Capillary LC-MS for high sensitivity metabolomic analysis of single islets of Langerhans

Reversed-phase, packed capillary liquid chromatography interfaced by electrospray ionization to mass spectrometry was explored as an analytical method for determination of metabolites in microscale tissue samples using single islets of Langerhans as a model system. With the use of a 75 microm inner diameter column coupled to a quadrupole ion trap mass spectrometer in full scan mode, detection limits of 0.1-33 fmol were achieved for glycoloytic and tricarboxylic acid cycle metabolites. Reproducible processing of islets for analysis with little loss of metabolites was performed by rapid freezing followed by methanol-water extraction. The method yielded 20 microL of extract of which just 15 nL was injected suggesting the potential for performing multiple assays on the same islet. Approximately 200 presumed metabolites could be detected, of which 22 were identified by matching retention times and MS/MS spectra to standards. Relative standard deviations for peak detection was from 7 to 18% and was unaffected by storage for up to 11 days. The method was used to detect changes in metabolism associated with increasing extracellular islet glucose concentration from 3 to 20 mM yielding results largely consistent with known metabolism of islets. Because most previous studies of islet metabolism have only observed a few compounds at once and require far more tissue, this measurement method represents a significant advance for studies of metabolism of islets and other microscale samples.