Life-time studies with capillary electrochromatography columns operated under different conditions

A test system has been established to permit the monitoring of the life-tim e performance of several reversed- phase capillary electrochromatography (C EC) columns. The retention factors, k(eec), peak symmetry coefficients, lam bda(eec), and column efficiencies, N, of three neutral n-alkylbenzene analy tes, namely ethyl-, N-butyl- and n-pentylbenzenes, were determined for Hype rsil 3 mu m n-octylsilica and n-octadecylsilica packed into CEC capillary c olumns of 100 mu m I.D., with a packed length of 250 mm, and a total length of 335 mm. The performances of these CEC capillary columns were examined f or a variety of eluents with pH values ranging between pH 2.0 - 8.0, simila r to those employed to study the retention behaviour of peptides that we ha ve previously reported. The relative standard deviation (RSD) of the retent ion factors (k(eec) values) of these n-alkylbenzenes, acquired with an elue nt of (25 mM Tris-HCl, pH 8.0,)-acetonitrile (1:4, v/v), when the CEC capil lary columns were used for the first time (virgin values), were 4% (based o n data acquired with 4 CEC capillary columns) for the n-octyl bonded silica capillary columns, and 6% (based on 8 columns) for n-octadecyl bonded sili ca capillary columns. The RSD values of the k(eec) values of the n-alkylben zenes for one set of replicates(n=6) with one CEC capillary column was <0.5 %. The theoretical plate numbers, N, for the virgin CEC capillary columns w ere ca. 60 000, whilst the observed N values for all new CEC capillary colu mns were greater than or equal to 40 000 for n-octyl bonded silica capillar y columns and greater than or equal to 50 000 for n-octadecyl bonded silica capillary columns. The peak symmetry coefficients, lambda(sym), of the n-a lkylbenzenes for virgin CEC capillary columns and for CEC capillary columns used for more than 1000 injections were always in the range 0.95-1.05. The experimental results clearly document that the life-time performance of th e CEC capillary columns depends on the eluent composition, as well as the n ature of the analytes to which the CEC capillary columns are exposed. (C) 2 000 Elsevier Science B.V. All rights reserved.