Pick the Vacuum Pump that Fits Your Needs!
Pal. No. 2,337,849
DUO-SEAL VACUUM
PUMPS are designed to fit almost every application better and at less cost . . . in the laboratory, industry and school. Ten different models to choose from... capacities from 21 to 1400 liters per minute . . . ultimate pressures from 2 x 10- 2 mm Hg down to 1 xlO—4mmHg and better . . . two-stage and singlestage mechanical pumps with vented exhaust, and mechanical and diffusion combination pumps. Over the years, Welch Duo-Seal Pumps have built a world-wide reputation for best performance, greatest reliability and quietest operation. When you use a Duo-Seal Pump, you're using the best!
,. . , For complete specifications and prices, ask for Duo-Seal Catalog
t SINCEI THE WELCH SCIENTIFIC COMPANY \ 1880 L ^ 1515 S e d g w i c k St., D e p t . 911, Chicago 10, III Circle No. 112 on Readers' Service Card 36 A
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ANALYTICAL CHEMISTRY
REPORT FOR ANALYTICAL CHEMISTS filiation or crystallization was required before assessment of a reaction product could be made. In spite of all this effort it was found in m a n y cases, whether through azeotroping or co-crystallization, t h a t some fractions could still be complex mixtures. Initial qualitative criteria for the purity of these fractions such as distillation temperature, melting point, or refractive index were misleading in a number of situations. With the advent of the chromatographic techniques such as G L C and thin layer, a tremendous simplification of the variables problem is now possible; qualitatively for the organic chemist, and qualitatively and quantitatively for the analytical chemist. In these laboratories as early as 1955 low temperature gas chromatography was modified to include a 350° C. flash heater and a 300° C.-column heating system. The technique has been used not only for the quantitative determination of components including isomers in mixtures, b u t also for p y rolysis kinetics. Non-aqueous reaction kinetics have been investigated using GLC in those systems where a t least one of the two starting materials, and one of the products are of low retention in the selected column system. In this m a n ner a complete analysis of components could be obtained a t any stage of the equilibrium. With more modern equipment it has been found possible to chromatograph multifunctional compounds some with melting points in excess of 250° C. G L C also was found to be applicable to direct plant sampling and analyses of highly reactive chemical compounds. By combining quantitative thin layer chromatography (for non-distillable components) and G L C , it became possible to completely assess a mixture of Sarin, other phosphonofiuoridates, chloridates. esters, and acids in a fraction of the time previously required for their analysis. In this case the information including isolated compounds (as discussed previously) was already available for "plugging" into the new techniques. However, when applied to more recent work, experience has shown t h a t analytical chromatographic techniques including preparative
separation systems (GLC and column) have resulted in a tremendous decrease of research time for the solution of process analytical problems. This is especially true when these techniques are used in conjunction with elemental or functional group methods and spectrophotometry. Paper chromatograms and electrophoregrams are being analyzed directly on the surface, or after elution, by colorimetry. Thin layer plates can be used directly for detection via reagent spray or ultraviolet lamp, and quantitatively when the bands are scraped and the absorbant removed by centrifugation or filtration. Under controlled conditions estimates can be made within the accuracy of spectrophotometric methods. Another separative technique t h a t was initially designed for biological materials is the countercurrent distribution method. Low molecular weight materials have been successfully separated from heterogeneous mixtures for further identification. To cite just a few examples, it was found t h a t the phenol and m-cresol content of a complex bacteriological mixture could be determined when the conventional sulfanilic acid method was used in conjunction with countercurrent distribution (27), and 2,4-dichlorophenoxyacetic acid in earth could be determined by solvent extraction, countercurrent separation, and ultraviolet absorption (28). Both of these analytical problems, although difficult at the time of the study, could have been solved very readily if thin layer and gas liquid chromatography had been available. Polarography, conductivity, and potentiometry, while not new techniques, are relatively recent when applied to the analysis of organic compounds, especially in non-aqueous system. The non-aqueous application to analysis has shown some promise in areas where polarography-amenable organic, or organometallic compounds are very unstable in water. In these cases an electrolyte such as lithium chloride or a quaternary amine salt is added to the organic solution. Conductimetric titrations have been found very useful when a conductor salt is the product, or when a conducting system is converted to
