Detectors for Mid-Infrared Microspectroscopy

The properties of MCT detectors depend on their composition, that is, their Hg Cd ratio. Narrow-band MCT detectors are typically about 50 times more sensitive than DTGS but do not respond to radiation below 750 cm h The cutoff can be extended to lower wavenumber but at the expense of sensitivity. Thus, mid-band MCT detectors have a cutoff of about 600 cm but their sensitivity is about half that of the narrow-band detector. Wide-band detectors cut off at 450 cm but are even less sensitive. Fortunately, few spectra of organic samples contain useful bands much below 700 cm , so FT-IR microscopes are almost invariably equipped with narrow-band MCT detectors. It should be noted that the response of narrow-band MCT detectors is nonlinear with radiation flux so that when large spatial regions are to be examined, the effect of this nonlinearity may become evident as a baseline offset [11]. 

The noise equivalent power (NEP) of an infrared detector is a measure of the noise generated by the detector and is given by 

From Equation 1.1, it can be seen that the area of any detector used for infrared microspectroscopy should be as small as possible. Provided that all the radiation that passes through the sample is focused on the detector, the use of a 0.25 mm detector gives an SNR that is four times greater than if a 

The SNR of an FT-IR spectrum (i.e., the reciprocal of the noise of a 100% line measured in transmittance) is given by the following equation [12]  

Mapping performed with a spatial resolution close to the diffraction limit can be very time consuming. For spectra measured when using sample apertures approaching the diffiaction limit ( 10 pm), even a 30 s collection may result in a spectrum with a rather poor SNR and may need an increased signal averaging. It may be noted that if the measurement of each spectrum takes 30 s and a 64 X 64 map is required at 10 pm spatial resolution, it would take over 34 h to acquire all the spectra required for the image  

The data acquisition rate (sampling frequency) for mid-IR interferograms is usually equal to the frequency, Hz, of the interferogram that is generated by a laser (usually a helium-neon laser at 632.8nm) simultaneously with the infrared interferogram. f is equal to the product of the wavenumber of the laser and the 

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