Globular proteins ranging in molecular mass from 5.7 to 669 kDa were separated and analyzed using an aerosol technique based on the electrophoretic mobility of singly-charged molecular ions in air. The ions were produced by electrospraying and drying 100-nm-diameter droplets of a liquid suspension of the proteins, using ionized air to remove the droplet charge due to the spray process. The electrophoretic mobility was measured using a modified commercial continuous-flow differential mobility analyzer operated near atmospheric pressure. An unmodified commercial condensation particle counter was used for detection. The concentrations analyzed ranged from 0.02 to 200 μg of protein/mL of buffer, with a liquid sample flow rate of approximately 50 nL/min. Sampling time of 3 min was used for each complete distribution measured. The electrophoretic mobilities measured were determined entirely from air flow rates, apparatus geometry, and applied potentials. Results were expressed as electrophoretic mobility equivalent diameters using a Millikan formula.

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A scanning mobility particle sizer (SMPS) allows size separation of gas phase particles according to their electrophoretic mobilities. The addition of an electrospray source (ES) recently allowed extension of SMPS analysis to the macromolecular range. We demonstrate here the application of ES-SMPS to nucleic acids analysis. Single- and double-stranded DNA molecules ranging from 6.1 kDa (single-stranded DNA 20 nucleotides in length) to 300 kDa (500 base-pair double-stranded DNA) were separated and detected by ES-SMPS at the picomole to femtomole levels. The measured electrophoretic mobility diameters were found to correlate with the analytes' molecular weights, while the peak areas could yield quantitative information. No fragmentation of DNA was observed under the conditions employed. Different apparent densities were observed for single-stranded and double-stranded DNAs, showing a different behavior for each type of biomolecule. The total analysis time was about 3 min/spectrum. Further optimization of ES-SMPS is expected to make it a fast and sensitive technique for biopolymer characterization.

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.