a photograph of the MTEC unit which mounts directly in the sample compartment of the FTIR's optical bench.

Fig. 3. MTEC Model 200 photoacoustic detector mounted on a Nicolet sample compartment baseplate.

The spectral range of a photoacoustic detector depends only on the transparency of the sample chamber window. With suitable windows a detector can operate from the UV to the far-infrared. The most common window material is KBr (UV through mid-infrared), but quartz (UV through near-infrared), CsI (UV - 200 cm^-1), ZnSe (near-infrared - 560 cm^-1), and polyethylene (far-infrared) are also frequently used.
The maximum MTEC sample holder dimensions are 10.7 mm in diameter by 9.0 mm deep. Most analyses require a much smaller sample volume. Sample holder inserts are provided to displace unused gas volume and boost the signal level which is proportional to the reciprocal of the gas volume.
Only the central region of the MTEC sample chamber is irradiated by the infrared beam because the photoacoustic detector's mirror causes a factor of two diameter reduction in the FTIR beam focal-spot diameter, this size is reduced to a 5 mm diameter inside the detector sample holder at the focal plane position which is approximately 1.0 mm to 1.5 mm below the detector window. Consequently, the sample volume that is actually analyzed is approximated by the focal area times the sampling depth, L, which