We report the performance of a slit virtual impactor at high particle mass concentrations (> 500 mu g/L). Transmission efficiency was determined on an individual particle basis by visualization of fluorescent particles embedded in a background matrix of Arizona road dust. No significant difference in transmission efficiency from dilute conditions was observed for a mass loading of 571 mu g/L. At greater mass loadings (> 1000 mu g/L), the device clogged before a statistically significant data set could be obtained. These results suggest that the performance of a slit virtual impactor is not degraded up to roughly 600 mu g/L, and may be extended to even larger mass loadings provided that clogging of the nozzle is minimized. (c) 2004 Elsevier Ltd. All rights reserved.