Electrophoresis-By-Accident in Electrospray Ion Source.

Preparing samples for analysis by mass spectrometry (MS) can be laborious and challenging. Hyphenated separation tools are convenient but costly and require much attention. Here, we present a facile low-cost online MS method for analysis of complex samples. It relies on a standard unmodified electrospray ionization (ESI) source, which incorporates polyether ether ketone tubing connected to a grounded metal union or diverter valve. The sample is infused by a syringe pump, and the flow is stopped. During the stopped-flow stage, ionic species are spontaneously separated due to the electric field between the ESI capillary and the grounded element. The flow is then restarted, and the separated species pass to the ion source. The signals of analytes are enhanced with respect to the signals obtained during direct infusion. This enhancement is attributed to the microseparation of analyte and matrix ions occurring in the sample flow line section. The signal change factors ranged from 0.74 to 295. The reported intrinsic property of ESI─operated in a run-stop-run sequence─has broad applicability in analysis of metabolites, peptides, and proteins in complex matrices. Given the tens of thousands of ESI-MS systems used worldwide, this fundamental finding could have broad applicability.