Anal. Chem. 1994,66, 3431-3435
Chemical Characterization of Single Particles by Laser Ablation/Desorption in a Quadrupole Ion Trap Mass Spectrometer John M. Dale,' Mo Yang, Wllllam B. Whltten, and J. Michael Ramsey Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennesee 3783 1-6 142
Particles are introduced into the center of the space surrounded by the hyperbolic electrode surfaces of a quadrupole ion trap mass spectrometer. Ions are desorbed or ablated from the surfaces of the particles with laser pulses from a Nd:YAG laser as the particles fall through the trap. The ions are trapped and subsequently mass analyzedusing the mass-selectiveinstability mode of operation of the ion trap. Desorption experiments were performed with 100 pm silicon carbide particles with an average of a few nanograms of adsorbed trimethylphenylammoniumchloride, tetraphenylphosphoniumbromide, or 2,4,6trinitrotoluene per particle. A conservative detection limit of 0.7 fm per particle was determined for tetraphenylphosphonium bromide. Ablation experiments were performed with -50 pm nickel particles for isotope ratio measurements with good agreement between accepted and average experimental values.
-
Although instrumentation and techniques can be considered to be well-developed for the physical characterization of particles, there is still a need for instrumentation to chemically characterize these materials. Understanding the chemical nature of particles is important for a number of branches of science. The characterization of particles has become increasingly important in material science as new material synthesis processes rely on microparticulate precursors.' Particles in the form of aerosols play a central role in atmospheric studies and are important for understanding environmental issues. The approaches currently used to chemically characterize particles have recently been reviewed by Niessner.' These methods can be classified as either optical techniques or vacuum techniques. The latter include various forms of mass spectrometry and photoelectron spectroscopy. Particles mounted on a substrate have been analyzed by secondary ion/neutral mass spectrometry using a quadrupole mass filter with special energy-selective ion optics.* Differentiallypumped, atmospheric-sampling, microparticle-beam generators have been used for the introduction of aerosol particles into the ionization region of quadrupole mass filters. The beam is produced by the expansion of the aerosol into the system vacuum through a capillary nozzle and skimmer. Individual particles contained in the beams have been volatilized and ionized sequentially for analysis by directing them either onto a hot filament3" or into the path of a high(1) Niessner, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 466. (2) Fichtner, M.; Lipp, M.; Goschnick J.; Ache, H. J. Surf. Interface Anal. 1991, 17, 151. (3) Stoffels, J. Int. J. Mass Specrrom. 1981, 40, 217. 0003-2700/94/0366-343 1$04.50/0 0 1994 American Chemical Society
energy pulsed laser beam.7 The use of a laser to produce ions from particles is an attractive approach. It provides the ability to either desorb molecules from the surface or vaporize an entire particle depending upon the energy of the laser pulse. Although used extensively, the quadrupole mass filter severely limits the ability toperform chemical characterizations on a single particle basis because it does not monitor all of the ions from one sampling of the particle. Later work used a Matauch-Herzog mass spectrometer with an focal plane array detector to allow simultaneous multiple ion monitoring.8 More recently, single particle analyses have been implemented using laser volatilization mass spectrometry and time-of-flight mass spectrometer^.^-*^ Laser desorption mass spectrometry has been demonstrated in a quadrupole ion trap with sample solutions that have been dried onto the end of a The probe is inserted through a hole in the central ring electrode in such a manner as to place the sample flush with the minimum radius of the electrode inner surface. The sample is vaporized with a pulsed laser beam entering the trap from the opposite direction. Multiple sampling can be made by rotating the probe. The work described in this paper focuses on the ability to perform laser desorption experiments and mass analysis characterizations of particles which are sampled at the center of the quadrupole ion trap. The ion trap has interesting features that make it attractive for this type of analysis. When the mass-selective instability mode of operation17 is used, ions of different masses generated in the ion trap from one ~
~
~~~
~
~
(4) Sinha,M.P.;Giffin,C.E.;Norris,D.D.;Estes,T.J.;Vilker,V.L.;Friedlander, S. K. J. Colloid Interface Sci. 1982, 87, 140. (5) Allen, J.; Gould, R. K. Rev. Sci. Instrum. 1981, 52, 804. (6) Sinha, M. P.; Platz, R. M.; Vilker, V. L.; Friedlander, S . K. Int. J. Mass Spectrom. Ion Processes 1984, 57, 125. ( 7 ) Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886. (8) Sinha, M. P. Aerosols: Formation and Reactioity. Proceedings of the 2nd International Aerosol Conference, Berlin, 1986; Pergamon: Oxford, 1986; p 875. (9) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63,
2069.
(10) Reents, W. D., Jr.; Swanson, A.; Downey, S.; Mujsce, A. M.; Muller, A. J.; Emerson, B.; Siconolfi, D. J.; Sinclair, J. D. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1993; p 640a. (11) Hinz, K. P.; Daufmann, R.; Spengler, B. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1993; p 946a. (12) Murray, K. K.; Russell, D. H. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1993; p 780a. (13) Johnston, M. V.; Carson, P. G.; Neubauer, K. R.; Wexler, S. S. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1993; p 944a. (14) Heller, D. N.; Lys, I.; Cotter, R. J. Anal. Chem. 1989, 61, 1083. (1 5) Glish, G. L.; Goeringer, D. E.; Asano, K. G.; McLuckey, S. A. Int. J. Mass Specrrom. Ion Processes 1989, 94, 15. (16) McLuckey, S . A.; Glish, G. L.; Kelley, P. E. Anal. Chem. 1987, 59, 1670.
Analytical Chemistry, Vol. 66, No. 20, October 15, 1994 3431
m Figure 1. Schematic of the experimental setup for laser desorption mass spectroscopy of falling particles.
laser pulse sampling of the particle can be confined to the trap and subsequently mass analyzed. Energetic ions, if trapped, will be quickly cooled by collisions with the helium buffer gas in the trap. Consequently, these ions will not cause the mass resolution of the analysis to be limited as is the case when a time-of-flight mass spectrometer is used.
EXPER I MENTAL SECT1ON The experimental work for this paper was performed with a Finnigan MAT Model 800 ion trap detector system (ITD). The standard ITD ion trap assembly consists of three cylindrically symmetric electrodes separated by ceramic spacers. The hyperbolic inner surfaces of the two end cap electrodes and the centrally located ring electrode define the volume of the trap. The minimum inner diameter of the ring electrode is 2.00 cm, and the axial distance between the end caps is 1.57 cm. The experimental setup as described below is shown schematically in Figure 1. The ion trap was removed from the ITD vacuum chamber and modified by drilling four 3.18 mm holes on perpendicular axes through the central plane of the ring electrode. The trap was remounted in a 15.2 cm vacuum cube with its symmetry axis in the vertical position. The trap mounting assembly was designed so that the holes in the ring electrode were each perpendicular to a vacuum cube side port, three of which were fitted with 15.2 cm viewport flanges. The fourth backside port was connected to a vacuum tee which had the ITD turbo pump mounted on the vertical leg. The two window-accessible collinear holes in the ring electrode are for laser access to the center of the trap. The third hole, facing the frontside port, is for photodiode detection of light reflected from the particles as they pass through the center of the trap. The trapping radio frequency (rf) voltage is applied through a 3.18 mm brass rod that extends from the ITD autotransformer to a high-voltage vacuum feed-through and then through the vacuum tee to the remaining hole in the ring electrode. After the trap was installed in the vacuum cube, it was necessary to retune the autotransformer for resonance by moving the input tap on the rf coil. (17) March, R. E.; Hughes, R. J. In QuadrupoleStorageMussSpectromefry.Vol. 102, Chemical Analysis; Winefordner, J. K., Ed.; Wiley-Interscience: New York, 1989.
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A particle dropping device was made to fit in a hole on the topside of the top end cap electrode where the electron gun assembly normally resides. The body of the device is a 17.4 mm diameter brass cylindrical hopper with a 2.35 mm hole that extends through the center of the hopper and terminates in a 0.56 mm hole at the bottom. A 0.46 mm diameter drill bit attached to an 8 mm diameter rod rests inside the hopper such that the end of the drill bit is flush with the bottom of the hopper. The bottom of the hopper is 12.2 mm above the center of the trap. A slot in the top of the 8 mm rod fits a mating piece that is attached to the shaft of a rotary vacuum feed-through. Particles of interest are loaded into the hopper and made to fall into the trap through a 1.2 mm hole in the center of the top cap electrode by rotating the vacuum feedthrough either by hand or with a stepping motor. A freely falling particle will reach the center of the trap in about 50 ms with a velocity of -0.5 mm/ms and will be observable through the holes in the ring electrode for about 6 ms. The second harmonic 532 nm radiation from a QuantaRay DCR-2A pulsed Nd:YAG laser is used to desorb or ablate and ionize material from particles as they fall through the center of the trap. The pulse for triggering the Nd:YAG laser is synchronized so that the laser fires as the particle passes through its beam path. The onset of the trigger is generated by light scattered from the particle as it passes through the beam of a 4 mW HeNe laser. The two beam paths are coplanar and propagate through the trap from opposing side ports with the HeNe beam parallel to and about 2 mm above the Nd:YAG beam. A falling particle traverses the distance between the two beams in about 4 ms. As the particle falls through the HeNe laser beam, the scattered radiation is detected by a photodiode, and the output is converted to a TTL pulse. When this pulse is coincident with an ion trapping gate from the ion trap control of the ITD, a TTL pulse is generated by the AND gate. The output of the AND gate, after passing through an adjustable delay generator, is used to trigger the Nd:YAG laser. The Nd:YAG laser fires about 3 ms after it is triggered. The displacement between the HeNe beam path and the Nd:YAG beam path is made a little larger than the distance that the particles fall during this 3 ms period. The additional distance allows the trigger to be delayed by an adjustable amount until the particles are consistently intercepted by theNd:YAG pulses. This condition is determined by monitoring the Nd:YAG radiation scattered from the particles with the photodiode. The magnitude of the photodiode pulse provides a measure of the degree to which the beam intercepted the particle and also shows a qualitative correlation to the total number of ions produced. Particles are consistently intercepted by the 10 ns pulse of the Nd: YAG laser after proper beam alignment and trigger delay adjustment. A Galileo Electro-optics Corp. Channeltron electron multiplier with conversion dynode, Model 4773, is mounted below the trap to detect ions as they are ejected through the holes in the bottom cap electrode. The perpendicular axis through the center of the entrance grid of the multiplier is aligned to be collinear with the symmetry axis of the ion trap. In this position, the vertical projection of the grid is intercepted by the conversion dynode, which is mounted at an angle below the grid. The channeltron is mounted adjacent to the open
face side of the conversion dynode. This configuration prevents any particle that might fall through one of the holes in the bottom cap electrode from dropping directly into the channeltron. Electrical connections for the high-voltage input, the signal output, the grid potential, and the conversiondynode potential are made through vacuum feed-throughs in the top flange of the vacuum cube where the rotary vacuum feedthrough is mounted. For positive ion detection, the grid was maintained at -500 V, and the conversion dynode was at ground. For negative ion detection, the grid was at +800 V, and the dynode was at +3000 V. The channeltron was set at -1600 V. A 10.2 cm diameter by 5.1 cm long cap welded to a 15.2 cm flange is mounted on the bottom port of the vacuum cube to provide additional space for the electron multiplier. A helium supply from an adjustable leak valve is connected to the cap to maintain a helium pressure of 0.13 Pa inside the vacuum cube. Helium atoms collide with the ions in the trap to provide increased resolution and sensitivity by dampening the ion trajectories.'* The ITD is tuned throughout the full mass range of the instrument using ions generated from perfluorotributylamine ((C4Fg)sN) so that adjacent peaks differing by 1 amu are resolved. Peak broadening may occur, however, if a particular laser desorption produces an ion population that is sufficiently high to generate a space charge. In this case, the leading and trailing edges of a major peak may be interpreted as separate masses by the ITD electronics and will not be distinguishable from true masses adjacent to the peak. The compounds used for the particle desorption experiments were trimethylphenylammonium chloride, tetraphenylphosphonium bromide, and 2,4,6-trinitrotoluene. The particles were made of silicon carbide that had been produced originally for use as an abrasive. A size distribution analysis of the particles showed that sizes from 98 to 120 pm accounted for the middle 60% of the particles by weight, with a median size of 112 pm. The surface area was 0.1 13 m2/g. The particles were coated with a quaternary salt by dissolving a weighed amount in methanol, combining the solution with a weighed quantity of particles, and evaporating the solvent. Acetone was used to dissolve the 2,4,64rinitrotoluene. The average quantity of material adsorbed per particle in each case was a few nanograms or a few tens of picomoles. This corresponds to about 10 monolayers on the surface of a particle based upon the surface area measurements. Nickel particles with cross-sectional diameters of 50 pm were used for the ablation experiments. The objective of these mass spectral measurements was to investigate the possibility for using laser ablation mass spectroscopy to perform isotopic analyses on individual particles in the ion trap.
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RESULTS AND DISCUSSION The mass spectrum shown in Figure 2 is a composite of results obtained from two trimethylphenylammonium chloride coated silicon carbide particles, each irradiated with a single 10 ns pulse from theNd:YAG laser. Due to the characteristics of the ITD electronics, the full mass range of ions ablated from one particle with one laser pulse cannot be covered with a single mass scan of the ion trap. In this case, an m / z scan (18) Stafford, G. C.; Kelly, P. E.; Syka, J . E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J . Mass Spectrom. Ion Processes 1984, 60, 8 5 .
600 500
1 K+
4
400
'-
5
(CH3)3C6H5N+
4
5 300 > J
Na+
;200
(CH3)( CHz)C6H5N+
G
100 0
Figure 2. Quadrupole Ion trap laser desorption mass spectrum of trimethylphenylammonlumchlorideadsorbedon siliconcarbide particles. 1200
K' 1000
.$ 800 5
i
9
600
$
J
$
400
%
200
0 0
1
50
100
150
200
250
300
350
400
m/z
Figure 3. Quadrupole Ion trap laser desorptlon mass spectrum of tetraphenylphosphonlumbromldeadsorbed on sillcon carbkle particles.
from 20 to 99 was taken for one particle and from 100 to 160 for a second particle. The consecutive m / z scans were then combined to provide the mass range displayed. The peak intensities are expressed as the digitized values of an integrated analog voltage which is proportional to the output of the electron multiplier detector. The parent cation for trimethylphenylammonium chloride, ( C H ~ ) ~ ( C ~ H S )isNobserved +, at m / z 136. The loss of a methane molecule accounts for the peak at m / z 120. The low mass peaks at m / z 23 and 39 are due to impurities of sodium and potassium, respectively, which seem to be omnipresent. The m / z values obtained in the manner described above with the ITD are reproducible from particle to particle, but the relative intensities are not because of the variables involved with each laser desorption, as described later. The more recent Finnigan MAT ITMS electronic ion trap controller provides for single scans from m / z 20 to 650 and will allow the acquisition of the true relative intensities of all of the ions from one particle with one laser pulse. The most satisfactory spectra for the coated particles were acquired with pulse energies of about 2 mJ (lo9 W/cm2). Higher pulse energies overloaded the ion trap and resulted in a decrease in the quality of the mass spectra. This is apparently due to space charge effects. The mass spectrum shown in Figure 3 is a composite of results from three tetraphenylphosphonium bromide coated silicon carbide particles obtained from m / z scans of 20-99, 100-249, and 250-400. The parent cation, (CaHs)$+, is observed at m l z 339. Peaks are also observed that correspond Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
3433
M = (NOe)3(CsHz)CH3* 80
20
40
60
80
100
120
140 160
180 200
220 240
m/z
Flgure 4. Quadrupole ion trap laser desorption mass spectrum of 2,4,6-trinitrotoluene adsorbed on silicon carbide particles.
to the loss of two benzene units at m / z 183 and the loss of three phenyl groups at m / z 108. The loss of a single phenyl group results in an odd electron ion and is occasionally observed in low abundance. The loss of two benzene groups instead of two phenyl groups was confirmed by secondary ion mass spectrometric analysis of the coated silicon carbide particles. The low mass peaks are again due to sodium and potassium. The mass spectrum shown in Figure 4 is a composite of results from two 2,4,6-trinitrotoluene coated silicon carbide particles obtained from m / z scans of 20-99 and 100-240. The parent molecular anion, ( N O ~ ) ~ ( C ~ H Z )-, CisHobserved ~' at m / z 227. Entity losses from the parent anion correspond to the loss of a H' at m / z 226, OH' at m / z 210, NO' at m / z 197, and NO2' at m / z 181. The NOz- anion is observed at m / z 46. To obtain an estimate for the detection limit of tetraphenylphosphonium bromide, experiments were performed with particle samples prepared as described previously with decreasing amounts of the salt until mass analysis of the particles no longer detected the parent ion peak. It is difficult toestablish theaccuracy of thevalueobtained for thedetection limit. This is due to the combined uncertainties of the amount of material that is adsorbed on a particular particle, the uniformity of the adsorption, the permanence of the adsorbed layer prior to analysis, the size and shape of the particle, the rotational position of the particle when it is struck by the laser pulse, the intensity of the laser pulse, and the position of the particle within the intensity profile of the pulse. The particles that we use have different sizes and angular shapes, and the particular surface of the particle that happens to be facing the oncoming laser pulse will be some unknown fraction of the total surface of the particle. In light of these qualifications, a conservative estimate of 0.7 fm per particle was made for the detection limit. The estimate assumes that all of the particles were 112 pm diameter spheres and that all of a 0.3 nm sample of tetraphenylphosphonium bromide was adsorbed with equal proportions on each of the estimated 0.4 X lo6 particles of the 1 g sample. The0.7 fm per particledetection limit is smaller by 4 orders of magnitude compared to earlier work for similar species evaporated onto a probe inserted up to the minimum radius of the ring e1ectr0de.l~It is equivalent to more recent results reported for biological molecules using a probe and matrix3434
Analytical Chemistry, Vol. 66,No. 20, October 15, 1994
assisted laser desorption (MALD).19 It is interesting to speculate on possible improvements in the sensitivity of MALD experiments by applying the technique to particles laser sampled at the center of the trap. We believe that further improvement in the sensitivity for particle analysis is possible. Our detection limit represents about 1O9molecules per particle. The laser beam should easily illuminate 1O8 of these molecules. We are detecting, however, only about 100 ions at this level of loading on the particle. We have shown in earlier work that ions generated by resonance enhanced multiphoton ionization of gas phase species can be trapped and mass analyzed with a 5-50% efficiency.20 For 100 ions detected, this represents 2000-200 ions produced, respectively. The 5-6 orders of magnitude difference between the number of molecules on the particle illuminated by the laser beam and the number of ions produced must, therefore, be explained by some combination of desorption, ionization, and trapping efficiencies. It has been concluded in earlier work that thermal ionization of quaternary ammonium salts is a qualitatively efficient soft ionization process.21 It is possible in the current work that the trapping efficiency is adversely affected by the presence of the particle in the trap causing possible perturbation of the trapping field. We are exploring further the significance of the various efficiencies involved and the implication of the particle's presence within the trap confines for some portion of the trapping period. The mass spectra presented for salts desorbed from falling particles are representative of the better spectra observed and compare favorably with the spectra obtained for the probe mounted samples15J6 and secondary ion mass spectroscopy data for the sameparticles. Spectra of thisquality areobserved with about 50% of the particles that are hit with the Nd:YAG laser. The other 50% of the experiments are either of low ion yield due to poor overlap of the Nd:YAG laser beam with the particle or of too high an ion yield, which overloads the ion trap and results in poor mass resolution from space charge effects. We have also shown that isotope ratios can be estimated from single particle, single laser shot measurements. A spectrum for a -50 pm nickel particle irradiated with a 10 ns pulse from the Nd:YAG laser is shown in Figure 5. The experimental isotope intensities (solid lines) for nickel and potassium are compared to the expected intensities (broken lines) obtained from the product of the total experimental count for each metal ion and the percent natural abundance for each isotope.22 A summary of the results for the isotopic analyses of 36 nickel particles is given in Table 1. Although the experimental average percent abundances found are reasonably close to the accepted values, it would be desirable to determine if the experimental conditions could be adjusted in some way to produce better statistics.
CONCLUSIONS We have demonstrated laser desorption mass spectroscopy for particles in an ion trap for both positive and negative ions. (19) Chambers, D. M.; Goeringer, D. E.; McLuckey, S.A,; Glish, G. L. Anal. Chem. 1993,65, 14. (20) Goeringer, D. E.; Whitten, W. B.; Ramsey, J. M. Inr. J. Muss Spectrom. Ion Processes 1991, 106, 175. (21) Glish, G. L.; Smith, D. H. I n f . J. Muss Spectrom. Ion Phys. 1983, 50, 143. (22) De Bievre, P.; Taylor, P. D. P. Int. J . Muss Spectrom. Ion Processses 1993, 123, 149.
500
Table 1. Nlckel Isotope Analysis Summary
isotope
natural ' abundance, %
abundance found, %
standard deviation
58 60 61 62 64
68.1 26.2 1.1 3.6 0.9
68.1 25.3 1.8 3.5 0.1
4.4 3.0
K+ 400
100
Ni+
1
0 20
1
30
L 40
50
L
60
70
80
90
1
2.0 1.1 0.6
been obtained for single laser shot desorption and ionization. The results of this work will be described in a later paper.
m/z
Figure 5. Isotope intensities for potassium 39 and 41 and nickel 58, 60, 61, 62, and 64 ablated from a nickel particle. Natural abundance, -; experimental, -.
--
It has also been shown that isotopic analysis of particles is possible by laser ablation mass spectroscopy in an ion trap. Improvements in the current apparatus promise to yield higher sensitivities for materials on the surface of particles. Combining particle levitation with ion trapping may allow multiple desorption experiments to be performed on a single particle and thus permit signal averaging. We have developed a laser desorption mass spectroscopy system for the analysis of single levitated particles in the quadrupole ion trap. Spectra have
ACKNOWLEDGMENT The authors express their appreciation to W. H. Christie and T. M. Rosseel for obtaining SIMS data on the particles investigated, to P. J. Todd for interpretation of some of the mass spectra, and to G. L. Glish and D. D. Goeringer for suggesting and providing the quaternary salts and the trinitrotoluene. Received for review April 7, 1994. Accepted July 0, 1994.'
Abstract published in Aduance ACS Abstracts. September 1, 1994.
Analytical Chemistry, Vol. 66,No. 20, October 15, 1994
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