Anal. Chem. 1002, 64, 1656-1659
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Factors Influencing the Atomization of Germanium in Graphite Furnace Atomic Absorption Spectrometry Yansheng Zheng' and Daliu Zhang
Department of Chemistry, Jilin University, Changchun 130023, People's Republic of China
Factors that influence the atomization of Ge in the graphite furnace have been studied. These include the presence of an oxidizing agent or an alkali and the modiflcatlon of the graphite tube surface. The graphite furnace a t m l c absorption spectrometry determination of Ge suffers from low sensitivity and poor reproducibility because Ge02 is reduced to volatile GeO when heated In the presence of carbon, which results in premature loss of Ge. This problem can be overcome by suppressing the reduction of Ge02 during ashlng and by atomizing the germanium oxides as they are vaporizing. Reductlon of Ge02 can be suppressed by the addition of oxkllzing agents, such as HCiO, or NHOs,or alkali to the sample solution or by the use of a W-coated or Zrtoated graphite tube.
INTRODUCTION
The determination of Ge by graphite furance atomic absorption spectrometry (GFAAS)has been described in the literature.l.2 However, there are only a few reports about the factors that influence the atomization of Ge. Johnson et aL3 pointed out that the reduction of GeO2 to GeO occurs on heating in the presence of carbon and that the premature loss of Ge as volatile GeO without undergoing atomization results in low sensitivity and poor reproducibility in the GFAAS determination of Ge. Sohrin et al.* suggested a few methods t o prevent the premature reduction of GeOs. Gao et aL2 reported that the loss of GeO was suppressed by using a Zrcoated graphite tube. Dittrich et reported that addition of some metal nitrates resulted in improved sensitivity. They reasoned that Ge was stabilized thermally and chemically in the graphite tube by cations that could participate in the formation of stable compounds, MGe03. Xuan et aL6studied the influence of Ni and Fe nitrates and HN03 on the atomization of Ge in the graphite furnace. They thought that the addition of Ni(N03)~ and Fe(N03)~raised the maximum permissible ashing temperature and thus reduced the premature loss of GeO, resulting in increased sensitivity of Ge determination. The addition of HN03 also enhanced sensitivity. Mino et al.' used X-ray diffraction methods to identify the composition of the residue left on the tube surface after thermal pretreatment. The results indicated that the residue formed after 700 "C pretreatment of NaOH or NaN03 solutions was (1)Ohta, K.; Suzuki, M. Anal. Chim.Acta 1979,104,293-297. (2)Gao, Y.;Ni, Z. Huazue Xuehao 1982,40,1021-1027. (3)Johnson, D. J.;West, T. S.; Dagnall, R. M. Anal. Chim.Acta 1973, 67,79-87. (4)Sohrin, Y.;Isshiki, K.; Kuwamoto, T.; Nakayama, E. Talanta 1987, 34., 341-344. -~~ (5)Dittrich, K.; Mandry, R.; Mothes, W.; Judelevic, J. G. Analyst 1985,110,169-175. (6)Xuan, W.; Li, J. Spectrochim. Acta 1990,4.53, 669-677. (7)Mino, Y.;Simomura, S.; Oh,N. Anal. Chim.Acta 1979,107,253259. ~~
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0003-2700/92/0364-1656$03.00/0
NazGeOs, but that it was GeO2 or GeO in other media. Wend8 and Kolbg studied the chemical reactions of Ge in graphite tube and found that the NazGeOg formed after thermal pretreatment in alkaline solution reduced to elemental Ge at higher temperatures through the carbon of the tube surface. Excess NaOH enhances the absorbance value of Ge by a factor of 2. They explained that the reason for this effect is in an addition-reduction process of GeO to Ge by metallic Na at temperatures higher than 1200 O C . In this paper the factors that influence the atomization of Ge in the graphite furnace have been studied. These include the presence of an oxidizing agent or an alkali and the modification of the graphite tube surface. EXPERIMENTAL SECTION Apparatus. A Hitachi 180-50atomicabsorption spectrometer and a GA-3 graphite furnace were used. Nz was used as the purge gas at a flow rate of 150 mL/min, and the purge gas was stopped at the atomization step. The slit width was 0.4 nm. A Ge hollow cathode lamp (made in China) was used as a light source and operated at 10 mA. A deuterium arc background system was used throughout. A Hitachi standard graphite tube was used for the study. The unresolved 265.118-265.158-nm resonance line pair of Ge was employed for all measurements. The absorption signals were recorded with XWT-164 (made in China)or RW-11T (madein Japan) strip chart recorder. Sample solution was injected into the graphite tube with a 20-pL Eppendorf micropipette. The temperature of the furnace was corrected with an Mt-2 optical pyrometer (made in China). The graphite furnace operating parameters were as follows: drying 80-120 "C,30 s; ashing 700 OC, 30 s; atomization 2600 "C, 10 s; cleaning 2800 O C , 3 8. Reagents. A germanium stock solution (1 mg/mL) was prepared by dissolving 0.1439 g of GeOz (99.99%)in 100 mL of 0.1 mol/LHN03anddilutingto 1OOOmLwithsubboilingdistilled water. The working solutionwas obtainedthrough serialdilutions of the stock solution with subboiling distilled water. All other chemicals used in this study were of analyticalreagent grade. Procedures. (A) Preparation of the W-Coated or ZrCoated GraphiteTube. A 20-pL solution of 10mg/mL (NH4)zWOr or ZrOClz was introduced into a new standard graphite tube (SGT), and the furnace was then heated to 2800 O C for 10 s. The above operation was repeated three times. A W-coated (WGT) or Zr-coated (ZrGT) graphite tube was then obtained. (B) Measurement of the Molecular Absorption of GeO. A Pt hollow cathode lamp or deuterium arc was used as a light source. A 20-pL solution of 200 pg/mL Ge was introduced into the graphite tube. The molecular absorption of GeO was measured during the vaporization step at 265.94 nm with the gas The furnace operation parameters were the same as above, but the vaporization temperature was varied,
RESULTS AND DISCUSSION Atomization Behavior of Ge. Figure 1 shows the Ge absorbances as a function of ashing temperature in various (8)Wendl, W.; Muller-Vogt, G. Spectrochim. Acta 1984,39E, 237242. (9)Kolb, A.; Muller-Vogt, G.; Wendl, W.; Stobel, W. Spectrochim. Acta 1987,42B, 951-957. 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, IS92 0.121
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Flgure 3. Absorbances of 8.0 ng of Ge and QeO (4 pg of Ge)vs ashlng tlme for ashlng temperatures of 700 and 800 O C In WGT. 700 " C (1) Ge. 800 "C: (2) Ge,(3)W.
Temperature
*C
Flgure 1. Absorbance of 8.0 ng of C3e a8 a functbn of ashlng tem perature: (1) SGT, (2) WGT, (3) ZrGT.
4;
sc
12C
153
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Time A Figure 2. Absorbances of 8.0 ng of Qe and GeO (4 of Qe) vs ashlng tlme for ashlng temperatures of 700 and 800 OC In SQT. 700 OC: (1) (2) W. 800 O C (3) Qe, (4) QeO.
a,
graphite tubes. The Ge absorbance was reduced sharply at an ashing temperature of about 800 "C in SGT. However, the maximum permissible ashing temperatures of Ge were raised to 800 and lo00 OC in WGT and ZrGT, respectively. Figure 1shows that the Ge absorbance signals were increased by nearly a factor of 8 and 5 in WGT and ZrGT, respectively. The reactions of Ge are reflected by the measurements of absorbance as a function of ashing time at ashing temperatures. The absorbances of Ge and GeO at ashing temperatures of 700 and 800 "C as a function of ashing time in SGT and WGT are depicted in Figures 2 and 3. It can be seen from Figure 2 that the amount of atomizable Ge increases with the increase of ashing time and remains constant after about 100s when the ashing temperature was 700 "C. On the contrary, the absorption signal of GeO decreases with the increase of ashing time and remains constant after about 4060 8. At an ashing temperature of 800 OC, a drastic decrease of Ge absorbance occurs for short ashing times and then
remains constant after about 40 8. However, at an ashing temperature of 700 "C, the absorption signal of GeO in WGT was not observed, and the Ge absorbance increases gradually with the increase of ashing time and remains constant after 1008. When the ashing temperature was 800 "C, the amount of atomizable Ge decreases with the increase of ashing time and remains constant after about 60 8. At the same time, the absorption signalof GeO appears a t 30 s and increasesabruptly after about 100 s (Figure 3). Kolb et al.9 pointed out that the GeO sublimed at temperatures higher than 700 OC and the metal was oxidized to volatile GeO a t temperature of 830 "C. We consider that the absorption signal of GeO in SGT belonged to the reduction of GeOz by the carbon of the tube surface when the ashing temperature was 700 "C. But when the ashing temperature was 800 "C, the absorption signal of GeO in WGT was due to the metal being oxidized to volatile GeO. These losses of atomizable Ge reduced the absorption signal of Ge at an ashing temperature of 800 "C. Therefore, we may conclude that the suppression of the formation of volatile GeO in the vapor phase caused by the use of WGT is the main factor for the enhancement of the Ge signal. Effects of Acids and Bases. Mino' and Sohrin' reported that the peak absorbance of Ge was increased obviously in the presence of HN03,HClO,, and alkali and that the addition of HC1 and HzSO4 suppressed the Ge signal. However, Pelieva et al.10pointed out that HN03suppressed the Ge signal. We have studied extensively the effects of inorganic acids and bases on the Ge signal in various graphite tubes. Acids. The experimental results show that the Ge signal was reduced by 78 and 38% at a concentration of 0.5 moUL HC1 or 1.0 moVL HzSO4 in SGT, respectively. But HNOa and HClOr greatly increased the Ge signal. Figure 4 shows the effects of various acids on the Ge signal as a function of their concentrations. The Ge absorbance increaseslarglywith the increases of HN03 and HCIOr concentrations and then increases gradually at concentrations higher than 0.15 and 0.05 mol/L in SGT. As shown in Figure 4 the effect of HN03 on the Ge signals in WGT and ZrGT was eliminated when its concentration was lower than 0.5 moVL, but the effect HClOr was reduced only. HN03 and HCIOr are oxidizing acids, and they could corrode the surface of tube. There is no doubt that the destruction of tube surface was caused by the corrosion of sample containing oxidizing acid during the atomization process and that this would result in the formation of the ~~~~
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(10) Pelieva, L. A.; Maytynenko, K. P. J. Appl. Spectrosc. (Engl. Trawl.) 1984,40, 33-37.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
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Acidic concentration mol/l F w r o 4. Absorbance of 8.0 ng of Ge as a functlon of HN03andHCi04 concentrations in various tubes: (1) %IO4 in SGT,(2) HN03 in SOT, (3) %IO4 in WGT, (4) HCi04 in ZrGT, (5) HN03 in WGT and ZrGT.
rough surface of the tube wall. That is, the oxidation of the tube surface and intercalation of oxygen into the graphite lattice by these oxidizing acids counteracts the reducing activity of the tube surface, suppressing the premature reduction of GeOz to GeO by the carbon. The effect of HC104 is more remarkable than that of HNOs, showing that HC104 intercalates oxygen more effectively into the graphite lattice. Thisprocess can result in the change of the surfacemorphology of the tube wall. On the other hand, L'vov et al.11 pointed out that the amount of the residual oxygen in the new graphite tube is 1.5-fold larger than that in an "old" tube. The oxygen decomposed from HC104or HN03adsorbed onto the surface of the tube wall, and subsequently it desorbed CO. Then more CO was formed in the graphite furnace in the presence of HC104 and HN03. The carbon monoxide is favorable for increasing the formation of Ge atoms in the vapor phase and decreasing the loss of the analyte as GeO(g). This would be confirmed by the effect of CO added to Nz at the back. Bases. The effects of several bases on the Ge signal as a function of ashing temperature are illustrated in Figure 5. The ashing curve of an aqueous ammonia Ge solution is similar to that in pure Ge solution (Figure 1). But the aqueous ammonia enhances the absorbance value by 20%. For the NaOH or KOH solution the maximum permissible ashing temperature was raised to 900 "C. The Ge absorbance as a function of NaOH concentration is illustrated in Figure 6. The absorbance shows first a rapid increase with increasing NaOH concentration and, for a 0.1 moVL solution of NaOH, results in a stabilization value. The alkali added to the sample solution resulted in the magnitude of signal enhancement of pure Ge solution similar to that shown in the modification of the graphite tube with refractory metals such as Zr or W. The above expermental results show that the atomization of Ge is relevant to Ge species formed in the solution and the morphology of Ge left on the tube surface after the ashing cycle. However, we found that there is the stronger background absorption in the NaOH solution, and the wavelength of the maximum background absorption peak is 245.0 nm. Therefore, the interference of background absorption must be considered when 265.94 nm is used as the measurement (11) L'vov, B.V.;Ryabchuk,G.N.Spectrochim.Acta 1982,37E,673685.
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Flguro 5. Absorbance of 8.0 ng of Qet as a functlon of ashing temperature in SGT: (1) 0.1 moi/L NaOH solution, (2) 0.1 moi/L KOH solution, (3) 0.1 moi/L aqueous ammonia solution.
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Concentration of NaOH moi/l Flguro 6. Absorbance of 8.0 ng of Ge as a function of NaOH concentration.
of GeO absorption in the NaOH solution. The experimental results show that the absorption signal of GeO in the vapor phase was not observed when the solution containing 200 r g l d Ge plus 0.1 moUL NaOH was vaporized. This indicates that the formation of GeO in the vapor phase is suppressed in the presence of NaOH. Barton et reported that GeOz is reduced by carbon under high temperature as shown in the following reactions: GeO, GeO,
+ C(s)
+ 2C(s)
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Ge(s)
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+ CO,(g)
Ge(s) + 2CO(g)
(1) (2)
The first, in which COZis a product, starts at 830 "C and is unimportant above 1100 "C. A second process in which CO is the product startsat 830 "C,proceeds most rapidly at 1100 "C,and is usually completed at 1180 "C.These reactions are apparently accompanied by subsequent reactions of the Ge produced with GeOz
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GeO,(s) + Ge(s) 2GeO(g) (3) Ge is thus transported from the reaction zone as GeO. The (12)Barton, L.;Heil, C.A. J. Less-CommonMet. 1970,22,11-17.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, lQQ2 1860
Table I. Influence of Carbon Monoxide on Ge Absorbance amt CO added to Nz (V/V % ) 0 10 20 30 35
0.50
re1 absorbance
i I
1.00 3.39 3.56 3.64 3.66
overall reaction most thermodynamically favorable at the higher temperature is a combination of reactions 2 and 3, i.e. GeO,(s) + C(s)
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GeO(g) + CO(g)
(4)
According to eq 3 the formation of GeO(g) is due to the interaction between Ge(s) produced from eqs 1 and 2 and GeOz(s)existing in the system studied. That is, the formation of GeO(g) is dependent on GeOz(s) existing in the system studied. In the alkaline solution NazGe03is left on the surface of tube after ashing cycle. At temperatures up to 830 "C, NazGeOsremains unchanged.13 This compound is reduced to elemental Ge at higher temperatures through the carbon of the tube surface as follows: Na2Ge03+ 3C
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Ge(s) + 2Na + 3CO
(5) Here eq 3 cannot occur in the graphite furnace because GeOz(8) was not formed on the tube surface after the ashing cycle. Thus, GeO(g) is not formed in the vapor phase in alkaline solution. This was proved by measuring the absorption of GeO. Effect of Carbon Monoxide in the Furnace. It can be seen from eq 4 that CO can influence the equilibrium of the reduction reaction of GeOzto GeO(g) by carbon. Therefore, the amount of CO in the furnace would effect a change in the absorption signal of Ge. In order to check the validity of the argument stated above, the influence of CO added to NZon the atomization behavior of Ge in SGT was studied. The experimental results show that the addition of CO resulted in the enhancement of Ge absorbance (Table I). The magnitude of the enhancement effect was closely related to the ashing temperature. Figure 7 shows that the Ge absor-
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Tcmpcrature
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Flgurr 7. Absorbances of 8.0 ng of Qe and GeO (4 pg of Qe) as a function of ashingtemperature in NPcontaining 20% (v/v) C O (1) Ge,
(2)-0.
bance increases with the increase of ashing temperature, reaches a maximum at an ashing temperature of 700 "C,and then decreases gradually. On the contrary, the absorption of GeO decreases with the increase of ashing temperature. The Ge absorbance increases by a factor of 3.56 whereas the GeO absorbance is reduced by 65% when 20% (v/v) of CO was introduced into NZand the ashing temperature was 700 O C . This phenomenon may support our hypothesis that it is favorable for the formation of Ge atoms in the vapor phase to increase amount of CO in the graphite furnace. It is concluded that the interference caused by the premature reduction of GeOz to GeO can be prevented by optimizing the heating program, and adding oxidizing acids or alkali to sample solution or by the use of a W-coated and Zr-coated graphite tubes. The suppression of the formation of volatile GeO in the vapor phase is the main factor for the enhancement of the Ge signal.
RECEIVED for review January 16, 1992. Accepted April 15, (13)Wendl, W. Freseniw' 2.Anal. Chem. 1986,323,726-729.
1992.
