Is that excitation  wavelength or emission wavelength? If I understood correctly your concern is in the region of 250-300 nm. Higher signal at the edge of excitation wavelength is due to Rayleigh scattering. Rayleigh scattering is prominent in the UV region because of its dependence with (1/lamda)4

http://pubs.acs.org/doi/abs/10.1021/ed081p705

Dear it's excitation wavelength and  2nd is the mid region i.e. 300-400 nm

If this is just DNA, then it should not be fluorescent at all. There may be a fluorescent contaminant in it. Alternatively, you may be seeing Raman scatter from the water. What was the emission wavelength setting? Have you taken an emission spectrum?

I agree with Adam, this looks like the tail of the raman peak. Try moving your excitation to lower wavelenghts and see if it moves