Thermal Gradients for the Control of Elution in RP-LC: Application to the Separation of Model Drugs

The use of temperature as a variable in liquid chromatography enables the facile alteration of eluotropic strength without the need to change solvent composition. The ability to change eluotropic strength via temperature alone means that thermal gradients can be used to mimic the effects of solvent gradients but without many of the unwanted effects of changes in solvent composition. Here we illustrate the use of thermal gradients as a means of controlling chromatographic separations using either constant flow or, with the flow rate increased to maintain isobaric conditions, constant pressure, performed using columns packed with 1.7 μm particles. A model is described that can be used to used to predict flow, pressure and temperature under gradient conditions. Practical experimental factors such as the need for post column cooling and the use of frit restrictors in order to obtain optimum results are described.

     

Application of Iso-Selective Gradient Elution for the Separation of Selected Phthalates

Abstract

In this paper, the overlapping resolution mapping scheme was applied to the gradient HPLC separation of selected phthalates. The procedure employs a minimum of seven experiments for the determination of the optimum mobile phase gradient for the desired separation. In the present work, solvent systems for iso-selective multisolvent gradient elution were used. The overlapping resolution mapping procedure was found to be a rapid and systematic approach for optimizing HPLC separations.     

Gradient Elution Centrifugal Partition Chromatography: Comparison with HPLC Gradients and use of Ternary Diagrams to Build Gradients

Abstract

Just as with HPLC, gradient elution provides to CPC an easy way to fractionate solutes of widely differing polarities and partition coefficients, and to reduce run times. The easiest way to design a gradient for CPC is to refer to ternary diagrams corresponding to the ternary liquid mixtures generally encountered in centrifugal partition chromatography.     

Application of the Solvatic Model for Prediction of Retention in RP-LC for Multi-Step Gradient Profiles

Application of the solvatic retention model of reversed-phase liquid chromatography was studied to predict retention of phenylisothiocyanate derivatives of amino acids from structural formulae and stationary and mobile phase properties. The gradient elution mode with methanol and acetonitrile aqueous mobile phases was used. It was shown that practically acceptable prediction or retention time values can be achieved after the first approximation step when experimental data of one run are used. The zero approximation level predictions—from structural formulae, column and mobile phase properties can be used as a “first guess” method from which further optimization can begin.

     

A Comparative Study of Elution Gradients in UPLC-TOF-MS-Based Metabonomics Research

Serum and urine samples from patients with type 2 diabetes mellitus and control samples were analyzed by UPLC-TOF-MS; fast and slow separation gradients were compared using both positive and negative ionization modes. The resulting data were analyzed using partial least squares discriminant analysis (PLS-DA), and models were developed to differentiate between patient and control samples. The models were evaluated using external test sets to classify their predictive ability. Under both fast and slow gradient conditions, the PLS-DA models generated using serum samples were more robust than those generated using urine samples, and the positive ionization mode produced better differentiation and higher classification rates than negative ionization mode. In addition, fast gradient conditions were found to have a comparable ability for differentiation to slow gradient conditions.     

离子色谱线性梯度淋洗分离阴离子的计算机模拟

建立了使用氢氧化钠淋洗液时用计算机模拟离子色谱梯度淋洗分离阴离子的新方法,研究了在线性梯度淋洗过程中不同时间阴离子在色谱柱中的位置及所在位置的淋洗液浓度,得出阴离子在不同时间的容量因子、迁移速度,通过积分得到保留时间。再根据离子色谱峰峰形变化的规律,得到色谱峰峰形的参数。模拟色谱图保留值与实验值的相对误差小于5%,模拟色谱图的色谱峰峰形与实验所得到的色谱图的色谱峰峰形也非常接近。     

Application of Thermal Analysis in The Characterization of Anti-hypertensive Drugs

The present work reports studies of the thermal behaviour of some anti-hypertensive drugs. Their purities were determined by DSC and specialized pharmacopeial methods. The activation energy values suggest the following sequence of satability: nifedipine>propanolol hydrochloride>captoril. Analysis of the DSC data indicated that the degrees of purity of nifedipine, captopril and propanolol hydrochloride were similar to those found by pharmacopeial methods BP 93 and USP 23. The simplicity, speed and low operational costs of thermal analysis justify its application in the quality control of pharmaceutical drugs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combined Effect of Temperature and Ternary Mobile Phase Composition on the Retention in Ternary Isocratic and Gradient Elution RP-LC under Isothermal Conditions. Application to the Retention Prediction of Four Macrolide Antibiotics

Retention models considering simultaneously ternary mobile phase organic contents and column temperature (T) were developed by a direct combination of equations expressing separately the dependence of the retention upon each of these factors. Thus, a combination of a linear dependence of the logarithm of the solute retention factor, ln k (T), against 1/T, i.e. ln k (T) = a + b/T, and of either a linear or a quadratic dependence of ln k upon the compositions of a binary mobile phase, gave two different expressions for the logarithm of the solute retention factor in terms of both temperature and organic contents in the ternary mobile phase. The effectiveness of the above models was tested in the prediction of isothermal retention of a mixture of four macrolide antibiotics under ternary isocratic and gradient elution conditions using mobile phases modified by methanol and acetonitrile. The limiting case of using ternary eluents with constant ratio of the concentrations of the two organic modifiers was also tested.     

Chromatographic Techniques for the Separation of Peptides: Application to Proteomics

The greatest challenge for proteomics is the inherently complex nature of cellular proteomes as they are highly dynamic entities. High performance liquid chromatography is an indispensable tool in proteomics research, providing high-speed, high sensitivity separation and good resolution of proteins and peptides. Chromatographic sciences have played an animated, bustling and critical role in many fields, the next challenging analytical project for the chromatographic scientists is in the area of proteomics. Which type of analysis best determines the optimal separation technique for any proteomic study? The aim of this review is to outline the different chromatographic strategies that have been employed for analysis of complex mixtures of proteins/peptides, highlighting the role of liquid chromatography coupled to mass spectrometry.     

Comparative Chiral Separation of Thalidomide Class of Drugs Using Polysaccharide-Type Stationary Phases with Emphasis on Elution Order and Hysteresis in Polar Organic Mode

The enantioseparation of four phthalimide derivatives (thalidomide, pomalidomide, lenalidomide and apremilast) was investigated on five different polysaccharide-type stationary phases (Chiralpak AD, Chiralpak AS, Lux Amylose-2, Chiralcel OD and Chiralcel OJ-H) using neat methanol (MeOH), ethanol (EtOH), 1-propanol (PROP), 2-propanol (IPA) and acetonitrile (ACN) as polar organic mobile phases and also in combination. Along with the separation capacity of the applied systems, our study also focuses on the elution sequences, the effect of mobile phase mixtures and the hysteresis of retention and selectivity. Although on several cases extremely high resolutions (R_s > 10) were observed for certain compounds, among the tested conditions only Chiralcel OJ-H column with MeOH was successful for baseline-separation of all investigated drugs. Chiral selector- and mobile-phase-dependent reversals of elution order were observed. Reversal of elution order and hysteresis of retention and enantioselectivity were further investigated using different eluent mixtures on Chiralpak AD, Chiralcel OD and Lux Amylose-2 column. In an IPA/MeOH mixture, enantiomer elution-order reversal was observed depending on the eluent composition. Furthermore, in eluent mixtures, enantioselectivity depends on the direction from which the composition of the eluent is approached, regardless of the eluent pair used on amylose-based columns. Using a mixture of polar alcohols not only the selectivities but the enantiomer elution order can also be fine-tuned on Chiralpak AD column, which opens up the possibility of a new type of chiral screening strategy.     

Separation of Enantiomers and Control of Elution Order of β-Lactams by GC Using Cyclodextrin-Based Chiral Stationary Phases

Enantiomers of 19 racemic β-lactams, with 3 and 4-position substitutions, were separated using gas chromatography. Excellent results were achieved on derivatized cyclodextrin-based GC chiral stationary phases (CSPs). All 19 compounds were baseline separated, most with high resolution factors. The Chiraldex G-TA was found to be the most powerful CSP with the broadest enantioselectivity, while Chiraldex B-DM produced the fastest separations for most of the compounds assayed. Results obtained in this work suggest that GC can serve as a potential method for the enantiomeric separation of sufficiently volatile solid β-lactams.     

Mutual Separation of Rare Earths in Monazite with Cation Exchange Elution Chromatography

Mutual separation of the individual rare earth elements (exception of cerium) in monazite from different districts was investigated by cation exchange elution method. Strong acid type cation exchange resin, Bio-Rad AG 50 Wx8 and eluting solution of a α-hydroxyisobutyric acid (α-HIBA) were used. Radioactivity tagged Eu-152, 154 or Tb-160 were used as radio active indicator for determination of the distribution coefficients by batch method or for the study of column elution conditions. By gradiently increase of the pH values from 3 to 5 in 0.3 M α-HIBA eluting solution, complete mutual separation of individual rare earth elements, exception of Dy and Y, were obtained. Dy and Y could not be separated by this scheme of separation and their elution zones were overlapped. Rare earth mixture samples of monazite from different districts were separated with this scheme and these results were compared. From this comparison followings were noticed; 1. Compositions of rare earth elements in monazite from different districts are evidently not alike. 2. Samples from Brazil and Southwestern Coast of Taiwan are much more alike in their compositions but not for those from Australia and Outskirt Island. 3. Sample from Outskirt Island has higher in contents of heavier rare earths and also Nd was higher than La.     

Thermal Gradients in TMDSC Samples: A comparison of theory and experimental data

Until recently, the issue of the thermal gradients within TMDSC samples remained mostly a subject of theory and mathematical models — only the phase lag was subject to experimental verification, as this information is readily available from the analysis software of most instruments. There was no method to verify the transient behaviour and temperature gradients within a sample without making costly and intensive modifications to the equipment. Recently, however, a group of researchers were able to experimentally measure thermal profiles as a function of sample thickness with a high-speed, high-resolution infrared camera mounted on the TMDSC cell. Therefore, this paper is dedicated to comparing the predictions of our three-dimensional model with this newly available data.This revised version was published online in November 2005 with corrections to the Cover Date.     

Coulometric Generation of Gradient Elution Programs: Separation of Amino Acids Without Buffer Mixing

Abstract

A coulometric cell and buffer system is reported which allows control of the pH of the mobile phase by control of the electrolysis current passed through the cell. The electrolysis cell is capable of varying the pH over the range of 2.5 – 12 without gas evolution at flow rates up to 1 ml/min. The pH is flow rate dependent in the configuration reported, but appropriate pH monitoring and electronic feedback can eliminate that problem. The use of coulometric pH control for the separation of amino acids without any buffer mixing is illustrated.     

Gradients for two-component quasirelativistic methods. Application to dihalogenides of element 116

The authors report the implementation of geometry gradients for quasirelativistic two-component Hartree-Fock and density functional methods using either the zero-order regular approximation Hamiltonian or spin-dependent effective core potentials. The computational effort of the resulting program is comparable to that of corresponding nonrelativistic calculations, as it is dominated by the evaluation of derivative two-electron integrals, which is the same for both types of calculations. Besides the implementation of derivatives of matrix elements of the one-particle Hamiltonian with respect to nuclear displacements, the calculation of the derivative exchange-correlation energy for the open shell case involves complicated expressions because of the noncollinear approach chosen to define the spin density. A pilot application to dihalogenides of element 116 shows how spin-orbit coupling strongly affects the chemistry of the superheavy p-block elements. While these molecules are bent at a scalar-relativistic level, spin-orbit coupling is so strong that only the 7p3/2 atomic orbitals of element 116 are involved in bonding, which favors linear molecular geometries for dihalogenides with heavy terminal halogen atoms.