Influence of pressure on the chromatographic behavior of insulin variants under nonlinear conditions

Effect of time-invariant pressure gradients on peak formation and efficiency in ultrahigh-pressure liquid chromatography

In chromatography, pressure can affect the retention factors of compounds significantly. In liquid chromatography, this effect is primarily related to the change in the molecular volume of solute during adsorption that is remarkably high for large biomolecules such as peptides and proteins. As a result, the migration velocities of chromatographic bands vary spatially through the column affecting the degree of band broadening. In this work, based on theoretical considerations, chromatographic efficiencies are studied under pressure-induced gradient conditions. The retention factor and migration velocity of different components are examined, and it is shown that components with the same retention time can have different migration patterns. The width of the initial band after injection is affected by the pressure gradient, providing significantly thinner initial bands for compounds with higher pressure sensitivity. In addition to classical band broadening phenomena, the influence of pressure gradients on band broadening is remarkable. The positive velocity gradient leads to extra band broadening. Our results clearly demonstrate that the zones are significantly wider at the end of the column if the change of molar volume of solute during adsorption is large. If the pressure drop is increasing, this effect becomes more significant. In the same time, the high release velocity of the bands somewhat counteracts the extra band broadening effect, however, it can not offset it perfectly. As a result, the separation efficiency of large biomolecules is decreased significantly due to the chromatographic pressure gradient. Under UHPLC conditions, the extent of apparent efficiency loss can reach up to 50% compared to the intrinsic efficiency of the column.

Theoretical study on solute migration and band broadening occurring in pressure-enhanced liquid chromatography

Upon recently studying the use of pressure gradients during liquid chromatography (LC), it was noted that pressure differentials across a column can have a significant impact on peak shape, not just retention as has been noted several times before. Theoretical models and thought experiments were performed here to more carefully study these effects. Two situations have been elucidated. The first is one that reflects a protein reversed phase separation wherein solute retention increases with pressure. In this condition, it has been found that a positive pressure gradient will result in band broadening while a negative pressure gradient will help yield sharper peaks. The second case that has come to be better appreciated is when solute retention decreases with pressure, which can occur in protein ion exchange (IEX) and hydrophobic interaction chromatography (HIC). In this situation, a positive pressure gradient will conversely result in peak sharpening, and a negative pressure gradient will introduce band broadening. These observations have facilitated making new fundamental understandings on pressurized separations which has in turn made it possible to begin envisioning new ways of and reasons for applying pressure enhanced LC methods.

Complex Protein Retention Shifts with a Pressure Increase: An Indication of a Standard Partial Molar Volume Increase during Adsorption?

Studies of protein adsorption on reversed-phase and ion exchange stationary phases demonstrated an increase in retention with increasing pressure, which is interpreted as a standard partial molar volume decrease during the transition of the protein from a mobile to a stationary phase. Investigation of the pressure effect on the retention of lysozyme and IgG on a cation exchange column surprisingly revealed a negative retention trend with the increase of pressure. Further investigation of this phenomenon was performed with β-lactoglobulin, which enabled adsorption to be studied on both cation and anion exchange columns using the same mobile phase with a pH of 5.2. The same surface charge and standard partial molar volume in the mobile phase allowed us to examine only the effect of adsorption. Interestingly, a negative retention trend with a pressure increase occurred on an anion exchange column while a positive trend was present on a cation exchange column. This indicates that the interaction type governs the change in the standard partial molar volume during adsorption, which is independent of the applied pressure. Increasing the protein charge by decreasing the pH of the mobile phase to 4 reversed the retention trend (into a negative) with a pressure increase on the cation exchange column. A further decrease of the pH value resulted in an even more pronounced negative trend. This counterintuitive behavior indicates an increase in the standard partial molar volume during adsorption with the protein charge, possibly due to intermolecular repulsion of adsorbed protein molecules. While a detailed mechanism remains to be elucidated, presented results demonstrate the complexity of ion exchange interactions that can be investigated simply by changing the column pressure.

Pressure-Enhanced Liquid Chromatography, a Proof of Concept: Tuning Selectivity with Pressure Changes and Gradients

Many chromatographers have observed that the operating pressure can dramatically change the chromatographic retention of solutes. Small molecules show observables changes, yet even more sizable effects are encountered with large biomolecules. With this work, we have explored the use of pressure as a method development parameter to alter the reversed-phase selectivity of peptide and protein separations. An apparatus for the facile manipulation of column pressure was assembled through a two-pump system and postcolumn flow restriction. The primary pump provided an eluent flow through the column, while the secondary pump provided a pressure-modulating flow at a tee junction after the column but ahead of a flow restrictor. Using this setup, we were able to quickly program various constant pressure changes and even pressure gradients. It was reconfirmed that pressure changes impact the retention of large molecules to a much greater degree than small molecules, making it especially interesting to consider the use of pressure to selectively separate solutes of different sizes. The addition of pressure to bring the column operating pressure beyond 500 bar was enough to change the elution order of insulin (a peptide hormone) and cytochrome C (a small serum protein). Moreover, with the proposed setup, it was possible to combine eluent and pressure gradients in the same analytical run. This advanced technique was applied to improve the separation of insulin from one of its forced degradation impurities. We have referred to this method as pressure-enhanced liquid chromatography and believe that it can offer unseen selectivity, starting with peptide and protein reversed-phase separations.

Effect of pressure on the retention of macromolecules in ion exchange chromatography

Shorter analysis times and greater resolving power are contributing factors for transfer of separation methods from an HPLC to a UHPLC system when performing analysis in biopharmaceutical or clinical research. The effect of pressure on separations in reversed phase chromatography is well described, however such investigations on ion exchange columns were previously not conducted. In this study we describe the effect of pressure on retention properties of proteins, oligonucleotides and plasmid DNA in ion exchange chromatography. Different column inlet pressures were obtained by coupling restriction capillaries with column outlet and performing separations at a constant temperature and mobile phase flow rate. Macromolecules were separated in isocratic mode as well as with various linear gradients of salt concentration at a constant pH value. The measured retention time increase was up to 80% for isocratic and 20% for gradient separations for a 500 bar increase in pressure. The effect of pressure was validated on a separate instrument after few months from initial experiments. The influence of pressure on retention properties seems to be dependent on the size, shape and flexibility of the macromolecule and causes different retention shifts when separating a sample with diverse analytes. Such changes in retention time can sometimes exceed the criteria set by European Pharmacopoeia (Ph. Eur.) for the allowable method adjustment and are thus considered to be a result of a different separation method. Therefore, the pressure effect that follows method transfer from HPLC to UHPLC conditions should not be neglected even for gradient separations in ion exchange chromatography, as the resulting retention change may cause revalidation of the separation method.

The importance of ion-pairing in peptide purification by reversed-phase liquid chromatography

The adsorption mechanism for three peptides was studied under overloaded conditions through adsorption isotherm measurements in the presence of an ion-pairing reagent, trifluoroacetic acid (TFA), on an end-capped C₁₈-bonded stationary phase. The overall aim of the study was to obtain a better understanding of how the acetonitrile and the TFA fractions in the eluent affected the overloaded elution profiles and the selectivity between peptides using mechanistic modelling and multivariate design of experiments. When studying the effect of TFA, direct evidence for ion pair formation between a peptide and TFA in acetonitrile-water solutions was provided by fluorine-proton nuclear Overhauser NMR enhancement experiments and the adsorption of TFA on the stationary phase was measured by frontal analysis. The adsorption isotherms for each peptide were then determined by the inverse method at eight TFA concentrations ranging from 2.6mM to 37.3mM (0.02-0.29vol-%) in isocratic elution. The equilibrium between the peptide ion and the peptide-TFA complex was modelled by coupling the mass-balance to reaction kinetics and determining separate adsorption isotherms for the two species. We found that a Langmuir isotherm described the elution profile of peptide-TFA complex well while the peptide ion was described by a bi-Langmuir adsorption isotherm since it exhibited strong secondary interactions. The elution profiles had an unfavorable shape at low TFA concentrations consisting of a spike in their front and a long tailing rear due to the secondary interactions for the peptide ion having very low saturation capacity. The acetonitrile dependence on the adsorption isotherms was studied by determination of adsorption isotherms directly from elution profiles obtained in gradient elution which enabled a broad acetonitrile interval to be studied. Here, it was found that the column saturation capacity was quickly reached at very low acetonitrile fractions and that there were significant variations in adsorption with the molecular weight. Finally, practical implications for method development are discussed based on an experimental design where gradient slope and TFA concentrations are used as factors.

Use of pressure in reversed-phase liquid chromatography to study protein conformational changes by differential deuterium exchange

The market of protein therapeutics is exploding, and characterization methods for proteins are being further developed to understand and explore conformational structures with regards to function and activity. There are several spectroscopic techniques that allow for analyzing protein secondary structure in solution. However, a majority of these techniques need to use purified protein, concentrated enough in the solution to produce a relevant spectrum. In this study, we describe a novel approach which uses ultrahigh pressure liquid chromatography (UHPLC) coupled with mass-spectrometry (MS) to explore compressibility of the secondary structure of proteins under increasing pressure detected by hydrogen-deuterium exchange (HDX). Several model proteins were used for these studies. The studies were conducted with UHPLC in isocratic mode at constant flow rate and temperature. The pressure was modified by a backpressure regulator up to about 1200 bar. It was found that the increase of retention factors upon pressure increase, at constant flow rate and temperature, was based on reduction of the proteins' molecular molar volume. The change in the proteins' molecular molar volume was caused by changes in protein folding, as was revealed by differential deuterium exchange. The degree of protein folding under certain UHPLC conditions can be controlled by pressure, at constant temperature and flow rate. By modifying pressure during UHPLC separation, it was possible to achieve changes in protein folding, which were manifested as changes in the number of labile protons exchanged to deuterons, or vice versa. Moreover, it was demonstrated with bovine insulin that a small difference in the number of protons exchanged to deuterons (based on protein folding under pressure) could be observed between batches obtained from different sources. The use of HDX during UHPLC separation allowed one to examine protein folding by pressure at constant flow rate and temperature in a mixture of sample solution with minimal amounts of sample used for analysis.

Influence of pressure and temperature on molar volume and retention properties of peptides in ultra-high pressure liquid chromatography

In this study, pressure induced changes in retention were measured for model peptides possessing molecular weights between ∼1 and ∼4kDa. The goal of the present work was to evaluate if such changes were only attributed to the variation of molar volume and if they could be estimated prior to the experiments, using theoretical models. Restrictor tubing was employed to generate pressures up to 1000bar and experiments were conducted for mobile phase temperatures comprised between 30 and 80°C. As expected, the retention increases significantly with pressure, up to 200% for glucagon at around 1000bar compared to ∼100bar. The obtained data were fitted with a theoretical model and the determination coefficients were excellent (r(2)>0.9992) for the peptides at various temperatures. On the other hand, the pressure induced change in retention was found to be temperature dependent and was more pronounced at 30°C vs. 60 or 80°C. Finally, using the proposed model, it was possible to easily estimate the pressure induced increase in retention for any peptide and mobile phase temperature. This allows to easily estimating the expected change in retention, when increasing the column length under UHPLC conditions.

Influence of phase type and solute structure on changes in retention with pressure in reversed-phase high performance liquid chromatography

The influence of pressure on the retention of several types of solute, including acids, bases and neutrals, was studied by the use of restriction capillaries added to the end of various monomeric and polymeric octadecylsilyl-modified 5μm particle size columns. Although it appeared that certain polymeric columns could give somewhat greater increases in retention with pressure, differences in behaviour between these different C18 columns were rather small. Differences in solute molecular size were most important in determining increases in retention with pressure. However, solute structure such as polarity and planarity were also influential. A prototype C30 column gave interesting selectivity changes between planar and non-planar solutes as a function of pressure. Considerable selectivity differences with pressure were shown when diverse mixtures of solutes were analysed. For the solutes studied, only minor effects of increased pressure on column efficiency and peak shape were noted.

The effect of pressure and mobile phase velocity on the retention properties of small analytes and large biomolecules in ultra-high pressure liquid chromatography

A possible complication of ultra-high pressure liquid chromatography (UHPLC) is related to the effect of pressure and mobile phase velocity on the retention properties of the analytes. In the present work, numerous model compounds have been selected including small molecules, peptides, and proteins (such as monoclonal antibodies). Two instrumental setups were considered to attain elevated pressure drops, firstly the use of a post-column restrictor capillary at low mobile phase flow rate (pure effect of pressure) and secondly the increase of mobile phase flow rate without restrictor (i.e. a combined effect of pressure and frictional heating). In both conditions, the goal was to assess differences in retention behaviour, depending on the type or character of the analyte. An important conclusion is that the effect of pressure and mobile phase velocity on retention varied in proportion with the size of the molecule and in some cases showed very different behaviour. In isocratic mode, the pure effect of pressure (experiments with a post-column restrictor capillary) induces an increase in retention by 25-100% on small molecules (MW<300 g/mol), 150% for peptides (~1.3 kDa), 800% for insulin (~6 kDa) and up to >3000% for myoglobin (~17 kDa) for an increase in pressure from 100 bar up to 1100 bar. The important effect observed for the isocratic elution of proteins is probably related to conformational changes of the protein in addition to the effect of molecular size. Working in gradient elution mode, the pressure related effects on retention were found to be less pronounced but still present (an increase of apparent retention factor between 0.2 and 2.5 was observed).

New trends in reversed-phase liquid chromatographic separations of therapeutic peptides and proteins: theory and applications

In the pharmaceutical field, there is considerable interest in the use of peptides and proteins for therapeutic purposes. There are various ways to characterize such complex samples, but during the last few years, a significant number of technological developments have been brought to the field of RPLC and RPLC-MS. Thus, the present review focuses first on the basics of RPLC for peptides and proteins, including the inherent problems, some possible solutions and some directions for developing a new RPLC method that is dedicated to biomolecules. Then the latest advances in RPLC, such as wide-pore core-shell particles, fully porous sub-2 μm particles, organic monoliths, porous layer open tubular columns and elevated temperature, are described and critically discussed in terms of both kinetic efficiency and selectivity. Numerous applications with real samples are presented that confirm the relevance of these different strategies. Finally, one of the key advantages of RPLC for peptides and proteins over other historical approaches is its inherent compatibility with MS using both MALDI and ESI sources.

Investigation of the effect of pressure on retention of small molecules using reversed-phase ultra-high-pressure liquid chromatography

The effect of inlet pressure on the retention of a series of low molecular weight acids, bases and neutrals, was investigated at constant temperature in reversed-phase liquid chromatography using a commercial ultra-high-pressure system (Waters UPLC instrument). For neutral compounds, relatively small increases in retention factor of up to approximately 12% for a pressure increase of 500bar were noted; the largest values were obtained for polar solutes, or solutes of higher molecular weight. Ionisable acids and bases gave much larger increases in retention with pressure, in some cases as high as 50% for a pressure increase of 500bar. Thus, such compounds could show increases in retention factor approaching 100% over the pressure range available in the commercial UPLC instrument. Due to these differential increases, significant selectivity effects can be obtained for mixtures of different types of solute merely by changing the pressure.

Mass-Transfer Kinetics in a Shell Packing Material for Chromatography

A shell particle consists of a solid, nonporous core that is surrounded with a shell of a porous solid having essentially the same physicochemical properties as those of the conventional porous particles used as packing media in chromatography. The diameter of the solid core and the thickness of its shell or the external diameter of the particle characterizes the chromatographic properties of the packing material. The potential advantage of this particle structure would be the shorter average path length experienced by solute molecules during their diffusion across the particles of packing material when they are retained. Compounds having slow pore diffusion would exhibit higher efficiencies on columns packed with shell than with conventional, fully porous particles. Using columns packed with Halo, a new type of porous silica shell particles, we assess the gain achieved with this principle for peptides of moderate molecular weights and for small proteins.

Effect of the mobile phase composition on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C18-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 2.5, 5, and 7.5% of acetonitrile (ACN) or 7, 10, 15, and 20% of methanol (MeOH). Most of these isotherms have two inflection points and three different parts. The low and the high concentration parts exhibit langmuirian behavior. The intermediate part exhibits anti-langmuirian behavior. The inflection points shift toward higher concentrations with increasing mobile phase concentration in ACN or MeOH, which causes the differences in the isotherm profiles. The nature of the organic modifier and its concentration affect only the isotherm profile and the numerical values of its parameters, not the nature of the best model, which is the bi-Moreau model in all cases. The isotherm profiles depend on the experimental conditions because they affect the intensity of the adsorbate-adsorbate interactions. Overloaded band profiles of tryptophan were recorded with the seven mobile phase compositions. They were used to determine the best values of the isotherm coefficients by the inverse method (IM) of chromatography. There is an excellent agreement between the values of these parameters obtained by FA and by IM. Increasing the concentration of either ACN or MeOH in the mobile phase causes a slight decrease in the saturation capacities of the low and the high energy sites, and in the adsorption constant of the low energy sites. The adsorption constant of the high energy sites increases with increasing concentration of either solvent or is little affected. The adsorbate-adsorbate interaction constants of both low and high energy sites increase for both solvents. Saturation capacities of the high energy sites are higher for ACN than for MeOH.

Critical contribution of nonlinear chromatography to the understanding of retention mechanism in reversed-phase liquid chromatography

The retention of most compounds in RPLC proceeds through a combination of several independent mechanisms. We review a series of recent studies made on the behavior of several commercial C18-bonded stationary phases and of the complex, mixed retention mechanisms that were observed in RPLC. These studies are essentially based on the acquisition of adsorption isotherm data, on the modeling, and on the interpretation of these data. Because linear chromatography deals only with the initial slope of the global, overall, or apparent isotherm, it is unable fully to describe the complete adsorption mechanism. It cannot even afford clues as to the existence of several overlaid retention mechanisms. More specifically, it cannot account for the consequences of the surface heterogeneity of the packing material. The acquisition of equilibrium data in a wide concentration range is required for this purpose. Frontal analysis (FA) of selected probes gives data that can be modeled into equilibrium isotherms of these probes and that can also be used to calculate their adsorption or affinity energy distribution (AED). The combination of these data, the detailed study of the best constants of the isotherm model, the determination of the influence of experimental parameters (e.g., buffer pH and pI, temperature) on the isotherm constants provide important clues regarding the heterogeneity of the adsorbent surface and the main properties of the adsorption mechanisms. The comparison of similar data obtained for the adsorption of neutral and ionizable compounds, treated with the same approach, and the investigation of the influence on the thermodynamics of phase equilibrium of the experimental conditions (temperature, average pressure, mobile phase composition, nature of the organic modifier, and, for ionizable compounds, of the ionic strength, the nature, the concentration of the buffer, and its pH) brings further information. This review provides original conclusions regarding retention mechanisms in RPLC.

Adsorption Mechanism in RPLC. Effect of the Nature of the Organic Modifier

The adsorption isotherms of phenol and caffeine were acquired by frontal analysis on two different adsorbents, Kromasil-C18 and Discovery-C18, with two different mobile phases, aqueous solutions of methanol (MeOH/H2O = 40/60 and 30/70, v/v) and aqueous solutions of acetonitrile (MeCN/H2O = 30/70 and 20/80, v/v). The adsorption isotherms are always strictly convex upward in methanol/water solutions. The calculations of the adsorption energy distribution confirm that the adsorption data for phenol are best modeled with the bi-Langmuir and the tri-Langmuir isotherm models for Kromasil-C18 and Discovery-C18, respectively. Because its molecule is larger and excluded from the deepest sites buried in the bonded layer, the adsorption data of caffeine follow bi-Langmuir isotherm model behavior on both adsorbents. In contrast, with acetonitrile/water solutions, the adsorption data of both phenol and caffeine deviate far less from linear behavior. They were best modeled by the sum of a Langmuir and a BET isotherm models. The Langmuir term represents the adsorption of the analyte on the high-energy sites located within the C18 layers and the BET term its adsorption on the low-energy sites and its accumulation in an adsorbed multilayer system of acetonitrile on the bonded alkyl chains. The formation of a complex adsorbed phase containing up to four layers of acetonitrile (with a thickness of 3.4 A each) was confirmed by the excess adsorption isotherm data measured for acetonitrile on Discovery-C18. A simple interpretation of this change in the isotherm curvature at high concentrations when methanol is replaced with acetonitrile as the organic modifier is proposed, based on the structure of the interface between the C18 chains and the bulk mobile phase. This new model accounts for all the experimental observations.

Effect of the Surface Heterogeneity of the Stationary Phase on the Range of Concentrations for Linear Chromatography

The range of sample sizes within which linear chromatographic behavior is achieved in a column depends on the surface heterogeneity of the RPLC adsorbents. Two widely different commercial adsorbents were tested, the end-capped XTerra-C18 and the non-end-capped Resolve-C18. Adsorption isotherm data of caffeine were acquired by frontal analysis. These data were modeled and used to calculate the adsorption energy distribution (AED). This double analysis informs on the degree of surface heterogeneity. The best adsorption isotherm models are the bi-Langmuir and the tetra-Langmuir isotherms for XTerra and Resolve, respectively. Their respective AEDs are bimodal and quadrimodal distributions. This interpretation of the results and the actual presence of a low density of high-energy adsorption sites on Resolve-C18 were validated by measuring the dependence of the peak retention times on the size of caffeine samples (20-microL volume, concentrations 10, 1, 0.1, 1 x 10(-2), 1 x 10(-3), 1 x 10(-4), and 1 x 10(-5) g/L). The experimental chromatograms agree closely with the band profiles calculated from the best isotherms. On Resolve-C18, the retention time decreases by 40% when the sample concentration is increased from 1 x 10(-5) to 10 g/L. The decrease is only 10% for Xterra-C18 under the same conditions. The upper limit for linear behavior is 1 x 10(-4) g/L for the former adsorbent and 0.01 g/L for the latter. The presence of a few high-energy adsorption sites on Resolve-C18, with an adsorption energy 20 kJ/mol larger than that of the low-energy sites while the same difference on Xterra is only 5 kJ/mol, explains this difference. The existence of adsorption sites with a very high energy for certain compounds affects the reproducibility of their retention times and a rapid loss of efficiency in a sample size range within which linear behavior is incorrectly anticipated.

Thermodynamic studies of pressure-induced retention of peptides in reversed-phase liquid chromatography

The pressure-induced retention of peptides on reversed-phase HPLC was studied by systematically changing organic solvent composition and temperature at both low (19 bar) and high (318 bar) pressures using a homologous series of hydrophobic poly-L-phenylalanine (n = 2-7) as the model compound. Based on van' t Hoff plots under different organic solvent compositions and pressures, the enthalpy change for the solute (deltaH) was determined. Moreover, both the enthalpy and entropy change for each phenylalanine residue (deltadeltaH and deltadeltaS), which corresponds to solute retention on a microenvironment along the depth of C18 chain, were also calculated by direct subtractions. Results indicate that under acetonitrile (ACN) compositions above 35%, the pressure caused deltadeltaS value to change from a negative to a positive value and both deltaH and deltadeltaH to change from a negative to a less negative value, all leading to a thermodynamic state closer to those under 35% acetonitrile composition. This implies that the pressure-induced retention observed in this study was an entropy-favored but enthalpy-unfavored process and was explained by pressure-induced desorption of solvent molecules that were associated with the stationary phase or with the peptide solute. Under 35% acetonitrile composition, however, it was found that neither deltadeltaH nor deltadeltaS value was significantly changed by the pressure. Whereas, both deltaH value and the intercept of van't Hoff plots under 35% acetonitrile composition were increased by pressure. This indicates that under low organic solvent composition, 35%, most of the acetonitrile molecules adsorbed on the surface of the stationary phase and only little solvent molecules were dissolved in the bulk stationary phase where the phenylalanine residues were partitioned. This study has provided new thermodynamic insights to the pressure-induced retention for peptides and proteins.

Influence of pressure on the retention and separation of insulin variants under linear conditions

The effect of pressure on the retention behavior of insulin variants in RPLC on a YMC-ODS C18 column was investigated under linear conditions. The retention factors of these variants increase nearly 2-fold when the average column pressure is increased from 55 to 250 bar while their separation factors remain nearly unchanged. This effect is explained by a change of the partial molar volume of the insulin variants associated with their adsorption that decreases from -99 to -80 mL/mol for mobile-phase concentrations of acetonitrile increasing from 29 to 33% (v/v). This volume change is much larger than the one observed with low molecular weight compounds. For the same pressure variation, the average number Z of acetonitrile molecules displaced from the protein and the stationary phase upon adsorption increases from 22 to 23.3. The pressure-induced relative increase of the term b[S]/[D0]z (which corresponds to the initial slope of the adsorption isotherm) is approximately twice as large for Lispro than for porcine insulin. Because the binding constant of insulin decreases with increasing pressure, this suggests that the number of binding sites on the stationary phase increases even faster. Finally, it was observed that the column efficiency at flow rates higher than 0.6 mL/min increases slightly with increasing pressure. It is suggested that these observations are also valid for other proteins analyzed in RPLC.

Pressure-induced effects in the heterogeneous adsorption of insulin on chromatographic surfaces

The effect of increasing the average column pressure (ACP) on the heterogeneous adsorption of insulin variants on a C18-bonded silica was studied in isocratic reversed-phase HPLC. Adsorption isotherm data of lispro and porcine insulin obtained for values of the ACP ranging from 57 to 237 bar were fitted to the Langmuir-Freundlich and the Tóth equation. The resulting isotherm parameters, including the equilibrium adsorption constant and the heterogeneity index, were next used for the calculation of distribution functions characterizing the energy of interactions between the adsorbed insulin molecules and the stationary phase. It was observed that increasing the pressure by 180 bar causes a broadening of the distribution functions and a shift of the position of their maximum toward lower interaction energies. These findings suggest that, under high pressures, the insulin molecules interact with the stationary phase in a more diversified way than under low pressures. Additionally, the most probable value of the energy of the insulin-surface interactions becomes lower when the ACP increases. The pressure-induced changes in the interaction of insulin variants with the hydrophobic surface are attributed to a possible conformational flexibility of the molecular structure of this protein.

Repeatability and reproducibility of high concentration data in reversed-phase liquid chromatography. I. Overloaded band profiles on Kromasil-C18

Single-component adsorption-isotherm data were acquired by frontal analysis (FA) for six low-molecular-mass compounds (phenol, aniline, caffeine, theophylline, ethylbenzene and propranolol) on one Kromasil-C18 column, using water-methanol solutions (between 70:30 and 20:80, v/v) as the mobile phase. Propranolol data were also acquired using an acetate buffer (0.2 M) instead of water. The data were modeled for best agreement between calculated and experimental overloaded band profiles. The adsorption energy distribution was also derived and used for the selection of the best isotherm model. Widely different isotherm models were found to model best the data obtained for these compounds, convex upward (i.e. Langmuirian), convex downward (i.e. anti-Langmuirian), and S-shaped isotherms. Using the same sample size for all columns (loading factor, Lf approximately 10%), overloaded band profiles were recorded on four different columns packed with the same batch of Kromasil-C18 and five other columns packed with different batches of Kromasil-C18. These experimental band profiles were compared to the profile calculated from the isotherm measured by FA on the first column. The repeatability as well as the column-to-column and the batch-to-batch reproducibilities of the band profiles are better than 4%.