Mass Transport Limited Conditions Research Articles

EWTN: Quantifying Mass Transport to Enable Water Electrolyzer Architectures with Low Flow-Rate Sensitivity

To develop cost-effective and high-performing polymer electrolyte water electrolyzers (PEWEs) for gigawatt-scale applications, researchers have focused on reducing precious metal catalyst loadings and optimizing porous transport layers. However, the performance of PEWEs is also affected by system architecture. Mass transport losses are dependent on localized architecture and material interactions. In-situ measurements, such as current density distribution maps have demonstrated advantages in understanding the intricate characteristics and influence of two-phase flow within PEWEs. This study proposes the parameter of effective water transport number (EWTN) as a quantitative tool to investigate such current density distribution (CDD) measurements for PEWEs. Results show that higher flow-rates have EWTN values of 0.95 and above, indicating no mass transport limitations; while lower flow-rates with large gradients CDD have EWTN values between 0.6-0.8, indicating mass transport limited conditions. The new analysis also identified a correlation between mass transport losses due to bubble accumulation, membrane hydration, and ohmic overpotentials. To address these limitations, an unitized pin-type LGDL/flow-field design was developed, which effectively prevents local gas phase accumulation, resulting in improved mass transport characteristics. The results of this work show reduced flow-rate sensitivity with the pin-type architecture and ∼13% increased performance at 0.24ml/min/cm2 .

Quantifying transport and electrocatalytic reaction processes in a gastight rotating cylinder electrode reactor via integration of Computational Fluid Dynamics modeling and experiments

Understanding the complexity of the multiple processes of mass, momentum, charge, and heat transport, and how these affect reaction kinetics at the electrode/electrolyte interface is one of the major challenges in the field of energy and catalysis. The rapid and rational scale-up of electrocatalytic systems to industrial scales require a detailed understanding of nonlinear transport-reaction processes, accessible only through the building of multi-physics models that capture with high fidelity the complexity of real-world devices. The gastight rotating cylinder electrode (RCE) reactor is a promising lab-scale tool that can decouple transport from intrinsic kinetics to generate data for first-principle models useful in the design of industrial, electrochemical reactors. Computational Fluid Dynamics (CFD) studies have previously been used to investigate the bulk flow in RCE reactors for simple corrosion and electroplating processes. However, the quantification of changes in local concentration within the viscous layer where catalysis takes place requires capturing the correct flow conditions inside the hydrodynamic boundary layer near the surface of the electrode. This requires simulations with spatial resolution in the nm and μm scale and temporal resolutions between ms and s scales that are similar to the timescales for reactions on the electrode surface. In this study, experimental electrocatalysis is combined with CFD modeling to elucidate and parameterize the hydrodynamics in a gastight RCE reactor. CFD simulations of the electrochemical ferricyanide reduction reaction under mass transport limited conditions are used to evaluate the validity of the CFD model parameters by comparing calculated dimensionless mass transport descriptors to dimensionless correlations obtained experimentally. Justifications for assumptions and details of the simulation methods used in this study are presented to provide a detailed understanding of the effect that each model parameter has on the ability to accurately simulate electrocatalysis in RCE systems. The simulation methodology reported here is a first step towards the development of multi-scale models for the study of transport dependent electrocatalytic processes, such as the electrochemical transformation of CO2 to fuels and chemicals.

Mass transport limitations in polymer electrolyte water electrolyzers using spatially-resolved current measurement

This work utilizes spatially-resolved current measurements to provide insight into mass transport limitations in electrolyzers that are not observable from traditional polarization measurement. In this study, two types of flow-fields (parallel and triple-serpentine) and two types of diffusion media (patterned porous thin titanium foil LGDLs and Ir-coated titanium felt PTLs) were examined. A non-uniform current distribution dominated by mass transport limitations was observed to be instigated by the restriction of liquid water transport to catalyst sites. Additionally, conditions are revealed which yield similar polarization performance but dissimilar current distributions. In such cases, the transport limitations for different architectures and porous media affect polarization in different regions of the active area. Furthermore, the triple-serpentine flow-field used in this study performs better than the parallel flow-field under mass transport limited operating conditions. This indicates that the parallel flow-field used in this study is more susceptible to starvation than the triple-serpentine flow-field for the electrolyzer studied.

A Robust and Quantified Analysis of Mass Transport and Its Application in Designing Next Generation Architecture for Polymer Electrolyte Water Electrolysis Cells

Investigations to better understand the nature of mass transport and two-phase flow within a polymer electrolyte water electrolyzer (PEWE) have gained popularity as researchers strive to improve system performance and efficiency. Along with traditional polarization experiments, the use of in-situ spatial and temporal current distribution measurement techniques has proven to be very attractive. Different methods for spatial current distribution measurements include the use of resistor networks, potential probes, and printed circuit boards (PCB) [1–3]. Using such techniques, Mench and co-workers have previously demonstrated [4–7] that, under mass transport limited conditions for any combination of flow-field and diffusion media, the current density distributions consistently exhibit a gradient – with high current towards the inlet and low towards the outlet. These contour plots showed certain dissimilarities; however, they serve as a qualitative picture to gauge cell performance and characterize mass transport. Therefore, in this work a robust analysis was developed that provides a quantitative understanding and better characterization of the mass transport limitation within the electrolyzer. A mass transport number (MTN) was defined and used to further characterize the influence of mass transport limitation experienced within an electrolyzer.Applying this new analysis to our the investigations revealed that the PEWE system performance using high-performance micro-patterned Ti-foil liquid-gas-diffusion-layer (LGDL) [8,9] was highly sensitive to the cell architecture and exhibited poor mass transport behavior using traditional channel based flow-fields. Compared with the conventional porous transport layer (PTL) morphology, the lack of in-plane permeability of the LGDLs caused faster onset of mass transport limitations at lower flowrates, local increase in HFR, and poor area utilization. Therefore, a unitized single flow-field-LDGL component that mitigates these problems has been designed. Through this work it will be shown that the flow-field is designed to have pins that allow for greater in-plane transport. The staggered pattern reduces pressure drop and promotes exchange in-between channels that assist in efficient bubble removal. Additionally, the cross-section of the pin, the pitch, and aspect-ratios are adjustable to maximize porosity for different LGDLs. Finally, making this into a single unitized component having a permanently attached LGDL will decrease interfacial contact resistances and improve cell performance (as shown in Figure 1).[1] T. V. Reshetenko, G. Bender, K. Bethune, R. Rocheleau, A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution, Electrochim. Acta. 88 (2013) 571–579. https://doi.org/10.1016/j.electacta.2012.10.103.[2] A. Phillips, M. Ulsh, J. Porter, G. Bender, Utilizing a Segmented Fuel Cell to Study the Effects of Electrode Coating Irregularities on PEM Fuel Cell Initial Performance, Fuel Cells. 17 (2017) 288–298. https://doi.org/10.1002/fuce.201600214.[3] C. Immerz, B. Bensmann, P. Trinke, M. Suermann, R. Hanke-Rauschenbach, Local Current Density and Electrochemical Impedance Measurements within 50 cm Single-Channel PEM Electrolysis Cell, J. Electrochem. Soc. 165 (2018) F1292–F1299. https://doi.org/10.1149/2.0411816jes.[4] F.H. Roenning, A. Roy, D. Aaron, M.M. Mench, Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers, ECS Meet. Abstr. MA2020-02 (2020) 2457–2457. https://doi.org/10.1149/MA2020-02382457mtgabs.[5] A. Roy, F. Roenning, D. Aaron, M.M. Mench, Quantifying Lateral Current Spread While Measuring Performance Using a Segmented Polymer Electrolyte Water Electrolysis Cell, ECS Meet. Abstr. MA2020-02 (2020) 2458–2458. https://doi.org/10.1149/MA2020-02382458mtgabs.[6] A. Roy, F. Roenning, D. Aaron, M.M. Mench, Local Two-Phase Flow and Performance in Polymer Electrolyte Water Electrolysis Cells, ECS Meet. Abstr. MA2021-01 (2021) 1190–1190. https://doi.org/10.1149/MA2021-01381190mtgabs.[7] A.Z. Weber, R.L. Borup, R.M. Darling, P.K. Das, T.J. Dursch, W. Gu, D. Harvey, A. Kusoglu, S. Litster, M.M. Mench, R. Mukundan, J.P. Owejan, J.G. Pharoah, M. Secanell, I. V. Zenyuk, A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells, J. Electrochem. Soc. 161 (2014) F1254–F1299. https://doi.org/10.1149/2.0751412jes.[8] J. Mo, Z. Kang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green, M.M. Mench, F.-Y. Zhang, Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting, Sci. Adv. 2 (2016) e1600690. https://doi.org/10.1126/sciadv.1600690.[9] Y. Li, Z. Kang, X. Deng, G. Yang, S. Yu, J. Mo, D.A. Talley, G.K. Jennings, F.-Y. Zhang, Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells, Electrochim. Acta. 298 (2019) 704–708. https://doi.org/10.1016/j.electacta.2018.12.162. Figure 1

High-Throughput Electrosynthesis with Flow-through Electrodes Made from Cu Nanowires

The high surface area per unit volume and large mass-transfer rates offered by three-dimensional porous electrodes have resulted in their use in a wide variety of electrochemical processes.1-4 Several types of porous electrodes are commercially available, including carbon paper, graphite felt, reticulated vitreous carbon (RVC), metal mesh, and metal foam. Metal foam offers relatively high conductivity (1.5x10-5 ohm m) but low specific surface area (<4x104 m-1),5 whereas carbon paper has one of the highest specific surface areas (up to 1.6x105 m-1) but lower conductivity (4.7x10-5 ohm m).6 An improved flow-through electrode would have the electrical conductivity of metals, a surface area greater than carbon paper, and a hydraulic permeability comparable to carbon paper. This presentation will describe the characteristics of a Cu nanowire electrode that has 15 times more surface area (2.4x106 m­-1) and is 32 times more conductive (1.4x10-6 ohm m) than carbon paper. The improvement in surface area is due to the small diameter of the nanowires relative to carbon fibers. The higher conductivity is due to the intrinsically higher conductivity of Cu, and the fact that the metal nanowires can be sintered together. The porosity of the nanowire electrode is 0.94, but its hydraulic permeability was 89 times lower than carbon paper. For Cu ion reduction, the Cu nanowire electrode can achieve the same single-pass conversion as carbon paper at flow rates up to 1000 times greater under mass transport-limited conditions, and 10 times greater under kinetically limited conditions. We will also report results for the hydrogenation of furfural and for water oxidation. Future work will focus on improving the hydraulic permeability of flow-through electrodes while maintaining a >10x increase in surface area relative to carbon paper.

Building upon the Koutecky-Levich Equation for Evaluation of Next-Generation Oxygen Reduction Reaction Catalysts

The performance of oxygen reduction reaction (ORR) catalysts has been substantially improved over the past several decades. These catalysts are evaluated for electrochemical activity in a rotating disk electrode (RDE) assembly using an oxygen saturated liquid electrolyte. Koutecky-Levich (K-L) analysis provides a simple and effective method to extract electrokinetic information by correcting for mass transport effects. We propose extensions to the K-L analysis to address some of the simplifying assumptions made during its derivation. In particular, we demonstrate that decreased concentrations of surface reactants contribute to measured overpotentials in a Nernst fashion, and can lead to an underestimation of catalytic activity. By applying a Nernst overpotential correction in conjunction with K-L analysis, more accurate measurements of the intrinsic reaction kinetics under mass transport limited conditions are possible. As the K-L method assumes a reaction order of unity, we also consider kinetic reaction order deviation from unity due to measurement conditions. We show that examining the dependence of the reaction order on overpotential can provide a straightforward technique to probe blocking effects of surface absorbents. We propose that these extensions to the K-L method can allow for increased versatility of the RDE technique for extracting electrokinetic parameters for ORR catalysts.

Kinetic Characterisation of a Single Chain Antibody against the Hormone Abscisic Acid: Comparison with Its Parental Monoclonal

A single-chain Fv fragment antibody (scFv) specific for the plant hormone abscisic acid (ABA) has been expressed in the bacterium Escherichia coli as a fusion protein. The kinetics of ABA binding have been measured using surface plasmon resonance spectrometry (BIAcore 2000) using surface and solution assays. Care was taken to calculate the concentration of active protein in each sample using initial rate measurements under conditions of partial mass transport limitation. The fusion product, parental monoclonal antibody and the free scFv all have low nanomolar affinity constants, but there is a lower dissociation rate constant for the parental monoclonal resulting in a three-fold greater affinity. Analogue specificity was tested and structure-activity binding preferences measured. The biologically-active (+)-ABA enantiomer is recognised with an affinity three orders of magnitude higher than the inactive (-)-ABA. Metabolites of ABA including phaseic acid, dihydrophaseic acid and deoxy-ABA have affinities over 100-fold lower than that for (+)-ABA. These properties of the scFv make it suitable as a sensor domain in bioreporters specific for the naturally occurring form of ABA.

Determination of the efficiency of methanol oxidation in a direct methanol fuel cell

The energy efficiency of a direct methanol fuel cell (DMFC) is dependent on the cell potential, crossover of methanol and oxygen, and the average number of electrons released per methanol molecule (nav) at the anode. The nav value is determined by the product distribution (CO2:formic acid:formaldehyde), and in the absence of methanol loss due to crossover can be determined from the dependence of the cell current on the flow rate of the methanol solution. The effects of crossover have been assessed here by analysis of methanol in the cell exhaust, and a method to correct for crossover is presented. Under mass transport limited conditions, the effect of crossover is shown to be negligible, and accurate nav values are obtained from the flow rate dependence of the current alone. However, analysis of methanol in the cell exhaust is essential for determining nav under mixed kinetic and mass transport control. The exhaust concentration, combined with the flow rate dependence of the current, also provides a measure of efficiency losses due to crossover.

Determining the affinity and stoichiometry of interactions between unmodified proteins in solution using Biacore

We describe a general Biacore method for measuring equilibrium binding affinities and stoichiometries for interactions between unmodified proteins and their unmodified ligands free in solution. Mixtures of protein and ligand are preequilibrated at different ratios in solution and then analyzed by Biacore using a sensor chip surface that detects only unbound analyte. Performing the Biacore analysis under mass transport limited conditions allows the concentration of unbound analyte to be determined from the initial velocity of binding. Plots of initial velocity versus the concentration of the varied binding partner are fitted to a quadratic binding equation to give the affinity and stoichiometry of binding. We demonstrate the method using soluble Her2 extracellular domain binding to monovalent, bivalent, and trivalent forms of an anti-Her2 antibody. The affinity we measured agrees with that obtained from conventional Biacore kinetic analysis, and the stoichiometries for the resulting 1:1, 1:2, and 1:3 complexes were confirmed by gel filtration with in-line light scattering. The method is applicable over an affinity range of approximately 100pM to 1μM and is particularly useful when there is concern that covalently modifying one or the other binding partner might affect its binding properties or where multivalency might otherwise complicate a quantitative analysis of binding.

The Influences of Water Vapor/Hydrogen Ratio, Gas-Flow Rate and Antimony on the Surface Oxidation of Trip Steels

In the current paper, we are reporting the results from an investigation of the surface and sub-surface oxidation of a TRIP steel containing 2 wt.% Mn and 0.5 wt.% Al with and without 0.03 wt.% Sb. The oxidizing conditions in the gas were successively varied in terms of the linear gas flow-rate and dew-point, from conditions were gas-phase mass transport limited conditions prevailed, to those were solid state processes became the rate determining conditions. It was found, that at sufficient low oxidizing conditions (defined as flow-rate/dew-point), the metal surfaces were clear of any external oxides, and as the oxidizing conditions were increased, Mn- and Si- oxide nodules formed along with magnetite. As the oxidizing conditions were increased further, a dense magnetite layer was present. The limits of the various regions were experimentally quantified and a proposed hypothesis for their occurrences is presented. No obvious effect of Sb was noted in this micro-structural research of the oxides that results from the various conditions investigated in this study.

Improving the Limit of Detection of Nanoscale Sensors by Directed Binding to High-Sensitivity Areas

The revelation of protein-protein interactions is one of the main preoccupations in the field of proteomics. Nanoplasmonics has emerged as an attractive surface-based technique because of its ability to sense protein binding under physiological conditions in a label-free manner. Here, we use short-range ordered holes with a diameter of approximately 150 nm and a depth of approximately 50 nm as a nanoplasmonic template. A approximately 40 nm high cylindrical region of Au is exposed on the walls of the holes only, while the rest of the surface consists of TiO2. Since the sensitivity is confined to the nanometric holes, the use of two different materials for the sensor substrate offers the opportunity to selectively bind proteins to the most sensitive Au regions on the sensor surface. This was realized by applying material-selective poly(ethylene glycol)-based surface chemistry, restricting NeutrAvidin binding to surface-immobilized biotin on the Au areas only. We show that under mass-transport limited conditions (low nM bulk concentrations), the initial time-resolved response of uptake could be increased by a factor of almost 20 compared with the case where proteins were allowed to bind on the entire sensor surface and stress the generic relevance of this concept for nanoscale sensors. In the scope of further optimizing the limit of detection (LOD) of the sensor structure, we present finite-element (FE) simulations to unravel spatially resolved binding rates. These revealed that the binding rates in the holes occur in a highly inhomogeneous manner with highest binding rates observed at the upper rim of the holes and the lowest rates observed at the bottom of the holes. By assuming a plasmonic field distribution with enhanced sensitivity at the Au-TiO2 interface, the FE simulations reproduced the experimental findings qualitatively.

Extracting kinetic rate constants from surface plasmon resonance array systems

Surface plasmon resonance imaging systems, such as Flexchip from Biacore, are capable of monitoring hundreds of reaction spots simultaneously within a single flow cell. Interpreting the binding kinetics in a large-format flow cell presents a number of potential challenges, including accounting for mass transport effects and spot-to-spot sample depletion. We employed a combination of computer simulations and experimentation to characterize these effects across the spotted array and established that a simple two-compartment model may be used to accurately extract intrinsic rate constants from the array under mass transport-limited conditions. Using antibody systems, we demonstrate that the spot-to-spot variability in the binding kinetics was <9%. We also illustrate the advantage of globally fitting binding data from multiple spots within an array for a system that is mass transport limited.

Biosensor‐based characterization of serum antibodies during development of an anti‐IgE immunotherapeutic against allergy and asthma

Antibody responses, induced in Cynomolgus monkey by recombinant IgE-derived immunotherapeutic protein against atopic allergies and asthma, were characterized using label-free, real-time protein interaction analysis. The effects of two different immunotherapeutic proteins were compared. Active concentrations of specific anti-IgE antibodies formed were determined in sera sampled at multiple time points, using conditions of total mass transport limitation that were proved to exist on the sensor surface. These concentrations varied from about 0.4 to 35 microg/ml among the monkeys and throughout the immunization period. Based on these concentrations, the rate and affinity constants for the binding of antibody populations to the antigen could be determined. The apparent equilibrium dissociation constant decreased during the immunization period, for all the monkeys, by a factor between 6 and 50, ending at values from approximately 2 x 10(-9) to approximately 2 x 10(-11) M among the animals. This affinity maturation was attributable to the changes in both rate constants, although the magnitude of the contribution of each constant depended partly on specimen, but primarily on the immunotherapeutic used. The immunotherapeutic proteins examined showed excellent immunogenic properties, providing the basis for a new and effective treatment for allergy and asthma.

Modeling the effect of particle-to-bubble adhesion on mass transport and reaction rate in a stirred slurry reactor: influence of catalyst support

The adhesion of catalyst particles to the gas–liquid interface significantly influences the rate of reaction in a three-phase gas-inducing stirred slurry reactor. For the Pd-catalyzed glucose oxidation reaction at mass transport-limited conditions, the experimental reaction rate is higher for lyophobic 3% Pd/C catalyst than for lyophilic 3% Pd / SiO 2 catalyst. This is attributed to a higher particle-to-bubble adhesion (PBA) of the Pd/C catalyst. The interfacial catalyst concentration is quantified by a PBA equilibrium parameter in a PBA isotherm. The classical resistances-in-series “GLS model” cannot describe the overall reaction rate. An additional gas-to-solid “GS model” is presented with a gas-to-solid mass transfer coefficient to describe the increased rate of reaction by the catalyst particles adhered to the gas–liquid interface. The PBA equilibrium parameter and the gas-to-solid mass transfer coefficient during reaction are estimated as a function of mixing intensity, oxygen partial pressure, and catalyst concentration. The combined GLS–GS model adequately describes the experimentally observed reaction rates.

The effect of mass transport limitation on the estimation of intrinsic kinetic parameters for negative order reactions

The kinetic rate parameters of a reaction rate model are usually estimated using reaction rate measurements obtained at intrinsic kinetic conditions, viz., without limitation of the reaction rate by mass transport, irrespective of the order of the reaction kinetics. For positive order kinetics, this is good practice and the kinetic rate parameters can be safely applied both at intrinsic and mass transport limited conditions. However, for negative order kinetics, erroneous results are obtained because the kinetic rate parameters estimated at intrinsic kinetic conditions fail to predict the reaction rate when mass transport limitation also plays a role. This is demonstrated for the simple case of Langmuir–Hinshelwood (LH) negative-order kinetics and for the, more complex, kinetics of the oxidation of methyl α -D-glucopyranoside on noble metal catalysts. It is concluded that for a negative-order reaction rate model, which is valid for a wide range of reaction conditions, the kinetic rate parameters need to be estimated both at intrinsic and mass transport limited conditions.

TG/DTA/MS of poly(methyl methacrylate)

Degradation of relatively large particle size, 0.5 mm of Type-G PMMA (Rohm and Haas) were conducted with thermogravimetric analysis and evolved gas measurements using quadrupole mass spectrometer under conditions of mass transport limitation. In addition, differential thermal analysis was performed in order to furnish information with regards to exothermic or endothermic reactions associated with the degradation. The tests were conducted in an inert environment of pure N2 and oxygenated environment. The results indicated one step degradation process in pure N2 and the degradation process is endothermic. As the O2 fraction increases the degradation process is transformed to exothermic.

The computer-aided design of a new hydrodynamic device for studying mechanisms of chemical transfer across the liquid|liquid interface

This article outlines the computer-aided design and development of a new hydrodynamic device for investigating the transfer of chemical species between two immiscible liquids. The technique is based on the confluence reactor design, recently introduced for mechanistic and kinetic analysis in single-phase solvent systems. In this article the formulation, numerical procedures and simulation results are presented for the device development. Finite element simulations are presented to establish the fluid dynamic properties of the solvents within the device. In addition, the concentration distribution and current/voltage characteristics of electroactive species introduced under mass transport limited conditions are presented. The simulations reveal the range of optimal transport rate/cell configurations for efficient detection of species partitioning between the two solvent phases.

Affinity analysis of non-steady-state data obtained under mass transport limited conditions using BIAcore technology.

Binding data obtained with Biacore instrumentation is often evaluated using a kinetic transport model where reaction rate constants and a mass transport coefficient are used to describe the interaction. Here the use of a simplified model, an affinity transport model, for determination of the affinity (K(D)) but not the kinetics (k(a), k(d)) has been investigated. When binding rates were highly governed by mass transport effects the two models returned the same affinity and gave similar residuals, but k(a) and k(d) values found with the kinetic transport model were unreliable. On the other hand the affinity transport model failed to describe the data when binding curves were less influenced by mass transport effects. Under such circumstances the kinetic transport model returned correct k(a) and k(d) values. Depending on the outcome of the analysis the affinity transport model can therefore be used to reduce uncertainties of the kinetic parameters or as an easy way to determine K(D) values from non-steady-state data. The use of the affinity transport model is illustrated with simulated data and with binding data obtained for the interaction between a 439 Da thrombin inhibitor and immobilized thrombin. Since it is more difficult to resolve high k(a) values for low molecular weight analytes, the affinity transport model may be particularly useful for affinity analysis involving fast reactions between such analytes and immobilized protein targets.

Active concentration measurements of recombinant biomolecules using biosensor technology.

Whereas the concentration of a biomolecule simply refers to the amount of chemical substance per unit of volume, its active concentration refers to a relational parameter that has meaning only with respect to the molecule's ability to interact specifically with one particular ligand. When proteins are studied in a biological context, it is the biologically active concentration that is relevant, and not the total concentration of correctly and incorrectly folded molecules. Using a biosensor instrument the concentration of active biomolecules in a preparation can be measured by injecting the preparation at different flow rates onto a sensor chip surface presenting a high concentration of a specific ligand. The method can be used under conditions of partial mass transport limitation and does not require a pre-established standard curve. When the method was used to measure the active concentration of several recombinant proteins it was found that the active concentration was much lower than the nominal concentration determined by conventional methods. The active concentration also depended on the ligand used in the binding assay, reflecting the fact that active concentration can only be defined with respect to one specific probe. Such discrepancies in concentration values, if undetected, may lead to erroneous conclusions regarding the properties and behaviour of recombinant proteins tested in different assays.

Active concentration measurements of recombinant biomolecules using biosensor technology

Whereas the concentration of a biomolecule simply refers to the amount of chemical substance per unit of volume, its active concentration refers to a relational parameter that has meaning only with respect to the molecule's ability to interact specifically with one particular ligand. When proteins are studied in a biological context, it is the biologically active concentration that is relevant, and not the total concentration of correctly and incorrectly folded molecules. Using a biosensor instrument the concentration of active biomolecules in a preparation can be measured by injecting the preparation at different flow rates onto a sensor chip surface presenting a high concentration of a specific ligand. The method can be used under conditions of partial mass transport limitation and does not require a pre-established standard curve. When the method was used to measure the active concentration of several recombinant proteins it was found that the active concentration was much lower than the nominal concentration determined by conventional methods. The active concentration also depended on the ligand used in the binding assay, reflecting the fact that active concentration can only be defined with respect to one specific probe. Such discrepancies in concentration values, if undetected, may lead to erroneous conclusions regarding the properties and behaviour of recombinant proteins tested in different assays. Copyright © 1999 John Wiley & Sons, Ltd.