Anal. Chem. 1997, 69, 2927-2930
Laser Time-of-Flight Mass Spectrometry of PAH-Picrate Complexes Steven M. Hankin,† Phillip John,*,† and Gerald P. Smith‡
Department of Chemistry and Edinburgh Surface Analysis Technology, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K.
The laser desorption/laser ionization time-of-flight (L2ToF), mass spectra of anthracene and the anthracene-picric acid charge transfer (C-T) complex have been compared at a desorption and ionization wavelength of 266 nm. Laser desorption/ionization spectra of anthracene were obtained at low temperatures (-30 °C) to minimize the interference from gas phase ionization. Positive ion mass spectra of the picrate C-T complex at room temperature comprise the parent ion of anthracene and were devoid of signals associated with the picric acid component. The L2ToF analyses of a mixture of volatile and involatile EPA priority PAHs in picric acid show that low molecular weight PAHs form involatile charge transfer complexes. The present method reduces the possibility of volatile PAH loss during mass spectrometric analyses in vacuo. Laser time-of-flight mass spectrometry has been established as a valuable analytical technique for the detection of polycyclic aromatic hydrocarbons (PAHs) with high spatial resolution.1-3 The technique has been applied to both terrestrial4-6 and extraterrestrial samples.7-9 The essence of the method is that a plume of neutral molecules is created on irradiation of a solid sample, in vacuo, with a pulsed UV or IR desorption laser. The intact molecules in the gas phase are subsequently ionized with a timedelayed second laser pulse. A number of advantages have emerged from the development of two-laser postionization (L2ToF) mass spectrometry. First, spatially and temporarily decoupling the desorption from the ionization step enables desorption powers to be used that are below the ionization threshold. Low power UV laser pulses (10-5 Pa), all show decreased volatility when complexed with picric acid. The evidence is clear from Figure 3 that the volatile components of PAH mixtures evaporate during mass spectrometric analyses at room temperature. On comparison of the data in Figures 3 and 4, the relative properties of the parent ions from the involatile PAHs are unaffected by the presence of picric acid. The enhanced signals in Figure 4 from the volatile PAHs suggest that their sublimation is inhibited. This is confirmed by the absence of gas phase spectra produced by irradiation with the ionization laser in the absence of desorption. We attribute the lack of volatility to the formation of charge (21) Van Vaeck, L.; Claereboudt, J.; De Waele, J.; Esmans, E.; Gijbels, R. Anal. Chem. 1985, 57, 2944-2951. (22) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1986, 58, 1102-1108. (23) Zenobi, R. Int. J. Mass Spectrom. Ion Processes 1995, 145, 51-77 and references therein.
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Analytical Chemistry, Vol. 69, No. 15, August 1, 1997
transfer complexes in the low molecular mass PAHs naphthalene, acenaphthene, acenaphthylene, fluorene, anthracene, phenanthrene, fluoranthene, fluoranthene, pyrene, and most probably also the higher molecular mass components. Whilst we have no direct evidence of this, the literature suggests that picrate complexes are formed from all priority PAHs.19 There is indirect evidence from the absence of anthracene in the gas phase that the concentration of free anthracene is negligible in the solid complex. While this is true for the isolated anthracene picrate crystals, the absence of gas phase spectra from the volatile components of the PAH mixture strongly suggests that all the PAHs are complexed in excess picric acid. In principle the laser desorption of anthracene picrate at 266 nm could lead to the presence of free anthracene (A), picric acid (P), and the charge-transfer complex [Aδ+‚‚‚Pδ-] in the laser plume. In the absence of the ionizing laser pulse, no ions were detected, eliminating total charge transfer and dissociation in the ground state or excited state complexes. The photophysics17 of C-T complexes suggests that absorption of 266 nm laser light may generate an excited state complex [A*‚‚‚P] through near resonance excitation of the anthracene S3 r S0 band at 252 nm. Electronic relaxation may then lead to the formation of a stable ion pair. Photofragmentation of the ion pair by the ionization laser pulse may produce A+ and P- as discrete ions detectable by timeof-flight mass spectrometry. This hypothesis is supported by our L2ToF spectra of anthracene picrate; the positive ion mass spectrum is characteristic of anthracene with the complementary negative ion mass spectrum containing ions exclusively derived from picric acid. Work is continuing on determining the mechanism of this process. Irrespective of the detailed mechanism, the formation of C-T complexes with picric acid or possibly other electron acceptors provides a simple route to the detection of volatile PAHs. The generality of the approach has been proven with other PAHs in mixtures typical of real environmental samples. In addition, we envisage the direct formation of picrate C-T complexes by coating environmental samples with a thin film of the acceptor. Conceivably, other approaches such as the formation of β-cyclodextrin inclusion compounds might also be employed and offer the prospect for selective complexation and mass spectrometric analysis. ACKNOWLEDGMENT The authors thank Heriot-Watt University for a Ph.D. Scholarship to S.M.H. We also acknowledge the help of Lis FernandezHaig and John Barraclough in preparing and characterizing the PAH-picric acid complexes. We acknowledge the help of Edinburgh Surface Analysis Technology in partially financing this work and in the use of equipment. Received for review February 27, 1997. Accepted May 15, 1997.X AC970222O X
Abstract published in Advance ACS Abstracts, July 1, 1997.
