INSTRUMENTATION N e w monograph offers wealth of information on vapor phase chromatography
Tphase
ECHNIQUES
in
the
field
of
vapor
chromatography are now so highly developed and there are so many excellent instruments for the automatic separation and analysis of complex mixtures that we are inclined to leave these matters to the experts. The Spring meeting of the AMERICAN CHEMICAL SOCIETY held in Dallas, Tex.,
left no doubt in anyone's mind about the great interest which this subject has aroused. It is particularly fortunate and timely that an excellent monograph has appeared on the subject. Our good friends, Courtenay Phillips. Fellow of Merton College and University Demonstrator in Inorganic Chemistry at Oxford, has sent us his monograph on "Cas Chromatography," Butterworths Scientific Publications, 88 Kingsway, London, W. C. 2 (1956). This 105-page monograph contains an astonishing amount of information and is liberally illustrated with pertinent diagrams and data. As the author states, "This book is intended to give a broad survey of the work in the field, but is especially designed with the object of assisting those who are using, or planning to use, gas chromatographic techniques themselves." The first chapter is concerned with definitions, an historical outline, a comparison with liquid chromatography, and a comparison with other analytical methods. Chapter 2 treats the theoretical principles such as retention time, theoretical plate treatment, competitive and cooperative effects, displacement analysis, and frontal analysis. Chapter 3 describes general apparatus for gas chromatography. This is particularly useful for the investigator who wishes to set up his own equipment and it contains almost all the information which he will require. There is no reference to commercially available instruments, and this is readily understood. Most of these were being developed while this monograph was in preparation. Despite the many elegant machines which are now available in the United States, practically all of them use the same principles and employ the differential thermal conductance cell for detection. VOLUME
Chapter 3 is, therefore, stimulating and useful in that it is not confined to what one may regard as standard practice. In the same sense, Chapter 4, dealing with vapor detectors, presents numerous techniques for detecting or characterizing the effluent. It includes a discussion of the relative advantages and limitations of the automatic recording buret, integral detectors such as the nitrometer, thermal conductivity cells, the gas density balance, the infrared analyzer, the hydrogen flame detector, and others. The surface potential detector, originally suggested by G. Phillips, / . Sci. Instr. 28, 342 (1951), is also described. This device has been investigated extensively by the author and his students at Oxford. The remarkable specificity which can be imparted to this detector by appropriate chemical treatment of the electrodes is fascinating and is of great interest aside from its practical value as a detector. We learned there for the first time of Scott's hydrogen flame detector. Hydrogen is used as the mobile phase. The exit gas from the column is burned at a small vertical jet, and a thermocouple junction is placed so as to be slightly above the normal hydrogen flame. When an organic vapor is present in the gas, the flame lengthens and engulfs the thermojunction. The output from the thermocouple is fed through a suitable potentiometric network to a recorder. A globule of alloy at the junction, of about 1.5-mm. diameter, increases the thermal inertia of the couple and increases the zero stability. Although the theory of the detector is not yet clear, it does depend upon the heat of combustion and rate of burning of the vapor. A linear relationship has been found between peak area and weight of a hydrocarbon vapor producing the peak. In its present form, the device will easily detect down to about 4 γ of a vapor. Chapters 5, 6. and 7 deal with gasliquid partition chromatography, separation efficiency in gas-liquid parti tion chromatography, and gas adsorp tion chromatography, respectively. Chapter 8 discusses the determination of distribution functions. Appendix I covers the properties of binomial distri
2 8, N O . 9, S E P T E M B E R
1956
by Ralph H. Müller
bution, and Appendix I I discusses the correction for gas compressibility. There are 81 literature references and a subject index. This excellent book is indispensable for anyone contemplating work in vapor phase chromatography. Likewise, it would seem to be required reading for the student who hopes to acquire an appreciation of modern analysis. It should find wide adoption in industrial and academic circles.
High Efficiency Light
Sources
Some startling results have been achieved in producing high efficiency light sources by means of microwave excitation. Although electrodeless gas discharge tubes have been in use for a long time, the efficiency with which electrical power is converted to luminous power has left much to be desired. According to Forrester, Gudmundsen, and Johnson, J. Opt. Soc. Amer. 46, 339 (1956), the efficiency of micro wave excitation is "amazing." They quote Jacobsen and Harrison as finding a 16-fold increase in intensity of the Mercury 5461 A. line at 3000 mc. over that obtained at 150 mc. for 40 watts of exciting power. The authors found that with an oscillator delivering about 1000 watts of power at 70 m c , they were never able to secure an intensity of the mercury green line of more than 0.002 watt per sq. cm., whereas at 2450 m c , intensities of the order of 0.025 watt per sq. cm. were attained at 95 watts of power. These high efficiencies were obtained by mounting an electrodeless discharge tube in a section of the inner conductor of a tunable length of coaxial line. This provided essentially perfect match ing between the oscillator and the light source tube, and permitted operation in 47 A
instrument abstracts
Cary Applied Physics
Corporation/Pasadena/California
Radioactivity Measurements Made Faster, Cheaper with Vibrating Reed Electrometer Measurement of radioactivity in radioisotope determination, reactor control, air contamination studies, oil well logging·, and other problems in volving precise measurement of small currents, voltages and charges, such as precise pH determination and mass s p e c t r o m e t r y , c a n n o w be m a d e faster, simpler and cheaper by using the Cary Model 31 Vibrating Reed Electrometer. Unusually high sensitivity plus high zero stability and ease of instal lation and operation are responsible for the greater speed and savings. The Model 31 detects as little as 10- 1 7 amperes, and measures up to ΙΟ -6 amperes with a precision of 1%. Zero drift is less than 0.2 mV in 24 hours and less than .02 mV per hour.
The Model 31 can be used in any laboratory and does not require costly, vibration-free mountings or other special conditions of installa tion or operation. For additional in formation on the Model 31, write for bulletin AC-17 today. I t gives you de tails on applications, references, per formance, o p e r a t i n g principle, speci fications, modifications, accessories. 'Wilzbach, Brown. Kaplan, Science. 118, 522-523 (1953) Wilzbach,Van Dvken. Kaplan.Anal. Chem..26.880 (1954) Wilzbach, Sykes, Science 120, 494-496 (1954).
C " , H 3 DETERMINATIONS SIMPLIFIED
One widespread application in which the Model 31 has been of par ticular value is in determining C 14 , H 3 , and S 3 5 . Wilzbach and his co workers at Argonne National Labo ratory have developed procedures* which simplify these determinations in a wide variety of organic com pounds. Samples a r e converted di rectly to a g a s suitable for measure ment with an ionization chamber and the Model 31. This simple procedure eliminates the necessity for use of a precipitate, with its inaccuracy and time-consuming, tedious preparation. Since as little as 10- 1 2 curies of radioactivity can be detected, use of expensive "tagged" materials can be greatly reduced, often enough to re turn the cost of the instrument in a relatively short time.
The Cary Model 31 Vibrating Reed Electro meter is capable of detecting a current as small as 1.0 X 10-1? amperes originating in a high impedance source. Charges as small as 5 X 10- ' *» coulombs and voltages as small as .02 mV can be measured.
CARY MODEL 36 VIBRATING REED AMPLIFIER IMPROVES MASS SPECTROMETER PERFORMANCE The Cary Vibrating Reed Amplifier, Model 36, is being used in an increasing number of mass spectrometer installa tions where high molecular weight anal yses make rapid scanning of m a s s num bers desirable. The Model 36 combines rapid response with high sensitivity. Response is critically damped, with an 0.1 sec. natural period (98.6 percent re sponse in 0.1 second). Thus a range of 100 mass numbers can be accurately scanned in as little as one minute. Sen sitivity and range are such that as little
as 10-15 amperes and up to 10-11 am peres can be measured to a reproduci bility of 0.2 percent without change of range. The stability of the Model 36 is superior too-zero drift is less than 10-15 amperes. The Model 31 is preferred for mass spectrometer applications where ex treme response speed is not required, such as isotope determinations. Sensi tivity of the Model 31 is 10-17 amperes, and like the 36 it has high stability-less than 5 χ 10-17 amperes zero drift.
Cary instruments SPECTROPHOTOMETERS VIBRATING
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48 A
INSTRUMENTATION a cavity of small diameter. This is of advantage where it is desired to run the light source between the poles of an electromagnet. To avoid wastage of power incurred when an attenuator is placed between the oscillator and the load, the oscillator was protected by a cutoff relay activated by reflected power. Of the many types of mercury discharge tube which were used, the most efficient was a quartz capillary 1 X 4 mm. in cross section, containing a very small amount of mercury. Smaller capillaries, down to 1 mm. in inside diameter, were unsatisfactory because the quartz would melt and seal itself off internally. At the very highest intensities, the mercury green line was no longer sharp. At maximum output, the line width, although greater than 1010 c.p.s., still looked sharp when observed visually through a spectroscope. However, un der conditions for which the line width was only 8 Χ 103 c.p.s., the intensity had dropped by a factor of only 6, yielding 0.004 watt per sq. cm. At high intensity levels, the authors found it necessary to use argon in the tubes at a pressure of approximately 4 mm. of mercury in order to prevent blackening of the tubes. The phenomenon is most interesting aside from its utility in providing high light intensities. One wonders to what degree the beam is modulated by these high frequencies. It does not follow that the per cent modulation should persist as frequency goes up. Un doubtedly, fairly high vapor pressures of mercury are developed. An oscillo graphic study of the phenomenon would be very interesting. During World War II, we had occasion to use this phenomenon, but for another purpose. We succeeded in measuring power very conveniently in a high power radar system by mount ing a neon-filled capillary tube in a slot in the wave guide. A simple phototube circuit measured the lumi nosity of the discharge tube and could be correlated accurately with more in volved water absorption measurements of power. Our achievement was short lived, however, because as the carrier left her dock, a sailor dropped a wrench on the neon tube and the fragile spares.
Differential
Refractometer
Phoenix Precision Instrument Co. of 3803-05 North 5th St., Philadelphia 40, Pa., has an elegant automatic re cording differential refractometer which plots refractive index changes in any nonopaque liquid stream, such as a still output or chromatographic eluate. The differential glass cell has a maxi mum holdup of 1 ml. A variable ANALYTICAL
CHEMISTRY
INSTRUMENTATION
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monochromatic light source permits operation at wave lengths of 406, 436, 546, and 578 mu. The limiting sensitivity of the refractive index difference is about 3 units in the sixth decimal place. The chart drive in the recording potentiometer is controlled by the volume of the sample passed. We hope to secure more detailed information on the construction and circuitry of this useful instrument for discussion in a later column. Microradiographs Notable advances are being made in high resolution microradiography using very low voltage x-rays. An interesting contribution has been made by Engstrôm and Grenlich of the Karolinska Institutet in Stockholm, Sweden, / . Appl. Phys. 27, 758 (1956). With their technique specimens no more than 4 microns thick are placed in direct contact with a "grainless" photographic emulsion and then exposed within the x-ray tube to soft x-rays. The resulting small x-ray image, the microradiograph, is then examined directly in an ordinary microscope or can be enlarged greatly by photomicrography. Successful microradiographs have been obtained with potentials as low as 200 volts, although the potential suitable for studies of single cells or of thin tissue sections is about 1000 volts. Light from the x-ray tube filament is filtered out by means of a 1000-A. thick aluminum window. Equations are given for computing the optimum wave length, and hence tube voltage, for securing pictures of maximum contrast. This can be calculated in terms of the relative mass absorption coefficients for different components of the sample. When monochromatic x-rays, suitably selected in relation to the characteristic absorption edges of the elements, are used, the technique allows the determination of the elementary composition of the sample. On the other hand, if a soft polychromatic x-ray spectrum is used, sample weights as small as 10~14 gram can be measured to within 10%. The absolute resolution of the x-ray image is comparable to that of the light microscope. Some time ago, we described equipment for microradiography which is manufactured by North American Philips. Although the above described technique has been applied primarily to biological studies such as cell mitosis, it would seem to have extensive analytical applications. When one is dealing with x-rays of the order of 50 A. wave length, it is at once evident that this brings one into the range of the very light elements which, in general, are not accessible to harder x-ravs. ANALYTICAL
CHEMISTRY
