2.2.2 Adsorption isotherms
Pre-weighted 0.8 g of activated silica gel (200-300 mesh) were added
into 20 mL γ-tocopherol solutions with different initial concentrations,
the conical flasks with cover were shaken (25 °C, 120 rpm) until
adsorption equilibrium was reached. The initial and equilibrium
concentrations of γ-tocopherol solutions were analyzed by HPLC. The
adsorption capacity of γ-tocopherol on silica gel was calculated
according to the following equation (1). Langmuir and Freundlich models
were employed to describe the adsorption behaviors.
\begin{equation}
Q_{e}=\frac{\left(C_{0}-C_{e}\right)\times V}{M}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(1\right)\nonumber \\
\end{equation}Langmuir equation:
\begin{equation}
Q_{e}=\frac{Q_{m}K_{L}C_{e}}{1+K_{L}C_{e}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\ \nonumber \\
\end{equation}Freundlich equation:
\begin{equation}
Q_{e}=K_{F}\ C_{e}^{\frac{1}{n}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\
\end{equation}Where, Qerefers
to the adsorption capacity (mg/g silica gel). C0and Ce stand for the initial concentration and
equilibrium concentrations of γ-tocopherol (mg/mL), respectively.V refers to the volume of the initial sample solution (mL), andM represents the weight of the tested adsorbent (g).Qm stands for the theoretically calculated
maximum adsorption capacity (mg/g). KL is the
Langmuir constant. KF and n are both the
Freundlich constants.
2.2.3
γ-tocopherol purification process
The pre-weighed mixed tocopherol sample was dissolved in three mL of
hexane to form a homogeneous solution which was then loaded onto a
chromatographic column (length and diameter, 60 × 3 cm) packed with
silica gel (200-300 mesh). Elution solvent was comprised of n-hexane and
ethyl acetate. Elution flow rate was controlled by a constant flow pump,
and a UV detector (wavelength, 290 nm) was employed to monitor the
elution processes. Elution fractions were collected in test tubes and
analyzed by using TLC.
Elution fractions containing only γ-tocopherol were combined into a
single mixture and the organic solvents were removed using a rotary
evaporator (IKA, Germany) under reduced pressure. The amount of purified
γ-tocopherol was calculated using mass balance, and the purity of
γ-tocopherol was analyzed by HPLC according to the method above
mentioned. The recovery yield and purity of γ-tocopherol were calculated
according to the following equations, respectively.
\begin{equation}
P\left(\%\right)=\frac{C\times V\times 10^{-6}}{W}\times 100\%\ \text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(4\right)\text{\ \ }\nonumber \\
\end{equation}\begin{equation}
R(\%)=\frac{W\times P}{L\times B}\times 100\%\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)\ \ \nonumber \\
\end{equation}Where C is the concentration of purified γ-tocopherol (µg/mL).V is the volume of the purified γ-tocopherol solution (mL).W is the mass of purified γ-tocopherol (g), P represents
the purity of γ-tocopherol (%), and B refers to the γ-tocopherol
content of mixed tocopherols (g/100g), and L is the loading mass of
column chromatography (g).