Capillary LC-MS2 at the Attomole Level for Monitoring and Discovering Endogenous Peptides in Microdialysis Samples Collected in Vivo

Fused-silica capillary LC columns (25-mu m i.d.) with 3-mu m-i.d. integrated electrospray emitters interfaced to a quadrupole ion trap mass spectrometer were evaluated for highsensitivity LC-MS2. Column preparation involved constructing frits by in situ photopolymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate, preparing the electrospray emitter by pulling the column outlet to a fine tip with a CO2 laser puller, and slurry-packing the column with 5-mum reversed-phase particles. Large-volume injections were facilitated by an automated two-pump system that allowed high-flow rates for sample loading and low-flow rates for elution. Small electrospray emitters, low elution flow rates, and optimization of gradient steepness allowed a detection limit of 4 amol, corresponding to 2 pM for 1.8 mu L injected on-column, for a mixture of peptides dissolved in artificial cerebral spinal fluid. The system was coupled on-line to microdialysis sampling and was used to monitor and discover endogenous neuropeptides from the globus pallidus of anesthetized male Sprague-Dawley rats. Time-segmented MS2 scans enabled simultaneous monitoring of Met-enkephalin, Leu-enkephalin, and unknown peptides. Basal dialysate levels of Met-enkephalin and Leu-enkephalin were 60 ± 30 and 70 ± 20 pM while K+-stimulated levels were 1900 ± 500 and 1300 ± 300 pM, respectively (n = 7). Data-dependent and time-segmented MS2 scans revealed several unknown peptides that were present in dialysate. One of the unknowns was identified as peptide I1-10 (SPQLEDEAKE), a novel product of preproenkephalin A processing, using MS2, MS3, and database searching.