Aerosol Analysis via Electrostatic Precipitation-Electrospray Ionization

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Aerosol Analysis via Electrostatic PrecipitationElectrospray Ionization Mass Spectrometry (EP-ESI-MS) Siqin He, Lin Li, Hongxu Duan, Amir Naqwi, and Christopher J. Hogan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01183 • Publication Date (Web): 29 May 2015 Downloaded from http://pubs.acs.org on June 4, 2015

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Analytical Chemistry

Aerosol Analysis via Electrostatic Precipitation-Electrospray Ionization Mass Spectrometry (EP-ESI-MS) 1

Siqin He1, Lin Li2, Hongxu Duan2, Amir Naqwi2, and Christopher J. Hogan Jr.1* Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA 2 MSP Corporation, Shoreview, MN, USA

Submitted to:


*To whom correspondence should be addressed: [email protected], Tel: 1-612-626-8312, Fax: 1-612-625-6069

ACS Paragon Plus Environment


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ABSTRACT Electrospray ionization (ESI) is the preferred mode of ion generation for mass analysis of many organic species, as alternative ionization techniques can lead appreciable analyte fragmentation. For this reason, ESI is an ideal method for the analysis of species within aerosol particles. However, because of their low concentrations (~10 µg/m3) in most environments, ESI has been applied sparingly in aerosol particle analysis; aerosol mass spectrometers typically employ analyte volatilization followed by electron ionization or chemical ionization, which can lead to a considerable degree of analyte fragmentation. Here, we describe an approach to apply ESI to submicrometer and nanometer scale aerosol particles, which utilizes unipolar ionization to charge particles, electrostatic precipitation to collect particles on the tip of a Tungsten rod, and subsequently, by flowing liquid over the rod, ESI and mass analysis of the species composing collected particles. This technique, which we term electrostatic precipitation-ESI-MS (EP-ESIMS), is shown to enable analysis of nanogram quantities of collected particles (from aerosol phase concentrations as low as 102 ng m-3) composed of cesium iodide, levoglucosan, and levoglucosan within a carbon nanoparticle matrix. With EP-ESI-MS, the integrated mass spectrometric signals are found to be a monotonic function of the mass concentration of analyte in the aerosol phase. We additionally show that EP-ESI-MS has a dynamic range of close to 5 orders of magnitude in mass, making it suitable for molecular analysis of aerosol particles in laboratory settings with upstream particle size classification, as well as analysis of PM 2.5 particles in ambient air.


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INTRODUCTION Electrospray ionization (ESI) enables mass spectrometric measurement of organic ions and macromolecules1,2 which would otherwise be difficult to ionize in the gas phase without substantial analyte fragmentation during ionization3,4. ESI is particularly well suited for analytes that can be concentrated in small volumes of ESI-suitable solutions2,5, and a number of analytes, particularly those of interest in proteomics and metabolomics6, do fit this criterion. However, there is a class of species, namely, analytes in ambient aerosol particles, for which ESI would be well suited but for which it has been difficult to apply7,8. Ambient particles, typically chemically complex condensed phase entities with characteristic sizes below 2.5 µm (i.e. PM 2.5)9, are of considerable interest due to their potential influences on radiative forcing10,11, visibility12,13, and human health14-16. The chemical composition of such particles is difficult to analyze, as particles persist in the atmosphere at mass concentrations of the order 10 µg/m3 or less in many environments. Nonetheless, particle chemical compositions have been analyzed both in ambient field studies and in simulated laboratory environments via aerosol mass spectrometers (e.g. the commercially available AMS from Aerodyne)17-19; in these instruments particles are deposited (subsequent to aerodynamic focusing) on a surface, the surface is heated to thermally volatilize species which were in particles, and electron ionization (EI) is used to ionize species of interest20. The combination of thermal volatilization and EI prohibits direct examination of the actual molecules composing ambient aerosol particles; fragmentation products are typically observed21. While alternative techniques have been developed (e.g. the thermal desorption chemical ionization mass spectrometer, TDCIMS22-25) utilizing chemical ionization (CI) instead of EI, they still rely on thermal volatilization and hence their use precludes direct measurement of completely unfragmented/unreacted species.



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Commonly applied chemical analyses for species within ambient aerosol particles are conceptually similar to the techniques applied for condensed phase organic species or those in solution prior to the advent of ESI; they involve use of thermal volatilization and either EI or CI, and rarely produce undissociated species17,22,26,27. As organic analysis in solution has benefited from ESI6, so too would ambient aerosol particle analysis. In fact, ESI-MS-MS analysis of the particles produced in chamber experiments of secondary aerosol formation has already yielded a considerable amount of information on the chemical structure of the organic species present in such particles28,29, and both ESI-MS30 and DESI-MS31 (desorption electrospray ionization-mass spectrometry) have been used for analysis of particles collected in field measurements. However, in these instances, ESI-MS could only be performed after collecting particles (submicrometer in size) for long periods of time (hours) onto filters and subsequently extracting particles, i.e. analyses were performed offline.

Several attempts have been made to develop systems

facilitating ESI-like ion production for species within ambient aerosol particles, enabling online analysis without the need for filter collection and extraction. In particular, Grimm et al32 showed that ESI can be performed directly from droplets in the gas–phase (i.e. field induced droplet dissociation), giving rise to ESI type ions directly from aerosol droplets. However, the technique is limited to larger, supermicrometer (>10 µm) liquid droplets, prohibiting its application for measurements of smaller micrometer and submicrometer particles. Peng et al33 demonstrated that proteins, introduced into the gas phase via matrix assisted laser desorption ionization (MALDI), could be uptaken into ESI generated droplets, and subsequently released from droplets as multiply charged ions though mixing an ESI generated droplet plume with a MALDI generated analyte plume (i.e. analytes were incorporated into droplets via droplet-analyte coagulation). Similarly, Shia et al34 showed that biomolecules (aerosolized via either laser


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desorption or by an ultrasonic nebulizer) could be collisionally incorporated into ESI like droplets, resulting in the eventual formation of multiply charged ESI-like biomolecular ions. While these studies clearly demonstrate the capture of aerosol particles by ESI generated droplets and subsequent ionization, unfortunately, these techniques are limited by collision kinetics between ESI droplets and aerosol particles. Calculations of particle collision rates35,36 demonstrate that gas phase based collision approaches place a high lower limit on the number concentration of aerosol particles needed to generate a sufficient number of ions for mass analysis, i.e. though extractive ESI techniques are commonly used for vapor phase species37-39, it is difficult to apply this technique to lower concentration aerosol particles. Recently, Horan et al.8 as well as Gallimore & Kalberer7 demonstrated that extractive ESI can be adapted to aerosol particles by colliding particles not only with ESI generated droplets, but also directly with the Taylor cone of a stably operating electrospray.

Because of the

significantly larger collision length (for diffusive capture) of a Taylor cone as compared to droplets, such systems are a more promising route to the production of ESI-type, unfragmented ions from aerosol particles than either field induced droplet ionization systems or more typical extractive ESI systems. However, the lower limit of detection (in total aerosol mass and in mass gas phase concentration) is still controlled by operating the electrospray continuously, as the concentration of analyte in solution is dependent on the ratio of the particle-Taylor cone collision rate to the ESI flowrate. The concentration limitations of continuous operation can be overcome by collecting particles electrostatically (which is more efficient that diffusion facilitated capture) for a prescribed period of time and subsequently using ESI for collected particles. In this work, we hence describe a modified technique to generate ESI ions from molecules in aerosol particles by devising a two-step collection and ionization scheme (electrostatic precipitation-electrospray



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ionization, EP-ESI). In it, aerosol particles are charged (via either a bipolar or unipolar ion generation source), deposited on a < 20 square millimeter area spot on a metal rod by electrophoretic motion in the gas phase, and after a prescribed amount of collection time, a stable electrospray is formed over the metal rod.

Collected molecules (from aerosol particles)

subsequently dissolve in the flowing liquid and are released as unfragmented ions from ESI generated droplets. Using both polydisperse and monodisperse aerosol particles in the 20 nm – 400 nm size range composed of cesium iodide, levoglucosan (a chemical tracer for biomass burning30,40), and levoglucosan with carbon nanoparticles, we show that EP-ESI can be used to detect nanograms of collected material with a dynamic range of roughly five orders of magnitude in collected mass, making EP-ESI usable for analysis of size-classified particle fractions in laboratory settings, or unclassified PM 2.5 particles sampled in field measurements.

EXPERIMENTAL METHODS System Description A schematic of the prototype EP-ESI ionization and collection system is provided in figure 1a, with a photograph of the collection rod/ESI system in figure 1b. An additional schematic with dimensions is provided in the supporting information. System operation for particle collection, dissolution, ion production, and mass measurement is carried out as follows: first, flow of particle-laden air (an aerosol, which can be mobility classified41 if desired) is pulled or pushed (depending on the desired operation mode) at 0.5-1.0 l min-1 into the ionization chamber (labelled as the “Corona-discharge Unipolar Ionization Chamber” in figure 1a). This chamber is employed because electrostatic deposition requires charged (ionized) aerosol particles, while in the ambient submicrometer and nanoparticles are predominantly neutral42.


In the

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ionization chamber, ions are generated via corona discharge (hence their chemical composition is dependent on the gas composition and tubing material used), and the discharge itself is generated by application of a high voltage (3 kV) to a tungsten 1/16” diameter needle, tapered at its edge for electric field enhancement. While the polarity of the ions generated is controllable, here we use positive ions only, as positive corona discharges are found to be more stable than negative corona discharges (there is less fluctuation in the ion current). Generated ions readily attached to aerosol particles43 and by controlling the flowrate of air through the ionization chamber, particle deposition within it can be minimized while the ionization efficiency (i.e. the fraction of particles leaving the ionizer electrostatically charged) can be optimized. Subsequent to the ionization chamber, the sampled aerosol is passed through 1/32” inner diameter stainless steel nozzle, labelled in figure 1b. The nozzle is grounded; meanwhile a negative voltage (typically -4 kV) is applied to a 1/16” diameter tungsten rod (also labelled in figure 1b as the “collection and spray rod”), facilitating deposition of particles onto the rod. The rod itself is sheathed with a plastic (PEEK) tube, such that only its rounded end is exposed to the aerosol. Therefore all particles are deposited onto a small area (approximately ~0.16 cm2, or 0.024 square inches).

Particle

ionization and collection proceeds for a predefined period (5-60 minutes in the present study), after which aerosol is no longer sampled, and the polarity of the voltage applied to the Tungsten is switched to positive 7 kV. Simultaneously, the rod is positioned close to the inlet of a mass spectrometer (here we use a QSTAR XL mass spectrometer, Applied Biosystems, Waltham, MA, USA with the EP-ESI chamber built via modification of a QSTAR XL IonSpray source) and a flow of liquid solution, controlled via a syringe pump (Harvard apparatus) at 25 µL min-1 is driven over the rod. As the liquid passes over the rod, collected molecules (from aerosol particles) dissolve in it and the high voltage applied relative to the surroundings (the mass



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spectrometer inlet is grounded) leads to the formation of a Taylor cone at the rod tip, shown in figure 1c; hence ESI is carried for collected molecules. The solution used for ESI can be tuned to target the ionization of specific analytes44-46 or to specifically prevent dissolution of species not of interest. The collection of mass spectra with high mass resolving power for generated ions then enables chemical (molecular) identification for collected species, with the temporal evolution of measured signal dependent upon the dissolution of collected molecules into the chosen ESI liquid.

Experimental Investigation To assess the capabilities of an EP-ESI prototype system, it was necessary to (1) characterize the performance of the unipolar ionizer employed as a function of particle size (diameter), (2) determine the deposition efficiency of particles onto the deposition rod, and (3) correlate time-integrated signal intensities with the measured mass of analyte deposited for variable deposition times as well as both polydisperse and monodisperse particles.

The

experimental details for (1) and (2) are provided in the supporting information, as are the results for (1). For (3), the EP-ESI chamber was attached to the QSTAR XL mass spectrometer to examine electrospray ionization of material deposited on the collection rod.

Polydisperse

particles were either sent directly into the EP-ESI chamber or were first sent into a differential mobility analyzer (DMA, model 3081, TSI Inc., Shoreview, MN) to examine monodisperse particles (the operating principle of a DMA is discussed in greater detail in the supporting information). In both instances particles were sent through the ionization chamber, operated with 0.5 µA current at 0.5 l min-1, which was also the deposition flowrate. These parameters were selected based on the results of ionization efficiency and deposition experiments. Three


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different types of test particles were used. First, CsI particles were generated with a Collision nebulizer (with aqueous CsI) and silica bead diffusion dryer. Second, levoglucosan (Sigma Aldrich) particles were produced via Collison nebulization and diffusion drying of an aqueous levoglucosan solution (3-10 mM). Again, both polydisperse and DMA-selected monodisperse particles were examined.

Finally, 3 mM aqueous levoglucosan was mixed with carbon

nanoparticles (Sigma Aldrich, < 500 nm), and the resulting suspension was nebulized and dried. In this final instance, polydisperse particles were examined, and the DMA was used to selected monodisperse particles with a mean diameter of 80 nm only. Particle collection in the EP-ESI system proceeded for 5-60 minutes. After the desired collection time, the EP-ESI chamber was sealed from the particle source, and the polarity of the collection rod was switched from negative to positive (to 7.2 kV) and it was repositioned near the mass spectrometer inlet.

To facilitate ESI, the cylindrical sheath tube surrounding the

collection rod was connected to a solvent feed with the pumping rate precisely controlled by a syringe pump (Model NE-1000, New Era Pump Systems Inc., Farmingdale, NY). In separate experiments, the solution was 1mM tetrahepytlammonium bromide in methanol (THABr), and the mass spectrometer was used to detect the THA+ ion. The position of the collection rod during ESI was optimized by maximizing the THA+ signal intensity with fixed mass spectrometer settings. For aerosol particle measurements, the solvent composition was selected to target specific analytes within the deposited particles; for Cs+ and (CsI)nCs+ ions47 within CsI particles, 1 M acetic acid in methanol was used, while for levoglucosan 10 mM NaCl in 95:5 methanol:water was employed. The latter was shown previously to lead to the production of the levogluson-Na+ ion in ESI30. In all instances, the solvent flowrate was 25 µL/min, necessary to maintain a stable electrospray over the collection rod (which is larger than most capillaries



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employed traditional ESI), and with liquid flowrates substantially smaller or larger we were not able to generate a stable Taylor cone. Between measurements, the collection rod was cleaned manually via washing with methanol and lint-free cotton tips.

RESULTS & DISCUSSION Ionized Particle Deposition Efficiency An important feature of EP-ESI is that it discriminates against vapor phase analytes by directing only ionized particles to the collection rod. As described in the supporting information, we examined the deposition efficiency (i.e. the ratio of the number particles deposited on the rod to the number incident on it) of both singly and multiply and charged particles, with results displayed in figures 2a & 2b. Specifically, the fraction of particles deposited onto the collection rod (i.e. the deposition efficiency) is plotted as a function of size for both singly and multiply charged particles in figure 2a with an applied voltage of -4 kV to the collection rod. Singly charged particles were obtained directly from the outlet of a DMA, while multiply charged were obtained by passing DMA selected through the ionization chamber at 0.5 l min-1 and with 0.5 µA of current. Therefore, multiply charged particle results apply for particles with size dependent charge distributions43, including some particles which may have remained singly charged. The results for singly and multiply charged particles are similar for particle diameters below 40 nm, indicating that the smallest particles examined did not attain multiple charges in the ionization chamber.

Particle deposition on the collection appears to be completely due to particle

electrophoretic motion; the deposition efficiency for singly charged particles is plotted as a function of the product of particle electrical mobility (Zp, defined in the supporting information) and the voltage applied (V) to the collection rod in figure 2b. The product of these two


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parameters is directly proportional to particle electrophoretic velocity (for a fixed nozzle to rod distance), and as deposition efficiency is directly proportional this product, it too is found to be directly proportional to the electrophoretic velocity. In total, deposition efficiencies vary from 20% for the largest, lowest mobility particles to upwards of 100% for high mobility particles. Although through alternative designs this efficiency could be improved upon (for example, by increasing the voltage applied to the collection rod, though corona discharge should be avoided), we show subsequently that when operating under the presented conditions, EP-ESI-MS enables analysis of nanograms of collected mass.

EP-ESI Measurements Representative log10 based size distribution functions (dN/dlogDp, in particles per cm3, which integrated in a particular log diameter range to yield the number concentration of particles in the specified range9) for CsI particles (20 mM in water), levoglucosan (3 mM in water), and levoglucosan mixed carbon nanoparticles (3 mM levoglucson with 10 mM on an elemental carbon basis), all produced by nebulization, are shown in figures 3a-c. These distributions were measured using a DMA and CPC (condensation particle counter) in series as a scanning electrical mobility spectrometer48.

The mode diameters in distributions are noted; all

distributions were highly polydisperse, which is commonly found during aerosol production with Collison nebulizers. Also displayed on plots are approximate mobility classification “windows” for particles transmitted through a DMA when set to transmit particles with the noted mobility equivalent diameter (i.e. spherical, singly charged particles). By use or omission of the DMA, we were able to challenge the EP-ESI-MS system with both polydisperse and monodisperse particles.



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For cesium iodide, with 15 minutes of collection for a polydisperse particle distribution, the accumulated mass spectrometer signal chromatograph and corresponding integrated mass spectrum obtained via ESI of precipitated particles are shown in figures 4a & 4b, respectively. At time = 0 minutes, 7 kV was applied to the collection rod and the flow of liquid for ESI was initiated. In all experiments, after 1-2 minutes, liquid arrived at the collection rod, leading the formation of a Taylor cone. Correspondingly, ions were detected in the mass spectrometer after 1-2 minutes. In all instances ions corresponding to Cs+ (m/z = 132.9) were detected. For higher deposited masses, (CsI)Cs+ (m/z = 392.8), (CsI)2Cs+ (m/z = 652.7), and (CsI)3Cs+ (m/z = 912.6) were detectable, and are labelled in the integrated mass spectrum. Also evident in the spectrum is the tetraheptylammonium+ ion (m/z = 410.6); this ion was present in all spectra measured after using it for signal optimization. Qualitatively, the results in figure 3 demonstrate that particles can be collected via electrostatic precipitation and subsequently ESI can be used for chemical analysis. The signal chromatographs for the (CsI)nCs+ with n = 0-3 ions are shown in figure 5 for this experiment. All chromatographs reveal a sharp peak in signal at the onset of the Taylor cone, with signal decay over time as cesium salt desorbs from the collection rod and is carried away by the ESI solvent (we note the total signal does not decay completely as long as the electrospray is operated). The detected signal for the larger cluster ions is found to decay faster than for the Cs+ ion, indicating that such cluster ions, which are presumably formed by the charged residue mechanism49 are only formed when the concentration of cesium iodide in solution is above a critical concentration. Integrating the signal from all ions containing Cesium (and accounting for multiple Cesium atoms in cluster ions) over the entire experiment yields a total detected cesium concentration. In figure 6a and 6b we plot this signal intensity as a function of exposed cesium iodide mass (product the mass concentration of particles, the


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flowrate through the nozzle, and the collection time) and the deposited cesium iodide mass (which is corrected for charging and deposition efficiencies, but not for transmission and detection efficiencies in the mass spectrometer), respectively. Both plots reveal a power law relationship between detected signal and aerosol particle mass over five orders of magnitude, with less than 2 ng of deposited CsI detectable (corresponding to gas phase concentrations of the order 10-2 µg m-3). Regression equations for these results are displayed (R2>0.93), and both plots reveal a scaling exponent less than unity (close to 0.6), indicating that as deposited mass increases, the system is less efficient in detecting analyte. Nonetheless, a clear correlation is evident between measured signal and gas phase analyte mass. Similar to cesium iodide results, for a polydisperse levoglucosan sample collected for 10 minutes, the corresponding signal chromatograph and integrated mass spectrum are shown in figures 7a & 7b respectively. In the mass spectrum, the levoglucosan+Na+ ion is the dominant species (m/z = 185.1), with the sodiated levoglucosan dimer (m/z = 347.2) also present. No fragment ions of levoglucosan were detected.

Again, as evidenced in figure 8, signal for

sodiated levoglucosan and sodiated levoglucosan cluster ions rise sharply at the onset of Taylor cone formation and decay over time.

Interestingly, for the experiment shown, signal

corresponding to the sodiated trimer ion is only detected for several seconds. Considering both pure levoglucosan particles and levoglucosan mixed with carbon nanoparticles, the integrated levoglucosan signal intensity (again accounting for clusters) is plotted as a function of exposed levoglucosan mass and deposited levoglucosan mass in figures 9a & 9b, respectively. As was found for Cesium, these results display a power law relationship between integrated signal and aerosol particle mass, and further reveal that nanogram quantities are detectable with a dynamic range of approximately 4 orders of magnitude in deposited analyte mass. Power law regression



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again reveals a scaling exponent near 0.6, suggesting this scaling originates from an instrinic property of the electrospray process or the mass spectrometer employed, as it is found for two very distinct analytes. The levoglucosan-carbon black samples have integrated signals which agree well with the integrated signals for levoglucosan only experiments; no ions were detected which could be attributed to the carbon nanoparticles. However, carbon nanoparticle deposition was apparent from visual examination of the collection rod, suggesting that the presence of these nanoparticles, which are insoluble in water and methanol, minimally influenced levoglucosan ionization in EP-ESI-MS.

Comparison to Alternative ESI-Based Techniques Based on these initial performance studies, we believe that EP-ESI-MS would be advantageous over other ESI based approaches for aerosol particles.

For example, filter

collection followed by ESI-MS is a three-step process, requiring collection, extraction, and subsequently mass analysis.

In the extraction step there can be loss of analyte, chemical

reactions, and further the introduction of contaminants into samples (e.g. artifacts have been to be up to 50% of the measured signal50). EP-ESI-MS is a two-step, extraction free process, and in this regard it is similar to TDCIMS based analyses23, only differing in that EP-ESI-MS utilizes an electrospray while TDCIMS utilizes chemical ionization. Extractive-ESI-MS, (which has been applied by Horan et al8 and Gallimore & Kalberer7 to aerosols), cannot discriminate for aerosol particles, as both particles and vapor phase analytes may collide with ESI droplets and the Taylor cone. In fact, because most extractive-ESI-MS applications focus on the analysis of vapor phase analytes37,39, it is likely that extractive ESI would be biased vapor phase analytes, which are higher number concentrations than aerosol particles and have collision frequency


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functions with electrospray droplets. Further, in our initial results, we show here an ability to detect analytes at gas phase mass concentrations of order 10-2 µg m-3 (based upon the deposition flowrate, deposition time, and particle concentration); extractive ESI-MS has only been applied to aerosol concentrations in excess of 10-1 µg m-3. DESI-MS51 also mitigates the need to use an extraction step and could be coupled with electrostatic deposition.

However, it is more

complicated than EP-ESI-MS in that in relies on droplet impingement and bounce from a surface for analytes to be incorporated into droplets (and subsequently analyzed); though if optimized it would likely permit similar measurements to EP-ESI-MS we suspect field implementation of EPESI-MS will be simpler.

Finally, because of its similarity to existing electrosprays in

commercial ESI-MS systems, we note that EP-ESI-MS could be combined with a variety existing mass spectrometers without the need to modify mass spectrometer operation. However, no ionization-mass analysis technique is without its disadvantages. Based on our measurements, we find that deposited analytes are not dissolved in the ESI liquid immediately, and there is a greater risk of sample carry-over in EP-ESI-MS (though in our experiments we did find that analyte signal intensities decreased below noise levels after ~10 minutes in all circumstances) as compared to extractive-ESI-MS (which has minimal sample carry-over risk). Though use of ESI minimizes analyte fragmentation, there are downsides to ESI. Only molecules which are soluble in the liquid used while simultaneously ionizable can be analyzed. In a given ESI solvent, not all analytes may be analyzed with the same efficiency44. Such features may complicate precise determination of the gas phase concentrations of certain analytes, or may make it such that multiple ESI liquids need to be used to detect all analytes of interest. Further, the detected signal in ESI is strongly dependent on the conditions of the Taylor



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cone, and quantitation in a field environment would likely require the use of internal standards, or calibration post measurement.

CONCLUSIONS The application of ESI-MS to aerosol particles is of continued interest for both laboratory and field studies. Here, we demonstrate that an electrostatic precipitation-ESI-MS scheme (EPESI-MS) can be developed and enables detection of nanograms of collected analyte mass via ESI-MS.

System characterization and proof-of-concept experiments were performed and

discussed. In total, we find separating collection and ionization into two steps (which is unique from alternative approaches to ESI-MS of aerosol particles7,8) can be used to improve the limit of detection (based upon gas phase mass concentration) for aerosol particles as compared to related extractive-ESI-MS techniques, and results suggests that the current incarnation EP-ESI-MS could be applied in P.M. 2.5 chemical analysis as well as laboratory studies of smaller particles. EP-ESI-MS provides orthogonal information to aerosol mass spectrometers, as ESI yields the molecular masses of species within aerosol particles while aerosol mass spectrometers yield information on analyte fragments.

While promising as an aerosol particle chemical

characterization technique, future work will need to be performed (both numerical modeling and experimental studies) to better understand the power law relationship between measured signal and deposited mass, to further improve the deposited mass limit of detection to the subnanogram level, to mitigate potential sample carry-over effects without manual cleaning, and to demonstrate that EP-ESI-MS can be used for molecular identification of other organic species in particles,in particular the molecules in secondary organic aerosols.

Such studies will likely

require the coupling of EP-ESI to tandem MS analysis (to infer molecular structure in chemically


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complex systems), as well as careful examination of proper solvent choices for oxidized organic species, and further examination of matrix effects on detected efficiencies.

ACKNOWLEDGEMENTS This work supported by the University of Minnesota MN-DRIVE Program.

SUPPORTING INFORMATION AVAILABLE A detailed system schematic, as well as a description of the ionization efficiency and deposition experiments is available online.



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(a.)

(b.)

(c.)

Figure 1. (a.) Schematic representation of the EP-ESI-MS system, including the unipolar ionization chamber, EP-ESI chamber, and mass spectrometer. (b.) Photograph of the EP-ESI nozzle, collection rod, and inlet for liquid flow. (c.) Photograph of the Taylor cone formed over the collection rod during ESI.



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Figure 2. (a.) The deposition efficiency of the EP-ESI system as a function of particle diameter. Singly charged particles were transmitted directly from a differential mobility analyzer while the multiply charged particles were passed through the unipolar ionization chamber. (b.) Deposition efficiency as a function of the product of the electrical mobility and applied voltage to the collection rod.


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Figure 3. The size distribution functions of aerosol particles composed of (a.) cesium iodide, (b.) levoglucosan, and (c.) levoglucosan mixed with carbon nanoparticles which were used in challenging the EP-ESI-MS system. Mode diameters in distributions are labelled. Also depicted (solid lines) are the approximate transmission windows of a differential mobility analyzer set to transmit 80 nm and 100 nm singly charged particles.



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(a.)

(b.)

Figure 4. (a.) The mass spectrometer total signal chromatograph for polydisperse CsI particles deposited for 15 minutes. (b.) The integrated mass spectrum for the same sample.


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Figure 5. Chromatographs for CsI cluster ions.



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Figure 6. Integrated cesium signal intensity from mass spectrometric measurements as a function of (a.) the mass of cesium iodide passed into the ionization chamber, and (b.) the mass deposited on the collection rod.


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(a.)

(b.)

Figure 7. (a.) The mass spectrometer total signal chromatograph for polydisperse levoglucosan particles deposited for 10 minutes. (b.) The integrated mass spectrum for the same sample.



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Figure 8. Chromatographs for levoglucoan cluster ions.


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(a.)

(b.)

Figure 9. Integrated levoglucosan signal intensity from mass spectrometric measurements as a function of (a.) the mass of levoglucosan passed into the ionization chamber, and (b.) the mass deposited on the collection rod.



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TOC Graphic:


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Aerosol Analysis via Electrostatic Precipitation-Electrospray Ionization

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