Some sources of persulfate contain high levels of nitrogen. When they are used to digest samples for total nitrogen determination, they can produce a high background. Too high of a background will effect quantification on the lower end of the method.
For potassium persulfate sources, we use Fluke persulfate from Sigma Aldrich, and EMSure from EMD. It is also possible to consider a sodium persulfate source. In addition to trying another source of persulfate, you could also consider recrystallizing existing persulfate. Please see attached links for reagents and attached document for information regarding persulfate crystallization.