No abstract

The continuous flow gradient and its effect on chromatographic parameters were investigated for the separations of inorganic anions on a monolithic porous disk with bonded hydroxyproline residues.The general definition of high-performance liquid chromatography assumes a maximum number of resolved chromatographic peaks in the shortest possible time. There are two main approaches to high-speed ion-chromatography (IC). One is the use of short columns (3-5 cm) packed with fine particles 1.5-3.0 µm in diameter. 1 A reasonable efficiency can be achieved with such columns in a short time, but a high backpressure does not allow a significant increase of the flow-rate of the eluent and hence a further decrease of the analysis time.Another possibility for achieving fast IC consists in the use of monoliths or continuous stationary phases. Monoliths have through pores, which result in a low backpressure, and mesopores providing a large surface area. The pores are highly interconnected and form a network of channels, 2 which allow a very large increase in flow-rate with little loss in efficiency, thus drastically reducing the time of analysis. For a monolithic column 5 cm in length with an internal diameter of 4.6 mm, the flow-rate can be increased up to 16 ml min -1 providing the separation of a mixture of anions in a minute. 3 Of course, gradient elution can also be used to reduce the time of chromatographic analysis. A rarely used alternative approach of flow programming has very limited applications in HPLC with packed columns because of a sharp increase of the backpressure, which limits the flow-rate to a relatively narrow range. 4,5 Note that a flow gradient can often be used only in tandem with a temperature gradient, 6,7 which diminishes the growth of backpressure.Schafer et al. 8 considered flow-gradient elution on monolithic silica-based Chromolith (Merck) columns with the use of a sharp increase in the flow rate to speed up the elution of one or two strongly retained peaks, which are widely separated from others on an isocratic chromatogram. The programming of flow-gradient elution for separation on monolithic columns could be more efficient. We studied the separations of inorganic anions on a chemically modified monolithic column with programmed flow gradients during the entire run.We used a Shimadzu LC-10AT pump (Germany) with programmed eluent flow rate, a Rheodyne 7125 injection valve (USA) and CD-510 conductivity detector (Aquilon, Moscow, Russia). The CIM ® (Convective Interaction Media) disk monolithic column consisted of a CIM ® Epoxy disk (thickness, 3 mm; diameter, 16 mm; active bed volume, 0.34 ml; macropore diameter, 1500 nm; mesopore diameter, 100 nm 9 ) and a CIM ® housing (BIA Separations, Slovenia).L-Hydroxyproline (Sigma) (pK 1 = 1.92; pK 2 = 9.73) was chosen as an ion-exchange functionality because of the tertiary nature of the amino group in the bonded state, which provides anion-exchange properties and high ion-exchange selectivity. 10,11 The attachment of L-hydroxyproline to the disk was perfo...

No abstract

One of the main tasks of analytical high-performance liquid chromatography (HPLC) is the resolution of the maximum number of chromatographic peaks within the minimum time. If the difference between the retention of the components is small, commonly the composition of the mobile phase can be optimized, ensuring a reasonable rate of analysis and the resolution of chromatographic peaks under isocratic conditions. However, in some cases the analyzed mixture can contain compounds that differ widely in retention, significantly increasing the time of analysis.To bring the chromatographic peaks closer to one another in the chromatogram and, correspondingly, to decrease the time of analysis, the concentration gradient of the mobile phase component having the higher eluting ability is used. The concentration gradient is formed using two high-pressure liquid pumps and a system for mixing and degassing the eluent, which substantially complicates the design of the chromatograph and increases its cost.A possible alternative is the gradient of the eluent flow rate, which makes it possible to optimize the separation process using one pump with a programmed flow rate. Another advantage of the flow-rate gradient over the concentration gradient is that the equilibration of the system is not required after each separation [1]. However, this technique is unsuitable practically for conventional packed chromatographic columns because an increase in the eluent flow rate is accompanied by a drastic increase in the pressure at the inlet of the column, which restricts the range of possible flow rates [2,3]. In some cases, an increase in pressure due to the creation of a flow-rate gradient in packed columns can be compensated for by the temperature gradient in the chromatographic column [4,5], in order to decrease the viscosity of the mobile phase and the pressure at the inlet of the column.Separation with the gradient of the eluent flow rate can be performed on monolithic porous columns due to their structure and unique hydrodynamic characteristics. Monolithic columns involve pores of two types. The macropores of the monolith are linked and form a channel network; mesopores form a developed surface. This structure provides efficient mass transfer and separation at a high flow rate of the eluent within a short time. A monolithic column with a length of 10 cm and a diameter of 4.6 mm allows the separation of five β -blockers within 1 min at a flow rate of the eluent of 9 mL/min, and the resolution of the column remains as high at a flow rate of 1 mL/min [1]. The use of a single increase in the flow rate of the eluent (a step gradient of the flow rate) from 2 to 5 mL/min for decreasing the elution time of strongly retained peaks on a Chromolith monolithic column (Merck) has been reported in the literature [1, 6].The optimization of the chromatographic separation is an important problem in the analysis of mixtures of the homologues of organic compounds, in particular, polycyclic aromatic hydrocarbons and di-n-alkyl phthalates. Carcinogenic and...

Significant deviations from classical van Deemter behaviour, indicative of turbulent flow liquid chromatography, has been recorded for mobile phases of varying viscosity on porous silica monolithic columns at elevated mobile phase flow rates.