Behaviour of Electronic Absorption Spectra in Supercritical CO2

When using UV-Vis detection for SFC of carotenoids, attention must be paid to spectral shifts that occur in supercritical fluids, in comparison to those measured in liquid solvents. This is particularly relevant as the effect can influence qualitative and quantitative results. The shape of the absorption spectra of all-trans-^-carotene, 15-cis-^-carotene and the xanthophylls, zeaxanthin, cathaxanthin and astaxanthin, which are nonacidic oxygen derivatives of the carotenes, are similar in both supercritical CO2 and hexane. The ^max of the five carotenoids however shifts to a shorter wavelength (hypsochromic shift) in supercritical CO2. Figure 5 shows the absorption spectra of all-trans-^-carotene in supercritical CO2 and in hexane. Table 3 lists the maximum absorbance wavelength of the selected carotenoids in supercritical CO2 and in hexane. Due to the relatively low solubilities of the xanthophylls in supercritical CO2 below 5000 psi pressure, comparison of the spectra has been carried out at 6000 psi and 35°C. While solvent-induced shifts of the visible (JBu + ) spectra of carotenoids in a range of polar and nonpolar organic solvents commonly used for HPLC are well established, changes in the supercritical CO2 density have also been observed to shift the Amax. Density is related to both pressure and temperature, hence a change in either of the two variables is known to affect the absorption maxima of carotenoids. An increase in pressure increases the visible imax of all-trans-^-carotene. Over the pressure range of 1500-6000 psi (at a constant temperature of 35°C), the position of this Amax shifts 7.0 nm to longer wavelength (430.0 to 437.0 nm) in a nonlinear manner.

The effect of variation in temperature on Amax has also been assessed in the range 25-50°C at a constant

Table 3 The maximum absorbance wavelength of carotenoids in hexane and in supercritical CO2 (6000 psi at 35°C)

Figure 5 Absorption spectra of all-trans-f-carotene in supercritical CO2 (continuous line) and in hexane (dashed line). Supercritical conditions: mobile-phase CO2, temperature 35°C, pressure 3000 psi. Reproduced with permission from Hui etal. (1994).

Table 3 The maximum absorbance wavelength of carotenoids in hexane and in supercritical CO2 (6000 psi at 35°C)

Carotenoid

^max (nm)

Hexane

CO2

All - trans-f-carotene

450.0

437.0

Lycopene

472.0

456.0

Astaxanthin

474.0

453.0

Canthaxanthin

464.0

452.0

Zeaxanthin

448.0

436.0

Figure 5 Absorption spectra of all-trans-f-carotene in supercritical CO2 (continuous line) and in hexane (dashed line). Supercritical conditions: mobile-phase CO2, temperature 35°C, pressure 3000 psi. Reproduced with permission from Hui etal. (1994).

pressure of 3000 psi and over this range only a 2.0 nm shift is observed. Thus, changes to both pressure and temperature can induce shifts in the (JBu + ) Amax of all-trans-^-carotene in supercritical CO2, although the former is much more significant. In addition to the main absorption peak in the visible region, cis-isomers of carotenoids exhibit a peak in the UV region, termed the cis-peak, which originates from the JAg + transition. The electronic absorption spectrum of 15-cis-^-carotene in supercritical CO2 shows a clear cis-peak in the region 250-350 nm. Unlike the behaviour of the main Amax in the visible region, the position of the cis-peak maxima does not alter as a function of temperature and pressure, but remains fixed, suggesting that the energies of the JBu+ and JAg+ states of carotenoids may not respond in the same manner to alternations in density.

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Solar Panel Basics

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