JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1 99 1 VOL. 6 669 Combination of Chemical Modifiers and Graphite Tube Pre-treatment to Determine Boron by Electrothermal Atomic Absorption Spectrometry Milagros Luguera Yolanda Madrid and Carmen Camara* Departamento de Quimica Analitica Facultad de Quimicas Universidad Complutense 28040 Madrid Spain Several ways of increasing the atomization efficiency in the determination of B by electrothermal atomic absorption spectrometry were evaluated the use of a chemical modifier (Ni Pd and ascorbic acid); pre- treatment of the graphite tubes with carbide forming elements (Zr Ta and V); and addition of NaF as a cleaning solution between measurements. The tubes were pre-treated by soaking in the carbide forming metal solution under vacuum drying for 4 h at 150 "C and finally subjecting them to slow electrothermal heating to at least 1600 "C in order to form the metal carbide on the inner side of the tube.The best results were obtained using Zr- treated tubes in conjunction with Ni as chemical modifier. This resulted in high sensitivity (80 pg) a negligible memory effect and an increased tube lifetime (200 atomization cycles of 3 s at 2650 "C). The method was successfully applied to determine B in waste and river waters. Keywords Pre-treatment of graphite tube; chemical modification; determination of boron; electrothermal atomic absorption spectrometry Conventional electrothermal atomic absorption spectrome- try (ETAAS) has a limited application in the determination of B because of the tendency of this element to form refractory carbides with the carbon of the graphite tube before atomization occurs.Among the problems associated with the formation of refractory carbides are loss of sensitivity and the occurrence of 'memory peaks' due to the atomization of residual boron carbides formed during previous determinations. This reduces the reproducibility of the analytical measurements. Furthermore the high vaporization temperature of boron carbide (about 2700 "C) reduces the tube lifetime.' Several methods have been reported in the literature to remove the problems mentioned above. Most of them attempt to prevent the formation of refractory carbides by means of a chemical modifier that inhibits this reaction such as Ca Mg Sr Ba Ni and La2-4 or a Ti and ascorbic acid m i x t ~ r e .~ Some methods involve pre-treating the graphite tube with elements that form carbides that are more thermodynamically stable than boron carbides. Others seek to destroy residual boron carbide by flushing the instrument with cleaning solutions between measure- ments such as NaF solution or CH30H-H2S04.1 The purpose of the present work was to evaluate the ability of the following procedures to increase the efficiency of B vaporization the use of chemical modifiers; pre- treatment of the graphite tube; flushing with cleaning solutions; and a combination of chemical modification and pre-treatment of the tube. Finally the method was applied to the determination of B in waste and river waters by ETAAS. Experimental Apparatus A Perkin-Elmer Model 1 1 OOB atomic absorption spectro- meter equipped with an HGA-400 graphite furnace atom- izer was used.A hollow cathode boron lamp was operated at 34 mA and the absorption was measured at 249.7 nm with a 0.7 nm slit-width. A deuterium lamp was used to correct for background absorption. * To whom correspondence should be addressed. Reagents All chemicals were of analytical-reagent grade or higher purity and de-ionized water from a Milli-Q system (Milli- pore) was used throughout. A 1000 mg 1-l stock standard solution of B was prepared by dissolving 2.86 g of H3B03 in 500 ml of de-ionized water and storing in a polyethylene bottle. Working solutions were prepared each day by diluting appropriate aliquots of the stock solution. Metal solutions used as chemical modifiers or as graphite tube pre- treatment agents were prepared from the salts or oxides of the pure metals depending on the species of interest.Tube Pre-treatment Procedure Both pyrolytic graphite and non-pyrolytic graphite tubes were immersed in 10 ml of a carbide forming metal solution (1 g 1-l for Zr and V and 10 g 1-l for Ta) in a bottle. A vacuum pump was connected for a few minutes to drive bubbles from the tubes and then atmospheric pressure was restored. The process was repeated until no more bubbles were formed. This procedure forces the carbide forming metal solution to flow into the pores of the tubes. The tubes were removed from the bottle and dried first at room temperature for 1 h and then in an oven at 150 "C for 24 h. The ends of the tubes were carefully polished with a soft laboratory tissue to remove any remaining salts that might alter the electrical conductivity between the graphite tubes and the graphite contact cones. The tubes were further dried by subjecting them to slow electrothermal heating to at least 1600 "C in order to form the metal carbide in the graphite tube. A similar procedure was followed with the L'vov platform.Sample Analysis Procedure River and waste water samples were collected in polyeth- elene bottles filtered and acidified with HN03 to pH<2. Nickel (to a final concentration of 1000 ppm) as a chemical modifier was added to the samples and the B was deter- mined by injecting 20 pl of the sample solution into a pyrolytic graphite coated graphite tube treated with Zr as described under Tube Pre-treatment Procedure. The graph- ite furnace temperature programme used is summarized in Table 1.The analytical peaks were recorded as integrated670 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Table 1 Graphite furnace temperature programme using Ni as a chemical modifier and a Zr-treated pyrolytic graphite coated graphite tube Air flow Step ture/"C time/s time/s ml min-l Tempera- Ramp Hold rate/ Drying 120 20 1 300 Ashing 900 20 20 300 Atomization 2700 0 3 0 Cleaning 2650 1 3 300 Cool down 30 1 5 300 absorbance. The concentration of B in the sample was obtained by extrapolation from the calibration graph. Results and Discussion Effect of Graphite Tube Type on Boron Atomization In order to study how the type of graphite tube employed affects the determination of B by ETAAS several types of tube (without pre-treatment) were tested non-pyrolytic graphite tubes; pyrolytic graphite coated graphite tubes; and L'vov platforms with pyrolytic graphite coated graphite tubes.Pyrolytic graphite coated graphite tubes gave a 75% higher sensitivity than L'vov platforms with pyrolytic graphite coated graphite tubes. Non-pyrolytic graphite tubes were totally unsuitable for the determination of low levels of B as they gave a poor sensitivity which could be due to the higher porosity that facilitates the formation of boron carbides and thus increases the memory effect. Although pyrolytic graphite coated graphite tubes were chosen for all further experiments the determination of B without a chemical modifier became very imprecise because of the poor sensitivity and the too wide analytical peaks that were obtained. Furthermore a large memory effect was obtained which made it necessary to apply 3-5 cleaning cycles between successive atomizations.This increased the analysis time and decreased the lifetime of the tubes (20 firings for a pyrolytic graphite coated graphite tube). Addition of NaF Cleaning Solution In order to prevent the memory effect 10 pl of 4% m/v NaF solution were injected between atomization cycles to transform the residual B that remained in the tube as B4C into the volatile species BF3. This decreased the memory effect so that it was only necessary to perform one cleaning cycle between measurements. Furthermore the sensitivity was significantly higher than for the conventional method. This could be explained by an interaction between the B and F remaining in the tube which facilitates B vaporiza- tion by the formation of the volatile species BF3.However this procedure was not sufficiently sensitive to determine B at low concentrations and the long analysis time and the short lifetime of the graphite tubes could not be overcome. Use of Ni Pd and Ascorbic Acid as Chemical Modifiers It is well established that elements such as Ca Ba Mg and Sr increase the efficiency of B vaporization owing to the formation of the respective borides which are more volatile than boron carbides. In this work the influence on the atomization signal of Ni and Pd (which are well known as universal modifiers) was studied both separately and in combination with ascorbic acid. The results displayed in Table 2 show that the use of these ~ Table 2 Relative sensitivity obtained with 20 pl of different chemical modifier solutions Relative Chemical modifier sensitivity B (1.0 pg ml-I) 1 Ascorbic acid (0.25% m/v) 45 Pd (1000 pg ml-I) 72 Ni (1000 pg ml-I) 170 Ni (1 000 pg ml-I) + ascorbic Pd ( 1000 pg ml-l) + ascorbic acid (0.25% m/v) 45 acid (0.25% m/v) 35 elements as chemical modifiers significantly enhances the signal Ni being the more effective. Other advantages are a lower background and a smaller memory effect i.e. it is not necessary to apply cleaning cycles between the measure- ments.The lifetime of the tubes increased by up to 100 firings without any significant loss in sensitivity in the determination of the B.The combination of Ni and Pd with ascorbic acid applied successfully in the determination of Se,6 gave worse results (Table 2) than those obtained using each element alone. Tube Pre-treatment Combined With the Use of Ni as Chemical Modifier In order to increase the efficiency of B atomization with respect to that obtained in the section above the pyrolytic graphite coated graphite tubes were pre-treated with car- bide forming elements. This pre-treatment has been suc- cessfully applied in the determination of other refractory metals such as Si and Al.7-9 Of the three tube pre-treatments tested (Zr Ta and V) the Zr-treated tube gave the best results. Although the characteristic mass obtained using Zr- treated tubes is higher than that obtained using Ni as a chemical modifier (Table 3) the former method increases the lifetime of the tube by up to 200 firings without deterioration. On considering these results both effects were combined for the proposed procedure.The results obtained using pyrolytic graphite coated graphite tubes pre- treated with several metals and Ni as chemical modifier are compared in Table 3 expressed in terms of characteristic mass and memory effect. The improvement obtained by the combination of Ni as chemical modifier and a Zr-treated pyrolytic graphite Table 3 Comparative results obtained in terms of sensitivity and memory effect using several procedures to determine B by ETAAS Type of pre-treatment of pyrolytic graphite Memory Characteristic Modifier coated graphite tube effect mass*/pg - - ++++t 24295 Ni - + +$ 143 ++ 338 Pd - Zr +§ 7 90 Ni Zr -7 88 Ni Ta 216 Ni V 126 - *The characteristic mass is defined as the mass of analyte in picograms required to give an integrated absorbance signal of 0.0044 absorbance seconds.t Very significant memory effect. # Significant memory effect. 0 Low memory effect. 7 No memory effet.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 67 1 0.200 I 0.175 0) 0.150 x $ 0.125 n 0 0.100 0) U 0.075 - 0.050 0) U 0.025 1 1 &50 2600 2650 2700 Temperatu rePC Fig. 1 Effect of atomization temperature on the signal for 20 pl of 0.1 pg ml-I of B using Ni as chemical modifier. A Untreated pyrolytic graphite tube; B Ta; C V; and D Zr-treated pyrolytic graphite coated graphite tubes coated graphite tube could be explained by the conjunction of two factors (z] B is stabilized as nickel boride thus reducing the tendency of B to react with the graphite; and (ii) Zr forms a stable carbide on the carbon surface and reduces the boron-carbon interaction.On the other hand the use of pyrolytic graphite coated graphite tubes pre-treated with carbide forming elements provides higher sensitivity than that obtained with un- treated pyrolytic graphite coated graphite tubes at the optimum vaporization temperature. Fig. 1 shows the different integrated absorbance values obtained using Ta- V- and Zr-treated pyrolytic graphite coated graphite tubes and Ni as chemical modifier at several atomization temperatures. The use of a Zr-treated L'vov platform gives lower sensitivity than that with treated pyrolytic graphite coated graphite tubes.Determination of Boron in River and Waste Water Samples The combination of a Zr-treated pyrolytic graphite coated graphite tube and Ni as chemical modifier was applied to the determination of B in river and waste water samples. The analytical conditions were those indicated in the procedure and the results obtained are summarized in Table 4. In no instance was a matrix effect observed and hence the standard additions method was not required. The procedure gives a high recovery of B as measured by adding known amounts of B to the real samples. In order to confirm the accuracy of the proposed method the samples were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) as an alternative tech- nique.The results show good agreement with those ob- tained by ETAAS (Table 4). The precision of the method (mean of ten values between batches expressed as relative standard deviation) was 6.1 ~ ~ Table 4 Determination of B in river and waste waters Concentration*/ Recovery ICP-AESI Waste water No. 1 1.26k0.02 9 6 k 4 1.18k0.06 Sample ml-I (%) pg ml-I No. 2 0.42 k 0.02 0.38 f 0.03 No. 3 0.25 k 0.03 0.3 1 k 0.02 River water No. 1 1.09k0.04 9 4 k 3 1.10k0.06 No. 2 0.63 k 0.04 0.70 k 0.06 No. 3 0.22 k 0.02 - * Mean value of six determinations k standard deviation. and 3.0% for 0.4 and 1.0 pg ml-l of B respectively. The detection limit calculated using the IUPAC recommenda- tion (30) was 0.07 pg ml-l and the determination limit (1 00) was 0.25 pg m1-I.Conclusion The proposed method for the determination of B by the combination of a chemical modifier Ni and a Zr-treated pyrolytic graphite coated graphite tube provides several advantages over the methods reported previously in the literature increased sensitivity no memory effect shorter analysis time and longer lifetime of the graphite tube. The method allows the determination of B in waste water samples. It was shown that it is not necessary to use the standard additions method which could be of interest for the routine analysis of samples of this kind. The authors thank M. Gormann for the revision of the manuscript and the Comisi6n Interministerial de Ciencia y Technologia (CICYT) (Project No. PB 880094) for financial support. References 1 2 3 4 5 6 7 8 9 Barnett N. W. Ebdon L. Evans E. H. and Ollivier P. Anal. Proc. 1988 25 233. Jyan Y. Yao J. and Huang B. FenxiHuuxue 1989,17,456. Norval E. Anal. Chim. Acta 1986 181 169. Van der Geugten R. P. Fresenius Z. Anal. Chem. 198 1,306 13. Goyal A. Patel D. and Sastry M. D. Anal. Chim. Acta 1986,182,225. Kwoles M. B. Broodie K. G. J. Anal. At. Spectrom. 1989,4 305. Runnels J. H. Merryfield R. and Fisher H. B. Anal. Chem. 1975,47 1258. Nater E. A. and Burau R. G. Anal. Chim. Acta 1989 220 83. Ng K. C. and Caruso J. A. Anal. Chim. Acta 1982,143,209. Paper I /0133 7H Received March 20th I991 Accepted August 14th I991