The RA-915+ mercury analyser employs differential atomic absorption spectrometry technique, which is implemented using the direct Zeeman effect (Zeeman Atomic Absorption Spectrometry using High Frequency Modulation of Light Polarization ZAAS-HFM).

A glow discharge mercury lamp is placed in a permanent magnetic field, whereby the 254-nm mercury resonance line is split into three polarized components, only two of those, + and -, which are circularly polarized in the opposite directions, being detected for analysis. After passing through a polarization modulator, which modulates the polarization at a frequency of 50 kHz and thus triggers the line components in turn, the radiation then passes through a multi-path cell, whose equivalent optical length is about 10 m.

Being equipped with narrow-band high reflectivity mirrors, the cell isolates solely the 254-nm resonance line and suppresses all the nonresonance and stray radiation. A logarithm of the intensity ratio of s+ and s-, which is proportional to the mercury atom concentration in the cell, is determined upon detecting the radiation by a photodetector and subsequent analog-digital conversion of its electric signal by a built-in microprocessor. The measurement results are read out from a built-in LC display, or are transmitted to a computer for further processing or data storage.

In this measurement technique, the analytical signal depends only on mercury concentration and is independent of the presence of dust, aerosols , and other foreign contaminants in the analytical cell.