The Use of Ultrashort Monolithic Porous Columns and an Eluent Flow Gradient for Chromatographic Separation of Polyaromatic Hydrocarbons

Abstract

An important task of state-of-the-art high performance liquid chromatography (HPLC) is to decrease the time of analysis. There exist several approaches to solving this task. The first one implies the use of short (3 − 5 cm long) columns packed with particles 1.5-3 µ m in diameter [1]. These tiny particles ensure an effective mass transfer and rather high column efficiency. Complete separation of five inorganic anions requires 2 min at a flow rate of 1 mL/min. However, the high pressure at the column inlet precludes the possibility of increasing the eluent flow rate and thus of reducing additionally the time of analysis. For separation at higher flow rates, it is necessary to raise the column temperature. The pressure at the column inlet is more than 10 MPa at a flow rate of 2.5 mL/min and a temperature of 45 ° ë .To induce chromatographic peaks in the chromatogram to move toward one another and, hence, to reduce the time of analysis, a concentration gradient is created for the mobile phase component having a higher elution ability. The concentration gradient is formed by means of a special system of mixing eluents and eluent degassing, which substantially complicates the design and increases the cost of a chromatograph.An important alternative is an eluent flow gradient, which allows one to optimize the separation using one pump with flow rate programming. An advantage of a flow gradient over a concentration gradient is elimination of the need to equilibrate the chromatographic system after each separation [2]. Unfortunately, for chromatographic columns packed with microspherical particles, the use of a flow gradient is restricted by the high flow resistance, which markedly narrows the range of variation of flow rates for these columns.A possible solution of the problem is the use of monolithic porous columns. These columns have two types of pores, namely, through (transport) pores that ensure a low pressure at the column inlet and mesopores that form a developed surface with a large area and ensure a high load capacity [3]. This structure allows one to reduce the time of chromatographic analysis five-to sixfold with respect to the classical columns packed with microspherical particles. Indeed, a 10-cmlong monolithic porous column 4.6 mm in diameter allows the separation of five β -blockers in 1 min at an eluent flow rate of 9 mL/min, the resolution of the column being retained at a level as high as that at a flow rate of 1 mL/min [2]. Using a monolithic porous column, one can further reduce the time of chromatographic analysis by decreasing the length of the column and by using simultaneously two gradient elution modes, i.e., both the concentration and the flow gradients [4].Analytical practice is often faced with tasks for which the efficiency of a conventional chromatographic column, 5-10 cm long, is excessive. The advent of