Effects of Conditions for Pyrolysis of Ascorbic Acid as a Chemical Modifier on the Vaporization Mechanism of Gold in Electrothermal Atomic Absorption Spectrometry ETSURO IWAMOTO*a , MIHO ITAMOTOa , KAZUE NISHIOKAa , SHOJI IMAIb , YASUHISA HAYASHIc AND TAKAHIRO KUMAMARUd aDepartment of Health Science, Hiroshima Women’s University, Hiroshima 734, Japan bDepartment of Chemistry, Faculty of Integrated Arts and Science, T he University of T okushima, T okushima 770, Japan cDepartment of Chemistry, Joetsu University of Education, Joetsu 943, Japan dDepartment of Chemistry, Faculty of Science, Hiroshima University, Higashi-hiroshima 739, Japan The eVect of the ramp and hold times and the temperature of 580 K; (ii ) active carbon species between 600 and 1100 K; and (iii ) thermally stable carbon species between 1200 and 2400 K.the pyrolysis step of ascorbic acid as a chemical modifier on AA signals for gold have been investigated. Three methods of Raman spectrometry17 also indicated that a disordered structure was observed for the wall surface when treated with pyrolysis for ascorbic acid were used: (1) pyrolysis of ascorbic acid before deposition of a gold solution on the platform ascorbic acid at high temperatures (1200–2150 K).Mechanistic studies of the adsorption and desorption of surface; (2) pyrolysis of ascorbic acid after ashing a gold solution; and (3) charring a solution containing both ascorbic gold on graphite by McNally and Holcombe18 and Fonseca et al.19 showed that evaporation of gold could occur from acid and gold.Although pyrolysis methods (2) and (3) gave a delayed single absorption peak for gold compared with that in microdroplets of various sizes or adsorbed atoms, depending on the analytical conditions, and that the tendency of the the absence of ascorbic acid, in pyrolysis method (1) a double peak appeared even at a high pyrolysis temperature of microdroplets to disperse on the graphite was lowered by mechanical roughing of the furnace surface, which may aVect 1500 °C.However, as the ramp time increased from 5 to 80 s from a drying temperature of 120 up to 1500 °C, the first peak the number of active sites on the graphite. A double peak or a peak with a shoulder for the atomization of gold has often increased and concomitantly the second peak decreased with an isosbestic point; the integrated absorbance remained been observed when the matrix composition of a sample was complex, containing organic compounds8,10 and heavy metals,9 constant. The concentration dependence of the gold signals indicates that a fractional-order of release is shown for the and when old tubes were used.11 Although the higher-temperature shift of the gold signal in first peak and a first-order process is obtained for the second, indicative of gold atoms adsorbed onto the active carbon ETAAS has been related to the active carbon or carbon residue, characterization of the ascorbic acid residue and the surface.From inspection of a scanning electron micrograph and Raman spectra of the pyrolysed ascorbic acid, it was clear mechanism of its interactions with gold atoms are still unclear with respect to the AA signal for gold. The optimal use of that a carbon film formed on the platform surface. It was concluded that use of a short ramp and hold time, even at ascorbic acid as a chemical modifier requires knowledge of the atomization mechanism of gold with ascorbic acid. 1500 °C, for the pyrolysis of ascorbic acid leads to the formation of active amorphous carbon enriched in micro-sized In the present work, an extensive examination of the eVects of temperature and ramp time for the pyrolysis of ascorbic pores (r<25 nm), where adsorption of gold atoms that give rise to the second absorption signal occurs. Furthermore, the acid on the AA signal for gold in ETAAS was carried out. Furthermore, the extent of formation of amorphous carbon, micro-sized pores are almost destroyed by treatment at temperatures higher than 1800 °C, resulting in graphitization which was determined by the pyrolysis conditions for the ascorbic acid, is shown to be related to the double peak for gold. of the carbon residue.Keywords: Ascorbic acid; chemical modifier; electrothermal EXPERIMENTAL atomic absorption spectrometry; gold; amorphous carbon; carbon film; Raman spectrometry Apparatus A Perkin-Elmer Model 4100ZL AA spectrometer equipped Ascorbic acid has been used as a chemical modifier for some with a longitudinal Zeeman-eVect background corrector system elements such as lead,1–6 tin6,7 and gold8–11 in ETAAS.An and an AS-71 autosampler was used. The graphite furnace increase in the pyrolysis temperature and enhancement in the used was a transversely heated graphite tube with an integral sensitivity for gold in the presence of ascorbic acid6,8–10 and L’vov platform. A Perkin-Elmer hollow-cathode lamp for gold proteins12 were ascribed to the formation of a carbon residue was used as the light source.as a result of thermal decomposition during the pyrolysis step. Instrumental operating conditions and a typical furnace The eVectiveness of ascorbic acid has also been discussed from programme are given in Table 1. the viewpoints of formation of active carbon species and reductive gases.13–16 Reagents Imai et al.17 reported that the decomposition of pyrolysed ascorbic acid, when investigated by ETV-ICP-MS, indicated An aliquot of a commercially available stock solution (1000 mg l-1 HAuCl4 in 1 mol dm-3 HCl; Hayashi Pure three signals in the atomization cycle which correspond to: (i ) gaseous compounds (hydrocarbons, CO and CO2) below Chemical Industries) was diluted with deionized water to a Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1293–1296) 1293Table 1 Instrumental operating conditions and temperature programmes for pyrolysis of ascorbic acid and atomization of gold Spectrometer— Wavelength 242.8 nm Bandpass 0.7 nm Lamp current 15 mA Signal type Zeeman AA Purge gas Ar THGA— Ar flow Ramp Hold rate/ml Step Temp./°C time/s time/s min-1 Read 1 80 1 20 250 — 2 120 5 30 250 — Fig. 1 Absorbance–time profiles for Au, varying the ramp time for 3 Various Various 5 250 — pyrolysis of ascorbic acid at 1500 °C according to the heating pro- 4 80 1 20 250 — gramme in Table 1; Au, 1.0 ng.Broken line, with ramp time 80 s in 5 120 5 30 250 — the absence of ascorbic acid; solid lines, in the presence of ascorbic 6 500 10 20 250 — acid. Ramp time: a, 5; b, 10; c, 20; d, 25; e, 40; f, 50; and g, 80 s. 7 2300 1 5 0 On 8 2500 3 1 250 — Thus, the second peak is a characteristic of gold with ascorbic Sequence of acid as a chemical modifier. It should be noted in Fig. 1 that actions— the first peak increases with increasing the ramp time and the 1 Inject 20 ml of 1%m/v ascorbic acid solution second peak concomitantly decreases with an isosbestic point, 2 Run THGA steps 1–3 with the peak area remaining constant.The eVect of pyrolysis 3 Inject 20 ml of 50 mg l-1 gold solution temperature for a long ramp time of 80 s with two diVerent 4 Run THGA steps 4 to end hold times, 5 and 60 s, on the first and second peak heights is indicated in Fig. 2. The second peak is stable below a pyrolysis temperature of 1000 °C, even for the longer hold time of 60 s.suitable concentration for use as a working standard solution. The above results indicate that the longer ramp and hold times Ascorbic acid was of analytical-reagent grade (Katayama for a pyrolysis temperature above 1200 °C suppress the appear- Chemical Industries). ance of the second peak, although rapid pyrolysis of ascorbic acid gives the second peak even at 1500 °C. It was found that Procedure no second peak was obtained at a pyrolysis temperature above 1800 °C. Three pyrolysis methods for ascorbic acid were used.(1) An aliquot of 20 ml of ascorbic acid (1% m/v) was deposited in the graphite furnace by the autosampler. It was pyrolysed Carbon Residue and Active Sites according to a heating programme consisting of the first three When ascorbic acid is heated at 400 °C or above on the steps of the THGA programme shown in Table 1. The temperagraphite surface, most of the carbon and hydrogen are dissi- ture and ramp time for step 3 was varied, e.g., 1500 °C with a pated as CH and CO by thermal decomposition of the ascorbic ramp time of 5 s.After cooling, 20 ml of the gold standard acid.8,17 Some residues of pyrolysed carbon remain on the tube solution (50 mg l-1) were injected by the autosampler followed surface.17 In Fig. 3 is shown a micrograph produced by SEM by atomization. In atomization step 7, a 1 s ramp time was of the surface of the platform on which ascorbic acid had been used because it gave a double peak with a deeper bottom pyrolysed at a temperature of 1500 °C with a 5 s ramp time between the two peaks than did the maximum power heating and a 5 s hold time, conditions which mainly give the second mode with a 0 s ramp.(2) The gold solution (20 ml ) was peak (Fig. 1). It can be seen that a carbon film is formed. The injected and ashed according to a heating programme conplatform with the carbon film was again installed in the sisting of 500 °C (ramp time 10 s, hold time 20 s) for step 3 in graphite tube and heated at 2300 °C for 3 s.Then the gold Table 1. After cooling, the ascorbic acid solution (20 ml ) was pyrolysed according to a heating programme at, for example, 1500 °C (ramp time 5 s, hold time 5 s) for step 6 in Table 1 followed by atomization step 7. (3) An aliquot of 20 ml of the solution containing gold and ascorbic acid was deposited in the furnace, ashed at 1500 °C (ramp time 5 s, hold time 5 s) and atomized according to the heating programme, skipping over steps 4–6 in Table 1.RESULTS AND DISCUSSION Pyrolysis Conditions The eVects of ramp time and temperature for the pyrolysis of ascorbic acid on the absorption signals for 1.0 ng of gold were investigated using pyrolysis method (1). Typical results at a constant temperature of 1500 °C and varying the ramp time between 5 and 80 s for step 3 are shown in Fig. 1. Double peaks appeared in the presence of ascorbic acid but not in the Fig. 2 EVects of temperature with a constant ramp time of 80 s for absence of ascorbic acid (broken line in Fig. 1). Hereafter the the pyrolysis of ascorbic acid on the first and the second peak heights; peak which appears initially will be referred to as the first peak Au, 1.0 ng. Hold time (60 s): %, first signal; #, second signal. Hold time (5 s): &, first signal; $, second signal. and the one which appears later is described as the second. 1294 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12into the transport pore (macropore, radius >25 nm) and the adsorbing pore (mesopore 1–25 nm, micropore 0.4–1 nm and submicropore <0.4 nm).21 A thermal dependence of the microscopic surface structure of carbon prepared from the pyrolysis of cyclodextrin and amylose was examined by Hosokawa and Yamaguchi.22 They reported that the pore size radius in cyclodextrin carbon treated at 1473K is distributed in the range 0.5–4.5 nm with a peak distribution at 0.7–1 nm, for which the pore volume is more than 1×10-3 cm3 g-1.However, when treated at 1873 K the distribution of pore sizes from 0.7 to 1 nm falls to a level of 0.2×10-3 cm3 g-1 in the case of the size for radii of 2.5–4.5 nm. This shows that higher temperature treatment results in a marked decrease in the number of micropores. This is in good agreement with a change in the microstructure of the carbon film formed with ascorbic acid in the platform furnace by heating at 2300 °C.Fig. 3 SEM micrograph of the carbon film obtained by pyrolysis of ascorbic acid (5% m/v, 20 ml ) at 1500 °C (ramp time 5 s and hold time Concentration Dependence of Absorption Profiles 5 s) on the platform. According to McNally and Holcombe18 and Fonseca et al.19 the order of atom release can be predicted from the concensolution (20 ml ) was deposited on it followed by atomization tration dependence of absorbance–time profile characteristics. according to the heating programme of steps 4–8 in Table 1.In the first-order process, analyte desorption takes place from It was found that no second peak was given but SEM a monolayer of analyte atoms which are physically sorbed or apparently gave an unchanged image of the carbon film, chemisorbed onto the wall surface, and in the fractional-order showing that it is thermally stable. process the analyte atoms are released from microdroplets. Information from Raman spectra is very useful for the They showed that for a zero- or fractional-order of release the discrimination of amorphous and graphite carbon.Raman peak maximum shifts to a later time with an increase in the spectra in the macro-mode (100 mm diameter laser beam) are amount of gold, while in a first-order signal process the peak shown in Fig. 4 for three types of platform surfaces which were maximum appears at the same time, independent of the amount prepared in the same way as for SEM, using the 5% m/v of gold deposited. Gold atoms in the gas phase are known to ascorbic acid solution (20 ml ).As shown in Fig. 4a for a new be produced by evaporation of the pure metal or microdropyrolytic graphite (PG) surface of the platform, two Raman plets.18,19 The microdroplets interact with the active sites on bands, a sharp band (E2g mode) near to 1580 cm-1 and the graphite surface and their mobility is reduced, thereby a broad band (disorder mode)20 with a weak intensity of preserving the larger sized droplets.19 The presence of a chemi- 1350 cm-1 were observed, showing that its graphite structure cal modifier can alter the release order of an analyte in a is almost complete.It is illustrated in Fig. 4c that for the graphite atomizer.9,11,23 Thomaidis et al.11 found that addition surface of a platform treated with ascorbic acid at 1500 °C of 5 mg of rhenium as a chemical modifier to the gold solution (ramp time 5 s, hold time 5 s) which gives the second peak, was suYcient to convert the fractional-order of release into a two broad Raman bands were observed at the same shifts as first-order process. The chemical modification eVect was those for the non-treated PG surface. However, once the explained by rhenium reacting with carbon atoms at the active surface had been heated at 2300 °C for 5 s, the two bands sites on the graphite surface, which decreases the number of (Fig. 4b) became sharper than those (in Fig. 4c) for the surface active sites, leading to easy dispersion of gold atoms; thus the treated with ascorbic acid at 1500 °C.Thus, it was shown that formation of highly dispersed atoms rather than microdroplets the graphite structure of the carbon residue becomes more is expected. ordered by pyrolysis at 2300 °C, and as a result no second Concentration dependence, in the present system, of the first peak is produced by this platform. and second peaks is shown in Figs. 5 and 6, respectively. In It is known that the adsorbing capacity of active carbon is dependent upon the pore size in the carbon, which is classified Fig. 5 Absorbance–time profiles for the first Au signal with masses Fig. 4 Raman spectra of pyrolysed ascorbic acid. a, Platform PG surface without the ascorbic acid residue; b, ascorbic acid pyrolysed varying between 0.1 and 1.0 ng, obtained using a heating programme with a temperature of 1700 °C (ramp time 60 s and hold time 5 s) for at 1500 °C (ramp time 5 s and hold time 5 s) followed by heating at 2300 °C for 5 s; and c, ascorbic acid pyrolysed at 1500 °C (ramp time step 3 in Table 1.Mass of Au: a, 1.0; b, 0.8; c, 0.6; d, 0.4; e, 0.2; and f, 0.1 ng. 5 s and hold time 5 s). Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1295Fig. 6 Absorbance–time profiles for the second Au signal with masses varying between 0.2 and 1.0 ng, obtained using a heating programme with a temperature of 1500 °C (ramp time 5 s and hold time 5 s) for step 3 in Table 1.Mass of Au: a, 1.0; b, 0.8; c, 0.6; d, 0.4; and e, 0.2 ng. Fig. 7 Absorbance–time profiles for Au for various ascorbic acid pyrolysis methods. (1) Pyrolysis method (2) (see the Experimental); and (2), pyrolysis method (3). Broken line, in the absence of ascorbic Fig. 5 the eVect of injecting various masses of gold between acid (mass of Au, 1.25 ng); solid lines, in the presence of ascorbic acid 0.1 and 1.0 ng is shown, after pyrolysing ascorbic acid at a (mass of Au: a, 1.25; b, 1.0; c, 0.75; d, 0.5; and e, 0.25 ng).temperature of 1700 °C, with a ramp time of 60 s and a hold time of 5 s, which gives the first peak. However, in Fig. 6 the which gives no second absorption signal, although the carbon eVect of injecting various masses of gold between 0.2 and film residue itself is thermally stable at that temperature. 1.0 ng is shown, after pyrolysing ascorbic acid at a temperature of 1500 °C, with a ramp time of 5 s and a hold time of 5 s, This work was supported by a Grant-in-Aid for the Scientific which gives the second peak.The results indicate that the Research No. 07404055 from the Ministry of Education, profiles of the first peaks correspond to the fractional-order Science and Culture in Japan. process as expected, and the second peak, for which the maximum is independent of concentration, is due to the first- REFERENCES order of release. Therefore, it can be deduced that in the presence of ascorbic acid the first peak signal is due to 1 Regan, J.G. T., and Warren, J., Analyst, 1976, 101, 220. atomization from microdroplets of gold and the second peak 2 McLaren, J. W., and Wheeler, R. 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A rapid heating rate for the 21 IUPAC Manual of Symbols and T erminology for Physicochemical pyrolysis of ascorbic acid, for example, 276 °C s-1 even at Quantities and Units, Butterworths, London, 1972. 22 Hosokawa, K., and Yamaguchi, M., T anso (Carbons), 1988, 1500 °C (hold time 5 s), gives an amorphous carbon film with 132, 17. micro- or submicro-pores during the thermal decomposition, 23 Qiao, H., and Jackson, K. W., Spectrochim Acta, Part B, 1991, which is when the gold atoms are adsorbed, leading to the 46, 1841. second signal with a first-order release of atoms. The gold– carbon interaction in micro-size pores is stronger than the Paper 7/01659J gold–gold interaction, resulting in dispersion of the gold atoms. ReceivedMarch 10, 1997 Thermal treatment of the amorphous carbon film at tempera- Accepted June 30, 1997 tures higher than 1800 °C gives rise to well-ordered graphite, 1296 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12