I say, and with God's help: Zakat is one of the obligatory acts of worship for a Muslim; one should not combine the intention of the obligatory with another intention, even if it is voluntary like charity. The reward for the intention of zakat for wealth is very great, unless you specify what the amount of zakat is, in which case your intention should be for zakat, and any excess can be intended as charity, and this is permissible. And God knows best.