Infrared microscopy 



The design requirements of infrared mi- forms a reduced image of the exit sht at the 

 croscopes, or more accurately infrared micro- sample space. The objective collects the en- 

 sampling attachments, have been rather ergy which has passed through the sample 

 thoroughly explored in recent years. All have and forms a magnified image of the sample 

 the common property of measuring, as a at an adjustable diaphragm. The energy then 

 function of wavelength, the infrared absorp- passes to a second field mirror which forms 

 tion of minute samples. The resulting ab- an image of the Littrow mirror near the cen- 

 sorption spectra are similar to the infrared ter of curvature of the thermocouple con- 

 spectra of macroscopic samples so that the denser, which forms a reduced image of the 

 well known applications of infrared (see En- slit on the thermocouple detector, 

 cyclopedia of Spectroscopy) — qualitative The microscope can be placed hi the spec- 

 and quantitative analyses and molecular trometer either between the source and the 

 structure investigations — are possible. Infra- entrance slit of the monochromator, or be- 

 red microscopes have unique importance tween the exit slit of the monochromator and 

 where (a) the amount of sample is small, the detector; the latter avoids difficulties in 

 (b) the dimensions of the sample are small samples sensitive to heat or photochemical 

 and (c) the sample is not homogeneous. As effects. Special precautions may be taken to 

 little as 0.1 microgram of sample in the field eliminate absorption from atmospheric wa- 

 of the microscope can yield useful spectra, ter vapor which may seriously interfere with 



Samples which have been studied using spectra for ver}' small specimens (5). 

 such equipment include natural and syn- A Cassegrain-type condenser permits the 

 thetic fibers, single crystals, biological tissue substitution of photoconductive cells, photo- 

 sections and bacterial cultures. Instruments multiplier tubes or other types of detectors 

 of this type allow the study of liquid extracts in place of the thermocouple, 

 and solutions in cells of extremely small vol- In practice the required thickness of the 

 ume. This means that it is now possible to sample is about the same as for macroscopic 

 use samples of compounds separated by work, frequently about 25 microns. The 

 chromatography. The technique for compress- minimum sample area is that required to 

 ing finely ground samples mixed with KBr provide sufficient energy for satisfactory de- 

 powder into optically clear pellets of optimum tection — that is, inversely proportional to 

 dimensions is now a very useful sampling source brightness, the transmission effi- 

 procedure for infrared microspectropho- ciency, the detector sensitivity and the 

 tometry. Polarization effects permit investi- square of the effective numerical aperture 

 gation of molecular orientation in dichroic of the rnicroscope objective. Thus the area 

 samples. may be decreased by the use of such intense 



In the Perkin-Elmer infrared microscopic sources as carbon arcs, zirconium arcs or 



attachment (4) to their infrared spectrom- tungsten glowers. Other controllable instru- 



eter, energy from the exit slit of the mono- mental factors of course provide limits of 



chromator is incident on a field mirror which size. If the sample does not cover the entire 



directs it upward and forms a reduced image field of the microscope, some radiation will 



of the pupil of the monochromator (Littrow reach the detector without being subjected 



mirror) near the convex mirror of the con- to absorption by the sample. This "dilution" 



denser, so that radiation from the entire use- of the spectra is also increased by aberra- 



ful slit is directed to the condenser, which tions and diffraction introduced by the ob- 



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