Direct detection of peptides and proteins on a microfluidic platform with MALDI mass spectrometry

The ability to detect and quantify proteins of individual cells in high throughput is of enormous biological and clinical relevance. Most methods currently in use either require the measurement of large cell populations or are limited to the investigation of few cells at a time. In this report, we present the combination of a polydimethylsiloxane-based microfluidic device to a matrix-assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOF-MS) that allows the detection of as few as 300 molecules at the peptide level and ~10(6) to 10(7) molecules at the protein level. Moreover, we performed an immunoassay with subsequent MALDI-TOF-MS to capture and detect insulin immobilized on a surface (~0.05 mm(2)) in this device with a detection limit of 10(6) insulin molecules. This microfluidic-based approach therefore begins to approach the sample handling and sensitivity requirements for MS-based single-cell analysis of proteins and peptides and holds the potential for easy parallelization of immunoassays and other highly sensitive protein analyses.