Anal. Chem. 2001, 73, 5762-5770
Laser-Induced Multidimensional Fluorescence Spectroscopy in Shpol’skii Matrixes with a Fiber-Optic Probe at Liquid Helium Temperature Adam J. Bystol,† Andres D. Campiglia,*,† and Gregory D. Gillispie‡
Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105, and Dakota Technologies, Inc., 2201A 12th Street N., Fargo, North Dakota 58102-1803
Reducing the sample temperature almost always improves vibronic resolution of polycylic aromatic hydrocarbons’ (PAHs) fluorescence and excitation spectra because the Boltzmann population distribution is narrowed. But the temperature effects on fluorescence are especially pronounced in the so-called Shpol’skii solvents, which are often n-alkanes.1,2 Freezing the PAH into a uniform matrix reduces both thermal and inhomogeneous band broadening and results in vibrationally resolved excitation and fluorescence spectra with unmatched information for PAH identification. In recent articles,3,4 we presented improved methodology for measuring Shpol’skii spectra at liquid nitrogen temperature. By using a bifurcated fiber-optic probe with the distal
end frozen directly into the sample matrix, we completely eliminated all the well-known disadvantages of conventional 77 K measurements. Upon sample excitation with a frequency-doubled tunable dye laser, whose bandwidth ( 50 µL. Figure 8 compares the 4.2 K spectra recorded from n-heptane and from n-heptane spiked with VMP ) 100 µL containing the four
Figure 8. The 4.2 K fluorescence spectra of 1.0 µg/mL benzo[k]fluoranthene, benzo[e]pyrene, perylene, and 0.1 µg/mL benzo[a]pyrene in (A) n-heptane spiked with 100 µL of PAHs in 60:40 acetonitrile/water (v/v) and (B) n-heptane recorded with the fiber-optic probe, spectrograph, and ICCD camera. Delay and gate times were 10 and 200 ns, respectively. Excitation wavelength was 287.0 nm. Peak assignments are as follows: benzo[e]pyrene (I), benzo[k]fluoranthene (II, VI, VII, IX), benzo[a]pyrene (III, IV, V, VI, VIII), and perylene (X). Spectrograph entrance slit, 25 µm.
PAHs. The spectral resolution of the four PAHs can be made at several wavelengths. CONCLUSIONS The fiber-optic probe is a unique tool for fluorescence measurements in Shpol’skii matrixes at helium temperature. It provides Analytical Chemistry, Vol. 73, No. 23, December 1, 2001
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the analyst with a rapid and simple freezing procedure to obtain accurate and reproducible spectra from both single-site and multiple-site PAH/n-alkane systems. The cryogenic probe and the combination of the high-resolution spectrograph and ICCD with the tunable pulsed laser excitation source are well suited for collecting 4.2 K WTM, EEM, and TREEM. The scanning capability and the narrow excitation bandwidth of the tunable dye laser allows one to selectively excite small wavelength shifts in PAH excitation spectra for analyte and site-selective excitation, EEM, and TREEM collection. Because of the spectrograph and the ICCD, WTM, EEM, and TREEM are rapidly collected. In comparison to stimulated Raman scattering with a Raman shifter, which has been applied for simultaneous wavelength excitation in TREEM analysis at room temperature,13 the tunable laser has the disadvantage of longer collection times because of sequential wavelength excitation. On the other end, its scanning capability is essential to take full advantage of the specificity obtainable from the highly resolved PAH excitation spectra in Shpol’skii matrixes. The excitation energy from the tunable dye laser promotes intense PAH fluorescence to recording spectra with optimum spectrograph slit widths for maximum spectral resolution. In comparison to 77 K, lowering the temperature to 4.2 K promotes significant spectral narrowing. As expected, increasing the delay after the excitation pulse deteriorates the LOD. However, our results demonstrate the possibility to time discriminate shortlived phenomena and still collect enough fluorescence emission to detect PAHs at the parts-per-billion level. The fluorescence lifetimes did not change much with lowering the temperature from 77 to 4.2 K. Surprising differences were observed from fluores-
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cence lifetimes of PAH molecules occupying different crystallographic sites in the frozen matrix. The lifetime differences from site to site were larger than their wavelength shifts. This fact helped us to identify the two crystallographic sites of B[a]P in n-octane, which had been previously reported as a single-site system. The analytical potential of site-selective lifetime analysis is still unfolded, but our results anticipate a promising contribution to enhancing the specificity of site-selective excitation. Spiking the mobile phase into the Shpol’skii solvent is a feasible approach for the direct analysis of HPLC fractions. The PAH fraction partitioning into the Shpol’skii solvent provides strong fluorescence signal, and the fraction of mobile phase partitioning into the organic layer does not deteriorate the resolution of Shpol’skii spectra. The fiber-optic probe allows direct access to the organic layer of the biphasic solution, which results in highquality Shpol’skii spectra. The spectral narrowing at 4.2 K notoriously enhances the specificity of WTM, EEM, and TREEM analysis. The 4.2 K laserinduced multidimensional fluorescence spectroscopy in Shpol’skii matrixes holds tremendous potential for the direct analysis of PAHs in complex environmental matrixes. ACKNOWLEDGMENT This research was partially supported by NSF-EPSCoR and by the NSF-SBIR program through contract DMI-9960722. Received for review July 24, 2001. Accepted September 23, 2001. AC010828J
