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DETERMINATION Raymond C. Smith, K. C. Pillai,” Scripps Institution of Oceanography, OF RUBIDIUM IN SEAWATER’ Tsaihwa J. Chow, and Theodore University of California, San Diego, R. Folsom La Jolla, California ABSTRACT Rubidium concentrations in seawater from various locations and depths to 3,900 m have been determined using two independent techniques--direct flame photometry and mass spectrometry. Results of these two techniques agree within estimated errors and indicate that the rubidium content in the oceans is uniform within H$% with values varying about 120 pg/liter. Sediment from the sea floor below a depth profile, analyzed for rubidium and potassium, gave a rubidium concentration factor of 150 and a potassimil to rubidium ratio of 1,270. INTRODUCTION Soon after his discovery of rubidium, Bunsen evaporated large quantities of seawater in an attempt to demonstrate its existence in the sea but obtained negative results ( Bunsen 1861) . Similarly, other early investigators (Grandeau 1863; Sonstadt 1870; Schmidt 1878; and Thompson and Robinson 1932) failed to detect rubidium in seawater. Results of more recent investigations, usually surface coastal waters, are summarized in Table 1. There is considerable variation in these values, from 35 to 450 ,g/liter, and there are no accurate representative values from various locations and depths in the oceans. However, previous analytical techniques have been cumbersome and time-consuming and did not lend themselves easily to routine analysis of samples. Direct flame photometric determinations of alkali-earth elements in mg/liter quantities have been developed, and alkali metals like lithium, rubidium, and cesium in the ,g/liter range have been assayed by mass spectrometry, neutron activation, and flame l The authors are indebted to their colleagues who collected the samples, especially Daniel M. Brown of R. V. Agussiz and Jan B. Lawson of R. V. Argo of the Scripps Institution. They also wish to thank Lawrence E. Finnin for contributing so much to the construction and improvements in the flame photometer. This work received substantial support from the U.S. Atomic Energy Commission, the Office of Naval Research, and the National Science Foundation. 3 On deputation from Atomic Energy Establishment Trombay, Bombay, on an IAEA Fellowship administered by the U.S. National Academy of Sciences-National Research Council. photometry after enrichment. The work described in this paper employs two independent techniques-direct flame photometry and mass spectrometry-to ascertain values for rubidium concentration in seawater. APPARATUS AND METHODS A stock solution containing 125.6 mg rubidium/liter was prepared by drying and weighing spectrochemically pure rubidium chloride and dissolving it in a known volume of double-distilled water. The stock solution was stored in a Teflon bottle, and rubidium standards were prepared from this by dilution. The Pacific Ocean seawater samples were collected in Nansen type bottles made of Lexan plastic and brought to the laboratory in polyethylene bottles. Those from the Indian and South Atlantic oceans were taken with a large metal sampler and stored in SO-liter polyethylene carboys. Salinities of these samples were determined by the conductivity bridge method. The rubidium content of each sample was then independently determined by both of the following techniques : 1) Flame Photometry: To make quantitative measurements of rubidium in seawater without recourse to chemical separation and concentration, a low-noise system with high signal-to-noise ratio was constructed with the following components : 226 Beckman hydrogen and housing Bausch and Lomb chromator atomizer-burner grating mono- DETERMINATION TABLE Date this 1. Rubidium OF Investigators Source Burkser, et al. 1933 1940 Goldschmidt, et al. * Kovaleva and Burkser 1942 1944 Ishibashi and Harada Borovick-Romanova 1955 1957 1959 1964 1965 Smales and Salmon Smales and Webster Ishibashi and Hara Bolter, Turekian and Schutz Average of this work paper (Goldschmidt corrected to 20 IN et al. mg/kg. 1933) 227 SEAWATER content of seawater reported 1932 * In the original was mistakingly RUBIDIUM by various investigators No. of samples of seawater Azov Sea Black Sea North Sea Black Sea Mediterranean Sea English Channel Japan Coast Japan Sea Barents Sea Greek Archipelago Okhotsk Sea Black Sea North Atlantic Ocean North Atlantic Ocean Japan Coast Atlantic Ocean Pacific, Atlantic and Indian a value Grating: 675 grooves/mm blazed for 0.7-1.6 ,J, Filter: Ednalite Duraklad R3 29F dark red Farnsworth (ITT) FW-118 photomultiplier with an S-l photocathode. Throughout the determination the flame photometer settings were as follows: Wavelength: scanned from 787 rnp to 803 rnp in 33 seconds Photomultiplier voltage: 1811 v Rubidium spectral line: 795 rnp Slit width: 0.1 mm Oxygen: 15 psi (1.06 kg/cm”) Hydrogen : 6.5 psi ( 0.46 kg/cm” ) The photometer was adapted from one designed for cesium analyses at Oak Ridge National Laboratory (Feldman and Rains 1964), but the electronic circuitry was simplified. Since the FW-118 photomultiplier has unique signal-to-noise properties ( Eberhardt 1963n, b, and c), it is an excellent low-noise amplifier. Use was made of this characteristic to eliminate intermediate amplifiers and the output of the photomultiplier was fed directly into a self-balancing potentiometric chart recorder (10 mv full scale) across a 6,700 ohm load resistor. A of 200 Kg/kg was given. Oceans In Goldschmidt Rubidium ,ug/liter 1 1 1 1 1 1 1 1 2 2 1 1 6 1 1 11 16 ( 1958, 200 320 200 450 340 240 35 210 200 190 180 100 120 121 190 125 120 p. 169 ), simple potentiometer circuit was used for background current and sensitivity adjustment. While input impedance of this circuit is higher than that recommended for the recorder, this did not lower the full scale response or increase the response time significantly. Since special difficulties attend elimination of flame background when it is impractical to prepare a blank solution, the standard additions technique (Chow and Thompson 1955) was used to eliminate interferences caused by changes in physical and chemical conditions of the seawater sample. Fifteen-ml aliquots of a seawater sample were pipetted into four different Pyrex bottles, and known quantities of rubidium standard were added in the ratio of 0, 1, 2, and 3. The volume of each solution was adjusted to 2,O ml with double-distilled water. The quantity of additions was approximately one-half, equal to, and one and one-half times the original rubidium present in the solution. This represented a compromise that minimized the number of standard additions to be made per sample, kept the standard deviation of the extrapolated intercept a minimum for given deviations in single intensity measurements, 228 2” 60 5 ii lx 40 RAYMOND C. SMITH, K. C. PILLAI, TSAIHWA 20 795 WAVELENGTH [wl FIG. 1. Schematic diagram of rubidium 795 rnp spectral line overlapping large potassium 766/769 rnp background line, -- net rubidium after background is subtracted. and retained the linearity of the emission vs. concentration curve. The net emission intensity of each of the four portions was measured as follows: After sufficient instrument warm-up, measurements of four aliquots of each of the samples were carried out rapidly in a uniform rhythm to avoid drift of sensitivity during measurements. The aliquots of the samples were analyzed in order of increasing concentration and repetitions were made by re-running the four aliquots. The atomizer was rinsed with warm distilled water between each measurement. This periodic rinsing helped to prevent clogging (more precisely the deposition of salt crystals on the atomizer tip) of the burner by sea salts. The stability of the instrument an d uniformity of burner atomization were checked at the beginning and end of measurements by running a standard rubidium solution. The net emission intensity plotted against rubidium concentration gives an intercept from which the rubidium content of the original sample can be determined. In this paper, a best-fit to a straight line and the extrapolated intercept were calculated numerically. J. CHOW, AND THEODORE R. FOLSOM To measure the net emission intensity of rubidium in the presence of large quantities of other elements requires care. In particular, since the ratio of potassium to rubidium is greater than three thousand in seawater, the “tail” of the potassium doublet at 766/ 769 rnp obscures the 780 rnp line of the rubidium doublet and gives a high background emission under the 795 rnp line. The 2.5 rnp half-intensity band-width of the grating monochromator could have been narrowed to lower this background intensity, but only with a sacrifice in photocurrent and hence in a lower signal-to-noise ratio. To measure the net emission intensity of rubidium in the presence of this high background response, which varies more or less linearly with wavelength, a chart recorder was used to plot total emission intensity from 787 to 803 rnp. Fig. 1 illustrates the features of such a record. The overlapping interference is corrected by measuring the peak height, h, from the top of the rubidium peak to a point directly below on a line representing the background emission of potassium. The height, h, is taken as the net emission intensity of rubidium. Shellenberger et al. (1960) showed that the emission intensity of rubidium is enhanced by the presence of potassium. Their data also show that after the addition of large quantities of potassium, further increases have relatively little effect on the rubidium emission. From these results and because there is a large excess of potassium in seawater, it is assumed that the amounts of rubidium added, in the standard additions technique, have negligible effect on the relative emission intensity of the rubidium line. Also, our measurements have shown that the added rubidium is not sufficient to change the potassium, and hence the background, emission intensity noticeably (less than 1%). Interferences that affect equally the emission intensity of radiation from the test element either originally present or added as the standard increment are referred to as multiplicative because, in principle, the interference could be corrected by multiplying the emission intensities by a DETERMINATION OF RUBIDIUM constant factor. Interfering substances that are corrected by the addition or subtraction of a constant factor are additive interferences. The standard additions technique permits determination of the slope of the emission intensity vs. concentration curve (working curve) in the presence of multiplicative interferences. But, if additive interferences are present and unaccounted for, the working curve will be translated parallel to itself up or down the emission intensity axis and the extrapolated value giving the original concentration will be in error. Our procedure, taking the net emission of rubidium as the height from the peak to a point directly below on a line representing the background emission, is a method of correcting for additive interferences, since the background, regardless of origin, is automatically subtracted. This is true except when, by some mischance, a background line peak lies within the instrument band-width of the analytical spectral line. Search of flame spectra tables (Hermann and Alkemade 1963) shows no known lines arising from elements sufficiently concentrated in seawater, that could affect the rubidium measurements. 2) Mass Spectrometry: Isotope dilution technique was performed using a 60” sector single focusing mass spectrometer with a 12-inch (30 cm) radius of curvature. An enriched “TRb tracer, obtained from the Oak Ridge National Laboratory, was dissolved in 1 N HCI to make up the spike solution containing 68 pg rubidium per ml. The isotope composition of the spike and of the common rubidium were determined as follows : “Rb atom % “5Rb atom % Tracer 99.19 0.81 Common 27.85 72.15 Seawater ( approx 100 ml) was introduced into a 250-ml Teflon beaker, and 100 ~1 of rubidium spike were added. The combined solutions were reduced to a small volume by evaporation. About 5 ,~l of this brine were placed and dried on the surface of a tantalum filament that had been carefully baked in vacuum at 2.5 amp until any rubidium which might have been initially IN 229 SEAM’ATER present on the filament was removed. No chemical separation or enrichment was required. Since no other chemical than the rubidium spike was used, the contamination could be controlled. The spectrometric procedure has been described by Chow and Goldberg ( 1962). The filament current was slowly raised to 0.8 amp, corresponding to a temperature of about 750C. At this temperature only the rubidium spectrum was observed. The interference from 87Sr was carefully checked by monitoring the @Sr and Y5r peaks. Throughout the mass spectrometric measurements, no strontium ion beam was detected. The electrometer shunts and atomic weight corrections were incorporated in the calculation. Since measurements were made with a Faraday cup collector, the square root of mass ratio correction was not necessary. The spike solution and seawater were mixed so that the ratio of sTRb to s5Rb was about one, which minimizes the error and facilitates measurement. The amount of rubidium in the sample was calculated from the following equation: M= m( iV x s5Rb, - s7Rb,) =Rb, - N x ““Rb, ’ where LM= rubidium in the sample (pg); m = amount of 8TRb tracer added (PdG N = observed t(‘iRb/s5Rb ratio with atomic weight correction; “SRb,s,S5Rb, = atom % of 87Rb and 85Rb in the rubidium tracer, respectively; 87Rblo,s5Rb, = atom % of 87Rb and X5Rb in the common rubiduim, respectively. RESULTS AND DISCUSSION In flame photometric analysis, random errors arise from a number of causes (Hermann and Alkemade 1963). The dominant source in this determination is the variation in emission intensity caused by the formation and dislodgment of salt deposits on the atomizer tip. This error might be re- 230 RAYMOND C. SMITH, K. C. PILLAI, TABLE 2. TSAIHWA Rubidium J. CHOW, content AND THEODORE Depth (m) FOLSOM of seawater Rubidium, Location R. pg/liter Normalized Temp. (Cl Flame photometer Mass spectrometer s ‘%g, Pacific Ocean 33”25’ N lat 122”36’ W long ( Agassix 80.80 ) 8 Ott 1963 0 49 98 291 485 972 1,943 2,906 3,920 19.44 16.30 12.01 7.30 5.69 3.90 2.14 1.64 1.50 33.09 33.02 33.04 34.03 34.17 34.56 34.61 34.66 34.69 120 113 112 115 117 109 110 112 122 129 121 118 114 113 111 112 122 119 132 124 122 118 118 112 112 118 122 34”04’ N lat 122”02’ W long ( Agassiz 82.47 ) 9 Ott 1963 0 474 18.46 6.81 33.59 34.25 127 128 N.D. N.D. 133 131 33.76 119 115 121 32”52’ 117”15’ ( S.I.O. 27 Aug N lat W long Pier ) 1963 0 South Atlantic Ocean 19”44’ S lat 12”54’ W long ( Lusiad 51) 23 June 1963 0 3,000+ - 36.50 34.90 144 121 135 118 134 120 S. Indian Ocean 18”Ol’ S lat 40”51’ E long ( Lusiad Hydro 2 ) 22 May 1963 0 2,500+ - 34.64 34.93 132 129 125 114 130 123 121 119 123 Average duced and the precision of measurement increased by chemical separation and concentration of rubidium, but our aim was a procedure suitable for routine analysis on unconcentrated seawater, and the precision of the present method is greater than all previous work except the single measurement of Smales and Webster ( 1957). By comparison, the much smaller concentrations of cesium occurring in seawater (about 0.4 pg/liter ) were determined with a precision of about +4% (Folsom, Feldman, and Rains 1964) using a similar photometer and the aid of concentrating steps. The precision of the extrapolated intercept value depends on the deviation in the individual intensity values, the number of individual intensity measurements, and the slope of the best-fit line. Choice of these variables has been discussed above. In the standard additions method, the prime consideration is directed to minimizing changes in burner characteristics during a determination while such changes between repetitions are of no consequence to the intercept value, since they are multiplicative in effect. In analyzing the data, the extrapolated intercept value and its standard deviation were calculated for each determination. At least three sets of measurements were made on each sample. The replicated intercept values were averaged. The standard deviations ranged from 2 to 10% with the average of the final rubidium values being about 6%. A careful study indicated that the possible systematic errors (e.g., non-linearity of the working curve, possible contamina- DETERMINATION OF RUBIDIUM tion, neglect of interferences) are small compared to the random errors. However, the effect of most of the possible systematic errors is to increase the measured amount of rubidium. In mass spectrometric analyses, random errors arise from chemical contamination, fluctuation of the ion beam intensity, and background noise. Because there is no chemical operation involved and the isotopes are measured with a ratio of about one, random errors are greatly reduced. A possible systematic error is isotopic fractionation during ion beam emission. As a test, the samples were “burned” for considerable lengths of time. No detectable change in isotope ratio was observed. The validity of the isotope dilution calculation was tested by mixing known amounts of standard solutions of common rubidium and the tracer in which the predicted X’Rb to “sRb ratios ranged from 0.75 to 3.5. By this test, the resulting ratios were actually determined. The uncertainty of the calculation was about 5%. It is a significant feature of this work that each sample was measured by two entirely independent analytical techniques. The results of these measurements are given in Table 2. The flame photometric analyses and the mass spectrometric analyses agree within the estimated precision (measure of that random errors ) , This is an indication the accuracy ( measure of systematic errors) is of the order of or smaller than the precision of the measurements. An analysis of the data gives an average for the rubidium content in the oceans as 120 -C 9 pg/liter and an average value for the rubidium to chlorinity ratio as 6.34 + 0.41 x lo-“. This agrees with the work of Smales and Webster ( 1957) and Bolter et al. (1964 ) and indicates that the rubidium content in the oceans is uniform within 28%. The flame photometer and mass spectrometer rubidium values have been averaged and then normalized to water of salinity 35;;‘(, in Table 2. The Pacific Ocean depth profile shows a slight trend toward maximum rubidium at the surface and bottom with a minimum at intermediate waters. Seawater Sediment Sediment Seawater IN 231 SEAWATER 0.109 16.3 150 390 20,700 3,580 1,270 53 The flame photometric procedure was also applied to the analysis of alkali metals in one sample of marine sediment. Sediment taken from a sample bottle which struck bottom at 3,929 m at the profile station in the Pacific Ocean was analyzed for rubidium and potassium content (Table 3). The average- concentration factor of rubidium in the sediment to that of seawater was found to be 150. This sediment had a potassium to rubidium ratio of 1,270 as compared to 3,580 for seawater. REFERENCES E., K. K. TUREKIAN, AND D. F. SCHUTZ. 1964. The distribution of rubidium, cesium and barimn in the oceans. Geochim. Cosmochim. Acta, 28: 1459-1466. BOROVICK-ROMANOVA, T. F. 1944. On the ruDokl. Akad. bidium content of sea water. Nauk SSSR, 42: 216-218. BUNSEN, R. 1861. Chemical analysis by spectrum observations. Phil. Mag., 22: 329-349. BUIIKSER, E. S., N. V. KOMA~I, S. G. RUBLEY, AND 1932. Researches exR. I. USTIL~VAKAYA. perimentales sur la determination du rubidium dans l’eau de la mer noire, de la mer d’azov et des limans cl’odessa. Tr. Biogeokhim. Lab. Akad. Nauk SSSR, 2: 65-84. CHOW, T. J., AND E. D. GOLDBERG. 1962. Mass spectrometric determination of lithium in sea J. Marine Res., 20: 163-167. water. AND T. G. THOMPSON. 1955. Flame phbtometric determination of strontium in sea water. Anal. Chem., 27: 18-21. EBERHARDT, E. N. 1963a. Noise in multiplier phototubes. 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