[stasy]

Hy, I need of simple and rapid method for urea analysis in fermentation broad. The filtrates are coloured and using colorimetric method is difficult. Any ideas?

[Andy TB]

These are a variety of test kits for urea available in the marketplace (Google urea and test kits) which generate a coloured species which is detected using a spectrophotometer. If your samples are coloured, a known addition method may be appropiate. Divide a sample into four and to three, add known increasing amounts of urea. Analyse the samples using the test kit and then plot amount of urea added vs absorbance. The x intercept will give the urea level in the original sample.
There are other methods out there - it all depends upon what you have available and how big your budget is (!). For example, HPLC would do the trick. I would suggest an amino column and (ideally) some universal detector such evalporative light scattering. You could try UV detection but it would need to be tuned to around 195nm as urea is not the best chromophore in the world. What you haven't mentioned are the expected levels of urea in your samples. As concentrations get lower, the analysis tends to get more tricky and expensive.