On-line large-volume electroextraction coupled to liquid chromatography-mass spectrometry to improve detection of peptides

Electroextraction (EE) takes place in a two-phase liquid-liquid system, consisting of an aqueous and an organic phase, where an applied electric field causes ions to be extracted from one phase into the other, to be concentrated close after the liquid-liquid interface. The extraction takes place in a wide-bore capillary that is connected to a 2-way 10-port switching valve, which serves to couple capillary EE (cEE) with LC-MS. In this set-up, volumes as high as 100 μL can be extracted, which is a ten times larger volume than has been reported, earlier. After a feasibility study using the cationic purple dye crystal violet, the method was coupled to LC-MS and large volume cEE of several model peptides was optimized. The cEE-LC-MS method had good repeatability, good linearity and LODs between 0.5 and 10nM. The whole procedure was automated and could be used routinely. Finally, the method was applied to plasma analysis and calibration curves of the relevant plasma peptides angiotensin 1 and 2 as well as the fragment angiotensin 2 (3-8) showed good linearity and repeatability; LOD values were 10-50 nM. Analysis of unspiked plasma resulted in 60 putative endogenous peptides, underlining the great potential of EE as on-line sample concentrating technique. On-line large volume cEE-LC-MS allows for enrichment, separation and detection of plasma peptides from large sample volumes, minimizes sample handling and can be an important step in full automation of analytical procedures.