Bipolar Charge Analyzer (BOLAR): A new aerosol instrument for bipolar charge measurements

The Bipolar Charge Analyzer (BOLAR, Dekati Ltd.) is a new commercial instrument developed to characterize bipolar charge and charge-to-mass ratio of powders and inhaled aerosols as a function of particle size. The instrument combines aerodynamic size classification with electrostatic precipitator based bipolar charge measurement. As a result bipolar charge and charge-to-mass ratios are obtained for 5 size fractions between zero and 11.6 µm. In this study we present the operation concept, design and the performance characteristics of the BOLAR. The instrument was calibrated using monodisperse particles in laboratory conditions. The performance of the instrument in inhaler measurement was tested by measuring aerosol produced by a dry powder inhaler (DPI). The DPI was found to produce noticeable levels of positively and negatively charged particles. For a single size fraction the largest observed positive and negative values were +300 pC and −200 pC, respectively, although the net charge was close to zero. By analyzing the mass of the particles collected by the individual parts of the BOLAR the size fractioned charge-to-mass ratios were calculated for the tested inhaler. The largest values were approximately ±20 pC/µg which was obtained for particles having a mean diameter of 1.8 μm. The number of elementary charges per particle ranged from ±500 up to ±8000 depending on the particle size.