INSTRUMENTATION Portable apparatus analyzes multicomponent mixtures by fractional separation of vapors in partition column
ι Ν M E past whenever we lectured on J- the future of instrumentation in chemical analysis it was almost inevita ble that the presiding officer would say, "from what the speaker has told us, it will not be long before the analyst will be able to drop a sample into a hopper and have the analysis come out printed on a piece of paper." However kindly and appreciatively intended, this remark always provoked a gale of laughter as if it effectively showed the reductio ad absurdum of the situation. We were often impolite enough to reply, "It should have been done long ago!" Every experienced analyst realizes
how very far we are from that type of automatic analysis, but even the casual reader can note the astonishing prog ress which is being made in that direction. It is obvious to all of us that almost any standard series of analytical operations can be mechanized and that the resources of electronics, servomechanisms, and related tech niques are adequate for the purpose. In some instances this could lead to elabo rate and even foolish gadgetry, the only justification for which might lie in the need for a large number of simple repetitive analyses. The opposite and highly successful approach recognizes that we have resources for the measure ment, control, and recording of almost any variable. The question then is— What can we measure or control which will give us a new analytical tool? Twenty-five years ago this would have been a fine example of putting the cart before the horse. In conservative cir cles it is still so considered. Fractional Separation of Vapors on Partition Column For some time, and in several of these columns, we have urged closer attention to some of the less conven tional aspects of chromatography, most of which are being pursued in Great Britain. The work on vapor-phase chromatography by Martin and his associates and that of Phillips and his
Figure 1.
by Ralph H. Mutter
students at Oxford have fascinatingpossibilities. It is with great satis faction that we find some valuable American developments in this field. Through the courtesy of Fred Wittner Advertising and the staff of the PerkinElmer Corp. we are informed that the Vapor Fractometer, a simple, low-cost, portable apparatus for the analysis of multicomponent mixtures of gases or volatile liquids, is now in production by by the Perkin-Elmer Corp., Norwalk, Conn. The new apparatus utilizes the fractional separation of vapors by means of a partition column. The effluent stream, with its constituent vapors appearing sequentially, is moni tored by a differential thermal con ductance cell and the analysis is pre sented directly on a recording potentiom eter. If the reader can tolerate oui· exuberance, which we hope is conta gious, these recordings are a delight to behold. Figure 1 shows a general view of the instrument, which weighs less than 50 ACETONE
Vapor Fractometer
Figure 3. Figure 2VOLUME
ETHYL ACETATE
Diagram of operation
2 7, N O . 6, J U N E
1955
P e r f o r m a n c e of i n s t r u m e n t
8 - c o m p o n e n t m i x t u r e . T e m p . , 50° C. Flow r a t e , 90 ml. per m i n u t e . Pressure, 20 lb. C o l u m n 4-foot, '/.i-inch d i a m e t e r . Carrier gas helium
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INSTRUMENTATION
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 a i l controls within easy reach of operator.
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FOR C O M P L E T E I N F O R M A T I O N . . . . . , · w .., ΛΛ η ... ι π ι • ~ . On the ναπαη V-4300B High Resolution n-m-r Spectrometer and associated Maqnet System, write the Special Products Division for: ° ' / ι THE
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pounds and is only 30 X 20 X 12 inches in size. Figure 2 is a block diagram showing the principle of operation. An inactive gas, preferably helium, is drawn from a tank, G, and reduced to an appropriate flow rate by regulator R. The sample should be introduced at a point as near the column as pos sible, A. A more easily controlled sample introduction scheme is afforded by the control valve which can be turned to the sampler S, wherein a liquid specimen in a capillary tube can be placed, or a small capsule containing a gas. The column is a temperaturecontrolled glass tube VVinch i.d. which is U-shaped, either 4 or 8 feet long. The column material consists of a mixture of 25% liquid with 75% of a supporting solid which is a small particle-size sieved diatomaceous earth (Celite). Packing of the column is done by means of a vibrating tool, so that maximum uniformity and com pactness can be obtained. The sensing detector, D, is directly connected to the exit of the column in order to minimize reshuffling of the components as they come off the col umn. The detector volume (0.5 cc.) is also kept to a minimum for the same reason. A flowmeter, F, at the exit of the sensing side of the detector is used to observe flow rates. The flowmeter covers a range of 0 to 200 ml. per minute. Both sides of the detector and the column are enclosed in a thermostated air chamber, T, accessible by means of a door. The temperature of this air bath can be set to any desired value in the range of room temperature to 150° C. and then held to 0.5° C. or better. A preheating circuit and blower are incorporated in the instrument to permit rapid changes in temperature setting. The detector circuits consist essen tially of a current adjustment for the two thermistors which are operated by standard batteries which are set to 3.5 volts. Balance and range adjust ments are also provided for setting the instrument to proper zero and full scale deflecting conditions. The out put of the special bridge circuit used is then directly connected to a standard 10-mv. recorder without need for inter mediate amplification. In developing this instrument, ex perience showed that an elution prin ciple combined with a partition liquid in the column was the best compromise between such alternatives as displace ment or straight adsorption. Figure 3 is a striking example of the performance of this instrument. While it is largely self-explanatory, it may be mentioned that the area under each component peak can be obtained in two simple and economic ways. The area ANALYTICAL
CHEMISTRY
INSTRUMENTATION
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can be approximated by multiplying the peak height by its half-band width, which gives very satisfactory results. It was also found that the retention times—i.e., distance of peak from the starting point— were somewhat pro portional to these half-band widths. Therefore, one can also obtain the area by multiplying the peak height by the retention time. This brief summary of the Vapor Fractometer has been derived from an extensive report by H. H. Hausdorff of the Perkin-Elmer Corp. More de tailed information will undoubtedly be forthcoming from this source. The Vapor Fractometer will find use wherever mixtures of gases or low boiling liquids are analyzed routinely. The method may also be used for the separation of unknown components from a mixture, which can then be qualitatively analyzed by infrared or mass spectrometry, or other means. Time for analysis varies from 0.5 to 5 minutes per component. The repro ducibility is better than 0.5%. As little as 1 mg. of sample per analysis is required. A few other characteristics or speci fications may be of interest: Amount of sample for full scale deflection : gases, 0.1 mg.; low boilers (25° to 50° C ) , 1 mg.; medium boilers (50° to 100° C ) , 5 mg.; high boilers (100° to 200° C ) , 10 to 20 mg. Sample volume required for multicomponent mixtures: liquids, 5 to 20 μΐ. gases, 100 to 500 μΐ. Op timum speed, 2 to 3 minutes per com ponent. Stability and noise, 0.25% of full scale. Reproducibility, 0.25 to 0.50% concentration even if flow rates, temperatures, and sample quan tities are varied over a wide range. Resolution (retention time Va band width), 15 for a 4-foot column length. For qualitative analysis the abscissa values for peaks are characteristic of the component and independent of other components present. For quan titative analysis, the area under each peak is proportional to sample quantity. Perfect symmetry of bands permits obtaining area proportionality by mul tiplying peak heights by half-band widths (or retention times). Thermal conductivity corrections can be neg lected if helium gas is used as a carrier. A large cylinder of gas can be used for 4 months with average daily use. This costs $16 for nitrogen or $18 for helium. The instrument is insensitive to vibration. The tentative price of the Vapor Fractometer is $1500 and deliveries are scheduled for September 1955. We congratulate Perkin-Elmer on this splendid example of automatic analysis. I t is bound to find many uses and applications.
For further information, circle number 36 A on Readers' Service Carl, page 39 λ
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