Quantitative analysis of pharmaceutically active peptides using on-capillary analyte preconcentration transient isotachophoresis

An on-capillary adsorptive phase in combination with capillary electrophore sis (CE), frequently referred to as preconcentration CE, for quantitative a nalysis of low peptide concentrations was developed. The capillary containi ng the on-line analyte preconcentrator can be constructed within 5 min from commercially available extraction disks. These disks contain poly(styrened ivinylbenzene) adsorbent particles incorporated in a matrix of inert Teflon , creating a mechanically stable sorbent. Therefore, no frits are needed in the capillary to hold the stationary phase in place. Several parameters, s uch as the required minimal elution volume, required elution strength, samp le application speed or ionic strength, and the capacity were investigated and special interest was given to the quantitative properties of the method . Instead of nL injections, volumes up to a least 25 mu L are possible, yie lding improvements in detection limits of 3-4 orders of magnitude. The obse rved limit of detection for both model peptides was 20 pg, corresponding to a 20 mu L injection of a 1 ng/ml solution of both model peptides. Using lo w-wavelength UV detection, reproducibility and linearity in the low nanogra m range were satisfactory. No influence of matrix salt concentrations was o bserved, extending the use of CE to all kinds of samples.