Spectroscopic and photochemical study of riboflavin

This work involved the use of UV-VIS spectrophotometric and spectrofluorometer to study the photoproperties and the determination of riboflavin in the vitamin synthetic standards either directly from the
riboflavin present in standard solution samples or indirectly by conversion of riboflavin to its photoderivatives. Standard solutions of riboflavin were prepared at different pH ranged from (2 -10) to get the pH at which maximum absorbance occurs. Riboflavin showed a maximum absorbance at 445 nm at pH7 after scanning in visible region. Thus this wavelength and pH were used to determine the riboflavin
concentration in different standard solution samples by spectrophotometric method. The recovery of this method was found to be 98.68% with relative error of 1.31%. Scanning of the emission spectra of riboflavin by spectrofluorometer showed (445 nm as excitation wavelength) a maximum emission at 520 nm,this wavelength was chosen for the determination of riboflavin at different standard solution, the recovery of this method was found to be 99.68% with relative error of 0.63%.Indirect determination of riboflavin was carried out after the conversion of riboflavin to its photoderivatives by irradiating the vitamin
solution with UV light.  Different standard solutions of riboflavin with the same concentration were prepared at different pH range from (2-10) to get the suitable pH at which lumiflvine and lumichrome formed, pH9 showed the disappearance of the 445 nm peak and the shift of 373 nm to 359 nm,which is indicative for the formation of lumiflavine. pH4 showed the disappearance of the 445 nm peak and the shift of 373 nm to 353 nm, which is indicative for the formation of lumichrome. Both lumiflavine and lumichrome were the main photoderivatives in basic and in acidic media respectively. The percentage of the degradation of each photoderivative was determined. The produced lumiflavine showed a maximum absorbance at 359 nm and a maximum emission at 465 nm. pH9 and a wavelength 359 nm and 465 nm were used to determine the concentration of riboflavin spectrophotometricaly and spectrofluorometricaly. The recovery of the spectrophotometric method was 95.99% with a relative error of 4%, the recovery of spectrofluorometric method was 96.42% with a relative error of 3.57%. pH4 and both  wavelengths (353 nm,422 nm) were used to determine the concentration of riboflavin spectrophotometricaly and spectrofluorometricaly. The recovery of the spectrophotometric method was 95.18 % with relative errors of 4.826%, and the recovery of spectrofluorometric method was 95.75% with relative a error of 4.25%.Spectrofluorometric method showed a better recovery with a less error than the spectrophotometric method.During the study of the photochemical properties of riboflavin, it was found that the degradation is first order reaction. The specific rate constant (k) at room temperature, was evaluated by monitoring the spectral changes during irradiation process.dThe quantum yield of the photodecomposition process is generally low and is greatly affected by the pH. The quantum yield of photodecomposition increases as the pH increase from 2-7 then begin to decrease as the pH increase from 8-10.