1) Thermometer ions are ions for which we know the threshold dissociation energy E0, which is the minimum energy required for dissociation to occur. Each selected thermometer ion has a different E0. E0 values can be either calculated, or measured with guided ion beam mass spectrometry (GIBMS) . E0 is easiest to calculate for a simple bond cleavage leading to a neutral loss. Quantum chemical calculations provide E0 values.