Low-picomolar analysis of peptides by on-line coupling of fritless solid-phase extraction to sheathless capillary electrophoresis-mass spectrometry

A novel fritless solid-phase extraction (SPE) microcartridge was designed for combination with sheathless capillary electrophoresis-mass spectrometry (sheathless CE-MS) employing a prototype porous-tip capillary for nanoelectrospray ionization (nanoESI). The inlet of the separation capillary (30 μm inner diameter (id), 150 μm outer diameter (od)) was inserted in a 4 mm long SPE microcartridge (150 μm id, 365 μm od) packed with a C18 sorbent of 55-105 μm particle size. Performance of the SPE-CE-MS system was evaluated using diluted solutions of the three opioid peptides dynorphin A (1-7) (DynA), endomorphin 1 (End1) and met-enkephalin (Met). Sample volumes of 1.5 μL were loaded on the SPE microcartridge and the retained peptides were eluted with 22 nL of an acidic methanol/water (60:40, v/v) solution. Using a pressure of 50 mbar during separation to speed up the analysis, good peptide resolution was obtained with acceptable plate numbers (between 53,000 and 92,000). Intraday relative standard deviations (% RSD) for peptide migration times and peak areas were below 4% and 9%, respectively. The SPE-CE-MS method showed good linearity in the 0.05-5 ng mL−1 range and limits of detection (LODs) were 10 pg mL−1. However, loading a larger volume of sample (8 μL), LODs could be decreased down to 2 pg mL−1 (2.2-3.5 pM). This represents an improvement of up to 5000-fold with respect to the LODs achieved by sheathless CE-MS without on-line preconcentration demonstrating the potential of on-line SPE for further enhancing sensitivity.