Enhancing the coverage of the urinary metabolome by sheathless capillary electrophoresis-mass spectrometry

Sheathless capillary electrophoresis-mass spectrometry (CE-MS), using a porous tip sprayer, is proposed for the first time for highly sensitive metabolic profiling of human urine. A representative metabolite mixture and human urine were used for evaluation of the sheathless CE-MS platform. For test compounds, relative standard deviations (RSDs) for migration times and peak areas were below 2% and 12%, respectively, and an injection volume of only ∼8 nL resulted in detection limits between 11 and 120 nM. Approximately 900 molecular features were detected in human urine by sheathless CE-MS whereas about 300 molecular features were found with classical sheath-liquid CE-MS. This difference can probably be attributed to an improved ionization efficiency and increased sensitivity at low flow-rate conditions. The integration of transient-isotachophoresis (t-ITP) as an in-capillary preconcentration procedure in sheathless CE-MS further resulted in subnanomolar limits of detection for compounds of the metabolite mixture, and more than 1300 molecular features were observed in urine. Compared to the classical CE-MS approaches, the integration of t-ITP combined with the use of a sheathless interface provides up to 2 orders of magnitude sensitivity improvement. Hence, sheathless CE-MS can be used for in-depth metabolic profiling of biological samples, and we anticipate that this approach will yield unique information in the field of metabolomics.