Electrospray-Condensation Particle Counter: A Molecule-Counting LC Detector for Macromolecules

A new detector for macromolecular separations is described. The detector counts individual macromolecules (molecular weights greater than about 10,000) and reports counts per second. The chromatographic effluent is electrosprayed, neutralized, and swept to the detector by a stream of air. The detector is a condensation particle counter that detects individual particles by light scattering from droplets condensed on the particles. When used as the detector for a size exclusion separation of proteins, the detector has a linear range of 4 orders of magnitude with detection limits as low as 0.1 microgram/mL. The detector can be directly interfaced (no makeup flow) with effluent flows as low as 10 nL/min. A Monte Carlo model based on size measurements of the electrosprayed droplets correctly predicts the observed detector behavior.