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A trans-2-NCA 0.5% solution was concentrated enough to absorb all UV-A radiation received from light source, as it showed absorbance values > 2 over the studied spectral range (320-400 nm), on the contrary of quinine monohydrochloride dihydrate solution (2% in water), the chemical actinometer proposed by ICH5'6 for calibrating UV radiation, as shown in Figure 3.

Similar exposure tests were carried out with 0.3%, 0.7%, 0.8%, 1.0% (w/v) methanolic solutions as an attempt to extend the wavelength absorption range and to observe the concentration effect on trans-2-NCA photoreactivity. At high concentration, solubility problems were observed and slightly worse linear responses were obtained, confirming the 0.5% solution as optimal.

Wavelength [nm]

Figure 3 Comparison between quinine monohydrochloride dihydrate (2% in water) and trans-2-NCA (0.5% in methanol) absorption spectra.

Wavelength [nm]

Figure 3 Comparison between quinine monohydrochloride dihydrate (2% in water) and trans-2-NCA (0.5% in methanol) absorption spectra.

Irradiation experiments were also performed with monochromatic lights at two different wavelengths, 313 and 365 nm respectively; A440 vs. absorbed photons (Nhv x 10~5) for both wavelengths is reported in Fig.4, while regression lines were compared in Table 2. The data showed that trans-2-NCA absorbance increase was linearly correlated to the number of photons emitted from the light source, which were all absorbed from the actinometer solution, and was not depending on the irradiation wavelength. These results suggested that A440 increase of trans-2-NCA solutions exposed to different polychromatic UV-A lamps, should be dependent only from the total number of photons emitted from each lamp and not from their spectral distribution in the UV-A range.

Nhv x 10-5

Figure 4. Changes in absorbance at 440 nm of 0.5% solutions irradiated at 313 and 365 nm, respectively, as a function of cumulative number of photons (Nhvx 10~5).

Nhv x 10-5

Figure 4. Changes in absorbance at 440 nm of 0.5% solutions irradiated at 313 and 365 nm, respectively, as a function of cumulative number of photons (Nhvx 10~5).

Table 2. Regression lines of Abs vs Nhvx 10~5, from monochromatic lights at 313 and 365 nm.
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