An Inexpensive, Easily Constructed Cryostat for Variable-Temperature Spectrophotometry Albert W. Burgstahler and Douglas S. McBaln The University of Kansas, Lawrence, KS 66045
Spectral measurements a t low temperature have many important theoretical and practical applications such as in the study of atomic and molecular structure, conformational equilibria, and kinetics of phenomena or reactions that are ntherwise ..--~-too fast to follow.' Unfortunatelv. .. most suectroscopic cryostats reported in the literature2 are rather expensive and comnlicated. sometimes reauirina so~histicatedshor, facilities for'their cbnstruction. Here we describe an economical. easilv made crvostat for use in standard UV-visible spectrophotu&%ersthat is a significant improvement of one nrieinallv devised for vuriable-temperature ORD-CU determi nation^.^ In the present design, provision has been made for adding reagents or withdrawing sample aliquots during operation. Automatic heaters also have been incorporated to prevent fogging of the windows in ambient air. Figure 1and its legend give details of construction, while Figure 2 shows a photograph of the completed unit with the sample elevator removed. The dimensions (in millimeters) are based on requirements for use in a Cary 14 or similar spectrophotometer and are about as small as can accommodate a standard 10-mm-path-length rectangular cuvette. The upper and lower sections of the outer part of the cryostat are assembled from half-inch-thick, packing-container-type Styrofoama glued together with GE or du Pont silicone seal~~~
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ant. Thicker walls improve the insulating efficiency,but the size of the unit is then increased. T o ensure a tight fit, the upper portion of the sample elevator is cast from polyurethane foam in the neck of the chamber in an aluminum foil mold suirounding the various parts and openings indicated in the figures. For proper po-
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Included in a presentation (by E. S. Huyser and D. S. McBain) before the Division of Organic Chemistry at the 184th National Meeting of the American Chemical Society, Kansas City, MO, Sept. 12-17. 1982; ORGN 142. ' Hoch, F. L., J. CnEM. EOUC., 32,469 (1955). (a)Greenstock,C. L., and Dunlop, I., Cryogenics, 13,4 (1973);(b) Crosby, G. A,, and Pankuch, 8.. Chem. Insfrum., 2, 329 (1979);(c) Aurich, F., J. Phys. E., 2, 109 (1969);(d) Sousa, J. A., and Weinstein, J., Rev. Sci. Insfrum., 34, 150 (1963). Burgstahler, A. W., and Weigel, L. O., Lab. Pract., 24, 812 (1975).
ELEVATOR
1 - 5 6 4 FRONT
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Figure 1. Constructiondetails of cryostat. All dimensions are in millimeters (to fir sample mmpartment of 14 spectmphotometer).A, Ma W m f m walls; 8, heating cable leads; C. elevatwguide;D. Frost& II heating cable; E, evacuatsd cwene: F, overflowpan; G, aluminum baseplate: H, mermocaupie leads; I, nihogen outlet;J, cannula; K, nitrogen inlet: L. foam rubber gasket; M, rigid polyurethane foam;N, vent; 0. serum cap; P, plastic cuvene retainer;a, rectangular sample cwene; R, plastic cwene shelf: S, plastic sample elevator.
Volume 61 Number 5 May 1964
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sitioning, the sample elevator slides into the unit on two plastic I-beam channels embedded in the sidewalls. An aluminum drip pan a t the bottom of the chamber prevents solvent damage to the Styrofoam in the event of minor spillage or leakage from the sample cuvette. For durability and protection aeainst scuffine. the outer surfaces of the crvostat are wranoed &h black plastic tape. At the top of the sample elevator a cannula passes through a small-diameter plastic tube which serves as hoth a gnide and a seal. For addition of a liouid the cannula is lowered into the cuvette; afterward it is wikdrawn to a position above the light nath. Addition and mixine of a second comoonent is also ef?ected in this manner. ~oiutionsof thermaily sensitive suhstances can he transferred into the cold cuvette under syringe pressure after having first been placed in a precooled vial covered with a serum c a n Precalihration of the vial euards against overfilling the cuvette. Alternatively, liquidsean he with a longadded or withdrawn hv a svrinee . - eauipped . .. needle. In the metal base of the cryostat are two slightly projecting screws that interlock with the two holes in the shelf plate of the spectrophotometer sample compartment through which the regular cuvette holder is normally mounted. The body of the cryostat projects from the sample compartment through n 60 ~ 6 0 - m m s(,uarr olwning in n si.parrrtely fahricntrd cover plate. In this way, correct and reproducible alignment of the ;ample cuvette in the light pathis assured. As in the original v e r ~ i o n the , ~ windows consist of two evacuated 10-mm-path-length cylindrical quartz cuvettes sealed with silicone glue into circular openings in the front and rear walls of the crvostat. Recessed around the outer edee of each window in the present design, however, is a single loop of Frostex" I1 self-regulating heating cahle (available at hardware stores) from which the outer covering has been removed to provide better sensine and heat transfer. 'This type of heating cahle consists of two wires separated by a semiconductor through which the temperatnre-dependent electrical circuit is completed, thus allowing use of &en very short leneths. The ends of the wires in each cahle are soldered hack o n 6 themselves to form a tight loop, and the lead wires are attached to these connections in parallel to a two-pronged male appliance plug. \Vith a transformer setting of 120 to 150 V, the cahles prevent foeeinr ~ even in ambient air without drv "" " of t h windows nitrogen purging of the instrument sample compartment. If foeeine does occur. brief warmine of the windows with an electric heat blowe; quickly correks the problem, generally without recurrence.
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Figure 2. Photograph of completed cryastat with sample elevator removed.
During use, cooling is achieved by passage of dry nitrogen gas through a coil of copper tubing (114 in. X 6 ft) immersed in liquid nitrogen or liquid air. The rate of cooling is readily controlled by a needle valve and pressure regulator attached to a nitrogen tank, and the temperature can he maintained within a deeree or two of anv desired value down to =I00 K (-173 OC) 6y adjustment ofthe flow of the cold nitrogen. A hwass inlet from the nitroeen tank to the crvostat oermits rapid rewarming. By heating the copper coil, ietermfnations above room temperature to =+65 "C can he made. Temperatures are conveniently measured by an iron-constantan thermocouple connected to a 3.5-digit millivolt meter with ice water as the reference j ~ n c t i o n o, ~r a suitable commercial device can he used. Acknowledgment We thank Earl S. Huyser, graduate research director of D. S. M., for encouraging this project, and Charles W. Duver of our Instrumentation Design Laboratory for valuable advice and assistance.
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"CRC Handbook of Chemisby and Physics." 62nd d.,CRC Press. Inc., Boca Raton. FL. 1981, p. E-107.
