ANALYST, JANUARY 1986, VOL. 111 19 Determination of Gallium in Phosphorus Flue Dust and Other Materials by Graphite Furnace Atomic Absorption Spectrometry David C. Barron and Benjamin W. Haynes US Department of the Interior, Bureau of Mines, 4900 Lasalle Road, Avondale, MD 20782, USA A simple, rapid method for the determination of gallium in inorganic matrices using Zeeman-corrected graphite furnace atomic absorption spectrometry (GFAAS) is described. The method uses an acid dissolution technique, Mg(NO& as a matrix modifier and the method of standard additions. The 294.4-nm wavelength is used to improve the signal to noise ratio without loss of sensitivity. Gallium was determined in phosphorus flue dust and also in coal fly ash, copper ore and steelmaking flue dust. The procedure is used as an alternative to neutron-activation analysis (NAA) and is intermediate in sensitivity between flame atomic absorption spectrometry and NAA.A comparison of GFAAS and NAA results is presented. Keywords: Gallium determination; graphite furnace atomic absorption spectrometry; matrix modification; mineral wastes An ongoing project of the Bureau of Mines is the recovery of gallium from high-volume mining and metallurgical wastes. A rapid method was needed to provide gallium concentrations when determining optimum parameters for its extraction and recovery. High-volume metallurgical wastes such as phospho- rus flue dust, coal fly ash and electric furnace steelmaking flue dust were studied as potential materials for gallium recovery. The determination of trace levels of gallium by neutron- activation analysis (NAA) and by graphite furnace atomic absorption spectrometric (GFAAS) analysis in conjunction with solvent extraction pre-concentration has been repor- ted.1-3 The method of Lo et aZ.,2 used in determining trace levels of transition metals in sea water, was applied to gallium using dithiocarbamate complexes to extract the gallium and subsequent back-extraction into nitric acid using Hg2+. This method gives good results, but the use of high concentrations of mercury in the graphite furnace and its subsequent vaporisation into the laboratory were not desired. The use of this same method for NAA using Pb2+ as a back-extraction agent was described by Yu and Wai.1 The use of NAA for determining gallium is very sensitive, but it is not generally available for routine use.The use of the alternative wavelength of 294.4 nm has been reported to give a sensitivity that is equal to or better than that of the primary wavelength of 287.4 nm for gallium in GFAAS.3 The use of the L’vov platform in this GFAAS method gives enhancement over normal tube wall atomisation of the analyte.4 Based on very limited testing, Botha and Fazakas3 suggested that ascorbic acid may be a suitable matrix modifier. This study was carried out with dilute aqueous solutions of gallium in 0.05% V/V nitric acid. Shan et aZ.5 used Ni(N03)2 as a matrix modifier and the 287.4 nm line. In their testing for suitable matrix modifiers, the relative sensitivities for Ni(N03)2 and Mg(N03)2 were similar, with Mg(N03) being slightly better.The samples used in their analysis were digested using HC104, thereby adding C1- to the matrix. The chloride effect was overcome in this method by adding a second modifier of NH4N03 to remove the chloride effect.5 The high volatility of chlorides6 in the graphite furnace was avoided in this proposed method, which required only the use of Mg(N03)2. To avoid the use of solvent extraction and mercury2 in the laboratory and chloride536 in the graphite furnace, a simple GFAAS procedure was desired. The availability of NAA was such that sample analysis would take several weeks. The proposed method uses a sample digestion and analysis procedure modified from Haynes ,7 polarised Zeeman back- ground-corrected GFAAS, and avoids the presence of chloride.Although only Zeeman GFAAS was used, standard GFAAS using deuterium arc background correction should also provide adequate precision. Experimental Reagents All acids used were of analytical-reagent grade. Distilled, de-ionised water (DDW) was used throughout. A 1000 mg 1-1 gallium standard (Aldrich Chemical Co.) was diluted to 1 mg 1-1 with 2% V/VHN03 in DDW. This solution could be used for up to a month without change in concentration. The matrix modifier was a 1% V/V solution of Mg(N03)2 in DDW. Instrumentation A Perkin-Elmer Model 5000 Zeeman atomic absorption spectrometer* and an HGA-400 graphite furnace controller were used. An AS-40 autosampler was used to inject the samples into the furnace. Pyrolytically coated graphite tubes and a L’vov platform were used for all determinations. A standard gallium hollow-cathode lamp was used at 294.4 nm, a current of 12 mA and a slit width of 0.7 nm.Sam p 1 e s Phosphorus flue dust samples were obtained from elemental phosphorus production furnaces in Tennessee and Idaho. The stainless-steel electric furnace dust was a blend of 12 dust samples from various furnaces.* A sample of mixed copper ore was obtained from the St. George Mine in Utah. The coal fly ash was National Bureau of Standards (NBS) Standard Reference Material (SRM) 1633a that contained a reference (non-certified) value for gallium of 58 mg kg-1. Procedure Dry the samples overnight at 140°C and store them in a desiccator to cool. Weigh 0.5 g of sample to the nearest 0.1 mg and place it in a Teflon beaker.Add 25 ml of DDW, 5.0 ml of HN03 and 5.0 ml of HF. Heat the samples on a hot-plate at moderate heat until dry, usually 1-1.5 h. Cool the samples and then add 20 ml of DDW and 5.0 ml of HN03. Re-heat the samples at low temperature for 5-10 min. Filter each sample into a 100-ml calibrated flask, washing the filter-paper thoroughly with DDW. Cool the samples to room tempera- * Reference to specific products does not imply endorsement by the Bureau of Mines.20 ture and dilute to 100 ml. Serial dilution of the sample may be required and HN03 should be kept at 1% V/V in these dilutions. Table 1 gives the graphite furnace conditions developed and used in this method. All readings are in absorbance units in the peak-area mode using a 5-s integration time.The argon purge gas is interrupted during atomisation. The sample volume is 20 pl and is followed by 5 pl of Mg(N03)2 solution. This 5-p1 volume provides 50 pg of Mg(N03)2 to the L'vov platform. The use of Mg(N03)2 as an ashing aid here is similar to its use in other methods.7.9 All samples are analysed by the method of standard additions using dilutions of the sample plus additions of 0, 5 , 10 and 15 yg 1-1 of gallium. Results and Discussion Optimisation of Parameters Initial work was performed using the primary gallium wavelength of 287.4 nm. Erratic results were obtained at this wavelength using Mg(N03)2 as the matrix modifier and high background absorbance readings were observed. Recent studies by Wibetoe and Langmyhrlo on spectral interferences in Zeeman background-corrected AAS indicated a back- ground overcompensation for gallium in the presence of iron.This introduces serious negative systematic errors at the 287.4-nm line. This overcompensation is due to the presence of an iron line close to the 287.4-nm gallium line whose 0 components overlap the gallium analyte line. Under the same conditions using deuterium arc background correction, these errors are positive. 10 The spectral interferences were avoided by using the alternative wavelength for gallium of 294.4 nm. The parameters shown in Table 1 were the result of tests to determine optimum charring and atomisation temperatures. Using 2 0 4 aliquots of 25 pg 1-1 aqueous gallium standard, a 2000 "C atomisation temperature and varying the charring temperature, the effect of Mg(N03)2 matrix modifier on gallium recovery was determined.The results are depicted in Fig. 1. Using the same parameters for the tests shown in Fig. 1 but with 20-pl aliquots of a dissolved phosphorus flue dust sample, the effect of Mg(N03)2 on gallium recovery was again determined and is shown in Fig. 2. Finally, using Mg(N03)2 as a matrix modifier and a 1200°C charring temperature, the effect of varying atomisation temperature on gallium recovery was determined for an aqueous gallium standard and a dissolved sample of NBS SRM 1633a coal fly ash. These results are presented in Fig. 3. From Figs. 1-3, an optimum char temperature of 1200°C and an optimum atomisation temperature of 2000°C were obtained. The results in Fig. 2 indicate that a slightly higher signal is obtained at lower temperatures by not using the Mg(N03)2 modifier in the phosphorus flue dust samples.However, the decreased background obtained with the modifier at the higher char temperature of 1200 "C gave better precision. This increased precision at the higher char temperature was determined to be more important than the small increase in signal intensity at lower char temperatures without the modifier. Therefore, Mg(N03)2 was used with all samples in this study. The maximum power heating mode was used in all tests to provide Table 1. Conditions in GFAAS for gallium at 294.4 nm Temperature/ Argon flow-ratel Step "C Ramp/s Hold/s ml min-1 1 160 1 60 300 2 1200 45 20 300 3 2000 O* 5 0 4 2650 1 5 300 5 20 10 5 300 * Maximum power heating mode.0.1 50 0.125 ~ 0.100 0 m + 0 0.075 2 0.050 0.025 n ANALYST, JANUARY 1986, VOI 1 I I I I 111 "400 600 800 1000 1200 1400 1600 1800 Tem peratu rePC Fig. 1. Effect of Mg(N03)2 on the recovery of gallium by varying the charring temperatures with atomisation at 2000 "C. Sample, a ueous standard solution. A, With Mg(N03)*; and B, without Mg(N8,), 1 0.175 400 600 800 1000 1200 1400 1600 Tern peratu rePC Fig. 2. Effect of Mg(N03), on the recovery of gallium by varying the charring temperature with atomisation at 2000 "C. Sample, phospho- rus flue dust sample. A , With Mg(N03),; and B, without Mg(N03)2 0.175 0.150 0.125 8 c 0.100 + s 0 2 0.075 0.050 0.025 0 A 1600 1800 2000 2200 2400 2600 Temperature/"C Fig. 3. Effect of Mg(N03)2 on the recovery of gallium by varying the atomisation tem erature with charring at 1200 "C.Samples: A, aqueous standart and B, coal fly ash, both with Mg(N03)2ANALYST, JANUARY 1986, VOL. 111 21 Table 2. Gallium concentrations in various samples determined by GFAAS and NAA Gdmg kg-1 GFAAS* NAAt Sample Matrix type N x (7 C.V., Yo N x PFD-2 Phosphorus flue dust 10 315 28 8.8 2 300 PFD-1 Phosphorus flue dust 8 512 43 8.4 2 505 2 530 PFD-3 Phosphorus flue dust 2 531 2 840 PFD-4 Phosphorus flue dust 2 830 NBS 1633a$ Coal fly ash 8 61 5 8.2 2 58 NDP NDQ ss-1 Stainless-steel flue dust 3 50 3.6 7.2 - 2 270 - - - - MCO-1 Mixed copper ore 2 253 - * N = Number of replicate samples; X = average of replicate gallium determinations; u = standard deviation; C.V. = coefficient of t Neutron-activation analysis performed under the direction of A.B.Whitehead, Salt Lake City Research Center. $ Not certified, NBS reference value 58 mg kg-1. 0 ND = Not determined. variation. rapid, even heating to the platform for atomisation with a final burnout temperature at 2650 “C to clean the graphite furnace prior to introduction of the next sample aliquot. To determine if standard additions methods were neces- sary, slopes of calibration graphs were determined for aqueous gallium standards, phosphorus flue dust samples and coal fly ash samples using the method of standard additions. Using linear regression analysis, sample solution graphs gave slopes different from that of the aqueous calibration graph. These results led to the use of the method of standard additions for all samples.Using 20 p1 of a 50 p1-1 (1.0 ng) gallium standard, a peak-area absorbance of 0.280 was routinely obtained. Analysis of Samples Using the above procedure, four phosphorus flue dust samples, one coal fly ash sample, one stainless-steel flue dust sample and one mixed copper ore sample were analysed for gallium. All samples except the stainless-steel flue dust were also analysed in duplicate by standard NAA procedures; comparative results are given in Table 2. The lack of certified standards for gallium in reference materials makes it difficult to assess the accuracy of this procedure, but the results of GFAAS and NAA compare well. The coefficient of variation was between 7.2 and 8.8. Conclusions This method for gallium determination using Zeeman- corrected GFAAS and the L’vov platform is intermediate in sensitivity between flame AAS and NAA.Although not as sensitive as NAA, this method is more rapid than NAA and can be performed in laboratories equipped with graphite furnace AAS. It does not require the use of a reactor or other associated equipment for storing and counting radioactive decay products. A lower limit of about 0.8 pg 1-1 of gallium can be determined in aqueous solution. The method emplovs a straightforward dissolution technique and the U V af Mg(NO& as a single matrix modifier. All samples involved in this study required standard additions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Yu, J. C., and Wai, C. M., Anal. Chem., 1984, 56, 1689. Lo, J. M., Yu, J. C, Hutchison, F. I., and Wai, C. M., Anal. Chem., 1982,54,2536. Botha, P. V., and Fazakas, J., Anal. Chim. Acta, 1984, 162, 413. Slavin, W., Manning, D. C., and Carnrick, G. R., At. Spectrosc., 1981, 2, 137. Shan, X.-Q., Yuan, 2.-N., and Ni, Z.-M., Anal. Chem., 1985, 57, 857. Slavin, W., Carnrick, G. R., and Manning, D. C., Anal. Chem., 1984, 56, 162. Haynes, B. W., At. Absorpt. Newsl., 1978, 17, 49. Law, S. L., Lowry, W. F., Snyder, J. G., and Kramer, G. W., “Characterization of Steelmaking Dusts from Electric Arc Furnaces,” Report of Investigation, RI 8750, National Bureau of Mines, Washington, DC, 1983,26 pp. Slavin, W., Carnrick, G. R., Manning, D. C., and Pruszkow- ska, E., At. Spectrosc., 1983, 4, 69. Wibetoe, G., and Langmyhr, F. S., Anal. Chirn. Acta, 1984, 165, 87. Paper A.5/194 Received May 28th, 1985 Accepted July 29th, 1985