Check your cuvettes, check your solvent and check your lamp. The purity of your simple should also be assessed.

ok thanks. The sample has a purity grade of 99.5 % so it possibly cant be the problem. The solvent was also scanned before and shows no absorbance above 220 nm. So it might be the cuvette or the lamp

I like to add another question: how reprodiucible is a spectrum at one given concentration?

What is the absolute absorbance? If the y-axis in the figure is the number directly from the spectrometer (the absorbance). It is a bit difficult to tell, but it is possible that very little light is hitting the detector at the lower wavelengths because the sample is strongly absorbing the light. If this is true, what you are seeing is essentially detector noise.

To test this possibility, try diluting the sample (by a factor of 10 - 1000). You will loose the longer wavelength peaks, but the shorter wavelength peaks might come back on scale. There are two pi-->pi* electronic transitions that could be quite intense, large \epsilon value (from the double bond and from the C=O).

If you plot the data as curves rather than point it might help see that.

Harry, i measured some spectra twice and there was some deviation of between 10- 20 % so i think its fairly reproducible

Thank you for the dilution suggestion. i'm not sure it's possible to measure the absolute absorbance. My supervisor told me only the relative absorbance can be measured with devices that have only one collimator and diffuser.

Try cutting the concentration, even non-quantitatively. See if the low wavelength peaks come in.

Inka

In addition to the answer by David:

Does the absosption in the "normal" region of 300-325 show Beer's law as a fucntion of concentration? I cannot see this from the very crude spectra you provide.

In the shorter wavelength region your absorbance is (much too) high for a proper spectrum for a single beam instrument