Anal. Chem. 1994,66, 3540-3542
Real-Time Measurement Capabilities Using Aerosol Time-of-Flight Mass Spectrometry T. Nordmeyer and K. A. Prather' Department of Chemistry, University of California, Riverside, California 9252 1
Thisnote describes the recent development of a new technique, aerosol time-of-fight mass spectrometry, that allows for realtime analysis of single particles. We report here the first results demonstrating the utility of this technique for real-timeanalysis of polydisperse systems of aerosols, determining both size and composition. Future calibration studies will follow, enabling complete analysis of complex atmosphericsystems, determining particle size distributions and, more importantly, the corresponding compositional variations for each particle size.
Aerosol Generator
U
n
Aerosol Interface
Circuit
Time-of-Flight Real-time aerosol analysis, determining the size and Mass Spectrometer [Particle Composition] chemical composition of individual particles, has been an elusive goal for aerosol researchers since the early 1 9 7 0 ~ . ' . ~ We recently reported a method for accomplishing this goal using aerosol time-of-flight mass spectrometry (ATOFMS), Oscilloscope a technique which combines aerodynamic particle sizing and time-of-flight mass spectrometry.3 In that paper, we described Figure 1. Block diagram of the aerosol timeof-fllght mass specan electronic logic circuit that was under development which trometer. would allow us to correlate particle size and composition measurements in the real-time analysis of individual particles. This external circuit has now been added, completing the initial of the experimental details will be given here. A block diagram stage of development of the system, which is now capable of of the experimental setup is shown in Figure 1. Particles of highly efficient aerosol analysis. To our knowledge, this is known size and composition are generated using a comthe first analytical technique capable of determining the size mercially available aerosol generator, whereupon they are and complete mass spectrum of an individual particle in real dried, neutralized, and then directed to the inlet of a particle time. This paper presents details of the real-time capability interface. After they are admitted into the interface, particles of this system. pass through a capillary and supersonically expand through The electronic circuit serves as a crucial link between the three differentially pumped regions separated by three particle sizing and composition analysis regions of the system skimmers to form a narrow particle beam. This beam of by tracking the individual aerosol particles through the system particles travels under vacuum through two low-power laser while providing the logic and timing necessary for efficient beams used to track the particles into the ionization region particle analysis.3 The implications of analysis using ATOFMS of the mass spectrometer. The signal from the tracking lasers are far reaching, potentially changing the direction of aerosol is sent to a custom electronic circuit used to measure the particle research by allowing the first direct investigations of a velocity and also to synchronize the arrival of the particle in multitude of atmospherically relevant processes in real time. the exact center of the ionization region of the mass Although techniques exist for determination of both size and spectrometer. A pulsed high-powered Nd:YAG and/or COz composition of single particles, these techniques are not in laser is then fired at the appropriate time to intercept the situ, and therefore sample integrity is often in question. As particle with the desorption/ionization pulse. Ions formed in a result, many atmospheric researchers have opted to use the laser desorption/ionization process are accelerated into computer models to simulate real systems because of the lack the time-of-flight drift region where they separate by massof adequate analytical techniques which could provide such to-charge ratio. The complete mass spectrum is obtained on inf~rmation.~ The ATOFMS system described herein will a digital oscilloscope interfaced to a personal computer. We ultimately allow establishment of the key correlation between are currently modifying a data acquisition program which particle composition and size. will allow each particle's mass spectrum to be stored with the In a previous paper, preliminary results and details of the corresponding aerodynamic particle velocity, which later will total ATOFMS system were presented, so only a summary be converted to particle size. Earlier attempts at determination of particle size and (1) Myers, R. L.; Fite. W. L. Emiron. Sci. Technol. 1975, 9, 334-336. composition showed some success. However, these techniques (2) Davis, W. D. Environ. Sei. Technol. 1977, 11, 587-592. (3) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994,66, 1403-1407. utilized a quadrupole mass analyzer and thus were unable to (4) Hanson, D. D.; Ravishankara, A. R.; Solomon, S. J . Geophysical Res. 1994, provide the entire mass spectrum for an individual p a r t i ~ l e . ~ . ~ 99, 3615-3629. 3540
Analytical Chemistry, Vol. 66,No. 20, October 15, 1994
0003-2700/94/03063540$04.50/0
0 1994 American Chemlcal Soclety
In addition,the timing schemes for laser desorption/ionization in these experiments were based on a fixed delay and were not suitable for polydisperse aerosol systems in which particles assume a range of velocities upon expansion into vacuum. Other researchers have incorporated time-of-flight mass spectrometryfor singleparticle compositionanalysis;however, these methods have either been off-line or incapable of determining the size and composition of single particles simultaneously. One method involves firing a high-powered laser at high repetition rates, randomly desorbing those particles that happen to be in the path of the laser at the time of firing.7 This method, in addition to being extremely inefficient,*does not allow for determination of particle size and therefore has limited utility. Other methods utilize a single low-powered continuous wave visible laser to detect an individual particle in the source region of the mass spect r ~ m e t e r . ~Upon J ~ detection, a high-powered laser is instantly fired to desorb the particle and obtain the mass spectrum. Determination of particle size from such experiments is difficult because the Mie scattering intensity is a complex function of particle size, index of refraction, and the angle of detection. In addition, laser beam inhomogeneity in single laser systems can yield misleading results, because particles of the same size possessunique trajectoriesand sample different portions of the beam, resulting in different scattering intensities. In our experiment, we utilize two low-powered continuous wave lasers separated by a fixed distance,determining particle size by measuring the transit time of the particle between the two lasers. The correlation between transit time and aerodynamic particle size can be established experimentally using particles of known size and composition. This method for particle sizing was first introduced by Dahneke in 1973.” Substantial research in this area has led to the commercialization of instruments designed to measure transit time using a dual tracking laser approach. In Figure 2, we show the measured relationship of transit time as a function of aerodynamicparticle size for (NH4)2S04 particles in the 1-10 pm size range. The error bars in this figure (one standard deviation) represent the distribution of velocities imparted to particles of the same size during the expansion process into vacuum.*2Ammonium sulfate particles of known density (1.8 g/cm3) were chosen initially for use in calibration studies because of this species’s ubiquitous presence in the atmosphere. The advantage of determining both aerodynamic size and composition simultaneously is that each particle can be associated with a particular group or class of particles with a known density. This knowledge of particle density allows for conversion from aerodynamic particle size to geometric particle size. Sinha,M.P.;Giffen,C.E.;Noms,D.D.;Estes,T. J.;Vilker,V. L.;Friedlander, S. K. J . Coll. Inter. Sci. 1982, 87, 140-153. Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886891. Reents, W. D.; Swanson, A.; Downey. S.; Mujsce, A. M.; Muller, A. J.; Emerson, B.; Siconolfi, D. J.; Sinclair, J. D. Proceedings of the ASMS Conferenceon Mass Spectrometry,San Francisco,CA, May 3&June4,1993; A bst r. 640a. We have experimentally measured a 10 000-fold increase in the number of mass spectra obtained when using our method of particle tracking as opposed to the random fire mode of operation. McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991.63. 2069-2073. Thomson, D. S.; Murphy, D. M. Appl. Opt. 1993,32,6818-6826. Dahneke, B. Nut. Phys. Sci. 1973,244.54-55. The error in the flight time of each individual particle is less than 2%.
2 2
150 0
4
6
IO
8
Aerodynamic Diameter (pm) FlgweP. TransitthnevsaerodynamicdiameterforindMduaiammonium sulfate particles. The error bars represent one standard deviation.
Laser fired too soon Laser fired at optimum time
U
I
’
Laser fired too late
Flgure 3. Graph showing desorption efficiency as a function of time delay between the particle arrival in the center of the source and Nd:YAG desorption pulse for three different particles sizes.
The efficiency of the timing circuit in determining the arrival of a particle to coincide with the Nd:YAG pulse is demonstrated for 1.O, 2.0, and 4.0 pm 2,4-dihydroxybenzoic acid particles in Figure 3. Particles that pass through both tracking beams are fired upon at the calculated arrival time, t = 0. The percentage of particles sized and tracked by the circuit for which a corresponding mass spectrum was obtained is shown as a function of the delay time, which is the delay between the predicted arrival of the particle in the center of the source of the mass spectrometer and the Nd:YAG pulse. It should be noted that as the Nd:YAG pulse is delayed either before or after the arrival of the particle, the probability for ionization decreases because the particles pass out of the “interception zone” defined by the overlap between the projected volume from the tracking lasers and the volume of the Nd:YAG laser beam. The fact that the optimum time delay is the same for both large and small particles demonstrates the ability of this technique to analyze polydisperse samples; if the interception time were calculated incorrectly, the peak center would shift with particle size, i.e., particle AnatytkalChemWy, Vol. 66, No. 20, October 15, 1994
3541
velocity. Note that larger particles travel more slowly and remain in the zone longer and therefore yield a longer time window for particle desorption/ionization. The fact that fewer than 100%of all particles tracked by the circuit are desorbed can be explained by the dispersive nature of the particle beam. This is further supported by the greater dispersion (less percentage ionization) seen for smaller particles. These results are typical for particle beams undergoing supersonic expansion through a capillary and will be studied in more detail to determine the optimum conditions for our system.13J4 In the current mode of operation, we obtain both the complete mass spectrum and size information for approximately 300 single particles in 1 min, using initial (Le., before expansion through capillary nozzle into particle interface) particle concentrations of approximately 102-1O3 ~ m - We ~. have observed desorption/ionization efficiencies up to 50% meaning one out of two particles that pass through both scattering lasers results in mass spectra. The total efficiency of analysis using ATOFMS is ultimately determined by the fraction of particles admitted to the particle interface which produce both mass spectra and size information. Several factors important in determining this efficiency are being explored. The final results will be presented in a full paper. Among those factors under study are the following: (1) particle transfer efficiency, (2) transmission efficiency through the particle interface, (3) spatial overlap of particles with tracking lasers, (4) projected overlap between the tracking lasers and the ionizing laser, ( 5 ) threshold levels used to discriminate between particles and background noise, (6) coincidence errors in the timing circuit resulting from multiple particle triggers, (13) Estes, T. J.; Vilker, V. L.; Friedlander, S.K. J . ColloidInterface Sci. 1983, 93, 8 6 9 4 . (14) Cheng, Y. S.;Dahneke, B. E. J. Aerosol Sci. 1979, 10, 257-274.
3542
Analflicai Chemistry, Vol. 66,No. 20, October 15, 1994
and (7) dead time in the timing circuit due to limitations of laser repetition rate (approximately 50%). Current plans for study of these factors include construction of x-y positioners for the tracking laser optics to allow real-time optimization of the particlelscattering laser overlap and projected particle beam overlap with the ionization laser. In addition, the dispersion of the particle beam for various particle sizes is being studied for different capillary and nozzle designs. In summary, we have combined the techniques of aerodynamic particle sizing with time-of-flight mass spectrometry and have shown the ability of this system to properly determine both size and composition for a range of particle sizes. The implications are numerous because now, for the first time, there is a method that can provide, in real time, both the size and the chemical composition of individual particles. In addition, the ability to determine these properties for polydisperse systems has also been demonstrated. Such analytical capability opens new areas for research in atmospheric studies, both for field sampling and for analytical studies of heterogeneous reactions. These types of studies will lead to improved models for atmospheric processes that are currently not well understood, such as ozone depletion. This method may also be adapted for on-line particle sampling in combustion processes such as diesel and gas engine emissions. Now that ATOFMS has proven to be a highly efficient method for single particle analysis, the next stage of development will involve performing calibration experiments using particles of varying size and chemical composition while optimizing the system for real-time particle analysis. Received for revlew M a y 16, 1994. Accepted July 5, 1994.' *Abstract published in Aduance ACS Abstracts. August 15, 1994.
