Our new modelling approach gets around some of the uncertainty in linking volatiles in apatite, to volatiles in melt. We find a good fit to the data, then use a new sensitivity analysis to identify other sets of parameters that could give an equally good fit. This allows us to understand the limitations of our results and brings confidence to our itnerpretations. We can also calcualte the amount of exsolved volatiles making up a pre-eruptive gas phase present in the magma storage reservoir. For the 12.9 ka Laacher See tephra, this amounts up to 187 Tg H2O, 26 Tg Cl and 1.5 Tg F. This work is currently in revision for Earth and Planetary Science Letters.