While this introduces the potential for some inaccuracies, including the elimination of the smallest particles
(we observed the loss of some particles of less than 20 nm axial length), it avoids the loss of large particles as well as the greater error introduced by the addition of debris such as salt or organic agglomerates (which can be up to 10 nm) or large agglomerateswhich have accumulated organic materials. Observed particleswere nearly always spherical (see Fig. 3), so the diameter was calculated based on the assumption that the threshold-area represented the largest circular cross-section
of a particle.