Michel Attoui

The mobility-volume relationship below 3.0 nm examined by tandem mobility-mass measurement

The validity of the Stokes-Millikan equation is examined in light of mass and mobility measurements of clusters of the ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMI-BF4) in ambient air. The mobility diameter dZ based on the measured mobility and the Stokes-Millikan law is compared with the volume diameter dv , which generalizes the mass diameter for binary substances such as salts. dv is based on the sum of anion and cation volumes in the cluster corrected for the void fraction of the bulk ionic liquid. For dv > 1.5 nm, d Z is within 1.4% of dv + 0.3 nm. For smaller clusters 3.84 and 14.3% deviations are observed at dv = 1.21 nm and 0.68 nm, respectively. These differences are smaller than expected due to a cancellation of competing effects. The increasing difference seen for dv < 1.5 nm is due primarily to the interaction between the cluster and the dipole it induces in the gas molecules. Other potential sources of disagreement are non-globular cluster geometries, and departures of the cluster void fraction from the bulk value. These two effects are examined via molecular dynamics simulations, which confirm that the volume diameter concept is accurate for EMI-BF4 nanodrops with dv as small as 1.6 nm.

Electrical Mobility Spectrometer Using a Diethylene Glycol Condensation Particle Counter for Measurement of Aerosol Size Distributions Down to 1 nm

We report a new scanning mobility particle spectrometer (SMPS) for measuring number size distributions of particles down to ∼1 nm mobility diameter. This SMPS includes an aerosol charger, a TSI 3085 nano differential mobility analyzer (nanoDMA), an ultrafine condensation particle counter (UCPC) using diethylene glycol (DEG) as the working fluid, and a conventional butanol CPC (the “booster”) to detect the small droplets leaving the DEG UCPC. The response of the DEG UCPC to negatively charged sodium chloride particles with mobility diameters ranging from 1–6 nm was measured. The sensitivity of the DEG UCPC to particle composition was also studied by comparing its response to positively charged 1.47 and 1.70 nm tetra-alkyl ammonium ions, sodium chloride, and silver particles. A high resolution differential mobility analyzer was used to generate the test particles. These results show that the response of this UCPC to sub-2 nm particles is sensitive to particle composition. The applicability of the new SMPS for atmospheric measurement was demonstrated during the Nucleation and Cloud Condensation Nuclei (NCCN) field campaign (Atlanta, Georgia, summer 2009). We operated the instrument at saturator and condenser temperatures that allowed the efficient detection of sodium chloride particles but not of air ions having the same mobility. We found that particles as small as 1 nm were detected during nucleation events but not at other times. Factors affecting size distribution measurements, including aerosol charging in the 1–10 nm size range, are discussed. For the charger used in this study, bipolar charging was found to be more effective for sub-2 nm particles than unipolar charging. No ion induced nucleation inside the charger was observed during the NCCN campaign.

Laboratory Verification of PH-CPC's Ability to Monitor Atmospheric Sub-3 nm Clusters

Gas-to-particle conversion takes readily place in the atmosphere. Detecting the initial clusters, which act as embryos for the newly formed particles, is beyond traditional aerosol instrumentation. Charged atmospheric clusters can be measured with air ion spectrometers, but typical state-of-the-art condensation particle counters, which detect both neutral and charged clusters, only see particles larger than 2.5 nm in diameter. In this study we present a modified pulse-height condensation particle counter (PH-CPC) and confirm by laboratory verification that it is capable of detecting charged clusters with electrical mobility equivalent diameter down to ∼1 nm. We show how the detection efficiency and the pulse heights depend on the calibration particle size, polarity and composition. The effect of butanol supersaturation on the PH-CPC counting efficiency is also discussed. Furthermore, we developed an inversion method for the data to obtain true particle size distribution from the measurement signal.

Transfer Functions and Penetrations of Five Differential Mobility Analyzers for Sub-2 nm Particle Classification

The transfer functions and penetrations of five differential mobility analyzers (DMAs) for sub-2 nm particle classification were evaluated in this study. These DMAs include the TSI nanoDMA, the Caltech radial DMA (RDMA) and nanoRDMA, the Grimm nanoDMA, and the Karlsruhe-Vienna DMA. Measurements were done using tetra-alkyl ammonium ion standards with mobility diameters of 1.16, 1.47, and 1.70 nm. These monomobile ions were generated by electrospray followed by high resolution mobility classification. Measurements were focused at an aerosol-to-sheath flow ratio of 0.1. A data inversion routine was developed to obtain the true transfer function for each test DMA, and these measured transfer functions were compared with theory. DMA penetration efficiencies were also measured. An approximate model for diffusional deposition, based on the modified Gormley and Kennedy equation using an effective length, is given for each test DMA. These results quantitatively characterize the performance of the test DMAs in classifying sub-2 nm particles and can be readily used for DMA data inversion.

Remarks on Ion Generation for CPC Detection Efficiency Studies in Sub-3-nm Size Range

To calibrate a newly developed condensation particle counter, samples of known chemical composition are needed as the chemistry plays a role in the activation process. For that, we have built a calibration setup and produced ammonium sulfate, sodium chloride, tungsten oxide, silver, alkyl halide, and ionic liquid clusters down to 1 nm in mobility diameter in positive and negative mode. The chemical composition of most negatively charged clusters was solved using high-resolution mass spectrometer and we identified about 70% of the total signal of the mass spectrometer. For the Airmodus Particle Size Magnifier, which was the instrument to be calibrated, we measured cutoff diameters of 1.1, 1.3, 1.4, 1.6, and 1.6–1.8 nm for negative sodium chloride, ammonium sulfate, tungsten oxide, silver, and positive organics, respectively. From the alkyl halide and ionic liquid experiments, we concluded that the composition plays a bigger role than the charge state of the cluster in the activation process. We also showed that relative humidity of the sample flow can change the detection efficiency of the Particle Size Magnifier, which adds some uncertainties to the measured number concentrations. Copyright 2013 American Association for Aerosol Research

A Medium Flow, High-Resolution Vienna DMA Running in Recirculating Mode

In this work we present the latest design of the Vienna type Differential Mobility Analyzer (DMA). The so-called UDMA is specially designed for high-resolution mobility measurements for the particle size range between 1 and 5 nm and has been optimized to be combined with a mass spectrometer to provide a well-defined mobility fraction of molecular clusters. As the UDMA is designed for this narrow size range it features high-resolution mobility measurements already at flow rates of approximately 700 L/min, marked as medium flow conditions. This readily allows operating the UDMA in a recirculating sheath air flow system with gas cleaning filters, as the control over the purity of the sheath air is crucial for operating the UDMA inline with a mass spectrometer or other carrier gas sensible devices. The performance of the newly developed UDMA was tested by the analysis of well-defined molecular clusters produced by an electrospray source and compared with one of the state-of-the-art high resolution DMAs also known as “Attoui DMA.” For the UDMA, a relative FWHH of the transfer function of 2.5% in mobility-size was found for the monomer clusters of Tetraheptylammoniumbromide (THAB) at 1.44 nm mobility equivalent diameter.

Determination of gas phase protein ion densities via ion mobility analysis with charge reduction

We use a charge reduction electrospray (ESI) source and subsequent ion mobility analysis with a differential mobility analyzer (DMA, with detection via both a Faraday cage electrometer and a condensation particle counter) to infer the densities of single and multiprotein ions of cytochrome C, lysozyme, myoglobin, ovalbumin, and bovine serum albumin produced from non-denaturing (20 mM aqueous ammonium acetate) and denaturing (1 : 49.5 : 49.5, formic acid : methanol : water) ESI. Charge reduction is achieved through use of a Po-210 radioactive source, which generates roughly equal concentrations of positive and negative ions. Ions produced by the source collide with and reduce the charge on ESI generated drops, preventing Coulombic fissions, and unlike typical protein ESI, leading to gas-phase protein ions with +1 to +3 excess charges. Therefore, charge reduction serves to effectively mitigate any role that Coulombic stretching may play on the structure of the gas phase ions. Density inference is made via determination of the mobility diameter, and correspondingly the spherical equivalent protein volume. Through this approach it is found that for both non-denaturing and denaturing ESI-generated ions, gas-phase protein ions are relatively compact, with average densities of 0.97 g cm(-3) and 0.86 g cm(-3), respectively. Ions from non-denaturing ESI are found to be slightly more compact than predicted from the protein crystal structures, suggesting that low charge state protein ions in the gas phase are slightly denser than their solution conformations. While a slight difference is detected between the ions produced with non-denaturing and denaturing ESI, the denatured ions are found to be much more dense than those examined previously by drift tube mobility analysis, in which charge reduction was not employed. This indicates that Coulombic stretching is typically what leads to non-compact ions in the gas-phase, and suggests that for gas phase measurements to be correlated to biomolecular structures in solution, low charge state ions should be analyzed. Further, to determine if different solution conditions give rise to ions of different structure, ions of similar charge state should be compared. Non-denatured protein ion densities are found to be in excellent agreement with non-denatured protein ion densities inferred from prior DMA and drift tube measurements made without charge reduction (all ions with densities in the 0.85-1.10 g cm(-3) range), showing that these ions are not strongly influenced by Coulombic stretching nor by analysis method.

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Tandem DMA Generation of Strictly Monomobile 1–3.5 nm Particle Standards

Generation of monomobile molecular standards by electrospray (ES) followed by classification in a differential mobility analyzer (DMA) fails at diameters above ∼2 nm because many clusters in different charge states z crowd in a narrow mobility range. Use of a second DMA (DMA2) in series (tandem) with DMA1 is very helpful because, unexpectedly, many multiply charged ions selected in DMA1 undergo spontaneous transitions, appearing as pure species at different mobilities in DMA2. Remarkably, for salt clusters of composition (CA) n (C+ ) z carrying z elementary charges and n neutral ion pairs, (i) ion evaporation (CA) n (C+ ) z →(CA) n –1(C+ ) z– 1+(CA)C+ and (ii) neutral evaporation transitions (CA) n (C+ ) z →(CA) n –1(C+ ) z+CA affect a substantial fraction of the clusters. Neutral evaporation (fueled by the Kelvin effect) is effective in isolating singly charged clusters, yielding mobility standards easily exceeding 2 nm. Ion evaporation (fueled by large electric fields) produces even larger well-resolved standards. Singly charged clusters of up to 2.5 nm rising in isolation result from metastable doubly charged parent ions (z = 2→1 transition). Isolated doubly charged ions of up to 3.5 nm arise from the z = 3→2 transition, but are harder to resolve from the products of higher initial charge states. We report tandem DMA measurements for electrosprayed nanodrops of two ionic liquids: EMI-Im and EMI-Methide, both based on the small cation EMI+ (1-Ethyl-3-methylimidazolium+) and two relatively large anions: Im− = (CF3SO2)2N−; Methide− = (CF3SO2)3C−. Some exploration on the effect of actively reducing the charge on the clusters as they pass between both analyzers is also included. Copyright 2013 American Association for Aerosol Research

Characterization of a Herrmann-type high-resolution differential mobility analyzer

ABSTRACT Aerosol instrument characterization and verification for nanometer-sized particles requires well-established generation and classification instruments. A precise size selection of sub-3-nm charged aerosol particles requires a differential mobility analyzer (DMA), specially designed for the sub-3-nm size range. In this study, a Herrmann-type high-resolution DMA developed at Yale University was characterized in various operation conditions. A relation between sheath flow rate and tetraheptylammonium ion (C28H60N+, THA+, 1.47 nm, mobility equivalent diameter) was established. The maximum particle size that the DMA was able to classify was 2.9 nm with the highest sheath flow rate of 1427 liters per minute (Lpm), and 6.5 nm with the lowest stable sheath flow rate of 215 Lpm, restricted by the maximum and minimum flow rates provided by our blower. Resolution and transmission of DMA are reported for tetrapropylammonium (C12H28N+, TPA+, 1.16 nm), THA+, and THA2Br+ (1.78 nm) ions measured with two different central electrodes and five different sheath flow rates. The transmission varied between 0.01 and 0.22, and the resolution varied between 10.8 and 51.9, depending on the operation conditions. Copyright