MCT DETECTOR (Mercury-cadmium-telluride)
For gas analysis, the nitrogen-cooled MCT detector has great advantages over detectors that operate at or near room temperature. For a given scanning time, an MCT detector will produce a spectrum with a noise level 10 to 100 times lower than the noise from a DTGS detector. This low noise has two important results. (1) It lowers the minimum detection limits for all compoundsbeing measured. (2) It widens the concentration range over which valid measurements can be made.
In measuring on solids and liquids, high absorbance values can be used, and the DTGS detector may be found to be satisfactory. In this case, quantitative measurements can be made with absorbance values from 1 absorbance unit (A.U.) down to the noise level, which may be 0.01 A.U. This is a factor of 100 in concentrations. In measuring gases, however, one can only work from about 0.1 A.U. down to the noise level of 0.01. This is only a factor of ten. For the same scanning time, the MCT detector will allow work down to the noise level of 0.0001 A.U., which represents a 100-fold extension of the range for valid concentration measurements.
Infrared Analysis, Inc. considers the use of a nitrogen-cooled detector to be nearly essential for good gas measurements. Pouring nitrogen into a dewar is a minor inconvenience that is far out-weighed by the advantages of having the wide range for valid measurements.