Solvent-strength selectivity in reversed-phase HPLC

Development and Validation of an RP-HPLC-PDA Method for Determination of Paracetamol, Caffeine and Tramadol Hydrochloride in Pharmaceutical Formulations

Paracetamol (acetaminophen) (PAR), caffeine (CAF) and tramadol hydrochloride (TRA) are important drugs widely used for many clinical purposes. Determination of their contents is of the paramount interest. In this respect, a quick, simple and sensitive isocratic RP-HPLC method with photodiode array detection was developed for the determination of paracetamol, caffeine and tramadol in pharmaceutical formulations. An improved sensitive procedure was also evolved for tramadol using a fluorescence detector system. A C₁₈ column and a mobile phase constituted by methanol/phosphate were used. LODs were found to be 0.2 μg/mL, 0.1 μg/mL and 0.3 μg/mL for paracetamol, caffeine and tramadol hydrochloride, respectively, using photodiode-array detection. Alternatively, LOD for tramadol decreased to 0.1 μg/mL with the fluorescence detector. Other notable analytical figures of merit include the linear concentration ranges, 0.8–270 μg/mL, 0.4–250 μg/mL and 1.0–300 (0.2–40) μg/mL, for the same ordered analytes (including the fluorescence detector). The proposed method was successfully applied for the quantitative determination of the three drugs in tablet dosage forms.

Analysis of the retention of tetracyclines on reversed-phase columns: Chemometrics, design of experiments and quantitative structure-property relationship (QSPR) study for interpretation and optimization

In this study, design of experiments was applied for the analysis of 6 reversed phase U-HPLC columns used for the separation of four tetracyclines (TCs): tetracycline, doxycycline, chlortetracycline and oxytetracycline in different elution conditions. In a first part, a fractional factorial design (2^4-1) was used to study the influence of four chromatographic parameters: column temperature, pH, flow rate and composition of the mobile phase (i.e. nature of the solvent used as the organic modifier), on the quality of the separation, which was evaluated in terms of peak width and resolution between two pairs of TCs. This experimental design revealed that the nature of the solvent: acetonitrile (ACN) or methanol (MeOH), and the mobile phase flow rate were the two main factors actually having the most influence on the quality of the separation. Moreover, these two factors presented an antagonistic influence according to the response considered: peak width or peak resolution. In order to understand this behavior, a Doehlert design was performed in the second part. It consisted in modeling the evolution of responses as a function of the two main factors: nature of the composition of the mobile phase (mix of ACN and MeOH, from 100% ACN to 100% MeOH) and mobile phase flow rate (from 0.3 to 0.8 mL min^-1). For all the reversed phase columns studied, an inversion of the elution order of TCs and an increase of the retention factors was observed according to the composition of the organic mixture at the end of the gradient. To understand the modification of the interactions implied in the various retention modes related to the selectivity of the organic solvents used, a quantitative structure-property relationship (QSPR) study was achieved. In this final study, the molecular descriptors of each TCs were connected to its retention factor.

Polar aprotic modifiers for chromatographic separation and back-exchange reduction for protein hydrogen/deuterium exchange monitored by Fourier transform ion cyclotron resonance mass spectrometry

Hydrogen/deuterium exchange monitored by mass spectrometry is an important non-perturbing tool to study protein structure and protein–protein interactions. However, water in the reversed-phase liquid chromatography mobile phase leads to back-exchange of D for H during chromatographic separation of proteolytic peptides following H/D exchange, resulting in incorrect identification of fast-exchanging hydrogens as unexchanged hydrogens. Previously, fast high-performance liquid chromatography (HPLC) and supercritical fluid chromatography have been shown to decrease back-exchange. Here, we show that replacement of up to 40% of the water in the LC mobile phase by the modifiers, dimethylformamide (DMF) and N-methylpyrrolidone (NMP) (i.e., polar organic modifiers that lack rapid exchanging hydrogens), significantly reduces back-exchange. On-line LC micro-ESI FT-ICR MS resolves overlapped proteolytic peptide isotopic distributions, allowing for quantitative determination of the extent of back-exchange. The DMF modified solvent composition also improves chromatographic separation while reducing back-exchange relative to conventional solvent.

Fabrication and chromatographic performance of porous-shell pillar-array columns

We report on a new approach to obtain highly homogeneous silica-monolithic columns, applying a sol-gel fabrication process inside a rectangular pillar-array column (1 mm in width, 29 microm in height and 33.75 mm in length) having a cross-sectional area comparable to that of a 200 microm diameter circular capillary. Starting from a silicon-based pillar array and working under high phase-separation-tendency conditions (low poly(ethylene glycol) (PEG)-concentration), highly regular silica-based chromatographic systems with an external porosity in the order of 66-68% were obtained. The pillars, 2.4 microm in diameter, were typically clad with a 0.5 microm shell layer of silica, thus creating a 3.4 microm total outer pillar diameter and leaving a minimal through-pore size of 2.2 microm. After mesopore creation by hydrothermal treatment and column derivatization with octyldimethylchlorosilane, the monolithic column was used for chip-based liquid-chromatographic separations of coumarin dyes. Minimal plate heights ranging between 3.9 microm (nonretaining conditions) and 6 mum (for a retention factor of 6.5) were obtained, corresponding to domain-size-reduced plate heights ranging between 0.7 and 1.2. The column permeability was in the order of 1.3 x 10(13) m(2), lower than theoretically expected, but this is probably due to obstructions induced by the sol-gel process in the supply channels.

Computerized design of separation strategies by reversed-phase liquid chromatography: development of DryLab software

The development of DryLab software is a special achievement in analytical HPLC which took place in the last 16 years. This paper tries to collect some of the historical mile stones and concepts. DryLab, being always subject to change according to the needs of the user, never stopped being developed. Under the influence of an ever changing science market, the DryLab development team had to consider not just scientific improvements, but also new technological achievements, such as the introduction of Windows 1.0 and 3.1, and later Windows NT and 2000. The recent availability of new 32-bit programming tools allowed calculations of chromatograms to be completed more quickly so as to show peak movements which result for example from slight changes in eluent pH. DryLab is a great success of interdisciplinary and intercontinental cooperation by many scientists.

Assay of 2,3-dihydroxybenzoic acid and related compounds in plant materials by high-performance liquid chromatography

Salicylic acid and its putative biosynthetic precursors were assayed isocratically by RP-HPLC with UV detection at 280 nm. Optimum resolution was provided by an HPLC mobile phase consisting of MeOH-1% aqueous HOAc (40:60, v/v), at pH 4. Furthermore, for the analysis of 2,3-dihydroxybenzoic acid (2,3-DHBA) in Catharanthus roseus cell cultures after elicitation, a mobile phase consisting of acetonitrile-1% aqueous HCOOH containing 0.25% trichloroacetic acid (1:5, v/v), at pH 2, was used. The recovery for the free form of 2,3-DHBA was about 80% after a one-step extraction of the cells. The detection limit of 2,3-DHBA was 3 microg by using saligenin as an internal standard.

An approach to the concept of resolution optimization through changes in the effective chromatographic selectivity

It is very common chromatographic practice to optimize resolution by making changes in selectivity by systematically varying key retention controlling factors. In many instances, a change in conditions merely results in monotonic, systematic variation in the relative retention of all pairs of peaks. Useful or "effective" changes in selectivity generally result when we see peak crossovers, changes in elution order or differential changes in band position of three or more peaks upon changing some operating condition. In this work, we demonstrate that changes in what we now call the effective selectivity can only take place when retention depends on a minimum of two solute molecular properties and further the dependencies must differ for the two sets of conditions. To verify our concept, real chromatographic data are examined from the viewpoint of linear solvation energy relationships (LSERs) and linear solvent strength theory. Five different RPLC stationary phases in different eluents are compared to elucidate the similarities and differences in their effective selectivities. Of major importance is our finding that the effective selectivity can only be understood when it is viewed in terms of the ratios of system-dependent interaction coefficients, such as the LSER coefficients, and not merely the absolute values of the coefficients. We confirm, both theoretically and experimentally, that a change in mobile-phase volume fraction and in column temperature is not as powerful a mechanism for tuning the effective selectivity as is a change in stationary-phase type.

Maintaining fixed band spacing when changing column dimensions in gradient elution

In gradient elution separations, it may be required to change either column length (to increase resolution or shorten run time) or column diameter (for an increase in sensitivity or for preparative separations). In either of these changes of column dimensions, it is usually desired to maintain the same relative band spacing (selectivity), so as to increase resolution in proportion to (column plate number)1/2 when increasing column length, or to maintain constant resolution when changing column diameter. A general rule for avoiding changes in band spacing in these situations is to maintain the quantity [(gradient time) x (flow-rate)/(column volume)] constant, while holding the initial and final gradient mobile phase compositions (%B) fixed. This rule is only valid as long as the equipment hold-up volume (dwell volume) is negligible, or if all sample components are strongly retained at the start of the gradient. When neither of the latter conditions apply, then significant changes in band spacing may result when changing column size. Rules are presented for recognizing this potential problem for a given sample/HPLC-equipment combination, and adjustments in separation conditions that can avoid this problem are discussed. Changes in band spacing as a result of change in column size are of special concern when developing procedures for preparative chromatography under gradient conditions.

Changing reversed-phase high performance liquid chromatography selectivity. Which variables should be tried first?

When carrying out HPLC method development, it is often necessary to vary the relative retention of the sample (values of alpha) by changing some experimental variable, e.g., solvent type, pH, etc. The choice of which variable will be most suitable for a change in selectivity depends on two conflicting goals: (a) the attainment of maximum changes in alpha for the better control of resolution and (b) the avoidance of practical problems associated with the use of a given variable to optimize selectivity. This study provides a quantitative evaluation of different variables for their effect on selectivity (alpha). Various practical problems which must be balanced against this ability of a variable to change value of alpha are also discussed. The selection of any two variables for their simultaneous use in controlling alpha is also examined.

Hydrophobicity estimations by reversed-phase liquid chromatography. Implications for biological partitioning processes

Liquid chromatography has long been used for the estimation of "hydrophobicity" of solutes of biological, environmental and agricultural interest. These measurements have taken the form of octanol-water partition coefficient estimation, or less often the more fundamental processes that the octanol-water partition coefficient is intended to model. Here we review both the chromatographic methods used for these estimations, their successes and failures, and discuss pertinent solution thermodynamics of the partitioning of small molecules between bulk phases, such as octanol and water, and between a bulk phase and an interphase, such as partitioning of solute molecules into lipid layers and biological membranes.

Implementation and use of gradient predictions for optimization of reversed-phase liquid chromatography of peptides. Practical considerations

The options in the implementation of gradient theory for optimization work are critically reviewed and evaluated for the case of the reversed-phase liquid chromatography of peptides. Various models are covered together with methods for the determination of model parameters. Approaches for calculating retention times and band widths from experimental data are discussed. Different kinds of extrapolation are compared with interpolation. This study was aimed at finding the best compromise between number of experiments, accuracy of predictions and simplicity of calculations. Implementation and the use of gradient predictions can be simple, and practical recommendations are given.

Effect of stationary phase solvation on shape selectivity in reversed-phase high-performance liquid chromatography

The effect of stationary phase solvation on reversed-phase chromatographic shape selectivity has been investigated using n-hexanol as an additive to methanol-water mobile phases. A wide range of mobile phase compositions was evaluated to normalize for solvent strength selectivity differences. Monomeric C18 stationary phases of both high and low bonding density were synthesized and used to correlate selectivity changes caused by stationary phase ordering with those seen by the addition of n-hexanol. The temperature dependence of retention and selectivity was also investigated using Van 't Hoff plots, which provided insight into the nature of selectivity behavior for estrogens and polyaromatic hydrocarbons. The results showed that using n-hexanol as a mobile phase additive did not provide higher shape selectivity, suggesting that changes in the solvation of the stationary phase did not impart a significant change in the level of surface ordering or morphology. However, n-hexanol did impart solvent selectivity changes in the separation of estrogen diastereomers that could prove useful in future methods development schemes.

Accurate determination of log k'w in reversed-phase liquid chromatography. Implications for quantitative structure-retention relationships

With increased understanding of the retention mechanisms of reversed-phase LC has come increased usage of the technique for the measurement of physico-chemical data, especially partitioning information that can be used for quantitative structure-activity relationships. However, the use of chromatographic retention requires that a standard set of mobile phase conditions be chosen. The choice of 100% water has theoretical advantages, as an aqueous phase-membrane phase is the most common system being modeled. However, experimental measurement of k' values with this mobile phase is difficult or impossible for most real solutes. Various retention extrapolation methods to 100% water have been proposed, but when compared, often yield different values for the same solute. Most of the extrapolation methods are based on the retention as a function of the mobile phase only. However, as the retention is controlled by solute partitioning between the mobile phase and stationary phase, stationary phase effects cannot be ignored. In this paper log k'w values extrapolated from different methods are compared to the measured values. Prediction of log k'w is attempted from the retention as a function of both the mobile phase and stationary phase. Solvatochromic analysis is used to deconvolute stationary and mobile phase effects. Log k'w values extrapolated from ET(30) plots are recommended as the most meaningful representation of retention for quantitative structure-retention relationships.

Computer simulation for the prediction of separation as a function of pH for reversed-phase high-performance liquid chromatography. II. Resolution as a function of simultaneous change in pH and solvent strength

The optimization of reversed-phase high-performance liquid chromatographic separation by the simultaneous variation of pH and solvent strength (%B) was studied for acidic (substituted benzoic acids) and basic samples (substituted anilines). The combination of these two variables was expected to be more useful than either variable alone. This proved to be the case for the benzoic acid sample, but not for the aniline sample. Column plate numbers were also studied for each sample and as a function of pH. With the exception of one compound (3,5-dimethylaniline) in one particular pH range (3.0-4.5), plate numbers of 12,000-20,000 were observed for each sample.

Dependence of retention on the organic modifier concentration and multicomponent adsorption behavior in reversed-phase chromatography

A three-parameter equation is derived to express the dependence of the logarithmic retention factor, kappa, on the volume fraction of the retention modulator, phi, in a binary eluent (such as the organic modifier in the hydro-organic eluents used in reversed-phase chromatography). It is based on the competitive binary adsorption isotherm of the eluite and the modulator generated by employing the ideal adsorbed solution (IAS) method. The equation is found to describe adequately the trends in the kappa-phi relationship experimentally observed in reversed-phase systems. Furthermore, the expression affords an estimation of the single-component adsorption isotherm of the eluite from the corresponding kappa versus phi plot and thus provides a simple means to gather data of importance in the design of separations by non-linear chromatography. For instance, the method can be used to determine whether a pair of eluite isotherms cross one another, a situation that could lead to difficulties in preparative separations. The inherent limitations of the IAS approach may restrict the usefulness of the expression in specific cases. Nevertheless, the approach presented here establishes an explicit, thermodynamically consistent link between the eluite-modulator multicomponent isotherm and corresponding plots and allows a rational description of the generally observed retention behavior in reversed-phase chromatography. The results of this work also illustrate the limitations of the competitive Langmuir isotherm, which is most frequently used to treat competitive adsorption, in the study of the kappa-phi relationship specifically and in investigating and modeling non-linear chromatography at large.

The use of a computer to select optimized conditions for high-performance liquid chromatography separation

Computer simulation allows the convenient prediction and optimization of HPLC separation as a function of various separation conditions. The use of retention and bandwidth relationships that have been validated for a broad range of chromatographic systems minimizes the number of experimental runs needed, especially for the new technique of restricted multi-parameter optimization. The chromatographer is free to use these procedures in a trial-and-error mode, or alternatively use can be made of resolution maps and other data summaries. "Gridding" experiments, based on the automated collection of chromatographic data, can be used to supplement predictions obtained from computer simulation.