and personal care products detectable amounts of titanium were found ranging from 0.02 to 9.0 mg TiO2/g product. The number-based size distributions for TiO2 particles in the food and personal care products showed that 5-10% of the particles in these products had sizes below 100 nm, comparable to that found in the E171 materials. Comparable size distributions were found using the three principally different analytical methods. Although the applied methods are considered state of the art, they showed practical size limits for TiO2 particles in the range of 20-50 nm, which may introduce a significant bias in the size distribution because particles <20 nm are excluded. This shows the inability of current state of the art methods to support the European Union recommendation for the definition of nanomaterials.

Biotechnological applications of bioluminescence and chemiluminescence

Recent progress in molecular biology has made available several biotechnological tools that take advantage of the high detectability and rapidity of bioluminescence and chemiluminescence spectroscopy. These developments provide inroads to in vitro and in vivo continuous monitoring of biological processes (e.g. gene expression, protein-protein interaction and disease progression), with clinical, diagnostic and drug discovery applications. Furthermore, combining luminescent enzymes or photoproteins with biospecific recognition elements at the genetic level has led to the development of ultrasensitive and selective bioanalytical tools, such as recombinant whole-cell biosensors, immunoassays and nucleic acid hybridization assays. The high detectability of the luminescence analytical signal makes it appropriate for miniaturized bioanalytical devices (e.g. microarrays, microfluidic devices and high-density-well microtiter plates) for the high-throughput screening of genes and proteins in small sample volumes.

The use of asymmetrical flow field-flow fractionation in pharmaceutics and biopharmaceutics

Field-flow fractionation (FFF) is a family of flexible analytical fractionating techniques which have the advantage that the separation of analytes is achieved, solely through the interaction of the sample with an external, perpendicular physical field, rather than by the interaction with a stationary phase. The rapid progress in pharmaceutical biotechnology goes along with an increasing demand in potent, high-efficient analytical methods. Thus, FFF techniques are gaining increasing attention for their ability to separate and characterize populations of polymers, colloids and particles of up to about 100 microm in size. It is the intention of this review to provide an overview on common FFF techniques, to summarize inherent advantages and limitations and to introduce both established and challenging applications in the (bio)pharmaceutical field. Thereby, asymmetrical flow FFF is addressed predominantly, since it is the most versatile applicable FFF technique.

A critical review of analytical ultracentrifugation and field flow fractionation methods for measuring protein aggregation

Analytical ultracentrifugation (AUC) and field flow fractionation (FFF) are 2 important biophysical methods for measuring protein aggregates. Both methods can separate protein monomer from its aggregate forms under a broad range of solution conditions. Recent advances in instrumentation and data analysis, particularly in the field of analytical ultracentrifugation technology, have significantly improved the capability and sensitivity of these biophysical methods for detecting protein aggregates. These advances have resulted in an increased use of these methods in the biopharmaceutical industry for characterization of therapeutic proteins. However, despite their many advantages over conventional methods, the difficulty in the use of the instrumentation and the complexity of data analysis process, have often hampered the widespread use and proper interpretation of data. This article reviews the recent progress in both technologies, and a few case studies are also presented to discuss their advantages and limitations.

Correlations between the dielectric properties and exterior morphology of cells revealed by dielectrophoretic field-flow fractionation

Although dielectrophoresis (DEP) has great potential for addressing clinical cell isolation problems based on cell dielectric differences, a biological basis for predicting the DEP behavior of cells has been lacking. Here, the dielectric properties of the NCI-60 panel of tumor cell types have been measured by dielectrophoretic (DEP) field-flow fractionation, correlated with the exterior morphologies of the cells during growth, and compared with the dielectric and morphological characteristics of the subpopulations of peripheral blood. In agreement with earlier findings, cell total capacitance varied with both cell size and plasma membrane folding and the dielectric properties of the NCI-60 cell types in suspension reflected the plasma membrane area and volume of the cells at their growth sites. Therefore, the behavior of cells in DEP-based manipulations is largely determined by their exterior morphological characteristics prior to release into suspension. As a consequence, DEP is able to discriminate between cells of similar size having different morphological origins, offering a significant advantage over size-based filtering for isolating circulating tumor cells, for example. The findings provide a framework for anticipating cell dielectric behavior on the basis of structure-function relationships and suggest that DEP should be widely applicable as a surface marker-independent method for sorting cells.

Enrichment of putative stem cells from adipose tissue using dielectrophoretic field-flow fractionation

We have applied the microfluidic cell separation method of dielectrophoretic field-flow fractionation (DEP-FFF) to the enrichment of a putative stem cell population from an enzyme-digested adipose tissue derived cell suspension. A DEP-FFF separator device was constructed using a novel microfluidic-microelectronic hybrid flex-circuit fabrication approach that is scaleable and anticipates future low-cost volume manufacturing. We report the separation of a nucleated cell fraction from cell debris and the bulk of the erythrocyte population, with the relatively rare (<2% starting concentration) NG2-positive cell population (pericytes and/or putative progenitor cells) being enriched up to 14-fold. This work demonstrates a potential clinical application for DEP-FFF and further establishes the utility of the method for achieving label-free fractionation of cell subpopulations.

Branching Features of Amylopectins and Glycogen Determined by Asymmetrical Flow Field Flow Fractionation Coupled with Multiangle Laser Light Scattering

The aim of this work was to characterize starch polysaccharides using asymmetrical flow field flow fractionation coupled with multiangle laser light scattering. Amylopectins from eight different botanical sources and rabbit liver glycogen were studied. Amylopectins and glycogen were completely solubilized and analyzed, and high mass recoveries were achieved (81.7-100.0%). Amylopectin Mw, RG, and the hydrodynamic coefficient nuG (the slope of the log-log plot of RGi vs Mi) were within the ranges 1.05-3.18 x 10(8) g mol(-1), 163-229 nm, 0.37-0.49, respectively. The data were also considered in terms of structural parameters. The results were analyzed by comparison with the theory of hyperbranched polymers (Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; Burchard, W. Macromolecules, 1977, 10, 919-927). This theory, based upon the ABC model, has been shown to underestimate the branching degrees of amylopectins. However, quantitative agreement with the data in the literature was found for amylopectins when using the ABC model modified by the introduction of a multiplying factor, determined from previously described amylopectin structures in terms of the number of branching point calculations.

Rationalizing nanomaterial sizes measured by atomic force microscopy, flow field-flow fractionation, and dynamic light scattering: sample preparation, polydispersity, and particle structure

This study aims to rationalize the variability in the measured size of nanomaterials (NMs) by some of the most commonly applied techniques in the field of nano(eco)toxicology and environmental sciences, including atomic force microscopy (AFM), dynamic light scattering (DLS), and flow field-flow fractionation (FlFFF). A validated sample preparation procedure for size evaluation by AFM is presented, along with a quantitative explanation of the variability of measured sizes by FlFFF, AFM, and DLS. The ratio of the z-average hydrodynamic diameter (d(DLS)) by DLS and the particle height by AFM (d(AFM)) approaches 1.0 for monodisperse samples and increases with sample polydispersity. A polydispersity index of 0.1 is suggested as a suitable limit above which DLS data can no longer be interpreted accurately. Conversion of the volume particle size distribution (PSD) by FlFFF-UV to the number PSD reduces the differences observed between the sizes measured by FlFFF (d(FlFFF)) and AFM. The remaining differences in the measured sizes can be attributed to particle structure (sphericity and permeability). The ratio d(FlFFF)/d(AFM) approaches 1 for small ion-coated NMs, which can be described as hard spheres, whereas d(FlFFF)/d(AFM) deviates from 1 for polymer-coated NMs, indicating that these particles are permeable, nonspherical, or both. These findings improve our understanding of the rather scattered data on NM size measurements reported in the environmental and nano(eco)toxicology literature and provide a tool for comparison of the measured sizes by different techniques.

Field-flow fractionation of proteins, polysaccharides, synthetic polymers, and supramolecular assemblies

This review summarizes developments and applications of flow and thermal field-flow fractionation (FFF) in the areas of macromolecules and supramolecular assemblies. In the past 10 years, the use of these FFF techniques has extended beyond determining diffusion coefficients, hydrodynamic diameters, and molecular weights of standards. Complex samples as diverse as polysaccharides, prion particles, and block copolymers have been characterized and processes such as aggregation, stability, and infectivity have been monitored. The open channel design used in FFF makes it a gentle separation technique for high- and ultrahigh-molecular weight macromolecules, aggregates, and self-assembled complexes. Coupling FFF with other techniques such as multiangle light scattering and MS provides additional invaluable information about conformation, branching, and identity.

Detection of nanoplastics in food by asymmetric flow field-flow fractionation coupled to multi-angle light scattering: possibilities, challenges and analytical limitations

We tested the suitability of asymmetric flow field-flow fractionation (AF4) coupled to multi-angle light scattering (MALS) for detection of nanoplastics in fish. A homogenized fish sample was spiked with 100 nm polystyrene nanoparticles (PSNPs) (1.3 mg/g fish). Two sample preparation strategies were tested: acid digestion and enzymatic digestion with proteinase K. Both procedures were found suitable for degradation of the organic matrix. However, acid digestion resulted in large PSNPs aggregates/agglomerates (> 1 μm). The presence of large particulates was not observed after enzymatic digestion, and consequently it was chosen as a sample preparation method. The results demonstrated that it was possible to use AF4 for separating the PSNPs from the digested fish and to determine their size by MALS. The PSNPs could be easily detected by following their light scattering (LS) signal with a limit of detection of 52 μg/g fish. The AF4-MALS method could also be exploited for another type of nanoplastics in solution, namely polyethylene (PE). However, it was not possible to detect the PE particles in fish, due to the presence of an elevated LS background. Our results demonstrate that an analytical method developed for a certain type of nanoplastics may not be directly applicable to other types of nanoplastics and may require further adjustment. This work describes for the first time the detection of nanoplastics in a food matrix by AF4-MALS. Despite the current limitations, this is a promising methodology for detecting nanoplastics in food and in experimental studies (e.g., toxicity tests, uptake studies). Graphical abstract Basic concept for the detection of nanoplastics in fish by asymmetric flow field-flow fractionation coupled to multi-angle light scattering.

Biophysical characterization of influenza virus subpopulations using field flow fractionation and multiangle light scattering: Correlation of particle counts, size distribution and infectivity

Adequate biophysical characterization of influenza virions is important for vaccine development. The influenza virus vaccines are produced from the allantoic fluid of developing chicken embryos. The process of viral replication produces a heterogeneous mixture of infectious and non-infectious viral particles with varying states of aggregation. The study of the relative distribution and behavior of different subpopulations and their inter-correlation can assist in the development of a robust process for a live virus vaccine. This report describes a field flow fractionation and multiangle light scattering (FFF-MALS) method optimized for the analysis of size distribution and total particle counts. The FFF-MALS method was compared with several other methods such as transmission electron microscopy (TEM), atomic force microscopy (AFM), size exclusion chromatography followed by MALS (SEC-MALS), quantitative reverse transcription polymerase chain reaction (RT Q-PCR), median tissue culture dose (TCID(50)), and the fluorescent focus assay (FFA). The correlation between the various methods for determining total particle counts, infectivity and size distribution is reported. The pros and cons of each of the analytical methods are discussed.

Determination of size distribution and encapsulation efficiency of liposome-encapsulated hemoglobin blood substitutes using asymmetric flow field-flow fractionation coupled with multi-angle static light scattering

In this study, we investigated the size distribution, encapsulation efficiency, and oxygen affinity of liposome-encapsulated tetrameric hemoglobin (LEHb) dispersions and correlated the data with the variation in extruder membrane pore size, ionic strength of the extrusion buffer, and hemoglobin (Hb) concentration. Asymmetric flow field-flow fractionation (AFFF) in series with multi-angle static light scattering (MASLS) was used to study the LEHb size distribution. We also introduced a novel method to measure the encapsulation efficiency using a differential interferometric refractive index (DIR) detector coupled to the AFFF-MASLS system. This technique was nondestructive toward the sample and easy to implement. LEHbs were prepared by extrusion using a lipid combination of dimyristoyl-phosphatidylcholine, cholesterol, and dimyristoyl-phosphatidylglycerol in a 10:9:1 molar ratio. Five initial Hb concentrations (50, 100, 150, 200, and 300 mg Hb per mL of buffer) extruded through five different membrane pore diameters (400, 200, 100, 80, and 50 nm) were studied. Phosphate buffered saline (PBS) and phosphate buffer (PB) both at pH 7.3 were used as extrusion buffers. Despite the variation, extrusion through 400-nm pore diameter membranes produced LEHbs smaller than the pore size, extrusion through 200-nm membranes produced LEHbs with diameters close to the pore diameter, and extrusion through 100-, 80-, and 50-nm membranes produced LEHbs larger than the pore sizes. We found that the choice of extrusion buffer had the greatest effect on the LEHb size distribution compared to either Hb concentration or extruder membrane pore size. Extrusion in PBS produced larger LEHbs and more monodisperse LEHb dispersions. However, LEHbs extruded in PB generally had higher Hb encapsulation efficiencies and lower methemoglobin (metHb) levels. The choice of extrusion buffer also affected how the encapsulation efficiency correlated with Hb concentration, extruder pore size, and the metHb level. The most optimum encapsulation efficiency and amount of Hb entrapped were achieved at the highest Hb concentration and the largest pore size for both extrusion buffers (62.38% and 187.14 mg Hb/mL of LEHb dispersion extruded in PBS, and 69.98% and 209.94 mg Hb/mL of LEHb dispersion extruded in PB). All LEHbs displayed good oxygen-carrying properties as indicated by their P(50) and cooperativity coefficients. LEHbs extruded in PB had an average P(50) of 23.04 mmHg and an average Hill number of 2.29, and those extruded in PBS had average values of 27.25 mmHg and 2.49. These oxygen-binding properties indicate that LEHbs possess strong potential as artificial blood substitutes. In addition, the metHb levels in PB-LEHb dispersions are significantly low even in the absence of antioxidants such as N-acetyl-L-cysteine.

Improving on SUVA 254 using fluorescence-PARAFAC analysis and asymmetric flow-field flow fractionation for assessing disinfection byproduct formation and control

Several challenges with disinfection byproduct (DBP) control stem from the complexity and diversity of dissolved organic matter (DOM), which is ubiquitous in natural waters and reacts with disinfectants to form DBPs. Fluorescence parallel factor (PARAFAC) analysis and asymmetric flow-field flow fractionation (AF4) were used in combination with free chlorine DBP formation potential (DBPFP) tests to study the physicochemical DOM properties and DBP formation in raw- and alum-coagulated waters. Enhanced coagulation with alum became more effective at removing DBP-precursors as the pH decreased from 8 to 6. AF4-UV(254) fractograms indicated enhanced coagulation at pH 6 preferentially removed larger DOM, whereas no preferential size removal occurred at pH 8. Fluorescence-PARAFAC analysis revealed the presence of one protein-like and three humic-like fluorophore groups; stronger linear correlations were found between chloroform and the maximum intensity (F(MAX)) of a humic-like fluorophore (r(2) = 0.84) than with SUVA(254) (r(2) = 0.51). This result indicated that the fluorescence-PARAFAC approach used here was an improvement on SUVA(254), i.e., fluorescence-based measurements were stronger predictors of chloroform formation.

Asymmetrical flow field-flow fractionation and multiangle light scattering for analysis of gelatin nanoparticle drug carrier systems

The physicochemical properties of nanosized colloidal drug carrier systems are of great influence on drug efficacy. Consequently, a broad spectrum of analytical techniques is applied for comprehensive drug carrier characterization. It is the primary objective of this paper to present asymmetrical flow field-flow fractionation (AF4), coupled online with multiangle light scattering detection, for the characterization of gelatin nanoparticles. Size and size distribution of drug-loaded and unloaded nanoparticles were determined, and data were correlated with results of state-of-the-art methods, such as scanning electron microscopy and photon correlation spectroscopy. Moreover, the AF4 fractionation of gelatin nanoparticulate carriers from a protein model drug is demonstrated for the first time, proposing a feasible way to assess the amount of loaded drug in situ without sample preparation. This hypothesis was set into practice by monitoring the drug loading of nanoparticles with oligonucleotide payloads. In this realm, various fractions of gelatin bulk material were analyzed via AF4 and size-exclusion high-pressure liquid chromatography. Mass distributions and high-molecular-weight fraction ratios of the gelatin samples varied, depending on the separation method applied. In general, the AF4 method demonstrated the ability to comprehensively characterize polymeric gelatin bulk material as well as drug-loaded and unloaded nanoparticles in terms of size, size distribution, molecular weight, and loading efficiency.

SEDFIT-MSTAR: molecular weight and molecular weight distribution analysis of polymers by sedimentation equilibrium in the ultracentrifuge

Sedimentation equilibrium (analytical ultracentrifugation) is one of the most inherently suitable methods for the determination of average molecular weights and molecular weight distributions of polymers, because of its absolute basis (no conformation assumptions) and inherent fractionation ability (without the need for columns or membranes and associated assumptions over inertness). With modern instrumentation it is also possible to run up to 21 samples simultaneously in a single run. Its application has been severely hampered because of difficulties in terms of baseline determination (incorporating estimation of the concentration at the air/solution meniscus) and complexity of the analysis procedures. We describe a new method for baseline determination based on a smart-smoothing principle and built into the highly popular platform SEDFIT for the analysis of the sedimentation behavior of natural and synthetic polymer materials. The SEDFIT-MSTAR procedure - which takes only a few minutes to perform - is tested with four synthetic data sets (including a significantly non-ideal system), a naturally occurring protein (human IgG1) and two naturally occurring carbohydrate polymers (pullulan and λ-carrageenan) in terms of (i) weight average molecular weight for the whole distribution of species in the sample (ii) the variation in "point" average molecular weight with local concentration in the ultracentrifuge cell and (iii) molecular weight distribution.

Multielement characterization of metal-humic substances complexation by size exclusion chromatography, asymmetrical flow field-flow fractionation, ultrafiltration and inductively coupled plasma-mass spectrometry detection: a comparative approach

The use of three different separation techniques, ultrafiltration (UF), high performance size exclusion chromatography (HPSEC) and asymmetrical flow field-flow fractionation (AsFlFFF), for the characterization of a compost leachate is described. The possible interaction of about 30 elements with different size fractions of humic substances (HS) has been investigated coupling these separation techniques with UV-vis absorption spectrophotometry and inductively coupled plasma-mass spectrometry (ICP-MS) as detection techniques. The organic matter is constituted by a polydisperse mixture of humic substances ranging from low molecular weights (around 1kDa) to significantly larger entities. Elements can be classified into three main groups with regard to their interaction with HS. The first group is constituted by primarily the monovalent alkaline metal ions and anionic species like B, W, Mo, As existing as oxyanions all being not significantly associated to HS. The second group consists of elements that are at least partly associated to a smaller HS size fraction (such as Ni, Cu, Cr and Co). A third group of mainly tri- and tetravalent metal ions like Al, Fe, the lanthanides, Sn and Th are rather associated to larger-sized HS fractions. The three separation techniques provide a fairly consistent size classification for most of the metal ions, even though slight disagreements were observed. The number-average molecular weight (Mn), the weight-average molecular weight (Mw) and the polydispersity (rho) parameters have been calculated both from AsFlFFF and HPSEC experiments and compared for HS and some metal-HS species. Differences in values can be partly explained by an overloading effect observed in the AsFlFFF experiments induced by electrostatic repulsion effects in the low ionic strength, high pH carrier solution. Size information obtained from ultrafiltration is not as resolved as for the other methods. Molecular weight cut-offs (MWCO) of the individual filter membranes refer to globular proteins and molecular weight information may therefore, deviate from that given by the other methods after calibration with polystyrene sulfonate (PSS) standards.

Analysis of whole bacterial cells by flow field-flow fractionation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The purpose of this study is to develop a novel bacterial analysis method by coupling the flow field-flow fractionation (flow FFF) separation technique with detection by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The composition of carrier liquid used for flow FFF was selected based on retention of bacterial cells and compatibility with the MALDI process. The coupling of flow FFF and MALDI-TOF MS was demonstrated for P. putida and E. coli. Fractions of the whole cells were collected after separation by FFF and further analyzed by MALDI-MS. Each fraction, collected over different time intervals, corresponded to different sizes and possibly different growth stages of bacteria. The bacterial analysis by flow FFF/MALDI-TOF MS was completed within 1 h with only preliminary optimization of the process.

On-Line Hollow-Fiber Flow Field-Flow Fractionation-Electrospray Ionization/Time-of-Flight Mass Spectrometry of Intact Proteins

Capabilities of mass spectrometry for the analysis of intact proteins can be increased through separation methods. Flow field-flow fractionation (FlFFF) is characterized by the particularly "soft" separation mechanism, which is ideally suited to maintain the native structure of intact proteins. This work describes the original on-line coupling between hollow-fiber FlFFF (HF FlFFF), the microcolumn variant of FlFFF, and electrospray ionization/time-of-flight mass spectrometry (ESI/TOFMS) for the analysis and characterization of intact proteins. The results show that the native (or pseudonative) structure of horse heart myoglobin and horseradish peroxidase is maintained. Sample desalting is also observed for horse heart myoglobin. Correlation between the molar mass values independently measured by HF FlFFF retention and ESI/TOFMS allows us to confirm the protein aggregation features of bovine serum albumin and to indicate possible changes in the quaternary structure of human hemoglobin.

Continuous flow microextraction combined with high-performance liquid chromatography for the analysis of pesticides in natural waters

Continuous flow microextraction (CFME) combined with high-performance liquid chromatography-ultraviolet (HPLC-UV) detection has been applied to the analysis of five widely used pesticides, simazine, fensulfothion, etridiazole, mepronil and bensulide, present at trace levels in water samples. CFME employs a single organic solvent drop positioned at the tip of a polyether ether ketone (PEEK) tubing, which is immersed in a continuous flowing aqueous sample solution in a 0.5-ml glass chamber. The PEEK tubing acts as the organic drop holder and fluid delivery duct. Analytes are partitioned between the organic drop and the bulk sample solution. Important extraction factors including type of solvent, its volume, sample solution flow rate, extraction time, its pH and addition of salt were investigated. All pesticides exhibit good linearity in the investigated concentration range of 25-250 ng ml(-1) with coefficients of determination (R2) ranging from 0.9879 to 0.9999 under the optimized conditions. Detection limits lower than 4 ng ml(-1) were obtained for all analytes. The method was evaluated by analyzing natural water sample collected from a reservoir in Singapore. This study for the first time demonstrated the compatibility of CFME procedure and HPLC separation.

Size-dependent sedimentation properties of nanocrystals

Centrifugation is an increasingly important technique for nanomaterial processing. Here, we examine this process for gold, cadmium selenide, and iron oxide nanocrystals using an analytical ultracentrifuge. Such data provide an accurate measure of the sedimentation coefficients for these materials, and we find that this parameter has a significant dependence on the size and surface coating. Conventional models for particle sedimentation cannot capture the behavior of these nanocrystals unless the density of the nanocrystals is described by a size-dependent term that accounts for both the inorganic core and the organic coating. Using this modification in the particle sedimentation framework, it is possible to estimate sedimentation coefficients from information about the nanocrystal core and surface coating dimensions. Such data are useful in choosing the speeds for a centrifugation process and are particularly important when bimodal nanocrystal distributions are present.

Hyperlayer hollow-fiber flow field-flow fractionation of cells

Interest in low-cost, analytical-scale, highly efficient and sensitive separation methods for cells, among which bacteria, is increasing. Particle separation in hollow-fiber flow field-flow fractionation (HF FlFFF) has been recently improved by the optimization of the HF FIFFF channel design. The intrinsic simplicity and low cost of this HF FlFFF channel allows for its disposable usage. which is particularly appealing for analytical bio-applications. Here, for the first time, we present a feasibility study on high-performance, hyperlayer HF FIFFF of micrometer-sized bacteria (Escherichia coli) and of different types of cells (human red blood cells, wine-making yeast from Saccharomyces cerevisiae). Fractionation performance is shown to be at least comparable to that obtained with conventional, flat-channel hyperlayer FIFFF of cells, at superior size-based selectivity and reduced analysis time.