To provide a mass spectrometer having a high sensitivity to dioxins. The line detector, for example a CCD camera or a linear array of reflection probes, is arranged in such a way that optical properties of the crystal bed can be detected continuously in a region running parallel to the longitudinal axis of the wash column, this region covering the desired setpoint position of the wash front or of the build-up front. In one embodiment, the present invention involves utilizing a transformation which evaluates a plurality of possible signal coefficients in order to find appropriate coefficients. The module includes a first optical device and a splitter in the transmission path and is coupled to a second optical device via the splitter. The second yoke portion connects the first columnar portion to the second and third columnar portions. The pedestal may also include fluid purge channels that may be configured to provide thermal isolation between the regions of the pedestal.