Is it possible to collect enough xylem sap from the sap wood by using a growth cone , I want to do some hydrogen and oxygen analysis as well as monitor the sap flow. There are many references where they cut suberized twigs first and then all leaves and green stem tissue were removed from them to avoid contamination of xylem water by isotopically enriched water and then placed in a capped vial, wrapped in parafilm and stored in the freezer until water extraction. If it is possible, will the isotope signals  be different from those measured in the traditional ways?