Anal. Chem. 1991, 63, 2069-2073
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On-Line Single-Particle Analysis by Laser Desorption Mass Spectrometry Sir: Although mass spectrometry has been successfully used to analyze particulate matter, a completely suitable method for performing on-line, single-particle analysis is not yet available (I). Single-particle analysis, unlike bulk analysis, allows microscopic variations in sample composition to be determined. When performed in an on-line mode, time-dependent phenomena can be studied as well. In this correspondence, we report an improved method for on-line single particle analysis based upon laser desorption mass spedrometry. This approach should find applications in a diverse range of areas such as the analysis of airborne particulate matter (2), clean room technology (31, the study of heterogeneous chemical reactions ( 4 ) , and the development of an improved sample vaporization method for liquid chromatography/mass spectrometry with a particle beam interface (5). On-line single-particle analysis by mass spectrometry is normally accomplished by sampling particles through a differentially pumped nozzle and impacting the particle beam onto a heated surface (6-16). In the surface ionization mode, ions emitted directly from the surface are detected (6, 7,4-12, 15, 16). The ionization yield depends upon the metal work function, surface temperature, and analyte ionization potential. In some cases, it can approach 100%. Surface ionization is highly selective toward atoms and molecules having ionization potentials below ca. 8 eV and can be regarded as a "soft" ionization method. More universal detection can be performed by 70-eV electron impact ionization of neutrals ejected by the particle-surface collision (8,13,14). This method, however, gives fairly extensive fragmentation and much lower ionization yields than surface ionization. Although surface ionization and electron impact ionization are complementary, neither is totally adequate. Another problem associated with on-line analysis is that each particle yields a burst of ions on the time scale of tens of milliseconds or less (11, 13). The transient nature of the signal makes it difficult or impossible to obtain a complete mass spectrum with scanning mass analyzers such as the quadrupole or magnetic sector. Yet, on-line analysis has been done exclusively with these analyzers. The consequences of using these analyzers are poor sensitivity and difficulty in performing multicomponent determinations. These problems could be substantially reduced by incorporating many of the features inherent to single-particle analysis by laser microprobe mass spectrometry (17-19). In the laser microprobe, ions are produced by one-step laser desorption/ionization of the particle and the entire mass spectrum is obtained for a single desorption event with a time-of-flight mass analyzer. Multicomponent determinations of both organic and inorganic constituents can be performed. For elemental analysis, a standard deviation of 10-20% can be achieved through the use of relative sensitivity factors. Unfortunately, the laser microprobe functions only in an off-line mode since the particle must be mounted on a solid substrate and the laser beam must be aligned to irradiate the particle. One attempt to perform laser desorption in an on-line mode has been reported (20). This experiment, however, was constrained by a prohibitively low irradiance for laser desorption, a complex arrangement for triggering the desorption laser, and the use of a scanning mass analyzer. Improvements to this approach have been suggested but not implemented (21). Our method for performing on-line single-particle analysis is shown in Figure 1. Particles enter the source region of a m a s spectrometer through a differentially pumped nozzle and 0003-2700/91/0363-2069$02.50/0
are detected in-flght by light scattering of a helium-neon laser beam. The scatter pulse promptly triggers an excimer laser that causes one-step desorption/ionization of the particle within a useful range of irradiances. A complete mass spectrum of the particle is subsequently recorded with a timeof-flight mass analyzer. This method retains sample integrity in two important ways. First, sample contamination and/or decomposition is reduced since particle-surface interactions are virtually eliminated. Second, the loss of volatile components is reduced since the particles spend less than 1ms under vacuum conditions before ionization. This setup is described along with a preliminary study of the characteristics of single-particle mass spectra.
EXPERIMENTAL SECTION Particles of known size and composition were generated by using a Model 3050 Berglund-Liu vibrating orifice monodisperse aerosol generator (TSI, Inc., St. Paul, MN). A solution containing the nonvolatile solute plus volatile solvent was fed into the generator at a rate of 0.11 mL/min by a syringe pump. The vibrating orifice, 10 pm diameter, was typically operated at 75 kHz. These conditions resulted in the formation of 36 pm diameter liquid droplets. The solvent was subsequently removed in a drying tube, yielding an aerosol of solute particles. Two low-level polonium sources were placed in the drying tube to reduce the charge on the aerosol particles and thus reduce the potential for particle agglomeration and wall loss. Particle size and monodispersion from the drying tube were verified by using a Climet Instruments (Redland, CA) particle analyzer. Initial experiments involved the generation of oleic acid particles from binary solutions of oleic acid and 2-propanol. Subsequent experiments were performed with multicomponent inorganic salt particles generated by dissolving the respective salts in a 50:Mmixture of water and ethanol. Solute particles generated in this manner are not necessarily spherical. The nominal particle diameters given in this correspondence are calculated on the basis of the solute mass in the original 36-pm liquid droplet, the solute density, and a spherical geometry. For multicomponent inorganic particles, the composition is given by the relative mole ratios of the salt originally dissolved in solution. These ratios are not meant to imply a specific distribution of phases in the particle. After leaving the drying tube, the particles passed along a 1-m length of 6.4 mm i.d. Teflon tubing into the mass spectrometer through a differentially pumped nozzle. Differential pumping was performed with an 11 L/s rotary vane pump (Alcatel, Hingham, MA). The first orifice in the nozzle had a 330 pm diameter, the second had a 200 pm diameter, and the distance between the two could be varied to maximize particle throughput and alter the pressure in the source region. Particle velocities in the mass spectrometer were estimated to be 200-300 m/s. Particle detection in the mass spectrometer was accomplished by monitoring scattered radiation from a helium-neon laser beam. The laser beam crossed the mass spectrometer perpendicular to the flight tube and was focused to a 100 pm diameter spot in the center of the source region. Scattered radiation was collected by a 6.4 mm diameter glass fiber (Schott Fiber Optics, Inc., Southbridge, MA) oriented perpendicular to the laser beam. A 5 mm diameter hole was drilled in the rear plate of the source region to allow scattered radiation to reach the fiber. Nickel-wire mesh (90% transmission) was placed over the fiber tip and pressed against the rear plate to minimize electric field distortions in the source region. The scatter signal was detected by a photomultiplier. The output signal from a discriminator was then used to externally trigger an excimer laser for laser desorption. The optical layout did not include any baffling along the helium-neon laser beam path. Consequently, stray radiation was high and it limited our ability to detect small particles. With this setup, 3 pm diameter particles gave a scatter peak with a sig0 1991 American Chemical Society
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Flgure 1. Laser desorption mass spectrometer for on-line single-
particle analysis. nal-to-noiseratio of 4. As will be discussed later, particles in the 3-10 pm diameter range gave such large ion currents from laser desorption that detector saturation was a significant problem. Therefore, the small particle limit to this method is currently determined by the sensitivity of the light-scattering step rather than laser desorption. Simple improvements in the optical layout would allow much smaller particles, probably on the order of 0.4 pm diameter or less, to be detected by light scattering. The small particle limit to ion detection by laser desorption is not currently known but is most likely below 0.4 pm diameter (17). Laser desorption was performed with a Questek Model 2110 excimer laser (Billerica,MA) at 193 nm. The excimer beam was roughly collimated by passing it through a series of apertures. It counterpropagated colinearly with the helium-neon laser beam and was focused to a spot 200 pm in the axis perpendicular to the flight tube by 270 pm in the axis along the flight tube. The beam dimensions in the focal region were measured by translating a knife edge across each axis to determine the points where the intensity decreased to 5% of its maximum value. The approximately rectangular shape was caused by the asymmetric beam divergence of the excimer laser. The two laser beams were initially overlapped by focusing them through a 200 pm diameter stainless steel orifice mounted in the center of the source region. After the orifice was removed, the beams were translated slightly to maximize the rate of particle detection and the magnitude of the mass spectrometer ion current. The spatial offset between the laser beams along the particle beam axis compensated for movement of the particle during the time delay between the scatter pulse and excimer laser pulse. The excimer was operated in the “Command Charge off” mode, meaning that the capacitors were maintained in a charged state until an external trigger was received. This mode of operation gave the smallest time delay between the scatter pulse and laser pulse. However, it also required that the laser be fired at a minimum of 10 Hz to prevent damage to the thryratron. For this reason, a central triggering system was designed to generate a trigger pulse for the excimer laser. An optoisolator (Questek Model 9205) was inserted between the central triggering unit and laser to eliminate rf noise in the trigger line. Once the excimer laser fired, a 20-ms “dead time” was required to recharge the capacitors in the laser. Any scatter pulses received during this time period were ignored. After the recharging period, an 80-ms window was available for firing the laser. If a scatter pulse was received during this window, then trigger pulses were immediately sent to the excimer laser and mass spectrometer data system. If no scatter pulse was received, then a trigger pulse was automatically sent to the excimer laser at the end of the 80-ms period and the entire process was repeated. The total delay time between the scatter pulse and laser pulse was 2.100 ps with a jitter of ca. 10 ns. The overall duty factor for particle detection and analysis was
Figwe 2. (a)Laser desorption mass spectrum of an 8 pm diameter oleic acid particle taken with a single laser pulse of 19Mm radiatlon and an energy of 290 pJ (ca.50 MWlcm2). (b) Backgwndspectrum taken with no particle in the laser beam path. The single partlcle and background spectra are offset, but the vertical scale expansions are the same.
limited by several factors. The aerosol generator typically produced IO6 particles/min. The pulse frequency from the discriminator ranged from 100 to 200 per minute depending upon the sensitivity level chosen for the discriminator. Therefore, approximately 1 in lo‘particles from the aerosol generator yielded a detectable scatter pulse. This ratio reflected the fraction of the total flow from the aerosol generator that was sent into the nozzle, the transfer efficiency of the nozzle, the overlap of the particle beam with the helium-neon laser beam, and the detection efficiency of the scatter pulse. (The transfer line and nozzle aeaembly used in these experiments were not designed to maximize particle throughput, so an increase in the fraction of particles detected may be possible.) Mass spectra were obtained for roughly half of the particles giving a detectable scatter pulse. This value was limited by the recharging time of the excimer laser and by the relative spatial positions of the helium-neon and excimer laser beams. Given the measured spot size and estimated particle velocity, the particle residence time in the excimer beam was on the order of 1 ps. The probability of ionizing more than one particle with a single laser pulse was very small since the particle detection rate was only 100-200 per minute. Excimer laser pulse energies of 14.0-340 pJ in the source region were used for laser desorption. These values corresponded to peak irradiances of ca. 23-60 MW/cm2 at the focal point. The pulse energy in the source region was adjusted by changing the output pulse energy of the laser. Attenuation of the output pulse energy was accomplished by passing the beam through a series of aperatures and fused-silica optical components. Mass analysis of the laser-desorbed ions was performed with a linear time-of-flight mass analyzer and a microchannel plate detector. The width of the source region was 11mm; the secondary acceleration region, 4 mm; the flight tube, 968 mm, and the postacceleration region 15 mm. The acceleration voltages were varied and are discussed later. The transient ion current from the detector was sampled digital directly by a Nicolet (Madison, WI) Model 450,200-MHz oscilloscope or by first passing the signal through an Analog Modules, Inc. (Longwood, FL)Model 383 logarithmic amplifier. T h e spectra were then transferred to a personal computer for data manipulation and plotting.
RESULTS AND DISCUSSION Several organic and inorganic particles which absorb 193-nm radiation were investigated. Figure 2 shows a mass spectrum of a single 8 km diameter oleic acid particle taken with a single excimer laser pulse at an energy of 290 pJ (ca. 50 MW/cm2). Also shown is a background spectrum that was obtained under similar conditions to the single-particle spectrum except that no particle was in the laser beam path. These spectra are
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smooth versions of the raw spectra using a Savitsky-Golay algorithm. Smoothing is necessary to reduce the amount of digitization noise associated with the limited vertical resolution (8 bits) of the digital oscilloscope. Figure 2 exhibits several noteworthy characteristics. First, relative to the 70 eV electron impact mass spectrum of vapor phase oleic acid, the single particle spectrum contains a greater fraction of high-mass ions both in the parent ion region (mlz 282) and in the region of water loss from the parent ion ( m / z 264). It is not clear whether ion current in the parent ion region is a result of Me+ (mlz 282), (M + H)+ (mlz 283), or a combination thereof. Second, the baseline in the single-particle spectrum is high. This could result from metastable decay of primary ions or from chemical noise (e.g. ion current at every mass) resulting from complex bond cleavages and recombinations accompanying laser desorption. The background spectrum does not contain an elevated baseline. Third and perhaps most significant is the relatively poor mass resolution. This aspect will be discussed in more detail later. Both the particle size and laser pulse energy were varied to gain further insight into the characteristics of laser desorption from single particles. Within experimental error, oleic acid particles in the 6.0-9.5 pm diameter range (a factor of 4 change in mass) gave the same absolute and relative ion currents. This observation suggests that only partial vaporization of the particles occurred. It is not a surprising result since a linear relationship between ion current and particle volume in laser microprobe mass spectrometry is normally limited to particle sizes below 1pm diameter (17). The laser pulse energy was varied to study changes in the amount of fragmentation and to determine the threshold for ion detection. Fragmentation increased with increasing laser pulse energy, though not dramatically. The parent ion current relative to the base peak decreased from 30% at 195 &/pulse to 10% a t 340 pJ/pulse. The threshold was found to be approximately 160 pJ/pulse (ca. 27 MW/cm2 at the laser focus). This irradiance is approximately 1order of magnitude above those reported for laser desorption from absorbing films with 193-nm radiation (22). However, the threshold in conventional laser desorption is strongly dependent upon the effective cross-sectional area for desorption (e.g. the laser spot size). When laser desorption occurs from a single particle, this area is given by the dimensions of the particle rather than the laser beam. The threshold observed for oleic acid single particles is similar to that from a laser microprobe having a similar effective area for desorption (23). Spectra were also obtained for particles generated from various combinations of KC1, NaC1, NH4Cl, (NH4)$04, and ZnSO1. Figure 3 shows two spectra of a 9 pm diameter particle generated by mixing (NH4)804, KCl, and NaCl in a 1.61.03.6 mol ratio in the aerosol generator. The spectrum in Figure 3a was taken by directly sampling the ion current vs time profile from the detector with the digital oscilloscope. The effect of digitizing noise is readily apparent. The spectrum in Figure 3b is for the same particle as in Figure 3a except that a portion of the ion current from the detector was sent into a logarithmic amplifier, digitized, and then converted back to a linear scale. Both simple cations (K+, Na+, and NH4+)and cluster ions (Na2Cl+,NaKCP, and K2Cl+)are observed in Figure 3. These ions are also found in the laser microprobe mass spectra of similar particles (8,24,25). The set of peaks at m / z 64,66, and 68 have not been conclusively identified. m/z 64 may correspond to SOz+, although this is unlikely since S+, SO+, and SO3+ are not observed. More likely, these ions are due to memory effects of zinc in the aerosol generator from previous experiments yielding 64Zn+,@Zn+,and @Zn+with ion abundances reflecting the natural isotopic distribution.
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Figure 3. Laser desorption mass spectra (193 nm; 190 pJ/pulse, ca. 32 MW/cm2)of a single 9 pm diameter particle containing (NH4)*S04, KCI, and NaCl in a 1.6:1.0:3.6 mol ratio: (a) linear spectrum; (b) spectrum taken with a logarithmic amplifier and converted back to a
linear scale.
Particle-to-ParticleVariations. Particle-to-particle (and hence laser pulse-to-pulse) variations in the mass spectra were briefly investigated by obtaining successive single-particle spectra with the Same nominal operating conditions. Absolute ion currents were fairly consistent from particle to particle, usually varying by no more than 25%. Approximately one particle in ten would give a much higher or, more likely, a much lower absolute ion current than the rest. Assuming that the aerosol was truly monodisperse, this variation could be explained by a relatively large change in the location of the particle within the focused excimer beam profile in the source region. Such a change could significantly alter the effective laser irradiance. Particle-bparticle variations of location in the source region were clearly manifested in both the oleic acid and salt particle spectra by a time shift of the ion peaks from one spectrum to the next. Two different source region configurations were used to study this phenomenon. The normal source configuration had a 700-V drawout potential over a 11-mm source region and a 2300-V, 4-mm acceleration region. The second configuration had a 3OOO-V drawout potential over the 11-mm source region and no further acceleration voltage. These configurations were modeled by using the method of Kinsel and Johnston (26)and with the ion trajectory program SIMION (Idaho National Laboratory, Idaho Falls, ID). Flight times were calculated for ion formation positions that varied over the 270-pm width of the excimer laser beam along the flight tube axis. Modeling confirmed that the normal source configuration approximately satisfied space focusing. For example, the flight time for the centroid of m / z 55 of oleic acid was predicted to vary by only 20 ns across the width of laser beam
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profile. In contrast, flight time variations of about 260 ns were predicted for the second ion source configuration. Variations in the transverse position of ion formation (i.e. perpendicular to the axis of the flight tube) changed the predicted flight time by less that 5 ns for all positions where ions could reach the detector. Particle-to-particle shifts in ion flight times were measured for at least 10 oleic acid particles using each source configuration. Observations of rf noise present in the detector signal before the mass spectrum confirmed that the jitter in the trigger circuitry and excimer laser firing was approximately 10 ns. The average time of the m / z 55 peak centroid was 12.560 ps with u = 22 ns for the normal source configuration and 16.156 ps with u = 220 ns for the second source configuration. The reasonable agreement between the predicted and measured values of u suggest that the primary cause for particle-to-particle time shifts was the location of the particle in the laser beam. Similar agreement between predicted and measured time shifts was observed in separate experiments using salt particles. Resolution. The mass resolution we observed for singleparticle laser desorption was consistently poorer than what we normally achieve with vapor-phase multiphoton ionization (27). Resolution in a time-of-flight mass analyzer has three major components: spatial, energy, and temporal (28). The poorer resolution in the single-particle spectra is most likely due to decreased energy resolution, since laser desorption is known to produce a wider distribution of ion kinetic energies than gas-phase ionization (29). This problem is even greater with our mass analyzer configuration since desorption occurs in the center of the source region and ions are created with initial velocity vectors both toward and away from the detector. Spatial resolution should be quite high since it is essentially determined by the particle diameter. Temporal resolution is ultimately limited by the length of the laser pulse (13 ns) and cannot account for all of the broadening observed. An accurate measure of resolution is difficult for the spectrum in Figure 2 owing to overlap of adjacent m/z peaks. This situation is less problematic for the salt particle spectrum in Figure 3. Figure 3b exhibits a mass resolution of 90 at m/z 39 based upon the equation R = t/2At, where t = flight time and At = full width at the peak base. The salt particles, however, gave such large ion currents that detector saturation and distorted peak shapes were severe problems. As a result, the resolution shown in Figure 3 could only be achieved by reducing the microchannel plate voltage and the laser pulse energy to near threshold. In order to gain a better understanding of the factors that affect resolution, ion peak widths were modeled for individual oleic acid and salt particle spectra taken with both ion source configurations. Ions were assumed to be produced over the duration of the laser pulse. The width of the ion formation region and the ion translational temperature were varied to investigate the contributions to peak width. The predicted peak width using the normal source configuration depended mostly on the ion translational temperature. On the other hand, the predicted peak width using the second configuration depended mostly on the width of the ion formation region and was nearly independent of ion translational temperature. Measured peak widths (half-maximum) in individual singleparticle spectra could be explained by using an ion formation region slightly larger than the particle diameter (10-20 pm wide) and an ion translational temperature between lo00 and loo00 K. The experimental peak profiles show a broad base that was not fully explained by the modeling. This contribution to the peak shape may due to space-charge repulsions, detector saturation, or a nonthermal ion kinetic energy distribution from laser desorption.
Digitization Noise. Digitization noise is not usually a factor in mass spectra averaged over many laser pulses. It can, however, play an important role in mass spectra taken with a single laser pulse. Figure 3a shows how digitization noise in the vertical scale can obscure weak features in a mass spectrum, particularly when the full scale expansion of the oscilloscope does not exactly match the height of the largest peak in the spectrum. Two separate strategies were briefly explored to overcome this problem. First, the dynamic range was extended somewhat as in Figure 2 by using a smoothing algorithm to act as a high-frequency digital filter. In relative terms, this approach should give analog-to-digital conversion with high precision for high-intensity ions, but low precision for low-intensity ions. Peak broadening from the smoothing process is unlikely (30). Figure 3b shows an alternative method for increasing the dynamic range. In this case, the ion current was sent into a logarithmic amplifier, digitized and stored. Later, the spectrum was converted back to a linear scale on a personal computer. The amplifier had an input range of 100 p V to 5 V and an output scale of 8 mV/dB, making a dynamic range of almost five decades accessible to the digital oscilloscope. This approach should give roughly constant precision for analog-to-digital conversion over several decades of signal intensity, though with a precision intermediate between the high and low ion intensity limits of the smoothing method. CONCLUSION Laser desorption mass spectra of single particles obtained by our on-line method are similar to mass spectra obtained by conventional laser desorption in terms of the types of ions observed, the threshold laser irradiance for ion detection, a small spatial region over which ions are formed, and a significant distribution of ion kinetic energies. Unlike conventional laser desorption, the location in the source region where desorption occurs changes from particle to particle with the on-line method. Therefore, a significant time shift of the ion peaks may be observed depending upon the source configuration used. Two relatively straightforward instrumental improvements would greatly expand the capabilities of this method: (1) improved stray light rejection in the scatter detection step so that submicron diameter particles can be studied and (2) increased mass resolution without inducing large particle-to-particle time shifts of ion peaks. ACKNOWLEDGMENT We gratefully acknowledge Chuck Wilson for loan of the aerosol generator and particle analyzer. Registry No. (NH&SO4, 7783-20-2; KC1, 7447-40-7; NaC1, 7647-14-5; oleic acid, 112-80-1. LITERATURE CITED Spurny, K. R. In Pbysical and Chemical Characterizationof IndhMual Airborne Particles; Spurny, K. R., Ed.; Ellis Horwood, Ltd.: Chichester, U.K., 1986; p 5. Baleman, T. F. Envivon. Sci. Tecbnd. 1988, 22, 361-367. Liking, 2.; Wei, G.-T.; Irwin, R. L.; Walton, A. P.; Michel, R. 0.; Sneddon, J. Anal. Chem. 1990, 62, 1452-1457. Rubel, G. 0.; Gentry, J. W. J . Pbys. Cbem. 1984, 86, 3142-3148. Winkler, P. C.; Perkins, D. D.; WHilams, W. K.; Browner, R. F. Anal. Chem. 1988, 60. 489-493. Davis, W. D. Environ. Sci. Tecbnol. 1977, 1 1 , 597-592. Davis, W. D. Environ. Sci. Techno/. 1977, 1 1 , 593-596. Allen, J.; Gould, R. K. Rev. Sci. Instrum. 1981. 52, 804-809. Stoffels, J. J. I n t . J . Mass Spectrom. Ion Processes 1981, 40, 217-222. Stoffels, J. J. Int. J . Mass Spectrom. Ion Processes 1981, 40, 223-234. Stoffels, J. J.: Lagergren, C. R. Int. J . Mass Spectrom. Ion Processes 1981, 40, 243-254. Meyers, R. L.; Fite, W. L. Environ. Sci. Tecbnol. 1975, 9 . 334-336. Sinha, M. P.; Griffin, C. E.; Norrls, D. D.; Estes, T. J.; Vllker, V. L.; Friedlander. S. K. J . CdloM Interface Sci. 1882, 87, 140-153. Sinha, M. P.; Platz. R. M.; Vllker, V. L.; Friedlander. S. K. Int. J . Mass Spectrom. Ion Processes 1984, 57, 125-133. Sinha, M. P.: Friedlander, S. K. Anal. Cbem. 1985, 5 7 , 1880-1883.
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Anal. Chem. 1991, 63, 2073-2075 (16) Slnha, M. P.; Friedlander, S. K. J . cdldd Interface Scl. 1986, 772, 573-582. (17) Kaufmann, R. L. In Physical and Chemical Chamcterizatkm of IndvvMuel Akbome ParMbs; Spurny, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K.. 1986; Chapter 13. (18) Wbser, P.; Wurster, A. In Physical and Chemical Chamctedzatbn of IndlbMual Akborne Pa&bs; Spurny, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K., 1986; Chapter 14. (19) De Waele, J. K. E.; Adams, F. C. In Physical and Chemical Chamctedzatbn of Indivklual Akttme Particles; Spumy, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K., 1986 Chapter 15. (20) Slnha, M. P. Rev. Scl. Instrum. 1984, 55, 886-891. (21) Marljnlssen, J.; Scarlett. 6.;Verheluen, P. J . Aerosol Sci. 1988. 79. 1307-13 10. (22) Spengler, 6.;Karas. M.; Bahr. U.; Hllbnkamp, F. J . Phys. Chem. 1987, 97, 8502-6506. (23) Karas, M.; Bachmann, D.;Hlllenkamp, F. Anal. Chem. 1985, 57, 2935-2939. (24) Wleser, P.; Wurster, R.; Seller, H. Atmos. Envlron. 1980, 74, 485-494. (25) Van Orleken, R.; Adams, F. I n Chemisw of MuNphase Atmospheric Systems; Jaeschke. W., Ed.; NATO AS1 Series Vol. 06; SprlngerVerlag: Berlin. 1986; p 81. (26) Klnsel, G. R.; Johnston, M. V. Int. J . Mass Spectrom. Ion Processes 1989. 97. 157-176. (27) Klnsel, (3. R.: Mowry, C. D.; McKeown. P. J.; Johnston, M. V. Int. J . Mass Spectrom. Ion Process8s 1991, 704, 35-44. (28) Opsal. R. 6.;Owens, K. G.; Rellly, J. P. Anal. Chem. 1985, 57, 1884- 1889.
(29) See, for example: Vertes, A.; Juhasz, P.; Ani. P.: Czltrovszky, A. Int. J . Maw Spectrom. IOn P r w 1986, ~ 83, 45-70. (30) Press, W. H.; Peukolsky. S. A. Compvt. Phys. 1990, 669-672.
To whom correspondence may be addressed.
P. J. McKeown M. V. Johnston* Department of Chemistry and Biochemistry University of Delaware Newark, Delaware 19716
D. M. Murphy* Aeronomy Laboratory NOAA/ERL Boulder, Colorado 80303 RECEIVED for review April 4,1991. Accepted June 24,1991. This research was supported in part by a grant to M.V.J. from the National Science Foundation (CHE9096266). P.J.M. acknowledges a research fellowship from the National Oceanic and Atmospheric Administration.
TECHNICAL NOTES Design and Evaluation of an Electrochemical Cell for the Study of Organometallic Complexes at Increased Gas Pressures James E. Anderson* and Eileen T. Maher Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167
INTRODUCTION Determination of the electron-transfer properties of organometallic compounds by electrochemical and spectroelectrochemical methods is an area of current interest (1-6). These studies often involve complexes containing gas-phase ligands such as carbon monoxide (CO) (1,7,8) or may examine the reactions of electrogenerated species with gases such as CO, (9, IO). The range and type of electrochemical experiments for systems that involve gas-phase species would be enhanced by an ability to perform measurements at increased pressures. For example, the concentration of a gas-phase reactant could be increased to allow observation of a specific chemical reaction. If the concentration of the gas could be systematically varied by controlling the pressure, titration experiments commonly used to examine ligand loss or addition coupled with electron transfer (11-13) could be easily performed. However, conventional electrochemical cells are not designed to allow measurements at pressures significantly greater than 1 atm (14, 15). Electrochemical cells have been designed to work at high pressures and in general are used to perform measurements in near-critical and supercritical fluids (16-18). These conditions typically require both very high pressures and temperatures such as 240 bar 400 K (18))and consequently, the cell is made from alumina and/or stainless steel components. These metal components are potentially reactive toward several different classes of organometallic compounds, and consequently, decomposition of the compound as well as degradation of the cell could occur. In addition, the extreme air sensitivity of some complexes would prohibit their use with the high-pressure cells that are described in the literature. 0003-2700/9 110363-2073$02.50/0
In this paper, we report the design and evaluation of an electrochemical cell that can be conveniently used at increased gas pressures for highly reactive organometallic complexes. With the exception of the platinum electrodes, the cell is made entirely from glass and pressures up to 125 psi above ambient pressure (Pa of approximately 9.5 atm) can be obtained. We will demonstrate the use of this cell by examining the electron-transfer properties of osmocene as a function of CO pressure.
EXPERIMENTAL SECTION (A) Apparatus. Figure 1 shows the design and the placement of the electrodes in the electrochemical cell. The cell walls are thick glass capable of withstanding pressures up to 200 psi, and the four inlets are ACE glass joints (Part 5027-20) designed for high-pressure applications. Three of the inlets are for the electrodes while the fourth is for the delivery of the reactant gas. The electrodes are platinum wire, sealed in glass, connected to copper wire via solder or Hg. The electrodes have a rim in the glass wall to mechanically prevent the electrode from slipping from the cell, while air tight seals are obtained via the ACE glass joints. The electrode separation is minimized by the shape of the cell and careful electrode placement. In our case, the working and reference electrodes are approximately 1.0 cm apart. The working electrode is a platinum disk,approximate area of 0.0020 cm2,while the auxiliary and reference electrodes are platinum wire. The Pt wire pseudoreference electrode could be easily replaced with a bridge (Pt wire sealed in glass) to accommodate a standard reference electrode. In addition to the high-pressure cell and the gas supply tank with regulator, our experimental setup has a Matheson pressure gauge (Part63-5622) and a pressure release valve. The pressure gauge is accurate to 0.5 psi. Electrochemical experiments were performed with a BAS-100A. All potentials are reported vs the Pt wire pseudoreference electrode 0 1991 American Chemical Society
