Electrospray ionizer for mass spectrometry of aerosol particles

Abstract

A device and method are disclosed to apply ESI-based mass spectroscopy to submicrometer and nanometer scale aerosol particles. Unipolar ionization is utilized to charge the particles in order to collect them electrostatically on the tip of a tungsten rod. Subsequently, the species composing the collected particles are dissolved by making a liquid flow over the tungsten rod. This liquid with dissolved aerosol contents is formed into highly charged droplets, which release unfragmented ions for mass spectroscopy, such as time-of-flight mass spectroscopy. The device is configured to operate in a switching mode, wherein aerosol deposition occurs while solvent delivery is turned off and vice versa.

Claims

1. A system for electrospray ionization of species in aerosol particles, comprising: a source for providing a sample of aerosol particles; an aerosol deposition apparatus including: a chamber for electrically charging the sample of aerosol particles; and a nozzle for directing the sample of aerosol particles to an electrospray tip; a solvent delivery mechanism for delivering a liquid to the electrospray tip; and a high-voltage source for setting the electrospray tip at a high voltage, in order to enable the release of charged droplets carrying dissolved contents of the aerosol particles.

2. The system of claim 1 configured to operate in a switching mode, with the solvent delivery mechanism turned off while the aerosol deposition apparatus is on and vice versa.

3. The system of claim 2, in which polarity of the high-voltage source has the capability of being switched between positive and negative polarities during periods of aerosol deposition being on and off.

4. The system of claim 1, in which the electrospray tip includes a metal rod.

5. The system of claim 4, wherein the nozzle is positioned for aerosol deposition to take place at the tip of the metal rod.

6. The system of claim 4, wherein the nozzle is positioned for aerosol deposition to take place on the metal rod at a point distal from the tip of the rod.

7. The system of claim 6, in which the metal rod is at least partially enclosed in a tube.

8. The system of claim 7 wherein the metal rod diameter is between 0.1 and 10 millimeters.

9. The system of claim 1, and further comprising a mechanism configured to provide the ions that have been released from the charged droplets into a mass spectrometer.

10. A method for electrospray ionization of species in aerosol particles, comprising: sampling aerosol particles from a gas flow; applying an electrical charge to the sampled particles; directing the sampled particles to an electrospray tip and allowing the particles to deposit on the electrospray tip; delivering a solvent to the electrospray tip for removing the deposited particles from the electrospray tip; and applying a high-voltage to the electrospray tip to release charged droplets carrying dissolved contents of the aerosol particles.

11. The method of claim 10 wherein the steps of allowing the particles to deposit on the electrospray tip and delivering the solvent are performed non-concurrently by operating in a switching mode such that when the particles are depositing on the tip, no liquid solvent is delivered and when liquid solvent is delivered to the tip no particles are being deposited.

12. The method of claim 11, where the polarity of the high-voltage applied to the electrospray tip alternates polarities between a negative polarity applied during periods of aerosol deposition and a positive polarity applied during periods of solvent delivery.

13. The method of claim 10, wherein the electrospray tip includes a metal rod.

14. The method of claim 13, wherein aerosol deposition takes place at the tip of the metal rod.

15. The method of claim 13, wherein aerosol deposition takes place on the metal rod at a point distal from the tip of the rod.

16. The method of claim 15, wherein the metal rod is at least partially enclosed in a tube.

17. The method of claim 14, wherein the metal rod has a diameter between 0.1 and 10 millimeters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of CAESI system together with a mass spectrometer for real-time chemical analysis of aerosol particles.

(2) FIG. 2A is a front perspective view of the front-end of CAESI.

(3) FIG. 2B is a magnified view of the tip of the collection rod, where electrospray liquid cone is formed.

(4) FIG. 3A illustrates a size distribution function of test aerosol particles of cesium iodide wherein mode diameters in distributions are labelled and solid lines illustrate the approximate transmission windows of a differential mobility analyzer (DMA) set to transmit 80 nm singly charged particles.

(5) FIG. 3B illustrates a size distribution function of test aerosol particles of levoglucosan wherein mode diameters in distributions are labelled and solid lines illustrate the approximate transmission windows of a differential mobility analyzer (DMA) set to transmit 100 nm singly charged particles.

(6) FIG. 3C is size distribution function of test aerosol particles of levoglucosan mixed with carbon nanoparticles which were used in challenging the CAESI-MS system. Mode diameters in distributions are labelled and solid lines illustrate the approximate transmission windows of a differential mobility analyzer (DMA) set to transmit 80 nm singly charged particles.

(7) FIG. 4 is a mass spectrum of polydisperse cesium iodide particles deposited for 15 minutes.

(8) FIG. 5A is a plot of cesium signal intensity as a function of the mass of cesium iodide passed into the ionization chamber.

(9) FIG. 5B is a plot of cesium signal intensity as a function of the mass of cesium iodide deposited on the collection rod.

(10) FIG. 6 is a mass spectrum of polydisperse levoglucosan particles deposited for 10 minutes.

(11) FIG. 7A is a plot of integrated levoglucosan signal intensity from mass spectrometric measurements as a function of the mass of levoglucosan passed into the ionization chamber

(12) FIG. 7B is a plot of integrated levoglucosan signal intensity from mass spectrometric measurements as a function of the mass deposited on the collection rod.

DETAILED DESCRIPTION

(13) FIG. 1 is a schematic of an embodiment of CAESI system 10 and a collection rod 12 of CAESI is shown in FIGS. 2A and 2B. Operating the CAESI System 10 for particle collection, dissolution, ion production, and mass measurement includes a flow of particle-laden air being pulled at a flow rate of approximately 0.5-1.0 liters/min into an ionization chamber 14, for example, a “Corona-discharge Unipolar Ionization Chamber”. The chamber 14 is employed to electrically charge aerosol particles and may be connected to a high voltage power supply 15, so the particles can be electrostatically deposited onto a collection rod 12. In the ionization chamber 14, ions are generated via corona discharge which is created by application of a high voltage, for example, 3 kV, to an approximately 1/16″ diameter tungsten needle, which may be tapered at its edge for electric field enhancement. While the polarity of the ions generated is controllable, it may be beneficial to ions of one polarity only, for example the use of positive ions only. Positive corona discharges are found to be more stable than negative corona discharges, which often exhibit significant fluctuations in the ion current. Airborne ions readily attach to aerosol particles. Loss of charged particles in the ionization chamber can be minimized by controlling the airflow rate.

(14) Subsequent to the ionization chamber 14, the sampled aerosol is passed through a nozzle 16 as illustrated in FIG. 2. The nozzle 16 is a small diameter nozzle, having an approximately 1/32″ inner diameter and is comprised of stainless steel. Deposition of particles onto the collection rod 12, which may be a tungsten rod having a diameter of approximately 1/16″, is facilitated by a negative voltage (typically approximately −4 kV) applied to the rod 12 by a second high voltage power source 17, while the nozzle 16 is kept grounded. The rod 12 is sheathed with a plastic (PEEK) tube 18, such that only a rounded end is exposed to the aerosol, allowing all particles to be deposited onto a small surface area (approximately ˜0.16 cm.sup.2).

(15) Particle charging and collection proceeds for a selected period of time ranging from approximately 5 minutes to 60 minutes, after which the aerosol is no longer sampled, and the polarity of the voltage applied to the tungsten rod 12 is flipped to positive 7 kV.

(16) Simultaneously, the rod 12 is positioned close to an inlet 22 of a mass spectrometer 24 (for example, the mass spectrometer 24 is a time-of-flight mass spectrometer with an ionization source such as a QSTAR XL mass spectrometer from Applied Biosystems, Waltham, Mass., USA with the CAESI chamber built via modification of a QSTAR XL IonSpray source) and a flow of liquid solution, controlled via a syringe pump 20 (Harvard apparatus) at approximately 25 microliters per minute is driven over the rod 12. As the liquid passes over the rod 12, the soluble content of the deposited aerosol particles is dissolved in the liquid and the high voltage applied to the rod 12 leads to the formation of a liquid cone at the rod tip 12, as shown in FIG. 2B. A fine liquid jet issues from the tip of the liquid cone and breaks up into highly charged fine droplets. As these droplets shrink by evaporation of the liquid content, the charge is concentrated on the molecules of the dissolved content leading to “soft ionization” (as opposed to “hard ionization” resulting from EI and CI) of the unfragmented molecules, which are then drawn into the mass spectrometer for identification through the measurement of mass-to-charge ratio.

(17) The solution used for ESI can be tuned to target the ionization of specific analytes 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 of the collected species, with the temporal evolution of measured signal dependent upon the dissolution of collected molecules into the chosen solvent.

EXAMPLES

(18) In preliminary evaluation, a CAESI chamber 26 was attached to the mass spectrometer 24 to examine electrospray ionization of material deposited on the collection rod 12. Polydisperse particles were either sent directly into the CAESI chamber or were first sent into a DMA to select either an 80 nm or a 100 nm monodisperse sample as reflected in FIGS. 3A, 3B and 3C. In both instances, particles were sent through the ionization chamber, 14 operated with 0.5 microampere current at 0.5 liters per minute, which was also the deposition flowrate. Three different types of test particles were used.

(19) Test Particles

(20) First, cesium iodide (CsI) particles were generated by nebulizing aqueous CsI with a Collison nebulizer and then drying out the droplets using a silica gel diffusion dryer.

(21) 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.

(22) 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.

Example 1

(23) Particle collection in the CAESI system proceeded for selected times ranging from 5-60 minutes. After the selected collection time, the CAESI chamber 26 was sealed from the particle source, and the polarity of the collection rod 12 was switched from negative to positive and it was repositioned near the mass spectrometer inlet. The optimal rod position was determined earlier by maximizing the signal intensity using a standard ESI solution. To facilitate ESI of aerosol content, the cylindrical sheath tube 18, surrounding the collection rod 12, was connected to a solvent feed with the pumping rate precisely controlled by a syringe pump 20. For aerosol particle measurements, the solvent composition was selected to target specific analytes within the deposited particles. For Cs.sup.+ and (CsI).sub.nCs.sup.+ ions released from CsI particles, 1 M acetic acid in methanol was used, while for levoglucosan 10 mM NaCl 95:5 methanol:water was employed. The latter was shown previously to lead to the production of the levogluson-Na.sup.+ ion in ESI. In all instances, the solvent flowrate was 25 microliters per minute, necessary to maintain a stable electrospray over the collection rod.

(24) FIGS. 3A-C illustrates the size distribution functions 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) respectively. All the particles were produced by nebulization. These distributions were measured using a DMA and a condensation particle counter (“CPC”) in series as a scanning electrical mobility spectrometer. The distribution function dN/d log D.sub.p, plotted in FIG. 3, can be integrated in a particular log diameter range to obtain the number concentration of particles in that specific range in particles per cubic centimeter. The mode diameters in distributions are marked on the plots. Also displayed on the 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, the CAESI-MS system is challenged with monodisperse or polydisperse particles respectively.

(25) At time T=0 minutes, 7 kV was applied to the collection rod 12 and the flow of liquid for ESI was initiated. In all examples, after approximately 1-2 minutes, liquid arrived at the collection rod 12, leading the formation of a liquid cone. Correspondingly, ions were detected in the mass spectrometer after approximately 1-2 minutes. With CsI particles, ions corresponding to Cs.sup.+ (m/z=132.9) were detected. For higher deposited masses, (CsI)Cs.sup.+ (m/z=392.8), (CsI).sub.2Cs.sup.+ (m/z=652.7), and (CsI).sub.3Cs.sup.+ (m/z=912.6) were also detectable, and are labelled in the integrated mass spectrum in FIG. 4. Also evident in the spectrum is the tetraheptylammonium.sup.+ ion (m/z=410.6), which was used earlier for determining the optimal ESI tip position.

(26) Qualitatively, the results illustrated in FIG. 4 demonstrate that particles can be collected via electrostatic precipitation and subsequently ESI can be used for chemical analysis. 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 FIGS. 5A and 5B, the signal intensity is plotted as a function of exposed cesium iodide mass (product of the mass concentration of particles, the 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 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. Regression equations to these results are displayed (R.sup.2>0.93), 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 analytes. Nonetheless, a clear correlation is evident between measured signal and gas phase analyte mass.

(27) For a polydisperse levoglucosan sample collected for 10 minutes, the integrated mass spectrum is shown in FIG. 6. In the mass spectrum, the levoglucosan+Na.sup.+ 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. 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 FIGS. 7A and 7B, 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 again reveals a scaling exponent near 0.6, suggesting this scaling originates from an intrinsic 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, did not influence levoglucosan ionization in CAESI process.

(28) The system and technique described throughout this disclosure, referred to as Charged Aerosol ElectroSpray Ionizer (CAESI), is shown to enable analysis of nanogram quantities of collected particles composed of cesium iodide, levoglucosan, and levoglucosan within a carbon nanoparticle matrix. It is further demonstrated that CAESI has a dynamic range of close to 5 orders of magnitude in mass, making it suitable for molecular analysis of aerosol particles in a variety of settings, including laboratory settings with upstream particle size classification, as well as analysis of PM 2.5 particles in ambient air.

(29) Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.