Dear Florencia
you can store the aqueous phase at 4° only few days, the time you need to hydrolyse the SA glucoside. For the step of extraction i preconise you to not store the powder at 4° treate your sample for extraction of metabolites . Another solution if you don't want to use them immediately is to store your samples at -80° before extraction but you must weight them before storing
best regards
Valerie