TM 93-10 Interferences in the Methylene Blue Active Substances [MBAS] Method, Techniques of Water-Resources Investigation [TWRI], [0-311-83], Book 5, Chapter A3.

The National Water Quality Laboratory (NWQL) investigated the interference of nitrate on the TWRI method for the determination of MBAS. This investigation was conducted in response to questions arising from the results of district-submitted QC samples spiked with various concentrations of nitrate but no other MBAS. These results indicated a positive response by nitrate on the MBAS method. The NWQL ran a series of tests to quantify the positive interference observed in the district samples. The data from these experiments did confirm that there is a positive interference by both nitrate and chloride on MBAS measurements. As a result, the NWQL has taken steps to verify a replacement 
method.