Biomedical subjects

Peter W Carr

Publications and source records attributed to Peter W Carr.

36 records · Page 2Linked to original sources

PubMed2005

The combined effect of silanols and the reversed-phase ligand on the retention of positively charged analytes.

The nature of the interaction of positively charged analytes with the surface of reversed-phase bonded phases has been investigated as a function of both pH and volume fraction of organic modifier. Studies of the combined effect of both the parameters have been previously reported by us, and the data presented here further demonstrate a multiplicative interaction between pH and the concentration of organic modifier in the mobile phase. Fitting of the data as functions of pH and eluent composition clearly shows that the hydrophobically assisted ion-exchange process dominates over a purely reversed-phase or a pure ion-exchange retention mechanism. The underlying theory is developed in detail, and the mechanism is elucidated using several reversed-phase packings of substantially different character.

Ligands↗

PubMed2004

Synthesis and characterization of hypercrosslinked, surface-confined, ultra-stable silica-based stationary phases.

The synthesis and chromatographic characterization of a highly crosslinked self-assembled monolayer (SAM) stationary phase whose acid and thermal stability were significantly improved relative to a sterically protected octadecylsilane (ODS) stationary phase were recently described [B.C. Trammell, L. Ma, H. Luo, D. Jin, M.A. Hillmyer, P.W. Carr, Anal. Chem. 74 (2002) 4634]. Unfortunately, this highly crosslinked SAM phase is much more silanophilic than a conventional sterically protected octadecyl silane phase. 29Si CP-MAS NMR analysis shows that the high concentration of silanol groups in the self-assembled monolayer causes the increased retention and poor peak shape of basic solutes. In this work dimethyl-chloromethyl-phenylethylchlorosilane (DM-CMPES), a silane with only a single reactive silyl chloride group was tested as an alternative to chloromethyl-phenyethyltrichlorosilane (CMPES) as the basis for forming the starting phase. Most importantly this "conventional" silanization step (i.e., a non-SAM silanization) was followed by a Friedel-Crafts reaction using aluminum chloride as the catalyst and styrene heptamer as the multi-valent crosslinker to form the surface DM-CMPES groups into a network polymer which is fully confined and attached to the surface. An octyl (C8) derivative of the hypercrosslinked (HC) dimethyl-chloromethyl-phenylethyl (DM-CMPES) surface-confined stationary phase was synthesized to demonstrate the potential of a Friedel-Crafts based approach to making high efficiency, acid and thermally stable polymerized phases on silica with selectivity closer to conventional aliphatic phases. The stability of the retention factors of these phases under very aggressive conditions (5%, (v/v) trifluoroacetic acid and 150 degrees C) are compared to that of a sterically protected octadecylsilane (ODS) phase. The comparisons show that the long term stability of highly crosslinked DM-CMPES phases in acid is superior to the conventional phase. The HC-C8 phase is even more stable in acid than the HC-styrene heptamer DM-CMPES phase on which it is based. Additionally, the efficiency and peak shape of several prototypical bases under acidic (0.1% TFA, pH 2.0) elution conditions are discussed. The column dynamics and thermodynamic characteristics of the HC-C8 phase were investigated to demonstrate the chromatographic utility of this ultra-stable phase. Inverse size exclusion chromatography and flow studies of the HC-C8 and the sterically protected C18 stationary phases indicate the absence of pore plugging and quite good (nearly 100,000 plates/m) chromatographic efficiency. Further chromatographic investigations show that the HC-C8 stationary phase behaves as a typical reversed phase material. The HC-C8 stationary phase offers unique chromatographic selectivity for certain classes of analytes compared to both alkyl and phenyl bonded phases.

Chromatography, Liquid↗

PubMed2004

Separation of racemic 2,4-dinitrophenyl amino acids on 9-O-(phenyloxycarbonyl)quinine-bonded carbon-clad zirconia in reversed-phase liquid chromatography.

Zirconia is known to be one of the best materials for the chromatographic support due to its excellent chemical, thermal, and mechanical stability. In this work, we report preparation and use of 9-O-(phenyloxycarbonyl)quinine-bonded carbon-clad zirconia (QNCZ) as a chiral stationary phase (CSP) for separation of N-(2,4-dinitrophenyl) (DNP)-amino acids (AAs) enantiomers in reversed-phase liquid chromatography. Retention and enantioselectivity of the QNCZ CSP were compared with those of quinine 3-triethoxysilylpropylcarbamate-coated zirconia (QNZ) and quinine 3-triethoxysilylpropylcarbamate-bonded silica (QNS). The QNCZ CSP showed in general the better enantioselectivity for most of the amino acids studied.

Amino Acids↗

PubMed2003

An ultra acid stable reversed stationary phase.

We report a new reversed phase liquid chromatography (RPLC) phase with remarkable acid stability using dimethyl(ethylphenylchloromethyl)chlorosilane (1), oligomeric polystyrene (PS), and octylbenzene (C8). This phase Si-1-PS-C8 was prepared using silica modification processes and Friedel-Crafts alkylation chemistry. Under highly aggressive mobile phase conditions, Si-1-PS-C8 exhibited remarkable stability as evinced by only minimal reduction in retention factor (k') after 1400 column volumes at pH = 0.5 and 150 degrees C. The peak shapes for a variety of basic solutes were symmetric using Si-1-PS-C8. Evidence for a highly cross-linked coating of the silica particles was observed using scanning electron microscopy. The remarkable stability of this phase is unparalleled as compared to all other RPLC phases reported to date.

Drug Stability↗

PubMed2003

Comparison of the chromatography of octadecyl silane bonded silica and polybutadiene-coated zirconia phases based on a diverse set of cationic drugs.

In this study, we compare the separation of basic drugs on several octadecyl silane bonded silica (ODS) phases and a polybutadiene-coated zirconia (PBD-ZrO2) phase. The retention characteristics were investigated in detail using a variety of cationic drugs as probe solutes. The ODS phases were selected to cover a relatively wide range in silanol activity and were studied with ammonium phosphate eluents at pH 3.0 and 6.0. Compared to any of the ODS phases, the PBD-ZrO2 phase showed very significant differences in selectivities towards these drugs. Due to the presence of both reversed-phase and ion-exchange interactions between the stationary phase and the basic analyte on ODS and PBD-ZrO2, mixed-mode retention takes place to some extent on both types of phases. However, very large differences in the relative contributions from ion-exchange and reversed-phase interactions on the two types of phases led to quite different selectivities. When phosphate is present in the eluent and adsorbs on the surface, the PBD-ZrO2 phase takes on a high negative charge over a wide pH range due to phosphate adsorption on its surface. On ODS phases, ion-exchange interactions result from the interactions between protonated basic compounds and ionized residual silanol groups. Since the pH of the eluent influences the charge state of the silanol groups, the ion-exchange interactions vary in strength depending on pH. At pH 6.0, the ion-exchange interactions are strong. However, at pH 3.0 the ion-exchange interactions on ODS are significantly smaller because the silanol groups are less dissociated at the lower pH. Thus, not only are the selectivities of the ODS and PBD-ZrO2 phases different but quite different trends in retention are observed on these two types of phases as the pH of the eluent is varied. More importantly, by using the large set of "real" basic analytes we show the extreme complexity of the chromatographic processes on the reversed stationary phases. Both the test condition and solute property influence the column performance. Therefore, use of only one or two probe solutes is not sufficient for column ranking.

Butadienes↗

PubMed2003

Effect of amine counterion type on the retention of basic compounds on octadecyl silane bonded silica-based and polybutadiene-coated zirconia phases.

In a previous paper, we compared the mixed-mode retention characteristics of cationic solutes on octadecyl silane-bonded silica (ODS) and polybutadiene-coated zirconia (PBD-ZrO2) phases. It is well recognized that both reversed-phase and ion-exchange interactions contribute to the retention of cations on ODS phases. The reversed-phase interaction results from the bonded hydrocarbon chain; the ion-exchange interaction originates in the ionized residual silanol groups. These two types of interactions also exist on the PBD-ZrO2 phase. The polybutadiene contributes to the reversed-phase interaction and the ionized zirconol, but primarily, the adsorbed Lewis base anions, such as phosphate or fluoride, contribute to the ion-exchange interaction. We have shown that on ODS phases, reversed-phase interactions are much more important, whereas the opposite is true of PBD-ZrO2 phases. In this work, we investigate the effect of several amine mobile phase counterions on the retention of cationic solutes on ODS and PBD-ZrO2 phases. The effects of the chain length and the type of amine (1 degree, 2 degrees, 3 degrees) counterion on the retention of basic compounds were studied. In contrast to older studies of type A silica-based phases, the results show that the chain length and type of the amine blocker do not have a large effect on the retention of basic compounds with the newer type B silica-based materials. However, on the PBD-ZrO2 phase, very striking differences in retention were observed with different amine counterions. We show that the molecular geometry of the amine counterion has a significant effect on the retention of basic solutes on the PBD-ZrO2 phase.

Amines↗

PubMed2003

Characterization and optimization of a chromatographic process based on ethylenediamine-N,N,N',N'-tetra(methylphosphonic) acid-modified zirconia particles.

The primary objective of work was to characterize, optimize and model a chromatographic process based on ethylenediamine-N,N,N',N'-tetra(methylphosphonic) acid (EDTPA)-modified zirconia particles. Zirconia particles were produced by spray-drying colloidal zirconia. Zirconia spheres produced were further classified, calcined and modified with EDTPA to yield a solid-phase support for use in bio-chromatography (r_PEZ). Specifically, the ability of r_PEZ to selectively bind and enrich IgG, IgA, and IgM from biological fluids was evaluated and demonstrated. To better understand the force of interaction between the IgG and the r_PEZ, the equilibrium disassociation constant (K(d)) was determined by static binding isotherms, as a function of temperature and by frontal analysis at different linear velocities. The maximum static binding capacity (Q(max)) was found to be in the range 55-65 mg IgG per ml of beads, and unaffected by temperature. The maximum dynamic binding capacity (Q(x)) was found to be in the range 20-12 mg IgG per ml of beads. The adsorption rate constant (k(a)) was determined by a split-peak approach to be between 982 and 3242 l mol(-1) s(-1) depending on the linear velocity. The standard enthalpy and entropy values were estimated for this interaction of IgG with this novel support.

Chromatography, Liquid↗

PubMed2003

Synthesis of micron diameter polybutadiene-encapsulated non-porous zirconia particles for ultrahigh pressure liquid chromatography.

In this study, 1-microm diameter polybutadiene-encapsulated non-porous zirconia particles were synthesized, slurry packed into 50-microm I.D. fused-silica capillary columns, and evaluated using ultrahigh pressure liquid chromatography. The dependencies of column efficiency and solute retention factor on pressure were investigated. Efficiencies as high as 280000 plates per meter were obtained for the separation of anti-inflammatory drugs at a pressure of 1351 MPa. Comparing the reversed-phase behavior of the polybutadiene-encapsulated non-porous zirconia with octadecylsilane bonded non-porous silica, greater selectivity was found using the zirconia-based material for the applications reported in this study. The encapsulated non-porous zirconia particles demonstrated excellent thermal stability in the separation of polycyclic aromatic hydrocarbons at a temperature of 100 degrees C and a pressure of 1351 MPa.

Anti-Inflammatory Agents↗

PubMed2003

Analysis and critical comparison of the reversed-phase and ion-exchange contributions to retention on polybutadiene coated zirconia and octadecyl silane bonded silica phases.

The two major modes of retention of basic compounds in reversed-phase liquid chromatography on both octadecyl silane bonded silica-based (ODS) and polybutadiene coated zirconia (PBD-ZrO2) materials are hydrophobic and ion-exchange (Coulombic) interactions. Although the influence of reversed-phase and Coulombic interactions on the chromatography of organic cations is qualitatively well recognized, the quantitative relationship between hydrophobic and ion-exchange interactions remains unclear. In this work, the retention mechanisms on both of the above types of phases were probed by studying the retention of a homologous series of p-alkylbenzylamines as a function of the ammonium concentration in the eluent. The various columns tested were studied in terms of plots of retention factor vs. the inverse of the displacingcation concentration. The analysis of such plots as well as plots of log k' vs. number of methylene groups in the solutes and plots of log k' vs. log[NH4+] clearly shows that at least two types of sites--a pure reversed-phase site and a "hydrophobically-assisted ion-exchange site" similar to the type of site described by Neue [J. Chromatogr. A 925 (2001) 49] are needed to explain the observations. In addition, we have found a quantitative measure of the relative amount of reversed-phase and ion-exchange interaction on a given solute has on a given stationary phase which allows unambiguous classification of columns. It is now clear that ion-exchange contributions to retention on PBD-ZrO2, sometimes exceeding 90%, are even more important than previously thought and relative to hydrophobic interaction much more significant on PBD-ZrO2 than on ODS type-B silicas.

Buffers↗

PubMed2003

Elevated-temperature ultrahigh-pressure liquid chromatography using very small polybutadiene-coated nonporous zirconia particles.

Capillary columns packed with small diameter particles typically lead to low permeability and long separation times in high-performance liquid chromatography. Ultrahigh pressures (>10,000 p.s.i.; 1 p.s.i. is identical with 6,894.76 Pa) can be used to overcome the limitations that small particles impose. Ultrahigh-pressure liquid chromatography (UHPLC) has demonstrated great potential for high-speed and high-efficiency separations. Decreasing the viscosity of the mobile phase by elevating the temperature could additionally reduce the pressure drop and facilitate the use of longer columns or smaller particles to achieve even higher total plate numbers. For this reason, we investigated the use of elevated temperatures in UHPLC. Water-resistant, flexible heater tape covered with insulation was used to provide the desired heat to the column. Polybutadiene-coated 1 microm nonporous zirconia particles were used because of their chemical stability at elevated temperature. A column efficiency as high as 420,000 plates m(-1) was obtained. The effects of temperature and pressure on the separation of parabens were investigated. Separation of five herbicides was completed in 60 s using 26,000 p.s.i. and 90 degrees C.

Butadienes↗

PubMed2002

Highly cross-linked self-assembled monolayer stationary phases: an approach to greatly enhancing the low pH stability of silica-based stationary phases.

A new type of silica-based stationary phase with dramatically improved acid stability compared to any currently available silica-based stationary phase has been developed. Superior low pH stability is achieved by first self-assembling a densely bonded monolayer of (chloromethyl)-phenylethyltrichlorosilane (CMPES). The self-assembly step is followed by a Friedel-Crafts cross-linking of the reactive moieties with their neighbors, by addition of secondary, cross-linkable aromatic reagents, or by both. This phase is not endcapped. Elemental analysis data shows that an aluminum chloride catalyst is very effective at bonding aromatic cross-linking reagents, such as styrene heptamer and triphenylmethane, to the reactive CMPES monolayer. The stability of the retention factor of decylbenzene on the cross-linked self-assembled CMPES phases is compared to a sterically protected C18 phase to illustrate its superior resistance to acid-catalyzed-phase loss. Inverse size exclusion chromatography and flow-curve comparisons of the cross-linked self-assembled CMPES and the sterically protected C18 stationary phases illustrate their similar chromatographic efficiency.

Journal Article↗

PubMed2002

Mixed-mode reversed-phase and ion-exchange separations of cationic analytes on polybutadiene-coated zirconia.

The retention and selectivity of the chromatographic separation of basic (cationic) analytes on a polybutadiene-coated zirconia (PBD-ZrO2) stationary phase have been studied in greater detail than in previous studies. These separations are strongly influenced by the chemistry of the accessible surface of zirconia. In the presence of buffers which contain hard Lewis bases (e.g., phosphate, fluoride, carboxylic acids) zirconia's surface becomes negatively charged due to adsorption of the buffer anion at the hard Lewis acid sites. Consequently, under most conditions (e.g., neutral pH), cationic analytes undergo both hydrophobic and cation-exchange interactions. This mixed-mode retention process generally leads to greater retention factors for cations relative to those on silica-based reversed phases despite the lower surface areas of the zirconia phase, but, more importantly, adsorption of hard Lewis bases can be used to control the chromatographic selectivity for cationic analytes on these zirconia-based stationary phases. In contrast to our prior work, here we show that when mixed-mode retention takes place, both retention and selectivity are easily adjusted by changing the type of hard Lewis base buffer anion, the type of buffer counter-ion (e.g., sodium, potassium, ammonium), the pH, and the ionic strength of the eluent as well as the type and amount of organic modifier.

Buffers↗

PubMed2002

High-speed liquid chromatography by simultaneous optimization of temperature and eluent composition.

Clearly, a major trend in liquid chromatography is to increase its speed to make it faster. Improving throughput for routine analysis of pharmaceutical release samples and stability assays are all key motivations for improving separation speed. Recent work has focused on the theoretical benefits of temperature on speed in liquid chromatography. We have shown that the 5-10-fold decrease in eluent viscosity that comes from a temperature increase of 175 degrees C over ambient, and the concomitant increase in analyte diffusitivity, act to dramatically decrease the time needed to generate a theoretical plate. Lower viscosities at elevated temperatures decrease the pressure drop across the column and allow the use of higher linear velocities as the pump pressure limit is approached. Simultaneously, faster analyte diffusion at higher column temperature improves efficiency at high eluent velocity conditions compared to the efficiency at lower temperatures at the same velocity. We find that higher temperature plays a central role in improving speed. In this work, we show that when the percent organic modifier in the eluent and column temperature are adjusted to keep retention factors fixed, highly efficient, subminute separations can be routinely achieved when a hot column is used at the maximum system back pressure. We find that the best way to facilitate such an optimization, assuming constant selectivity, is to use a very retentive column so that one can work at both high temperature and high volume fraction of organic modifier to achieve the lowest possible eluent viscosity. We have also analyzed the effect that key extracolumn contributions have on column selection and system design.

Chemistry, Pharmaceutical↗

PubMed2002

Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical-conditions model for retention prediction in reversed-phase liquid chromatography.

This paper describes two new retention models for predicting retention under different reversed-phase liquid chromatography (RPLC) conditions. The first one is a global linear solvation energy relationship (LSER) that expresses retention as a function of both solute LSER descriptors and mobile phase composition. The second is a so-called "typical-conditions model" that expresses retention under a given chromatographic condition as a linear function of retention under different so-called "typical" conditions. The global LSER was derived by combining the local LSER model and the linear solvent strength theory (LSST) of RPLC. Compared to local LSER and the LSST models, the global LSER model requires far fewer retention measurements for calibrating the model when different solutes and different mobile phase compositions are involved. Its fitting performance is equal to the local LSER model but worse than that of LSST. The poor fit of the global LSER results primarily from the local LSER model and not from the LSST model. The typical-conditions model (TCM) was developed based on a concept of multivariate space that is conceptually compatible with LSER. However, no LSER descriptors are used in the TCM approach. The number of input conditions needed in the typical-conditions model is determined by the chemical diversity of the solutes and the conditions involved. Principal component analysis (PCA) and iterative key set factor analysis (IKSFA) were used to find the number of typical conditions needed for a given data set. Compared to LSER, LSST, and global LSER, the typical-conditions model is more precise and requires fewer retention measurements for calibrating the model when different solutes and different stationary and/or mobile phases are involved.

Chromatography, Liquid↗

PubMed2002

Use of DRYLAB to compare octadecylsilane and carbon supports for reversed-phase chromatography of triazine herbicide test solutes.

One of the most important aspects of method development in reversed-phase high-performance liquid chromatography (RPLC) is the accurate prediction of the resolution as one or more experimental variables (usually the mobile-phase composition) are changed. By use of commercially available computer simulation programs, e.g. Drylab 2000, data from a few initial experimental runs are used to predict how the separation will vary with composition so that a computer program can then rapidly locate the optimum conditions. Assumption of a linear relationship between log k' (retention factor) and phi (volume fraction of organic modifier in the element) minimizes the number of initial runs required for a separation and is usually adequate when the range of phi is not large. In this work the linearity of plots of log k' against phi for octadecylsilane-coated silica (ODS) and carbon-coated zirconia (C-ZrO(2)) columns were compared and the plots were used to determine whether prediction of the separation of triazine herbicides was adequate. Analysis of the slopes and intercepts of such plots are presented for both types of columns.

Chromatography, High Pressure Liquid↗

PubMed2002

Study of polar and nonpolar substituted benzenes and aromatic isomers on carbon-coated zirconia and alkyl bonded phases.

Retention factors of polar and non-polar mono- and di-substituted benzene derivatives were measured on carbon coated zirconia (C/ZrO2) and an alkyl modified silica using water-acetonitrile mobile phases. Published data on porous graphitic carbon phases (PGC) were used to facilitate comparisons between the two types of carbon media. This work showed that retention on both C/ZrO2 and PGC is much more sensitive to the solute polarizability, dipolarity, and shape than on aliphatic phases. For simple disubstituted benzenes there was no general clear cut advantage in terms of chromatographic selectivity to using a carbon-based phase over a bonded phase silica; however, the selectivities towards such isomers are quite different on the two types of media. In contradistinction to their effect on alkyl bonded phase retention, addition of a dipolar substituent and weak hydrogen bond acceptor to a benzene ring almost always increases the solute's retention on C/ZrO2 and PGC.

Benzene Derivatives↗

PubMed2002

A study of the critical criteria for analyte stability in high-temperature liquid chromatography.

There are three major impediments to the use of high-temperature ultrafast liquid chromatography. First, the stationary phase must be thermally stable. Over the past decade, a series of thermally stable, highly efficient stationary phases have been developed that can withstand temperatures exceeding 200 degrees C. Second, the temperature mismatch between the incoming eluent and the column must be minimized (<5 degrees C), because such a mismatch is a very serious cause of peak broadening, especially in ultrafast separations. The thermal mismatch problem can be significantly ameliorated at high column linear velocities by using narrow-bore columns (2.1-mm i.d.). Third, analytes that are exposed to high temperatures must be thermally stable on the time scale of the chromatographic run. We report here a study of the ability of a number of pharmaceuticals to withstand superambient temperatures on the time scale of fast separations. We propose criteria by which a particular analyte may be rejected as a candidate for high-temperature analysis, and we demonstrate that complex molecules are amenable to quantitation, even at temperatures in excess of 100 degrees C in the aqueous media. We also show that as the time an analyte spends on hot column decreases, the extent of on-column reaction decreases for those analytes that do react. Although the seminal work of Antia and Horvath addresses these issues from a theoretical perspective, we hope to further alleviate fear of the use of high temperatures in liquid chromatography through the empirical approach used here.

Algorithms↗

PubMed2002

Mechanistic implications of the equality of compensation temperatures in chromatography.

A common interpretation of the observation that two processes exhibit similar compensation temperatures in an enthalpy-entropy plot is that the two processes occur via the same "mechanism". We show that this interpretation is not rigorously allowed. In fact, the only thing that can be concluded from the observation of identical compensation temperatures is that the relative contributions of enthalpy and entropy to the overall free energy are the same in the two processes. Since it is possible that two processes occur via different mechanisms that, by chance, result in the same relative blends of enthalpy and entropy, the observation of identical compensation temperatures cannot be used as evidence for mechanistic identity. If two processes exhibit different compensation temperatures, however, it can logically be concluded that the two processes are mechanistically distinct.

Chromatography↗