Using Microprobe-Capture In-Emitter Elution to Directly Couple Label-Free Optical Sensing Technology with Mass Spectrometry for Top-Down Protein Analysis

Affinity capture of an analyte by a capture agent is one of the most effective sample preparation approaches for protein analytes. We describe a new affinity capture technique for top-down protein analysis, microprobe-capture in-emitter elution (MPIE), which can directly couple a label-free optical sensing technology (next-generation biolayer interferometry, BLI) with MS. In MPIE, an analyte is first captured on the surface of a microprobe while the capture process was monitored in real-time by BLI, and then eluted from the microprobe inside an electrospray emitter. When electrospray is established from the emitter to a mass spectrometer, the analyte is immediately ionized through electrospray ionization (ESI) for MS analysis. By this means, next-generation BLI and MS are directly coupled in the form of MPIE-ESI-MS. The performance of MPIE-ESI-MS was demonstrated by the analysis of Aβ 1-40 and Tf using both standard samples and human specimens. In comparison to the conventional affinity capture techniques such as bead-based immunoprecipitation, MPIE innovated the affinity capture methodology by introducing real-time process monitoring and providing binding characteristics of analytes, providing more information-rich experimental results. Thus, MPIE is a valuable addition to the TD-MS sample preparation toolbox, and more applications of MPIE-ESI-MS in top-down protein analysis can be expected.