INSTRUMENTATION Resistance thermometer, instrument for determining freezing point, rotating mirror frame c a m e r a , gas chromatography
precise determination of colligaTHE tive properties retains its importance for the analyst as either a direct or supplementary method of characterization. Freezing point or melting point determinations are valuable methods for identification or for the estimation of purity. Determination of Freezing Point An elegant means for determining the freezing points of solutions in very small volumes of liquid has been described recently by J. A. Ramsay and R. H. J. Brown of the University of Cambridge [J. Sci. Insfr., 32, 372 (1955)]. Their method works best with volumes of the order of 10 _ 3 to 1 0 - 4 cu. mm. of liquid and its accuracy in terms of standard deviation is ± 0.003 e C. for freezing point depressions of the order of 1° to 2° C. Although this technique was developed by zoologists, and is a matter of considerable importance in invertebrate physiology, the authors point out that the high precision is largely due to the use of small samples and the technique is to be recommended even when economy of solution is not a consideration. Their improved instrument makes use of the principle of Drucker and Schreiner, who showed that the usual difficulties of supercooling can be avoided by first freezing the sample, then warming slowly and observing microscopically the disappearance of the last ice crystal. The temperature at which this occurs is the true freezing point, provided certain precautions have been observed. The instrument makes use of alcohol—dry ice for the initial freezing operation, contained in two concentric vessels. The inner vessel has conducting brass rods of varying length sealed in the bottom and heat conduction with the outer and colder vessel can be controlled by the depth of immersion of these rods. For the heating
operation which follows freezing of the sample an efficient combination of electrical grid heater, motor-stirrer, and set of baffles is provided. The disappearance of the last ice crystal is followed by a microscope in transmitted light. The temperature is read on a thermometer with 0° to —6° C. range graduated in hundredths and capable of being raised to room temperature. An appropriate electrical network in the heater circuit piwides a wide choice of heating rates. The final approach to the end point is achieved by having a more or less constant temperature prevailing and then to raise the temperature in minute steps by tapping a series resistance shorting switch for short periods. The technique for sample mounting is also described, in which very thinwalled silica capillaries are filled with tiny droplets of the solution. These are alternated with tiny amounts of liquid paraffin, so that duplicate or triplicate samples of the unknown may be mounted in the same capillary. A single example of the power of this method is quoted, even though it may have no immediate interest for the analyst. I t has been possible to isolate single sea urchin eggs in liquid paraffin and to determine the freezing point depression of their contents. Resistance Thermometer Another development contributing to the rapid measurement of temperature is a resistance thermometer with a high speed of response. The general requirements which led to the development of this device were a relatively large surface area, short thermal path, and small heat capacity. As described by W. T. Bane and E. R. Dymott of the National Physical Laboratory [J. Sci. Instr., 32, 403 (1955)], a resistance thermometer wound in the form of an electrical-resistance strain gage was developed. Some 43 cm. of nickel wire
V O L U M E 2 7, NO. 12, D E C E M B E R
19S5
by Ralph H
Mullet
of diameter 0.0025 cm. was wound on a thin flat form of impregnated paper. Thin metal strips were spot-welded to the ends of the winding for connections. The entire winding was covered with a single layer of the same paper and the unit was bonded in a hot press. The resistance of the thermometer was about 100 ohms and the over-all dimensions were about 2.5 X 0.5 cm. and 0.013 cm. in thickness. The thermometer element was connected in one arm of a d.c. bridge network with a bridge excitation of 5 ma. The bridge output after amplification was connected to a high speed recorder. The response time was determined by plunging the thermometer into hot water. From the recorder trace, it was shown that the time constant was approximately 0.07 second. The response time in stirred air was found to be 1.5 seconds. Absolute sensitivity to temperature changes is not given by the authors, but this is easily inferred from the temperature coefficient of resistance of nickel, which is 0.004 to a first approximation. This is the intrinsic sensitivity, which of course can be extended by the amplification. Gas Chromatography Some months ago we described the Perkin-Elmer Fractometer, an instrument for automatic analysis on the basis of vapor-phase or gas chromatography. We have received a 31-pago bulletin from this company, written by H. II. Hausdorff, which we assume is generally available; if not, we hope 33 A
INSTRUMENTATION
Parr
Oxygen Bomb Sulfur Apparatus •
For rapid, accurate determination of Sulfur, Halogens, Arsenic, Boron and many other elements in combustible organic materials. Meets A.S.T.M. Requirements for: D-I29-5I, D 271-48, D 808-5IT and D 833-46T.
The Series 1900 apparatus consists of a PARR Oxygen Bomb and all accessories for the combustion of analytical samples when calorific measurements are not required. Either single or double valve bombs can be furnished. Ask your PARR dealer f o r information and prices or write the factory.
PARR EST. 189?
·
INSTRUMENT CO.,
MOLINE. ILLINOIS
MAKERS OF CALORIMETERS AND PRESSURE REACTION EQUIPMENT
For further information, circle number 34 A-1 on Readers' Service Card, page 39 A
OHAUS
INDUSTRIAL BALANCES
easier, faster reading, greater sensitivity/ longer l i f e for fREi folder write to
OHAUS SCALE CORPORATION 10S0 COMMERCE AVE. UNION, NEW JERSEY
it will be published elsewhere. This is a compact and very informative summary of the theory, uses, instrumentation, and general practice of gas chromatography and lists 61 references to the technical and scientific literature. After a concise explanation of the general principles of chromatography, the main topic of gas chromatography is treated. There is a brief treatment of the various schemes which have been proposed and used for detecting the effluent from a column. These include thermal conductivity, vapor-density balances, photoelectric titration, surface potentials, dielectric constant, heat of adsorption, infrared absorption, and the H 2 flame. The author has set up eight criteria of the ideal detector. 1. Good signal to noise (or stability) ratio. 2. Small volume to ensure fast response and high resolution. 3. Simplicity of operation. 4. Preferably should give equal signal amplitude independently of nature of component to be analyzed. 5. Reasonable cost. 6. Simple construction. 7. Rugged and stable operation. 8. Applicability to a broad range of compounds. Thermal conductance, especially in the improved, small-volume cell type and using thermistors, seemed to satisfy more of the above criteria better than any other scheme, although for very specific applications, some of the alternative methods have anywhere from 10 to 1000 times the sensitivity. The remainder of this informative bulletin is concerned with operational and manipulative details for quantitative analysis of mixtures. In our opinion the instrument manufacturers of this country are doing a splendid and increasingly important job in disseminating scientific and technical information. Our sympathy for prospective authors of textbooks on analysis increases in exactly the same ratio. It will be increasingly difficult for the average author to attain competence in the bewildering array of new analytical instruments, nor will it suffice for him to dismiss the elaborate electronic and mechanical gadgetry with a wave of the hand and try to convince students that the main question is the fundamental chemistry of the method. The progressive instrument manufacturer in preparing descriptive literature, faces the entire problem. If he does not do a complete job of explaining fundamental theory, application, construction of the instrument, operation, interpretation, precision and accuracy, maintenance, repair, and servicing, then there is a fair chance that his instrument will not find wide use. Analysts are heavily
For further information, circle number 34 A-2 on Readers' Service Card, page 39 A
34 A
ANALYTICAL
CHEMISTRY
INSTRUMENTATION indebted to our more progressive in strument manufacturers who are con tributing in this way to the collection of definitive information on the newer methods of analysis. The delineation and recording of data at extremely high speed continue to occupy the attention of all scientists. The chemist has countless problems in volving the same requirements and to a large degree, kinetic studies have always been limited by the available means for following the process. While oscillographic methods have always been in the forefront, direct photo graphic methods have the distinct advantage of yielding a large amount of information, much of which can be examined at leisure, after the event.
N-M-R Solves Chemical Puzzles . . . No instrument offers greater advantages for advanced study of chemical systems than Varian's V-4300B High Resolution n-m-r (nuclear magnetic resonance) Spectrometer. This "nuclear detective" is now being used by scientists in many well known universities and research centers for determining molecular structures, analyzing functional groups and monitoring chemical reactions. In all these applications it provides non-destructive observation of undisturbed chemical systems, requires a minimum of sample preparation time. Here are some advanced features of the V-4300B High Resolution n-m-r • • •
Spectrometer:
Exclusive Varian sample spinner permits fine separation of closely spaced resonances. Complete Varian magnet system available to provide unsurpassed field stability and homogeneity. New console mounting puts all controls within easy reach of operator.
THIS IS N-M-R AT WORK JH1K5RUM
OF UNOUJÇ ACID
DATA
Sample: 25% Linoleic acid in CCU Volume: 0.03 ce. i
ofsuchagroupml.noleicacKlisrewa.ea
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peak Β in the n-m-r spertrum Peatt , CH> correspond respectively ' ° ^ X „ d , aliphatic CHS groups adjacent to ont)» ο»· £ ™ * ° ° s while far off groups, and the ter mina m t h y u r » Ρ resonariCe to the left of the trace, the ca'ooxy, ρ ^ ffom
Signal Observed: H
Frequency: 30 mc. Field: 7050 gauss SweepRate:1.5milligauss/sec,
would · Ρ Ρ « ' · ^ Μ ί d hnolenic acids, and S r a S 0 a U n t n c h e S m i c a l " n the paint industry. F
B2AÎ
+Λ
+2 FOR C O M P L E T E
INFORMATION...
On the Varian V-4300B High Resolution n-m-r Spectrometer and associated Magnef System, write the Special Products Division for:
THE MARK OF LEADERSHIP
VA
V A R I A N P A L O
A L T O
4,
_. n _ . . • n-m-r Table
associates C A L I F O R N I A
MICROWAV Ε Τ U BES — SPECI AL PRODUCTS. For further information, circle number 36 A on Readers' Service Card, page 39 A
36 A
Rotating Mirror Frame Camera The famous million frame per second camera has been in use for 5 years at the Los Alamos Scientific Laboratory, primarily for the investigation of ex plosive and related phenomena. It has been described in considerable de tail recently by Berlyn Brixner [/. Opt. Soc. Amer., 45, 876(1955)]. The design and construction of this rotating mirror frame camera are described. Twenty-five consecutive pictures 20 mm. in diameter can be obtained on a strip of 35-mm. film. A resolution of at least 20 lines per mm. is obtained on a moderately fast film. Since the "seeing" time of the camera amounts to only 12% of its cycle of operation, accurate synchronization and the use of explosive initiators are required. A number of photoelectric trigger and pulse generating circuits are available for the rapid firing of electric detonators and synchronization within 2 micro seconds. The camera employs a two-faced rotating mirror driven at 5000 revolu tions per second and a refocused re volving light beam. There are 25 fixed framing lenses in a 90° quadrant or 100 per circle. To avoid multiple exposures a very fast shutter is re quired. The shutter is made by det onating a primacord explosive fuse at a point near the window of the camera. The explosion of the primacord pro duces an opaque cone-shaped shock wave enclosing a smoke cloud. This opacity persists until the smoke is dis sipated by convection or by shock waves from the phenomenon under investiga tion. Other cameras of this kind have been made with picture sizes ranging from 6 to 35 mm. and number of frames from 48 to 170 and with frame frequencies from 5 Χ 104 to 1.5 X 10' per second. Synchronization difficulties can be ex pected to increase in proportion to speed and number of desired frames. ANALYTICAL
CHEMISTRY
