Determination of Thallium by Electrothermal Atomic Absorption Spectrometry with a Metal Atomizer Masami Suzuki and Kiyohisa Ohta Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho, Tsu, Mie-ken 514, Japan

Thalliumbestimmung dureh AAS mit elektrothermischer Atomisierung in einem metallischen Atomizer

Zusammenfassnng. Zur AAS-Bestimmung von Thallium wird eine Molybd/in-Mikrorthre als Atomizer empfohlen. Zusatz von geringeren Wasserstoffmengen zum Argongas ergab eine hthere Absorption. In Gegenwart von Thioharn- stoff erfolgte durch Wasserstoff-Zugabe keine Anderung des Absorptionswertes in reinern Argon, obwohl die Peaktempe- ratur in einen niederen Bereich verschoben wurde. Fiir h t - here Absorption und zur Verminderung yon Sttrungen dutch Begleitelemente wurden hthere Erhitzungsgeschwin- digkeiten empfohlen. Sttrungen durch Begleitelemente im Bereich von 50 ng konnten mit Thioharnstoff verringert werden. Die Zugabe von Thioharnstoff erwies sich ebenfalls als wirksam fiir 500 ng Cadinium, Zink oder Kupfer, wfih- rend sie fiir dieselben Mengen an Calcium, Magnesium, Chrom, Bismut oder Blei ungen/igend war. Mit hoher Erhit- zungsgeschwindigkeit und Thioharnstoffzusatz konnte eine Sttrung durch Chlorid im Bereich von 500 ng reduziert wer- den. Die absolute Empfindlichkeit ffir Thallium (0,0044 abs) betrug 1,2 pg.

Summary. Electrothermal atomization of thallium in a mo- lybdenum microtube atomizer is described. The addition of a low flow rate of hydrogen to argon purge gas resulted in higher peak absorption. In the presence of thiourea the addition of hydrogen did not alter the peak absorption value in pure argon, although the peak temperature shifted to lower region. A high heating rate of the atomizer was re- commended for higher peak absorption and improvement of interferences from concomitants. The interferences by concomitants at 50 ng level were modified with thiourea. Modification with thiourea was effective even for in- terferences from 500 ng of cadmium, zinc or copper, while it was insufficient for 500 ng of calcium, magnesium, chro- mium, bismuth or lead. Provided the atomization of thallium was carried out at high heating rate and on addition of thiourea as a matrix modifier, the interference from chlo- ride (500 ng) was modified. The absolute sensitivity (0.0044 abs) for thallium was 1.2 pg.

Introduction

Environmental concern has grown over thallium because of its toxicity. Furnace atomic absorption spectrometry ap-

Offprint requests to: M. Suzuki

Fresenius Z Aria! Chem (1985) 322:480--485 �9 Springer-Verlag 1985

pears to be suitable for the determination of traces of this element. However, only few papers have described the thal- lium determination with this technique. Slavin and Manning applied platform atomization for suppressing interferences from chlorides [1]. Matrix modifiers have also been de- scribed [2, 3]. Schmidt and Dietl recommended the use of zirconium coated graphite tubes for atomization of thallium [4]. Atomization under pressure improves the linearity of the calibration curves for thallium [5].

The present work was undertaken in an attempt to obtain some understanding of the atomization characteristics in a metal microtube atomizer.

Experimental

Apparatus

Atomic absorption measurements were made with a Nippon Jarrell-Ash 0.5 m Ebert-type monochromator coupled to an R-928 photomultiplier tube (Hamamatsu Photonics K.K.). The signal from the photomultiplier was amplified by a fast- response amplifier (time constant 3 ms). Signals from the amplifier were fed to a microcomputer (SORD M223) through an AD converter (Datel ADC-HX 12 BGC) and multiplexer (Datel MX-808). The microcomputer used was mounted with an 8-bit microprocessor (Zirog Z-80) as the central processing unit and equipped with 64 kbytes of main memory. The memory capacity was 64,000 words, and the basic cycle time was 500 gs. The BASIC program was ope- rated by an interpretive routine. The memory capacity of the floppy disc used as sub-memory was 350 kbytes. The signals were also monitored on a Memoriscope (Iwatsu MS- 5021) which had a time constant of I ~ts.

The molybdenum microtube atomizer (24 mm long and 2 mm i.d.) and atomization chamber (300 rnl) have been described [6]. A microtube was made from 0.05 mm thick molybdenum sheet (Rernbar Co., USA). The argon used as purge gas in the atomization chamber was mixed with hydrogen. Two light apertures (0.5 and 5 mm diameters) were positioned in front of the monochromator entrance slit so that the light beam from the hollow cathode lamp passed through the microtube and through the apertures.

A thallium hollow cathode lamp (Hamamatsu Photonics K.K.) was used for atomic absorption measurement at 276.8 nm.

The tube temperature was measured with a photodiode (Hamamatsu Photonics K.K.). The signal from the photo- diode was calibrated with an optical pyrometer (Chino Works, Model 760). Corrections were made for non-black- body emissivity. The signal from the photodiode was fed to

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the microcomputer and to the Memoriscope, and recorded simultaneously with the absorption signals.

All sample solutions were injected into the microtube atomizer by use of a 1 gl glass micropipet.

Reagents

A thallium stock solution (1 mg T1/ml) was prepared by dissolving high-purity thallium nitrate in 0.7 M nitric acid. Working solutions were prepared immediately before use by diluting appropriate volumes of the stock solution. All reagents used were of analytical reagent grade.

Procedure

A 1 gl volume of thallium solution was transferred into the microtube, dried at 100 ~ C for 2 s, and then atomized by heating to a final temperature of 2,000 ~ C using an appro- priate heating rate. All the atomization signals were stored in a microcomputer. Signals were subject to background (noise) subtraction and smoothing [7]. Programs were written for data manipulation and presentation. For obtaining the energies involved in the atom formation pro- cess a logarithmic plot of the measured absorbance vs. the reciprocal absolute temperature and the evaluation of the energies from the slope of this plot by a least-squares analysis [8] were programmed. The absorbance values from the initial 80 ~ 120 ms portion of the atomization profiles were em- ployed.

Results and Discussion

Atomization Characteristics o f Thallium

The addition of hydrogen to the argon purge gas served to improve atomization of thallium with a molybdenum microtube atomizer. Figure I shows the dependence of ato- mic absorption profiles for thallium on flow rates of argon and hydrogen as purge gases. Higher peak absorption of thallium was observed on addition of a low flow rate of hydrogen to the argon purge gas accompanying temperature shift to lower region. Poor absorption was shown in pure hydrogen. The effect of thiourea on atomization of thallium is of interest. In this case, the addition of hydrogen did not alter the peak absorption value in pure argon, although the shift of peak temperature remained (Fig. 1). On the basis of these results, all subsequent measurements were made with argon (480 ml/min)-hydrogen (20 ml/min).

In order to understand the atom formation processes of thallium with and without thiourea, the activation energies were calculated. The observed energies of atomization of thallium were 192 and 384 kJ/mol with and without thio- urea, respectively. The value (384 k J/tool) correlates with the dissociation energy of T1-O (378 kJ/mol). The proposed

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atomization process for thallium is the gas phase dissocia- tion of T10(g). Reduction reaction with hydrogen appears to play no role in the formation of atoms. The value (192 kJ/ mol) corresponds to the heat of formation of gaseous thallium from elementary thallium (182 k J/tool). Thallium sulfide, formed from the thiourea complex, vaporized at more than 300~ and decomposed completely to form thallium at higher temperature.

Hydrogen also served to prolong the life time of the atomizer resulting in protection of the atomizer from oxida- tion by traces of oxygen in the atomization chamber.

The heating rate of the microtube atomizer affects the shape of the atomization profile and the peak absorption. Figure 2 presents the atomization profiles observed at dif- ferent heating rates. An increase in the peak absorption with increasing heating rates of the microtube was observed, but

the appearance temperature was independent of the heating rate of the microtube. An increase in the peak absorption can result from an increase in the rate of atom formation. However, the atomization at high heating rates (3.8 and 2.8 ~ C/ms) resulted in an increase in background signals and poor reproducibility of analyte signals. The heating rate of 2.4 ~ C/ms was recommended for the present microtube atomizer. The heating rate had a significance for in- terferences from concomitants as stated below.

The absolute sensitivity was calculated from the peak absorption and defined as the mass of thallium which gives an absorption of 1% (0.0044 abs). The value was computed from the slope-intercept equation for the linear portion of the calibration curve. Under the optimum experimental conditions the absolute sensitivities of thallium with and without thiourea were 1.1 and 1.2 pg, respectively. The rela-

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Fig. $A, B CRT display showing the effect of palladium as a matrix modifier on the atomization of thallium (50 pg) in the presence of sodium chloride (500 ng). A Heating rate 2.4 ~ C/ms; B heating rate 0.7 ~ C/ms; a T1 alone; b T1 and NaC1; c T1 and Pd (500 ng); dT1, NaC1 and Pd (500 ng); e temperature increase. Gas flow rates as in Fig. 2

Fig. 6A, B CRT display showing the effect of lead (50 ng) on the atomization of thallium (50 pg) and its modification with thiourea A No thiourea added; B thiourea (5 gg) added; a T1 alone; b T1 and Pb; e Pb alone; d temperature increase. Heating rate 2.4 ~ C/ms; gas flow rates as Fig. 2

tire standard deviations (10 replicates) were 4.1 and 3.7% for pure and thiourea-containing thallium solutions (50 pg T1), respectively.

Interferences

The interference of halides in the atomization of thallium in the graphite furnace has been noted and attempts have been made to reduce the interferences [2, 3]. The effect of sodium chloride on atomization of thallium in a microtube atomizer are presented in Fig. 3. The distortion and temperature shift of the thallium profile resulted from the presence of sodium chloride. The addition of thiourea diminished the inter- ference from chloride considerably although the temper- ature shift of the thallium profile remained, To confirm the effect of the heating rate on tile interference from sodium

chloride and its decrease with thiourea, the atomization of thallium was carried out at lower heating rate. At lower heating rate the interference of sodium chloride on thallium was complex, and the decrease with thiourea was insuffi- cient. At a high heating rate this effect was more satisfactory for 50 ng of sodium chloride. When thallium was atomized in the presence of chloride, an activation energy value for 380 kJ/mol was obtained, in agreement with T1-C1 bond energy of 368 kJ/mol. Atomization appears to proceed via thermal dissociation of the chloride. If thiourea was added, the activation energy value of 163 k J/tool was obtained even in the presence of chloride, corresponding to the value in the absence of chloride. L'vov described that the addition of lithium reduced the interference of sodium chloride on thallium, reducing the extent of the reaction between thallium and chloride by the formation in the vapor phase

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Fig. 7A, B CRT display showing the effect of cadmium (500 rig) on the atomization of thallium (50 pg) and its modification with thiourea. A No thiourea added; B thiourea (5 gg) added; a T1 alone; b T1 and Cd; e Cd alone; dtemperature increase. Heating rate and gas flow rates as in Fig. 6

Fig. 8A, B CRT display showing the effect of calcium (500 ng) and magnesium (500 ng) on the atomization of thallium (50 pg). A No thiourea added; B thiourea (5 gg) added; a T1 alone; b T1 and Ca; c Ca alone; dT1 and Mg; e Mg alone; f temperature increase. Heating rate and gas flow rates as in Fig. 6

of relatively stable LiC1 [2]. The effectiveness of lithium as a matrix modifier was tested in a microtube atomizer. The results are shown in Fig. 4. Lithium did not act as a modifier for the interference of chloride in the present atomizer. San Xiao-quan et al. [3] used palladium as a matrix modifier for the interference of halides in the determination of thallium by graphite furnace atomic absorption spectrometry. This technique was also attempted in a microtube atomizer. Figure 5 presents the effect of palladium on the atomization of thallium in the presence of sodium chloride. From the results it is evident that palladium does not prevent the interfering effect of sodium chloride in the present atomizer.

Possible interferences of various elements were also ex- amined. The elements tested were magnesium, calcium, chromium, nickel, copper, cadmium, zinc, lead and bismuth. Each concomitant (at 50 ng level) depressed the thallium

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absorption; the degree of interference was dependent on the concomitants. Thiourea reduced the interferences from the concomitants. Figure 6 demonstrates the effect of thiourea as a matrix modifier. The small absorption, due to decompo- sition products of thiourea, appeared at a lower temperature region. When thiourea was added as a matrix modifier, the ashing step prior to atomization was important for better reproducibility of the atomization profiles of thallium. Ashing at 100 ~ C for 2 s was recommended in the present atomizer. Long ashing resulted in poor reproducibility of the atomization profiles of thallium, although small absorp- tion at lower temperature region disappeared. The matrix modification technique with thiourea was also described for cadmium, lead, copper and bismuth [9].

The interferences were not insignificant for the 500 ng level of concomitants. Almost all the concomitants tested

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had a depressive effect on the thallium absorption. Of these the interferences from cadmium, zinc and copper were re- duced with thiourea, but the decrease was insufficient for calcium, magnesium and chromium (Figs. 7 and 8). When calcium (500 ng) was present the addition of thiourea re- sulted in an increased thallium absorption accompanying a temperature shift to lower region. The interference from lead (500 ng) was considerable. With an increasing amount of thallium an apparent enhancement of thallium absorption was found in the presence of lead (Fig. 9). However, the reproducibility of the profiles of thallium in the presence of lead was poor. The measured atomization energy of thallium in the presence of lead, 217 kJ/mol, differed from that for pure thallium. This may clarify the different atomization processes for both cases. There is no satisfactory explanation for this interference at the present stage. The interference of lead (500 ng) on the thallium absorption could not be modified even by the addition of thiourea. In this case, the measured activation energy value of 134 kJ/mol was obtained. This value indicates that the atomization process of thallium in the presence of lead (500 ng), even by addition of thiourea, differs from that for the thallium thiourea complex. However, the lack o f thermodynamic data does not allow any speculation on the atomization process. Bismuth (500 ng) shifted the peak temperature of the thallium profile. The interference from bismuth was also insufficiently modi- fied with thiourea.

Koir tyohann et al. showed a severe depression effect of perchloric acid on thallium absorption [10]. A depressing effect of perchloric acid (0.05%) was also observed for

atomization of thallium in a molybdenum microtube atom- izer.

Conclusions

The heating rate of the microtube is significant for the atom- ization of thallium, especially in the presence of concomi- tants. At a higher heating rate a reduction of interferences from concomitants can be expected. Thiourea acts as a matrix modifier at the 50 ng level of concomitants. However, reduction is insufficient for some concomitants at the 500 ng level. Separation procedures may be required for samples which contain extremely high concentrations (500 ng) of calcium, magnesium, lead or bismuth.

References

1. Slavin W, Manning DC (1980) Spectrochim Acta 35 B:701 2. L'vov BV (t 978) Spectrochim Acta 33 B : 153 3. Xiao-quan Shan, Zhe-ming Ni, Li Zhang (1984) Talanta 31 : 150 4. Schmidt W, Dietl F (1983) Fresenius Z Anal Chem 315:687 5. Fazakas J (1982) Anal Lett 15 : 1523 6. Ohta K, Suzuki M (1979) Fresenius Z Anal Chem 298:140 7. Suzuki M, Ohta K, Yamakita T (t 981) Anal Chim Acta 133 : 209 8. Sturgeon RE, Chakrabarti CL, Maines IS, Bertels PC (1976)

Anal Chem 48 : 1792 9. Suzuki M, Ohta K (1983) Prog Anal At Spectrosc 6:49

10. Koirtyohann SR, Glass ED, Lichte FE (1981) Appl Spectrosc 35:22

Received March 29, 1985

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