Comment submitted by Nadejda Vilissova, Applications Chemist, Thermo Fisher Scientific

We'd like to obtain clarification regarding the Electrode method (4500-NH3 D-1997 or E-1997) for the Kjeldahl Nitrogen-Total (TKN) analysis in the 40 CFR Part 136 Proposed Rule. The Ion Selective Ammonia Electrode method is the determinative method for both manual and automated digestions in MUR 2007. In the proposed TKN analytical procedures (MUR 2010), which are described in item 31 of the Table IB-List of Approved Inorganic Test Procedures (pages 58046 and 58047), the Electrode Method is allowed when manual digestion is utilized, but it is not listed in the section, which describes the Automated Methods for TKN that do not require manual distillation, for example, when the lab uses an automated block digester. We believe that the Electrode method is suitable and can be used after either the manual or the automated digestion. Could you explain why the block digester potentiometric method, which was included in the 40 CFR Part 136 published in March of 2007 (see image 1 below), is not listed in the proposed rule (image 2 below)?