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Nov 15, 1996 - The ability of time-of-flight mass spectrometry to examine the relative amounts of each arsenic oxide present in a series of mixtures i...
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Anal. Chem. 1996, 68, 4052-4059

Speciation of Arsenic Oxides Using Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Todd M. Allen,† Dawit Z. Bezabeh,† Catherine H. Smith,† Eileen M. McCauley,† A. Daniel Jones,‡ Daniel P. Y. Chang,§ Ian M. Kennedy,⊥ and Peter B. Kelly*,†

Department of Chemistry, Facility for Advanced Instrumentation, Department of Civil and Environmental Engineering, and Department of Mechanical and Aeronautical Engineering, University of California, Davis, California 95616

Positive and negative ion mass spectra of arsenic trioxide (As2O3) and arsenic pentaoxide (As2O5) have been obtained by single-step laser desorption/ionization time-offlight mass spectrometry. Pulsed UV radiation at 266 nm was used for the simultaneous desorption and ionization of the solid sample. High-mass cluster ions that are unique to the oxidation state of each oxide sample appear in the negative ion mass spectra. The As2O3 produces As3O5-, while the As2O5 yields As3O8-. The formation of unique negative cluster ions presents the capability for arsenic oxidation state speciation by laser desorption/ ionization mass spectrometry. The ability of time-of-flight mass spectrometry to examine the relative amounts of each arsenic oxide present in a series of mixtures is discussed. Application of our speciation technique to a model incinerator sample is demonstrated. The presence of arsenic compounds, such as the arsenic oxides, arsenite, and arsenate, in the environment is of concern because of their carcinogenic and mutagenic properties.1-3 Biological activity is dependent on the chemical form of the arsenic, where trivalent arsenic is considered to be more toxic and potentially more carcinogenic than pentavalent arsenic.4 Thus, speciation of chemical form is important for an accurate evaluation of potential human health hazards. Toxic source emissions from the incineration of waste containing arsenic compounds result predominantly in formation of the trivalent and pentavalent arsenic oxides, As2O3 and As2O5, in airborne particulate matter.5-8 Other major sources of airborne arsenic oxides (trivalent and pentava†

Department of Chemistry. Facility for Advanced Instrumentation. § Department of Civil and Environmental Engineering. ⊥ Department of Mechanical and Aeroneutical Engineering. (1) Biological and Environmental Effects of Arsenic, Fowler, B. A., Ed.; Elsevier: Amsterdam, 1983. (2) Cullen, W. R.; Reimer, K. J. Chem. Rev. 1989, 89, 713-764. (3) Medical and Biological Effects of Environmental Pollutants: Arsenic; Committee on Medical and Biological Effects of Environmental Pollutants, National Research Council; National Academy of Science: Washington, DC, 1977. (4) Penrose, W. R. CRC Crit. Rev. Environ. Control 1974, 465. (5) Chesworth, S.; Yang, G.; Chang, D. P. Y.; Jones, A. D.; Kelly, P. B.; Kennedy, I. M., Combust. Flame 1994, 98, 259-266. (6) Wu, C. Y.; Biswas, P. Combust. Flame 1993, 93, 31-40. (7) Report to the Air Resources Board on Inorganic Arsenic Part A: Public Exposure to Airborne Inorganic Arsenic in California; California Air Resources Board: Sacramento, CA, March 1990; pp A1-A57. (8) Tillman, D. A. Trace Metals in Combustion Systems; Academic Press, Inc.: San Diego, CA, 1994; pp 35-38, 48. ‡

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lent) include the combustion of wood, pesticide use,7 and geothermal emissions.7,9 Total arsenic content in environmental samples can be determined by many current analytical techniques including numerous mass spectrometric techniques.10-18 Speciation of arsenic is possible when certain techniques, like GC, HPLC, or selective hydride generation are coupled with atomic absorption spectroscopy (AAS), ICP, or ICPMS.10,19-22 However, these techniques involve lengthy procedures requiring preconcentration, extraction, separation, and/or derivatization. The coupling of lasers as an ionization source with mass spectrometric techniques23,24 provides molecular, fragment, and cluster ion information about organic and inorganic materials which is not easily obtained by conventional techniques. Recent work in rapid single-particle mass spectrometry (RSMS) performed by Johnston et al. examined the ability to speciate chromium and sulfur in aerosols based on the formation of cluster ions unique to specific oxidation states.25 We have been investigating the potential of laser desorption/ ionization (LDI) time-of-flight mass spectrometry (TOF-MS) as a rapid screening method for arsenic oxidation state speciation.26 LDI-TOF-MS offers the advantage of minimal sample preparation to examine low-volatility solid compounds. (9) Solomon, P. A.; Altshuler, S. L.; Keller, M. L. Air Waste 1993, 43, 765768. (10) Schoene, K.; Steinhanses, J.; Bruckert, H.; Ko¨nig, A. J. Chromatogr. 1992, 605 (2), 257-262. (11) Story, W. C.; Caruso, J. A.; Heitkemper, D. T.; Perkius, L. J. Chromatogr. Sci. 1992, 30 (11), 427-432. (12) Michel, P.; Averty, B.; Colandini, V. Mikrochim. Acta 1992, 109, 35-38. (13) Howard, A. G.; Comber, S. D. W. Mikrochim. Acta 1992, 109, 27-33. (14) Litzow, M. R.; Spalding, T. R. Mass Spectrometry of Inorganic and Organometallic Compounds; Elsevier: Amsterdam, 1973. (15) Norin, H.; Christakopoulos, A. Chemoshphere 1982, 11 (3), 287-298. (16) Luten, J. B.; Riekwel-Booy, G.; Greef, J. v. d.; Noever de Brauw, M. C. Chemosphere 1983, 12 (2), 131-141. (17) Norin, H.; Christakopoulos, A.; Sanstrom, M.; Ryhage, R. Chemosphere 1985, 14 (3/4), 313-323. (18) Cullen, W. R.; Dodd, M. Appl. Organomet. Chem. 1989, 3, 401-409. (19) Siu, K. W. M.; Gardner, G. J.; Berman, S. S. Rapid Commun. Mass Spectrom. 1988, 2 (4), 69-71. (20) Cullen, W. R.; Eigendorf, G. K.; Pergantis, S. A. Rapid Commun. Mass Spectrom. 1993, 7 (1), 33-36. (21) Ban, V. S.; Knox, B. E. J. Chem. Phys. 1970, 52 (1), 248-253. (22) Torralba, R.; Bonilla, M.; Pe´rez-Arribas, L. V.; Palacios, A.; Ca´mara, C. Spectrochim. Acta 1994, 49B (9), 893-899. (23) Lasers and Mass Spectrometry; Lubman, D. M., Ed.; Oxford University Press: New York, 1990. (24) Zenobi, R.; Zare, R. N. In Advances in Multi-Photon Processes and Spectroscopy, Vol. 7; Lin, S. H., Ed.; World Scientific Press: Singapore, 1991; Chapter 1. (25) Neubauer, K. R.; Johnston, M. V.; Wexler, A. S. Int. J. Mass Spectrom. Ion Processes 1995, 151, 77-87. S0003-2700(96)00359-9 CCC: $12.00

© 1996 American Chemical Society

The interaction of a pulsed laser beam with a solid sample causes a variety of processes to occur, including surface heating, phase changes, desorption, and ionization. The leading edge of the laser pulse is absorbed by the sample and heats the sample rapidly to vaporize the solid phase material into a gas phase plume above the sample surface. The gas phase plume contains neutral species, electrons, and molecular and atomic ions, where the dominant particles produced are neutral species. Ionization mechanisms also include the gain or loss of electrons as well as combination reactions resulting from numerous collisions between cations, anions, and neutrals in the gas phase plume.27,28 The formation of positive ions occurs by photoionization, ion-pair formation,29 protonation,30,31 and ion-molecule recombination reactions in the gas phase plume directly above the sample surface.32 Negative ions are formed by either electron attachment or by ion-molecule reactions.33 Simultaneous laser desorption and ionization at 266 nm of the arsenic oxides produces dominant cluster and fragment ions containing different ratios of arsenic and oxygen. Both positive and negative ion mass spectra are presented for arsenic trioxide and arsenic pentaoxide using UV laser radiation of 266 nm. The relative intensity of the clusters depends on the stability of the ion structure, the laser power density, and the interaction of the laser beam with sample surface. Interpretation of the mass spectra is simplified by the monoisotopic nature of arsenic. In addition to the mass spectra of the pure arsenic oxides, negative ion mass spectra are also presented for a series of mixtures containing arsenic trioxide and arsenic pentaoxide acquired using UV laser radiation at 266 nm. The formation of unique negative cluster ions for arsenic trioxide and arsenic pentaoxide upon laser desorption/ionization allows for the speciation of arsenic oxidation state and represents the most promising results to date for inorganic arsenic speciation by laser mass spectrometry. EXPERIMENTAL SECTION Caution: Arsenic oxides are toxic and mutagenic. Each sample should be treated as a potential health hazard, and care should be exercised to minimize exposure during handling and use. Arsenic trioxide (analytical reagent grade) was purchased from Mallinckrodt. Arsenic pentaoxide (99%) was purchased from Strem. The chemicals were used as received. Laser desorption/ionization mass spectra were obtained using a custom-built linear time-of-flight mass spectrometer with a source region built by R. M. Jordan Co. (Grass Valley, CA). Two oil diffusion pumps equipped with cryotraps separately pump down the source and flight tube regions to 10-7 Torr. Samples were (26) Allen, T. M.; McCauley, E. M.; Smith, C. H.; Kelly, P. B.; Jones, A. D.; Chang, D. P.; Kennedy, I. M. Laser Desorption Ionization Time-of-Flight Mass Spectrometry as an Environmental Screening Technique for Arsenic Containing Compounds. Proceedings of the 42nd ASMS Conference on Mass Spectrometry and Allied Topics, Chicago, IL, May 29-June 3, 1994; p 272. (27) Novak, F. P.; Balasanmugan, K.; Viswanadham, K.; Parker, C. D.; Wilk, Z. A.; Mattern, D.; Hercules, D. M. Int. J. Mass Spectrom. Ion Phys. 1983, 53, 135. (28) Hercules, D. M.; Day, R. J.; Balasanmugan, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982, 54, 280A. (29) Stoll, R.; Rollgen, F. W. Z. Naturforsch. 1982, 37a, 9-14. (30) Parker, C. D.; Hercules, D. M. Anal. Chem. 1986, 58, 25. (31) Chiarelli, M. P.; Sharkey, A. G.; Hercules, D. M. Anal. Chem. 1993, 65, 307. (32) van der Peyl, G. J. Q.; Isa, K.; Haverkamp, J.; Kistemaker, P. G. Org. Mass Spectrom. 1981, 16, 416. (33) Viswanadham, K.; Hercules, D. M. Anal. Chem. 1988, 60, 2346.

introduced through a fast load-lock arrangement which allows for the aluminum probe to be inserted from behind the repeller plate such that the sample surface is coplanar with the plate surface and in electrical contact with the repeller plate. To ensure more representative mass spectra, sufficiently thick solid samples, which consisted of arsenic trioxide or arsenic pentaoxide, were ground into a fine crystalline powder by mortar and pestle prior to being mounted onto an aluminum probe with double-sided adhesive. To change samples, the laser-irradiated sample on the probe is removed from the mass spectrometer. The old sample and double-sided adhesive are replaced by a new piece of double-sided adhesive, which is applied to and covers the sample surface (25 mm2) of the aluminum probe. Samples are then pressed firmly with a spatula to the double-sided adhesive in order to prevent the physical movement and loss of the sample during loading and rotation of the probe in the mass spectrometer. The probe is rotated during data acquisition so that different spots on the sample can be interrogated and averaged within a single mass spectrum. The location of the laser irradiation on the sample can also be adjusted such that different spots on the sample can be analyzed during the rotation of the probe. Averaging over several spots on the probe is important for obtaining semiquantitative and/or quantitative results from samples of heterogeneous composition (e.g., mixtures of arsenic trioxide and arsenic pentaoxide). In the laser pulse energy range used for generating mass spectra (described below), no background mass spectrum is observed when using only a blank of double-sided adhesive on the probe. Pulsed UV radiation at 266 nm is produced by a Q-switched Quanta Ray DCR-3 Nd:YAG laser equipped with a KD*P harmonic generation crystal. The UV laser beam has an 8 ns pulse width and is focused into the source region through a 250 mm S1-UV quartz lens to a circular spot of ∼160 µm diameter. UV laser energy is measured at the optical window of the source region using a Molectron Model J9LP joulemeter interfaced to a PcJ meter board (Q&A Instruments) installed in a DOS-based microcomputer. Thermal desorption generated by the impact of the UV beam on the sample produces a mix of ions, neutrals, and electrons in the plasma above the surface. The ions are produced by multiphoton ionization, chemical ionization, and/or collisionally induced processes. The output of the laser power was attenuated to 2-10 µJ/pulse, which produces maximum power densities of 106-107 W/cm2 at the focus, with neutral density filters. Ions, positive or negative, formed during each laser pulse are accelerated into the flight tube region by the Wiley-McLarenbased ion optics,34 where they are detected by a dual-microchannel plate detector (MCP). The current generated at the MCP detector is 50 Ω terminated into a 100 MHz DSP 2001 transient recorder and is digitized with 8-bit precision. In a typical experiment, data are collected and signal averaged by a DSP Model 4101 averaging memory over 100-500 laser pulses and then transferred by a CAMAC interfaced to an 80386 microcomputer. Data acquisition and analysis are handled by software developed in-house. RESULTS AND DISCUSSION Generation of a representative mass spectrum in laser desorption/ionization time-of-flight mass spectrometry is dependent on a number of factors, including laser pulse energy, the (34) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150-1157.

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Figure 1. Positive ion mass spectra, displaying the cluster and fragment ions obtained with 266 nm laser excitation of pure crystalline (a) arsenic trioxide (As2O3) and (b) arsenic pentaoxide (As2O5).

interaction between the laser beam and the sample, and the stability of the ion structure. Laser desorption/ionization mass spectra presented in this work were averaged for 500 laser pulses per spectrum in order to improve signal quality. A single laser pulse on the sample surface was also able to generate a representative mass spectrum. However, consecutive mass spectra obtained from a single laser pulse on the same spot on the sample surface demonstrated some variability in relative ion intensities, which was due in part to a 10% laser power fluctuation from pulse to pulse. The reproducibility of relative ion intensities with variation in laser pulse energy has been examined. As the laser pulse energy increases from 2 to 10 µJ/pulse, variations (5%) in the relative ion abundance in the As3Ox- region of the mass spectra remain insensitive to changes in laser pulse energy. The threshold of ion formation due to laser irradiation of the sample is higher for arsenic pentaoxide than for arsenic trioxide. All positive and negative ion mass spectra in this study were acquired within the range of laser pulse energies (2-10 µJ/pulse). Positive Ions of Arsenic(III) and -(V) Oxides. The positive ion mass spectra generated from arsenic trioxide and arsenic pentaoxide are shown in Figure 1. The most intense peak observed for both oxides is AsO+. Since AsO has an electronic absorption at 266 nm,35 an expected result is an enhanced resonant ionization of AsO. The spectrum of the trioxide also shows additional peaks corresponding to As3O4+, As5O7+, and As5O8+, while the pentaoxide shows additional peaks at As3O4+, As3O5+, As5O7+, and As5O8+. Additionally, the positive ion mass spectrum of the arsenic pentaoxide can show attachment of hydrogen to clusters (HAs2Ox+ and HAs4Ox+). The hygroscopic nature of the arsenic pentaoxide crystals accounts for the observation of water or hydrogen in the spectrum. Although the As5O8+ and As5O7+ cluster ions appear to be very weak in intensity for the arsenic trioxide and the arsenic pentaoxide in Figure 1a and b, respec(35) Kushawaha, V. S.; Asthana, B. P.; Pathak, C. M. J. Mol. Spectrosc. 1972, 41, 577-583.

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tively, greater laser pulse energies variably produce a more intense As5O8+ or As5O7+ cluster ion. The positive ion mass spectra of arsenic trioxide and arsenic pentaoxide contain no unique cluster ions to be used as a means of identification of As2O3 and As2O5 with 266 nm excitation.26 Characteristic Positive Ions of Arsenic Trioxide. The positive ion LDI spectra for crystalline As2O3 are characterized by three dominant ions: AsO+ (m/z ) 91), As3O4+ (m/z ) 289), and As5O7+ (m/z ) 487), which have been previously observed and reported by other workers.36 The higher mass ions were most likely formed by collisions of the fragment ion AsO+ with the neutral As2O3 and/or As4O6 species within the gas phase plume generated by the laser and have the general formula [AsO(As2O3)n]+, where n ) 0-2. According to the valence model developed by Plog et al. to describe ion yields in secondary ion mass spectrometry (SIMS) experiments,37 for ions with the general metal oxide formula MmOnq, the fragment valence, K, of the metal atom is given by the equation, K ) (q + 2n)/m.37 The three major ions observed for As2O3 have a fragment valence of K ) 3, where K corresponds to the valence state of the arsenic in the original compound and in the observed cluster and fragment ions. The formation of positive cluster ions with the general formula [AsO(As2O3)n]+ can be correlated to the molecular structure of the arsenic trioxide in the crystal and the vapor phase. The two most common forms for the molecular structure of arsenic trioxide can be found as either the arsenolite or the claudetite. Arsenolite is the most common form of arsenic trioxide, but claudetite is the most thermodynamically stable form at standard temperature and pressure. The basic molecular unit of the arsenolite structure in the vapor phase consists of As4O6 molecules up to 800 °C, where dissociation of the As4O6 molecules to As2O3 occurs.38 Vaporization of claudetite, which is composed of AsO3 units bonded together as continuous double layers of AsO3/2 coordination polyhedra,38 results in the formation of As2O3 molecules and As4O6 molecules. Upon laser desorption/ionization, arsenic trioxide reproducibly forms characteristic fragment and cluster ions in the mass spectra which are related to the molecular structure. Based on the molecular structure of the vapor phase above either form of arsenic trioxide, arsenolite or claudetite, the characteristic arsenic-oxygen cluster ions observed in the positive (and negative) ion mass spectra are believed to be cagelike molecular structures and/or chains resembling those of coordination polyhedra. Characteristic Positive Ions of Arsenic Pentaoxide. The positive ion LDI spectrum obtained by using 266 nm light on crystalline As2O5 samples is also characterized by three dominant ions: AsO+ (m/z ) 91), As3O4+ (m/z ) 289), and As5O8+ (m/z ) 503). Several less abundant cluster ions are also formed: HAs2O3+ (m/z ) 199), As3O5+ (m/z ) 305), HAs4O7+ (m/z ) 413), HAs4O8+ (m/z ) 429), As5O7+ (m/z ) 487), and As5O9+ (m/z ) 519). In the neutral pentaoxide molecule, arsenic has a fragment valence of K ) 5, which is indicative of a pentavalent oxidation state for arsenic. The fragment valence of the two lower mass ions (91 and 289), however, is 3, which indicates a trivalent oxidation state for arsenic. Different arsenic oxidation states present within the same cluster ion are indicated by a nonintegral fragment valence, (36) Michiels, E.; Bijbels, R. Anal. Chem. 1984, 56, 1115-1121. (37) Plog, C.; Wiedmann, L.; Benninghoven, A. Surf. Sci. 1977, 67, 565. (38) Becker, K. A.; Plieth, K.; Stanski, I. N. Prog. Inorg. Chem. 1962, 4, 1-71.

Figure 2. Negative ion mass spectra, revealing the cluster and fragment ions obtained with 266 nm laser excitation of pure crystalline (a) arsenic trioxide (As2O3) and (b) arsenic pentaoxide (As2O5).

as in the case of As5O8+, where K ) 3.4. The two less abundant cluster ions (m/z ) 305, 519) have fragment valences of 3.6 and 3.8, respectively, which again is indicative of different arsenic oxidation states present within the same cluster ion. For the less abundant clusters which demonstrate hydrogen or water attachment, the hydrated cluster ion is believed to contain different arsenic oxidation states. The formation of the dominant positive cluster ions with the general formula [AsO(As2O3)n]+ can also be correlated to the molecular structure of the arsenic pentaoxide in the gas phase. The true crystalline structure and formula of arsenic pentaoxide are unknown;39 however, the gaseous phase above solid As2O5 has been investigated and found to contain As4Ox molecules (x ) 6-10) and O2.40,41 The structure of arsenic pentaoxide is thermodynamically unstable and begins to decompose into arsenic trioxide and oxygen molecules around the melting point temperature of 315 °C. Thus, the positive laser desorption/ionization mass spectra of arsenic pentaoxide are expected to show that the predominant cluster and fragment ions are similar to those observed for the trioxide. Negative Ions of Arsenic(III) and -(V) Oxides. Figure 2a and b shows negative ion mass spectra of As2O3 and As2O5, respectively. Both of the oxides in the negative ion mass spectra demonstrate a more diverse set of ions than those in the positive ion spectra. As in the positive ion spectra, some similar negative ion fragments and clusters are observed in both the As2O3 and As2O5 . In both oxides, there are two abundant fragment ions, AsO2- and AsO3-, produced from laser irradiation of the solid sample. The abundant low-mass fragment ions are capable of recombining with the neutral species in the plume of gas phase and condensed phase material directly above the sample surface. As a result of the ion-neutral collisions and attachment in the (39) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; John Wiley & Sons: New York, 1988. (40) Dictionary of Organic Compounds, Vol. 1; Chapman & Hall: London, 1992. (41) Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley & Sons: New York, 1984; Vol. 3, p 254.

plume, both the As2O3 and the As2O5 negative ion spectra contain high-mass cluster ions which are unique to their respective oxides. For example, the intense As3O8- peak (AsO3- + As2O5) seen in Figure 2b is unique to the pentaoxide. Similarly, the As3O5- peak (AsO2- + As2O3) is observed only in the trioxide. These two ions also maintain the original stoichiometry of their respective oxide,37 and thus allow speciation of the arsenic in these compounds. Additional evidence concerning the oxidation state can be obtained by examining the ratio of the relative intensities of AsO2- to AsO3-, with the former peak being the more intense in the trioxide and the latter peak being the more intense in the pentaoxide. Characteristic Negative Ions of Arsenic Trioxide. The negative LDI spectra collected at 266 nm for crystalline As2O3 are characterized by four dominant fragment and cluster ions: AsO2(m/z ) 107), AsO3- (m/z ) 123), As3O5- (m/z ) 305), and As3O6(m/z ) 321), where the most intense peak is AsO2-. Prior to laser irradiation, arsenic is in the trivalent state, but after laser irradiation, the four dominant negative ions are found to have fragment valences of K ) 3, 5, 3, and 3.6, respectively. The fragment ions AsO2- and As3O5- maintain the original trivalent state (where K ) 3) of the arsenic, where As3O5- is unique to the trioxide and is used specifically as a marker to indicate the presence of the trioxide. The negative cluster and fragment ions for the pure arsenic oxides have not been as well studied or characterized as the positive ions. However, by analogous substitution of AsO2- and AsO3- for AsO+ in the general formula, the negative ion general formulas [AsO2(As2O3)n]- and [AsO3(As2O3)n]- can then be used to describe the dominant negative ions observed above. For n ) 0 and 1, the four major clusters and fragments (m/z ) 107, 123, 305, and 306) observed are accounted for in the negative ion spectra. The width of the ions shown in the mass spectrum for arsenic trioxide in Figure 2a may be a result of fragmentation of higher mass metastable clusters in the acceleration region of the time-of-flight mass spectrometer. Characteristic Negative Ions of Arsenic Pentaoxide. The negative LDI spectra at 266 nm for crystalline As2O5 are characterized by four dominant fragment and cluster ions: AsO2- (m/z ) 107), AsO3- (m/z ) 123), As3O7- (m/z ) 337), and As3O8- (m/z ) 353). One less abundant ion is also observed: HAs2O6- (m/z ) 247). The most intense peak is AsO3-. By substitution of As2O5 in the negative ion general formula, [(AsO2)v(As2O5)n]- and [(AsO3)v(As2O5)n]- can then be used to describe the dominant negative ions observed for arsenic pentaoxide. For n ) 0 and 1 (v ) 1), four of the five ions (107, 123, 337, and 353) can be accounted for in the negative ion spectra. Mass 247 is accounted for when AsO3- encounters HAsO3 in the gas phase plume in order to form HAs2O6-. The hygroscopic nature of arsenic pentaoxide can account for the observation of hydrogen attachment to a cluster ion. The four dominant ions did not exhibit the hydration effect. Prior to laser irradiation, arsenic is in the pentavalent state, but after laser irradiation, the four dominant negative ions are found to have fragment valences of K ) 3, 5, 4.3, and 5 with respect to increasing mass-to-charge ratio. The fragment ions AsO2- and As3O8- maintain the original pentavalent state (fragment valence of K ) 5) of the arsenic, where As3O8- is unique to the pentaoxide, and like As3O5- for the trioxide, As3O8is used specifically as a unique marker for the pentaoxide. There are additional higher mass clusters formed for the arsenic Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

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pentaoxide at As4O(9,11)-, where n ) 1 and v ) 2, and also at As4O(10)-, where n ) 1 and v ) 2 (AsO2- and AsO3-). Other higher mass clusters are also formed at As5O(12,13)-, where n ) 2 and v ) 1. With low laser pulse energy, the intensities of the As4Ox- and As5Ox- ions were minimal and thus are not reliable markers for arsenic pentaoxide. The width and low intensity of the high-mass ions ( As3Ox-) may be a result of fragmentation of the higher mass metastable clusters in the acceleration region of the time-of-flight mass spectrometer. The formation of the arsenic-oxygen fragment and cluster ions relates to the stability of the electronic configuration associated with the number of arsenic atoms and the oxidation state of the arsenic present within the structure. Ions with odd numbers of arsenic, which are closed-shell ions, were predominantly formed over those with even numbers of arsenic, which are open-shell ions. Arsenic pentaoxide formed a greater variety of positive and negative ions than arsenic trioxide, which may be due to decomposition of the pentaoxide during vaporization.33-35 Mixtures of Arsenic(III) and -(V) Oxides. The negative ion mass spectra demonstrate a more diverse set of clusters unique to each arsenic oxide than the positive ion mass spectra. Thus, the speciation LDI-TOF-MS experiments were performed using the negative ion mode. The clusters As3O5- and As3O8- are used as indicators of the oxidation state of the arsenic, (III) or (V), present within a mixture of the two arsenic oxides. Mixtures were prepared by separately weighing out appropriate amounts of the pure solids, arsenic trioxide and arsenic pentaoxide, and then mixing them together before they are ground into a fine crystalline powder by mortar and pestle. The series of mixtures are reported as mole percent ratios of arsenic(III) trioxide to arsenic(V) pentaoxide. Mixture 100/0 corresponds to 100% arsenic(III) and 0% arsenic(V) in Figure 3a, 80/20 in b, 60/ 40 in c, 40/60 in d, 20/80 in e, and 0/100 in f. The mass spectra in Figure 3 have been expanded to examine the 280-380 m/z range with variation of the arsenic(III) to arsenic(V) ratio in a series of mixtures. The relative intensities of AsO2- and AsO3indicate the presence of arsenic(III) and -(V) oxides in the mixture but are not the most reliable measure of the percent composition of the arsenic(III) and -(V) oxides within the mixtures, and thus they are not used as a means of distinguishing between the different ratios of arsenic oxidation states. The relative signal intensities of the As3Ox- provide a reliable measure of the percent composition of the arsenic(III) and -(V) oxides within the mixtures. From the progression of oxide mixtures shown in Figure 3a-f, the intensity of As3O5- (*) relative to the other As3Ox- is observed to be diminishing, while the relative intensity of As3O8- (#) is growing. The data presented in Figure 3 indicate that there is a direct relationship between the relative intensity of the As3Ox- peak and the amount of each arsenic oxidation state. Calibration Curves of the As3Ox- Ions. Calibration curves for the As3Ox- (x ) 5-8) cluster ions in Figure 4 were generated from the mass spectral series of arsenic oxide mixtures illustrated in Figure 3, and depict a monotonic relationship between the intensity of the respective peaks and the percent composition of arsenic trioxide or arsenic pentaoxide within the mixture. Each point in Figure 4 represents the integrated peak area intensity averaged over nine mass spectra of the same mixture. Error bars for each point indicate one standard deviation in peak area intensity and are dependent on the composition, which is depend4056 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

Figure 3. Series of negative ion mass spectra acquired from 266 nm laser excitation of arsenic trioxide (As2O3) and arsenic pentaoxide (As2O5) mixtures, where the mole ratios of arsenic(III)/arsenic(V) were (a) 100/0, (b) 90/10, (c) 70/30, (d) 30/70, (e) 10/90, and (f) 0/100. From the progression of oxide mixtures shown in a-f, the intensity of As3O5- diminishes while the intensity of As3O8- increases.

ent on the physical heterogeneity of the mixture. Other factors, i.e., laser power fluctuations, also contribute to the variability in relative cluster ion intensities indicated by the error bars. The fraction of As3Ox- (y axis) is plotted as the ratio of the individual integrated peak areas to the sum of the four peak areas in the As3Ox- region of the mass spectra. The composition (x axis) is based on the mole percent of arsenic(III) oxide present within the mixture of arsenic trioxide and arsenic pentaoxide. The composition (mole percent) of arsenic(V) oxide, f, can be calculated as 100% less the mole percent of arsenic(III), t, yielding f ) 100% - t. Calibration curves of the arsenic(III/V) oxide mixtures have been established for each of the four As3Ox- (x ) 5-8) cluster ions. Although the As3O5- and As3O8- cluster ions were uniquely produced for a single oxidation state (III and V, respectively), the effects on the calibration curves from other ions such as As3O6and As3O7- cannot be ignored. The As3O6- and As3O7- cluster ions each contain both arsenic oxidation states and were formed from the pure arsenic oxides as well as the arsenic oxide mixtures. Each calibration curve was calculated independently and represents the computer-generated least-squares fit to the data. Since each of the four As3Ox- cluster ions contains three arsenic atoms, one would naively expect that the relative intensity of the As3Ox- peaks (IAs3Ox-) would be dependent on the number of arsenic atoms (mole fraction of the arsenic) in the trivalent (III), t, or pentavalent (V), f, oxidation state and would therefore reflect the relative amounts of arsenic(III) and arsenic(V) in the

Figure 4. Calibration curves for the unique marker ion As3O5-, the cluster ion As3O6-, the cluster ion As3O7-, and the unique marker ion As3O8- based on a comprehensive series of arsenic(III) and -(V) oxide mixtures. Error bars represent a single standard deviation from the average As3O5- peak area in the mass spectra of the mixtures.

sample. The relative intensity of As3O5-, where all three arsenic atoms are in the trivalent (III) oxidation state, to As3Ox- should be proportional to the mole fraction of the arsenic oxidation state within that cluster ion, which would be arsenic(III) cubed, or t3 for (IAs3O5-). The relative intensities of As3O6-, As3O7-, and As3O8could be naively predicted in a similar fashion. The relative intensities of As3O6- should be proportional to the mole fraction of arsenic(III) squared, times the mole fraction of arsenic (V), t2f. The other relative intensities would be tf 2 for IAs3O7- and arsenic(V) cubed, f3, for IAs3O8-. The naive prediction about the relative intensities of the As3Oxcluster ions, however, does not correlate with the observed results because the laser desorption/ionization process can convert a fraction of one arsenic oxidation state to the other. Thus, cluster ions containing both arsenic oxidation states may be formed, even when starting with a pure sample of one oxidation state. Conversion of the arsenic oxidation state may result from the numerous gas phase reactions occurring above the sample surface during desorption and ionization by the laser and thermal decomposition driven by the laser. For the series of arsenic oxide mixtures in Figure 3, the intensity of the unique marker ion, As3O5-, relative to the other As3Ox- ions is fit to a cubic, t3, relationship, as was naively expected. However, the curve for As3O6- is fit to linear responses in mole fraction of arsenic(III), t. Linear relationships are not expected from a probability standpoint (naive assumption), and the mechanism of ion formation is not fully understood at this time. It is interesting to note that the cluster ion As3O6- is observed for both of the pure arsenic oxides. The curve for As3O7- appears to follow an unusual (constant - t3) response instead of the statistical tf 2 behavior. The As3O8- cluster ion is the unique marker ion for arsenic(V). The As3O8- ion was expected to fit a cubic, f 3, response rather than a linear, f, response since all three arsenic atoms in the cluster have the arsenic(V) formal oxidation state. Analysis of the nonstatistical cluster ion yields is currently being investigated and will be presented in future work.

The observed ratios of the individual peak areas of the As3Oxions to the total peak area yield a measure of the mole percent of the arsenic oxidation state within the mixture. Obtaining quantitative ion yields for the mixture of oxides is made difficult by physical inhomogeneity in the mixtures. The physical heterogeneity of the sample, as well as the shot-to-shot fluctuation of laser pulse energy contributes to the error bars shown in Figure 4. Application of Speciation Technique. Many of the current analytical techniques for determination of the oxidation states of arsenic involve lengthy procedures requiring extraction, separations, derivatization, and/or digestion10,19-22 Speciation of trivalent and pentavalent arsenic by differential pulse polarography (DPP) is possible since trivalent arsenic is electroactive, whereas pentavalent arsenic is not. Pentavalent arsenic may be obtained either by reduction to trivalent arsenic followed by analysis by DPP or by taking the difference of trivalent arsenic, acquired from DPP, from the total arsenic on the filter. The total arsenic on the filter sample was ∼3 mg of arsenic (trivalent and pentavalent) per gram of filter, as determined by X-ray fluorescence (XRF). There was difficulty in quantitatively extracting arsenic embedded on and within the air filter sample; thus, the analysis by DPP produced unreliable results. Laser desorption/ionization time-of-flight mass spectrometry, however, demonstrates the ability to rapidly speciate and determine a semiquantitative amount of the trivalent and pentavalent arsenic oxides on an air filter sample from a model incinerator without the need for labor-intensive procedures necessary for other analytical techniques. Laser desorption/ionization (LDI) has been applied to a sample from a model incinerator. An air filter, located in the exhaust fume hood of a model incinerator, was used to trap the particulate effluent generated under a variety of combustion conditions during a set of burns of cacodylic acid ((CH3)2As(O)OH).5 Ethylene and a hydrogen/nitrogen mixture were the two fuels used for various combustion trials.5 After exposure to a variety of combustion conditions, the air filter was removed from the model incinerator and prepared for Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

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Figure 5. Negative ion mass spectrum acquired from 266 nm laser irradiation of a stack air filter, which was used for trapping the particulate effluent from a model incinerator. Numerous carbon clusters (Cx-, x ) 2-8) indicative of soot from combustion were observed in the low-mass region. In the high-mass region, arsenic oxide clusters (As3O5-, As3O6-, and As3O7-) and abundant fragment ions (AsO2- and AsO3-) were formed by laser desorption/ionization without interference from other species.

analysis by LDI-TOF-MS. A small section (25 mm2) of the large filter (61 600 mm2) was removed from the central region and was applied without further preparation directly to the sample probe with double-sided adhesive. A similar sized section of a blank filter was also prepared for analysis. Figure 5 is a negative ion mass spectrum acquired by LDI from the exposed air filter sample. The mass spectrum was averaged for 50 laser pulses. The byproducts of the model incinerator trapped on the air filter do not cause any observable interference with the formation or detection of the As3Ox- ions. Numerous carbon clusters (Cx-, where x ) 2-8) indicative of soot from combustion were observed in the low-mass region. Features in the 200-250 m/z range may be due to either polycyclic aromatic hydrocarbon (PAH) species42-45 or As2Ox- clusters. From our studies of soot from the model incinerator, the m/z range above 300 in the negative ion mode is free of interference from PAHs. The presence or absence of sodium is one of the features which influence the products of combustion,8 so the effects of large amounts of sodium on the formation of the As3Ox- has been evaluated. Negative ion mass spectra have been obtained for sodium arsenite (NaAsO2), and the presence of sodium did not prevent the formation of the As3O5- and As3O6- ions. The formation of the As3O5- and As3O6- ions is believed to result from recombination reactions of AsO2 and AsO3 within the gas phase rather than from intact molecules desorbed from the NaAsO2 sample. The blank filter produced no background signal in the (42) Dotter, R. N.; Smith, C. H.; Young, M. K.; Kelly, P. B.; Jones, A. D.; McCauley, E. M.; Chang, D. P. Y. Laser Desorption Time of Flight Mass Spectrometry of Nitrated Polycyclic Aromatic Hydrocarbons. Anal. Chem., submitted. (43) Bezabeh, D. Z.; McCauley, E. M.; Jones, A. D.; Kelly, P. B. Negative Ion Laser Desorption Ionization Time of Flight Mass Spectrometry of Nitrated Polycyclic Aromatic Hydrocarbons. J. Am. Soc. Mass Spectrom., submitted. (44) Bezabeh, D. Z.; McCauley, E. M.; Kelly, P. B.; Jones, A. D. Laser Desorption Ionization TOF Mass Spectrometry of Nitrated Polycyclic Aromatic Hydrocarbons. Proceedings of the 42nd ASMS Conference on Mass Spectrometry and Allied Topics, Chicago, IL, May 29-June 3, 1994; p 1119. (45) Bezabeh, D. Z.; Kelly, P. B.; Jones, A. D. Laser Desorption Ionization TOF Mass Spectrometry of Hydroxy-Nitro-Polycyclic Aromatic Hydrocarbons. Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 21-26, 1995; p 962.

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mass regions of the negative ion spectra where the arsenic oxide fragment and cluster ions were observed. The observation of the high-mass arsenic cluster ions at m/z ) 305, 321, and 337 in addition to the abundant fragment ions at m/z ) 107 and 123 is of importance. The relative ratio of the fragment ions, AsO2- to AsO3-, is indicative of the presence of arsenic oxides. As shown above, mass 305 corresponds to the unique marker peak, As3O5-, of arsenic trioxide. From the calibration curve in Figure 4, the area under the As3O5- marker peak relative to the integrated intensity for all As3Ox- features corresponds to about 72% arsenic(III) oxide and 28% arsenic(V) oxide. Very little of the marker ion, As3O8-, is observed in the mass spectrum for the air filter sample, but the formation of As3O7-, which consistently accompanies As3O8-, is a strong indication that there is some pentavalent arsenic trapped on the air filter. From the As3O7- calibration curve in Figure 4, the relative area under the As3O7- peak corresponds to about 85% arsenic(III) and 15% arsenic(V). The relative area of the As3O6peak indicates an arsenic(III)-to-arsenic(V) ratio of >85. The broad range in determining the amount of trivalent or pentavalent arsenic was expected due to the error associated with physical inhomogeneity in the air filter sample from a model incinerator. The negative ion mass spectrum of Figure 5 illustrates that arsenic oxide cluster and fragment ions were formed by laser desorption/ ionization in a complex mixture. We are able to determine the mole percent composition of arsenic(III) and -(V) oxides as a combustion exhaust diagnostic. CONCLUSION Application of laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) to pure arsenic oxides yields speciesspecific information about the arsenic oxidation state. Of the numerous ions formed from laser desorption/ionization, the ability to distinguish the relative amount of an arsenic oxide present within a series of mixtures has been demonstrated by the formation of the unique marker ions, As3O5- and As3O8-, which maintain the initial arsenic oxidation state of the molecular compound. Calibration curves for a semiquantitative analysis of samples have been developed. The unusual behavior observed for the As3Ox- ions in the calibration curves is not completely understood but offers the potential for further investigation of the data using multivariate analysis. The LDI-TOF-MS technique is limited by physical inhomogeneity in a sample; however, application of LDI to a model incinerator air filter sample demonstrates the ability to form and detect the unique marker ions within a complex mixture. The initial oxidation state of the arsenic has a profound effect on the relative amounts of the different clusters formed, indicating a potentially new methodology of generating unique MmOnq metal oxide cluster ions. The formation of unique negative cluster ions for the trivalent and pentavalent arsenic oxides by laser desorption/ionization (at 266 nm light) time-of-flight mass spectrometry represents the most encouraging results for application of the method as a rapid screening technique for inorganic oxidation state speciation by laser mass spectrometry. Determination and speciation of other metal oxidation states by LDI-TOF-MS may be possible, based on our success with arsenic oxides. ACKNOWLEDGMENT We gratefully acknowledge support from the NIEHS Superfund Basic Research Program (Grant 3-P42-ES04699). The authors

thank Peter Schiffman and Sarah Roeske of the Geology Department at the University of California at Davis for the use of and help with the X-ray fluorescence. The authors also thank Dr. Amy Witter for her help with the analysis of the filter samples by differential pulse polarography.

Received for review April 15, 1996. Accepted August 15, 1996.X AC960359Z X

Abstract published in Advance ACS Abstracts, October 1, 1996.

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