Small molecules such as NO, OH, I2, S2, SO2, NO2, and HCHO display discrete sharp electronic rotational‐vibrational lines. Utilizing the Zeeman effect, one component of an atomic emission line can be made to exactly coincide with a sharp molecular line while the other component is shifted so that there is no overlap with this line. When large molecules undergo electronic transitions it is not possible to resolve the rotational structure in the vast majority of cases. Therefore, the difference in absorption between the σ± components is proportional to the density of the molecules showing sharp absorption but is not influenced by other molecules. These phenomena can be used to achieve highly sensitive and selective detection of small molecules.