The present invention concerns a method for determining at least one absorption band in a spectrum, the method at least comprising the steps of:—providing a measured absorption spectrum from the sample,—providing a calculation spectrum,—from the calculation spectrum, extracting at least one absorption band,—calculating a residual spectrum by removing each extracted absorption band from the calculation spectrum, testing whether a predefined stop criterion is fulfilled by the residual spectrum,—if the stop criterion is not fulfilled, using the residual spectrum as the calculation spectrum and iterating the extracting step, the forming step, the calculating step and the testing step, and—if the stop criterion is fulfilled, outputting each extracted absorption band.