388 Analyst, June, 1968, Vol. 93, fifi. 388-393 The Use of a Non-absorbing Reference Line in the Simultaneous Determination of Platinum, Rhodium, Palladium and Gold by Atomic-absorption Spectroscopy BY P. B. ZEEMAN AND J. A. BRINK* (The Merensky Institute for Physics, The University of Stellenbosch, Stellenbosch, South Africa) A method for determining platinum, rhodium, palladium and gold simultaneously on a direct-reading spectrometer with a non-absorbing reference line is described. The concentration ranges of the working curves were as follows: platinum, rhodium and palladium 0 to 20 p.p.m. and gold 0 to 5 p.p.m. The detection limit for rhodium and palladium was 0-1 p.p.m., for platinum 0.5 and for gold 0.01 p.p.m. The reproducibility of the results was satisfactory. METHODS for the determination of the noble metals have already been described by Lockyer and Hames,l Strasheim and Wessels,2 Zeeman3 and others.A method is described here in which aqueous solutions of the noble metals were diluted with acetone before being sprayed into the absorbing flame. The advantages of the use of a non-absorbing reference line were previously discussed by Menzies.4 APPARATUS- A multi-element hollow-cathode lamp of the water-cooled type described by Zeeman and Butler6 was used as the light source. The cathode body was made of platinum metal 0-1-mm thick, while strips of palladium and rhodium were soldered with gold to the inside of the platinum cylinder, thus no base metals were introduced into the cathode, and a purer spectrum was obtained.The hollow-cathode lamp was connected to a glass vacuum system so that the carrier gas could be easily changed whenever necessary. A series of experiments, with neon, helium, argon, xenon and krypton as carrier gases, was carried out to investigate the purity of the spectrum in the region of the resonance lines of the noble metals. The results, which agreed with those of Jones and Walsh,6 showed that a neon-filled hollow- cathode lamp had the most satisfactory intensity and stability. The lamp operated under a pressure of 1.6 mm of mercury and 50-mA current, which was supplied by a stabilised d.c. power source. The absorption burner was of the type used by Zeeman and B ~ t l e r , ~ the various inlets of which are shown in Fig. 1. Light from the hollow-cathode lamp, C, is directed over the absorption flame, B, and focused on the entrance slit of the spectrometer, A, as described below.The Zeiss atomiser, H, sprayed the solution into the spray chamber, E, from where it proceeded to the flame. Propane - butane gas, under controlled pressure, is fed in through G. This gas can also be fed in at full pressure through F, to keep the flame burning when the introduction of organic solvent is stopped. The atomising air at 20 p.s.i. is introduced at M, and pre-heated at K to 200" C by a copper spiral inside a cylindrical electrical furnace. Extra air at a pressure of 0.4 kg per cm2 was introduced through L, and fed into the spray chamber through the normal gas inlet of the Zeiss atomiser. Further air was fed into the chamber through the normal condensed solution outlet of the spray chamber via N, because no liquid condenses when organic substances are sprayed as described above.This extra air is essential to obtain a clear soot-free flame when burning organic liquids. The pressure of this extra air is read on the gas-flow manometer, 0, and regulated by the valve, P, which is fed through inlet, Q. The light from the cathode was focused on the entrance slit by means of two quartz lenses. The first, a cylindrical lens of focal length 10 cm, was placed, with its axis horizontal, * Present address : National Physics Research Laboratory, Council for Scientific and Industrial Research, Pretoria, South Africa. 0 SAC and the authors. EXPERIMENTALZEEMAN AND BRINK 389 near the window of the hollow-cathode lamp.The second lens was spherical, with focal length of 30 cm, and was placed about halfway between the source and the slit, which were 120 cm apart. This arrangement just filled the grating with light. An entrance slit width of 30 p was used. The absorption burner was placed about 5 cm from the lamp. As a result part of the flame was focused on the entrance slit. t ‘F Fig. 1. Schematic drawing of the atomic-absorption equipment used (lettered parts of the apparatus are referred to in the text) The spectrometer used was a Bausch and Lomb, type 18000, direct-reading grating monochromator. The grating is in the Paschen mounting with radius 1.5 metres. Normally the instrument is fitted with R.C.A. 1P28 photomultipliers, but these were replaced by E.M.I.tubes, types 9601 B and 9526 B, for higher sensitivity. The power unit for the photomultipliers was modified so that the sensitivity of the five channels used could be varied separately. The instrument is equipped with automatic termination of the integration. The latter is terminated when a fixed voltage is reached on the integrating condenser of a pre- selected line. This results in a constant-intensity reading for one of the spectral lines, and is usually reserved for the internal standard line in emission analysis. In the present investi- gation it was coupled to the non-absorbing reference line. Five photomultipliers were used, one for the resonant line of each of the four noble metals and one for the reference line. PROCEDURE FOR TAKING MEASUREMENTS- In preliminary work photographic records were made with 35-mm film in the focal plane of the spectrometer.The spectra obtained were studied with the view of selecting the best non-resonant noble-metal line to be used as reference for absorption measurements. The platinum line 3042A was selected because it was reasonably intense and free from other interfering lines. A line belonging to one of the noble metals was chosen as reference line to simplify the analytical procedure. The resonance lines used were gold 2428 A, palladium 3404 A, platinum 2659 A and rhodium 3434 A. These lines were also used by Strasheim.2 In ordinary measurements the “100” reading is adjusted as best possible when the solvent is sprayed, and it is then assumed that the lamp’s intensity will stay constant while the standard or sample solution is sprayed afterwards.In the procedure described here, the emission of the hollow-cathode lamp is monitored during the whole period of every measurement by integrating the emission of the non-absorbing line. This applies to the period when the solvent is sprayed, as well as when the solutions are sprayed thereafter. Hence the uncertainty regarding the “100” reading during this latter period disappears. In the method finally adopted, 95 per cent. acetone was aspirated until the pre-set count for the non-absorbing line was reached, when the integration stopped automatically and the readings of the four noble-metal elements could be taken. Solutions containing the noble-metal standards or samples were then sprayed and the counts taken in a similar way.The counts obtained indicate the intensities or transmissions of the various lines. The sensitivity of the reference detector was adjusted so that the integration time was about 20 seconds. During this time 5 ml of solution were nebulised.390 ZEEMAN AND BRINK: NON-ABSORBING REFERENCE LINE IN THE [Analyst, VOI. 93 REPRODUCIBILITY OF THE HOLLOW-CATHODE LAMPS AND DETAILS OF THE ANALYTICAL PRO- The absolute intensities of the lines showed short-term fluctuations and also a slow decrease in intensity with time. This was caused by impurities gathering in the filler gas. When the ratios of the intensities of the different resonant lines to the intensity of the monitor were calculated, an improvement in both sources of error resulted.After a warming-up period of 45 minutes, the decrease in intensity of the resonance lines amounted to 3.5 per cent. every 5 minutes. The intensity ratios, however, decreased only 1 per cent. during 90 minutes. The ratios mentioned above were plotted as ordinates against time, and the curves obtained are shown in Fig. 2. They show that a warming-up time of 10 minutes is CEDURE- 0.61 I. I I I I I . I I I 1 I 0 5 10 I S 20 25 30 35 40 45 50 55 Time, minutes Fig. 2. Time variation of the intensity ratios for the palladium 3404 A. gold 2428 A. platinum 3042 A’ B’ platinum 3042 A’ various elements: A, platinum 2659 A . and D, rhodium 3434 A platinum 3042 A’ platinum 3042 A needed for the intensity ratios of the platinum and rhodium lines to reach stability, and 45 minutes for the palladium and gold.The improvement in the short-term fluctuations are shown in Table I. In agreement with Butler and Strasheim,’ significantly lower co- efficients of variation were obtained with the integrating method. The detailed procedure for calculating results is given here for gold. Let T be the number of counts on gold channel when 95 per cent. acetone is sprayed. Let Tm be the number of counts now observed on the monitor channel. Let T A ~ be the number of counts on the gold channel when the solution is sprayed. Let T’m be the number of counts now observed on the monitor channel. Let T’ be the number of counts that would have been observed on the gold channel if 95 per cent. acetone had been sprayed during this period. = constant. counts for line channel counts for monitor channel It was proved above that T T’ Tm Tm Therefore - = constant =I.T A ~ x 100 percent. T’ The percentage transmission for gold = - - - T ~ ~ ’ T ’ m x 100 per cent. Tm x 100 per cent. (from the above) Tlm T T/Tm But with the automatic stopping of the integration process, T‘m = Tm. Therefore percentage transmission for gold = - x 100 per cent. TAU TJune, 19681 DETERMINATION OF PLATINUM, RHODIUM, PALLADIUM AND GOLD 391 By a similar calculation the percentage transmission can be obtained for the other elements. It must be realised that although the percentage transmission is thus calculated simply by taking a ratio as proved, and the monitor count apparently plays no r61e, it did so in fact, as the periods of integration varied slightly, depending on the lamp operation.TABLE I SENSITIVITY AND REPRODUCIBILITY OF THE DETERMINATIONS Detection limit Resonance (99 per cent. wavelength, transmission), Element A p.p.m. Gold .. .. 2427.950 0.0 1 Platinum . . .. 2659454 0.5 Palladium . . . . 3404.580 0.1 Rhodium . . .. 3434.893 0.1 Coefficient of variation at 50 per cent. transmission Absolute intensities, Relative intensities, per cent. per cent. 4-6 2.3 3.8 1.4 3-1 1.6 5.1 2.4 A r \ STANDARDS- The standards used were prepared from “Specpure” (Johnson, Matthey & Co.) solutions, which were of the ammonium chloro type. The stock solution was obtained by diluting the smallest convenient volume of aqueous solution containing 8000 p.p.m. of noble metal with analytical-reagent grade acetone to give a concentration of 400 p.p.m.of noble metal, which was thus in a 95 per cent. acetone medium. The various standards were obtained by diluting this stock solution with 95 per cent. acetone solution. The concentration ranges used were as follows: platinum, rhodium and palladium 0 to 20 p.p.m., and gold 0 to 5 p.p.m. RESULTS- In the preliminary investigations, the various elements were determined separately. The working curves are given in Fig. 3. The curves for rhodium and palladium are non- linear throughout, that for platinum is linear, while the curve for gold is linear for concen- trations up to 1 p.p.m. and then bends downwards. Repeated determinations of the four elements were made and the detection limits are given in Table I. d o 0 5 20 n.- 540 ul .- 60 m 0 2 4 6 8 10 I2 I4 16 18 20 I,: p.p.m.--, Fig. 3. Working curves obtained when elements were analysed separately Standard solutions were then prepared containing the four elements in solution, and the same amcentration ranges as above were used. Prior to the final measurements, the influence of the various elements on one another was investigated in the normal way by varying the concentration of one element in a constant composition of the others. It was found that there was no interference from palladium and gold, which is in agreement with Strasheim and Wessels’ observations2; however, platinum was suppressed, and rhodium, which was suppressed below 5 p.p.m., was enhanced above 5 p.p.m. The influence mentioned above, however, was not serious as is to be expected at the392 ZEEMAN AND BRINK: NON-ABSORBING REFERENCE LINE I N THE [Artdyst, Vol.93 low concentrations involved. The working curves obtained, with the four elements simul- taneously in solution, do not differ much from those when the elements were treated separately (Fig. 4). With palladium and gold they were the same, while the absorption was suppressed with platinum, and for rhodium it was suppressed below 5 p.p.m. and enhanced for higher concentrations. The sensitivity and reproducibility were the same as those given in Table I. 0 2 4 6 8 10. I2 14. 16 I8 20 p.p.m.- Fig. 4. Working curves obtained when elements were analysed simultaneously The non-linear working curves for rhodium and palladium were caused by the emission of O-H bands in the flame.These bands are degraded to the red. The (0, 1) band has heads at 3428 A and 3432 A. Hence rhodium determinations are strongly influenced by these, and palladium to a lesser degree. DETERMINATION OF THE NOBLE-METAL ORES- The accuracy of the above method was tested by analysing four samples supplied by a gold mining company. The samples were dissolved and the noble metals extracted from the base metals to avoid interference effects that occur, especially when these metals are present in high concentration. The noble-metal content was then determined by the atomic- absorption method described above. The results are given in Table 11, together with the concentrations given by the mine laboratories, and those obtained by emission analysis (horizontal arc) of the same samples carried out in our laboratory.TABLE I1 RESULTS OBTAINED FOR ORE SAMPLES BY VARIOUS METHODS Sample 3A M 8 3F R1 3A M 8 3F R1 3A M S 3F R1 3A M 8 3F R1 Concentration by present method, Element p.p.m. Rhodium 121 42 68 79 Palladium 528 443 429 563 902 82 1 799 782 48 39 44 51 Platinum Gold Concentration given, p.p.m. 96 49 85 83 550 468 489 530 933 805 83 1 7 10 43 48 43 42 Concentration by spectroscopic method, p.p.m. 115 40 72 75 510 450 450 560 880 835 807 770 49 57 39 50June, 19681 DETERMINATION OF PLATINUM, RHODIUM, PALLADIUM AND GOLD 393 CONCLUSION The work described shows that a non-absorbing monitor line can be successfully used in atomic-absorption measurements. The agreement between the values obtained by the different analytical methods in Table I1 is satisfactory, especially as the spectroscopic methods were not then used on a routine basis. It also shows that the integrating method described can be a practical way of determining more than one element simultaneously. We wish to thank the South African Council for Scientific and Industrial Research for the grant without which it would have been impossible to carry out the research. REFERENCES 1. 2. 3. 4. 5. 6. 7. Lockyer. R., and Hames, G. E., Analyst, 1959, 84, 385. Strasheim, A., and Wessels, G., Appl. Spectrosc., 1963, 17, 65. Zeeman, P. B., NaudB, W. J., and van der Westhuysen, 0. A., Tydskr. Natuurwetenskap#e, 1964, Menzies, A. C., Analyt. Chem., 1960, 32, 898. Zeeman, P. B., and Butler, L. R. P., Afipl. Spectrosc., 1962, 16, 120. Jones, W. G., and Walsh, A., Spectrochim. Ada, 1960, 16, 249. Butler, L. P. R., and Strasheim, A., Ibid., 1965, 21, 1207. 4, 202. First received November 19th, 1964 Amended February 8th, 1968