Anal. Chem. 1995,67,4330-4334
Laser Desorption Tandem Mass Spectrometry of Individual Microparticles in an Ion Trap Mass Spectrometer Mo Yang, John M. Dale, William B. Whitten,* and J. Michael Ramsey Chemical and Analytical Sciences Division, Oak Ridge National Laboratoty, Oak Ridge, Tennessee 37831-6142
A technique is described for tandem mass spectrometry of individual microparticles in an ion trap mass spectrometer. The particles are sampled by laser desorption/ ablation after being injected into the center of the ion trap electrodes. The method is especially effective for the detection of trace levels of involatile organic molecules. Test particles were 100 pm silicon carbide grains coated with tetraphenylphosphoniumbromide. Mass spectra of the primary ions and fragment ions following collisioninduced dissociation were measured. There is growing interest in methods for analyzing individual microparticles, both for their characterization and for detecting trace quantities of involatile substances.' Such measurements are especially important when variations in composition must be determined or when only a small fraction of the particles contain a substance of interest. When particles are measured individually, only the background from a single particle will interfere in a given determination. In contrast, the sensitivity of a collective measurement will often be limited by the instrumental dynamic range if only a few of the particles contain the target substance. In this paper, we describe a method for performing laser ablation/ desorption mass spectrometry on microparticles in an ion trap mass spectrometer. Involatile organic substances are detected with high specificity by tandem mass spectrometry of the ions from individual particles. Early mass spectrometric measurements on individual microparticles were performed with a heated filament to evaporate a portion of the particulate Ionization in some cases occurred on contact; in other experiments, a supplemental electron beam was employed. The mass spectrometers were either magnetic sector instruments or linear quadrupole mass filters. This is a powerful technique when real-time monitoring of airborne particles is required at only one or a few values of m/z.ll Off-line measurements on individual particles have been made by laser (1) Spumy, K R In Physical and Chemical Characterization oflndiuidual Airborne Particles; Spumy, K R., Ed.; Ellis Honvood, Ltd.: Chichester, 1986; p 5. (2) Davis, W. D. Enuiron. Sci. Technol. 1977,11, 593; 1977,11, 597. (3) Allen, J.: Gould, R. K Reu. Sci. Instrum. 1981,52, 804. (4)Stoffels, J. J. Int. J. Mass Spectrom. Ion Processes 1981,40, 217: 1981,40, 223. (5) Stoffels, J. J.; Lagergren, C. R. Int. J. Mass Spectrom. Ion Processes 1981, 40, 243. (6) Meyers, R L.; Fite, W. L. Environ. Sci. Technol. 1975,9, 334. (7) Sinha, M. P.; G ~ nC., E.; Noms, D. D.; Estes,T. J.:Vilker,V. L.: Friedlander. S. K. J, Colloid Inteiface Sci. 1982,87, 140. (8) Sinha, M. P.; Platz, R. M.; Vilker, V. L.; Friedlander, S. K. Int. J. Mass Spectrom. Ion Processes 1984,57, 125. (9) Sinha, M. P.; Friedlander, S. K Anal. Chem. 1985,57, 1880. (10) Sinha, M. P.: Friedlander, S. K. J. Colloid Intetface Sci. 1986,112, 573.
4330 Analytical Chemistry, Vol. 67,No. 23,December 7, 7995
microprobe analysis, secondary ion mass spectrometry, and optical measurements12and more recently by laser ablation in an ion trap mass spectrometer.13J4 A recent spate of articles has appeared describing on-line single-particle measurements with pulsed laser desorption and ionization (see, for example, Prather et all5 and Hlnz et all6 and references therein) building on the pioneering work by Siha17and McKeown et The on-line laser ionization experiments have mostly used time-of-flight mass spectrometry to analyze the particles following laser sampling. The experiments in the present investigation were carried out with an ion trap mass spectrometer. Although it is somewhat slower than a time-offlight instrument, the ion trap has the important capability of allowing tandem mass spectrometry for molecular samples. Ions of a single mass-to-charge ratio can be selectively trapped and collisionally dissociated and the resulting fragment ions subsequently mass analyzed. The high storage and detection efficiency of the ion trap permits this process to be repeated several times if needed.lg This additional information, which cannot be obtained with conventional time-of-flight mass spectrometry, enhances the specificity of a determination. In the present investigation, we have devised a method for analyzing a batch of collected microparticles by injecting them one by one into the center of the ion trap electrodes, where they can be sampled by laser desorption/ablation. Mer a storage time of a few milliseconds, the resulting ions are either mass analyzed directly by a mass-selective instability determinationz0or analyzed by tandem mass spectrometry as described above. EXPERIMENTAL SECTION In the experiments described in ref 13, particles were introduced into the ion trap by dropping them from a hopper mounted above the electrodes. A serious problem experienced with this particle hopper system was that occasionally several particles would be released with somewhat different velocities or arrival (11) Stoffels, J. J.; Wacker, J. W.; Kelley. J. M.: Bond, L. A,; Kiddy, R A,; Brauer, F. P. Appl. Spectrosc. 1994,48, 1326. (12) Reference 1, Chapters 13-15. (13) Dale, J . M.; Yang. M.; Whitten, W. B.: Ramsey, J. M. Anal. Chem. 1994, 66, 3431. (14) Yang. M.: Dale, J. M.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1995, 67, 1021. (15) Prather, K A; Nordmeyer, T.; Salt, K Anal. Chem. 1994,66. 1403. (16) Hlnz. IC-P.; Kaufmann, R; Spengler, B. Anal. Chem. 1994,66,2071. (17) Sinha, M. P. Reu. Sci. Instrum. 1984,55, 886. (18) McKeown, P. J.; Johnston, M. V.: Murphy, D. M. Anal. Chem. 1991,63, 2073. (19) Louris. J. N.; Brodbelt-Lustig, J. S.; Cooks, R G.; Glish, G. L.; Van Berkel, G. J.; McLuckey, S. A. Int. J. Mass Spectrom. Ion Processes 1990,96, 117. (20) Stafford, G. C.; Kelley, P. E.; Syka, J . E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984,60, 85.
0003-2700/9510367-4330$9.00/0 0 1995 American Chemical Society
PARTICLE
2
Launch
Laser Desorption
PHOTODIODE PARTICLE DETECTOR
CHANNELTRON ION DETECTOR
RESERVOIR
RF SCAN for MS
2I
SOLENOID
Figure 1, Particle launcher mounted below the ring electrode. The rotational symmetry axis of the ion trap is horizontal.
times. If the desorption laser pulse sampled the first falling particle and produced desorbed ions, the following particles would adversely affect the mass spectrum. Single particle drops are hard to achieve with this system because of variations in the size and shape of the particles. The task is even more difficult when the particles carry organic substances on their surface that make them adhere to one another. A solenoid-driven particle launcher mounted at the bottom of the ion trap, as shown in Figure 1,has proven more successful at delivering single particles. The principle of the particle reservoir is similar to that of a salt shaker, in that only a certain number of particles can be ejected from the reservoir each time. The solenoid controls the launching speed so that only the fastest particle arrives at the center. The other particles return to the container without reaching the center of the ion trap. The NdYAG laser fires at this arrival time to desorb material from the particle. The time to reach the trap center depends only on the initial velocity, controlled by the solenoid, and is therefore independent of particle size. Since the arrival time is reproducible and the particle stays inside of the laser beam area for a few milliseconds, the probability of the desorption laser striking the particle is very high, so that optical particle detection and laser triggering are not required. A HeNe laser, not shown in Figure 1,is used to adjust the solenoid drive to the desired level. In this way, reliable desorption mass spectra have been achieved for up to 60%of the particle launchings. The ion trap electrodes and control electronics were manufactured by Finnigan MAT for their ion trap mass spectrometer (ITMS) . The ring electrode was modilied by drilling several holes, two for the laser beam and one for particle introduction,as shown in Figure 1. With ITMS software developed by Finnigan MAT, we can set up a scan function for the sequence of ionization, mass selection, collisional dissociation, and mass analysis of the resulting fragment ions. Figure 2 shows the time sequence of the particle launch, desorption ionization, and radio frequency (I$ scan function for the MS/MS measurement. The ion trap was usually run at a rate of 4-5 scans/s, with the particles launched synchronously so that they arrived at the trap center while the ion trap was in the ion trapping mode. (The electron gun normally used for electron impact ionization was no longer present. The internal gate pulse could be used, however, to determine the start
RF SCAN for MS/MS Figure 2. Time sequence of particle launch, laser desorption ionization, and rf scan function for mass selection, tickling, and daughter mass scan.
of the rf scan.) The duration of the ion trap rf scan, from the beginning of the electron gate pulse to the end of the fragment mass scan, was typically 160 ms. Tickle conditions for a selected ion mass were found by trial and error. The resonance frequency and tickling voltage used in this experiment were very close to those used by Johnson et a1.2l Detailed methods for MS/MS measurements in an ion trap are described elsewhere.22 The second harmonic output from a Nd-YAG laser, a Quanta Ray DCR-2, was used for desorption/ionization of the microparticles. The laser pulses at 532 nm were defocused to avoid excessive production of ions and possible rf discharges. The laser beam diameter at the center of the trap was 1.7 mm, and the laser intensity was estimated to be about 7.8 mJ mm-2. The delay time of the Nd-YAG laser flashlamp discharge after a laser trigger is about 3 ms, and the Q-switching is delayed further by 200 pi. The fourth harmonic output from the laser at 266 nm was used to ionize residual hydrocarbon molecules in some experiments. The Q-switching is controlled externally by an electronics circuit that generates a switching pulse at any desired rf phase. With this circuit, we can investigate trapping efficiency when the ions are created under maximum or zero electric field within the trap. The Q-switch trigger pulse timing is obtained by counting cycles of the rf voltage to -200 p s and adding an additional delay to generate a laser pulse with the desired phase. W laser ionization of background hydrocarbon gases as a function of the rf phase angle has revealed that about 50% more ions are detected for ionization at 90 or 270", where the magnitude of the field is a maximum, than at 0 or 180", where the field strength vanishes. In contrast to ions produced by particle laser ablation at trap center, background ions are generated at any position along the laser beam passing diametrically through the ring electrode. Those produced near the electrode will presumably be lost if the field is of the wrong sense. Most of the mass spectra in this experiment were measured at a phase angle of 270", when the ring electrode is at maximum positive potential. (21) Johnson, J. V.; Pedder, R E.; Yost, R. k Int. /. Mass Spectrom. Ion Processes
1991,106, 197. (22) McLuckey, S. A; Glish, G. L.; Van Berkel, G. J. Int. J. Mass Spectrom. Ion Processes 1991,106, 213.
Analytical Chemistry, Vol. 67, No. 23, December 7, 1995
4331
3000
1
Ion Counts: 4803
P h4PC
I
~
Phf+ 7520
1 '
0
1
1
I
0
100
200
300
400
ION MASS [ m k ]
Figure 3. Typical primary mass spectrum of TPPB laser-desorbed from a Sic microparticle.
19645
Tetraphenylphosphonium bromide VPPB) , obtained from Aldrich Chemicals, was dissolved in acetone. Silicon carbide (Sic) particles (120.grit) were drenched with this solution and dried in a desiccator. A size distribution for the particles obtained by sieving showed that 60% of the particles were between 98 and 120 pm. The average sample mass per particle is estimated to be 6 x g. For these measurements, the ion trap was filled with 0.1 Pa of helium buffer gas. A membrane manometer with an automatic valve controller served to maintain constant pressure.
0 100
0
200
300
ION MASS [m/z] Figure 4. Fragmentation patterns of primary mass spectra of TPPB
RESULTS In Figure 3, we show a typical mass spectrum of TPPB desorbed from a Sic microparticle by a 532 nm laser pulse, displaying parent ion and fragmentation peaks plus the ubiquitous potassium and sodium peaks. The degree of fragmentation apparent in the primary mass spectrum is found to be inversely related to the total number of ions produced in the ion trap. Figure 4 shows mass spectra associated with three different total ion counts, all measured under the same nominal conditions. At low ion number, the fragmentation peaks are larger than or comparable to the parent ion peak. The center curve shows a fragmentation pattern at a medium ion number. When the ion number is very high (bottom curve), virtually no fragmentation peaks are visible compared to the parent ion peak. We find that the mass spectra of TPPB-coated particles with approximately the same number of total ion counts display similar fragmentation patterns. The rf phase dependence of ion collection efficiency is not as conclusive with particle measurements as it was with ions generated from the hydrocarbon background because of the large particle-to-particle variation in ion count. This variation in total ion yield per particle for a typical series of measurements is shown in Figure 5. We attempted to measure the rf phase dependence of the primary mass spectrum by firing the laser at phase angles of 0, 90, 180, and 270" of the rf cycle. The large pulse-to-pulse variation due to other effects obscured any dependence of the total ion count on rf phase. However, by comparing mass spectra with comparable total ion count numbers, we find that slightly fewer fragment ions are observed with the laser firing at an rf phase angle of 0" than at 270". For measurements of MS/MS fragmentation, ions of all masses except the parent ions are ejected. These ions are "tickled" by 4332
Analytical Chemistry, Vol. 67, No. 23, December 1, 1995
for different total ion count numbers. ao
,
I
m
I
0
5
10
15
20
25
30
PARTICLE LAUNCH Figure 5. Total ion yield per particle in a typical series of measurements. applying an rfvoltage to the endcap electrodes for 20 ms, causing dissociation through collisions with the buffer gas. The masses of the fragment ions are subsequently determined by ramping the ring rf voltage. The mass selectivity for the parent ion can be tested by turning off the tickle voltage. Without tickling, only the parent peak is observed, independent of the total ion count number. However, the fragmentation pattern of tickled parent ions is again found to be dependent on the total ion count number. Figure 6 shows the variation of the pattern of fragment ions for dfierent total ion counts, with other conditions unchanged. When the ion count number is low, almost all of the parent ions are dissociated, while very little fragmentation appears for a high ion count number. The relative fragmentation is consistent for those
Dissociation
detection sensitivity for analytes adsorbed on a microparticle surface is expected to be very high for this reason. The relative yield of fragment ions was observed to be lower at high total ion count for both primary and MS/MS spectra. For the primary ion spectra, this pattern change could be due to either a smaller percentage of fragment ions being generated at high total count or a lower trapping efficiency for these fragments. There is not enough information from the present experiments to decide whether either or both mechanisms are responsible for the observed dependence. In the region qz < 0.4, where a pseudopotential model is applicable, theory predicts a greater trap depth for lighter so these would have to acquire considerably greater kinetic energy in fragmentation than the heavier parent ions to produce the observed spectra. Space charge calculations2*show that the maximum number of ions that can be trapped varies inversely with mass for constant trapping voltage. This dependence implies similarly that lighter fragments should dominate the mass spectrum when a large number of ions are trapped. Since the opposite occurs, it seems likely that fragmentation of the parent ions is inhibited as the total ion count increases. A similar dependence has been observed in matrixassisted laser desorption/ionization experimentsz5and attributed to rotational and vibrational cooling during the expansion of the ablating species. It is possible that expansion cooling is reducing the fragmentation in the present experiments as well, although in this case the expansion would be homogeneous, i.e., with no matrix, and collisions with neutral TPPB molecules would presumably produce the cooling. Although the dependence of fragmentation pattern with total ion count for the MS/MS spectra is qualitatively similar to that of the primary ion spectra, the conditions responsible are quite different, with the fragmentation now due to collisions with the helium buffer gas. It is most likely that the observed dependence is due to detuning of the tickle frequency as the trap is filled with ions. Such detuning is now well e s t a b l i ~ h e d . ~ ~ ~ ~ ~ One serious problem with the laser desorption process is that too many ions are sometimes produced in the ion trap, resulting in nonnegligible space charge distortions. Another problem is the fluctuation of total ion number from particle to particle. This fluctuation is probably because of irregularities in particle shape and surface conditions or in the thickness of the sample layer. However, very consistent fragmentation mass spectra have been observed in both primary and secondary mass spectra selected for the same total ion number. Reducing shot-to-shot variations in total ion count in the ion trap w illbe very important for obtaining reproducible and reliable fragmentation mass spectra. In laserinduced desorption ionization, controlling the generation of ions is difficult because the nanosecond pulse duration is too short for present day feedback technology. However, it should be possible to regulate the number of ions in the trap after the desorption/ionization but prior to collisional dissociation. Broadband excitation techniques are now available to circumvent the
25
Ion Counts:
I
I
Precursor Ion
,
b
3942
I
6148
10111
120
200
280
360
ION MASS [mk] Figure 6. MS/MS spectra of TPPB for different total ion count numbers.
determinations that give approximately the same total ion counts. DISCUSSION The experiments described above show that laser ablation mass spectrometry in an ion trap mass spectrometer can be an effective method for characterizing collected microparticles. The MS/MS capability of the ion trap considerably enhances our ability to positively identify a particular organic substance that might be present on the particle. By measuring the particles one by one, we can explore the variation in particle composition or detect substances present in only a few particles with higher sensitivity than by measuring the entire sample at once. However, the uncertainty in the total ion count to be expected from each particle event makes MS/MS with discrete frequency tickling more difficult. The sample introduction system with microparticles launched from below the ion trap has been shown to be very reliable and reproducible. The probability of obtaining sufficient ions from a single particle to measure a mass spectrum in the ion trap has been increased to about 60%, as shown in Figure 5. There is a large variation in total ion count from particle to particle, due to differences in sample loading, laser fluence, and ~rientation.~~ The laser pulses of 532 nm were defocused so that the laser intensity was barely enough to produce desorbed ions. Even close to the threshold for desorption/ionization,however, some particle events produced enough ions, 104-105, to overload the trap and distort the mass spectrum. With a focused laser beam, ion counts several orders of magnitude higher than this could be obtained. The
(23) Dehmelt, H. G . Adu. At. Mol. Phys. 1967, 3, 53. (24) March R E.; Hughes, R J. Quadrupole Storage Mass Spectroscopy; Wiley: New York, 1989; p 192. (25) Reilly, P. T. A.; Reilly, J. P. Rapid Commun. Mass Spectrom. 1994, 8, 731. (26) Todd, J. F. J.; Waldren, R M.; Freer, D. A; Tumer, R. B. Int. J. Mass Spectrom. Ion Phys. 1980, 35, 107. (27) Yates, N. A.; Yost, R. A; Bradshaw, S. C.; Tucker, D. B. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 1991; p 132.
Analytical Chemisfty, Vol. 67, No. 23, December 1 , 1995
4333
shift in ion resonance frequency with ion c o ~ n t . ~With ~ - these ~~ enhancements, ion trap laser desorption tandem mass spectrometry should be a practical technique for chemical analysis of microparticles.
Sciences, under Contract DE-AC05840R21400 with Lockheed Martin Energy Systems, Inc. The authors thank D. E. Goeringer and S. A. McLuckey for helpful advice concerning the operation of the ion trap mass spectrometer and P. T. A. Reilly for discussions about expansion cooling.
ACKNOWLEDGMENT
This work was sponsored by the US.Department of Energy, Office of Research and Development and Office of Basic Energy (28) McLuckey, S. A,: Goeringer. D. E.; Glish, G. L.Anal. Chem. 1992,64,1455. (29) Guan S.: Marshall, A G. Anal. Chem. 1993, 65, 1288. (30) Julian, R. IC: Cooks, R. G. Anal. Chem. 1993, 65, 1827.
4334 Analytical Chemistry, Vol. 67,
No. 23, December
1, 1995
Received for review July 7, 1995. Accepted September
12, 1995.@ AC9506746 %Abstractpublished in Advance ACS Abstracts, October 15, 1995.
