We propose a new functional for the charge and mass identification in Delta E-E telescopes. This functional is based on Bethe's formula, allowing safe interpolation or extrapolation in regions with low statistics. When applied to telescopes involving detectors delivering a linear response, as silicon detectors or ionization chambers, a good mass and charge identification is achieved. For other detectors, as caesium-iodide used as a final member of a telescope, a good accuracy is also obtained except in the low residual energy region. A good identification is however recovered if a non-linear energy dependence of the light output is included.