A field evaluation of versatile aerosol concentration enrichment system (VACES) coupled to a rapid single-particle mass spectrometer (RSMS-3) was conducted as part of the U.S. Environmental Protection Agency Supersite program in Pittsburgh during March 2002. RSMS-3 hit rate increases were measured, and possible particle composition changes introduced by the VACES were examined in the single-particle mass spectra. The hit rates increased by 5--20 times at particle sizes ranging from 40 to 640 nm. VACES only enhances the hit rate by about a factor of 2 for large particle sizes because the RSMS-3 flow rates for these particles did not match the optimum operating condition of VACES. During the 3 days of measurements most of the particles were a mixture of carbonaceous material and ammonium nitrate with a variation across the spectrum from particles that were mostly carbonaceous to particles that were mostly ammonium nitrate. Both ambient and concentrated carbonaceous and ammonium nitrate composition distributions were indistinguishable with RSMS-3, suggesting that VACES introduces an insignificant artifact for those particles.