Instrumentation Walter C. McCrone McCrone Associates Chicago, III. 60616
Microscopy A large proportion of industrial processes takes place at temperatures higher or lower than room temperature. They often involve phase or composition changes or, at least, dynamic changes in physical form. Microscopists believe that all such processes can be better understood, controlled, and improved if they can be observed microscopically. Almost since the first microscopist looked at a magnified image, he has wanted to observe the effects of heat and cold. Even Antony Van Leeuwenhoek studied the effects of heat on the growth of aquatic insects. Important facts can occasionally be learned by studying materials at room temperature after exposure to higher or lower temperatures. Generally, however, a microscopist prefers to watch the process microscopically as it occurs. Obviously, he cannot take his microscope into an electric furnace to watch the reaction between graphite and sand to make carborundum. He can, however, duplicate these and other conditions on a small scale and observe the processes taking place at reasonable magnifications. In this way he can study the baking of cake, cooking of meat, thermally induced detonation of lead azide, action of glass melt on a refractory, devitrification of glass, and the formation of silicon carbide from sand and charcoal. He uses small amounts of material and small amounts of heat so that no harm is done to delicate optics and sensitive human bodies. I know of no industrial process that cannot be performed on a microscope stage and observed microscopically. Hot and cold stages for both light and electron microscopy have been developed by many microscopists. These accessories are often very simple, and relatively few have ever been produced commercially. Temperature itself should not be a problem since objects from liquid helium temperatures (—268.6°C) to flame temperatures (about 4500°C) can be studied microscopically. Materials of construction and protection of micro-
scope optics also become problems above the range of commercially available hot stages (about 2000° C). Most microscopists construct their own hot and cold stages when their needs fall outside the - 5 0 ° to +2000°C range. Thermocouples can be used for temperature sensing up to about 3000° C (W/W-Re), and optical pyrometers beyond that point. Thermistors and thermometers are often used near room temperature, and platinum resistance thermometers can be used up to about 1500°C. Two unusual high temperature thermometers based on gallium and tin have also been used. Gallium with a liquid range from 29.7-1600°C has been used in a quartz capillary thermometer over the range 30-1000°C. Tin, with a liquid range from 231.82260° C, has been used in a graphite thermometer using a capacitance clip around the graphite to detect the molten tin meniscus; it is useful to about 1700°C. Thermocouples and platinum resistance thermometers have largely replaced these more heroic thermometers. Optical Microscopy
The behavior of substances when heated or cooled is often best viewed at low magnifications. The light microscope should always be used unless higher resolution (scanning or transmission electron microscopes) is justified. Specimen preparation is generally much easier for light microscopy. Some substances, e.g., volatile liquids, cannot be studied under high vacuum; hence, the light microscope must be used. Cold Stages. Both hot and cold stages are relatively easy to make—so much so that most microscopists have made one or more, and the literature contains descriptions of many hundreds of them. We will start with the lowest attainable temperatures and proceed step by step (by degrees?) to the highest temperature hot stages. Cold Gas Stages. One class of temperature stages utilizes a flowing gas
as heat transfer medium. They are usually used only below room temperature and are designed for different temperature ranges: to —260°C with liquid helium, to —185°C with liquid nitrogen, to —170°C with liquid air, or to —70°C with solid carbon dioxide. All have the common requirement that they must be very compact and very well insulated especially with liquid helium and liquid nitrogen or air; otherwise, not only will the consumption of liquefied gas be excessive, but the lowest temperatures will be unattainable. To ensure adequate cooling capacity, this cooling liquid must evaporate at a rate sufficient to maintain the specimen at the desired low temperature. Ideally, however, the Dewar of liquefied gas is insulated but with a small electric heater immersed in the Dewar itself. In this way the liquid can be evaporated at a rate controlled by the heater in order to control, in turn, the temperature of the specimen. If the entire stage and Dewar are completely insulated, the expensive liquefied gas can be conserved and used only when, and as, needed. A cold stage of this type was described by McCrone and O'Bradovic (1956). In that particular paper the desired low temperature of —100° C was easily attained without much insulation. A feature of this stage is its thinness; this permits use of high numerical aperture objectives and condenser as well as phase contrast. It is also fitted with an electrically conductive microscope slide to permit better temperature
Table I. Temperature of Liquefied Gas and Inversion Temperature Temp, °C Gas
Argon Air
Nitrogen Hydrogen Helium
Bp
-186 —194 —196 —253 —270
Inversion
>200 202
ca. 200 -69
—253
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975 · 1279 A
control through the ambient range and a high temperature range up to +100°C. There are, to my knowledge, no commerically available stages of this design. There is, however, a simple Leitz cold stage (Heating and Cooling Microscope Stage 80) that can be in sulated for use at temperatures lower than the advertised —20°C and can also be used with any standard micro scope optics. Joule-Thomson Stages. Another class of cold stage utilizes liquid heli um, hydrogen, argon, nitrogen, or air, but the respective gas is liquefied in the stage itself by Joule-Thomson ex pansion through a fine orifice. The temperature required determines the gas selected (Table I). Temperatures at or near the boiling point of the liq uid can be attained in a well-designed Joule-Thomson cold stage. The signif icance of the inversion temperature is that the gas cannot be liquefied by a Joule-Thomson expansion if its start ing temperature exceeds that figure. Argon, air, and nitrogen can therefore be liquefied starting with the gas at room temperature. Hydrogen, how
ever, must be precooled to —69°C be fore expansion, and helium must first be cooled to — 253° C. It is relatively simple to precool gaseous hydrogen with liquid air or liquid nitrogen be fore Joule-Thomson liquefaction of the hydrogen. Helium would require precooling by hydrogen before it could be liquefied. Although cryogenic cold stages sound formidable to most microscopists, the devices available are rela tively easy to use. One is the Hymatic Minicooler (Nicholds et al., 1973) marketed in the U.S. by Bendix (Bendix Instruments and Life Support Di vision, Davenport, Iowa 52808). A sec ond essentially identical device is mar keted by EMI in England under the name Emicooler. Both devices con duct the gas through finned metal capillaries in such a way that the gas exiting through the final 20-jum orifice flows back over the fins to precool the incoming gas. With a flow rate of about 0.4 cfm of gas at an initial pressure of 4000-6000 psi, liquefied gas can be observed at the orifice in a few seconds. These de vices have one disadvantage due to the
1280 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
tiny 20-μπι orifice; they clog easily with tiny particles or with solid CO2 or ice. These materials must be com pletely removed from the entering gas stream to ensure continuous troublefree operation. A molecular sieve re moves not only particles but CO2 and water. If oil-pumped nitrogen in large cylinders is used, there is little clean up required, and the molecular sieve has a long life. The Minicooler as usually furnished is best adapted for microscopical study of opaque objects or, at least, by reflected light. Bendix will, however, provide other configurations so that transmitted light can be used. The Emicooler can be used either for transmitted or reflected light although the polymer used for the stage is not
isotropic, nor can it usually be turned to an extinction position common to both top and bottom plates. A small thermocouple bead is easily imbedded in either stage just below the speci men. The latter should be mounted on the stage plate itself or on a tiny frag ment of coverslip. To work at liquid hydrogen temper atures (20°K) requires precooling of the hydrogen gas with a Dewar of liq uid air. The hydrogen passes through a copper coil immersed in the liquid air immediately before the cooling stage. These stages are especially useful because, with their low heat capacity, they can be heated and, especially, ' cooled very rapidly. We have observed many phase transitions of such sub
stances as methylene iodide (—6°C), carbon tetrachloride (—46°), and methanol (—113°C). Most of the oper ations possible above room tempera ture (McCrone, 1957) can be carried out with these (ultra) cold stages. Fig ure 1 shows a fusion preparation of aniline between crossed polars during crystallization from the melt at -6.1°C. Peltier Devices. The Peltier effect has been known for many years and is the basis of the common thermocou ple. Two dissimilar metals in contact will develop an electrical potential be tween them if the junction is heated or cooled. The converse is also true, but with any ordinary metals the effect is too small to be useful in a microscope cold stage. However, the relatively re
cent development of semiconductors has made this application of Peltier cooling practical. To accomplish this, a junction is formed from two semi conductor crystals, one a p-type and the other an η-type. When a current passes across this junction, either heating or cooling occurs, depending on the direction of the current. The degree of heating or cooling is a func tion of the current being carried and the resistance of the semiconductor. Usually, single crystals of bismuth telluride or antimony telluride are used as the semiconductor, and the current requirements range up to 25-30 amps at 1-2 V. At these amp erages, resistance heating is a limiting factor. Most thermoelectric junctions in use will produce a maximum tem perature difference, under optimum conditions, of at least 65° C. Since one end of each crystal becomes hot and the other cold, one must prevent con duction of heat from the hot end to the cold end by circulating cold water in contact with the hot junction or simply with copper cooling fins. A more elegant way to cool the hot junc tion is the use of a second thermoelec tric cooling unit. One could consider cascading several more units to thus gain additional cooling. However, very little additional cooling is effected be yond two to three units; moreover, each additional cooling unit must be larger than its predecessor. This, plus the power supply problem, usually limits the design to a two-unit system. The system we use (Markussen and McCrone, 1965) has one unit of two pand η-pairs cooled by a second unit
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975 ·
1281 A
Figure 1 . Aniline crystallizing from melt at —6.1°C
and gives specimen temperatures containing eight p- and η-pairs. Units down to about - 6 0 ° C. containing eight pairs can be pur Mettler FP-5 as a Cold Stage. The chased at relatively low cost already Mettler FP-5 (discussed below under assembled. Our two-stage unit has a hot stages) can be used as a cold stage cooling water coil to remove heat from down to — 20° C by circulating cooled the lower hot junction, and the entire . gas into the fan system. The digital assembly is insulated with plastic temperature readout functions over foam in a small box fashioned from the range - 2 0 ° to 300°C; hence, posi polystyrene. The microscope objective tive temperature control over the en fits through an O-ring lining a circular tire range can be obtained by having a opening cut in the center of the poly base temperature of less than — 20° C styrene top, thus sealing the stage in the cooling gas. If short gas lines from easy access to moisture in the air and good insulation are used, dry ni which would condense on, and ob trogen passing through a copper coil in scure, the preparation. a dry ice cooling bath can be used ef A 2-mm light well is drilled vertical fectively. Mettler will modify their ly through the centers of the two ther stage so that the range becomes —50° moelectric units, taking care to avoid to 270°C (or 30-350°C) on request. the semiconductor crystals so that Quick-Chill Aerosols. Several de transmitted light may be used. Any vices to chill cocktail glasses are now desired temperature from —50° to +100°C can be quickly set and held by being marketed as bar accessories. The inexpensive refills for these are manipulation of the variable voltage small, pressurized aerosol cans con and the direction of the current flow. taining liquefied gases such as dichloBoth controlled heating and cooling rodifluoromethane. The discharge are easily managed, and equilibrium from these cans lowers the tempera melting points in which the crystal ture of a microscope slide to — 50° C in and melt are maintained in dynamic a few seconds. equilibrium are easily carried out. Carbon Dioxide Stages. The tem Some of the uses for these products perature range for commercially avail in microscopy include rapid crystalli able stages begins at about — 50°C al zation of low melting compounds or though some can be used at lower crystallization from solution. Monoclitemperatures by using lower boiling nic sulfur is easily obtained by chilling liquids as coolants. Both Reichert and a solution of sulfur in aniline previous Leitz produce stages designed for car ly saturated at 75-100° C. The solu bon dioxide. They also have facilities tion-phase tranformation to the stable for heating so that positive tempera orthorhombic phase is then observed ture control is possible near room tem on the spontaneously warming slide perature. Reichert makes the Kofler on the microscope stage. Customarily, cold stage which is similar to the Ko this· procedure is used only as a quick fler hot stage but with a circulating qualitative procedure, but the rate of gas chamber below the specimen reheating to room temperature can be stage. A heating unit, limited to slowed, if not controlled, by placing +80° C, is also incorporated to give the preparation on a metal heat-sink positive response near ambient tem before cooling, by making a small cell perature. Any circulating gases at any to cover the preparation during warm temperature can be used with these ing or by successive properly timed stages. Carbon dioxide is convenient short bursts of aerosol. 1282 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
Hot Stages. Hot stage methods are simply research and analysis proce dures which involve heating or cooling a compound or mixture before or dur ing microscopical examination. Hot stages are easily made for studies at temperatures up to about 3000° C (Table II). There is almost no limit to the range of problems that can be in vestigated microscopically. The publi cations most useful to the microscopist who specializes in hot stage meth ods can be listed as follows: purity de termination, mixture analysis, charac terization and identification of fusible compounds and mixtures, composition diagram determination (two-compo nent systems and systems of three or more components), polymorphism, measurement of physical properties (molecular weight, crystal growth rates, crystal morphology, and crystal optics), recrystallization and grain growth, and crystal growth kinetics. Fairly often the microscopist's re search problem falls straightforwardly into one category in a list of "unit op erations". He may be asked to develop a fast method for analyzing binary mixtures of isomeric compounds, per haps position isomers, with which in frared absorption would have trouble. Simple measurement of the refractive index of the melt may then suffice. The method is based on the fact that the refractive indices of binary solu tions are linearly related. The hot stage then becomes the means of con verting a binary solid mixture to a liq uid so that refractive index can be measured. A simple hot stage proce dure for determining the refractive in dices of such liquid samples at specif ic, controlled temperatures is based on the usual immersion method. Further random perusal of research areas brings out the important prob lem in the pharmaceutical industry of marketing a given drug in its most ef ficacious form. This usually means the most soluble form to rapidly achieve high blood levels. It thus becomes im portant to know whether a crystal form more soluble than the ordinarily stable form exists. This could be a metastable phase, an amorphous phase, or a solvate. Hot stage methods Table I I . Melting Points of Metals Used as Hot Stage Heating Elements Metal
Mp. °C
Platinum Iridium Molybdenum Zirconium Tantalum Rhenium Tungsten Carbon
1769 2410 2610 2980 2996 3180 3410 3700
are ideal for elucidating the possibili ties for each drug composition. One can quickly determine both the rela tionship between different phases ob tained in the laboratory and whether the compound has other commercially useful forms. Similar problems exist in the field of high explosives, although here the most stable phase is usually the de sired production form—generally less sensitive to impact and denser than the other phases. It is important to know whether, and how, less stable phases can be formed. The hot stage microscopist can determine the num ber of polymorphs, observe their tem perature stability ranges, predict the likelihood of obtaining each during manufacture or use, and tell how to stabilize the desired phase. Another problem facing many in dustries is the chemical instability of a new compound. The compound may be a potential new drug, explosive, dye, rubber accelerator, etc.; but it may decompose during manufacture, storage, or use. The hot stage furnish es a procedure for measuring chemical instability quantitatively. This, in turn, furnishes a means of evaluating batches and of studying the effect of stabilizing additives. Chemical insta bility is measured by the decomposi tion rate. A convenient way to follow decomposition is to determine the melting point as a function of time. This procedure is routine with the hot stage since operating at the melting point accelerates decomposition. In normal use, the hot stage temperature is continuously manipulated down ward to follow the equilibrium be tween one last unmelted crystal and the melt. The temperature of the preparation as a function of time indi cates the decomposition rate. In certain systems, e.g., ρ-aminosal icylic acid, decomposition is far too rapid to be measured easily at the melting point, and the procedure is adapted for lower temperatures. The time for complete melting (TCM) is determined at a particular tempera ture. The impurity builds up as the sample decomposes at a characteristic rate. This progressively lowers the melting point, and complete melting finally occurs, all at a uniform temper ature. The temperature can be chosen so that elapsed time is in the range of a few minutes. Samples can then be compared on the basis of the time for complete melting; longer time indi cates greater stability. The effect of additives on chemical stability is also easily evaluated with this same proce dure. As with cold stages the commercial hot stages cover the range near room temperature. Above about 2000° C microscopists must make their own
stages, and many prefer to make their own stages in the lower temperature range. Possibly the largest single group of papers on microscopical top ics is the one covering hot stages. Probably the first hot stage was a strip of copper extending over the edge of the microscope stage so that it could be heated with a candle or micro gas flame. A specimen on the other end of the strip could then be ob served microscopically. Later, Otto Lehmann introduced a small gas flame immediately under the center of the stage to heat directly a metal sam ple holder. Since then hot stages have become more sophisticated, versatile, and expensive. Hot- Wire Stage. Still relatively simple are the hot-wire stages. An electrically heated wire held just above a microscope coverslip can be used to impose a temperature gradient on any specimen. Jones (1968) has used such a device to study composi tion diagrams, and Hartshorne (1975) has more recently applied an im proved device to the study of phase transitions. He has been able to make quite accurate (±1-2°C) measure ments of phase boundaries by cali brating the system with a compound having two to three known phase tran sitions. Two preparations, one known and the other unknown, are arranged side by side in the field of view of a low-power objective (30-50 mm focal length), and the positions of the tran
1284 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
sitions for each on an eyepiece mi crometer scale are recorded. Plotting of the known transition data against eyepiece scale divisions gives a cali bration curve for the unknown. Hart shorne finds that the plot of log (t — tr) vs. S is a straight line. S is the dis tance of the transition from the wire, t is the temperature at the transition, and tr is room temperature. Thin-Metal Stage. Hartshorne and Stuart (1970) have designed two thinmetal stages useful for orthoscopic and conoscopic observations from room temperature to about 150°C at numerical apertures to about 0.50. This NA can be exceeded somewhat if a long working distance condenser and objective are used. Oil Bath Microscope. This setup, difficult to describe as a hot stage mi croscope, does qualify as an apparatus for the study of microscopic objects as a function of temperature. It consists of a heated thermostated oil bath in which a long flat-bottomed sample container is positioned so that an in verted microscope can be used to ob serve (through the bottom of the oil bath, oil, and tube) any particles sus pended in a liquid inside the tube. Its special purpose (Teetsov and McCrone, 1965) is the study of poly morphic transformations and mea surement of transition temperatures. Excess solute suspended in a saturat ed solvent can be agitated at any tem perature up to about 200°C with con-
Figure 2. Oil bath with inverted microscope for study of crystals at temperatures f r o m 30-250°C
tinuous observation of the crystalline phase present. In the region of the transition, the temperature can be held constant to make sure which phase is stable and the temperature at which two phases are in equilibrium with the saturated solution, i.e., Ttr. The agitator in the sample tube can be stopped periodically for inspection of the settled crystals (Figure 2). A 35-mm camera was used when photo micrographs were desired. This appa ratus has considerable advantage for solution phase transformation studies over trying to seal a coverslip, capil lary, or tiny cell in a hot stage. It is also easy to add excess solute or to seed the suspension at any time. Electrically Coated Microscope Slide. If a microscope slide is coated with a conductor of electric current, but with a reasonable electrical resis tance, a current can be passed through the coating and thereby heat the slide. Such electrically coated (EC) slides can be prepared by evaporating onto the slide a very thin metal coating, e.g., platinum or by hydrolyzing tin chloride with steam on the glass sur face. The latter gives the most inert and stable coating and has been used to make heated windshields and other heating equipment or elements. We use these EC slides as heating ele ments in the -100° to +100°C cold stage (McCrone and O'Bradovic, 1956). The EC slides are also useful as low temperature heaters by them selves on a microscope slide. A thin strip of conducting paint on either end of the slide serves as an electrical con tact for two wires going to a variable voltage control. A recent application of the EC slide was to speed up evaporation of water in microchemical test cells (McCrone, 1971). This, in turn, increases the speed of microchemical tests. It can also be used to speed up the micro sol ubility test as carried out by Laskowski (1965). Kofler Hot Stage. Thousands of Kofler hot stages are in use through out the world today. Developed by Ludwig Kofler (1931, Figure 3), this well-engineered stage has been re sponsible for making hot stage meth ods a science rather than a hobby. Only when the Mettler hot state ap peared nearly 40 years later with digi tal temperature readout, programmed heating rates, and no vertical heating gradient did the Kofler hot stage sud denly become outclassed. Although the Kofler stage costs nearly an order of magnitude less than the Mettler stage, the latter has become the in strument of choice for those who can afford it. A careful microscopist can still do with the Kofler stage every thing a good microscopist can do with the Mettler. Operations, routine with
the Mettler, require more skill and care with the Kofler and often 2-10X the work time. The Kofler stage can be purchased from C. Reichert, William A. Hacker, or Arthur H. Thomas in essentially identical designs covering the range from room temperature to 350-360° C. Arthur H. Thomas furnishes the stage either with thermistor readout or two thermometers (30-230°C and 60350°C). Although heat baffles are fur nished, the Kofler stage, like all other stages (including the Mettler), must be carefully calibrated (Julian and McCrone, 1971). Mettler FP-5 Hot Stage. The Met tler hot stage is a superb instrument. Many of the difficulties encountered with the Kofler and other hot stages are due to the vertical temperature gradient. The Mettler stage gives a uniform temperature through the preparation volume by sandwiching it between two identical heating ele ments. A fan blows cool air around the outside of the heating elements and inside the case to keep the case cool and to cool the stage more rapidly. A platinum resistance thermometer is embedded in the lower heating ele ment to ensure accurate temperature sensing. A variety of linear heating rates from 0.2-10°C/min are available over the range from - 2 0 ° to +300° C. A reversing switch permits the same range of linear cooling rates. The tem perature readout is digital, and there is a memory bank for remote record ing of three temperatures during an experimental run. The Mettler hot stage is especially useful in police enforcement laborato ries. A good example is the compari son of glass samples, one from the
scene of a crime and the other from a suspect's shoes, clothing, car, or apart ment. A few tiny glass flakes are mounted in a liquid (e.g., Cargille) of known refractive index, dispersion, and temperature coefficient of index and heated to a series of constant tem peratures two degrees Celsius apart. After a few seconds equilibration at each temperature, the index matching wavelength is determined and record ed on a Hartman net. Approximately 2 min are required per data point using the Mettler (if the Kofler hot stage is used, 10-15 min/data point would be required). If two glass samples give closely similar data, they are very like ly identical. Tiny differences proving nonidentity are easily detected. Leitz -20° to +350° C Stage. Leitz furnishes a stage equivalent to the Ko fler stage and with several special fea tures. It can be placed on almost any polarizing microscope, but it is also furnished with a simple stand espe cially suited to its use. One feature is a "drawing camera" eyepiece designed . to superimpose any portion of the thermometer in the field of view for easy reading of temperatures. It also has a cooling chamber for carbon diox ide or other cold gases or liquids. The three thermometers cover the ranges: - 2 0 ° to +110°C, +100° to 230°C, and +220° to 350°C. Stanton-Redcroft 1000° C Stage (HMS-5). An excellent stage with pro vision for linear temperature program ming (l-100°C/min heating or cool ing) is furnished by Stanton-Redcroft (Copper Mill Lane, London SW17 0ΒΝ, England). The stage itself can be replaced with DTA or TGA systems to take full advantage of the temperature control unit. See Charsley and Kamp
Figure 3. Kofler 350°C hot stage as arranged for use; melting point standards, cooling block, and sublimation cells also shown Courtesy of Arthur H. Thomas Co.
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 14, DECEMBER
1975 ·
1285 A
(1971) for full details. A unique fea ture of the stage is the use of reflected light measurement as a function of temperature to show phase changes. Leitz 1350°C Stage. Maria Kuhnert-Brandstatter (1968) has done an excellent job of evaluating the Leitz 1350°C hot stage. She finds it to be reasonably accurate (±5-10°C) if fre quently calibrated and if used with an inert atmosphere, preferably argon. She reports that nitrogen as an "inert" gas reacts with some substances when heated; for example, barium chloride gives barium nitride. Other problems to be faced, e.g., composition of slides and covers, are also discussed. A major disadvantage of the stage for trans mitted light microscopy is the inabili ty to move the preparation once in the stage. A 2-mm circle, the area of the light well, is the total preparation area. Reichert 1600°C Stage (Vacutherm). A stage similar to the Leitz 1350° C stage is produced by C. Rei chert (Gabier and Wurz, 1959). It uses molybdenum, tantalum, or tungsten heating elements and is restricted to the examination of opaque specimens. This stage has more recently been modified for vacuum operation in stead of an inert gas. It is now called the Vacutherm. Union 1800°C Stage. The Union Optical Co. of Tokyo, Japan, produces a 1800° C reflected light vacuum stage similar to the Vacutherm. They note, however, that temperatures to 2300° C can be attained by use of optical pyrometry for temperature sensing. A vacuum of 10 - 6 torr is claimed as well as a 15-sec heating rate to 1800° C. Metal Ribbon Stages. The idea of electrically heating a thin metal strip doubling as a specimen support and heating element is capable of infinite variation. The metal itself may be var ied as well as the power supply and housing. The choice of metal depends not only on its melting point but on its oxidation stability. Most metals ex cept the noble elements like platinum, iridium, and gold oxidize in air on heating. Rhenium, for example, can be heated in air to 2000°C or somewhat higher before beginning to oxidize; tungsten oxidizes even more readily as does graphite. If it is necessary to ex ceed about 2500°C with, say molybde num, it is necessary to operate either with an inert, preferably argon, atmo sphere or with a good vacuum. Jack Dodd (1965) has designed an interesting, yet simple, 2000°C stage. The heating element is a tiny wire loop which acts also as a resistance thermometer. Platinum or iridium is ideal since neither requires an inert atmosphere. The loop itself is so small that the amount of heat is minimal, and almost any containing material
can be used. He used a machined lavite block which can be easily drilled to permit vacuum or inert gas operation. The sample is held in the wire loop mechanically or, once melted, by sur face tension. The stage is simple to op erate, and an equilibrium melting point on a single crystal of quartz was easily measured at 1721° ± 3°C. An advantage of Dodd's stage is that the null detector used to measure the re sistance of the loop becomes more ac curate at high temperatures, i.e., the null becomes sharper. The accuracy therefore, in effect, remains constant at about ±3°C over the entire temper ature range. Alison Instruments (Division of Ali son Engineering Ltd., Great Yar mouth, England) have marketed a very fine 1800° C hot stage related in principle to Jack Dodd's platinum loop device. Both designs use the heating unit as the temperature sens ing element: Dodd by using the plati num heating loop as a platinum resis tance thermometer and Alison by making the temperature sensing ther mocouple a resistance heating loop. Both cycle the two functions many times each second. The Alison instru ment is a beautifully engineered hot stage system with an integral micro scope and digital temperature read out. The sample and thermocouple mass are so small that only 3-4 W are needed for heating to 1800° C, and cooling time from 1800° to 700° C is only about 0.5 sec. This instrument is the Cadillac (Rolls-Royce?) of the high temperature hot stages (Figure 4). High Temperature Vacuum Stage. Mike Bayard of our laboratory was once asked to photograph tungsten metal surfaces at high magnifications (400-500 X) under high vacuum at
Figure 4. Alison 1 8 0 0 ° C hot stage
1286 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
temperatures up to 2500°C. We decid ed the easiest way to do this was to heat a tungsten strip directly and read the temperature by optical pyrometry. A simple microscope was mounted on the top of a large vacuum stage with a quartz plate as coverslip. A long work ing distance 4-mm objective was used. Many problems arose and were solved before suitable pictures were obtained. One such was clouding of the quartz window by sublimed tungsten. A mag netically operated "window shade" solved this problem. One problem which one might think would not arise is illumination of the self-luminous specimen. Yet, object detail is not well shown by self-luminosity, and we were able to achieve good detail by taking the picture with a very high intensity Xenon arc, thereby overpowering the self-luminosity. The vacuum problem was solved with an oil-diffusion pump; a vacuum of 5 Χ 1 0 - 7 was easily main tained for 2 hr at 2400°C. The problem of the window compo sition was solved by choosing silica be cause of its low temperature coeffi cient of expansion and high melting point (1728°C). In other applications of this stage, we were faced with the need for other high melting inert win dows. Polished sapphire plates cut from artificial boules were satisfactory up to about 1800° C; they are, how ever, biréfringent. Later an almost universally suitable material was found, crystalline cleavage plates of periclase (magnesium oxide). These cleavage plates are isotropic, clear, colorless, and melt at 2800°C. We obtained a good supply as a special favor from Gordon Finlay, then at the Norton Co. in Niagara Falls, Ont. Transfer Lens System. When the total amount of heat is small, even at ultrahigh temperatures, there is no
great risk to the microscope optics or stage materials. Some hot stages do generate sufficient heat to damage mi croscope objectives. A solution to this problem is to use a transfer lens to "look a t " the hot object and form an image on which the microscope objec tive can be focused. The resolution of such a transfer lens is limited by its aperture; hence, it should be a short focus, low f-stop system. We have used camera or projector lenses of almost any set of characteristics. A wide angle, e.g., 25-mm focal length f:1.2 lens operating 50 mm from the hot specimen, will deliver a cool image 50 mm on the other side of the lens. An interesting application of this kind of system is the work of Henry Bauman at the Carborundum Co. Using a transfer lens made especially for his work by the American Optical Co., he was able to study crystalliza tion of titanium dioxide from a glass, furnace refractory lining dissolving in glass melts, melting of aluminum and mullite, sublimation of alumina, boil ing of silica and synthesis of silicon carbide from silica sand and carbon. He used a resistance wound micro fur nace for temperatures up to 1500°C and a conduction-heated graphite rod with an opening therein as a cell for higher temperatures up to 2700°C. Although we know of no microscopy at temperatures higher than 2700°C, there seems no reason to doubt it could be done whenever the need
References Charsley, E. L., and A.C.F. Kamp, "Ther mal Analysis," Vol 1, Proceedings Third ICTA DAVOS, 1971. Dodd, J. G., Microscope, 14, 302 (1965). Gabier, F., and W. Wurz, Metali, 9, 819-23 (1959). Hartshorne, Ν. Η., Microscope, 23,177-90 (1975). Hartshorne, Ν. Η., and A. Stuart, "Crystals and the Polarizing Microscope," Arnold, London, England, 1970. Jones, F. T., Microscope, 16, 37 (1968). Julian, Y., and W. C. McCrone, ibid., 19, 225-34 (1971). Kofler, L., and H. Hilbck, Mikrochemie, 9, 38(1931). Kuhnert-Brandstatter, M., Microscope, 16, 257-65 (1968). Laskowski, D. E., Anal. Chem., 37,174 (1965). Markussen, J., and W. C. McCrone, Micro scope, 14, 395-402 (1965). McCrone, W. C , and S. M. O'Bradovic, Anal. Chem., 28,1038 (1956). McCrone, W. C , "Fusion Methods in Chemical Microscopy, Wiley, New York, N.Y., 1957. McCrone, W. C , Microscope, 19, 235-41 (1971). Nicholds, K. E., et al., Hymatic Cryogenic Symposium, November 1973. Teetsov, Α., and W. C. McCrone, Micro scope, 15,13-29 (1965). Based in part on R. Z. Muggli and W. C. McCrone, "Non-Ambient Temperature Micro scopes," Proceedings IMS/ASM symposium, June 29-30, 1975, Minneapolis, Minn., Plenum Press, in press.
GOW-MAC the T.C. Detector Specialists can keep your GC in business. The best way to save money is pre ventive care of your T.C. detector. A cell using hot wire filaments operates at elevated temperatures, the wires, too, are hot due to the current passed through them, therefore subject to oxidation. Many things can oxidize the detector elements: samples, leaks, in adequate purge. When filaments are oxidized their resistance will change and the bridge is out of balance. Certain filaments resist oxidation more than others under different conditions. Other things which unbalance bridge circuits are corrosion, vibration, column bleed, carbon deposits, etc. GOW-MAC can help by the availability of numerous fila ment materials and years of experience in solving such problems. Thermistors are also available in various resistance ranges. GOW-MAC can supply detector elements for most of the gas chromatographs in production as well as those that are no longer manufactured.
Four ways to solve the problem and save time and money: 1. Purchase detector elements in pairs. If you have several T.C. instruments and want to re-filament your own detector this is fast and the least expensive approach. You can have elements in stock or, we can ship from stock. Even our representatives have emergency stock for customers in trouble. 2 . Purchase a quad, (four matched ele ments). This is for those who have one or two units and want to refilament the cell themselves. With a quad, anyone can re-filament his own d e t e c t o r . A l l four e l e m e n t s a r e
GOW MAC
matched so you don't have to pick two matched pairs to make a wellbalanced bridge. 3. The easiest, not the quickest or least expensive, is to take the cell block out of the detector oven and send it to us. We will clean, refilament, check for leaks, drift and noise for $110.00*. This takes at least 48 hours, so allow one week for us to receive and return. 4 . You can also purchase a spare detec tor and keep it on the shelf. Most de tectors can be supplied by GOW-MAC for about $150.00. Take your pick, by ordering directly from GOW-MAC you can save either money, time, or both. We're prepared to send time and money saving information: 1. Filament Bulletin listing 8 different materials available for detector ele ments including thermistors. 2. Cell Bulletin which lists 12 T.C. detectors from process to micro, 4 types of Gas Density Detectors. 3. Our Filament Replacement Chart. This lists all major GC manufacturers, instrument models and the GOW-MAC replacement elements. 4 . General Service Bulletin—tells how to re-filament a cell, discusses main tenance and trouble shooting as well as recommended operating condi tions for T.C. cells. 5. Price List—this will save you money. We'll even throw in the booklet, "Selecting a GC Column" written for GOW-MAC by Dr. H. McNair. DON'T WAIT! *W or WX filaments for most detectors.
GOW-MAC INSTRUMENT CO. 100 Kings Road, Madison, N.J. 07940 Telephone: 201/377-3450 Telex: 136331 Shannon Free Airport, Co. Clare. Ireland
CIRCLE 97 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 14, DECEMBER 1975 · 1287 A
