Interference of Lithium in Atomic Absorption Spectrometry A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia IN ANALYSIS OF SOLID materials by the atomic absorption method, the sample must first be brought into solution. If this can be achieved successfully, then a wide variety of metals often can be determined by the atomic absorption method from the one solution procedure. For geological samples, fusion techniques are frequently used to obtain the sample in solution form. There are two obvious requisites for a particular fusion method to be suitable : Decomposition must be complete and the fusion process should not introduce any elements to be determined. Sodium and potassium are commonly occurring elements in geological samples and determination of these elements is -often required. Most fusion methods, however, utilize sodium or potassium salts such as the hydroxides, carbonates, peroxides, etc. which eliminates the possibility of determining one or the other of these elements without a separate fusion. Lithium, however, is not as commonly required in routine analysis, and use of lithium salts in the fusion method will often avoid the necessity of a second fusion. Lithium fusions can therefore be quite advantageous. The fusion method developed by Biskupsky ( I ) using lithium fluoride and boric acid has been found by the authors to provide virtually complete decomposition of a wide variety of rock types and is particularly suitable for geochemical analysis. For instance Bond and coworkers (2) recently compared this method with several well-documented fusion techniques, and found that the lithium fluoride-boric acid or sodium fluorideboric acid methods were the most satisfactory, little or no residue being left after a single fusion. As a fusion method prior toanalysis of a wide range of metals by atomic absorption, including sodium, the method is therefore particularly suitable. Sodium and potassium can often interfere in atomic absorption analysis (3). The low ionization potentials of sodium and potassium, relative t o other elements, causes an enhancement in the absorption of the other elements if the latter are ionized in the flame being used, but less preferentially than sodium or potassium. Table I shows the ionization potentials and percentage ionization of the alkali metals in various flames ( 4 , 5). Interference due t o lithium ionization would probably be less than that due t o the other alkali metals; however, when one considers the very high concentrations of lithium introduced by the lithium fluoride-boric acid fusion, significant ionization interference would be expected. To utilize the advantages of this fusion method with atomic (1) V. S. Biskupsky, Anal. Chim. Acfa,33, 333 (1965). (2) A. M. Bond, T. A. O’Donnell, A. B. Waugh, and R. J. W. McLaughlin, ANAL.CHEM.,42,1168 (1970). (3) J. Ramirez-Mufioz, “Atomic-Absorption Spectroscopy,” Elsevier, Amsterdam, 1968, p 269. (4) J. A. Dean, “Flame Photometry,” McGraw-Hill, New York, N. Y., 1960, p 42. ( 5 ) W. H. Foster, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1959. 134
Table I. Ionization Potentials and Percentage Ionization of Alkali Metals in Various Flames Values as Reported in Ref 4 % Ionization in different f l a m e s Hydrogen- AcetyleneIonization potential Air-propane oxygen oxygen Element (eV) (2200 “K) (2450 OK) (2800 OK) Li 5.37 0.01 0.9 16.1 Na 5.12 0.3 5.0 26.4 K 4.32 2.5 31.9 82.1 Rb 4.16 13.5 44.4 89.6 cs 3.87 28.3 69.6 96.4 Table 11. Reagents, Resonance Lines, and Flames Used for Each Element Element Reagents used Line, A Flame( s) Zinc(I1) sulfate 2138 Zn 2794 Mn Manganese(I1) sulfate Lead(I1) nitrate 2170 Pb 3132 Mo Molybdenum trioxide 2551 Sodium tungstate W 3642 Potassium titanium(1V) Ti oxalate 3601 Zirconyl chloride Zr octahydrate 3073 Hafnium nitrate Hf
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absorption spectrometry, it is therefore necessary to know to what extent the interference occurs and for those cases where it occurs, how to eliminate it. These aspects of lithium interference were therefore investigated with a number of elements and the results are reported in this paper. EXPERIMENTAL All chemicals used were of reagent grade purity. Lithium was added as both the chloride and perchlorate salt, for tests on lithium interference. Table I1 summarizes the chemicals used to provide the elements on which interference tests were carried out along with the resonance lines and types of flame used. The atomic absorption spectrophotometer used in this work was a modified Techtron Model AA-3 instrument. Techtron burners AB-50 (nitrous oxide-acetylene) and AB-51 (airacetylene) were used with the AA-5 model nebulizer and spray chamber. Small bore hollow cathode lamps manufactured by Atomic Spectral Lamps, Melbourne, were used in conjunction with the AA-5 power unit and lamp turret. The operating currents for the lamps and the monochromator slit widths used in each case were those suggested by the lamp manufacturer. RESULTS AND DISCUSSION Enhancement of Zinc Absorption by Lithium. According to Fuwa et al., (6), a solution containing 100 ppm lithium (6) K. Fuwa, P. Pulido, R. McKay, and B. L. Vallee, ANAL.CHEM., 36,2407 (1964).
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(as the chloride) did not affect the absorbance reading of a standard 0.2 ppm zinc solution in the hydrogen-air flame. However, we have found that as little as 5 pprn lithium caused significant enhancement of the absorbance of a 0.5 pprn zinc solution in the air-acetylene flame. Typical results are shown in Table 111. The enhancement reaches a maximum value at about 1000 ppm lithium and thereafter remains virtually constant. Similar results were obtained in the hotter nitrous oxideacetylene flame. The presence of lithium was estimated t o potentially provide up t o a 15 % error in the determination of zinc if calibration curves were prepared in the absence of lithium and n o correction were t o be applied for lithium interference. Enhancement of Lead Absorption by Lithium. Dagnall and West (7) reported no interference o n lead by a 1000-fold excess of lithium, in the air-propane flame. I n the air-acetylene flame, we have found that lithium causes considerable enhancement of the absorbance of a 5-ppm lead solution, as shown in Table IV. Again a maximum enhancement at about 1000 ppm lithium was found, and similar results were observed i n the nitrous oxide-acetylene flame. Enhancement of Manganese Absorption by Lithium. Lithium was found t o enhance the absorbance of a 3.0-ppm manganese solution in the air-acetylene flame. Typical results are shown in Table V. Enhancement of Other Elements Absorption by Lithium. Results similar to the above were found for titanium, zirconium, and hafnium in the nitrous oxide-acetylene flame. No enhancement of molybdenum or tungsten was observed in either the air-acetylene or nitrous oxide-acetylene flames.
Table 111. Enhancement of Zinc Absorbance by .Lithium Zinc concn, Lithium added, PPm PPm Av absorbancea 0.5 0 0.132 0.5 5.0 0.141 0.5 20.0 0.144 0.5 35.0 0.146 0.5 500 0.149 0.5 1000 0.150 0.5 2000 0.150 Average of three determinations.
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Table IV. Enhancement of Lead Absorbance by Lithium Lead concn, Lithium added, ppm PPm Av absorbancea 5.0 0 0.300 5.0 600 0.319 5.0 800 0.323 5.0 loo0 0.325 a Average of three determinations. Table V. Enhancement of Manganese Absorbance by Lithium Manganese concn, ppm Lithium added, ppm Av absorbance“ 3.0 0 0.303 3.0 800 0.313 3.0 loo0 0.314 Average of three determinations.
ELIMINATION OF LITHIUM INTERFERENCE Ionization interference can be overcome by using the addition method of analysis or by using standards containing a similar concentration of the interfering element t o the standard solutions. Alternatively, as the absorbance of elements was observed t o be independent of lithium concentration above 1000 ppm, addition of a standard lithium concentration above this value t o all test and standard solutions provides a third and probably the simplest and most convenient method for elimination of interference (8,9). The seriousness of this interference and the effective manner in which it can be overcome is demonstrated by the following set of results for determination of zinc in an impure willemite (ZnzSiOJ sample. Using standards containing zinc only, the result of five determinations was 58.3 =t0 . 6 x Zn. By using the addition method on the same sample, a value of 55.6 i 0.9% Z n was obtained. I n the solution analyzed by the addition procedure, the concentration of lithium from the fusion was about 10 ppm, and when zinc standards containing 10 ppm lithium were used, the result was 55.8 i 0.5% Zn. Addition of 2000 ppm lithium t o both standards and unknown solution gave 55.7 i 0.5%. As this sample contained high concentrations of zinc, the fusion solution was diluted prior t o the zinc determination by atomic absorption spectrometry, and the concentration of interfering lithium is not very high. However, even the potential interference that can be caused by 10 pprn lithium is significant, and it can be readily visualized that the apparent result for zinc when it is only a trace con(7) R. M. Dagnall and T. S. West, Tulunfu, 11, 1553 (1964). (8) J. E. Allan, Spectrochim. Acta, 18, 605 (1962). (9) E. E. Angino and G. K. Billings, “Atomic Absorption Spectrometry in Geology,” Elsevier, Amsterdam, 1967, p 37.
stituent of the rock sample and no dilution of the fusion solution is possible, could be exceedingly high unless the lithium interference is eliminated. When the willemite sample was analyzed for manganese and lead, all three methods were again successful in removing the interference. CONCLUSIONS A by no means exhaustive study of the interference of lithium on the determination of other elements has shown that significant interference occurs in many cases. If a fusion method such as the lithium fluoride-boric acid one is used for destruction of the solid prior t o analysis of the sample with the atomic absorption method, then the possibility of lithium interference should be investigated. If present, it can be eliminated simply by the methods described above. The simplest approach, however, seems to be t o add a large concentration of lithium t o all test and standard solutions, as above about 1000 ppm, absorbance becomes independent of lithium concentration for all elements examined in this work. The interference appears t o be due to ionization of lithium and the mechanism is undoubtedly the same as that commonly observed with sodium and potassium. Allan (8) has stated that ionization interference is minimized in cool flames, so the lack of interference reported for lithium o n the absorbance of zinc (6) and lead (7) may be due t o the use of cooler hydrogen-air and air-propane flames (See data in Table I). RECEIVED for review August 18, 1970. 25,1970.
Accepted September
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