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Fernandez de la Mora J.,Yale University | Perez-Lorenzo L.J.,Yale University | Arranz G.,SEADM | Amo-Gonzalez M.,SEADM | Burtscher H.,University of Applied Sciences and Arts Northwestern Switzerland
Aerosol Science and Technology | Year: 2017

Two fast electrometer circuits (1011 and 1012 V/A) are installed in a Faraday cage having a relatively small residence time. Removing readily distinguishable occasional spikes, the root mean square (r.m.s.) noise level at 1012 V/A is 0.11 fA when acquiring data at 1 Hz. This value is close to the expected thermal resistor noise at room temperature (0.09 mV). Both electrometers exhibit a 20 ms flow-related delay, followed by respective half-height rise-times of ∼4 and 25 ms. Fast high-resolution mobility spectra in the 1–2 nm size range are acquired with electrosprayed tetraheptylammonium ions by combining these electrometers with a high-speed DMA. At 1012 V/A, there is no ion mobility peak distortion when acquiring data with discrete voltage steps and dwelling 100 ms at each voltage. With the 1011 V/A electrometer, the DMA voltage VDMA is continuously swept up and down over 600 V in a triangular wave, at up to 1200 V/s. A shift ΔVDMA in the peak center is apparent, with little peak shape distortion. ΔVDMA is symmetric with respect to up or down sweep, and linear with sweep frequency, corresponding approximately to a pure delay Δt = 25 ms. This peak displacement may be offset by adding the correction ΔVDMA = Δt (dVDMA/dt) to the measured peak voltage. Extrapolating the measurements made here over a mobility range Zmax/Zmin of 4 to a much wider mobility range of 300 typical of aerosol studies, we conclude that almost undistorted high-resolution mobility spectra may be acquired in 1.3 s. Copyright © 2017 American Association for Aerosol Research © 2017 American Association for Aerosol Research

Fernandez de la Mora J.,Yale University | Barrios-Collado C.,SEADM
Aerosol Science and Technology | Year: 2017

The few clusters [B− nA+ n+1]+ (n = 0,1) with resolvable mobilities formed in electrosprays of large salts have been used for nanoparticle instrument testing and calibration at sizes smaller than 2 nm. Extensions of this modest size range by charge reduction with uncontrolled gas phase ions has resulted in impure singly charged clusters. Here, we combine two oppositely charged electrosprays of solutions of the same salt B−A+, including: (CnH2n+1)4N+Br− (n = 4,7,12,16), the large phosphonium cation (C6H13)3(C16H33)P+ paired with the anions Im− [(CF3SO2)2N−] or FAP− [(C2F5)3PF3 −], and the asymmetric pair [1-methyl-3-pentylimidazolium+FAP−]. Both polarities are simultaneously produced by this source in comparable abundances, primarily as singly charged A+ nB− n ± 1, with tiny contributions from higher charge states. Some but not all of these clusters produce narrow mobility peaks typical of pure ions, even beyond n = 43. Excellent independent stable control of the positive and the negative sprays brought very close to each other is achieved by isolating them electrostatically with a symmetrically interposed metallic screen. Two nanoDMAs covering the size range up to 30 nm (Halfmini and Herrmann DMAs, with classification lengths of 2 and 10 cm) are characterized with these standards, revealing resolving powers considerably higher than previously seen with unipolar electrospray sources. The bipolar source of pure and chemically homogeneous clusters described permits studying size and charge effects in a variety of aerosol instruments in the 1–4 nm size range. © 2017 American Association for Aerosol Research

Amo-Gonzalez M.,SEADM | Fernandez de la Mora J.,Yale University
Journal of the American Society for Mass Spectrometry | Year: 2017

The differential mobility analyzer (DMA) is a narrow-band linear ion mobility filter operating at atmospheric pressure. It combines in series with a quadrupole mass spectrometer (Q-MS) for mobility/mass analysis, greatly reducing chemical noise in selected ion monitoring. However, the large flow rate of drift gas (~1000 L/min) required by DMAs complicates the achievement of high gas purity. Additionally, the symmetry of the drying counterflow gas at the interface of many commercial MS instruments, is degraded by the lateral motion of the drift gas at the DMA entrance slit. As a result, DMA mobility peaks often exhibit tails due to the attachment of impurity vapors, either (1) to the reagent ion within the separation cell, or (2) to the analyte of interest in the ionization region. In order to greatly increase the noise-suppression capacity of the DMA, we describe various vapor-removal schemes and measure the resulting increase in the tailing ratio, (TR = signal at the peak maximum over signal two half-widths away from this maximum). Here we develop a low-outgassing DMA circuit connected to a mass spectrometer, and test it with three ionization sources (APCI, Desolvating-nano ESI, and Desolvating low flow SESI). While prior TR values were in the range 100–1000, the three new sources achieve TR ~ 105. The SESI source has been optimized for maximum sensitivity, delivering an unprecedented gain for TNT of 190 counts/fg, equivalent to an ionization efficiency of one out of 140 neutral molecules. © 2017, American Society for Mass Spectrometry.

Rus J.,SEADM | Moro D.,SEADM | Sillero J.A.,SEADM | Sillero J.A.,Yale University | And 5 more authors.
International Journal of Mass Spectrometry | Year: 2010

Recent progress in adding a mobility dimension to preexisting API-MS systems without modifying the MS itself is discussed, based on inserting a differential mobility analyzer (DMA) as part of the MS's atmospheric pressure ion source. Design criteria leading to high DMA resolving power R and transmission efficiency η are discussed. Various DMA prototypes have been interfaced to several triple quadrupoles, a single quadrupole and a quadrupole-TOF, all demonstrating R>50 and η>50%. We obtain two-dimensional DMA-MS spectra of the multiply charged clusters formed in electrosprays of concentrated solutions of tetrahexylammonium bromide (A+Br-). These reveal systematic loss of (ABr)A+ fragments from unstable multiply charged clusters, and provide mobility measurements in air on mass resolved (ABr)n(A+)z clusters with n>100 and z up to 10. Well-defined bands of ions not individually resolved are clearly visible at considerably larger n and z values. © 2010 Elsevier B.V.

Rawat V.K.,University of Minnesota | Vidal-De-Miguel G.,SEADM | Vidal-De-Miguel G.,ETH Zurich | Hogan C.J.,University of Minnesota
Analyst | Year: 2015

Low field ion mobility spectrometry-mass spectrometry (IMS-MS) techniques exhibit low orthogonality, as inverse mobility often scales with mass to charge ratio. This inadequacy can be mitigated by adding vapor dopants, which may cluster with analyte ions and shift their mobilities by amounts independent of both mass and mobility of the ion. It is therefore important to understand the interactions of vapor dopants with ions, to better quantify the extent of dopant facilitated mobility shifts. Here, we develop predictive models of vapor dopant facilitated mobility shifts, and compare model calculations to measurements of mobility shifts for peptide ions exposed to variable gas phase concentrations of isopropanol. Mobility measurements were made at atmospheric pressure and room temperature using a recently developed transversal modulation ion mobility spectrometer (TMIMS). Results are compared to three separate models, wherein mobility shifts due to vapor dopants are attributed to changes in gas composition and (I) no vapor dopant uptake is assumed, (II) site-specific dopant uptake by the ion is assumed (approximated via a Langmuir adsorption model), and (III) site-unspecific dopant uptake by the ion is assumed (approximated via a classical nucleation model). We find that mobility shifts in peptide ions are in excellent agreement with model II, site-specific binding predictions. Conversely, mobility shifts of tetraalkylammonium ions from previous measurements were compared with these models and best agreement was found with model III predictions, i.e. site-unspecific dopant uptake. © The Royal Society of Chemistry.

Martinez-Lozano P.,National Research Council Italy | Rus J.,SEADM
Journal of the American Society for Mass Spectrometry | Year: 2010

Sarcosine, an isomer of L-alanine, has been proposed as a prostate cancer progression biomarker [1]. Both compounds are detected in urine, where the measured sarcosine/alanine ratio has been found to be higher in prostate biopsy-positive group versus controls. We present here preliminary evidence showing that urine samples spiked with sarcosine/alanine can be partially resolved in 3 min via tandem differential mobility analysis-mass spectrometry (DMA-MS). Based on the calibration curves obtained for two mobility peaks, we finally estimate their concentration ratio in urine. © 2010 American Society for Mass Spectrometry.

Vidal-De-Miguel G.,SEADM | De La Mora J.F.,Yale University
Aerosol Science and Technology | Year: 2012

All known multi-stage aerodynamic lenses able to focus sharply a wide range of particle sizes include decelerating regions where the flow becomes turbulent at Reynolds numbers Re typically of 100 or less. Here, we propose a design for a focusing concentrator operating laminarly at Re of many thousands. The particles are accelerated by the gas through a continuously converging ladder of smooth contractions, each designed such that: (1) the flow remains laminar at substantial Re; (2) a certain band of particle sizes is focused in each contraction, without substantial defocusing of larger particle focused in 1 or several preceding contractions; (3) the form, length, and ratio of entry to exit diameter of each contraction, as well as the number of consecutive contractions are chosen such that all particles within a given relatively wide range of sizes are focused at the end of the ladder of contractions into a relatively narrow focal region. The focusing virtual impactor formed by coupling this device to a perforated surface could provide a powerful particle concentrator. © 2012 Copyright Taylor and Francis Group, LLC.

Attoui M.,University Paris Est Creteil | Fernandez-Garcia J.,Yale University | Cuevas J.,SEADM | Vidal-de-Miguel G.,SEADM | De la Mora F.J.,Yale University
Journal of Aerosol Science | Year: 2013

The charge distribution of polystyrene nanoparticles electrosprayed from l-methyl-2-pyrrolidone (NMP) dimethylammonium formate (10/1 vol) is studied by a continuous tandem differential mobility analyzer (CTDMA) technique. Two DMAs operated in series as narrow band mobility filters are scanned in a quasi-continuous fashion providing highly informative two-dimensional (2D) spectra. The first DMA selects naturally charged particles, which are then partially neutralized by a radioactive source and subsequently analyzed in the second DMA. The mobility pairs found in both DMAs finally yield the charge and size distribution. A 1 mM solution of relatively mono-disperse polystyrene with an average mass of 34.5 kDa forms polymer aggregates, including from 1 to 4 single molecules, leading to discrete particle sizes of 4.7, 5.95, 6.80 and 7.5 nm in diameter. Observed charge states are unusually low relative to previously studied water-soluble polymers such as polyethylene glycol. Large particles are originally charged in such a fashion as to give an almost constant mobility (in air) of 0.23 cm2/V/s, implying a critical electric field for ion evaporation from a polystyrene sphere of some 0.7 V/nm. This is the first measurement of this critical field for nonpolar nanoparticles, and results in a value which is three times smaller than those previously measured for polar materials. © 2012 Elsevier Ltd.

Attoui M.,University Paris Est Creteil | Paragano M.,Yale University | Cuevas J.,SEADM | De La Mora J.F.,Yale University
Aerosol Science and Technology | Year: 2013

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. © 2013 American Association for Aerosol Research.

Martinez-Lozano Sinues P.,National Research Council Italy | Martinez-Lozano Sinues P.,ETH Zurich | Criado E.,SEADM | Vidal G.,SEADM
International Journal of Mass Spectrometry | Year: 2012

The fact that electrosprays of pure solvents can efficiently ionize gas-phase analytes has been known for decades, although this method has not been widely exploited. With the advent of ambient mass spectrometry, this approach is becoming increasingly popular. However, the mechanism by which vapors become ionized in the encounter with electrospray plumes remains largely unknown. This has been our motivation in this study in which we have exposed a set of amine vapors towards electrosprays of water, water/methanol (1/1) and methanol. The ionized vapors were characterized via ion mobility-mass spectrometry. We further tested a recently developed model to predict ionization probabilities for the encounter and charge transfer of vapors with ions or charged droplets emerging from an electrospray source. We found: (i) the highest sensitivity with water sprays and the poorest with methanol; (ii) an approximate correlation of sensitivity with vapor mass, being the heaviest species insoluble in water; (iii) different electrical mobility spectra for the same compounds ionized from the liquid phase and from the gas phase (i.e. one main feature for gas-phase and 3 features for liquid-phase); (iv) a closer agreement with the model for ion-molecule reactions than for droplet-vapor charge exchange. We conclude that the analytes could not possibly be dissolved in the droplets to be reemitted as if they were originally present in the electrospray solution. Our observations suggest that the vapors are ionized via ion-molecule reactions. © 2011 Elsevier B.V. All rights reserved.

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