Multielement Trace Analysis of Geological Materials with Solvent Extraction and Flame Atomic Absorption Spectrometry Philip Hannaker and T. C. Hughes* Department of Geology, School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia
The development of a precise analytlcal method Is reported for the determlnatlon of the elements Cu, NI, Co, Cr, Ag, Pb, BI, Cd, Zn, Mn, Au, TI, Sb, Ga, and Mo In geologlcal materials. Results are quoted for USGS standard rocks and are compared with publlshed values; practlcal Ilmlts of detection are generally better than 0.5 ppm. The method utlllres solvent extractlon separatlon and preconcentratlon techniques (chloro Complexes, dlethyldlthlocarbamate, and 8-hydroxyqulnollne chelates are used wlth methyl Isobutyl ketone and n-butyl acetate as solvents), wlth elemental determlnatlon by flame atomlc absorptlon spectrometry. Analytlcal accuracy Is achleved by the chemlcal ellmlnatlon of chelate forming Iron and manganese. The rock dlssolutlon, chelatlon, extraction, and measurement procedures are reported, together wlth partitlon data, for a range of pH, Eh, and Ionic strengths.
Atomic absorption spectrometry (AAS) has become a favored technique for elemental determinations in geochemical distribution and exploration studies. The technique is relatively simple and reliable in operation and the inherent micro (ppm) detection sensitivity for many elements of geochemical interest, together with the speed of analysis, has assisted in its acceptance. AAS is subject to matrix interfering effects which can normally be more easily controlled with flame techniques rather than with the alternative furnace atomization methods. These interferences have been extensively investigated and methods now exist to minimize or correct for their effects (Amos et al. ( I ) , Price (2),Slavin ( 3 ) ,Rubeska and Moldan (4)). A number of authors have reported specific elemental interferences. Govett and Whitehead (5) noted that for the elements Pb, Zn, Ni, and Co, which are used extensively in exploration geochemical studies, extreme caution must be used when employing the standard practice of varying sample weight and dilution of a sample solution to suit the expected elemental concentration. Foster (6)has tabulated the effect of high Ca content on trace Ni and P b data, where Ni at 4 ppm showed an apparent 13 ppm and P b a t 20 ppm gave a 32 ppm result. The main source of interference at low concentrations of trace elements (up to 5 ppm) and low concentrations of major elements (up to 5000 ppm total metal content) is considered to be enhancement due to background or nonatomic absorption. This effect can be corrected for using the hydrogen or deuterium continuum, or by the use of a nonabsorbing wavelength. Matrix matching and standard addition techniques can further improve the analytical precision. At higher concentrations of the matrix materials, suppression effects in the flame and a reduction of atomization efficiency in the nebulizer occur. When these conditions predominate, the use of the standard “background corrector” methods for total molecular absorption results in an overall negative error. SAMPLE PREPARATION USING SOLVENT EXTRACTION METHODS The use of solvent extraction techniques in AAS to eliminate undesirable matrix effects has been widely reported
(Koirtyohann and Wen (7); Kinrade and Van Loon (8)).This relatively simple technique has the further advantage of significantly improving AA detection sensitivities by (a) allowing elemental preconcentration and separation, and (b) presenting the elements of interest to the flame in an aqueous free solvent which can possess excellent atomization and combustion characteristics (Lakanen (9);Culp et al. (10)). For AAS measurements at very low levels (below 1to 5 ppm), some sample pre-treatment is required and the adoption of solvent extraction techniques has been shown to give accurate, reproducible results together with simple, rapid analytical processing and low chemical blanks. This paper describes an investigation into the use of three solvent extraction systems for geological samples. The method has been developed for the determination of Cu, Ni, Co, Cr, Ag, Pb, Bi, Cd, Zn, Mn, Au, T1, Sb, Ga, and Mo in a wide range of materials. Results are quoted for USGS standard rocks and are compared with published values. A number of chelate and ion association solvent extraction systems were considered. Based on previously reported work (Morrison and Freiser (11), Stary (12),Zolotov (13)),cost availability and ease of handling, sodium diethyldithiocarbamate (NaDDC) and 8-hydroxyquinoline chelates and chloride ion association complex systems were adopted. Methyl isobutyl ketone (MIBK) (Kuwata et al. (14); Chau and Lum-Shue-Chan (1511, and n-butyl acetate were chosen as the nonaqueous solvents. NaDDC has been widely used for metal spectrometric determinations, particularly for river and seawater trace metal analysis (Welcher (16),Lacoste et al. (In, Kuwata et al. (14),Nix and Goodwin (18)). Mixed chelating agents such as ammonium pyrrolidine dithiocarbamate (APDC) and diethyl-ammonium diethyldithiocarbamate (DDDC) (8)were found to be unnecessary; the use of NaDDC with control of pH and general extraction conditions was adequate for the reproducible extraction of the chelate complexes from geological samples. 8-Hydroxyquinoline is known to readily form extractable complexes ((II), (15),Butler and Mathews (19)). The use of nonspecific chelating agents for XAS preconcentration processes can be limited by the presence of relatively high concentrations of complex forming elements. In geological materials, Fe is often present a t the 10-20% level and Mn occurs from minor amounts up to approximately 1% . Both of these elements form DDC complexes which have a saturation effect on the organic phase during solvent extraction. Their presence effectively reduces the partition of minor and trace elements into the MIBK, particularly for those chelates of lower stability. A light scattering process (Billings (20)) also becomes apparent a t high complex concentrations when measuring organic phase constituents by AAS. The combination of poor extraction reproducibility and variable light scattering effects leads to a major loss of accuracy and detection capability. Dilution of the organic phase, although reducing the interfering effects, results in an unacceptable loss of sensitivity. A quantitative chemical separation process was developed to eliminate Fe and Mn during the solvent extraction procedure. The Fe was removed in two stages, first using the ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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chloride system with control of HC1 concentration and second, using 8-hydroxyquinoline with control of pH. The Mn was held in the aqueous phase during the NaDDC extraction by the use of EDTA. The importance of investigating extraction parameters, particularly pH and aqueous and organic phase compositions, to optimize elemental separations has been stated by Kinrade and Van Loon (8). Partition coefficients and extraction rates were therefore examined by the use of radiotracers. These were prepared by thermal neutron irradiation of pure elements in the HIFAR reactor at Lucas Heights, Sydney. The isotopes used were 51Cr,6oCo,65Zn,%Mo,llOAg, l13Sn,? 3 n , and 124Sb. The radioactive tracers were added to crushed mineral samples and the dissolution and separation processes were followed by monitoring the associated y activity after each step. The optimization of extraction times and the number of equilibration steps needed was readily assessed using the tracers. Further tracer work showed that some Hg and Ge was lost from geological materials during the dissolution step when HF/HC104 was used. The complete replacement of HC104 by HzS04eliminated the Hg loss (Ehmann and Lovering (21)) but the presence of insoluble sulfates resulted in the possibility of trace element occlusion by absorption into the precipitate and the determination of Hg was therefore omitted from the present scheme.
EXPERIMENTAL Reagents. The reagents used were “analar” grade with the exception of n-butyl acetate which was reagent grade. All water used was distilled and deionized (DDI). Solutions. Buffer. Dissolve 102 g potassium biphthalate in 500 mL water. Extract once with 15-mL MIBK. 8-Hydroxyquinoline.Dissolve 1g 8-hydroxyquinolinein 100 mL MIBK. NuDDC. Dissolve 6 g NaDDC in 100 mL water. Filter the resultant solution and extract three times with 15-mL portions of MIBK. Standard Solutions. Stock Standard Solutions. Dissolve appropriate metal or stoichiometricsalt (“analar”or 99.9% purity) to prepare stock standard solutions at a concentration of 1000 PPm. Instrumentation. The AAS used was a Techtron Model AA-3 with a Varian Techtron 1M-6D Indicator Module. The pH measurements were made using a Radiometer Type TTT-1 Titrator-pH meter.
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Figure 1. pH dependence of extractions with NaDDC showing the effect of EDTA-Pb and Bi
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Figure 2. pH dependence of extractions with NaDDC showing the effect of EDTA-Cd and Co
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DISCUSSION Elimination of Fe Interference. The extraction of Fe(II1) as a chloro complex into an organic solvent is well known (McKaveney and Freiser (22)). The use of this process was investigated as a means of removing the majority of the Fe before addillg the NaDDC complexing agent. There are a number of elements which can be extracted with Fe from chloride solutions;they include Sb(V),k ( I I I ) , Ga(III), Ge(IV), Au(III), Mo(V1) and Tl(II1) (11). The partition coefficient and extraction efficiency of these ion association complexes depends essentially on HC1 concentration and the organic solvent used. A number of systems were therefore examined to investigate the possibility of using the Fe removal step as an integral part of the preconcentration solvent extraction scheme. After removal of the Fe, it was found that Au, T1, and Sb could be measured in the organic phase. Mo and Ga required a further solvent extraction step; they were found to closely follow Fe(II1) in C1-ion association systems and they required a chelate formation step before their separation could be achieved. Other elements which form extractable chloro ion association complexes were not considered in the present study. The efficiency of extraction of metal-chloro ion association complexes into three solvents of suitable volatility and combustion characteristics, namely, ethyl acetate, methyl 1486
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NO. 11, SEPTEMBER 1977
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Flgure 3. pH dependence of extractions with NaDDC showing the effect of EDTA-Zn and Ni
isobutyl ketone and n-butyl acetate, was examined. Morrison and Freiser (11) and Edwards and Voigt (23) reported a general increase in partition coefficients with increasing HC1 concentration for these systems and this was observed with all of the solvents used. It was found that for acid concentrations greater than 4 M HC1, although the elemental extraction was essentially complete, the solubility of the organic solvents in the strongly acid aqueous solutions became a serious problem, except with n-butyl acetate. This ester was therefore used for the quantitative extraction of the elements
minor and trace elements in rocks is restricted by the ready formation of large amounts of the relatively unstable Mn-DDC chelate. The initial high Mn complex concentration and build up of decomposition products makes the general DDC extraction unreliable for accurate AAS measurement of other chelate forming elements (e.g., Cu, Ni, Co, Cr, Cd, Ag, Pb, Bi, and Zn). Hague et al. (25) stated that Mn can be strongly masked with EDTA so as to restrict the formation of other extractable Mn complexes. This was found to be true even in the presence of concentrated NaDDC solutions (e.g., 2000 mg/L Mn was not precipitated from a 100-mL acidic solution containing EDTA in the presence of 10 ml of 6% w/v DDC solution). The effect of EDTA on the formation and extraction of DDC complexes was investigated for the elements Mn, Cu, Ni, Co, Cr, Cd, Ag, Pb, Bi, and Zn. The atomic absorbance of solutions containing EDTA was compared with similar solutions which were EDTA free. The results are shown in Figures 1, 2, 3, and 4. It was observed: (1)That EDTA completely inhibited the formation of Mn-DDC chelates in acid solutions. (2) The presence of EDTA had little effect on the DDC complex partition coefficient of the elements Pb, Bi, Cd, Cu, Cr, and Ag over the pH range 1.5 to 7 . (3) Zn and Ni showed very poor extraction characteristics above pH 2 but for lower pH solutions, EDTA had little effect. (4) Co showed a constant and reproducible (ca. 20%) reduction in absorption signal. Stability of DDC Complexes at pH 2. The stability of DDC chelates of Cu, Ni, Co, Cr, Cd, Ag, Pb, Bi, and Zn with respect to time was fully investigated. The metal chelate stabilities were examined in aqueous solutions a t pH 2 and also after extraction into MIBK with removal of the aqueous phase. In both experiments, the metals were prepared a t approximately 300 ppb in 100 mL of solution in the presence of 10 mL 0.1M EDTA and 3 mL of phthalate at a pH 2.0 f
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Flgure 4. pH dependence of extractions with NaDDC showing the effect of EDTA-Ag and Cu
Fe(III), Au(III), Tl(III), Sb(V), Mo(VI), and Ga(II1) from 8.0 M HCl solutions. Back extraction with 1 M HC1 was found to completely remove the Fe, Mo, and Ga leaving the other extracted elements in the n-butyl acetate. Au, T1, and Sb could then be measured directly by AAS. Separation of Mo and Ga from Fe. Fe, Mo, and Ga are known to form extractable complexes with &hydroxyquinoline (Luke and Campbell (24),Welcher (16)).However, in acidic solutions (pH < 1.6) in the presence of ascorbic acid, Fe(I1) does not react and Mo and Ga can be quantitatively extracted as 8-hydroxyquinoline chelates into MIBK, leaving the Fe in the aqueous fraction. The organic phase can then be analyzed directly by AAS for Mo and Ga. Elimination of the Mn Interference. The use of metal DDC chelates as a means of preconcentrating a number of
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D i s s o l v e rock ( 2 g) i n HF, "03, HClO4, H2SO4 i n Teflon (PTFE) crucible. Evaporate t o i n c i p i e n t d r y n e s s , t a k e up i n 100 m l 8 M H C 1 c o n t a i n i n g f r e e C12. E x t r a c t w i t h 2 x 10 m l n-butyl a c e t a t e . Combine and s e p a r a t e phases.
Organic phase
Aqueous phase
Wash w i t h 3 x 10 m l 1M H C 1 Organic
Evaporate t o (10 m l . Make up t o 200 m l w i t h D D I water. Take a p p r o p r i a t e a l i q u o t s , d i l u t e each t o -80 ml w i t h D D I w a t e r . Add 3 m l phthalate buffer, 10 m l of 0.1M EDTA. Note t o determine Mn t h e EDTA i s n o t added. Adjust pH t o 2.0 5 0.1. Make up t o 100 m l 6% w/v NaDDC s o l u t i o n , e x t r a c t w i t h 25 m l MLBK and a l l o w phases t o s e p a r a t e . I
Aqueous ( c o n t a i n s Fe)
Transfer t o a stoppered t e s t t u b e determine Au, T 1 , Sb by AAS.
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Add Ascorbic Acid, d i l u t e t o 90 m l w i t h D D I w a t e r , a d j u s t pH t o 1.6 5 0.1. Add o x i n e i n MIBK, shake f o r 15 mins and s e p a r a t e phases.
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I I T r a n s f e r t o a stoppered t e s t t u b e determine G a , Mo by AAS
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determined s e p a r a t e l y w i t h EDTA a b s e n t . The measurement i s made immediately a f t e r extraction.
Flgure 5. Summary of analysis procedure ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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Table I. Elemental Concentrations in USGS Standard Rocks AGV-1, ppm BCR-1, ppm Measured Quoted Measured Quoted Element concn valuea concn valuea 4.5 0.69 4.01 Sb 0.63 1.31 1.0 T1 0.42 0.30 Mo 3.63 1.1 3.35 2.3 24.1 20.5 Ga 20.0 20 0.95 ppb BDL~ 0.6 ppb Au BDL~
