Anal. Chem. 1996, 68, 3238-3243
Spatially Resolved Time-of-Flight Mass Spectrometry of Polycyclic Aromatic Hydrocarbons: Quantification Studies Steven M. Hankin, Phillip John,* Alexander W. Simpson,† and Gerald P. Smith
Department of Chemistry, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K.
Deuterated polycyclic aromatic hydrocarbons (PAHs) have been employed as surrogate internal standards for the quantitative analysis of PAHs by laser postionization time-of-flight mass spectrometry. Experiments were performed on intimate mixtures of chrysene and chrysened12 which were desorbed from thin films at 266 nm using a tightly focused (1-2 µm) Nd-YAG laser beam. The neutral molecules within the laser-desorbed plume were ionized by a frequency-doubled dye laser at 260 nm. Under soft ionization conditions, the mass spectrum comprised peaks associated with the parent ion envelope with negligible amounts of fragment ions at lower masses. The ratio of the peak areas of the parent ions of chrysene and chrysene-d12 was proportional to their relative molar concentrations in the standard solutions. With the use of chrysene-d12 as an internal standard, the determined concentration of chrysene in an NIST standard was in good agreement with the certified value. Fossil fuel combustion is largely responsible for the environmental burden of polycyclic aromatic hydrocarbons (PAHs) within urban areas.1 There is growing concern about the adverse effect on human health of respirible particulates,2 with attention currently being paid3 to airborne particulates of mean aerodynamic diameters of less than 10 µm (PM2.5s and PM10s). The potential mutagenic and carcinogenic effects4 of these ubiquitous environmental pollutants have warranted their extensive analysis by a range of analytical techniques and methodologies.5 While significant progress has been made in the extraction, characterization, and quantification of PAHs in environmental samples, the analysis of individual PM10s remains challenging. A number of sophisticated mass spectrometric techniques have been adapted for the microanalysis of solids with high spatial resolution. Foremost among these are the related techniques of laser ionization mass spectrometry6 and secondary ion mass spectrometry.7 These surface analysis techniques rely on a single† Present address: NEC Semiconductors Ltd., Livingston EH54 8QX, Scotland. (1) Bjorseth, A., Ed. Handbook of polycyclic aromatic hydrocarbons; Marcel Dekker: New York, 1983. (2) Holgate, S. T. Asthma and Outdoor Air Pollution; HMSO: London, 1995. (3) Pope, C. A.; Dockerty, D. W.; Schwartz, J. Inhalation Toxicol. 1995, 7, 1-18. (4) International Agency for Research on Cancer. Monographs on the Evaluation of Carcinogenic Risks to Humans; IARC: Lyon, France, 1989; Vol. 32, Part 1. (5) Bartle, K. D.; Lee, M. L.; Wise, S. A. Chem. Soc. Rev. 1981, 10, 113-158. (6) Vertes, A., Gijbels, R., Adams, F., Eds.; Laser Ionization Mass Analysis; Wiley: New York, 1993. (7) Benninghoven, A.; Ru ¨ denauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; Wiley: New York, 1987.
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step desorption process using a pulsed laser or focused ion beam respectively to create ions which are subsequently mass analysed. However, both techniques suffer from serious drawbacks when applied to the desorption of organic molecules. Signals may arise from ubiquitous surface contaminants along with extensive analyte fragmentation, resulting in extraneous lower mass ions. The latter feature is detrimental to the characterization of complex mixtures in that the parent ions may be obscured or, at least, in that their assignments are ambiguous. The mass spectra can be simplified by spatial and temporal separation of the desorption and ionization steps using two lasers. Thus, the two-step laser desorption/laser ionization technique coupled with time-of-flight mass separation, L2TOF, becomes a powerful method8-13 for the analysis of organics with high spatial resolution. Variations in the L2TOF mass spectrometric techniques adopted to date differ in terms of the type of laser employed for the desorption and ionization steps. Zare et al.8 first described the use of a pulsed CO2 laser at 10.6 µm for the desorption of molecules with minimal fragmentation. In practice, the adoption of wavelengths in the infrared region8,11-13 restricts the spatial resolution to greater than ∼40 µm. The ultimate diffraction-limited spot size (∼2λ) is governed by the aberrations in the infrared lenses. In contrast, instruments with a higher spatial resolution9,10 rely on shorter desorption wavelengths. DeVries et al.9 achieved this with a KrF (248 nm) excimer laser for desorption and fixed wavelength ArF and NdYAG lasers at 193 and 266 nm, respectively, for ionization. Several recent publications have highlighted the potential application of L2TOFMS to the qualitative analysis of environmental matrices.14-16 The quantification of metal isotope ratios17 and organic analytes18 using single-laser desorption mass spectrometry has been investigated. However, due to the dependence of the ion signal on a (8) Zare, R. N.; Hahn, J. H.; Zenobi, R. Bull. Chem. Soc. Jpn. 1988, 61, 87-92. (9) DeVries, M. S.; Elloway, D. J.; Wendt, H. R.; Hunziker, H. E. Rev. Sci. Instrum. 1992, 63, 3321-3325. (10) Odom, R. W.; Schueler, B. In Lasers and Mass Spectrometry; Lubman, D. M., Ed.; Oxford University Press: Oxford, 1990; pp 105-137. (11) Kovalenko, L. J.; Maechling, C. R.; Clemett, S. J.; Philippoz, J.-M.; Zare, R. N.; Alexander, C. M. O. Anal. Chem. 1992, 64, 682-690. (12) Voumard, P.; Zhan, Q.; Zenobi, R. Rev. Sci. Instrum. 1993, 64, 2215-2220. (13) Dale, M. J.; Jones, A. C.; Pollard, S. J. T.; Langridge-Smith, P. R. R.; Rowley, A. G. Environ. Sci. Technol. 1993, 27, 1693-1695. (14) Dale, M. J.; Jones, A. C.; Pollard, S. J. T.; Langridge-Smith, P. R. R. Analyst 1994, 119, 571-578. (15) Dale, M. J.; Downs, O. H. J.; Costello, K. F.; Wright, S. J.; Langridge-Smith, P. R. R.; Cape, J. N. Environ. Poll. 1995, 89, 123-129. (16) Zhan, Q.; Voumard, P.; Zenobi, R. Rapid Commun. Mass Spectrom. 1995, 9, 119-127. (17) Koumenis, I. L.; Vestal, M. L.; Yergey, A. L.; Abrams, S.; Deming, S. N.; Hutchens, T. W. Anal. Chem. 1995, 67, 4557-4564. (18) Wilk, Z. A.; Viswanadham, S. K.; Sharkey, A. G.; Hercules, D. M. Anal. Chem. 1988, 60, 2338-2346. S0003-2700(96)00356-3 CCC: $12.00
© 1996 American Chemical Society
Figure 1. Schematic diagram of the L2TOF mass spectrometer.
number of factors, including laser pulse energy and duration, ionization volume, and the efficiency of ion transmission, the reproducibility remains poor. The similarity in the behaviors of deuterated PAHs and their protio analogues, in conjunction with their inherent mass difference, serves as the reason for their use as surrogate internal standards in mass spectrometry. This paper reports on the quantitative analyses of chrysene using the perdeuterated analogue, highlighting the spatial resolution and sensitivity advantages of the L2TOF technique. EXPERIMENTAL SECTION The laser desorption/postionization time-of-flight mass spectrometer used in these experiments is a modified version of a commercial laser microprobe instrument (Kratos Analytical Ltd., Model LIMA 401L). The spectrometer is fitted with a highmagnification optical microscope for sample imaging. A schematic diagram of the instrument is provided in Figure 1. The main chamber and flight tube were maintained at a pressure better than 10-7 Torr by rotary-backed turbomolecular pumps (Balzers, 170 L/s). Manipulation of the samples on the x, y, and z axes of the rotary carousel was achieved by precision translation stages. The desorption laser output was provided by the fourth harmonic output (266 nm) of a pulsed Nd-YAG laser (Quanta-Ray DCR11, Spectra-Physics). Removal of the 532 nm component was achieved with a pair of dichroic mirrors (Tec-Optics Ltd). Achromatic focusing of the desorption beam normal to the sample surface was accomplished using a Cassegrain objective (focal length, 14 mm). Typical spot sizes at the focus are in the range of 1-2 µm diameter. Larger diameters can be achieved by moving the sample relative to the Cassegrain lens. A He/Ne laser coincident with the Nd-YAG optical path is used for targeting the analysis location. Representative sampling of the specimen is obtained by the translation of the sample relative to the desorption laser. The pulse energies at 266 nm were varied over the range 0.05-1 mJ by altering the delay between the lamps and Q-switch
firing. The Nd-YAG laser was operated at a repetition rate of 1 Hz with a pulse width of 10 ns. The desorbed neutrals were ionized by the frequency-doubled (KD*P crystal) output of an excimer (Questek Series 2000, 308 nm, 80-100 mJ/pulse) pumped dye laser (Quanta-Ray PDL-2, Spectra-Physics), operating on Coumarin-334 (Radiant Dyes Chemie) dissolved in HPLC grade methanol. The dye laser was tuned over the range 506-537 nm (λmax ) 520 nm) with pulse energies of 1.8-2.5 mJ at 1 Hz repetition rate, and a pulse width of 10-25 ns. The frequencydoubled output was tunable over the range 253-268 nm. The 520 nm fundamental was spatially resolved from the 260 nm second harmonic by a Pellin-Broca prism. The pulse energy of the 260 nm frequency-doubled output was varied over the range 1-140 µJ/pulse, at a repetition rate of 1 Hz, as measured by a power meter (Scientech Model 362). Selected L2TOFMS experiments have been conducted using the fourth harmonic output (266 nm) of a pulsed Nd-YAG laser (Quanta-Ray DCR-11) for postionization. The laser fluence values reported have not been corrected for reflection losses. The focus of the ionization laser was located ∼0.2-0.3 mm above the sample surface. The beam was focused into the center of the ablation plume using a fused silica UV grade planoconvex lens (focal length, 150 mm; diameter, 25 mm). The focused beam waist of the ionization laser is ∼10 µm in diameter in the current optical system. The ions within the laser-generated plume travel in the direction of the flight tube axis and pass through a precision-drilled hole in the Cassegrain lens. Synchronous firing of the Nd-YAG laser and the excimer laser was controlled using an in-house-built variable time delay unit (0-50 ms). The Nd-YAG laser was set in free running mode, with the TTL lamp synchronous output used to trigger the delay unit, the output of which was used to trigger the excimer laser. In this arrangement, the timing jitter between the desorption and ionization laser beam pulses was better than 50 ns. A delay time distribution of the chrysene molecular ions was measured from a plot of signal intensity at m/z ) 228 as a function of the delay between the desorption and ionization laser pulses. Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
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Figure 2. Delay time distribution of chrysene using a desorption wavelength of 266 nm and an ionization wavelength of 260 nm.
Under standard conditions, the parent ion signal attained a maximum at around 0.4 µs delay for the desorption of neutral molecules at 266 nm. As shown in Figure 2, the signal intensity subsequently exhibited a slow decline. During the analyses, the time delay was adjusted to maximize the signal. The postionization signal was contingent on the presence of both laser pulses; removal of either eliminated the signal. Acceleration of the ions was achieved using a three-plate Einzel lens assembly maintained at potentials of about +2.0, -2.5, and +2.0 kV, respectively, located ∼4 mm in front of the sample surface. The sample stub is an integral part of the ion extraction region and is maintained at a potential of +2.7 kV for postionization experiments. Mass separation of the ions was achieved using a 2 m long flight tube fitted with a reflectron, resulting in an ion flight path of ∼4 m. The ions were detected by an electron multiplier (Thorn-EMI EM119/1) located at the end of the ion flight path, with the amplified signal being fed directly into a 175 MHz transient digitizer (LeCroy 9400A). Data acquisition was initiated by the TTL Q-Switch synchronous output from the desorption laser. The observed flight time for the chrysene molecular ion at a mass of 228 amu is ∼68 µs. This corresponds to the sum of the neutral desorption and ion flight times intrinsic to laser desorption/postionization. The digitized data are then stored on a computer (Hewlett-Packard series 300, type 320) using HewlettPackard software (HP Basic 5.0). The resolution (M/∆M) of the instrument, using a reflectron in the flight path and determined from the carbon cluster peaks, varied with mass but was of the order of 800-900 at m/z ) 228. The resolution in the singlelaser ionization experiments was ∼700, using the isotope pattern of molybdenum in the range 92-100 amu. It should be noted that the instrument could be operated with postionization of neutrals or single-laser desorption/ionization by simply blocking the dye laser beam and increasing the Nd-YAG laser power to beyond the ionization threshold. The small target spot size and viewing capability of this microprobe technique is well suited to the analysis of micrometer-sized particulates such as atmospheric aerosols. Moreover, the low laser powers required for the desorption of neutrals allows multiple shots on a single location, eliminating the need for sample replenishment. Chrysene (Aldrich, 98%) and chrysene-d12 (Aldrich, >98 atom % D) were used without further purification. The chemical and isotopic purity was checked by high-resolution capillary gas 3240 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
Figure 3. Single-shot L2TOF mass spectrum of chrysene and chrysene-d12 from a thin film showing the respective parent ions at m/z ) 228 and 240. The desorption and ionization energies were
