Evaluation of fritless solid‐phase extraction coupled on‐line with capillary electrophoresis‐mass spectrometry for the analysis of opioid peptides in cerebrospinal fluid

Fritless SPE on‐line coupled to CE with UV and MS detection (SPE‐CE‐UV and SPE‐CE‐MS) was evaluated for the analysis of opioid peptides. A microcartridge of 150 μm id was packed with a C₁₈ sorbent (particle size > 50 μm), which was retained between a short inlet capillary and a separation capillary (50 μm id). Several experimental parameters were optimized by SPE‐CE‐UV using solutions of dynorphin A (DynA), endomorphin 1 (End1), and methionine‐enkephaline (Met). A microcartridge length of 4 mm was selected, sample was loaded for 10 min at 930 mbar and the retained peptides were eluted with 67 nL of an acidic hydro‐organic solution. Using SPE‐CE‐MS, peak area and migration time repeatabilities for the three opioid peptides were 12–27% and 4–5%, respectively. SPE recovery was lower for the less hydrophobic DynA (22%) than for End1 (66%) and Met (78%) and linearity was satisfactory in all cases between 5 and 60 ng/mL. The LODs varied between 0.5 and 1.0 ng/mL which represent an enhancement of two orders of magnitude when compared with CE‐MS. Cerebrospinal fluid (CSF) samples spiked with the opioid peptides were analyzed to demonstrate the applicability to biological samples. Peak area and migration time repeatabilities were similar to the standard solutions and the opioid peptides could be detected down to 1.0 ng/mL.