Playing devil advocates here, I have also seen PO4 lower in my triton ICPs , but am I getting exactly 10ml in the curvette? Am I getting all of the powder out of the sachet? Am I getting all of the powder 100% dissolved? Am I getting the indexing position of the curvette lined up exactly correctly before and after shaking? Is the curvette exactly spotless? Then add in the tolerance of the checker if everything i mentioned above is perfectly done and I can see why there are differences... who is correct triton or a Hanna in my hands (and yours).. think I’d err on the side of triton.