Is there a submit handler for custom fields?
You can use hook_field_update in your module where you define your field..


  Define custom update behavior for this module's field data.
  
  This hook is invoked from field_attach_update() on the module that
  defines a field, during the process of updating an entity object
  (node, taxonomy term, etc.). It is invoked just before the data for
  this field on the particular entity object is updated into field
  storage. Only field modules that are storing or tracking information
  outside the standard field storage mechanism need to implement this
  hook.