ANALYTICAL CHEMISTRY
1202 analysis of the data, however, involve several unique factors. When the same increments of potassium are added to the two series containing different amounts of unknown, and the areas of the resulting spots are measured, the rate of increase of the areas within the two series should be equal, and the plotted results should produce two parallel lines. It is therefore easy to devise a series of simultaneous equations which could be solved for the potassium content of the unknown. The equation presented is based on these considerations and provides a simplified method for calculating the result. The quantitation of results obtained by paper chromatography is extremely subject to errors due to the effects of foreign materials in the sample, and a method based on the principle of sample distribution employed here is essential for a suitable degree of accuracy, as pointed out by Berry and Cain ( 2 ) . The concentration of the extraneous materials ia a sample spot on paper may markedly influence the degree of spreading of any partirular constituent. Consequently, comparison cannot validly be made of the areas of urinary potassium spots with standard spots in which large concentrations of other materials are absent. I t is essential that the potassium standard be influenced in its “spreading” and migrating characteristics in the same manner as is the potassium in the unknown sample. This may be achieved by preparing the standard solution so that it contains representative amounts of the major constituents of urine. Such a procedure is analogous to that employed in the flame photometry of complex mixtures, where the reference standard is made up with foreign materials present so as to resemble the type of unknown under study. The use of the unknown itself as a base for the standard, however, presents R more accurate means of achieving this effect.
Sodium lead cobaltous hexanitrite has been previously used for the detection of potassium by microcrystallographic means ( 5 ) , under which circumstances ammonium, cesium, lithium, rubidium, thallium, arsenic, antimony, calcium, chromium, iron, magnesium, silver, and tin are reported to interfere with the results. Under the conditions of the test described here, none of these materials interferes, although some difficulty may result when these elements are present in very high amounts. Using the method described, potassium in concentrations of about 0.1% may be determined with an error of less than 10% on a sample of 0.075 ml. of solution. Although the accuracy of the present method is thus not so great as that of some other methods, it seems adequate for many analyses in which greater precision is not required, and where limitations exist on equipment, sample size, and time. LITERATURE CITED (1) Adams, M. F., and St. John, J. L., IND.ENO.CHEM.,ANAL.ED., 17,435-6 (1945). (2) Berry, H. K . , and Cain, L., Arch. Biochem., 24, 179-89 (1949). (3)Cotton, R. H., IND.ENO. CHEM.,ANAL.ED.,17, 734-8 (1945). (4) Folch, J., and Lauren, M.. J . Biol. Chem., 169, 539-49 (1947). (5) Frediani, H. A., and Gamble, L., Mikrochemie, 29, 22-43 (1941). (6) Kailmann, S., IND.ENG.CHEM.,ANAL.ED., 18, 678-80 (1946). (7) Keiley. 0. J., Hunter, A. S., and Sterges, A. J., Ibid., 18, 319-22 (1946). (8) Parks, T.D., Johnson, H. O., and Lykken, L.,Ibid., 20, 822-5 (1948). (9) Tinsley, J., Analyst, 74, 167-78 (1949). (10) Weaver, J. R., and Lykken, L., ANAL. CHEM.,19, 372-6 (1947).
RECEIVED December 27, 1940.
Road-Table Discusebn
FLAME PHOTOMETRY Digest of stenographic report of round-table discussion held by Division of’ Analytical Chemistry, 117th Meeting, A.C.S., Houston, Tex., March 1950 Moderator: W. G . SCHRENK, Department of Chemistry, Agricultural Experiment Station, Manhattan, Kan.
F
LAME photometry qffers excellent possibilities for the development of analytical procedures for those metallic ions which can be excited by the relatively low excitation levels available in the flame. The procedure is not new and has been used for years as a qualitative test for ions such as potassium and sodium. Interest in quantitative procedures, however, has increased rapidly in the past few years. This has been due, first, to the development and availability of instruments, designed for this purpose (I, 2 , 4 , B ) , and secondly, to the need for simpler and more accurate methods for the determination of the ions of the alkali metals. Particular attention has been given to the determination of sodium and potassium because of their importance in biological systems. Other elements, however, can be determined by this procedure. Of particular interest a t the discussion were calcium and magnesium. Although flame methods appear promising with regard to the determinations of these elements, any new technique involves certain difficulties not found in established methods. As a result discussion centered around two generral topiqs concerning this method of analysis. First, were factors hfluencing instrument stability and precision and, second, were the preparation of samples and calibration techniques.
FACTORS INFLUEYCING INSTRUiMEYT REPRODUCJBILITY
Included among the factors which govern the reproducibility of any given instrument are such items as the atomizer system, air supply, gas supply, and the stability of the electronic amplifier circuit, including the photocell. Discussion brought out the point that atomizer construction is critical, Atomizers of apparently identical construction require separate calibration and therefore are not entirely interchangeable. The atomizer described by Weichselbaum and Varney (6) was mentioned, but no performance data were available other than those described by them in their paper. Several people mentioned the need for extreme cleanliness in handling the atomizer. It was pointed out that if the atomizer does not drain properly erratic results will occur. Some workers clean the atomizer with distilled water and alcohol after each determination, followed by periodic cleanings with other materials. Others routinely clean after a definite number of determinations. It was also pointed out that the type of sample plays a role in determining the number of analyses which can be made before cleaning is required. Biological samples, in general, tend to reduce the number of times the atomizer can be used before thorough
1203
V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0 cleaning is required. All agreed that cleanliness with respect to the atominer is a critical point in flame photometry. Air supply also needs to be controlled. If the air supply in the laboratory is not clean it should be filtered. Dust particles may cause two effects: fist, they may clog the orifice of the atomizer, and secondly, they may cause luminous spots to appear in the Eame. Both effects are undesirable. Fumes from a chemical laboratory may @use difficulty also. One report was made of erratic behavior c,used by mercury vapor in the air. I t follows, therefole, that care is necessary with the air supply with regard to elements which aSe not usually considered in flame photometry. Adequate control of air, gas, and oxygen pressures is necessary also. I t has been shewn ( 1 , 3) that these factors influence the sensitivity of the analytical procedure and therefore not only need adequate control, but also must be easily reset to predetermined values. Diaphragm-type pressure regulators do not give as steady a Eow of gas as is required. If used as pressure reducers, they should be followed by some additional control such as a needle valve. Discussion also indicated that electronic amplifier tubes can cause errors in analyses. These errors are due in part to the lack of stability and uniformity of the tubes. Errors may also be due to variations in line voltage when instruments are alternating current operated. Tube errors can be minimized by buying on the open market and selecting by trial and error those that function best in the instrument. Line voltage variation can be stabilized in part by the use of “constant voltage” transformers. One reference was made to literature (3’) which reported on a modification of an exsiting electronic circuit which gave improved performance. Several other factors which were discussed briefly included atomizer chambers, the use of alcohol in the test solution, and surface tension effects, all of which appear to be related in instrument stability. The use of isopropyl alcohol in the test solution increases the sensitivity. The heated jacket on the atomizer chamber also incresses sensitivity and gives more reproducible results. The effect is apparently one of getting more of the saniple into the flame. The alcohol apparently increases the volatility of the sample and the heated atomizer chamber tends to prevent condensation of the atomized sample before it reaches the flame. Surface tension apparently affects the rate of atomization of the aample and consequently the sensitivity. This effect can be minimized by using dilute test solhtions, adding alcohol, and eliminating as far as possible the larger organic molecules which may be present in some cases. CALIBRATION AND SAMPLE PREPARATION
Special care is required in the preparation of standard solutions for the calibration of flame photometers. I t was suggested that consideration be given spectroscopically pure samples for such calibrations. Some laboratories reported that they run a standard for every four or five unknowns. Others calibrate their instruments a t the beginning and end of each series of determinations. Another technique reported was that in which two standards are run for each unknown, one of higher and the other of lower concentration than the unknown. This technique permits interpolation of results on the unknown. The accuracy of the determination is influenced by the composition of the test solution. Thus, it has been shown (8,S, 6) that extraneous elemer-ts may influence the intensity of emission of the test element. In cases where the extraneous elements are present in small amounts standards containing only the test element may be used. I t was pointed out, however, that such standard solutiom have limited application. In most cases it was felt desirable to make standard solutions of approximately the composition of the unknown. Included among the types of samples which were being analyzed
for sodium and potassium were such substances as soil, food products, blood serum, urine, plant tissue, and water. Disrussion emphasized the need for careful sample preparation. Most organic substanres need to be eliminated by ashing before analysis. In some cases a simple extraction process can be used-for example, potassium can be quantitatively extracted from many dried plant materials with hot water. Care in checking such a method is required, however, because oil-bearing substances such as cottonseed meal cannot be handled by this prccedure. Samples of blood serum also require special care. If serum is not removed from the blood soon after the blood is drawn, erroneous results on potajsium will be obtained. Anticoagulants cause errors on sodium analysir. Hemolysis also may cause errors in the determination of potassium. If liquid biological materials can be diluted sufficiently, ashing may not be required. Other laboratories routinely eliminate all organic substances by ashing. If the surface tension of the fluid is close to that of water, it is possible that some organic material is permi3sible. No mention was made during the discussion regarding the internal standard technique which may be used with some equip ment. This method of improving accuracy should not be overlooked, however, by those who may be starting work in this field. Most laboratories mere using flame methods primarily for the determination of sodium and potassium, although some were considering calcium and magnesium also and indicated considerable interest in their determination. In answer to questions concerning flame methods for calcium and magnesium it was pointed out that errors were greater and sensitivity less than for sodium and potassium. Suggestions regarding changes in instruments to accommodate such determinations included (1) a better lens system between the flame and the photocell, (2) an improved amplifier circuit, and (3) an increase in flame temperature. In answer to a question concerning the use of flame photometric techniques for the continuous determination of calcium and magnesium and water it was suggested that if total calcium and magnesium was desired a technique using a resin exchange might possibly be developed. Readings could then be mrrde on sodium and translated to combined calcium and magnesium content. The lack of sensitivity in the determination of magnesium waa a140 discussed. At low magnesium levels the errors are considerably larger than in the determination of sodium and potassium. It was suggested that a hotter flame might increase sensitivity. This could possibly be accomplished by the substitution of an acetylene-oxygen mixture to replace a gasoxygen mixture in the burner. It was also pointed out that such a change would increase the hazard of backfiring in a burner and that a new burner design might be of help. Among those taking evtensive part in the discussions were V. W. Meloche of the University of Wisconsin, 13. A. Frediani of Merck and Co., Inc., W. R. Lowstuter of Hercules Powder Companyi J. H. Cast of Baylor University, and C. E. Bills. Thanks are due them for the success of the discussion period as well as to all others who attended and took part in the round table discusjion. LITERATURE CITED
(1) Barnes, R. B.. Richardson, D., Berry, J. W., and Hood, R. L., IND.ENC).CAEM.,ANAL.ED., 17, 605 (1945). (2) Berry. J. W., Chappell, D. C., and Barnes, R. B., Ibid., 18, 19 (1946).
(3) Bills, C, E.. McDonald, F.’ G., Niedermeier, W., and Schwarts, M. C., ANAL.CHEM., 21, 1076 (1949). (4)
National Technical Laboratories, South Pasadena, Calif.,
Beekman Bull. 167-C (1918). ( 5 ) Parks, T. D., Johnson, H. O., and Lykken, L., ANAL.CHEM:, 20, 822 (1948). (6) Weichselbaurn. T. E.. and Varney, P. L., Pzoc. Soe. Ezptl. Bwl. Med., 71, 570 (1949). RECEIVSU Augurt 4. 19.50.
