Heated Infrared Cell for Investigation of Solids in a Controlled Atmosphere Theodore Wydeven and Mark Leban Ames Research Center, NASA, Moffett Field, Calg. 94035
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RECENTWORK (1-4) has stimulated interest in the infrared disk technique for studying the mechanisms and the thermal decomposition of solids. In the samples were heated for a known time and then quenched outside the spectrophotometer; the concentrations of infrared active product and reactant species were determined, intermittently, by infrared analysis. This note describes the construction and advantages of an infrared cell for studying the decomposition of solids temperature range of 25 to 500" C in a controlled atmos The cell can easily be accommodated by any spectrophotometer sample compartment comparable in size to the Beckman IR-9. This cell permits continuous in situ quantitative analysis of infrared active reactants and products. O
CELL CONSTRUCTION
Figure 1 is a perspective view of the cell and the pellet holder assembly. The overall dimensions of the cylindrical cell are 5.4 cm in diameter and 12.4 cm in length. The optical path length is 11.9 cm. The cell body (A), the end plate (B), and the pellet holder assembly (C, C-1) for 13-mm diameter pellets as well as the window retainers were machined from stainless steel. Two 1.6-mm diameter holes and a slot (C-2) were machined into the pellet holder assembly to allow gases to escape from inside the assembly during evacuation. The heating coi ts of B. & S. 40-gauge Nichrome V heating wire total resistance of 28 ohms at 25 " C. The heater wire was enclosed in a 1-mm diameter stainless steel tube and insulated from it by magnesium oxide (Ceramo Wire, Thermoelectric Co., Inc., Saddlebrook, N. J.). The heater coil is wrapped, evenly spaced and noninductively, around the center section of the cell and held firmly in place by spot-welded stainless steel straps. Alumino-silica fibers are used for insulation (E). The insulation is held in place by a thin aluminum cover (F). Sample temperature is measured with a chromel-alumel thermocouple
Figure 1. Perspective view of infrared cell for studying solids
(GI. Power for heating the cell was supplied to the windings by an SCR power supply and controlled with an electronic temperature controller (West Instrument Corp., Schiller Park, Ill.) used in conjunction with an iron-constantan thermocouple ( H ) located directly beneath the windings. Thermocouple connectors are secured to the top of the cell (see Figure 2). (Under the operating conditions used most extensively to date, i.e., 100" to 200" C and 200 torr helium in the cell, the pellet temperature was a constant 6" C lower than the temperature directly beneath the furnace windings.) Vacuum seals are made with Viton O-rings ( I ) and high vacuum grease. A high vacuum monel metal bellows valve (see Figure 2) was attached to the vacuum port (J). Polished KRS-5 windows (K), 25.2 mm in diameter and 2 mm thick,
Figure 2. Illustration of assembled cell. In the foreground is a pellet holder assembly
(1) K. 0. Hartman and I. C . Hisatsune, J. Phys, Chem., 69, 583 (1966). (2) Ibid.,70, 1281 (1966). (3) Ibid., 71, 392 (1967). (4) F. E. Freeberg, K. 0. Hartman, I. C. Hisatsune, and J. M. Schempf, Zbid.,71, 397 (1967).
are maintained at ambient temperature during high temperature operation of the cell by flowing tap water (-280 cc/minute) through the cooling channels (L) which were connected in series (see Figure 2). A modified Beckman track-mounted IR gas cell holder (see Figure 2) is used to retain and align the cell in the spectrophotometer. VOL. 39, NO. 13, NOVEMBER 1967
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DISCUSSION
(5-7)) have limited applicability and do not offer the many ad-
The cell described can provide controlled sample temperatures from 25" to 500" C while maintaining the cell windows at room temperature. Temperature control to 10.5" C throughout the range is achieved with the cell pressurized to 200 torr with helium which is used to increase thermal conductivity. The usable pressure environment in the cell ranges from IOF6torr to the high pressure limit of the window material. Operation in high vacuum reduces the temperature control response. (For example, with the cell evacuated to 3 x 10-6 torr the time required to reach 390" C was 21 minutes. When the cell was pressurized to 5 torr with helium, the time required to reach 390" C was only 3 minutes.) The unit requires approximately 280 watts to operate at a sample temperature of 500' C. The transmittance through a 0.14-mm thick pellet of undiluted silver carbonate was reduced by a factor of approximately 4 when the pellet was located in the cell as compared to being located in the normal position in the spectrophotometer-i.e., the most concentrated portion of the light beam. Approximately 50 % of the IR energy loss when using the cell is due to reflection from the KRS-5 windows. The remaining loss in energy is due to vignetting, absorption by the cell windows, and probably a small amount of scattering. However, in a study recently completed in this laboratory on the thermal decomposition of silver carbonate, it was found that ample IR energy was transmitted through the undiluted pellets in the cell to give a sharp signal even during single beam mode of operation with the second light chopper stopped (stopping the second chopper prevented furnace and pellet emission from affecting the sample signal at high temperatures). Other infrared cells, described previously in the literature
vantages of the cell described in this paper. The cell can easily be accommodated by the Beckman IR-9, it has a wide temperature range, and samples can be studied in a controlled atmosphere. The design of the pellet holder permits easy assembly of the cell and also results in reproducible alignment and location of the pellet in the IR beam. The O-ring seals, rather than epoxy resin seals, not only give high vacuum seals but also allow for easy replacement of the cell windows, The cell is suited to studying the decomposition kinetics of solids for several reasons. It has a rapid response time (only 4 minutes are required to go from 25 O to 450" C with the cell pressurized to 200 torr with helium); therefore, a significant amount of sample is prevented from decomposing before reaching the temperature of an experiment. The water-cooled ends of the cell are spacious enough to accommodate a sorbent for removing gaseous reaction products-e.g., carbon dioxide was adsorbed on a molecular sieve during the decomposition of silver carbonate, and thereby recombination was prevented from occurring. The concentration of products and reactants can be determined continuously, introducing less error into the kinetic data. ACKNOWLEDGMENT
The authors acknowledge the assistance of Joel Leonard in designing the cell. RECEIVED for review May 3,1967.
Accepted August 14,1967.
( 5 ) L. Bertsch and H. W. Habgood, J. Phys. Chem., 67, 1621 (1963). (6) J. K. A. Clarke and A. D. E. Pullin, Trans. Faraday SOC.,56, 534 (1960). (7) F. R. Harrison and J. J. Lawrance,J . Sci. Instr., 41, 693 (1964).
Simple Effluent Splitter for Measurement of Electron Capture/Flarne Ionization Response Ratios Walter L. Zielinski, Jr., Lawrence Fishbein, and Richard 0. Thomas Bionetics Research Luborutories, Inc., Fulls Church, Vu. DUALCHANNEL gas chromatography was early suggested by Lovelock ( I ) , expanded by Amy (2), and ultimately appeared on the commercial market from several of the leading gas chromatograph manufacturers. The purpose of this paper is to describe how, with several simple modifications, two standard laboratory chromatographs will provide qualitative information on injected solutes via measurement of electron captureiflame ionization response ratios. EXPERIMENTAL
Apparatus. The effluent end of a 6-foot by '/(-inch coiled borosilicate glass column mounted for on-column injection (kit from Applied Science Labs.. State College, Pa.) in an F & M Model 1609 flame ionization gas chromatograph (Figure 1) was fitted via a short length of Teflon tubing to a 1-inch piece of an 18-gauge stainless steel needle seated in a l/ls-inch brass Swagelok tee (D). The remaining arms of this tee similarly contained 1%gauge stainless steel needle protrusions to which l/8-inch 0.d. Teflon tubing was attached.
(1) J. Lovelock. ASTM Meeting, East Lansing, Mich., 1962. (2) Y. W. Amy and K. P. Dimick, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy,March 1963.
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ANALYTICAL CHEMISTRY
The lower leg of the tee led directly to the flame detector (F). The side arm led to a Whitey l/*-inch Swagelok 316 stainless steel micrometer control valve Model 22RS4 with a 0.020-inch orifice. This was coupled to the hydrogen air inlet of an Aerograph HY-FI Model 600-B containing a Model 600-D electrometer and 250-millicurie titanium tritide concentric tube electron capture detector. The air inlet (Z) adjacent to the hydrogen inlet on this instrument was capped off. The side arm lead from the 'jle-inch tee was wrapped with a 120 W 61asCol Cal Cord insulated with asbestos tape, from the column housing to the entrance of the electron capture chromatograph chassis, excluding the needle valve. The heating cord was controlled from a powerstat. The signal from the flame detector was attenuated to a 0-1mV Sargent recorder Model SR-20. The signal from the electron capture cell was recorded on a 0-1-mV Brown Model 153 recorder. Chart speed for both recorders was 0.5 inch per minute. Operating conditions for sample analysis were set as follows: ELECTRONCAPTURE. Nitrogen carrier 20 psig, range 1, attenuation 8, injection port 245" C, column (%foot by l/&ch coiled borosilicate glass packed with 4% QF-1 fluorosilicone on 80jlOO mesh acid-washed, DMCS-pretreated Chromosorb G) temperature 128" C. Column temperature was maintained by a Barber-Colman Capacitrol Model 293C.
