have on these peaks. Figure 7 shows that the system using 1.5 times the stoichiometric amounts of CL gives a ratio of the 120' C. peak to the 350" C. about twice that of the system employing a stoichiometric amount of CL. This peak ratio was not noticeably affected by a change in concentration when MA was used. This indicates that when larger than stoichiometric amounts of the two polymerizing agents were used, CL effected the cleavage of more epoxy rings than did MA. Figure 8 gives typical DTA curves for Epon S28/MA and Epon 828/CL for samples which were extensively cured and finely pulverized before subjecting to DTA. The low temperature exothermic peak has disappeared, but the usual high temperature one is still present for each of the systems. This indicates that it is very difficult for a stoichionietric amount of either M A or CL to wart with all of the epoxy groups. This is cvidently due to the decreasing mobility of the molecules as well as to the decreasing concentration of the reactive groups. I n Table I, comparison of the uItimate weight losses involved in the UC Ilntlo isomcr,'CL and UC Endo isomer/ MA q.atcms serves to corroborate the indication of the corresponding DTd curves (Figure 4) that X d n-as more reactive than CL. The n-eight loss of the former system was almost the same :is that of thcx uncatalyzed IIJ..,Polymer Sei. 28,447 (1958). ( 1 0 ) Ibid., p. 453. ' 1 1 ) Parker, R. E., Issacs, N. S.,Chem. Revs. 59, 737 (1959). (12) Peytral, E., Bull. SOC. chim. 39, 306 (1926). (13) Smothers, Is'. J., Chiang, T.,"Ilif-
ferential Thermal .Inalysis," Chemical Pub. Co., Kea York, 1958. RECEIVED for review February 15, 1960. Accepted June 21, 1960. Presented i n part, Division of Paint, Plastics, and Printing Ink Chemistry, 136th Mreting, ACS, Atlantic City, N. J., September 1959.
Trace Analysis Summer Symposium of the Analytical Division, Houston, Tex., 1960 Summary prepared b y WARREN
T
W. BRANDT, Department of Chemistry, Purdue University, Lafayette, Ind.
1960 Summer Symposium sponsored 11). ASALYTICAL CHEJIISTRY and the Division of Analytical Chemistry was devoted to n e x approaches to an old analytical prob1c.m: trace analysis. Each of five half-day sessions was devoted to a separate instrumental approach t o the determination of trace constituents. The coverage included a survey of past usage of the technique to determine trace amounts, followed by a look at the new developments and sonie specific applications. The number HE
of sessions-five-is grossly inadequate to cover all of the techniques which are anicnable to application in trace analysis. Each represented an area in which recent advances appear to be improving its utility for determining small percentages of material-inorganic and organic. X-RAY EMISSION SPECTROGRAPHY
H. A. Liebhafsky, General Electric Co. and chairman of the Division of
Analytical Chemistry, opcwd the program with an introduction to trace analysis by x-ray emission spectrograph!.. He pointed out that x-ray emission spcrtrography has become a powerful mrthod for trace determinations, mainly for tn o reasons. X-ray spectra are simple arid methods of x-ray detection have improved to the point where intensit!, measurements can be made easily and reliably by counting the quanta emitted by elements present in microgram or even smaller amounts. VOL. 32, NO. 12, NOVEMBER 1960
1595
E. L. Gunn, Humble Fksearch Laboratories, reviewed the great diversity of successful applications for x-ray emission spectrography in the petroleum industry. The use of a low-atomicnumber diluent, such as a mixture of lithium carbonate and starch, to reduce absorption and enhancement effects was described, as was the determination of platinum in reforming catalysts. The spectrography of “spots” is now being investigated at Humble as a method of determining traces of iron, nickel, and vanadium in catalytic cracking feed stock oils. A &gram sample of oil is charred and the trace metals are eventually concentrated in a spot on filter paper by evaporation of a hydrochloric acid solution. Micrograms of the metals can be determined with good precision, and the method consequently reaches well down into the parts-permillion range. The direct determination of nickel in hydrocarbons near the 0.1-p.p.m. range was reported by W. H. King, Jr., Esso Research and Engineering Co. In an investigation noteworthy for the very high ratio (500 to 1) of background to analytical-line intensity, it proved possible to obtain reliable results by using a special spectrograph that featured simultaneous integration of background and analytical-line intensities produced by a common x-ray source, curved crystals for maximum intensity, and long counting times. With the scattered background very near (within about 0.01 A.) the analytical line serving as reference standard, changes in the hydrocarbon matrix were satisfactorily compensated. The accuracy obtained provided reliable results at the 0.1p.p.m. level. By relating trace determination to the determination of film thickness, H. A. Liebhafsky, General Electric Research Laboratory, showed why these determinations become simpler as the sample size decreases. A spot on paper or Mylar resembles a film upon a substrate. Such samples are ideal for establishing whether the ultimate precision set by counting statistics can actually be realized, and for testing the law of combining errors. Both conditions having proved applicable to zinc and strontium spots in the microgram range, it was possible to show that reliable conclusions could be drawn about the presence of manganese in spots containing &s little as 0.003pg. of the element. RADIOACTIVATION ANALYSIS
The radioactivation analysis session featured papers citing the progress of radioactivation analysis in, present-day research especially concerned with trace element behavior. The information given in each paper attempted to satisfy such questions as “How practical is it?” 1596
ANALYTICAL CHEMISTRY
and “Aren’t the skill and equipment required to complete an analysis beyond most analytical chemists and their laboratories?” Some perspective as to the place of radioactivation analysis in analytical chemistry was achieved. I n general, radioactivation analysis can be applied to the determination of most elements. It is very specific, since the induced radioisotopes for these elements are never exactly the same with regard to half-life and radiation characteristics, so that identification of each radionuclide and its origin and subsequently the determination of the amount of stable element reacting with nuclear particles to produce the radioactivity can be made. Potentially, radioactivation analysis is probably more consistent than any other analysis method in its approach to being a very sensitive analytical tool. For some elements, it has been possible to determine microgram amounts by the use of a low intensity neutron source (of the Ra Be type) having only a neutron flux of 106n/sq. cm./second. For most of the other elements, irradiations in neutron flux areas of a nuclear reactor having at least 1014n/sq. cm./second have shown that as few as 1O’O atoms of an element can be determined. I n all applications, contamination effects from environmental and other external sources during the analysis are negligible, unless the contamination has occurred before the irradiation. Although nuclear reactors and cyclotrons have been used most frequently in radioactivation analysis, other types of nuclear particle-generating sources are coming into use in certain laboratory areas. Low-intensity neutron sources made of Ra-Be, Pu-Be, or Am-Be are now used. Van de Graaff accelerators and modified Cockcroft-Walton apparatus (fast neutron generators) are available and are being used in a t least semiroutine applications. I n general, the costs of equipment are comparable to those usually associated with spectroscopy instrumentation. As far as ability to undertake a radioactivation analysis, there appears to be no real difficulty in training personnel. Most analysts now engaged in radioactivation analysis have had previous experience in conventional laboratories. Sample handling and processing of sample materials are common to most ordinary laboratory practices. Radioactivity measurements and radiation safety are probably the only criteria thal require some instruction. Most radioactivity analyses are completed by either beta or gamma counting and most laboratories have access to this type of equipment. Radiation safety requires a judicious handling of the irradiated materials in order to assure personnel safety and uncontaminated iaboratory environments.
Besides being a usable tool for trace elements, radioactivation analysis is finding application to problems concerned with macro element concentration. It has also been used to determine particle size distributions, grain-boundary segregates, isotope compositions, and metal and oxide film thickness, and in element diffusion studies. G. W. Leddicotte, Oak Ridge National Laboratory, described some of these applications and showed that, in most instances, samples could be handled in a routine manner through the use of modern ideas in radiation detection and multisample irradiation techniques. The increasing application of activation analysis in the petroleum industry was itemized by V. P. Guinn, Shell Development Co. Published results include the determination of a variety of metals in many problems of interest. One example cited was the determination of very small traces of arsenic in platformer seeds. Routine activation analyses are now carried out for both micro and macro constituents. The more recent developments discussed cover three areas: instrumental methods based on y-ray spectrometry, the use of accelerators as neutron sources, and the extension of activation analysis to routine macro-level elements. One of the examples cited was the routine determination of beryllium by the Shell Development Co. using a 3-M.e.v. Van de Graaff electron accelerator as a thermal neutron source. Applications in the more specialized area of inorganic geochemistry were presented by J. W. Winchester, Department of Geology and Geophysics, AI. I. T. The analytical problems in this area stem from several sources : the chemical inertness of silicate minerals, the lack of satisfactory methods for nonmetallic constituents, the trace nature of most elements in the earth’s crust, and the increasingly frequent need for the determination of isotopic composition. Radioactivation analysis presents a promising general approach to these problems. Sensitivity, wide applicability, and lack of interference make it particularly useful. Many activating particles may be used. The availability of this technique presents considerable promise as a valuable adjunct to other methods in rapidly advancing many areas of geochemistry. The presence of large amounts of sodium and chloride in biological fluids was cited by J. H. Dowling, U. S. Xavy, Bethesda, Md., as a prime limitation in the determination of trace elements. Ion exchange techniques have been utilized to advantage. By running urine or plasma through a strongly basic anion exchange resin, chloride, bromide, and iodide are retained. The chloride
is then selectively eluted by ammonium thiocyanate. By activation of the remaining resin, iodide and bromide are determined with an accuracy t o 2 and 10 fig., respectively. R. E. Jervis, University of Toronto, discussed the application of radioactivation techniques to qualitative analysis in the metal industry. By combining systematic group separation with w a y spectrometry many special metals were identified. From the quantitative standpoint the method’s sensitivity aids in evaluating purification steps. The thickness of oxide films has been determined and various applications have been found in studies of metallic corrosion. In general, the symposium gave much information that could be used by other analytical chemists interested in the use of radioactivation analysis in their problems. I t could be concluded that radioactivation analysis has a very vital position in the field of analytical chemistry. EMISSION SPECTROSCOPY
Although this year marks the 100th anniversary of the discovery of spectroscopy by Bunsen and Kirchhoff, the frontiers of this valuable multielement method are continually being extended by new thoughts and developments. These refinements of technique now bring the level of trace spectroscopy to the “parts per billion” range, making it one of the most useful and convenient trace analytical tools at present available. George H. Morrison described a research program a t the General Telephone and Electronics Laboratories which has resulted in an emission spectrographic technique wherein the sensitivity for many elements has been increased by several orders of magnitude. Morrison revealed that refinements in each of the three major components of emission spectroscopy-the light source, the optical system, and the detectorhave yielded increased sensitivity, the most marked results being achieved through a better understanding of the volatilization-excitation processes taking place in the light source. This important increase has been achieved by better control of impurity volatilization from a solid matrix using a d.c. arc in an argon atmosphere. Better temperature control results from the combined use of high currents (15 to 23 amperes), an undercut carbon anode, and a flowing argon atmosphere. Optimization of exposure time to that just necessary for complete volatilization of the more volatile impurities, or minimization of exposure for the more refractory elements to that just necessary to produce a signal with the standard of lowest concentration, has led to substantial increases in signal to noise ratios. Increased spectrographic speed has
also led to increased sensitivity, and Morrison described results obtained with a new large-aperture (f/6.4) grating spectrograph. Typical results reported include the determination of mercury, phosphorus, boron, and other elements in semiconductor materials and high purity metals down to 5 parts per billion. Refinements in instrumentation to extend spectrographic sensitivity were described by A. J. Mitteldorf, Spex Industries, who stated that the adjustment of instruments to their optimum performance and the judicial use of accessories were often overlooked. For example, when a spectrograph is used a t higher orders and is focused quantitatively, signal to noise ratios as well as absolute sensitivity to radiation are increased. Trace analysis by flame spectrometry has been used for some time for the determination of a few elements; however, the use of the cyanogen-oxygen flame has greatly extended the applicability of the technique. According to Bert L. Vallee, Harvard Medical School, the high temperature of the source (4800’ K.) makes poss:ble the excitation and detection with high sensitivity of many elements not readily excited by conventional flames. Vallee described a new burner which has been developed, the characteristics of which are dictated by those of the flame. Various applications in analytical spectroscopy were presented. I n addition to efforts to extend the sensitivity of spectroscopy, oxygen, nitrogen, and hydrogen have now joined the list of elements which can be determined in metals by emission spectrographic methods but with a reduced time requirement. The key to the spectrographic method is that the lines of these elements show up with remarkable sensitivity in spectra of arc discharges between carbon electrodes in pure rare gas atmospheres. V. 4.Fassel, Iowa State University,-described a technique for extracting or evolving the oxygen content of a sample while a t the same time reducing the volatilization of the metal. The metal sample is placed in a cavity of a carbon supporting electrode which serves as the anode of a d.c. arc discharge in pure argon. The molten metal dissolves carbon from the electrode wall, which serves to reduce the oxide, nitride, or hydride impurity on the metal to form carbon monoxide, and molecular nitrogen and hydrogen, These gases are then dissociated by the arc discharge and excited to give their characteristic emission spectra. Using these techniques, the oxygen and nitrogen content of high alloy steels, and the oxygen content of niobium, vanadium, tantalum, copper, and nickel have been determined in the concentration range of 0.0005 to 0.5 weight %.
ELECTROCHEMICAL METHODS
W. D. Cooke, Cornell University, discussed the present status of electroanalytical chemistry and ventured the opinion that such methods are not as widely applied aa current research in the field would indieate. Some of the ressons proposed to explain this discrep ancy include poor presentation of electrochemical principles in our colleges, which often leaves the student with an uncomfortable lack of confidence in the field. Secondly, since electrochemid procedures are more narrow in scope than other techniques, such aa spectroscopy, their application does not immediately come to mind when faced with an analytical problem. Cooke presented some work on high sensitivity polarography, performed while on sabbatical leave at Oak Ridge National Laboratories. It was reported that a conventional dropping mercury electrode offers a much higher sensitivity than is generally realizeed. By electronically filtering out the drop polarographic wave, much more stable operation could be attained. Under these conditions, nonreproducible transients caused by “capillary noise” became the limiting factor in further extending the sensitivity. A discussion of the source of this phenomenon followed, and a slightly modified capillary waa found which was free of these fluctuations. Using these modified capillaries, it was possible to work at concentrations of the order of lO-’M, which is a hundredfold increase in sensitivity over conventional polarography. F. A. Scott, General Electric’s Hanford Laboratory, presented da% on a new technique called voltage scanning coulometry, which avoids some of the problems inherent in controlled potential coulometric methods. It differs from contmlled potential methods in that the voltage is scanned through the region of the half-wave potential while the current is recorded. The cell contains an electrode of relatively large area, and the solution is efficiently agitated so that complete reduction is attained a t the scan rates used. The technique has the advantage that blank corrections are simplified, and previously discharged species do not interfere, since they are completely removed from solution before the desired peak is recorded. In the case of ferric ion, the detection limit was 0.025 mg. of iron in a 5 m l . sample, but Scott felt that this might well be lowered. Irving Shain, University of Wisconsin, discussed various aspects of electrochemical stripping analysis. The basic principle of such methods involves the plating of an oxidizable or reducible species at an electrode surface aa a preconcentration step. Once an appreciable portion of the material has been accumulated, it can be suddenly stripped VOL. 32, NO. 12, NOVEMBER I960
1597
off the electrode by a change in voltage. This results in a very favorable signal-tonoise ratio which extends the sensitivity limits beyond those of most other electrochemical methods. Shain noted that a mercury electrode, such as a hanging drop, is often used in such work because of its favorable hydrogen overvoltage. Because of the limited volume of the hanging mercury drop electrode, the diffusion of metal into the mercury results in fairly concentrated amalgams with relatively short pre-electrolysis time. A mathematical evaluation of this diffusion process was presented. The high sensitivity of stripping analysis was illustrated by showing that large current peaks could be obtained for the heavy metals which were present in a 10-mg. sample of “spectrographically pure” potassium chloride. One of the limitations of stripping procedures is the difficulty in purifying necessary reagents. GAS CHROMATOGRAPHY
The currently popular subject of gas chromatography provided the concluding session of the programs because of increased application to the determination of traces. S. Dal Nogare, Du Pont, described his early work in the field, in which the usual type of chromatographic equipment was upgraded to provide morc sensitivity. He discussed the pertinent fact,ors to consider in obtaining such results. Primarily, an optimization of operating conditions is involved which permits the attainment, of the full sensitivity possible for katharometer detectors. The theoretical limitation based on thermal conductivities is usually not approached in normal operation. He illustrated his points with examples of direct parts per million determination of impurities in various organic products. ,J. J. Kirkland, Du Pont, has utilized conventional gas chromatographic equipment combined with preconcentration of the desirrd material to determine fracbional parts per million of biochemicals routinely in natural biological samples. Th(J inherent sensitivity and separating p o w r of the gas chromatograph permit very simple preconcentration steps to be used. This frequently provides considerable saving of time compared to alternative methods. He citcd an illustration of the determination of 0.3 to 0.6 p.p.m. of two herbicides in a variety of soil and crop matrices with good success. In addition, to the sensitivit,y obtained, t.he method distinguished between a herbicide pair prcvioiisly indistinguishable by other means. Another example dealt with the determination of less than 1 p.p.m. of a test biochemical in 3 ml. of animal blood.
1598
ANALYTICP.1 CHEMISTRY
On the basis of the limits of sensitivity now obtainable with ionization detectors, it becomes possible to consider determining parts per billion in a sample without any preconcentration. D. A. M. Mackay, Evans Research and Development Corp.! considered this alternative approach to high sensitivity gas chromatography. He had found that each of the commercial high sensitivity detectors has particular advantages which made it preferable for certain types of samples and problems. Each has certain sensitivities or insensitivlties to various types of compounds which provide these advantages. Ideally, a detector should have optimum sensitivity for the compound sought, while giving no response to the bulk of the sample. This situation is, in fact, obtainable in some circumstances. Mackay’s illustrat’ions were chosen from a wide variety of studies in the area of odor and flavor identification with and without preconcentration steps. I n cases in which the flavor or odor is due primarily to a simple or small number of constituents, striking results can frequently be obtained. Where the property brcomes a complicated function of a large number of constituents, the chromatograph may provide the separation and quantitative measurement,, but thc over-all problem may still bc very difficult. Richard Kicsrlbach. Du Pont,, presented two topics for consideration: the advantages of shielded thermiet]ors and an innovation in gas chromatographic theory. By shielding thermistors from the direct gas flow with porous nickel sheeting, sensitivity to flow disturbances was reduced by a factor of 100. This results in much improved detector stability. The improvement and refinrment of the theory of gas chromatography are topics of considerable current interest and importance. For the second subject Kieselbach discussed his recrnt verification of an extension of the Van Deemter equation by W. L. Jones, Du Pont. TKO new terms were introduced to explain bett’er the behavior of theoretical plate height with theory. These new terms involve gas diffusivity which has previously appeared in Golay’s theoretical treatment of capillary columns but has not, been used in packed column studies. The new form of the equation is H =A
+ ij/p + + Dp + Ep Cp
where €1, A , R, and fi are thc Van Deemter symbols. The C term is slightly modified, while D brings in the effects of diffusion through the moving gas phase to and from the liquid surface. E represents the effect of diffusion in the relatively stagnant gas within the porous packing. C and -0
become significant a t some particular value of k . the partition ratio. C is important for earlier peaks and insignificant for the late ones, where D becomes the main limitation in H . Essentially this demonstrates that gaseous diffusion is almost wholly responsible for the efficiency of the column for very early and for late peaks. This improvement in the understanding of the basis of co!limn efficiency points the way for improved operation of packed columns. Kieselbach is not completely satisfied and feels that still another term may be necessary before the theory is completrly adequate. This last term may ne11 bring the theory for packed and capillary columns very nearly into unity.
Corrections Spectrophotometric Determination of Small Amounts of Uranium with 8-QuinoIinol In this article by Kenji Motojima
et nl. [ A N a L . CHEK 32, 1083 (1960)], on page 1085, Table V. column 2, Uranium Sdded should read Amount Rare Earth Element Added.
Spectrophotometric Determination of Iron and Copper with Methyl-2-pyridyl Ketoxime and Their Simultaneous Determination in Mixtures I n this article [XNAL. CHEW 32, 1196 (1960)] on page 1196 the name of the senior author t.hould be D . Banerjea instead of D. K. I3anrrlen.
Determination of Aromatic Aldehydes by Near-Infrared Spectrophotometry In thi? article li? R 11. Poaers, J. L. Harper. and Ban Tal [ A N ~ L . CHEM.32, 1287 (i960)l on page 1288 the legmcis for Figurl. I an? 2 should be reversed.
