Inner Filter Effect in the Fluorescence Emission Spectra of Room Temperature Ionic Liquids with-β-Carotene

Steady-state fluorescence spectroscopy is highly sensitive an analytical fluorescence detecting tool limited to diluted samples. The serious problem of quantitative analysis is dominant in the samples with optical density higher than 0.05 (10 mm cell) detected in the right angle cell geometry, because of the two kinds of inner filter effects IFEs (Lakowicz, 2006; Li & Hu, 2007; Kao et al., 1998). The primary inner filter effect (PIFE) defined as the decrease in the intensity of the excitation beam at the point of observation because of the chromophore optical absorption in the excitation region. In the fluorescence emission experiment, an apparent decrease in the fluorescence emission quantum yield and/or distortion of band shape as a result of re-absorption of emitted radiation is observed. Finally, some molecules of the investigated samples will be excited by the less intense light. For example the excitation light intensity (Io) at center of 10 mm cuvette is (0.88 Io) for (OD = 0.1). The secondary inner filter effect (SIFE) takes place when the fluorescence intensity decreases as a result of the chromophore absorption in the emission region. More serious is PIFE than SIFE because excitation wavelengths are always shorter than emission wavelength, especially in organic samples. Generally, the relationship between fluorescence intensity (IF) and fluorophore concentration (c) is logarithmic (Guilbault, 1990) IF = Io F (1e- l c), where (Io) is the incident light intensity, (F) is the fluorescence quantum efficiency, ( is the molar extinction coefficient, (l) is the cell path length. In the concentrated sample the excitation radiation is absorbed significantly by the fluorophore or other chromophores in the cell and the (PIFE) plays a significant role. For dilute solutions (absorbance <0.01) fluorescence is uniformly distributed. The above equation is reduced to the linear form: IF = k Io F l c with approximately 1% error, where (k) is proportionality constant. For the fluorescence experiment it is always important to establish either by calculation or by measurement the concentration value (Cmax) at which a plot of fluorescence emission against concentration becomes nonlinear. This value can be calculated from equation: Cmax = 0.05/ l, where: symbols ( and (l) have the same meaning as in the other equations. From the nonlinear calibration curves (fluorescence intensity versus fluorophore concentrations) the information of the presence of the IFEs in