Versatile Vapor Cell for Use in Infrared Spectroscopy K. J. Sterling and R. J. Haines Research and Process Development, South African Iron and Steel Industrial Corp., Ltd., Pretoria, Republic of South Africa
THEVAPOR pressure of an organic, organometallic, or inorganic compound will obviously determine the temperature at which its vapor state infrared spectrum is measured. Whereas some compounds have a negligible vapor pressure at ambient temperature others are extremely volatile. A cell suitable for measuring the spectra of various types of compounds should thus be operable over a wide range of temperature and also be evacuable. Its versatility would also be increased by having a variety of sample introduction systems and demountable windows. A cell which displays the above characteristics is described. It will fit most commercial infrared spectrophotometers. DESCRIPTION OF CELL
The cell is of stainless steel and is roughly T-shaped as illustrated in Figure 1. Two flanges ( B ) fit the ends of the body ( A ) and contain the windows (D)in their center. These are held in position by retaining rings (C), a vacuum-tight seal being obtained by placing lead O-rings on both sides of the window. Because window material is very fragile and expensive the following procedure for securing the windows into position without breakage was adopted. The lead O-rings were first pressed into position by substituting a metal disk for the metal halide window. The disk was then replaced by the window and with the prepressed O-rings in position the retaining ring’s Allen socket screws were tightened progressively until a vacuum-tight seal was obtained. This
was achieved by a torque wrench, the final torque being approximately 4 kg cm. The two flanges are screwed to the ends of the cell body with lead O-rings providing hermetic seals. The vacuum port ( E ) consists of a stainless steel tube (H) connected to a glass vacuum stopcock ( K ) through a Kovar metal-glass joint (L). To prevent sample material condensing on cold spots at the stopcock, a shuttle-valve is placed between the cell and the metal-glass joint. It is constructed as shown in Figure 2; the housing ( M ) is of stainless steel while the shuttle ( N ) is made of silver steel. Control of the shuttle in operating the valve is effected by means of a permanent magnet. The sample chamber ( F ) is sealed by means of a plate (G) and a lead O-ring as shown. Figure 1 illustrates the plate in its simplest form but several accessories for introducing the sample may be attached, thereby increasing the versatility of the cell. Figure 3 shows one type of accessory. It consists of a bent glass tube, sealed at one end, and joined to the flange through a Kovar metal-glass joint. This accessory is suitable for introducing compounds with high vapor pressure at room temperature into the cell. The sample in the tube is kept cool while the cell is evacuated. A pyrolyzing attachment is shown in Figure 4. It consists of a filament (c), which acts as the sample holder, and two electrodes one of which (b) is a cold finger. While the cell is evacuated and heated to constant temperature, the sample is kept cool by passing a coolant through the cold finger. The sample is then pyrolyzed by passing a high current at low voltage
ALL DIMENSIONS IN MILLIMETERS
Figure 1. Vapor cell A. B.
C. D.
E. F. G. H.
Cell body Window flange Retaining ring Window Vacuum port Sample chamber Cover plate Stainless steel tube VOL. 40, NO. 8, JULY 1968
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ALL DIMENSENS N MILLIMETERS
K
Figure 2. Shuttle valve
H.
Stainless steel tube B12 ground glass cone K. Vacuum stopcock L. Kovar metal-glassjoint M. Stainless steel housing N . Silver steel shuttle 1.
Figure 4. Pyrolysis unit a.
b. c. d. e.
Stainless steel cold finger Stainless steel electrode Filament Electrical insulating material 5 BA locking nuts
by means of a ceramic heating mantle. Thermostatic control of the furnace is achieved by connecting two thermocouples in series to a galvanometer switching unit.
DISCUSSION
Figure 3. One type of accessory for cell a. Borosilicate glass sample compartment 6. Kovar metal tube
through the filament. This adaption may also be used for compounds which decompose slowly in the vapor state, the spectrum being measured as soon as the sample is volatilized. Further accessories include numerous attachments for collecting fractions from a gas chromatograph. The cell body and the tube to the shuttle valve are heated
The design of the cell was kept simple to overcome workshop difficulties while stainless steel was chosen as the basic material to minimize corrosion problems. Lead O-rings are the solution to obtaining vacuum-tight seals. They are nonvolatile and may be used up to temperatures of 200 "C. An important asset is their high thermal conductivity, good heat transfer being obtained from the cell to the window. Furthermore, they are easily made by fusing the ends of the required length of lead wire in a small flame. Should temperatures of higher than 200 "C be required, O-rings of pure aluminum may be used. In its present form, the cell may be evacuated to mm Hg while it has been found to hold a vacuum of 10-3 mrn Hg over 24 hr.
WAVELENGTH (MKRONSJ
Figure 5.
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ANALYTICAL CHEMISTRY
Infrared vapor spectrum of ",Cl
measured at 180 "C
FREQUENCY (CM')
Figure 6. Vapor spectrum of the pyrolyzate of a polyvinyl chloride-vinyl acetate plastic, pyrolyzed at approximately 300 "C and measured at 120 "C
The flanges (6) are readily interchangeable, and in this way a stock containing various types of window material may be kept. A particular window is kept indefinitely on its flange unless repolishing is required. the 's to introductory systems. Several of these have already been discussed but obviously more could be constructed as required. With these units being interchangeable the cell may be used for various type studies. This is clearly demonstrated by Figures 5 and 6, which give the spectra of ammonium chloride
vapor, measured at 18OoC and the pyrolyzate of a polyvinyl chloride-vinyl acetate plastic. ACKNOWLEDGMENT The authors thank the personnel of the instrument workshop of the Research Department for the construction of the cell and the management of the South African Iron and Steel Industrial Gorp., Ltd., for permission to publish this work. RECEIVED for review February 23, 1968. Accepted March 19, 1968.
A Polytetrafluoroethylena Diffusion Cell Nancy W. Alcockl Kanematsu Memorial institute, Sydney Hospital, Sydney, Australia
A DIFFUSION CELL made from Teflon (tetrafluoroethylene, Du Pont) is described. A series of the individual units clamped tightly together and lightly greased is completely gas tight. The cell has been used to isolate inorganic fluoride from sodium fluoride solutions. Although Singer and Armstrong ( I ) suggested the use of Teflon for making diffusion cells, they found it to be unsatisfactory for the estimation of fluoride (2). It is probable that they carried out the diffusion reaction at a temperature at which fluorine was released from the Teflon. As I was unable to consistently recover all of the available fluoride from a solution of sodium fluoride by a diffusion technique using polypropylene microdiffusion cells (Aloe Scientific Co.), a cell made from Teflon with a basically different and more reliable method of sealing has been designed and found to be ideal. With the conditions used-diffusion for 16 hours at 55 "C in the presence of 1.3N NaOH and 55% (WjW) HCIOa Present address, Sloan-Kettering Institute for Cancer Research, New York, N. Y. 10021 (1) L. Singer and W. D. Armstrong, ANAL. CHEM.,26, 904 (1954). (2) L. Singer and W. D. Armstrong, University of Minnesota, personal communication, 1966.
-there was no release of fluoride from the tetrafluoroethylene and the recovery of 0-8/1g of fluoride was 100%. The isolated fluoride was measured by the method of Wharton (3) using the [4,5-dihydroxy.3-(p-sulfophenylazo)-2,7,naphthalene-disulfonic acid]-zirconium oxychloride complex (SPADNS) described by Bellack and Schouboe (4). EXPERIMENTAL Apparatus and Its Care. THE CELL UNIT. The design of an individual unit is shown in Figure l a ; a cross section of the cell drawn to scale is shown in Figure lb. A center lathe was used to machine the cells from a solid cylinder of Teflon (supplied by Ludowici & Son, Sydney, Australia) 2 inches in diameter. When in use a series of the cells was assembled so that the base of one formed the top of the next as shown in Figure 2. The cells were clamped together as illustrated. SEALINGTHE UNITS. After assembling the units and fixing them in the cell holder, a thin film of Dow Corning silicone grease was smeared over the entire outside curved surface. Although the cells, if perfectly machined, may be self sealing when heated to 55" C, the temperature for diffusion, a slight inconsistency is recovery of fluoride from solution indicated that greasing was necessary. (3) H. W. Wharton, ANALCHEM.,34, 1296 (1962). (4) E. Bellack and-P. J. Shouboe, ibid., 30,2032 (1958). VOL. 40, NO. 8, JULY 1968
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