Anal. Chem. 2001, 73, 4977-4982
On-Line Identification Method in Column Liquid Chromatography: UV Resonance Raman Spectroscopy Reyer J. Dijkstra, Cornelius T. Martha, Freek Ariese, Udo A. Th. Brinkman, and Cees Gooijer*
Department of Analytical Chemistry and Applied Spectroscopy, Division of Chemistry, Free University, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
Ultraviolet resonance Raman spectroscopy (RRS) is presented as a novel identification tool for conventional-size column liquid chromatography (LC). The on-line coupling was made using a standard Z-shaped flow cell. A continuous-wave frequency-doubled argon ion laser operating at a wavelength of 244 nm was used for excitation. “On-thefly” resonance Raman spectra of four model compounds, fluorene, phenanthrene, fluoranthene, and pyrene, were recorded after a standard acetonitrile/water reversedphase LC separation. When applying a large-volumeinjection procedure (32 mL), detection limits were at the nanogram per milliliter level. The results indicate that UVRRS gives detailed spectral information at an appropriate sensitivity level so that coupling with LC becomes feasible. For many classes of compounds, Raman spectroscopy (RS) provides detailed vibrational information and has therefore a high identification potential, especially if aqueous solutions have to be analyzed. There is an obvious need to couple identification techniques such as Raman and Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) to column liquid chromatography (LC). The hyphenated systems so created provide structural details complementary to those of LCmass spectrometry (MS), currently a routine technique. Unfortunately, RS is very insensitive, which is a serious problem for most real-life applications. As shown in the recent literature, this problem can at least partly be overcome by using a detector cell with a path length as large as 30-50 cm, based on a liquid-core waveguide (LCW).1-4 Although the results obtained with the LCW are rather encouraging, it is clear that LC-RS will never become suitable for trace analytical problems. To achieve that goal, more sophisticated approaches to increase the Raman signal intensity should be used, i.e., resonance RS (RRS) or surface-enhanced (resonance) RS. The latter has successfully been coupled to LC in an at-line mode; i.e., after analyte deposition on a substrate,5-7 on-line coupling is not straightforward but has been reported as well. 8 (1) Holtz, M.; Dasgupta, P. K.; Zhang, G. Anal. Chem. 1999, 71, 2934-2938. (2) Dijkstra, R. J.; Bader, A. N.; Hoornweg, G. Ph.; Brinkman, U. A. Th.; Gooijer, C. Anal. Chem. 1999, 71, 4575-4579. (3) Marquardt, B. J.; Vahey, P. G.; Synovec, R. E.; Burgess, L. W. Anal. Chem. 1999, 71, 4808-4814. (4) Dijkstra, R. J.; Slooten, C. J.; Stortelder, A.; Buijs, J. B.; Ariese, F.; Brinkman, U. A. Th.; Gooijer, C. J. Chromatogr., A 2001, 918, 25-36. 10.1021/ac0103569 CCC: $20.00 Published on Web 09/11/2001
© 2001 American Chemical Society
In RRS, the laser excitation wavelength coincides with an electronic absorption band of the analyte molecule; as a result, some of the vibration bands will be 102-106 times enhanced. The enhancement affects only the vibrations that couple to the chromophoric group involved in the electronic transition. In general, only totally symmetric vibration modes are enhanced. Jones and Asher have shown that for pyrene different vibrations are enhanced depending on the electronic state or even the vibronic state that is reached after excitation, i.e., on the laser wavelength used. 9 The emphasis of the present paper is on LC-RRS in the deep UV (
