Analytica Chimica Acta, 173 (1985) 321-325 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

Short Communication

MECHANISM OF INTERFERENCE ELIMINATION BY THIOUREA IN ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY

MASAMI SUZUKI*, KIYOHISA OHTA and KAORU ISOBE

Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho, Tsu, Mie-ken 514 (Japan)

(Received 2nd October 1984)

Summary. The mechanism of interference elimination by thiourea in electrothermal atomization is discussed. Activation energies of atomization were measured. The experi- mental values for bismuth, lead, copper and cadmium were not altered in the presence of concomitants, provided that thiourea was added before atomization. These elements form complexes with thiourea which are converted to sulphides during the charring stage. Atom formation occurs from the sulphides without compound formation between analyte and concomitants.

Matrix interference effects are well known in electrothermal atomic absorption spectrometry (a.a.s.). Matousek [l] has reviewed such inter- ferences and their elimination. For the purpose of overcoming matrix inter- ferences, a variety of chemicals, including ascorbic acid, has been applied. Most partially overcame the interference effects. No clear account, however, has been given of the mechanism of such elimination of interferences.

Suzuki and Ohta [2] showed that thiourea improved the atomization profiles and removed the effects of different species in electrothermal a.a.s. of bismuth, antimony, copper, cadmium and lead. This reagent also proved to be effective for eliminating matrix interferences. Thiourea appears to have a different mechanism for the elimination of matrix interferences than other substances. This investigation provides some evidence about this mechanism.

Experimental Appamtus. Atomic absorption measurements were made as described

previously [3]. The output signal from the amplifier was fed to a micro- computer (Sord M223) through an AD converter (Date1 ADC-HX-12BGC) and multiplexer (Date1 MX-SOS) [3]. The signals were also monitored on a Memoriscope (Iwatsu MS5021). Light sources were hollow-cathode lamps (Hamamatsu Photonics K.K.). The wavelengths used were 233.06, 228.80, 324.75, 217.00, 328.07 and 217.59 nm for bismuth, cadmium, copper, lead, silver and antimony, respectively.

A molybdenum microtube (20 mm long, 1.5 mm i.d.) was used as the atomizer. This was machined from molybdenum sheet (0.05 mm thick;

0003-2670/85/$03.30 o 1986 Elsevier Science Publishers B.V.

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Rembar Co.). The microtube atomizer was enclosed in a Pyrex chamber (300 ml) which had two silica end-windows to allow transmission of the light beam. The chamber was purged with argon (480 ml min-‘) and hydrogen (20 ml min-‘).

The atomizer temperature was measured with a photodiode (Hamamatsu Photonics K.K., S641) [3]. The temperature measured was corrected for non-black body emissivity by use of Planck’s radiation formula. The signal from the photodiode was calibrated with an optical pyrometer and was recorded simultaneously with the absorbance signal. Samples were injected into the microtube from a l-111 glass micropipet.

A Rigaku X-ray diffractometer was used for identification of the residues formed when metal solutions were heated with and without thiourea at 570 K before atomization.

Reagents. All reagents used were of the highest available purity. The stock solutions (nitrates) were prepared from pure metals with the exception of antimony, which was prepared from antimony(II1) potassium tartrate. All working solutions were prepared just before use by dilution with deionized water.

Procedures. Atomization was achieved by heating a l-p1 sample to a final temperature of 2000-2260 K for 2 s after drying at 370 K for 10 s and charring at 570 K for 10 s. The heating rate was 1.8-3.4 K ms-‘. All the absorbance signals during atomization were stored in the microcomputer. Signals were smoothed after subtraction of background (noise) [3]. To determine activation energies, the slope of a logarithmic plot of the mea- sured absorbance vs. the reciprocal absolute temperature was evaluated by least-squares analysis [ 41. The precision of the measurements was < 11%.

Results and discussion Information can be obtained from atomization energies which may help

to elucidate the atomization process. Atomization energies, therefore, were measured in order to clarify the mechanism of the interferences and their removal with thiourea. The activation energies were determined by the method proposed by Sturgeon et al. [4]. This approach assumes that there is an analyte surface/gas phase equilibrium within the atomizer and that the rate of production of observable atoms is characterized by a unimolecular rate constant. The difference between the temperature of the gas phase in the microtube and that of the tube itself is negligible at the heating rate used [2]. Therefore, it seems reasonable to expect that the physical and chemical processes in the microtube attain equilibrium before the atoms are lost.

Silver, cadmium, copper, lead and antimony interfere in bismuth atomiza- tion [2]. Figure 1 depicts the interference of cadmium on bismuth atomiza- tion and its elimination with thiourea. The signals did not return to the baseline without thiourea. Although this tailing is characteristic of bismuth, its cause is uncertain. Thiourea greatly improved this tailing. Table 1 presents the activation energies, E,, obtained by atomizing bismuth and concomitant

323

A

1770 K

a, k C

K

An. *

-Ei

1270

75 A

1770 K

a 50 k 1270 K

25 b

910 K

0 05 1

B

Time . set Time , set

Fig. 1. Cathode-ray tube display showing the interference of cadmium on bismuth absorb- ance, and its elimination with thiourea: (A) no thiourea added; (B) thiourea (5 rg) added. Traces: (a) 0.25 ng Bi; (b) 0.25 ng Bi and 0.25 pg Cd; (c) temperature.

Fig. 2. Cathode-ray tube display showing the interference of magnesium on bismuth absorbance, and its elimination with thiourea: (A) no thiourea added; (B) thiourea (5 rg) added. Traces: (a) 0.25 ng Bi; (b) 0.25 ng Bi and 0.25 rg Mg; (c) temperature.

TABLE 1

Appearance temperatures and activation energies for bismuth, lead, copper and cadmium

Concomitant addeda

Without thiourea With thiourea

Tauu (E) E, (kJ mol-I) Tauu (E) E, (kJ mol-‘)

Bismuth -

Cd cu Pb Ag Sb

Lead -

Bi Cd

Copper -

Cd Sb Bi

Cadmium -

Pb cu Bi

910 322 950 209 925 182 920 208 910 95.7 930 206 900 141 925 205 915 196 925 209 920 248 930 204

1280 401 1060 185 1200 140 1060 187 1150 200 1055 191

1180 196 1325 326,163 1325 142 1350 326,200 1300 216 1325 323, 200 1270 101 1340 330,191

680 275 710 209 690 210 685 199 710 128 690 192 702 182 685 194

alOOO-fold wt. amount with respect to the amount of the element measured (250 pg Bi, 25 pg Pb, 50 pg Cu, or 10 pg Cd).

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elements in the absence and presence of thiourea. The values of E, represent the energy required in the rate-determining step to produce metal atoms. As can be seen from Table 1, the activation energy of bismuth in the presence of concomitant elements differs from that of bismuth alone. The experi- mental activation energies of bismuth in the presence of lead, silver and antimony are close to the bond energies of Bi-Pb (134 kJ mol-‘), Bi-Ag (192 kJ mol-‘) and Bi-Sb (251 kJ mol-‘) [5]. The lack of literature values does not allow the assignment for activation energies for bismuth in the presence of copper and cadmium. The interferences from concomitant elements appear to result from the slow vaporization of compounds formed between bismuth and concomitant elements. When thiourea was present, the experimental activation energies of bismuth in the presence of concomitant elements were identical with the value for bismuth alone, which corresponded to the heat of vaporization of bismuth, Bi(1) + Bi(g) (210 kJ mol-‘). The atomization process for bismuth in the presence of thiourea is hydrogen reduction of bismuth sulphide followed by vaporization of the free metal [6]. The X-ray diffraction pattern showed the formation of sulphide on heating of the bismuth/thiourea complex at 570 K.

Table 1 also shows the activation energies for lead, copper and cadmium in the presence of concomitant elements. Various values were obtained, depending on the concomitant elements. But, provided that thiourea was used during atomization, the values were identical with those for the elements alone, irrespective of the concomitant elements. The E, value of 185 kJ mol-’ for lead correlates well with the heat of vaporization of the element (183 kJ mol-‘). For copper, the two values 326 and 163 kJ mol-’ correspond to the heat of vaporization of Cu(s), 337 kJ mol-‘, and the Cu-Cu bond energy, 195 kJ mol-‘, respectively. This results from vaporization of copper formed by hydrogen reduction of copper sulphide [ 61. The formation of the sulphide in the charring stage was identified by the X-ray diffraction pattern. An E, value of 184 kJ mol-’ for cadmium correlates well with the CdS bond energy, 201 kJ mol-‘. The atomization process of cadmium in the presence of thiourea is thermal dissociation of cadmium sulphide [6] . The activation energies of lead, copper and cadmium in the presence of concomitants with- out thiourea were difficult to relate to the thermodynamic data except for Pb-Bi because of the lack of literature values.

The interference mechanism of concomitant elements may be considered as follows. The analyte and concomitant elements form compounds such as intermetallic compounds, which delay analyte vaporization. As noted above, the formation of the sulphides of analyte and concomitant elements proved to be effective for modification of interferences from concomitants. This may be due to the ready release of analyte elements from their sulphides.

In these experiments, both the analyte and concomitant elements formed thiourea complexes. The elimination of an interference from a concomitant, however, occurs if either analyte or concomitant forms a complex with thiourea. For example, a severe interference was encountered from magnesium

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on the atomization of bismuth, but this interference was eliminated by thiourea, as shown in Fig. 2. Magnesium does not form a thiourea complex. The similarity of the atomization profiles in the presence of thiourea indi- cates that the rate of formation of analyte atoms in the microtube is not changed by the presence of concomitants. The activation energy was the same both in the absence and presence of the concomitant element.

To confirm the process of interference elimination with thiourea, atomiza- tion was tested with a microtube atomizer with double indentations [7] . First the analyte and concomitant elements were placed in one indentation and thiourea in the other. In this case, thiourea had no effect. But when analyte, concomitant and thiourea were mixed in one of the indentations, the interference was eliminated. These results suggest that the formation of a thiourea complex of the analyte has a fundamental significance for inter- ference removal. The favorable effect of thiourea on the analyte alone was also observed when mixing was done before atomization, but not in atomiza- tion from separate indentations.

Other compounds, e.g., ascorbic acid and EDTA, have been used for elimination of matrix interferences. For ascorbic acid, Regan and Warren [8] proposed a mechanism involving the accelerated reduction of metal oxides by carbon generated in the pyrolysis of ascorbic acid. In contrast, the effect of thiourea is attributed to formation of the sulphide of the analyte or concomitant, or both, during the charring stage. This is supported by the similar effect of thioacetamide. The effect of thiourea was also observed for atomization of cadmium from a carbon surface. For this experiment, carbon was deposited on the molybdenum atomizer as a hard coating, by the thermal decomposition of acetylene added to the argon purge gas, at high temperature.

REFERENCES

1 J. P. Matousek, Prog. Anal. Atom. Spectrosc., 4 (1981) 247. 2 M. Suzuki and K. Ohta, Prog. Anal. Atom. Spectrosc., 6 (1983) 49. 3 M. Suzuki, K. Ohta and T. Yamakita, Anal. Chim. Acta, 133 (1981) 209. 4 R. E. Sturgeon, C. L. Chakrabarti and C. H. Langford, Anal. Chem., 48 (1979) 1792. 5 R. C. Weast, CRC Handbook of Chemistry and Physics, 54* edn., CRC Press, Cleveland,

OH, 1979. 6 M. Suzuki and K. Ohta, Anal. Chim. Acta, 151(1983) 401. 7 K. Ohta and M. Suzuki, Talanta, 23 (1976) 560. 8 J. G. T. Regan and J. Warren, Analyst (London), 101(1976) 220.