All working standard solutions for calibration curves were prepared
by diluting the stock solutions with the extraction solutions
at eight different concentrations (0.01–20 lg/mL). The linear standard
calibration curves (r2P0.999⁄⁄) of vitamin B1, B2, B3, B5 and
B6 were generated by injecting 0.01–20 lg/mL of standard solutions
in 20 lL. All stock and working solutions were stored at
4 C before being used.