A method to exclude nitrate from Kjeldahl digestion of plant tissues

A micro‐Kjeldahl method suitable for use with a commercial block digestor is described which minimizes NO3‐N reduction during digestion and also reduces foaming. One hundred mg plant tissue are pretreated with 3 ml H2O2 at 100° C for 15 min. This is followed by addition of concentrated H2SO4, K2SO4, and catalyst, and digestion at 360° C for 1 hr. With H2O2 added after the H2SO4, 40% of the NO3‐N was reduced over a range in the tissue from 0.25% to 1.5% NO3‐N. With the H2O2 pretreatment essentially none of the NO3‐N was reduced. Total nitrogen, (reduced N determined by the proposed method plus nitrate, measured separately) was analyzed for 9 species ranging in NO3‐N from 0 to 0.73% and in total N from 1.4 to 4.4%. The results agree well with the AOAC method 2.052 (1).