Influence of column radial heterogeneity on peak fronting in linear chromatography.

Using numerical calculations of elution peak profiles, an explanation of the fronting behavior of elution peaks in linear chromatography was found in certain radial distributions of the mobile phase flow velocity and local bed efficiency. Fronting peaks are observed only if the flow velocity is higher in the wall region than in the center part of the column and the local efficiency is lower near the wall than in the center. By contrast, tailing or symmetrical peaks are observed if only the flow velocity or the local efficiency are radially heterogeneous. The degree of peak fronting increases with increasing amplitude of the radial distributions. The influence of the radial heterogeneity of the flow velocity on the degree of peak fronting is more severe for high than for low efficiency columns. An equation is suggested to correlate peak fronting behavior for columns of different efficiencies and a procedure proposed for the estimation of the radial distributions of the flow velocity and the local efficiency by analyzing some characteristics of asymmetric peaks.     

Axial and radial diffusion coefficients in a liquid chromatography column and bed heterogeneity

The axial and transverse diffusion coefficients of a band of iodine in a chromatographic column were measured optically as a function of time. It was found that the axial diffusion coefficient remains constant even when the edges of the sample band get close to the wall. By contrast, the radial diffusion coefficient decreases progressively with increasing time when the edges of the sample band leave the core region and begin to diffuse inside the wall region. The local axial and transverse diffusion coefficients of the band decrease from the column center toward the wall. Hence, the increase in local height equivalent to a theoretical plate observed in the region close to the wall must be explained by increasing mass transfer resistances and degree of heterogeneity of the bed.     

Structural radial heterogeneity of a silica-based wide-bore monolithic column

The radial distribution of the main characteristics (elution time and standard deviation) of the elution profiles of a flat injected band recorded at the exit of a monolithic column were determined. These distributions provide the radial distributions of the average mobile phase velocity, the elution time and the maximum height of the peak of an analyte, the column efficiency and the analyte concentration. The band profiles were measured at the exit of a 10-mm i.d., 100-mm long silica-based monolithic column. An on-column local electrochemical amperometric detector allowed the recording of the elution profiles at different spatial positions throughout the column cross-section. The local spatial distribution of the mobile phase velocity does not follow a piston-flow behavior but exhibits radial heterogeneity. The local efficiency near the wall is lower than that near the column center. The radial distribution of the maximum concentration of the peaks varies throughout the column exit section, partially due to the radial variations of the column efficiency. These results might explain the rather large value of the A term of the Van Deemter or the Knox equations reported previously for monolithic columns.     

线性非理想条件下液相色谱柱末端峰形规律

 色谱流出曲线的二阶中心矩μ２和三阶中心矩μ３以及描述峰形非对称程度的偏态系数∑ｋ＝μ３／μ１．５２是反映色谱峰形的重要参数。从液相色谱过程动力学方程出发，运用电子计算机证明了在线性非理想条件下高效液相色谱体系中不同保留值组分在柱末端峰形的分布基本一致的结论。     

Peak symmetry as a measure of column performance in high-performance liquid chromatography

Summary During a column-packing method development study, 114 reverse phase and 40 silica gel columns were packed. A relationship between peak symmetry and column efficiency was observed which was found to be useful when evaluating column performance.     

Characteristics of elution profile in radial chromatography under linear conditions

Radial chromatography is one of the most efficient tools for the fast analytical and preparative separation of complex samples. Compared to conventional axial liquid chromatography, it has obvious advantage espe- cially in the preparation of biological sa…     

Peak spreading in linear gradient elution chromatography with a thin monolithic disk

The peak spreading of DNAs of various sizes [12-mer, 20-mer, 50-mer and 95-mer poly(T)] in linear gradient elution (LGE) chromatography with a thin monolithic disk was investigated by using our method developed for determining HETP in LGE. Electrostatic interaction-based chromatography mode (ion-exchange chromatography, IEC) was used. Polymer-based monolithic disks of two different sizes (12 mm diameter, 3mm thickness and 0.34 mL; 5.2 mm diameter, 4.95 mm thickness and 0.105 mL) having anion-exchange groups were employed. For comparison, a 15-μm porous bead IEC column (Resource Q, 6.4mm diameter, 30 mm height and 0.97 mL) was also used. The peak width did not change with the flow velocity for the monolithic disks where as it became wider with increasing velocity. For the monolithic disks the peak width normalized with the column bed volume was well-correlated with the distribution coefficient at the peak position K(R). HETP values were constant (ca. 0.003-0.005 cm) when K(R)>5. Much higher HETP values which are flow-rate dependent were obtained for the porous bead chromatography. It is possible to obtain 50-100 plates for the 3mm monolithic disk. This results in very sharp elution peaks (standard deviation/bed volume=0.15) even for stepwise elution chromatography, where the peak width is similar to that for LGE of a very steep gradient slope.     

Peak capacity in unidimensional chromatography

The currently existing knowledge about peak capacity in unidimensional separations is reviewed. The majority of the paper is dedicated to reversed-phase gradient chromatography, covering specific techniques as well as the subject of peak compression. Other sections deal with peak capacity in isocratic chromatography, size-exclusion chromatography and ion-exchange chromatography. An important topic is the limitation of the separation power and the meaning of the concept of peak capacity for real applications.     

Peak area determination and energy dependence of gamma- and X-ray peak tailing

Two analytical shape functions representing the shapes of - and characteristic X-ray photopeak sides obtained with a Ge(Li) detector are described. The photopeak centroid is accurately determined by using these functions. Energy dependence of photopeak tailing at the lower energy side of the photopeaks are obtained by the least-squares fitting method. The present method can be used for the analysis of partly overlapped photopeaks, one side of which is well resolved, with a sufficient accuracy.     

Peak doubling in preparative liquid chromatography

Summary Peak doublets in LC are ascribed to longitudinal column zones of different permeability leading to a double front in the column.     

Monolithic column in gas chromatography

Monolithic columns invented in chromatographic praxis almost 40 years ago gained nowadays a lot of popularity in separations by liquid chromatographic technique. At the same time, application of monolithic columns in gas chromatography is less common and only a single review published by Svec et al. [1] covers this field of research. Since that time a lot of new findings on application and properties of monolithic columns in gas chromatography have been published in the literature deserving consideration and discussion. This review considers preparation of monolithic columns for GC, an impact of preparation conditions on column performance, optimization of separation conditions for GC analysis on monolithic columns and other important aspects of preparation and usage of monolithic capillary columns in GC. A final part of the review discusses the modern trends and possible applications in the future of capillary monolithic columns in GC.     

Surface heterogeneity and surface area from linear inverse gas chromatography. Application to glass fibers

A new method of measuring surface heterogeneity of non-porous adsorbents by inverse gas chromatography (IGC) is developed. In contrast with the methods described in the literature which are based on nonlinear chromatography (thermodynamics of adsorption) this method is based on linear chromatography (kinetics of adsorption). The mass balance equation of linear chromatography with axial diffusion term and the Langmuir kinetic equation on an open (non-porous) heterogeneous surface are solved by the method of Laplace transforms. The expressions for four semi-invariants of an elution profile are obtained. These are linear combinations of the moments of the distribution of sites in residence time of adsorbed molecule with coefficients determined from elution profile of non-adsorbing gas. Four semi-invariants of elution profiles of benzene and methane on a column packed with E-glass fiber were determined experimentally at temperatures around 100 degrees C. The mean, the standard deviation of the normal distribution in adsorption energy as well as the sticking coefficient and pre-exponential factor of the Frenkel equation are determined for benzene on E-glass. A method of the specific surface determination from linear IGC is proposed.     

Peak compression in semi-preparative supercritical fluid chromatography

Summary Peak compression of a dihydropyridine drug, clevidipine, is obtained in both analytical and semi-preparative scale supercritical fluid chromatography, resulting in extremely high apparent efficiencies. The observed effect, when utilising a carbon dioxide/2-propanol mobile phase with a bare silica stationary phase, is achieved when the retention of the clevidipine peak is controlled to coalesce with a system peak, generated as a result of having water in the sample. Apparent efficiencies of 350,000 and 170,000 plates meter⁻¹ were obtained when 0.25 and 0.5 mg, respectively, are directly injected to a 200×4.6 mm ID 5 μm Hypersil silica packed column. The effect was extended to a semi-preparative system where apparent efficiencies in the region of 2,000,000 plates meter⁻¹ were observed when 0.3 mg of a clevidipine sample containing 80% water was injected to a 250×10 mm I.D. column containing 5-μm Hypersil silica particles.