Envlron. Scl. Technol. lQQ3,27, 1693-1695
Appllcation of Two-step Laser Mass Spectrometry to the Anaiysls of Polynuclear Aromatic Hydrocarbons in Contaminated Soils Mlchael J. Dale, Anlta C. Jones, Slmon J. T. Pollard, and Patrlck R. R. Langrldge-Smith'
Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, U.K. Alan 0. Rowley 218 Dlnerth Road, Rhos-on-Sea, Coiwyn Bay, Clwyd, LL28 4UH, U.K.
Introduction Contaminated site remediation is a multistage process that commenceswith an initial indication of contamination and is concluded with the long-term monitoring of remedial measures (1-3). Critical to the successful completion of the process is the informative nature of the site assessment data. This information ultimately determines the level of confidence with which risks to human health and the environment can be adequately assessed and managed (46). Hydrocarbon-contaminated soils, such as those found a t former manufactured gas plant sites, creosote woodpreserving plants, and petroleum refineries pose specific problems for reliable chemical characterization because of the presence of multimedia contamination, the existence of a complex matrix of individual contaminants exhibiting a diverse range of physiochemical properties, and the expensive analytical procedures often required (4,7).Such sites have generated growing concern throughout industrialized countries because the chemical contaminants identified on-site almost invariably include the polynuclear aromatic hydrocarbons (PAHs), several high molecular weight analogues of which pose a toxicological hazard due to their documented carcinogenic activity (8). Analytical techniques such as conventional gas chromatography-mass spectrometry (GC-MS) for hydrocarbon-contaminated soils require lengthy and involved sample cleanup procedures which can render them costprohibitive for extensive contaminated site assessment. Delay times between sampling in the field and the receipt of analytical data can result in sampling protocols being less focused than they would be if even a semiquantitative estimate of the extent of contamination were readily available. In acknowledgement of these difficulties, considerable effort has recently been focused a t developing and validating field techniques capable of analyzing contaminants on-site without the need for sample cleanup. On-site screening and field techniques allow the rapid feedback of information t o field personnel during the sampling procedure (9). Under certain sampling protocols, the screening of large numbers of samples prior to indicator compound(s) analysis will result in significant cost savings throughout the risk assessment and risk management process. Among the analytical methods currently under evaluation for on-site analysis are thermal desorption-gas chromatography-mass spectrometry (TE-GC-MS), ultraviolet fluorescence, and remote laser-induced fluorescence (RLIF) techniques (9-15). In this paper, we describe initial investigations into the use of laser desorption laser photoionization time-of-flight mass spectrometry (L2TOFMS) for the direct determination of PAH analytes in solid waste matrices. This relatively new technique has potential as an on-site field 0013-936X/93/0927-1693$04.00/0
0 1993 American Chemical Soclety
screening tool, offering the ability to determine a wide variety of organics contained in a wide variety of matrices. It involves a two-step process in which the first laser desorbs primarily intact neutral molecules from the sample and the second laser causes "soft ionization" of selected classes of compounds which are identified by time-of-flight (TOF) mass spectrometry (16-18). This method of mass analysis, with its advantages of rapid, multiplexed data aquisition is ideally interfaced to the pulsed lasers used for sample desorption and photoionization. Previous work performed using this two-stage methodology has demonstrated its ability to determine analytically important organic compounds. These have included peptides (191, porphyrins (20), commercial dyestuffs (211, PAHs (22), and polymers (23). More recently, Kovalenko et al. have used a similar methodology to investigate the PAH components of selected meteorites (24). As a novel approach to sample screening, this circumvents many problems associated with more conventional analytical techniques, allowing determination of semivolatile, involatile, and thermally labile species without the need for extensive extraction and separation procedures. The results presented here demonstrate the capability of LZTOFMS to analyze PAHs in a rapid, highly selective, and sensitive manner directly from the contaminated soil matrix. Experimental Section Soils. The six soil samples used in this study were obtained from a contaminated former coal gasification plant and coal-tar distillery in the UK. Samples were taken from shallow trial pits (0.5 m depth) and consisted of three grab samples taken a t 120° from each other prior to being composited into approximately 5-kg total samples. Samples were air-dried a t ambient temperatures in a forced draught. After the resulting aggregates were broken up, the entire sample was passed through a 10-mesh sieve (12.00 mm), the fraction passing through was then homogenized by being riffled down to a 50-g subsample (25). Previous gas chromatography-mass spectrometry has determined the concentration of individual PAHs in these samples to range from 1 and 400 ppm. Sample preparation for the L2TOFMS technique consisted of grinding the soils further into a fine powder, binding the particles together with a drop of glycerol, and applying the homogeneous paste to a 1.5 X 40 mm2 slot in the sample probe. The sample surface open to the desorption laser beam was then dried with a dusting of alumina-type H. Instrumentation. A detailed account of the experimental apparatus has previously been reported (21). The instrument is composed essentially of three separate high vacuum chambers: the desorption chamber, ionization chamber, and a TOF mass spectrometer. Envlron. Scl. Technoi., Vol. 27, No. 8, 1993
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Mass (amu) Flgure 1. LTOFMS spectrum of ca. 1 mg of contaminated soil produced by 193-nm photoionization.
In the desorption chamber, the output from a pulsed C02 laser (Alltec 854MS, 10.6 pm wavelength) is focused, using a NaCl lens, onto a stainless steel sample probe. The sample probe is translated orthogonally to the incident desorption beam, both extending the lifetime of the sample and exposing a fresh area of sample to each laser shot. The neutral molecules thus desorbed are entrained in a pulsed supersonic helium molecular beam and transported from the desorption chamber into the ionization chamber. Here, 193-nm photons (ArF line, Lumonics TE-860T laser) or 266-nm photons (fourth harmonic, J K HY750, Nd:YAG laser) are used to induce the multiphoton ionization (MPI) of the desorbed molecules. These ionizing wavelengths provide selective ionization of aromatic species over aliphatic components in a mixture of organic materials. Judicious control of the ionizing laser fluence, typically of the order ca. 2-5 X 105 W cm-2,provides "soft ionization" conditions so that the parent ions of the polynuclear aromatic compounds almost exclusivelydominate the mass spectra (17,18). The laser-generated photoions are mass separated in a home-built reflectron TOF mass spectrometer (resolution typically ca. 500) and detected by a dual chevron-type microchannel plate (MCP) detector. Experimental control and data aquisition are performed using a CAMAC-based system and interfaced to a Dell system 325 personal computer with in-house custom software. The entire experiment is performed a t a repetition rate of 10 Hz. Typically, data from 200 laser shots were accumulated to enhance the overall signal to noise ratio. Using the present experimental configuration, the entire procedure, from subsample preparation to obtaining a mass spectrum, can be performed within a 10-min period.
Results and Discussion Each of the six contaminated soil samples were examined using both 266-nm and 193-nmlaser photoionization. The mass spectrum of one of the soil samples produced using 193-nm photoionization is shown in Figure 1, and an expansion of the region containing the PAH peaks is shown in Figure 2. Similar spectra were obtained for all the soil sample8 investigated. Figure 1 represents the total 1694 Environ. Scl. Technol., Voi. 27, No. 8, 1993
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Mass (amu) Figure 2. LTTOFMS mass spectrum of contaminated soil expanded to clearly show the resolvedmass spectral peaks in the regioncontaining the PAHs. Mass identificationsare 1, 178, phenanthrenelanthracene; 2, 190, 4Hcyclopentaphenanthrene; 3, 202, fluoranthenelpyrene;4, 228, chrysenelbenzo[a]anthracene; 5, 252, benzo[e]fluoranthene/ benzo[a]pyrene; 6,276, indeno[ 1,2,3-cd]pyrenelbenzo[ghdperylene; 7,300, coronene. Insert: expansion of 195-235 amu reglon showing a series of alkylated pyrenelfluoranthene species.
accumulated ion signal on summing of 200 laser shots. These fingerprint mass spectra were obtained in consecutive experiments; the entire preliminary screening of the samples was completed within 90 min. Under the prevailing experimental conditions, a 200-shot spectrum represented the total ion signal obtained on interrogating ca. 1mg of soil. Samples of up to 50 mg can be interrogated in one continuous scan of the present sample probe. Examination of 50-mg quantities of material provides a better average PAH concentration and reduces subsampling errors. The PAH signals are observed in the region between 100 and 400 amu. Signals a t masses higher than 300 amu indicate the presence of PAHs with more than seven fused rings. Peaks a t masses less than 100 amu are generally the result of ionization of elemental species, e.g., Na and K, or attributed to fragmentation of higher molecular weight species. This low mass region of the mass spectrum is dominated by a peak a t 39 amu, corresponding to the atomic mass of potassium. From Figure 2 it is possible to clearly identify strong ion signals which correspond to the masses of parent PAH skeletons. The dominant masses of 178, 202, 228, 252, and 276 amu correspond to phenanthrene/anthracene, fluoranthene/pyrene, chrysene/benz[al anthracene, benz[a]fluoranthene/benzo[al pyrene, and indeno[1,2,3-cdlpyrene/benzo[ghilperylene. Other peaks of significant intensity correspond to further PAH compounds, e.g., 4Hcyclopentaphenanthrene at 190 amu and coronene a t 300 amu. It is also possible to identify a series of peaks with mass separations of 14 amu, characteristic of a homologous series of successively alkylated PAHs, e.g., for pyrene/ fluoranthene a t 216 and 230 m u . This is shown more clearly in the insert in Figure 2. The absence of significant soil matrix peaks is striking. This eliminates the need for the analysis of blank samples. The use of ultraviolet (UV) radiation for ionization of the desorbed species means that photoionization mass spectra will only contain peaks from
organic species which contain a UV chromophore. There is further selectivity of detection in the specific choice of the wavelength of the ionizing radiation. Photoionization at 266 nm is well known to enhance the detection of PAHs over other organic analytes such as porphyrins (24). The choice of 193-nm photons, which are of higher energy, leads to the ionization of higher mass species not observed in the spectra obtained a t 266 nm. Differences in the relative abundances of the same compound in different soil samples can be directly estimated from the peak intensity. We have also determined the concentrations of particular PAHs in the soils using a series of standard additions, in a procedure similar to that used by Hahn et al. (26). The results obtained by this methodology were consistent with previously determined concentrations obtained using gas chromatographymass spectrometry. The results show clearly that the technique is able to detect PAHs contained in the soil matrices a t the ppm level. Preliminary instrument characterization, using pure PAHs as analytes, showed minimum detection limits for a number of these species using 193-nm photoionization were in the subpicomole region. It is important to note that the parent ion peak intensities are not only proportional to the concentration of the compound in the samples but also depend on the overall ionization efficiency, which is largely determined by the absorption cross-sections for that compound a t the laser wavelength employed. Furthermore, the different PAHs may have different desorption efficiencies. Therefore, the relative peak intensities do not necessarily directly reflect the relative concentrations of the different PAHs in the matrix. These results demonstrate that L2TOFMS is a fast, effective, and sensitive analytical tool for the direct determination of PAHs in complex solid environmental matrices. Furthermore, as a quantitative analytical tool for the prediction of PAH distributions in soils, L2TOFMS could provide an effective alternative to more conventional approaches. Investigations are presently underway to determine the efficacy of the technique for the analysis of PAHs in a variety of different environmental media and to explore its potential as a rapid, on-site screening tool. Acknowledgments We are grateful to B P International Ltd., Unilever plc, and the Science and Engineering Research Council for financial support. M.J.D. holds a SERC/Unilever CASE studentship, and A.C.J. is a SERC Advanced Fellow. Literature Cited (1) Smith, M. A. J.Inst. Water Environ. Manage. 1991,5,617-
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(2) CCME. National Guidelines for Decommissioning In-
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(4) Pollard, S. J. T.; Hrudey, S. E. Site Evaluation Needs for the Assessment of Contaminated Land in Alberta, Canada; University of Alberta, Environmental Health Program; Contract Report for Alberta Environment Help End Landfill Pollution Project, Edmonton, Alberta, Canada, 1992. (5) Lord, D. W. In Reclaiming Contaminated Land; Cairney, T. C., Ed.; Blackie Publishers: Glasgow, 1987; pp 62-113. (6) Risk Assessment Guidance for Superfund Vol.1,Human Health Evaluation Manual (Part A) Interim Final; U.S. Environmental Protection Agency,Office of Emergencyand Remedial Response: Washington, DC, 1989; EPA/540/189/002. (7) Pollard, S. J. T.; Hrudey, S. E.; Fuhr, B. J.; Alex, R. F.; Holloway, L. R.; Tosto, F. Environ. Sci. Technol. 1992,26, 2528. (8) US. Department of Health and Human Services Toxicological Profile for Polycyclic Aromatic Hydrocarbons. TP90-20; Agency for Toxic Substance and Disease Registry. US. Public Health Service. US. Government Printing Office: Washington, DC, 1990. (9) Robbat, A., Jr.; Liu, T.-Y.; Abraham, B. M. Environ. Sci. Technol. 1992, 64, 358. (10) Junk, T.; Shirley, V.; Henry, C. B.; Irvin, T. R.; Overton, E. B.; Zumberge, J. E.; Sutton, C.; Worden, R. D. 2nd International Symposium on Field ScreeningMethods for Hazardous Wastes and Toxic Chemicals; U.S. EPA/U.S. DOE 1991; pp 327-338. (11) Theis, T. L.; Collins, A. G.; Monsour, P. J.; Pavlostathis, S. G.; Theis, C. D. Topical Report GRI-91/0228;Prepared for Remediation Technologies, Inc., for Gas Research Institute Contract No. 5086-254-1334; 1991, p 47. (12) Murphy, T. P.; Brouwer, H.; Fox, M. E.; Nagy, E. Water Pollut. Res. J. Can. 1991, 26, 1-16. (13) Mellone,A.; Smith, B. W.; Winefordner, J. D. Talanta 1990, 37,111-118. (14) Chudyk, W. Environ. Sci. Technol. 1989,23, 504-507. (15) Lieberman, S. H.; Theriault, G. A.; Copper, S. S.; Malone, P. G.; Olsen, R. S.; Lurke, P. W. 2nd International Symposium on Field Screening Methods for Hazardous Wastes and Toxic Chemicals; U.S. EPA/U.S. DOE: 1991; pp 57-65. (16) Weyssenhoff, H. V.; Seltze, H. L.; Schlag, E. W. 2. Naturforsch. 1986,40A, 674. (17) Tembreull, R.;Lubman, D. M. Anal. Chem. 1986,58,1299. (18) Engelke,F.;Hahn,J.H.;Henke, W.;Zare,R. N.Ana1. Chem. 1987,59, 909. (19) Grotemeyer, J.; Boesl, U.; Walter, K.; Schlag, E. W. Org. Mass Spectrom. 1986,21, 645. (20) Grotemeyer, J.; Lindner, J.; Koster, C.; Schlag, E. W. J. Mol. Struct. 1990, 217,51. (21) Dale, M. J.; Jones, A. C.; Costello, K. F.; Cummings, P. G.; Langridge-Smith, P. R. R. Anal. Chem. 1993,65,793. (22) Costello, K. F.; Dale, M. J.; Donovan, R. J.; Jones, A. C.; Keenan, G. A,;Langridge-Smith, P. R. R. 12thInternational Mass Spectrometry Conference, Amsterdam, 1991. (23) Redpath, C. R.; Dale, M. J.; Jones, A. C.; Taylor, M. J.; Rollins,K.; Langridge-Smith, P. R. R. 19thAnnual Meeting of the British Mass Spectrometry Society,University of St. Andrews, St. Andrews, Sept 13-16, 1992. (24) Kovalenko,L. J.;Maechling,C. R.; Clemett, S. J.;Philippoz, J.-M.;Zare,R. N.;Alexander, C. M. O’D. Anal. Chem. 1992, 64, 682. (25) Rowley, A. G. Personal Communication, 1992. (26) Hahn, J. H.; Zenobi, R.; Bada, J. L.; Zare, R. N. Science 1988,239, 1523. Received for review December 22, 1992.Revised manuscript received March 12, 1993. Accepted March 16, 1993.
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