Bionanoparticles as Candidate Reference Materials for Mobility Analysis of Nanoparticles

Condensable Vapor Sorption by Low Charge State Protein Ions

Measurement of the gas-phase ion mobility of proteins provides a means to quantitatively assess the relative sizes of charged proteins. However, protein ion mobility measurements are typically singular values. Here, we apply tandem mobility analysis to low charge state protein ions (+1 and +2 ions) introduced into the gas phase by nanodroplet nebulization. We first determine protein ion mobilities in dry air and subsequently examine shifts in mobilities brought about by the clustering of vapor molecules. Tandem mobility analysis yields mobility-vapor concentration curves for each protein ion, expanding the information obtained from mobility analysis. This experimental procedure and analysis is extended to bovine serum albumin, transferrin, immunoglobulin G, and apoferritin with water, 1-butanol, and nonane. All protein ions appear to adsorb vapor molecules, with mobility "diameter" shifts of up to 6-7% at conditions just below vapor saturation. We parametrize results using κ-Köhler theory, where the term κ quantifies the extent of uptake beyond Köhler model expectations. For 1-butanol and nonane, κ decreases with increasing protein ion size, while it increases with increasing protein ion size for water. For the systems probed, the extent of mobility shift for the organic vapors is unaffected by the nebulized solution pH, while shifts with water are sensitive to pH.

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.

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.

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

The chronic bee paralysis virus (CBPV), extracted from sick or dead bees, was studied by mobility measurements via electrospray charge reduction with a differential mobility analyzer (DMA) of unusually high resolution. Three different particles are observed. The most abundant one contributes a mobility peak at 38.3 nm, approximately as expected for CBPV. The peak is very sharp in spite of the nonisometric nature of CBPV. We also observe a previously unreported weaker well-resolved shoulder 4.8% more mobile, perhaps due to empty (genome-free) particles. Another sharp peak appearing at approximately 17.51 nm is likely associated with the known icosahedral CBPV satellite (CBPVS). The 17.51 and 38.3 nm peaks offer size and mobility standards much narrower than previously reported at any size above 5 nm, with relative full peak width at half-maximum (FWHM) in mobility approaching 2% (∼1% in diameter). Slight but clear imperfections in the DMA response and the electrospraying process suggest that the real width of the viral mobility distribution is less than 2%.

Virus Size Analysis by Gas-Phase Mobility Measurements: Resolution Limits

Electrospraying (ES) dissolved viral particles, followed by charge reduction and size analysis with a differential mobility analyzer (DMA), offers a flexible size-analysis tool for small particles in solution. The technique relies on pioneering work by Kaufman and colleagues, commercialized by TSI, and often referred to as GEMMA. However, viral studies with TSI's GEMMA have suffered from limited resolving power, possibly because of imperfections in either the instrument (DMA or charge reduction) or the sample solution preparation. Here, we explore the limits of the resolution achievable by GEMMA, taking advantage of (i) cleaner charge reduction methods and (ii) DMAs of higher resolving power. Analysis of the literature provides indications that mobility peak widths (fwhm) of 2% or less may be achieved by combining careful sample preparation with improved instrumentation. Working with purified PP7 bacteriophage particles small enough to be classifiable by existing high-resolution DMAs, we confirm that fairly narrow viral mobility peaks may be obtained (relative full width at half-maximum fwhm <5%). Comparison of spectra of a given apian virus sample obtained with TSI's GEMMA and our improved instrumentation confirms that one critical limitation is the DMA. This is further verified by narrow peaks from murine parvovirus, norovirus, and encephalomyelitis virus samples, obtained in our improved GEMMA with little sample preparation, directly from infected cell cultures. Classification of purified large (60 nm) coliphage PR772 particles leads to broad peaks, due to both viral degradation and limited intrinsic resolution of the DMAs used to cover the range of such large particles. We conclude that improved DMAs suitable for high-resolution analysis of particles larger than 30 nm need to be developed to determine the intrinsic mobility width of viral particles.

Mobility Analysis of Proteins by Charge Reduction in a Bipolar Electrospray Source

Analysis of large electrosprayed biopolymers by electrical mobility alone is greatly facilitated by reducing their charge to unity. Here, we combine within a single chamber positive aqueous electrospray (ES), producing multiply charged protein cations, with negative methanolic ES, yielding small singly charged anions. Use of a 100 mM triethylammonium formate buffer in both solutions yields very small drops. The two sprays are decoupled electrostatically by an interposed 50% transparent, grounded metallic grid. This screen is readily crossed by the ions, resulting in substantial charge reduction. In spite of the grid, the aqueous spray is easily destabilized by the presence of anions in the positive ES region. Nonetheless, practical ES stabilization is achieved by using relatively small capillary tips (∼15 μm) in the positive emitter. Protein peaks obtained are as narrow as those previously reported via charge reduction with a radioactive Ni-63 source. Controlling the position of the negative ES permits spanning the full range of charge states, from high natural values to predominantly singly charged ions, even for large proteins such as immunoglobulin G (∼150 kDa).