The detector element in an MCT detector comprises mercury, cadmium, and tellurium. The two MCT detector types are created by adjusting the mixing ratio of the mercury and cadmium. However, the sensitivity drops as the wavenumber range expands. So, when selecting your detector, select Type 1 to give priority to sensitivity over a narrower measuring range, or select Type 2 to study target samples with peaks near the 700 to 650 cm-1 range.
As a reference, the diagram below shows the spectra of polymers with absorption in the 700 to 650 cm-1 range. From top to bottom, these polymers are polystyrene (PS), polyethylene (PE), polybutadiene (BR), polyvinyl chloride (PVC), and polyvinylidene chloride (PVDC). The absorption near 700 to 650 cm-1 results from C-H out-of-plane vibrations and C-Cl stretching vibrations.