Chemical Characterization of Individual, Airborne Sub-10-nm Particles

A nanoaerosol mass spectrometer (NAMS) is described for real-time characterization of individual airborne nano- particles. The NAMS includes an aerody...
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Anal. Chem. 2006, 78, 1750-1754

Chemical Characterization of Individual, Airborne Sub-10-nm Particles and Molecules Shenyi Wang, Christopher A. Zordan, and Murray V. Johnston*

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716

A nanoaerosol mass spectrometer (NAMS) is described for real-time characterization of individual airborne nanoparticles. The NAMS includes an aerodynamic inlet, quadrupole ion guide, quadrupole ion trap, and time-offlight mass analyzer. Charged particles in the aerosol are drawn through the aerodynamic inlet, focused through the ion guide, and captured in the ion trap. Trapped particles are irradiated with a high-energy laser pulse to reach the “complete ionization limit” where each particle is thought to be completely disintegrated into atomic ions. In this limit, the relative signal intensities of the atomic ions give the atomic composition. The method is first demonstrated with sucrose particles produced with an electrospray generator. Under the conditions used, the particle detection efficiency (fraction of charged particles entering the inlet that are subsequently analyzed) reaches a maximum of 10-4 at 9.5 nm in diameter and the size distribution of trapped particles has a geometric standard deviation of 1.1 based on a log-normal distribution. A method to deconvolute overlapping multiply charged ions (e.g. C3+ and O4+) is presented. When applied to sucrose spectra, the measured C/O atomic ratio is 1.1, which matches the expected ratio from the molecular formula. The spectra of singly charged bovine serum albumin (BSA) molecules are also presented, and the measured and expected C/N/O atomic ratios are within 15% of the each other. Also observed in the BSA spectra are signals from 13C and 32S which arise from 40 and ∼34 atoms per molecule (particle), respectively. Potential applications of NAMS to atmospheric chemistry and biotechnology are briefly discussed.

Airborne nanoparticles, defined as particles with diameters under few tens of nanometers, have important consequences for human health and the environment.1 Particles in this size range can be emitted directly into air by energetic processes such as combustion, or they may be formed in situ by gas-phase oxidation of precursors such as sulfur dioxide or volatile organic compounds. As the use of nanomaterials in common products and processes increases, new avenues for nanoparticle release into the atmosphere may become important. * Corresponding author. E-mail: [email protected]. Telephone: 302.831.8014. Fax: 302.831.6335. (1) Biswas, P.; Wu, C.-Y. J. Air. Waste Manage. Assoc. 2005, 55, 708-746.

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Once in the atmosphere, nanoparticles can influence global climate by increasing the number of cloud condensation nuclei.2 They also represent a significant health hazard.3 Inhalation of nanoparticles has been shown to induce a variety of adverse responses associated with oxidative stress, pulmonary inflamation, or both.4 Epidemiological studies have linked ambient ultrafine particles with respiratory and cardiovascular problems in the human population that increase morbidity and mortality.5 Ambient nanoparticle number concentrations and size distributions change quickly with time and are highly dependent on proximity to a nanoparticle source, atmospheric conditions, or both.2,6 The chemical composition of these particles is largely unknown owing to limited measurement capabilities. Refractory nanoparticles have been collected on a grid and analyzed off-line by methods such as transmission electron microscopy.7 Some semivolatile components have been identified by collecting charged nanoparticles on a probe at ambient pressure, which is subsequently inserted into the source region of an atmospheric pressure chemical ionization mass spectrometer and heated.8 Knowledge of the chemical composition of airborne nanoparticles and how it evolves with time is crucial for understanding their ultimate health and environmental impacts. Over the past decade, many approaches have been developed for real-time analysis of airborne particles by mass spectrometry.9-11 When these methods are extended to nanoparticles, two main challenges arise. First is achieving efficient sampling of particles from ambient pressure into the high-vacuum environment of a mass spectrometer source. Submicrometer-size particles can be tightly focused into the center of a gas flow using a series of aerodynamic lenses.12,13 When the aerosol expands into a vacuum, (2) Kulmala, M.; Vehkamaki, H.; Petaja, T.; Dal Maso, M.; Lauri, A.; Kerminen, V.-M.; Birmili, W.; McMurry, P. H. J. Aerosol Sci. 2004, 35, 143-176. (3) Kaiser, J. Science 2005, 307, 1858-1861. (4) Oberdorster, G.; Oberdorster, E.; Oberdorster, J. Environ. Health Perspect. 2005, 113, 823-839. (5) Pope, C. A.; Burnett, R. T.; Thun, M. J.; Calle, E. E.; Krewski, D.; Ito, K.; Thurston, G. D. J. Am. Med. Assoc. 2002, 287, 1132-1141. (6) Sioutas, C.; Delfino, R. J.; Singh, M. Environ. Health Perspect. 2005, 113, 947-955. (7) Burleson, D. J.; Driessen, M. D.; Penn, R. L. J. Environ. Sci. Health 2004, A39, 2707-2753. (8) Smith, J. N.; Moore, K. F.; McMurry, P. H.; Eisele, F. L. Aerosol Sci. Technol. 2004, 38, 100-110. (9) Sullivan, R. C.; Prather, K. A. Anal. Chem. 2005, 77, 3861-3886. (10) Noble, C. A.; Prather, K. A. Mass Spectrom. Rev. 2000, 19, 248-274. (11) Johnston, M. V. J. Mass Spectrom. 2000, 35, 585-595. (12) Liu, B. Y. H.; Ziemann, P. J.; Kittleson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 293-313. (13) Liu, B. Y. H.; Ziemann, P. J.; Kittleson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 314-324. 10.1021/ac052243l CCC: $33.50

© 2006 American Chemical Society Published on Web 02/16/2006

Figure 1. Basic design of the nanoaerosol mass spectrometer.

the particles detach from the gas flow and are efficiently sampled through differentially pumped apertures into the mass spectrometer source region. This approach becomes inefficient for particle sizes under ∼20 nm in diameter as Brownian motion and diffusion inhibit formation of a tightly focused beam.14 Potential aerodynamic designs around this problem have been developed, but such an inlet has not yet been interfaced to an aerosol mass spectrometer.15,16 The second problem is the small particle mass. If particles are collected on a probe, vaporized, and ionized, typically several picograms of material are required.8,17 Since the mass of a single 10-nm-diameter particle is only on the order of 1 ag, over 1 million particles must be collected to achieve a detectable signal. Singlenanoparticle detection can be achieved by irradiating the particle in-flight with a high-energy pulsed laser beam.18,19 The problem encountered here, however, is a low duty factor for detection as it is hard to time the arrival of the particle with the laser pulse. In most real-time single-particle mass spectrometers, the arrival of the particle is indicated by light scatter when the particle traverses a continuous laser beam. This approach is not applicable to nanoparticles as the amount of light scattered by a single particle is too small to detect.20 In the work reported here, a nanoaerosol mass spectrometer (NAMS) is described that gives the atomic composition of individual nanoparticles in real time with reasonably high detection efficiency. This method shows promise not only for ambient nanoparticle characterization but also for applications to bionanotechnology. EXPERIMENTAL SECTION The NAMS is shown schematically in Figure 1. The main instrumental components are an aerodynamic inlet, quadrupole ion guide, quadrupole ion trap, and time-of-flight mass analyzer. Sinusoidal potentials between 75 and 300 kHz are applied to the (14) Zhang, X.; Smith, K. A.; Worsnop, D. R.; Jimenez, J. L.; Jayne, J. T.; Kolb, C. E.; Morris, J.; Davidovits, P. Aerosol Sci. Technol. 2004, 38, 619-638. (15) Wang, X.; Kruis, F. E.; McMurry, P. H. Aerosol Sci. Technol. 2005, 39, 611-623. (16) Wang, X.; Gidwani, A.; Girshick, S. L.; McMurry, P. H. Aerosol Sci. Technol. 2005, 39, 624-636. (17) Oktem, B.; Tolocka, M. P.; Johnston, M. V. Anal. Chem. 2004, 76, 253261. (18) Carson, P. G.; Johnston, M. V.; Wexler, A. S. Rapid Commun. Mass Spectrom. 1997, 11, 993-996. (19) Reents, W. D.; Schabel, M. J. Anal. Chem. 2001, 73, 5403-5414. (20) Su, Y.; Sipin, M.; Furutani, H.; Prather, K. A. Anal. Chem. 2004, 76, 712719.

ion guide and trap using a custom-built power supply (Pacific Northwest National Laboratory, Richland, WA). Prior to entering the mass spectrometer, the aerosol is brought to an equilibrium charge distribution with a radioactive neutralizer (NRD LLC, Grand Island, NY). The fraction of particles receiving a single charge of the desired polarity depends on the particle diameter.21 The charging efficiency was measured for the setup used in this work by inserting an electrostatic deflector in the aerosol flow. The particle number concentration was measured with a nanoscanning mobility particle sizer (nano-SMPS; model 3936, TSI, Inc., St. Paul, MN) before the deflector, after the deflector, and after the deflector followed by recharging with a second neutralizer. From this combination of measurements, it was found that ∼5% of 10-nm (mobility)-diameter particles received a single charge of the desired polarity. The aerosol (both charged and uncharged particles) initially passes through a flow-limiting orifice into an aerodynamic inlet at 2 Torr. The inlet is based on the design originally developed by Liu et al.12,13 to focus particles between 30 and 500 nm in diameter. In the NAMS, only the last focusing elements of the Liu et al. design are used (the final lens aperture, capillary, and critical orifice). This configuration provides moderate focusing of particles below 50 nm and more importantly little focusing of particles above 50 nm in diameter. The particle beam exiting the critical orifice of the aerodynamic inlet traverses a short (1 cm) differentially pumped region and passes through a second orifice into the quadrupole ion guide. The quadrupole ion guide consists of four cylindrical rods (10mm diameter, 12-cm length, ro ) 4 mm) and is differentially pumped. An argon gas leak maintains a pressure of ∼0.02 Torr in the ion guide. At this pressure, the stopping distance22 of 10nm-diameter particles is ∼11 cm, which closely matches the length of the ion guide. In this way, particles are collisionally cooled as they are focused through the lens so that the ion kinetic energy at the exit is determined primarily by the static potentials at which the ion lens and ion trap are floated. Collisional cooling is essential for the next stage (ion trapping). For example, 10-nm particles exiting the critical orifice of the aerodynamic inlet are accelerated to ∼350 m/s,14 which corresponds to a kinetic energy of 200 eV. This energy is too large for efficient trapping under the conditions used. Charged particles focused through the quadrupole ion guide subsequently enter the quadrupole ion trap (ro ) 10 mm; zo ) 7.1 mm). An argon gas leak into the trap maintains a pressure of ∼10-4 Torr. Collisional cooling at this pressure is sufficient to trap the (low kinetic energy) particles, which subsequently undergo additional collisions until they are focused to the center of the trap. Since the ablation laser beam is also focused in the center of the trap, the probability of hitting a trapped particle is maximized. Trapped particles are ablated with a Nd:YAG laser at 532 nm, 150 mJ pulse energy, and 5 ns pulse width (model CFR 400, Big Sky Lasers, MT) The laser beam is tightly focused, giving a fluence on the order of 100 J/cm2. This fluence reaches the “complete ionization limit”19,23-25 where each particle is thought (21) Covert, D.; Wiedensohler, A.; Russell, L. Aerosol Sci. Technol. 1997, 27, 206-217. (22) Reist, P. C. Aerosol Science and Technology; McGraw-Hill: New York, 1993. (23) Reents, W. D.; Ge, Z. Aerosol Sci. Technol. 2000, 33, 122-134.

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Table 1. Ionization Energiesa (eV) of Four Elements, Grouped into Similar Energy Ranges energy range element

I

II

III

C N O S

(+2) 24.4 (+2) 29.6 (+2) 35.1 (+2) 23.3 (+3) 34.8

(+3) 47.9 (+3) 47.5 (+3) 54.9 (+4) 47.3

(+4) 64.5 (+4) 77.5 (+4) 77.4 (+5) 72.7

a From: CRC Handbook of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton, FL, 1996.

Figure 2. Low m/z region of the mass spectrum of ∼10-nm-diameter sucrose particles, average of 50 individual spectra.

to be completely disintegrated into individual atoms that are quantitatively converted to singly and multiply charged positive ions. Under these conditions, the relative peak areas of the various atomic ions give a quantitative measure of the atomic composition. Shortly before the laser fires, the end caps of the ion trap are pulsed to high voltages so that the ions produced by laser irradiation are extracted into a reflecting-field time-of-flight mass analyzer (R. M. Jordan Co., Inc., Grass Valley, CA). A particle is considered “hit” and the mass spectrum saved when the ion signal increases above a threshold level. Because there is little background in the multiply charged m/z region, the threshold level is determined primarily by detector and electronics noise. Particles in the 10-nm size range are produced with an electrospray aerosol generator (model 3480, TSI, Inc.). Particles are generated from solutes dissolved in 18 MΩ water. Ammonium acetate is also added to the solution as needed to achieve an adequate conductivity. Particle size distributions and number concentrations from the generator are measured with the nanoSMPS. RESULTS AND DISCUSSION Figure 2 shows the multiply charged ion region of the averaged mass spectrum (50 single particle spectra) of ∼10-nm-diameter sucrose particles that were generated from the electrospray aerosol generator using an ammonium acetate buffer.26 The mass of a single particle is ∼5 × 10-19 g or 300 kDa. The rf potentials used in this experiment (136 kHz and 1340 V0-P for the quadrupole ion guide; 125 kHz and 2200 V0-P for the quadrupole ion trap) were selected to focus and trap ∼10-nm-diameter particles. The ion trap was floated at a static potential such that particles entering the trap had ∼7 eV of kinetic energy. When either rf potential was turned off, the particle hit rate (particles hit per unit time) decreased by ∼2 orders of magnitude. When both potentials were (24) Lee, D.; Park, K.; Zachariah, M. R. Aerosol Sci. Technol. 2005, 39, 162169. (25) Mahadevan, R.; Lee, D.; Sakurai, H.; Zachariah, M. R. J. Phys. Chem. A 2002, 106. (26) Chen, D.-R.; Pui, D. Y. H.; Kaufman, S. L. J. Aerosol Sci. 1995, 26, 963977.

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turned off, no particles were detectedsa minimum of 3 orders of magnitude decrease. Major peaks in Figure 2 include C2+ to C4+ (6.0, 4.0. and 3.0 m/z), O2+ to O4+ (8.0, 5.3, and 4.0 m/z), and H+ (1.0 m/z). Minor peaks include multiply charged 13C ions (6.5 and 4.3 m/z) and N ions from residual buffer in the particles at (7.0 and 4.7 m/z). To determine a carbon to oxygen (C/O) mole ratio from the relative signal intensities of the corresponding ions, the C3+ and O4+ signal intensities at 4.0 m/z must be deconvoluted. A similar problem would exist between S4+ and O2+ in particles containing sulfur and oxygen. The deconvolution approach adopted in this work is based in Table 1, where the second through fourth ionization energies of C, N, O, and S are grouped into three energy ranges. Singly charged ions are not considered in this analysis because of background signal at these m/z values (see below). Carbon, oxygen, and nitrogen have roughly equivalent second, third, and fourth ionization energies. If we assume that the energy distribution imparted to these atoms in the laser-induced plasma is independent of the specific element, then the fraction of atoms reaching each charge state should be the same:

NC(+n) NC(total)

)

NN(+n) NN(total)

)

NO(+n) NO(total)

(1)

where NC(+n) is the number of carbon atoms having a +n charge, NC(total) is the total number of carbon atoms, etc. Thus, each charge state gives an independent measure of atomic composition:

NC(+2) NO(+2)

)

NC(+3) NO(+3)

) etc.

(2)

Equation 2 provides a way of averaging together the compositions determined from each charge state, and it allows overlapping signals from different charge states (e.g., C3+ and O4+ at 4.00 m/z) to be deconvoluted. Sulfur is more problematic and will be discussed later as its second and third ionization energies are closely spaced and the m/z of each even charge state overlaps with a charge state of oxygen. When this approach is applied to the spectrum in Figure 2, the C/O atomic ratio averaged over the +2 to +4 charge states is 1.1, which matches the expected ratio from the molecular formula (C12H22O11). The H+ signal intensity is not considered in this work because of potential problems associated with transmission of such a low m/z ion through the mass analyzer.

Figure 3. Low and high m/z region of the mass spectrum of ∼10nm-diameter sucrose particles, average of 50 individual spectra.

Figure 5. (a) Mass spectrum of an individual, singly charged BSA molecule. (b) Averaged mass spectrum of 50 BSA molecules. Figure 4. Particle detection efficiency (fraction of particles entering the mass spectrometer inlet that are subsequently detected) vs mobility diameter. The width of the distribution corresponds to a geometric standard deviation of 1.1.

While approximately correct, the complete ionization limit as described above does not take into account processes such as ion-electron recombination, ion-neutral reactions, and ionization of background gas (including argon, which is added to the ion trap to collisionally cool the particles) with electrons produced directly by the plasma or by laser radiation striking a metal surface. While these processes lead to background signal, it is generally found at higher m/z values than the multiply charged ions from the particles. Figure 3 shows an expanded m/z range of the sucrose spectrum in Figure 2. Atomic and molecular fragments from these background processes are readily observed in the 12-18 and 26-32 m/z ranges. In addition, strong peaks are observed for Ar+ and Ar2+. Finally, there is a weak background signal at 2 m/z, which is assigned to H2+. The particle size range trapped and detected in the sucrose experiments was determined by size-selecting particles with a differential mobility analyzer (from the nano-SMPS) prior to entering the mass spectrometer. The detection efficiency, defined as the fraction of particles entering the inlet that are subsequently

“hit” was determined with the aid of a condensation particle counter (also from the nano-SMPS) to measure the number concentration of the aerosol being sampled. As shown in Figure 4, the detection efficiency reaches a maximum of 10-4 at 9.5 nm in diameter and the size distribution (assuming a log-normal distribution) has a geometric standard deviation of 1.1. The size range trapped can be shifted to higher and lower values by changing the frequency or amplitude of the potentials applied to the ion guide and trap. The ability of NAMS to detect individual macromolecules is illustrated in Figure 5. Figure 5a is the mass spectrum of a singly charged bovine serum albumin (BSA) molecule (1.1 × 10-19 g; 69 kDa; dm ) 7 nm; C3071H4826N816O927S40). The signal-to-noise ratios of the main peaks are quite high (>500), suggesting that much smaller molecules or particles should be detectable. Figure 5b is the average of 50 individual spectra. Although the relative peak areas vary substantially among the individual spectra, the averaged spectrum gives C/N/O peak area ratios (3.3/1.0/1.4) that are within 15% of the correct atomic ratios (3.8/1.0/1.2). Also noteworthy in Figure 5b are minor peaks due to S (10.6 m/z) and 13C (6.5 and 4.3 m/z), which arise from 40 and ∼34 atoms per molecule, respectively. That such small numbers of atoms per molecule can be detected is not surprising given that Analytical Chemistry, Vol. 78, No. 6, March 15, 2006

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(1) the microchannel plate detector can detect single ions, (2) the mass analyzer transmission efficiency is over 50%, and (3) virtually all atoms in the molecule are ionized. The S3+/C2+ signal intensity ratio is in line with the elemental composition as would be expected by the magnitudes of the respective ionization energy groupings in Table 1. However, the expected S/C ratio is 0.0075 while the measured ratio is 0.012. While the amount of S2+ produced could not be determined because it overlaps with the large O+ and background signals at 16 m/z, including S2+ in the calculation would increase the difference between expected and measured ratios. Neither is the magnitude of the 13C signal a quantitative measure of atomic composition: the expected 13C2+/ 12C2+ ratio is 0.011 while the measured ratio is 0.016. The reason for these discrepancies is not clear at this time, although it may be related to difficulty in establishing a correct baseline for these weak intensity peaks. Each individual spectrum in this work is thought to arise from a single particle/molecule because (1) differential mobility analysis of the electrospray-generated aerosols indicates few (if any) doublets or multiplets (e.g., particles containing two or more BSA molecules), (2) the spectra of size (mobility) selected sucrose particles between 8 and 12 nm show total peak areas that increase linearly with particle volume and the peak areas of comparably sized sucrose and BSA particles are similar, and (3) typically only 1 out of every 50 laser shots registers a particle hit, which suggests based on Poisson statistics that few hits arise from multiple particles in the trap. The detection efficiency (10-4) and charging efficiency (0.05) currently achieved permit characterization of aerosols with concentrations in excess of about 105 particles/cm3, which is sufficient for laboratory studies of new particle formation and possibly ambient measurements during periods of high particle concentrations. However, several avenues for improvement exist. The charging efficiency can be increased 1 order of magnitude (27) Chen, D.-R.; Pui, D. Y. H. J. Nanopart. Res. 1999, 1, 115-126. (28) Quarmby, S. T.; Yost, R. A. Int. J. Mass Spectrom. 1999, 190/191, 81102.

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with an optimally designed unipolar charger.27 Replacing the current aerodynamic inlet with one optimized for nanoparticle focusing15,16 will minimize particle losses at the entrance to the quadrupole ion guide. Based on the geometry of the interface between the aerodynamic inlet and quadrupole ion guide, we expect that only 1-10% of the particles leaving the aerodynamic inlet are currently focused through the ion guide. Improvements in the trapping efficiency of high m/z ions are also possible.28 Increasing the overall efficiency of particle analysis by 2 orders of magnitude will allow ambient particle measurements to be performed at the 103 particles/cm3 level or perhaps lower, which would be sufficient for many atmospheric measurements. While the emphasis of NAMS development has been on ambient nanoparticle characterization, applications to bionanotechnology are also possible. Protein molecules encoded by unique isotopes, elements, or both can be selectively detected and counted. Protein assemblies and individual virons may also be trapped and analyzed. Finally, airborne manufactured nanomaterials can be characterized over a wide range of elements and compositions. ACKNOWLEDGMENT The authors thank David Prior of the Pacific Northwest National Laboratory for design and construction of the rf power supplies used in this work. Although the research described in this paper has been funded by the United States Environmental Protection Agency through grant R829622010, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Partial support from National Science Foundation grant CHE-0517972 is also acknowledged.

Received for review December 19, 2005. Accepted February 1, 2006. AC052243L