Prediction (3 b) is shown in Fig. 8a for the same value C = 1.55 previously inferred from cluster studies, and for two values of FWHM ∞ . This quantity was associated in the prior cluster **data** with any possible non-ideal broadening mechanism from geometrical or flow imperfections in the DMA. This was then justified because the cluster standards are believed to be strictly monomobile. The same has never been established for any viral standard, so that now FWHM ∞ combines nonidealities in the response of the instrument with the perhaps finite width of the mobility distribution of the viral particles. It is noteworthy that, once the diffusive and q/Q corrections Fig. 6. Comparison of the measured DMA dimensionless shape factors K(α) [filled circles, including the theoretical (0,1) value] and the approximate prediction (1c) for slowly varying shapes. have been included in the theory, it fits the **data** with a value of FWHM ∞ approximately half-way between 0% and 1%. In other words, even if the DMA had a perfectly ideal response, we can state that the intrinsic width of the size distribution of this virus is about 0.5%. Alternatively, even if the virus was perfectly monodisperse, the resolving power of the DMA for non-diffusing particles at q/Q→0 would be about 200! These inferences are based on the linear-left criterion to determine the resolution. The choice is justified because our goal is to gauge the imperfections on either the viral size distribution or the DMA response, rather than the evident imperfections in the electrospraying process responsible for the asymmetry of the peaks. We conclude from the analysis of the **data** for the CBPV particle that both the DMA and the particle standard are closer to the ideal than could have been anticipated.