Microchip Liquid-Phase Ion Trap for Online Mass Spectrometry Analysis

Liquid-phase ion trapping (LPIT) was proposed recently for ion manipulations in liquid channels. When coupled with mass spectrometry, LPIT exhibits considerable potential in applications such as target enrichment and bottom-up proteomics. However, further evolution of LPIT techniques requires flexible field designs including electric and fluid fields. In this study, LPIT was constructed and implemented on microfluidic chips. Utilizing conductive polymers, nonlinear potential distribution was achieved in the liquid channel, enabling the focusing of ions at distinct locations based on their effective charges and hydrodynamic radii. The integration of an electrospray ionization source facilitated coupling of the LPIT chip with a mass spectrometer. The working mechanism and parameter optimizations were explored through a combination of theoretical analysis and numerical simulations. Experiments showcased the LIPT chip's proficiency in enrichment and separation capabilities. The detection sensitivity of the following mass spectrometer could be improved by ∼10-fold. A good linearity (R² > 0.99) was obtained for reserpine in the range of 1-100 ng/mL. The separation capability was demonstrated using a mixture of 11 amino acids.