Could be, though the KCl is a better standard. We know how much KCl we have, and the natural abundance of K40 is well-known. You'd need a dedicated campaign to

Could be, though the KCl is a better standard. We know how much KCl we have, and the natural abundance of K40 is well-known. You'd need a dedicated campaign to assess the potassium abundance in the "standard" banana before you could use it at better than ~100+% errorbars. The salt-substitute shaker is probably a <5% standard, and we've been using the same one for years.... :).