ANALYTICAL CHEMISTRY
1554 obtained. Those materials resulting from processing the initial samples contained very large quantities of some of the metals listed in Table 11. LITERATURE CITED (1) Beck, G., Anal. Chini. Acta, 1 , 69 (1947).
Beck, G., Mikrochemie w r . Mikrochim. Acta, 27, 47-51 (1939). Ibid., 34, 282-5 (1949). Beck, G., Mikrochim. Acta, 2 , 9-12 (1937). Carey, R. B., and Rogers, H. R., J . Am. Chem.Soc., 49, 2 1 G 1 7 (1927). (6) Fischer, W,, and Bock, R., 2. anorg. u. allgem. Chern., 249, 146 (1942). (2) (3) (4) (5)
(7) Fischer, W., Steinhauser, O., Holmann. E., Bock, E., and Borchers, P., Z . anal. Chem., 133, 57-72 (1951). ( 8 ) Kronstadt, R., and Eberle, A. R., Atomic Energy Commission Rept. RMO-838 (1952). (9) Kuznetsov, V. I., J . Ge7~.Chem. C.S.S.R.. 14, 897 (1944). (10) Lundell, G. E. F., and Hoffman, J. I., "Outlines of Methods of Chemical Analysis." D. 118, Wiley, X e w York. 1948. (11) Miller, C. C., J . C'hemSoc., 1947, 1347. (12, Peppard, D. F.. Faris, J. P., Gray, P. R., and Mason, G. I T . , J . Phgs. Chem.. 57, 294-301 (1953). (13) Welcher, F. J.. "Organic Analytical Reagents," vols. I-IV, Van Nostrand, S e w York, 1948. (14) Wenger, P., Duckert, I < . . and Rusconi, Y., Helv. Chim. A d a , 28 872-5 (1935). R E C E I V Efor D review M a y 1 7 , 1955.
Accepted July 21, 1955.
Spectrographic Determination of Rubidium in Plant and Animal Tissues B. L. GLENDENINGI, D. B. PARRISH,
and W. G. SCHRENK
Kansas State College, Manhattan, Kan.
Because wet chemical methods generally are unsatisfactory, rubidium usually is determined hy spectrographic methods. Excitation of prepared samples of natural products by an electric arc or spark produce spectral lines of other elements that interfere with reading those of rubidium. This difficulty may be reduced by using flame excitation and a spectrograph of high dispersion. If the potassium concentration of the sample is high, as usually occurs in plant tissue, interference due to scattered radiation from intense potassium lines may be reduced by a mask, which prevents the energy from the potassum lines striking the spectrographic plate. Under proper conditions the method presented here is satisfactory for as low a concentration as 1 p.p.m. of rubidium in plant or animal tissue. The rubidium content of a number of human and animal foods was determined. Soybeans contained the largest concentrations of rubidium, 160 to 225 p.p.m., dry basis. I n wheat flour, bread, and oleomargarine,less than 1 p.p.m. was found.
C
HEMICAL methods for determination of rubidium are difficult, because of the tedious procedures required to effect separations from other alkali metals. I n fact no published procedure has been found for quantitative separation of cesium and rubidium (14). Colorimetric methods also have not been found in the literahre. Only spectrographic methods appear t,o have heen used for determinations of the rubidium cont,ent of plant and animal tissues. l l a n y diffic ilties, however, have arisen in development of spectrographic methods for the quantitative determination of this element, although it was one of the first to have been discoverpd by spectrographic means (16 ) . Two sets of spectral lines of rubidium are sufficientl), intense for andytical purposes, the violet pair a t 4201.9 and 1215.6 .4. and the red pair a t 7800.2 and 7947.6 .4. Arc, spark, gas discharge, and flame techniques have been used for excitation of ruhidium in spectrographic Lvork. Sheldon and Ramage (1.5) rolled samples of plant tissues in ashless filter paper and burned them in a n arc before the slit of a spectrograph, rubiduisn being one of the elements determined. Strait ( 1 7 ) employed the condensed spark between an upper pointed electrode and a loner cylindrical electrode of copper upon which 0.04 ml. of plant sap had been deposited. The rubidium lines a t 4215.6 and 4201.8 A . 1 Present address, Laboratory Division. Kansas State Board of Health. Topeka, Kan.
\?ere used with indium line a t 4101.8 ri. serving as the internal standard. Bertrand, Bertrand, and Court?; ( 4 ) used a method of background correction to overcome the difficulties encountered in arc excitation. They used the 4201.9 A . line of rubidium. Halperin and Sambursky (IO)also employing arc excitation, chose to read the 7800.2 and 7947.6 A. lines in attempting to overcome interference from other elements. Cohn ( 8 ) excited rubidium by placing the powdered sample in a gas discharge tube. Vanstone and Philcox (18) pressed dried plant materials into a cellophane tube, which was fed into an oxyacetylene flame for excitat,ion. Rusanov and Vasil'ev ( I f ) introduced powdered materials into the oxpcetylene flame by means of an air stream. Borovik-Romanova ( 6 ) claimed both the acetone lamp and the direct current arc were suitable sources of excitation for spectrogrvphic determination of rubidium using the lines a t 7800.23 and 7947.68 A. Flame rather than arc excitation has the advantage of producing spectral lines of the alkali metals along with relatively few lines of interfering elements (9, I O ) . Cyanogen bands also are eliminated by flame excitation methods. Because the spectral lines of potassium (7664.9 and 7699.0 -4.) and rubiduim (7800.2 and 7947.6 a,)are relatively close together, the Beckman DI; flame photometer or similar instrument doee not permit sufficirnt rePolution for the determination of rubidium in the presence of potassium. Freytag (9) reported considerable interference of potassium and lithium in determination of rubidium using a Zeiss flame photometer. Instruments with interference or absorption filters likewise do not give sufficient resolution to permit the determination of rubidium in the presence of potassium.
Table I. Sanipla 1
2 3
? ti
7
8
9 10 11
12 a
Rubidium Content of Experimental Feed Samples" Calcd. R b Added. '% R b Detd., o/o 0 000 0 000 0 01 0 01 0 IO 0 IO 0 20 0 20 0 30 0 3r) 0 40 0 40
0 000 0 000
0 0 0 0 0
009 011 14 14 21
0 11 0 30
0 30 0 37 0.40
Feed compounded of casein. sucrose. oil, vitamins, and minerals
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 Replica of Determining Rubidium Content of Rat Liver Tissue"
Table 11.
Trial No. 1 2 3 4 5 6
7 d 9 10
11 12 13 14 55
R b %, D r y Basis
Plate No. 98 99 100 101 122 123 124 125 126 127 129 130 133
Av. Std. dev. a
1 .oo 1.22 1.06 0.96 1.10 1.13 1.09 0.91 1.09 1.20 1.18 1.20 1.24 1.16 1.15 1.11 0.031
155.5 the elements most likely to be present in appwciable quantities in the ash of plant and animal tissues, the effects of these elements on line intensities or rubidium were investigated. It was found that the intensities of the rubidium lines were enhanced by sodium and/or potassium. Calcium and magnesium however, had no apparent effect. For this reason, all standards were prepared containing quantities of sodium and potassium a t approximately the concentrations present in the samples for analysis. Calcium and magnesium were not added to standards. When analyzing samples having an exceptionally high rubidium content, the concentrations of rubidium, sodium, potassium, and lithium were increased t o compensate for the shorter exposure required. The concentrations of the first three mentioned elements in the standards were selected to be in the approximate ranges as in samples analyzed
Liver from rat receiving experimental rations containing Rb.
This laboratory has used the Beckman flame attachment (Model 10,300) as a n excitation unit and a high dispersion spectrograph (Bausch & Lomb large Littrow) for the determination of sodium, potassium, and calcium (fa,13). It appeared desirable to investigate the use of this equipment for the determination of rubidium. A special mask was devised for use with samples containing relatively large amounts of potassium to prevent scattered radiation from potassium interfering with reading the intensity of the rubidium line a t 7800.2 A,; thus making this line of rubidium available for analyses under these conditions. EXPERIMENTAL PROCEDURES
Preparation of Samples. Five-gram samples of plant or animal tissues were weighed into platinum dishes and dried a t i05" C. The samples were dry-ashed at 550' C. They then were covered with 1 ml. of a solution of hydrochloric acid (1 t o l ) , and after approximately 1 hour were heated until only a dry residue remained. Distilled water was added, the solution filtered, and the filtrate made to 10 ml. This filtrate was the stock solution, which waB dlluted for spectrographic anahses. Preparation of Standards. Several series of standards Lvere used, the selection depending on the contents of rubidium, sodium, and potassium. As it was found that the intensitv of the rubidium line of standards was increased by presence of sodium and potassium, these ions were added t o standard solutions. Lithium served as the internal standard. -111 salts were added as chlorides. Tvpical of the standards used were solution. containing 0.4 mg. per mi. of potasdium, 0.1 mg per ml. of sodium, 0,0125 me;. per ml of lithium with the rubidium Poncentration varying from 0.000 to 0.100 mg. per ml. Effect of Extraneous Elements. The possible effects of extraneous elements on line intensities of test elements are well known in emission spectroscopy ( 7 ) . Such effects were investigated with respect to the proposed method of analysis for rubidjum, Knee sodium, potassium, calcium, and magnesium are
Instruments, Accessories, and Operational Data. SPECTROG R ~ P H Bausch . and Lomb large Littrow, quartz prism. SETTIXG. Focus 8, tilt 310, wave-length range 3600 t o 8500 A. OPTICS. Cvlindrical lens before slit, stainless steel concave mirror back of flame. SLIT. Height 1.5 mm. width 65 t o 90 microns, depending on concentrations. EXPOSURE. 45 t o 180 seconds, depending on concentrations. EXCITATION.Beckman flame photometer excitation unit, IIodel 10,300. Natural gas fuel, pressure 5 cm. of a manometer fluid (specific gravity 1.04). Oxygen pressure, 40 inches of water. Air pressure on aspirator 16 pounds per square inch. Rate of feed, approximately 0.2 ml. per minute. PHOTOGRAPHIC PLATES.Eastman 1-Ar plates, developed according to recommended procedures. L \ l a s ~ . A black paper mask, fitted over the photographic plate in a position so that potassium lines 7664.9 and 7699.0 A . were masked, was used when analyzing - - samples - of potubsium content greater than 1 mg. per ml. SPECTRALLINES. Rubidium 7800.2 and 7947.6 A., lithium 6707.8 A. DESSITOMETER, ARL-Dietert. Analytical Curves. Spectra of a number of standards, the concentrations of which covered the range of those of the samples, were recorded on each plate which also contained spectra of samples. From densitometer readings and emulsion calibration curves, intensity ratios rubidium t o lithium of standards were calculated. A calibration curve was plotted on a log-log scale, placing the ratio rubidium t o lithium on one axis and concentration on the other. From this curve the concentratioris of rubidium in the samples were determined. It also is possible to determine sodium and potassium on the same plate with rubidium if the concentration ranges of sodium and potassium give appropriate line intensities. Effect of Masking. When the concentrations of rubidium and potassium are simjlar, no masking of potassium lines is necessary. However, samples containing small quantities of rubidium, as in many plant tissues, require extended exposure times. Because of the high potassium content in plants this may result in the
Table 111. Comparison of Duplicate Determinations of Rubidium in Soybeans Sample 8- 1 5-2
5-3 S-5 S-6 5-7
5-8 S-0 S-10 S-11 S-12 S-13 S-14 S-15 S-16
No.
Description Aerial part Aerial part Aerial part Seeds (commercial) Seeds (commercial) Meal (solvent extracted) Seeds (commercial)
{EA) (Chief)
(Adams) (Lincoln) (Wabash) (Wabash) (Wabash)
Trial 1 160 230 230 180 230 250 250 200 170 180 180 190 200 200 200
P.P.M. R b , D r y Basis Trial 2 Mean Dev. from mean 210 185 25 280 255 25 280 255 25 170 175 5 240 235 5 260 260 210 150 190 180 180 170 190 210
255 255 205 160 185 180 185 185 105 205
5 5 5 10 5 0 5 15 5 5
ANALYTICAL CHEMISTRY
1556 Table IV.
Rubidium Content of Some Human Foods and Animal Feeds, Dry Basis Product Tomato, whole fruit Beef, rib muscle Tea, Orange Pekoe Molasses Liver, beef Coffee, ground Cocoa Milk, dry skim Pork, steak Sweet potato Coconut. shredded Peanut butter All-bran cereal Ham Beans, dry lima Bpple Pecan, meat Dates Rice, white Banana Squash Cabbage Turnip Raisins Orange, peeled Tapioca Rye bread Oats. rolled Onion Cheese Flour, white wheat White bread
Cottonseed meal Prairie hay Red clover Alfalfa pellets
Rubidium Content, P.P.M. ’ 140 140 110 92 90 89 81 79 78 63 58 57 55 52 51 50 41 33 27 18 17 12 11
7 6 4 4 2 1 1
