Lawrence Verbit and Thomas R. Halbert'
University of N e w York at Binghamton Binghamton, N e w York 13901
State
I
A Versatile Variable Temperature Microscope Stage
The advantages of studying the thermal behavior of compounds under the microscope have been pointed out over the yearsZ but the method has not come into widespread use among chemists. In part, this may be attributed to the fact that few chemistry students receive any exposure to the use of the microscope in undergraduate or graduate courses. However, the increasing use of the microscope by chemists in such diverse and burgeoning fields as forensic science and environmental pollution, to cite just two examples, argues for some experience with chemical microscopy during a student's training. Microscopic examination has such salient advantages as the use of very small amounts of material, the ease with which impurities may be detected, and the increased ability under high magnification to recognke and classify thermal behavior such as melting point as well as other types of phase transitions. Most chemistry departments and research lahoratories have access to a good quality microscope. However, for many purposes the chemist does not wish to examine his sample a t room temperature but needs the capability to raise or lower the temperature as re-
quired. The general absence of such temperature regulating devices in most chemistry laboratories is a contributing reason for the neglect of microscopic examination. Temperature regulating devices for microscopes are classified as either hot or cold stages; hot stages for the study of substances which are solids a t room temperature and cold stages for those which are liquid above approximately -40°C. Hot stages are more common and several designs are described in the l i t e r a t ~ r e . ~Commercially available temperature regulating stages suffer from a t least two disadvantages; they are expensive (>$250), and they are usually designed to fit special microscopes. We describe below a versatile and conveniently
'Undergraduate summer research student from Rutgers University. H. F., "Micrn~copyfor Chemists," Dover PuhSCHAEPFER, lications, New York, 1966; JOHNS,I. B., "Labor&toryMmual of Microchemistry," Burgess Publishing Co., Minneapolis, Minn., C. W., "Handbook of 1941, p. 53; CRAMOT,E. M. AND MASON, Chemical Microscopy" (2nd Ed.), Vol. 1, John Wiley & Sans, Ino.,New York, N. Y. 1938, pp. 198-209. a Two of the less complicated hot stage designs are given by: W. A., J. CHEM. GRAY,G. W., Nature, 172,1137 (1953); BONNER, E ~ u c .23,601 , (1946). The hot stage described more recently in TnIs JOURNAL by Glasser and Miller is essentially a microfurnace: L. AND MILLER,R. P., 3. CAEM.EDUC.,42,91 (1965). GLABSER,
TOP VlEW
i
r
TOP V l E W
rHANDLE
FlOURE I. HEROSCOPE STAGE. OFHC CCQPER. IDIMENSIONS IN MILLIMETERS)
SIDE V l E W
BOTTOM VIEW
Figure 1.
Microscope stage, OFHC copper (dirnsnsionr in millimeten).
Figure 2. millimeters).
Cover block, OFHC copper, except handle (dimensions in
Volume 48, Number I I , November 1971
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constructed variable temperature microscope stage which combines the functions of a cold and hot stage. It covers the temperature range from -130 to +300° and is small enough to fit within the focusing region of most common microscopes. Working drawings for construction of the stage are shown in Figures 1 and 2. The main part of the stage consists of a block of oxygen-free, high conductivity (OFHC) copper r o d 4 A circular recess was drilled in the top to accommodate an ordinary round cover glass for melting point determinations, with a niche provided as shown in Figure 1 to allow for convenient removal or introduction of a sample while the stage is still hot. A 6.5-mm hole was drilled as shown for insertion of a thermometer. However, the size of the hole is easily modified if one wishes to use a thermocouple or other type of temperature measuring device. The block is heated by a simply installed electrical heating element connected to a variable transformer. A standard 200 W Chromalox ring heater5 was fitted into the large diameter recess at the bottom of the block. The rather long (13-mm) terminals were cut down to 6 mm and connected to asbestos insulated wire6 which was plugged into a Variac. Once the block has been heated to a high temperature, differential expansion and contraction causes the heating element to be held firmly in the block. A transite insulating board having a 5-mm hole through the center was affixed to the bottom of the block by means of three screws. This effectively prevented heat transfer from the block to the body of the microscope. The block is cooled by the passage of a cold, inert gas, e.g., nitrogen, through thin copper tubing which is coiled tightly around the outside of the circular stage. The '/s-in. 0.d. tubing was wound as snugly as possible, care being taken not to cover the thermometer well. The tubing was connected via Swagelok fittings to a cooling coil immersed in a Dewar flaslc and attached to a nitrogen cylinder. Use of appropriate coolants such as dry ice or liquid nitrogen in the Dewar flaslc serves to cool the nitrogen gas flowing through the tubing. The gas exiting from the block serves as a very convenient source of an inert atmosphere for sensitive samples as described below. The entire assembly was mounted on the rotating stage of an A 0 Spencer binocular polarizing microscope. The cooling coil on the side of the block was insulated by wrapping with wet asbestos tape. After the tape has dried, the thermometer hole is easily opened with scissors. An added advantage of the asbestos tape is that it forms a very strong bond where it joins the microscope stage a t the bottom of the block. This eliminated the necessity of having to bolt the assembled unit to the microscope as is usually done.
774 / Journal of Chemical Education
A problem inherent in many previous heating stages which use cover slips is that the top cover glass suffers localized cooling. Sublimation of the sample may occur making the upper cover glass opaque. The copper cover block detailed in Figure 2, when placed on top of the sample cover slips, eliminates this problem through efficient heat transfer from the main block. Details of operation of the variable temperature microscope stage will be given here. Firstly, we have found it very useful to enclose the lower part of the microscope in an appropriately sized polyethylene chamber such as a Glove-Bag.' An inert atmosphere is maintained using the nitrogen gas exiting from the block's cooling coil. Many compounds react with oxygen or moisture in the air upon heating so that the routine use of an inert atmosphere allows us to eliminate these variables. I n addition, the inert enclosure eliminates the problem of mositure condensation when working a t low temperatures and does away with temperature fluctuations due to air currents. When working at low temperatures, nitrogen gas should be allowed to flow for a short while before cooling is started in order to flush out any moisture in the copper tubing. The use of liquid nitrogen in the Dewar flask allows a working temperature of -lOO°C to be obtained within 30 min. If it is desired to rise slowly to a specific temperature and then hold there, heating is best effected by keeping the cooling gas stream on and heating by means of the Variac controlled heating element. If one wishes to observe thermal behavior as a function of temperature, a convenient rate of temperature rise is obtained by slowing down or stopping the flow of gas through the cooling coils. One has the option, of course, of beginning at ambient temperature and carrying out observations as the temperature is lowered. For measurements above room temperature it is convenient to calibrate the Variac settings in terms of the stage thermometer. Although the Chromalox heating element will go much higher than 300°C, this temperature is suggested as a safe upper limit because of the excessive amount of heat which would have to be dissipated. This work was done as part of a research project supported by a grant from the National Institute of General Medical Sciences. Available from local metal supply companies. A piece 3 in. in diameter and 2 in. long eost $12 including a cutting charge. Available from Edwin L. Wiegmd Co., Pittsburgh, Pa. 15208 at a. eost of approximately $5. 'Catalog number 3CSI-16; Edwin L. Wiegmd Ca., Pittsburgh, Ps. 15208, a.ptpproximately $0.17/ft, Instruments for neseareh and Industry, Cheltenham, Pa. 19012.
