High-temperature Reflectance Spectroscopy

Practically all the studies in reflectance spectroscopy (it should be noted that the term reflectance spectroscopy used here will denote diffuse reflectance spectroscopy only) have been carried out at ambient temperatures, or in some cases at subambient temperatures. The latter would most probably be used in single-crystal studies for the elucidation of hot bands, that is, transitions which originate from vibrationaily excited ground states. However, in many cases, a great amount of additional information on a chemical system can be obtained if the reflectance spectrum of a compound is obtained at elevated temperatures. Normally, temperatures in the range from 100 to 300°C have been used, although there is no reason why higher temperatures could not be employed. 

The use of a heated sample holder to contain the compound under investigation has been described by several investigators. Asmussen and Anderson 

Kortum (2) measured the reflectance spectrum of mercury(II) iodide at 140° but did not describe the heated sample block or other experimental details. Another heated block assembly was described by Hatfield et al. 

A modification of the preceding sample holder was described by Wendlandt and George (12) and by Wendlandt (13). The circular aluminum disk of the holder was heated by means of a cartridge heater element inserted directly behind the sample well. Two Chromel-Alumel thermocouples were placed in the block, one adjacent to the heater and the other in the bottom of the sample well so as to be in intimate contact with the compacted sample. The block thermocouple was used to control the temperature programmer. 

Still another heated sample holder was described by Wendlandt and. Hecht (1). It consisted of a block of aluminum, 50 mm in diameter by 25 mm thick, into which was machined a 25-mm by 1-mm deep sample well, A 35-watt stainless steel sheated heater cartirdge embedded in the main block of the holder was used as the heater. The same two-thermocouple systems, one for the temperature programmer and the other for sample temperature, was employed. For samples which evolved gaseous products, a Pyrex or quartz cover glass was used to prevent contamination of the integrating sphere. 

---

Kortiim (90), and Klier (91) and reviews on the subject have been published by Terenin (92) and by Leftin and Hobson (93). The potentialities of high-temperature reflectance spectroscopy (HTRS) and of dynamic reflectance spectroscopy (DRS), as described by Wendlandt (94), should be emphasized in connection with surface chemical problems. In the HTRS technique, reflectance spectra are recorded at constant elevated sample temperature, whereas in the DRS technique, the temperature is uniformly changed (usually increased) and the reflectance recorded at a fixed wave number. The results of DRS experiments if carried out under vacuum or with an inert carrier gas may, therefore, valuably, complement thermogravimetric or TPD experiments. 

A. high-temperature REFLECTANCE SPECTROSCOPY and DYNAMIC REFLECTANCE SPECTROSCOPY... 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

---