Accelerated Articles Anal. Chem. 1995, 67, 1021-1025
Laser Desorption Mass Spectrometry of a Levitated Single Microparticle in a Quadrupole ion Trap Mo Yang, John M. Dale, William B. Whitten,* and J. Michael Ramsey Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142
Techniques for microparticle levitation in a vacuum ion trap and switchingof electricfields from particle levitation mode to ion trapping mode prior to laser desorption/ ionization and mass analysis are described. Single microparticles of rubidium chloride and copper oxide with a size of -20 pm have been levitated in a quadrupole ion trap and ablated by a Nd/YAG laser (532 nm)followed by ion mass analysis in the same ion trap. The mass spectrum of an organic sample (tetraphenylphosphonium bromide) that was deposited on the surface of a Sic microparticle has been similarly measured. Chemical characterization of individual microparticles is becoming increasingly important for detecting the presence of involatile trace substances in the environment. Laser desorption mass spectrometry has been shown to be a powerful method for analysis of airborne particles on-line'-5 or from a batch of collected microparticles.6 Although these techniques interrogate particles individually, they are not designed to analyze every particle in a sample. Thus, a somewhat different approach must be used if at most a few microparticles are available for analysis. The purpose of this paper is to describe a technique for analyzing single microparticles in which a quadrupole electrodynamic trap is used for both particle levitation and mass analysis by laser desorption mass spectrometry. (1) Sinha, M. P. Reo. Sci. Instrum. 1984, 55, 886-891. (2) McKeown, P. J.; Johnston, M. V.;Murphy, D. M. Anal. Chem. 1991, 63, 2069-2073. (3) Reents, W. D., Jr.; Swanson, A; Downey, S.; Mujsce, A N.; Muller, A J.; Emerson, B.; Siconolsi, D.; Sinclair, J. D. Proceedings, 4lstASMS Conference on Mass Spectromety and Allied Topics, San Francisco, 1993; p 640. (4) Hinz, IC-P.; Kaufmann, R; Spengler, B. Anal. Chem. 1994,66,2071-2076. (5) Nordmeyer, T.;Prather, K. A. Anal. Chem. 1 9 9 4 , 66, 3540-3542, Prather, K A; Nordmeyer, T.;Salt, K Anal. Chem. 1994,66, 1403-1407. (6) Dale, J. M.; Yang, M.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 1994, 66,3431-3435.
0003-2700/95/0367-1021$9.00/0 0 1995 American Chemical Society
Laser-induced desorption and ionization of molecules adsorbed on a surface is a useful approach for generating ions of involatile or thermally labile molecules. When an intense laser beam is incident on a surface, ionization generally occurs just above the surface following the desorption of neutral species? Thus, one laser pulse can desorb and ionize the molecules simultaneously. Laser desorption mass spectrometry of microparticles falling through an ion trap mass spectrometer has been previously reported.6 A microparticle can be levitated either optically using a laser beams or electrodynamically in a quadrupole trap.9-14 Since the irregular shape of solid microparticles results in inhomogeneous radiation pressure for optical levitation, electrodynamic levitation is preferable. Methods of charging and levitating microparticles in an electrodynamic trap were developed at about the same time as those for trapping ions.9J5 While a microparticle is suspended within a trap in preparation for mass analysis, it is amenable to other methods of chemical and physical characterization. Measurements of the absolute charge and mass of a levitated particle have been reported.I6 Levitated microparticles can also be studied by optical absorption, fluorescence, or Raman spectros~opy.~~-~~ (7) Yang, M.; Reilly, J. P. J. Phys. Chem. 1990, 94, 6299-6305. (8) Ashkin, A Phys. Rev. Lett. 1 9 7 0 , 2 4 , 156-159. (9) Weurker, R F.; Shelton, H.; h g m u i r , R. V.J. Appl. Phys. 1 9 5 9 , 3 0 , 3 4 2 349. (10) Frickel, R B.; Shaffer, R E.; Stamatoff, J. B. U S . Army Technical Report ARCSLTR-77041, 1978. (11) Davis, E. J. Langmuir 1 9 8 5 , 1, 379-387. (12) Brandt, E. H. Science 1 9 8 9 , 2 4 3 , 349-355. (13) Amold, S.; Folan, L. M. Rev. Sci. Instrum. 1 9 8 7 , 58, 1732-1735. (14) Davis, E. J.;Buehler, M. F.; Ward, T. L. Reo. Sci. Instrum. 1990,61.12811288. (15) Vedder, J. F. Rev. Sci. Instrum. 1 9 6 3 , 34, 1175-1183. Arnold, S. J. Colloid Interface Sci. 1983, 91, (16) Philip, M. A,; Gelbard, T.; 507-515. (17) Amold, S.; Pluchino, A B. Appl. Opt. 1982, 21, 4194-4196.
Analytical Chemisty, Vol. 67, No. 6, March 15, 1995 1021
While essentially any electrode geometry with a nonzero quadrupole field at the trap center may be used for particle levitation,2lthe hyperbolic geometry used by the commercial ion trap mass spectrometers produces a nearly pure quadrupole field throughout the trap interior that is important for some of the mass spectral methods.22 The ion trap consists of two end cap electrodes and one ring electrode between them. A radio frequency (rf) high voltage is generally applied to the ring electrode for ion trapping. An ion produced inside of the ion trap then oscillates according to the oscillating electric field. The displacement of the oscillating particle can diverge (ion ejection) or converge (ion trapping), depending on the frequency and voltage applied on the electrodes. Molecular ions and charged microparticles behave in the same way. However, since the masses of ions (-100 amu) and microparticles (1015m u ) are very different, the required electric fields for particle levitation are different from that for ion trapping. The ring electrode voltage is usually at audio frequency in this case. Furthermore, if the particle is to be centered in the trap, an additional dc electric field is required to compensate for the gravitational force on the particle. Thus, to combine particle levitation with ion mass spectrometry, the trap conditions must be rapidly altered when the ions are generated. The theory of particle levitation in hyperbolic electrodynamic traps has been developed by Frickel et a1.lo and by Davis,I2while more recently, Hartung and Avedisian reviewed the theoretical and experimental work on electrodynamic traps of various geometries.21 Because the electric field for particle levitation breaks the symmetry of the trap, the potential distribution cannot be calculated analytically for hyperbolic electrodes. However, numerical methods can be used to calculate the potential and electric field within the trap to any desired precision. The motion of both microparticles and ions in a quadrupole field is described by the same Mathieu equations,22
d2z dz 2 m-+~-++&[U-Vcos(wt)l~=O de dt 20
(1)
where m and Q are the mass and charge of the particle, k is the drag coefficient, and U and V are the dc and ac voltages on the ring electrode. The quantities ro and 20 are the distances from the trap center to the ring and end cap electrodes, respectively, and w is the frequency of the ac voltage, V. For microparticle levitation, an additional voltage between the end caps balances the gravitational force. As long as the oscillation amplitude is small, these two forces add to zero and can be neglected in eqs 1and 2. The condition for stability is also the same when there is no damping-undamped particles will be trapped when (18) Ng, IC C.; Whitten, W. B.; funold, S.; Ramsey, J. M. Anal. Chem. 1992, 64, 2914-2919. (19) Barnes, M. D.; Ng, K. C.; Whitten, W. B.; Ramsey, J. M.Anal. Chem. 1993, 65, 2360-2365. (20) Tang, I; Munkelwitz, H. R. J Colloid Intelface Sci. 1 9 8 4 , 98, 430-438. (21) Hartung, W. H.; Avedisian. C. T. Proc. R. SOC. London A 1992,437,237266. (22) March, R E.; Hughes, R J. Quadrupole Storage Mass Spectromety. Chemical Analysis; Winefordner, J. K, Ed.; Wiley-Interscience: New York, 1989 Vol. 102.
1022 Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
is satisfied?* The microparticles are first trapped at atmospheric pressure. Here, the drag force can be sufficiently large to aifect the phase angle between the motion and the applied f0rce.2~The stability conditions must be calculated numerically in this case. After the particle is trapped, however, the pressure in the chamber is reduced to less than 1 Pa and eqn 3 is applicable. While both ions and charged microparticles obey the same equations of motion, the large difference in mass-to-charge ratio results in different trapping conditions for the two cases. The required voltage for ion trapping is typically a few hundred volts at the frequency of 1.1 MHz employed in the commercial trap used for this study, while that for particle levitation is -lo2 V in the frequency range of 30-100 Hz. The well depth for ions in an rf-only ion trap is at most a few electronvolts?2 Thus, the application of dc or low-frequency fields of tens of volts will preclude the storage of ions in the trap. On the other hand, it is possible to trap charged microparticles in the presence of an rf field. Since the force is proportional to 0 . - ~ , 2 an ~ audio frequency field will dominate if both are present. Thus a microparticle can be levitated with electric fields of both frequencies applied, but ion trapping is not possible in the presence of the electric field for particle levitation. To measure the mass spectrum of ions desorbed from a levitated microparticle, the dc and low-frequency ac electric fields must be switched to zero prior to ion generation. The experimental methods presented below demonstrate one way to accomplish the alteration in trap conditions. EXPERIMENTAL SECTION The particle levitator and mass spectrometer used the electrodes, electronics, and software from a commercial ion trap mass spectrometer (ITMS) manufactured by Finnigan Mat. The electrode assembly was mounted in a &ii. cube evacuated by a turbomolecular pump. The minimum background pressure was 4x Pa. A bypass gas line permits the system to be evacuated slowly and provides constant buffer gas pressure during measurements. Four 3.18-mm holes were drilled in the ring electrode for the laser beam and for the observation of particle motion. The ITMS was operated in a mode usually employed when the instrument is used as a mass-selective detector for a gas chromatograph. In this mode, the instrument makes repeated mass spectral scans over a predetermined mass interval until stopped. The electron gun that normally generates ions was removed, however, so that particles could be introduced through the top end cap. Therefore, the recorded “chromatogram” consists mostly of baseline determinations with one valid spectrum obtained after the laser was fired. The switching of ac and dc voltages is accomplished by four high-speed high-voltage transistors controlled by a timing circuit. A block diagram of the electronics is shown in Figure 1. The function generator coupled with a linear amplifier provides ac voltages with variable frequency. When the pressure has been reduced to a level suitable for a mass spectral determination, the timing control circuit is manually activated to monitor the clock pulses from the function generator and electron gate pulses from (23) Amold, S. Spectroscopy of Single Levitated Micron Sized Particles. In Optical Efects Associated with Small Partidesk Barber, P. W., Chang, R. IC, Eds.; World Scientific; Singapore, 1988.
AC 30-100H~
FUNCTION GENERATOR I
I
--
4
1
Figure 1. Block diagram of the experiment showing the circuitry for particle levitation and ion mass spectrometry.
A.
CLOCK
0.
CLOCKl4 4
C . STARTING PHASE D.
E-GATE
E.
(CAND D)
- VARIABLE
A I
+
i 3.2ms
A 2.5
F.
LASER DESORPTION
G.
SWITCHING TIME
I i-
ms
VARIABLE
Figure 2. Timing diagram for particle release and laser triggering. The clock signal is at the same frequency and phase as the ac levitating voltage, applied common mode to the end cap electrodes. This frequency is first divided by 4 to preclude two laser pulses in a single mass determination. The start pulse, C, is delayed from the leading edge of (6) as desired. E-gate is a signal from the ITMS electronics indicating that the rf voltage on the ring electrode is suitable for ion storage prior to mass analysis. The delay between (E) and (F) is the internal delay of the Quanta-Ray NdNAG laser. The pulse, G, grounds the end cap electrodes just prior to the laser pulse for ion storage and mass analysis, zeroing the audio frequency and dc fields within the trap.
the ITMS electronics. If conditions are optimum for laser ablation mass spectrometry, the control circuit switches the ac and dc voltage to zero and triggers the YAG laser. The time sequence of the pulses is shown in Figure 2. A complete description of the electronics circuitry will be published elsewhere. A length of &gauge Teflon tubing with a thin Teflon plunger inside was used to charge the particles. Rubbing the Teflon surface results in concentrated surface charge at the end of the tube. The charged end of the Teflon tube is used to pick up a number of particles and bring them to the top of the ion trap. The plunger inside the tube pushes and drops the particles through the hole in the top end cap electrode. A high-voltage ring electrode helps to increase the charge on the particle. Although the amount of charge on each particle varies, some of the particles have charge/mass ratios that produce stable tra-
jectories given the applied dc and ac voltages. If several particles are trapped simultaneously, they can be ejected one by one by varying the ac voltage or frequency until only one particle remains. Since the maximum voltage (dc plus peak of ac voltages) must not exceed the gaseous discharge breakdown voltage during the evacuation of the trap, the ac voltage was fixed at -100 V and the ac frequency varied from 30 to 100 Hz to establish stable particle levitation. The gravity balance of the particle can be adjusted by controlling the dc voltage between the two end cap electrodes. The dc voltage between the two end cap electrodes was usually in the range of 10-30 V. While the particle was levitated at the center, the ion trap chamber was evacuated slowly with a rotary roughing pump. About 20 min was typically required to reach the 0.1 Pa gas pressure range. The pressure inside the ion trap was then maintained at 0.13 Pa with a turbomolecularpump and a helium leak. A Nd/YAG laser, Quanta Ray Model DCR-2A, with a second harmonic generator provided laser light at 532 nm. The laser was triggered by timing control electronics. The laser beam was focused by a lens with a focal length of 1 m. The focused beam waist was -300 pm. The laser beam was aligned to intercept the levitated particle by reducing the pulse energy to -1 ,uJ and observing the scattered light through a telescope. The laser pulse energy for desorption and ionization was typically a few millijoules. RESULTS AND DISCUSSION
The mass-tocharge ratios of the levitating particles were calculated from the dc voltage between the end caps while the particle was levitated at the center of the ion trap. From ref 18 , the electric field at the center of a hyperbolic trap is given by CoW/2z0,where W is the dc voltage between the two end caps at balance and COis the first term in the series expansion for the potential in spherical harmonics. Numerical results include 0.8 by Philip et and, more recently, 0.802 787 f 0.000 091 by Hartung and Avedisian.21 A value of 3/4 can be obtained analytically by evaluating the h-st four terms of the Legendre polynomial expansion at (fro, 0) and (0, Experimental values for CM (24) Yang, M., unpublished result.
Analytical Chemistry, Vo/. 67, No. 6, March 15, 7995
1023
63
cu
2000 v)
!Z 1500
II
3 0
g
1000
0 500
20
0
40
60
0
100
80
100
ION MASS [m/z]
200 250
200
250
ION MASS [mk]
[
85
Rb
2 2ooo
v)
53 150
150
-
-
0
;
K
100 -
0,D
87
Rb
0 500 -
0
0' 0
20
40
60
80
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ION MASS [m/z]
compiled by Hartung and Avedisian range from 0.71 to 1.17.21 Deviations in trap geometry from the ideal, i.e., holes in the electrodes, are most likely responsible for the differences between experimental and theoretical results. Lacking experimental data for our electrode configuration, we therefore take the mass-tocharge ratio to be approximately
(4)
where g is the acceleration of gravity. For example, if a 10-V potential applied between the two end cap electrodes stabilizes a particle at the center of a standard ion trap where zo = 0.707 cm, then the mass-to-charge ratio of the particle is calculated to be m/Q = 57 kg/C = 5.5 x lo9 amu/e. The size of the levitated particle can be estimated by visual observation through a telescope. Since the density of the particle is known, the net charge on the particle can be calculated. The mass of a typical levitated particle in this experiment was of the order of a few nanograms with a surface charge of -1.6 x C or 105 unit charges. The particle mass and charge were different for each particle. In the present experiment, the voltages on the electrodes were set to certain values, so that only particles that had charges in the appropriate range could be caught in the ion trap. Thus, only a fraction of the sample particles were trapped on the first attempt. The untrapped particles could in principle be recovered and the process repeated until successful. 1024 Analytical Chemistry, Vol. 67, No. 6, March 15, 1995
300
350
400
ION MASS [m/z]
Figure 3. (a) Mass spectrum of laser-produced ions from a 50-ng CuO particle. The data are recorded with unit mass resolution. (b) Mass spectrum of ions from a 20-ng crystal of RbCI. Both particles were individually levitated within the ion trap prior to the laser pulse.
m/Q x 0.4W/z,g
250
Figure 4. Mass spectra of ions desorbed from Sic particles coated with tetraphenylphosphonium bromide: mass scan (a) from 100 to 250 amu and (b) from 250 to 400 amu. A different particle was trapped for each scan.
To test the system with inorganic microparticles, individual CuO and RbCl crystals with a size of -20 ym (approximately 50 and 20 ng,respectively) were levitated followed by laser desorption mass spectromeby. The mass spectrum of a levitated copper oxide microparticle is shown in Figure 3. The intensities of the two isotopes have a 1.9 ratio compared to the known value of 2.2. The total ion count for this mass scan was 9377. It was often observed that the mass peaks broaden at higher ion numbers, as shown in this mass spectrum. (The ITMS samples the mass spectrum at integral values of m/z only. We assume that masses not expected to be present that appear adjacent to prominent peaks, such as at m/z 61, 62, and 64,are really tails of the larger peaks). Figure 3b shows the mass spectrum of a levitated rubidium chloride microparticle. The intensities of the two rubidium isotopes have a 2.8 ratio compared to the known value of 2.6. The total ion counts in this mass scan was 418, so that better resolved mass peaks were recorded. The potassium peak is an impurity that seems to be ubiquitous. For the preparation of organic test samples, Sic microparticles of an average size of 100 ym were added to a tetraphenylphosphonium bromide (TPPB) solution in acetone and the liquid was evaporated. Single Sic microparticles with adsorbed TPPB were levitated and irradiated with the desorption laser followed by ion mass analysis. The mass spectra of two levitated TPPB-coated microparticles are shown in Figure 4a,b. The upper mass spectrum (a) was measured in the mass range from 100 to 250 while the lower one (b)was measured in the mass range of 250400. The parent cation, (c~Hj)&'+, is observed at m/z 339. Peaks also are observed that correspond to the loss of two benzene units
at m/z 183 and the loss of three phenyl groups at m / z 108. This investigation has shown that it is possible to both suspend an individual microparticle and analyze that particle by laser ablation/desorption mass spectrometry in the same electrodynamic trap. Techniques for microparticle levitation in an evacuated quadrupole ion trap and for switching the electric fields from levitation mode to ion trapping mode have been developed. S i i l e microparticles of CuO and RbCl(-20 pm) have been levitated in the quadrupole ion trap and ablated by a Nd/YAG laser followed by ion mass analysis in the same ion trap. Mass spectra of an organic sample, tetraphenylphosphonium bromide, laserdesorbed from the surface of levitated Sic single microparticles have also been measured by the same method. Two areas need further development to make the technique practical for routine analysis of specific individual microparticles. The first is to automate the charging and trapping process so that particles of widely varying mass-to-charge ratio can be captured. An optical feedback technique controlling the levitating dc electric field such as that described by h o l d z 3could perhaps be used to increase the trapping probability. The second area concerns the large variation in the number of ions produced with one laser pulse. The problem here is to limit the total number of ions (space charge effects reduce the achievable mass resolution when the
ion trap contains a large number of ions) without losing important information. When only a small mass range must be analyzed, such as for isotopic analysis of one element or for comparing particles of similar composition, several methods are available to eject most of the irrelevant ions or selectively accumulate ions of i n t e r e ~ t ?If~ion , ~ ~production could be monitored on a nanosecond time scale, it might also be possible to modulate the laser intensity to limit the total ion count. A mode-locked laser pulse train might be a better ablation source in this case than a single Q-switched pulse of equivalent energy. These enhancements appear feasible with present technology if there is s d c i e n t demand for such an instrument.
(25) Julim, R IC, Jr.; Cox, IC; Cooks, R G. Anal. Chem. 1993,65, 18271833. (26) Goeringer, D. E.; Asmo, IC G.; Mchckey, S. A; Hoekman, D.; Stiller, S. W. Anal. Chem. 1994,66, 313-318.
AC941130R
ACKNOWLEDGMENT
This research was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, and Office of Reserach and Development. Oak Ridge National Laboratory is managed by Martin Marietta Energy Systems, Inc., for the US.Department of Energy under Contract DE-AC05840R21400. Received for review November 23, 1994. January 16, 1995.@
Accepted
@Abstractpublished in Advance ACS Abstracts, February 15, 1995.
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