A microchip emitter for solid phase extraction – gradient elution – mass spectrometry
A microchip electrospray emitter with a magnetic bead trap has been designed for solid phase extraction-gradient elution-mass spectrometry (SPE-GEMS). The goal of this method is the detection of analytes at low concentrations and it is here demonstrated using reverse phase coated magnetic beads (Mbs) for the preconcentration and detection of the peptides. The sample is passed through the chip, and the peptides are retained and enriched in the trap. After washing, the peptides are released sequentially by stepwise gradient elution and electrosprayed for mass spectrometry analysis. This approach allows effective sample desalting, enrichment, sequential elution and MS detection without the introduction of an additional separation step after SPE. Efficient preconcentration of model peptides by SPE and sequential release and analysis of peptides by GEMS were demonstrated for diluted sample solutions within the range of 1 µM to 10 nM. Fortified human blood serum, protein digest and fractions collected after protein digest OFFGEL separation were analysed by SPE-GEMS allowing the detection of low abundance peptides usually not observed by direct mass spectrometry analysis. A mathematical model for gradient elution is proposed.