Question

A vitamin has both an absorbance maximum at 210 nm and a fluorescent emission maximum at 420 nm. Two standard curves were prepared using the pure vitamin and measuring absorbance at 210 nm and fluorescent emission at 420 nm. A clear fruit extract (water was used for extraction) was analysed by uv and fluorescence and the concentration determined with the standard curves. The results showed 3 times more vitamin by uv analysis compared to fluorescent analysis.

Give a possible reason for these results (5marks).

Answer 1

for a compound generally uv and fluorescence are proportional but here two possibility of this difference.

1. there is some other compound present that has significant absorbance in that region. because when the extraction has been done by water other compound may dissolved and the sample for the analysis do not contain pure vitamin but pure vitamin was used for the calibration curve. In addition, the impurity compound is non fluorescent in nature or has fluorescent in other region that is more far from the observed region. here the uv shows more concentration and fluorescent shows less concentration.

2. another possibility is inappropiate sample preparation. if the sample concentration is to high, then aggregation between the molecules may occur and in that case self fluorescnce quenching occur. in that case also low fluorescent concentration will be recorded.