Impact of Ionic Strength of Carrier Liquid on Recovery in Flow Field-Flow Fractionation

Utilization of AF4 for characterizing complex nanomaterial drug products: Reexamining sample recovery and its impact on particle size distribution as a quality attribute

Asymmetrical flow field-flow fractionation (AF4) with multi-detection has continued to gain wider acceptance for characterizing complex drug products. An important quality attribute for these products is the measurement of the particle size distribution (PSD). Current limitations of established procedures (e.g., dynamic light scattering) for accurately determining PSD can be overcome by AF4. However, while gaining acceptance this technique has not been fully adopted within the pharmaceutical industry. A technical understanding of fundamental operational factors is necessary for the successful application of utilizing any emerging technology. For example, recovery (R% = AS/AD*100, where AS and AD are the peak areas from the concentration detector with and without the crossflow field, respectively) is one factor that is used to assess the robustness during AF4 method development, but currently little is known about the interplay between analyte recovery and PSD. This work highlights factors that impact calculated AF4 recovery, and how differences in analyte and absolute recovery ultimately influence the PSD of nanoparticle size standards and complex drug product formulations such as emulsions and liposomes. Factors like ionic strength, buffer composition, and analyte chemistries, which are the most common factors associated with changes to R% in AF4, contributed to changes in AS. While AD is not typically examined in detail, the selection of the concentration detector (UV or dRI) along with their instrumental parameters (e.g., wavelength, attenuation value, linear range) and sample preparation was shown to under- or over-estimate AD thus changing R%. Examining both components of R% and their contributions to analyte and absolute recovery show that decreases in analyte recovery may not be exclusively due to sample loss but could be influenced by changes in analyte-membrane interactions or analyte instability. Because of this, four relationships between recovery and PSD were defined. While R% is used as a tool for assessing AF4 methodology, the factors investigated through this work warrant further considerations when establishing an appropriate R% threshold.

Size exclusion chromatography and asymmetrical flow field-flow fractionation for structural characterization of polysaccharides: A comparative review

Natural polysaccharides exhibit a wide range of biological activities, which are closely related to their structural characteristics, including their molecular weight distribution, size, monosaccharide composition, glycosidic bond types and spatial conformation, etc. Size exclusion chromatography (SEC) and asymmetrical flow field-flow fractionation (AF4), as two potent separation techniques, both harbor potential for continuous development and enhancement. This manuscript reviewed the fundamental principles and separation applications of SEC and AF4. The structural information and spatial conformation of polysaccharides can be obtained using SEC or AF4 coupled with multiple detectors. In addition, this manuscript elaborates in detail on the shear degradation of samples such as polysaccharides separated by SEC. In addition, the abnormal elution that occurs during the application of the two methods is also discussed. Both SEC and AF4 possess considerable potential for ongoing development and refinement, thereby offering increased possibilities and opportunities for polysaccharide separation and characterization.

Characterisation of locust bean gum with asymmetric flow field-flow fractionation (AF4) and light scattering

We present a detailed characterisation of locust bean gum (LBG), an industrially significant galactomannan, utilising asymmetric flow field-flow fractionation (AF4) and light scattering. Molecular weight and size determination of galactomannans is complicated by their tendency to aggregate, even in dilute solutions; AF4 allows us to confirm the presence of aggregates, separate these from well-dispersed polymer, and characterise both fractions. For the dispersed polymer, we find Mw=9.2×105 g mol-1 and Rg,z=82.1 nm; the distribution follows Flory scaling (Rg∼Mν) with ν∼ 0.63, indicating good solvent conditions. The aggregate fraction exhibited radii of up to 1000 nm and masses of up to 3×1010 g mol-1. Furthermore, we demonstrate how both fractions are influenced by changes to filtration procedure and solvent conditions. Notably, a 200 nm nylon membrane effectively removes the aggregated fraction; we present a concentration-dependent investigation of solutions following this protocol, using static and dynamic light scattering, which reveals additional weak aggregation in these unfractionated samples. Overall, we demonstrate that AF4 is highly suited to LBG characterisation, providing structural information for both well-dispersed and aggregated fractions, and expect the methods employed to apply similarly to other galactomannans and associating polymer systems.

The Power of Field-Flow Fractionation in Characterization of Nanoparticles in Drug Delivery

Asymmetric-flow field-flow fractionation (AF4) is a gentle, flexible, and powerful separation technique that is widely utilized for fractionating nanometer-sized analytes, which extend to many emerging nanocarriers for drug delivery, including lipid-, virus-, and polymer-based nanoparticles. To ascertain quality attributes and suitability of these nanostructures as drug delivery systems, including particle size distributions, shape, morphology, composition, and stability, it is imperative that comprehensive analytical tools be used to characterize the native properties of these nanoparticles. The capacity for AF4 to be readily coupled to multiple online detectors (MD-AF4) or non-destructively fractionated and analyzed offline make this technique broadly compatible with a multitude of characterization strategies, which can provide insight on size, mass, shape, dispersity, and many other critical quality attributes. This review will critically investigate MD-AF4 reports for characterizing nanoparticles in drug delivery, especially those reported in the last 10-15 years that characterize multiple attributes simultaneously downstream from fractionation.

Structural Studies Reveal that Endosomal Cations Promote Formation of Infectious Coxsackievirus A9 A-Particles, Facilitating RNA and VP4 Release

Coxsackievirus A9 (CVA9), an enterovirus, is a common cause of pediatric aseptic meningitis and neonatal sepsis. During cell entry, enterovirus capsids undergo conformational changes leading to expansion, formation of large pores, externalization of VP1 N termini, and loss of the lipid factor from VP1. Factors such as receptor binding, heat, and acidic pH can trigger capsid expansion in some enteroviruses. Here, we show that fatty acid-free bovine serum albumin or neutral endosomal ionic conditions can independently prime CVA9 for expansion and genome release. Our results showed that CVA9 treatment with albumin or endosomal ions generated a heterogeneous population of virions, which could be physically separated by asymmetric flow field flow fractionation and computationally by cryo-electron microscopy (cryo-EM) and image processing. We report cryo-EM structures of CVA9 A-particles obtained by albumin or endosomal ion treatment and a control nonexpanded virion to 3.5, 3.3, and 2.9 Å resolution, respectively. Whereas albumin promoted stable expanded virions, the endosomal ionic concentrations induced unstable CVA9 virions which easily disintegrated, losing their genome. Loss of most of the VP4 molecules and exposure of negatively charged amino acid residues in the capsid's interior after expansion created a repulsive viral RNA-capsid interface, aiding genome release. IMPORTANCE Coxsackievirus A9 (CVA9) is a common cause of meningitis and neonatal sepsis. The triggers and mode of action of RNA release into the cell unusually do not require receptor interaction. Rather, a slow process in the endosome, independent of low pH, is required. Here, we show by biophysical separation, cryogenic electron microscopy, and image reconstruction that albumin and buffers mimicking the endosomal ion composition can separately and together expand and prime CVA9 for uncoating. Furthermore, we show in these expanded particles that VP4 is present at only ~10% of the occupancy found in the virion, VP1 is externalized, and the genome is repelled by the negatively charged, repulsive inner surface of the capsid that occurs due to the expansion. Thus, we can now link observations from cell biology of infection with the physical processes that occur in the capsid to promote genome uncoating.

Wide-Range Size Fractionation of Graphene Oxide by Flow Field-Flow Fractionation

Many interesting properties of 2D materials and their assembled structures are strongly dependent on the lateral size and size distribution of 2D materials. Accordingly, effective size separation of polydisperse 2D sheets is critical for desirable applications. Here, we introduce flow field-flow fractionation (FlFFF) for a wide-range size fractionation of graphene oxide (GO) up to 100 μm. Two different separation mechanisms are identified for FlFFF, including normal mode and steric/hyperlayer mode, to size fractionate wide size-distributed GOs while employing a crossflow field for either diffusion or size-controlled migration of GO. Obviously, the 2D GO sheet reveals size separation behavior distinctive from typical spherical particles arising from its innate planar geometry. We also investigate 2D sheet size-dependent mechanical and electrical properties of three different graphene fibers produced from size-fractionated GOs. This FlFFF-based size selection methodology can be used as a generic approach for effective wide-range size separation for 2D materials, including rGO, TMDs, and MXene.

Analysis of complex particle mixtures by asymmetrical flow field-flow fractionation coupled to inductively coupled plasma time-of-flight mass spectrometry

Asymmetrical flow field-flow fractionation (AF4) hyphenated with inductively coupled plasma-mass spectrometry (ICP-MS) has been widely used to characterize metal containing particles. This study demonstrates the advantages of coupling AF4 with ICP-time-of-flight mass spectrometry (ICP-TOFMS) in standard and single particle modes to determine size distribution, elemental composition, and number concentration of composite particles. The coupled system was used to characterize two complex particle mixtures. The first mixture consisted of particles extracted from micro-alloyed steels with two size populations of different elemental composition. The second mixture consisted of particles extracted from soil spiked with various engineered nanoparticles (ENPs). The equivalent hydrodynamic sizes of individual micro-alloyed steel particles were up to 6 times larger than the sizes determined by single particle (sp)-ICP-TOFMS. The larger AF4 sizes were attributed to the presence of a surface coating, which is not reflected in the core size determined by sp-ICP-TOFMS. Two particle populations could not be separated by AF4 due to their broad size distributions but were resolved by sp-ICP-TOFMS using their unique elemental signatures. Multi-angle light scattering and ICP-TOFMS signals of soil suspensions increased with the spiked ENP concentrations. However, only after conducting full element screening and single particle fingerprinting by ICP-TOFMS could this increase be attributed to enhanced extraction efficiency of natural particles and the risk for false conclusions be eliminated. In this study, we describe how AF4 coupled to ICP-TOFMS can be applied to study complex samples of inorganic particles which contain organic compounds.

An effective solution to simultaneously analyze size, mass and number concentration of polydisperse nanoplastics in a biological matrix: asymmetrical flow field fractionation coupled with a diode array detector and multiangle light scattering

To accurately understand the biological pollution level and toxicity of polydisperse nanoplastics, an effective solution is presented to separate polydisperse nanoplastics and detect their size, mass and number concentration in a biological matrix by asymmetrical flow field fractionation coupled with a diode array detector and a multiangle light scattering detector.

AF4-DAD-MALS is proposed to separate polydisperse nanoplastics and detect their size, mass and number concentration in a biological matrix.

Separation and characterization of liposomes using asymmetric flow field-flow fractionation with online multi-angle light scattering detection

Liposomes, mainly formed by phospholipids and cholesterol that entrapped different compounds, were separated and characterized using asymmetric flow field-flow fractionation (AF4) coupled with a multi-angle light scattering detector (MALS). AF4 allows the separation of liposomes according to their hydrodynamic size, and the particle size can be estimated directly by their elution time. Besides, different synthesized liposome suspensions of liposomes with different species encapsulated in different places in liposomes were prepared with analytical purposes to be studied. These liposomes were: empty liposomes (e-Ls), magnetoliposomes (MLs) with Fe₃O₄@AuNPs-C12SH inside the lipid bilayer, and long-wavelength fluorophores encapsulated into the aqueous cavity of liposomes (Ls-LWF). The optimization process of the variables that affect the fractionation has been established. The separation effectiveness has been compared with the results achieved with a photon-correlation spectroscopy analyzer based on dynamic light scattering (DLS) and transmission electron microscopy (TEM), used in self-assembly structures characterization. In all cases, three different classes of liposomes have been obtained; two are commonly appaired in all studied samples, while only a third class is characteristic for each of the liposomes. This mean that the proposed methodology could be used for identifying liposomes according to the encapsulated material.

Asymmetrical flow field-flow fractionation to probe the dynamic association equilibria of β-D-galactosidase

Protein dynamics play a significant role in many aspects of enzyme activity. Monitoring of structural changes and aggregation of biotechnological enzymes under native conditions is important to safeguard their properties and function. In this work, the potential of asymmetrical flow field-flow fractionation (AF4) to study the dynamic association equilibria of the enzyme β-D-galactosidase (β-D-Gal) was evaluated. Three commercial products of β-D-Gal were investigated using carrier liquids containing sodium chloride or ammonium acetate, and the effect of adding magnesium (II) chloride to the carrier liquid was assessed. Preservation of protein structural integrity during AF4 analysis was essential and the influence of several parameters, such as the focusing step (including use of frit-inlet), cross flow, and injected amount, was studied. Size-exclusion chromatography (SEC) and dynamic light scattering (DLS) were used to corroborate the in-solution enzyme oligomerization observed with AF4. In contrast to SEC, AF4 provided sufficiently mild separation conditions to monitor protein conformations without disturbing the dynamic association equilibria. AF4 analysis showed that ammonium acetate concentrations above 40 mM led to further association of the dimers ("tetramerization") of β-D-Gal. Magnesium ions, which are needed to activate β-D-Gal, appeared to induce dimer association, raising justifiable questions about the role of divalent metal ions in protein oligomerization and on whether tetramers or dimers are the most active form of β-D-Gal.