Accurate Sizing of Nanoparticles Using a High-Throughput Charge Detection Mass Spectrometer without Energy Selection

Abstract

The sizes and shapes of nanoparticles play a critical role in their chemical and material properties. Common sizing methods based on light scattering or mobility lack individual particle specificity, and microscopy-based methods often require cumbersome sample preparation and image analysis. A promising alternative method for the rapid and accurate characterization of nanoparticle size is charge detection mass spectrometry (CDMS), an emerging technique that measures the masses of individual ions. A recently constructed CDMS instrument designed specifically for high acquisition speed, efficiency, and accuracy is described. This instrument does not rely on an ion energy filter or estimates of ion energy that have been previously required for mass determination, but instead uses direct, in situ measurements. A standardized sample of ∼100 nm diameter polystyrene nanoparticles and ∼50 nm polystyrene nanoparticles with aminefunctionalized surfaces are characterized using CDMS and transmission electron microscopy (TEM). Individual nanoparticle masses measured by CDMS are transformed to diameters, and these size distributions are in close agreement with distributions measured by TEM. CDMS analysis also reveals dimerization of ∼100 nm nanoparticles in solution that cannot be determined by TEM due to the tendency of nanoparticles to agglomerate when dried onto a surface. Comparing the acquisition and analysis times of CDMS and TEM shows particle sizing rates up to ∼80× faster are possible using CDMS, even when samples ∼50× more dilute were used. The combination of both high-accuracy individual nanoparticle measurements and fast acquisition rates by CDMS represents an important advance in nanoparticle analysis capabilities.

Dynamic Energy Measurements in Charge Detection Mass Spectrometry Eliminate Adverse Effects of Ion–Ion Interactions

“…However, the increased measurement errors observed for the ions in Figure c,d do not meaningfully affect the mass resolution. The full width at half-maximum of the mass distribution measured for the ∼100 nm diameter nanoparticle sample used here is ∼70 MDa, making the small increase in uncertainty originating from small interferences negligible. In general, the analytes most amenable to CDMS analysis are high mass (1 MDa+) and/or have intrinsically high heterogeneity and therefore have sample-based mass resolution limitations .…”

“…Experiments were performed using custom-built CDMS instruments, ,, and signals were analyzed using methods that have been described in detail elsewhere. ,,,, Briefly, ions are formed by nanoelectrospray ionization using borosilicate emitters with tip diameters of 1.5–5 μm. Ions travel through guiding ion optics and multiple stages of differential pumping until they reach an electrostatic conetrap that contains a cylindrical detector electrode.…”

“…Ions pass through cylindrical or other, similar “enclosing” pickup electrode(s) and induce a current proportional to the ion charge that is not sensitive to ion position or velocity, − enabling higher-precision charge measurements than FTICR or Orbitrap-based methods. State-of-the-art CDMS instruments incorporate this detection electrode between the trapping electrodes of an electrostatic ion trap. − In this configuration, the charges of individual ions can be resolved directly from signal amplitude measurements , with a precision of ±0.2 e demonstrated in a 1.5 s trapping period . The downstream electrode potential of the electrostatic ion trap is briefly lowered to let an ion beam pass through the detection electrode and is increased again to trap ions.…”

Dynamics of Rayleigh Fission Processes in ∼100 nm Charged Aqueous Nanodrops

“…The nanospheres were trapped for 1 s and analyzed using a STFT segment length of 50 ms and a 5 ms overlapping step. Additional instrumental details and a more in depth description of the analysis are given elsewhere …”

“…Standard analysis of CDMS data is described extensively elsewhere. − , As a charged nanodrop passes through the detector cylinder embedded within the electrostatic trap, it induces a current that is converted into a voltage signal by a charge-sensitive preamplifier (Amptek A250 CoolFET, Bedford, MA) inside the vacuum chamber where the electrostatic trap is located. The pulse amplitudes of this signal are proportional to the charge of the nanodrop.…”