Determining Methemoglobin in BlOOdby Zero-Crossing-Point First-Derivative Spectrophotometry

We determined methemoglobin in blood by zero-crossing-point first-derivative spectrophotometry. After lysis of erythro-cytes, hemoglobin was converted into oxyhemoglobin and the first-derivative spectrum was recorded between 405 and 425 nm. At the exact point where the first-derivative spectrum of oxyhemoglobin was zero (“zero-crossing point”), the first-derivative value of oxyhemoglobin and methemoglobin mix-ture was proportional to the methemoglobin concentration. The standard curve was linear for all proportions of methe-moglobin. Within-assay precision (CV) was 3.4 % for a 20% methemoglobin content. Correlation with results by the Eve-lyn and Malloy method was very good for high proportions of methemoglobin (>10%), but the proposed technique was far better for low methemoglobin percentages because of its linearity, its high sensitivity, and its low detection limit