On-line isotachophoretic sample focusing for loadability enhancement in capillary electrochromatography–mass spectrometry

The use of isotachophoretic (ITP) sample focusing to improve the detection limits for the analysis of charged compounds in capillary electrochromatography (CEC) is described. A coupled-column set-up was used with a 220-μm inner diameter capillary, in which counterflow ITP focusing was performed, connected via a T-junction to a 75-μm inner diameter CEC capillary. As is illustrated, the use of ITP focusing resulted in a dramatic reduction of the sample concentration detection limits. To demonstrate the performance of the ITP–CEC combination, several cationic low-molecular mass compounds in a plasma and urine matrix are analysed using UV-absorbance and mass spectrometric detection. A linear calibration curve was constructed over three decades and detection limits in the low nmol/l range were found for academic samples, using UV-absorbance detection.