does any one know how to detect benzoic acid by fluorescence detector? what is the excitation wavelength and emmision wavelength? i need suggestion

Since this is a potentially ionic species, the lambda max may vary with both pH and organic content. In acidic (pH~3) mobile phase, 15% acetonitrile, I see a lambda max at about 230 nm. I also see a lower absorbance maximum (about 8% the extinction coefficient of the 230 maximum) at about 275 nm.

Wavelengths in the regions of the absorbance maxima should prove suitable for excitation wavelengths, however, you'll have to check the spectrum in your own mobile phase.

Choose an excitation wavelength, then scan your sample to find a corresponding maximum for the emission wavelength. You may want to repeat this for a few choices of excitation wavelenght, across both absorbance peaks.

Choosing absorbance and emission maxima may result in the largest and most stable signals, however there can be a number of other considerations. For example, it may be advisable to choose an excitation or emission wavelength off the maximum if other components of the sample absorb at the wavelength of the maximum.

Does benzoic acid even fluoresce? My recollection from long-ago spectroscopy courses was that fluorescence is most likely in rigid, polycyclic compounds because the the energy absorbed by the incoming photon doesn't have allowed vibrational modes to bleed off into. In a small molecule like benzoic acid, the absorbed energy is dissipated before it can be re-emitted as a photon. (If anyone wants to clarify or correct this muddled explanation, please feel free to jump in!).

Apparently the above understanding is not always correct, as another coworker posted in this forum that aniline has fluorescence. The logical question is: what kinds of compounds have fluorescence?
Could someone refer me to literature as to what kinds of compounds have fluorescence?

Many thanks in advance.