Analytica Chimica Acta, 182 (1986) 225-229 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Short Communication

ATOMIC ABSORPTION SPECTROMETRIC STUDIES OF THE ATOMIZATION OF BORON FROM A CARBON ROD ATOMIZER

NEELAM GOYAL, A. R. DHOBALE, B. M. PATEL and M. D. SASTRY*

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085 (India)

(Received 30th May 1985)

Summary. Boron (< 20 fig ml-l) in aqueous solutions gives no absorbance but addition of ascorbic acid, especially with titanium greatly enhances the signal, leading to a detection limit of 0.2 pg ml-’ boron. The presence of uranium (< 10 mg ml-‘) only slightly decreases the boron signal.

Atomic absorption spectrometry (a.a.s.) with electrothermal atomization is quite useful for the determination of traces of metals in nuclear reactor fuels such as uranium(IV) oxide and mixed uranium(IV)/plutonium(IV) oxide [l-6]. However, a few elements such as boron and gadolinium, which are crucial for the quality assurance of nuclear fuels, pose problems in determi- nations below their specified upper limits. This is presumably due to the inefficient atom formation of these elements on account of factors such as carbide formation, the large dissociation energies of their oxides and possible loss as volatile products [7] . Therefore, there is a need to understand the atomization mechanism for these elements which may help in optimizing conditions of high temperature reactions leading to larger atom densities in the carbon rod atomizer (CRA). Strong interelement effects on the atom density of boron particularly by alkaline earth elements have been reported [8-101.

The results of studies of the determination of boron by a.a.s. with a car- bon rod atomizer in the presence of titanium and ascorbic acid are presented here. The effect of uranium on the atomization of boron is also investigated.

Experimental Apparatus. A Varian-Techtron atomic absorption spectrometer (model

AA-6) equipped with a carbon “rod” atomizer (CRA-63) and BC-6 back- ground corrector was used. The atomizer portion was enclosed in a glove box to facilitate handling the radioactive samples. A boron hollow-cathode lamp operated at 15 mA was used. The atomization program was: ashing, 30 s at 400°C; atomization, 4 s at 2850°C (manufacturer’s values). Peak-height signals were measured.

0003-2670/86/$03.50 0 1986 Elsevier Science Publishers B.V.

Preparation of standards and samples. Spectroscopically-pure chemicals and high-purity preanalysed uranium were used in the preparation of stock solutions for the elements of interest. Quartz double-distilled water and elec- tronic-grade nitric acid were used in their preparation. Three sets, each of seven graded multi-element lo-ml standard solutions, were prepared contain- ing boron in the range O-10 pg ml-’ and concomitant elements (Ag, Al, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, MO, Ni, Si, Sn, V and Zn) in the range 0.01-10 pg ml-‘. All solutions in a given set contained 250 mg of ascorbic acid, or 250 mg of ascorbic acid and 100 pg of titanium, or 100 mg of uranium in addition to these amounts of ascorbic acid and titanium. The blank solu- tions for all the three sets were prepared with the same reagents and starting materials, but without the boron and concomitant elements. Freshly prepared standard solutions were used each time.

Multi-element reference standards, obtained from Jarrell-Ash and Spex Industries, were suitably diluted and mixed with appropriate amounts of titanium and ascorbic acid to serve as reference samples for boron deter- mination. Aliquots (5 ~1) of an intermediate-concentration standard were used in optimization of experimental parameters.

Results and discussion Figure 1 shows the influence of 0.25% ascorbic acid and titanium on the

atomic absorption of 5 pg ml-’ boron. Atomization of boron is markedly increased in the presence of ascorbic acid. However, further increase of ascorbic acid concentration increases the viscosity of the solution and con- tributes to inaccuracy in sample injection.

The presence of trace metals also significantly affects the atomization of boron. Strong interelement effects, particularly that of calcium, have been reported by van der Geugten [8] and Szydlowski [ 91, suggesting that cal- cium (l-2 mg ml-‘) and barium in the presence of calcium enhance boron absorbance. Such effects were investigated. Titanium was found to have the most significant effect in increasing the boron absorbance, in the absence or presence of ascorbic acid (Fig. 1).

A series of standards containing the concomitant elements, titanium and

B+Ascorblc ocld B+Ti+ Ascorbic oad

Fig. 1. Effect of 10 pg ml-’ titanium and 0.25% ascorbic acid on the absorbance of 5 Mg ml-’ boron. The shaded portion shows the contribution of the non-specific absorbance at 249.7 nm.

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ascorbic acid as well as boron was analysed. A linear calibration graph was obtained in the range of 0.2-10 ccg ml-’ (Fig. 2). The reference standards were analysed with reference to this graph. The results (Table 1) were in good agreement with the certified values. The absolute 20 limit of detection, 0.2 pg ml-’ in 5 ~1 of sample (i.e., 1 ng of boron), compares favourably with that reported by Slavin et al. [lo] who used calcium as a matrix modifier, viz. 50 ng ml-’ in 20 ~1 of sample (again 1 ng of boron). Other workers [8,9] reported 20-25 ng ml-’ as the limit of detection for a 204 sample. This improvement is probably due to more efficient atomization of boron under their experimental conditions.

Atomization mechanism. The ln(absorbance) vs. l/T plot (T is the surface

_---iI 2 4 6 8 IO

Concentration (pg ml?

0.0 2 4 6 8 IO 12 14 16 18 20

Uranium concentration (mg ml-‘)

Fig. 2. Calibration graphs for boron: (A) standards contain boron and the concomitant elements given in the text; (B) standards used for (A) with addition of 10 mg ml-’ ura- nium.

Fig. 3. Effect of uranium on the absorbance of boron at 249.7 nm: (0) specific absor- bance; (a) non-specific absorbance.

TABLE 1

Comparison of the boron concentratron found in certified standard solution@

Sample Boron concentration (pg ml-l) Sample Boron concentration (pg ml-‘)

Expected Found Expected Found

M-Spex 0.25 0.26 B-Spex 2.50 2.50 M-Spex 0.50 0.50 B-Spex 4.00 3.40 M-Spex 2.00 2.25 Jarrell-Ash 2.50 2.50

std. 4

aWith addition of 0.25% ascorbic acid and 10 rg ml-’ titanium. The Jarrell-Ash standard was diluted I-fold.

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temperature of the graphite furnace as given by the manufacturer) was linear for boron in aqueous samples containing titanium and ascorbic acid. The slope gave an activation energy of 93 + 2 kcal mol-‘. In the presence of ascor- bic acid only, boron absorbance was decreased, but the linear In(absorbance) vs. l/T plot gave an activation energy of 88 f 3 kcal mol’. These values com- pare reasonably well with the reported value of 97 + 3 kcal mol-’ for the sub- limation of solid boron [ 111, This strongly suggests that the formation of boron atoms under the present conditions is primarily due to the sublima- tion of solid boron; the presence of ascorbic acid enhances the formation of solid boron and the presence of titanium further improves the conditions for solid boron formation. A detailed understanding of the role of ascorbic acid and titanium in this process is impossible from these studies, as they do not throw light on any reaction intermediates. It is possible, however, that the titanium decreases boron carbide formation.

Determination of boron in uranium. With a view to application of this approach to nuclear fuel analysis, the determination of boron in the presence of uranium was studied. Earlier studies [l-3, 61 showed that uranium has a marked effect on the atomization mechanisms of various analytes. Therefore, experiments were done to investigate the effect of uranium on the absorbance of 5 pg ml-’ boron. The absorbance decreased with increasing uranium con- centration up to 8 mg ml-‘, above which the absorbance was unaffected (Fig. 3). Non-specific absorbance from uranium increased at higher concen- trations of uranium, thereby limiting the permissible uranium concentration to 10 mg ml-’ (Fig. 3). The calibration graph for boron in the presence of 10 mg ml-’ uranium (Fig. 2) is linear for 0.4-10 pg ml-’ boron. If a uranium solution is diluted to a concentration of 10 mg ml-‘, then boron can be determined in the concentration range 40-1000 pg g-‘. This is much larger than the fractional pg g-l boron concentrations found in most nuclear fuels. This necessitates separation of boron from uranium in order to make the present method applicable for quality control of nuclear fuel materials.

The authors are grateful to Dr. M. V. Ramaniah, Director, Radiological Group and Dr. P. R. Natarajan, Head, Radiochemistry Division, for their constant encouragement.

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