JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 679 Determination of Cadmium by Electrothermal Atomic Absorption Spectrometry Using Palladium and Tartaric Acid as a Mixed Chemical Modifier and a Tungsten-foil Platform with the Possibility of Standardless Analysis Ma Yizai Li Zhikun Wang Xiaohui Wang Jiazhen and Li Yongquan Institute of Analysis and Measurement Chinese Research Academy of Environmental Sciences Beijing 1000 12 China Palladium and tartaric acid as a mixed chemical modifier is a better modifier for the determination of Cd in complex environmental samples than Pd and tartaric acid (or ascorbic acid) used as separate modifiers. The mixed modifier can effectively eliminate matrix effects and the two peak signals observed in the atomization of complex environmental samples.It has also been found that a W-foil platform has a longer lifetime than a Ta-foil platform. A comparison of the calculated and experimental values obtained for the characteristic mass m, with a W-foil platform in a pyrolytic graphite coated graphite tube using a mixed modifier shows that an atomization temperature higher than T(set) = 1673 K and T(effective) = 1530 K can be used for the determination of Cd. Quantitative analytical data for Cd in sea-water and in solutions of reference materials (pork liver wheat powder peach leaf tea leaf tea tree leaf cabbage water sediments coal fly ash soils and rock) by standardless analysis are presented (LOD = 0.3pg; RSD < 5%). Keywords Nectrothermal atomic absorption spectrometry; chemical modifier; cadmium; environmental samples Ma et al.' have previously described the determination of Cd in environmental samples by electrothermal atomic absorption spectrometry (ETAAS) using a Ta-foil platform with the pos- sibility of standardless analysis.The highly resistant Ta-foil surface is preferable for the determination of Cd especially when large amounts of various mineral acids and perchloric acid are used for sample decomposition. Ma and Cheng developed a new atomizer a pyrolytic graphite coated graphite tube lined with tungsten (WPGT) for the determination of C O ~ Pd,3 Y,4 Ba In Pb Cu Ni Rb Au Cd Mn Li Cr and Sb.' The common oxides and carbides of W sublime at moderate temperatures making the W surface 'self-cleaning' during high-temperature treatment.In the present study a new W-foil platform is used for the determination of Cd in environmental samples. Shan and Ni6 have reported the use of Pd as a modifier for the determination of Hg Pb Sb Bi Se and Te. It has been ascertained that the reduction of Pd begins around a minimum temperature of 600"C.7 Shan et aZ.* found Pd and ascorbic acid to be a more effective modifier than Pd or ascorbic acid alone; ascorbic acid reduces Pd ions in solution to elemental Pd. Zhuang et a2.' used Pd and citric acid as a mixed chemical modifier for the determination of Zn and Cd using wall PGT atomization. The aim of this work was to utilize Pd and tartaric acid (TA) as a mixed chemical modifier and a W-foil platform for the determination of Cd in complex environmental samples with the possibility of standardless analysis.Experimental Apparatus A Hitachi Model 28000 d.c. magnet Zeeman-effect atomic absorption spectrometer was used for the determination of Cd at the resonance line of 228.8 nm with a spectral bandwidth of 0.4 nm. The Cd hollow cathode lamp was operated at a current of 2.5mA. Samples of 0.010ml were introduced into the furnace using Eppendorf microlitre pipettes fitted with dispos- able polypropylene tips. The temperature measurements were made using an electro-optical pyrometer and chart recorder. The argon sheath gas flow rate was 3 1 min-' in the 'gas stop' mode during the atomization step. The PGTs were made by the Beijing Research Institute of Materials and Technology Ministry of Astronautics. The dimensions of the PGTs are inner radius 2.95 outer radius 4.05 and length 28 mm.The W-foil platform (WFP) is 4 x 9 mm with a 1 mm rim a thickness of 0.22 mm and a mass of 230 mg. The WFP is inserted into the centre of a PGT. The furnace heating programme consisted of drying at 80-120°C €or 20 s ashing at 300°C for 20 s (all ashing times 20 s) atomization at 1400 "C for 20 s 1500 "C for 20 s (for Table l) 2000 "C for 10 s 2100 "C for 10 s and 2600 "C for 5 s and cleaning at 2400°C for 3 s. Reagents A stock solution of Cd (0.500 mg ml-l) was obtained from the Chinese National Environmental Monitoring Centre and stored in an ampoule. Tartaric acid (10 mg ml-l) was prepared by dissolving the materials in concentrated ammonium solution. A solution of Pd (as PdCl,); (5 mg ml-l) was prepared by dissolving a suitable amount of PdCl (spectroscopic-grade Beijing Chemical Company) in 0.3 mol 1-' HNO,. The 0.5 mg ml-' solution of Pd was obtained by diluting the above solution with concentrated ammonium solution.The mixed solution of 10 mg ml-1 TA+O.5 mg ml-I Pd was obtained by dissolving the TA in the solution of 0.5 mg ml-1 Pd. Sample Dissolution Procedures Geochemical and water sediment samples Samples of 100 mg of reference materials (RM) of geochemical samples Rock GSR1 soil samples ESS3 893 Tibetan soil and water sediments GSD1,2,4,5,6,9 10 11 and 81-101 (obtained from the Institute of Physical and Chemical Prospecting Institute of Analysis and Measurement of Minerals and Rocks Beijing China) and Coal Fly Ashes National Institutue of Standards and Technology (Gaithersburg MD USA) Standard Reference Material (SRM) 1633a and 82-201 Coal Fly Ash (Research Centre for Eco-Environmental Sciences Academia Sinica China) were decomposed under pressure in poly(tetrafluorethy1ene) (PTFE) crucibles with the addition of 4 ml of 67% HNO 6 ml of 35% HF and 4 ml of 72% HC10,680 JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL.9 and placed in a heating vessel at 180°C for 8 h followed by evaporation of the solution to near dryness. The residues were dissolved in 5 ml of 0.8 mol 1-' HNO and the solutions filtered into 50 ml calibrated flasks. The concentrations of the final solutions were 2.00 mg ml-' of Cd. Biologicul sunzples Samples (1 g) of biological RMs were placed in PTFE crucibles and 5 ml of HNO,+ 1 ml of HF were added.The crucibles were capped and after being stored overnight the samples were further digested by the addition of 4 ml of HClO and heated in an oven. The solutions were then evaporated to near dryness followed by the addition of 5ml of 4mol 1-' HNO,. The solutions were transferred into 25 ml calibrated flasks and diluted with de-ionized water. The RMs used were cabbage pork liver (Ministry of Commerce China) tea tea tree leaf peach leaf (Research Centre of Eco-Environmental Sciences Academia Sinica) rice flour (Beijing Centre of Environmental Monitoring) and wheat flour (Ministry of Grain China). The concentrations of the final solutions were 40.0 mg ml-' of Cd. Results and Discussion Role of Mixed Modifier (Pd + TA) in WFP for Determination of Cd Palladium is a good retarding modifier of Cd because of the formation of the Pd-Cd alloy.Unfortunately Cd is a very volatile element so is lost easily at low temperatures before Pd can be reduced to the metallic form. Clearly to overcome the problem of loss of Cd Pd must be present in reduced metallic form as early as possible in the ashing stage. The addition of a reducing agent' is an essential and effective way to achieve the optimum result for Pd as a modifier. This has previously been shown by Shan et d8 and Zhuang et aL9 using ascorbic and citric acid respectively as reducing agents. In this paper a mixed modifier (Pd + TA) in a WFP is used for the determi- nation of Cd the mixed modifier solution is stable for several months without precipitation of the Pd metal.Fig. 1 illustrates the Cd absorbance profiles of 50pg of Cd for an aqueous solution in the absence of modifier and in the presence of 0.010 ml of 0.5 mg ml-' Pd in aqueous solution in 10 mg ml-' TA or 0.5 mg ml-' Pd+ 10 mg ml-' TA. Fig. 2 illustrates the absorbance profiles for 33.5 pg of Cd for a pork liver solution at a concentration of 50 mg ml-'. The ashing temperature is 50O0C which is the maximum ashing tempera- ture that can be used without significant loss of Cd in aqueous solutions in the absence of a modifier. The results reveal that in the presence of a Pd modifier the first peak which is thought to correspond to the dissociation of CdO is weak for 50 pg of Cd in aqueous solution and becomes stronger in the pork liver solutions. The second peak is believed to correspond to the atomization of the Pd-Cd species.The absorbance profiles for 50pg of Cd in aqueous and pork liver solutions for the mixed modifier (Pd + TA) and without modifiers reveal only one peak. The peak position without modifiers corresponds to the first peak (dissociation of CdO) and the peak position for the mixed modifier (Pd + TA) corresponds to the second peak (atomization of the Pd-Cd species). Evidently the addition of 10 mg ml-' of TA reduces all CdO species to Pd-Cd species so that the first peak disappears in atomization. Thus the reduction of Cd could also be promoted at lower temperatures in the presence of 10 mg ml-' of TA in a WFP. Theoretical Calculation of the Characteristic Masses and Atomization Effeciencies of Cd at Different Atomization Temperatures Calculation of the characteristic masses was performed as previously described' and a table of the calculated values mo (cal) is given as Table4 in ref.1. The calculated atomization 0.4 0.2 a 0 + a C m 2 0.4 0.2 0 (iai/'/+ A 0 kkx - ( 6 ) ' B' A' 0 5 ' 10' 16 Tirne/s Fig. 1 Atomization signals for 50 pg of Cd in H,O with W platform atomization. Injection volume 0.010 ml. Atomization temperature T(set) 1 2600; 2 2000; and 3 1400°C. Modifiers used (a) A 100 pg of tartaric acid; B 5 pg of Pd + 100 pg of tartaric acid; and (b) A' no modifier; and B' 5 pg of Pd 0.4 0.2 8 0 m n A' B' I 0 -7- 5 10 16 Tirne/s Fig. 2 Atomization signals for Cd in 50 mg ml-' SRM solution of Pork Liver (33.5 pg of Cd) with W platform atomization. Injection volume 0.010 ml.Atomization temperature T(set) 1 2600; 2 2000; and 3 1400°C. Modifiers used (a) A 100 pg of tartaric acid; and B 5 pg of Pd + 100 pg of tartaric acid; and (b) A' no modifier; and B' 5 pg of Pd modifierJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 68 1 efficiency (&IA) is defined as follows1o &IA= 100[rno(cal)/m,(exp)] where m,(exp) is the experimental characteristic mass. Ma et al.l have previously discussed the temperature dependence of atomization efficiencies of Cd in a Ta-foil platform inserted into a PGT &IA is near 100 when 0.5 mg ml-1 Pd is used as a modifier and atomization tempera- tures higher than T(set)= 1673 K and T(eff) = 1530 K are used where T(set) is set from the 28000 instrument and the effective atomization temperature T(eff) = 1/2[T(max) + T(end)].Atomization efficiencies can be used for the evaluation of the loss of Cd during ashing with the WFP and the dependence of the efficiency of Cd atomization on atomization temperature with the WFP. Ashing Temperature Dependence of Atomization Efficiencies of Cd with the WFP The dependence of atomization efficiencies of Cd with the WFP on ashing temperature are shown in Figs. 3-6. The high atomization temperature T(set) = 2373 K T(eff ) = 2100 K was used in order to obtain an &IA value near 100. T(set)= 1773 K T(eff) = 1550 K was used for sea-water in order to achieve low background absorption values. Fig. 3 shows the dependence of &IA on ashing temperature without a chemical modifier. It can be seen that the loss of Cd occurs at temperatures above 773 K.For selected sample solutions which have serious matrix effects the following values of &IA(%) can be reached 76 (sediment GSD10); 71 (1 mol 1-1 HClO,) 65 (tea tree leaf) and 47 (sea water) Fig.4 shows the dependence of &IA on ashing temperature when using 10 mg ml-1 of TA as modifier. The loss of Cd with the TA modifier occurs at temperatures above 773 K as observed in the absence of a modifier. Using 100 hq 50 0 x-x-x-x 500 1000 1500 2000 2500 Ashing temperature (T,,,) and atomization temperature (Teff)IK Fig. 3 Ashing (left-hand curves) and atomization (right-hand curves) temperature dependence of atomization efficiencies using a WFP without chemical modifier. Injection volume 0.010 ml. A 20 pg of Cd in H,O; B 20 pg of Cd in 1 mol 1-' HC104; C 20 pg of Cd in sea- water; D 40 mg ml-l solution of tea tree leaf RM (9.2 pg of Cd); and E 2.0 mg ml-' solution of water sediment RM ESDlO (22 pg of Cd) / 5--x-x-x-x I 0 2500 500 1000 1500 2000 Ashing temperature (T,,,) and atomization temperature (T,dIK Fig.4 Ashing (left-hand curves) and atomization (right-hand curves) temperature dependence of atomization efficiency using a WFP with 10 mg ml-' of tartaric acid as modifier. Injection volume 0.010 ml. Curves A-E as in Fig. 3 I L I I I j 500 1000 1500 2000 2500 Ashing temperature (TSet) and atomization temperature (T,,)/K 0 ' ' Fig. 5 Ashing (left-hand curves) and atomization (right-hand curves) temperature dependence of atomization efficiency using a WFP with 0.5 mg ml-' of Pd.Injection volume 0.010 ml. Curves A-E as in Fig. 3 I 100 1 hm 50 n " 500 1000 1500 2000 2500 Ashing temperature (T,,,) and atomization temperature (Teff)/K Fig. 6 Ashing (left-hand curves) and atomization (right-hand curves) temperature dependence of atomization efficiency using a WFP and 0.5 mg ml-' of Pd+ 10 mg ml-' of tartaric acid as mixed modifier. Injection volume 0.010 ml. Curves A-E as in Fig. 3 TA as modifier the values of &IA(%) are lower than those achieved without a modifier 59 (sediment GSD10); 48 (1 mol 1-1 HC104); and 41 (tea tree leaf). Using TA as a modifier the strong background absorption values can be depressed the over-corrected background correction in Zeeman AA is lower in TA than when no modifier is used and the value of &IA is enhanced to 79% for sea-water.Fig. 5 shows the depen- dence of &IA on ashing temperature using 0.5 mg ml-1 of Pd as modifier. The loss of Cd with the Pd modifier occurs at temperatures above 1073 K this value is 300 K higher than that with no Pd modifier. Using Pd as a modifier in the range 773-1073 K the values of &IA are near 100% and the matrix effects for 1 mol 1-1 HC104 sediment GSDlO and tea tree leaf can be eliminated effectively. However for an ashing tempera- ture lower than 773 K the matrix effects are still serious. Because of the over-corrected background correction for sea- water the &IA is low (71%). Fig. 6 shows the dependence of &IA on ashing temperature using 10 mg ml-1 TA + 0.5 mg ml-1 Pd as mixed modifier. Using a mixed modifier the values of &IA are near 100% in the range 473-1073 K and the matrix effect can be eliminated effectively. The over-corrected background correction for sea-water using a mixed modifier is the same as that using a TA modifier is 80%.From Figs. 3-6 it can be seen that the best modifier is 0.5 mg ml-' Pd+ 10 mg ml-' TA. Atomization Temperature Dependence of Atomization Efficiencies of Cd with the WFP The dependence of atomization efficiencies of Cd on atomiz- ation temperature with the WFP are shown in Figs. 3-6. The optimum ashing temperature used in Figs. 3-6 is 773 K. Fig. 3 shows that the value of &IA for 20 pg of Cd in H,O decreases below 1500 K (&IA= 91Y0) without a chemical modifier. A possible explanation for this is that some Cd is lost from the682 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL.9 Table 1 Determination of Cd in solutions of RMs with the WFP and 0.5 mg ml- ' Pd+ 10 mg ml-' TA as mixed modifier. Injection volume 0.010 ml ashing temperature 573 K; atomization temperature T (set)= 1773 K T(eff)= 1550 K; m,(cal)=0.377 pg m,(exp)=0.377f0.010 pg dA = 100k 2.7%. Results based on eight measurements per sample Standard or sample Pork liver Cabbage Wheat flour Rice flour Peach leaf Tea Tea tree leaf Rock GSRl Tibetan Soil Soil ESS3 Soil 893 Coal Fly Ash NIST SRM 1633a Sediment 8 1 - I01 Sediment GSD 1 Sediment GSD2 Sediment GSD4 Sediment GSD5 Sediment GSD6 Sediment GSD9 Sediment GSDlO Sediment GSD 1 1 1 mol dm-3 HC104 1 mol dm H2S04 1 mol dm-3 HN03 1 rnol dm-3 HCl 82-201 Concentration of sample/ng ml- ' 50 40 40 40 40 40 40 10 10 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 10 Reference value (ppm) 0.067 0.029 0.03 1 0.020 0.020 0.032 0.023 0.030 0.08 1 0.044 1.24 0.16 1 .o 2.4 0.088 0.065 0.19 0.82 0.43 0.26 1.1 2.3 - - -.- . Concentration of Cd/ng ml - 3.35 1.16 1.24 0.80 0.80 1.28 0.92 0.30 0.162 0.44 2.48 0.32 2.0 4.8 0.176 0.13 0.38 1.64 4.3 0.52 2.2 4.6 5.0 5.0 5.0 5.0 Concentration of Cd determined*/ng ml - ' 3.52 k 0.090 1.22 0.046 1.30 f 0.067 0.84 f 0.049 0.85 f 0.01 7 1.30 f 0.049 0.90 f 0.030 0.298 f O.OO85 0.170 -t 0.0035 0.42 f 0.029 2.35 kO.061 0.34f0.018 1.90 f 0.063 4.84 f 0.155 0.187+0.0115 0.1 35 f 0.0044 0.40 f 0.01 2 1.63 + 0.096 4.41 f 0.184 0.54 & 0.039 2.05 f 0.097 4.67 f 0.188 5.0 f 0.15 4.48 f 0.25 5.08 f 0.07 5.0 f0.15 * Error limits are f 1 standard deviation. atomizer in a molecular form at low atomization temperature. The values of dA for Cd in solutions of 1 mol 1-' HC104 sediment RM GSDlO and tea tree leaf RM were low even when using a high atomization temperature [T(set)= 2873 K T(eff)=2500 K] the values of E ' ~ were 83 80 and 70% respectively and a sharp reduction in dA was obtained at lower temperatures. Because the 1 moll-' HC104 and the RM solutions contained perchloric acid the probable explanation is that Cd can be lost from the atomizer as gaseous cadmium chloride at low temperatures. The low dA for Cd in sea-water can be influenced by two factors firstly the formation of gaseous cadmium chloride during atomization and secondly the strong background absorption of the sea-water matrix at high atomization temperatures which causes over-correction even when using the Zeeman spectrometer. It can be seen from Fig.3 that the use of a chemical modifier is necessary for the determination of Cd in different environmental samples with a WFP. Fig. 4 shows the dependence of atomization efficiencies of Cd on the atomization temperature with the WFP using 10 mg ml- of TA as modifier. Using a TA modifier the matrix effects are more serious than those observed when no chemical modifier is used the values of dA(%) are 65 (Sediment RM GSDlO) 60 ( 1 mol 1-' HC104) and 52 (tea tree leaf RM) at T(eff ) = 2500 K. The over-correction of background absorption for sea-water is low in comparison to the case without a modifier dA is 30% for sea-water using the TA modifier.Figs. 5 and 6 show the dependence of atomization efficiencies of Cd on the atomization temperature using 0.5 mg ml-.' Pd as a modifier and 0.5 mg ml- ' Pd + 10 mg ml- TA as a mixed modifier respectively. The values of dA(%) are 80 [T(eff)= 1300 K] 90 [T(eff)= 1400 K]. 100-105 [T(eff)> 1550 K] for all sample solutions which have serious matrix effects. Because of over-correction of the strong background absorption of sea- water the values of dA are low for sea-water 72% (using Pd modifier) and 80% (using Pd + TA mixed modifier). From Figs. 3-6 it can be seen that the Pd modifier and the Pd + TA mixed modifier are the best modifiers for the determi- nation of Cd in environmental samples and that matrix effects can be eliminated effectively.Determination of Cd in Solutions of Environmental Sample RMs with the WFP using Pd + TA Mixed Modifier As has been shown the use of 0.5 mg ml-' of Pd + 10 mg ml-' of TA as a mixed modifier at high atomization temperatures with the WFP can eliminate matrix effects effectively. I n Table 1 the results are shown for the determination of Cd in solutions of various environmental RMs. The results obtained are in good agreement with the certified or reference values and indicate that the use of 0.5 mg ml-' of Pd+ 10 mg ml-' TA with the WFP is a practical and reliable procedure for routine analyses. The results in Table 1 were obtained by standardless analysis and provide strong evidence for the applicability of the standardless analysis approach. Stability of Integrated Absorbance Characteristic Mass Values over the Lifetime of a PGT with the WFP for the Determination of Cd Nitric and perchloric acids result in a signal depressing effect and corrosion of the surface of the pyrolytic graphite platform and PGT but have no effect on the surface of a WFP. This is particularly advantageous for the routine analysis of environ- mental biological and geological samples dissolved in nitric and perchloric acids. The values of rn,(exp) were stable over 200-300 firings.Because the protection of the PGT provided by argon was not good in the graphite furnace of the 28000 spectrometer atomization temperatures changed significantly after 200-300 firings and the values of mo were not stable.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL.9 683 Conclusions Nitric and perchloric acids have no effect on the surface of a WFP. This is particularly advantageous for the routine analysis of environmental biological and geological samples dissolved in nitric and perchloric acids from the point view of stan- dardless or absolute analysis. Many sample solutions contain- ing perchloric acid suffer from serious matrix effects with W platform atomization and 0.5 mg ml-1 Pd+ 10 mg ml-I TA can be used to eliminate these matrix effects. The dependence of &IA on ashing temperature shows a plateau in the range 473-1073 K. The dependence of &IA on atomization tempera- ture reaches a plateau at temperatures higher then T(eff)= 1550 K T(set)= 1773 K and &IA is 100-105%. The possibility of developing a standardless analysis for the determination of Cd in environmental samples with a WFP and 0.5mgml-I Pd + 10 mg ml-I TA as mixed modifier was discussed. The authors are grateful to the National Natural Science Foundation of China for financial support of this research project. 1 2 3 4 5 6 7 8 9 10 11 References Ma Y.-z. Bai J. 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