Singularly Narrow Viral Size and Mobility Standards from the 38.3 nm Chronic Bee Paralysis Virus and Its 17.5 nm Satellite

How Narrow Is the Gas Phase Mobility Distribution of Enveloped Viruses? The Case of the Φ6 Bacteriophage

We use the Φ6 bacteriophage previously exploited as a BSL-1 surrogate of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome (MERS) coronavirus to obtain the first high-resolution gas phase mobility spectra of an enveloped virus. The relative full width at half-maximum found for the viral mobility distribution (FWHM_Z < 3.7%) is substantially narrower than that reported by prior mobility or microscopy studies with other enveloped viruses. It is nevertheless not as narrow as that recently found for several non-enveloped viruses (FWHM_Z ≈ 2%), presumably due to particle to particle variability of enveloped viruses. This 3.7% is an upper bound to the actual width. Nevertheless, the well-defined mobility peaks obtained indicate that gas phase mobility analysis is a more discriminating methodology than that previously demonstrated for physically based non-genetic viral diagnostic of enveloped viruses. These results are obtained by analysis of the original cell culture medium containing the virus, purified only by passage through a 0.22 μm filter and by dialysis into a 10 mM aqueous ammonium acetate buffer. We confirmed that this buffer exchange preserves infectivity. Therefore, the 63.7 nm mobility diameter found, although smaller than the 75 nm previously inferred by microscopy, corresponds to the full particle including the envelope.

Heterogeneous nucleation measurements in a sheathed planar diffusive condensation particle counter

HYPOTHESIS

While the lack of efficient tools yielding controllable uniform saturation ratios (S) has delayed basic experimental heterogeneous nucleation studies, common diffusive condensation particle counters (DCPCs) could fill this gap if their S-variation were minimized by increasing the proportion of sheath gas (σ) surrounding a central core of purified clusters.

ANALYSIS

We measure the activation probability P of Tetraheptylammonium Bromide cluster cations (THA-Br)_n-1THA⁺ in Kanomax's fast CPC while controlling S through the saturator and condenser temperatures (T_s, T_c), varying σ, and changing the size (n) of purified salt clusters via high resolution mobility selection.

FINDINGS

Experimental curves P(T_s,n) obtained in 1-butanol/air at fixed T_c (13 °C) and variable n and T_s (3 ≤ n ≤ 16; 30 ≤ T_s ≤ 40 °C) rise sharply versus both n and T_s. Their steepness increases five-fold with increasing σ to about σ = 75%, with little effect thereafter. Measurements changing S would yield size distributions of unknown aerosols at fairly high resolution. Comparing P(T_s,n) data with predictions from capillary theory suggests that basic heterogeneous nucleation measurements can be carried out, but instrument improvements are still needed.

Exceeding a resolving power of 50 for virus size determination by differential mobility analysis

A recently described DMA designed for high resolution viral particle analysis (Perez-DMA; Perez-Lorenzo et al, 2020) is modified to decrease the relative peak full width at half maximum (FWHM) below previously achieved ≈3.3%. The electrode radii at the outlet slit (R ₁  = 1.01 cm; R ₂  = 2 cm) and the working length are almost unchanged (L = 114.9 vs. 116 mm). The laminarization trumpet and the radius of the curve merging the trumpet to the working section are both considerably widened to improve gas flow laminarization. DMA evaluation with salt clusters is improved by reducing the flow resistance at the gas outlet, to reach substantially larger sheath gas flow rates Q near 1700 L/min. Tests with tetraheptylammonium bromide clusters with a center rod diverging at 3° demonstrate FWHM<2.7%, without indications of performance loss due to turbulence even at 1700 L/min. Correcting these high flow rate data for diffusive broadening reveals a maximal DMA FWHM in the limit of non-diffusing particles and zero sample flow, FWHM_∞ = 1.8%. An uncorrected peak width approaching 2% is independently demonstrated at much lower flow rates of sheath gas with two recently described bee virus particle standards having singularly narrow size distributions at mean diameters of 38 and 17 nm. Correcting raw 38 nm particle peak widths for broadening due to diffusion and aerosol to sheath gas flow rate ratio q/Q shows an even more ideal response with FWHM_∞<1%, where this value includes nonidealities in the DMA as well as possible lack of monodispersity in the viral particles.