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Vapor phase matrix extraction of high purity di-boron trioxide and trace analysis using electrothermal AAS
Analytica Chimica Acta 546 (2005) 229–235
Vapor phase matrix extraction of high purity di-boron trioxide and trace analysis using electrothermal AAS K. Dash, S. Thangavel, S.M. Dhavile, S.V. Rao, S.C. Chaurasia, J. Arunachalam ∗ National Centre for Compositional Characterisation of Materials (CCCM), Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, India Received 28 January 2005; received in revised form 30 April 2005; accepted 4 May 2005 Available online 24 June 2005
Abstract A method has been developed for the determination of Al, Cd, Cr, Cu, Fe, Mg, Mn, Ni, Sb, Sn and Zn at trace levels in high purity di-boron trioxide using ETAAS. The boron trioxide matrix was eliminated as trimethyl borate ester in a multiplex vapor phase matrix extraction (MVPME) device using a mixture of glycerol and methanol. In this MVPME device, in situ reagent purification, sample digestion and simultaneous matrix elimination were achieved by a single step in closed condition, which in combined effect reduce the process blanks. The matrix extraction procedure allows determination of trace elemental impurities by electrothermal atomic absorption spectrometry (ETAAS) with fast furnace analysis (without an ashing step and modifier) and calibration against aqueous standards. The performance and accuracy of the vapor phase matrix elimination technique are compared to those of suprapur grade hydrofluoric acid solution in two ways; (i) matrix separation as BF3 over hot plate and (ii) in situ matrix elimination inside graphite furnaces. The method detection limits calculated from blank samples are in the range of 0.5 (Ni) and 2.9 (Al) ng g−1 . Thus the MVPME-based sample preparation approach is well suited for the trace analysis of high purity di-boron trioxide used in microelectronics applications. © 2005 Elsevier B.V. All rights reserved. Keywords: Di-boron trioxide; Multiplex vapor phase matrix extraction; ETAAS; Trace analysis
1. Introduction Di-boron trioxide (B2 O3 ) is used as a network former [1] in optical fiber, where stringent purity levels with particular reference to transition metal impurities, which act as trap centers are required. Further, highly pure grade B2 O3 is required in the electronics industry as a liquid encapsulent [2] for the Czochralski type growth of group iii–v semiconductor crystals. Electrothermal atomic absorption spectrometry (ETAAS) is one of the most sensitive techniques for the determination of trace impurities and its power of detection extends to the low picogram range. In boron materials, however, the matrix at higher concentrations reduces the atomic absorption signal for the transition metal impurities [3–5] and thus the optimal power of detection and best accuracy may not be achieved in ∗
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presence of this matrix. In spite of the remarkable progress achieved by applying stabilized temperature platform furnace (STPF) concept, chemical modifiers and powerful Zeeman background correction, the presence of matrix continues to be a fundamental analytical problem for trace and ultra trace analysis. One approach to circumvent such problems is to perform some form of analyte–matrix separation prior to ETAAS measurements, which may also permit the use of lower dilution factor, and hence improve the limit of detection of the method. Some reports describing the boron matrix removal prior to instrumental detection have been published. Tompuri and Tummavuori [3] reported in situ matrix (H3 BO3 ) removal during the pyrolysis step using hydrofluoric acid as a chemical modifier. Smolander et al. [5] removed boric acid by ion exchange and esterification with ethanol. Esterification with ethanol produced too strong a background that required the sample to be diluted to 1:10. Hence, this method requires extensive sample preparation and may not be suitable for ultra trace analysis. Further, Yu et al. [6] reported that the
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reaction of boric acid with methanol gives highly volatile borate ester, which readily evaporates. However, handling trimethyl borate ester in an open laboratory is hazardous [7] and storage of methanol digest in autosampler cups was problematic because the analyte concentration increased during the sequence of automatic sample injection, owing to solvent evaporation. Alternately, off line volatilization of trimethyl borate ester could degrade the detection limits by contamination encountered in a normal laboratory or from reagents used. In such circumstances, the accurate determination of analytes at ng g−1 levels in high purity B2 O3 is mostly limited by contamination encountered in sample preparation steps rather than by the sensitivity of ETAAS. So, the reliability and validity of the analytical data strongly depend on the degree to which contamination can be minimized. The high and/or random procedural blank problems from reagents or from laboratory environment can be surmounted by vapor phase digestion (VPD) [8,9]. In an earlier study [10] methanol was used for boric acid matrix volatilization, where the non-ionic nature of the solvent (methanol) and lower process blanks were advantageous for the conductivity detection of trace ions by suppressed ion chromatography. In the present work, multiplex vapor phase matrix extraction (MVPME) is combined with fast furnace analysis [11–15] with the aim of determining trace impurities in boron trioxide. In this closed vapor phase digestion assembly in situ reagent (methanol) purification, sample digestion and simultaneous matrix (B2 O3 ) elimination are achieved in a single step. The results obtained by MVPME are compared to those of suprapur grade hydrofluoric acid solution in two ways: (i) matrix separation as BF3 over hot plate and (ii) in situ matrix elimination inside graphite furnace. In the described digestion device eight samples (including three blanks) can be handled in a batch, thus improving the sample throughput.
2. Experimental 2.1. Reagents and chemicals Ultrapure water with a specific resistance of 18 M cm or greater was obtained through a Milli-Q water purification system. Hydrofluoric acid (GR and suprapur grade) and nitric acid (suprapur grade) from Merck, Germany were used. Glycerol and methanol (both AR grade) were from SD Fine Chemicals, India. Standard solutions of 1000 mg l−1 were prepared from respective nitrate salts (GR grade, Merck, India) for Al, Cd, Cr, Cu, Fe, Mg, Mn, Ni, and Zn, whereas for Sb and Sn, respective chloride salts (Titrisol grade, Merck, Germany) were used. The multi-element calibration standard contained 50 g l−1 each of Al, Ni, Sn and Sb, 20 g l−1 (Fe, Ni, Cr and Cu), 5 g l−1 (Cd, Mg and Zn) and 10 g l−1 of Mn. Five point calibration curves were prepared for each element with the autosampler by dilution in tube. A sample volume of 20 l was used.
2.2. VPD apparatus The high purity boron trioxide samples were digested in the MVPME assembly (Fig. 1). The polypropylene (PP) vessel is partitioned into sample compartment (upper portion) and reagent compartment (lower portion) by placing a perforated sample rack made out of PP sheet. Through wholes (8 mm) were drilled to channelise the methanol vapors into the sample compartment. The vessel and the lid have air-locking arrangement through a rib in the lid and corresponding recess in the vessel. To ensure that the assembly is completely leak-proof, it was covered with a 0.2 mm thick circular Teflon sheet with the lid pressed into the vessel. Polyflouroalkoxy (PFA) vials (7 ml, Cole Parmer, NY, USA) were used as sample containers. To clean the digestion assembly a nitric acid solution (1:1) was taken in the reagent compartment, capped and kept on IR hot plate (temperature ≈90 ◦ C) for 2 h. After cooling, it was thoroughly rinsed with DI water. PFA containers were soaked for 24 h in 2% nitric acid rinsed several times and were dried in class 10 clean bench before use. The micropipette tips and autosampler cups were kept in warm 20% (v/v) suprapur nitric acid for 24 h and washed with ultrapure water for several times. 2.3. Instrumentation All the measurements were carried out using an Analytikjena model ZEEnit 65 (Germany) graphite furnace atomic absorption spectrometer equipped with an MPE 60 autosampler. The spectrometer was provided with both Zeeman (transverse variable magnetic field) and deuterium–HClbased background correction systems. The deuterium device was used through out this work, the Zeeman corrector being used only for comparison purpose. Hollow cathode lamps of various origins were used. For volatile elements (Cd, Mg, Sb, Sn, Zn) pyrolytically PIN platform (part no. 407-152.011) tubes were used. The other elements (Al, Fe, Cr, Ni, Cu, Mn) were atomized directly off the wall of the pyrolytically coated tubes (part no. 407-152.013). Details of other instrumental parameters and fast furnace programme are given in Table 1. 2.4. Sample preparation 2.4.1. Sample digestion in MVPME assembly Sample vials of PFA (perfluoroalkoxy, Cole parmer, USA) were cleaned with DI water and dried to ensure that there was no residue. Boron trioxide samples (0.5 g each) were weighed into the PFA vials in a class 1000 clean room and were placed in the grooves (22 mm) of the sample rack. Thoroughly mixed glycerol–methanol (2:1, 300 ml) was poured into the reagent compartment. The vessel was capped and the assembly was kept on IR hot plate (surface temperature ≈90 ◦ C) for 8 h. The assembly was allowed to cool, PFA vials were removed from the sample rack. The trace metals were leached into 1 ml of 1% nitric acid (by an eppendorf micropipette) prior to their determination by ETAAS.
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Fig. 1. Matrix extraction device.
2.4.2. Sample dissolution in HF 0.5 g of boron trioxide in a PFA vial was dissolved in 3 ml of suprapur grade HF and the resulting solution was analyzed for trace elements.
Table 1 Instrumental and fast furnace programme conditionsa Element
Al Cu Cr Cd Fe Mg Mn Ni Sb Sn Zn
Wavelength (nm)
309.3 324.8 357.9 228.8 248.3 285.2 279.5 232.0 217.6 224.6 213.9
Temperature (◦ C) Drying (ramp: ◦ C s−1 )
Atomization (ramp: ◦ C s−1 )
240 (15) 300 (15) 300 (20) 120 (25) 240 (20) 240 (20) 240 (20) 240 (20) 140 (25) 240 (20) 120 (15)
2400 (1600) 2300 (1600) 2500 (1600) 1600 (1200) 2200 (1600) 2200 (1500) 2400 (1600) 2400 (1600) 1900 (1000) 2300 (1500) 1800 (1000)
a Thermal pretreatment: drying, hold 15 s, for all elements. Atomization: hold 4 s, read 4 s, for all elements. Cleaning: 100 ◦ C more than the respective atomization temperature, ramp 1 s, hold 3 s, for all elements.
2.4.3. Matrix volatilization by HF In order to compare the results, conventional matrix volatilization was used. One gram sample was accurately weighed into PFA vial and to this 6 ml of suprapur grade HF was added. The resulting boron matrix BF3 was volatilized in a laminar clean flow bench over an IR hot plate. After drying up, the residues were leached into 1 ml of 1% nitric acid and analyzed by ETAAS. 2.4.4. Open volatilization with CH3 OH Methanol (10 ml) was taken in a PFA vial and was evaporated in a laminar flow clean bench (class 10) and the residue was leached into 1 ml of 1% (v/v) nitric acid and analyzed to find out the process blank values.
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The completely dry and empty condition inside the PFA container makes it possible to leach the trace impurities accurately into 1 ml of 1% nitric acid solution.
3. Results and discussions 3.1. Principles of matrix extraction in closed MVPME assembly
3.2. Advantages of MVPME The polypropylene vessel is divided into reagents compartment and sample compartment (Fig. 1). On heating the MVPME assembly, vapors of methanol from the glycerol–methanol mixture are channeled into the sample compartment, which reacts with B2 O3 in the PFA vial forming trimethyl borate as follows: B2 O3 + 6CH3 OH ↔ 2B(OCH3 )3 ↑ + 3H2 O
(1)
Further, the three water molecules produced in reaction (1) react with B2 O3 as follows: B2 O3 + 3H2 O → 2H3 BO3
(2)
Again the boric acid formed in reaction (2) reacts with methanol vapors as: H3 BO3 + 3CH3 OH ↔ B(OCH3 )3 ↑ + 3H2 O
(3)
Reactions (1) and (3), where trimethyl borate is formed are reversible reactions. So the progress of those reactions depends upon the facility with which water or the trimethyl borate or both can be removed from the reaction zone. Vapors of trimethyl borate (bp = 68.7 ◦ C) formed in reactions (1) and (3) react with glycerol (present in the reagent compartment) to form a polyolborate complex [16] which is a monobasic acid and again three molecules of CH3 OH are released as shown in the following reaction:
The use of MVPME results in a controlled transfer of methanol vapors (on heating) from the reagent compartment to the sample compartment, which effects the dissolution and thus the impurities from the reagent are not added into the sample. Additionally the PFA containers are separated by a distance from the glycerol–methanol solution, which ensures that creeping of methanol is completely eliminated. The glycerol (bp 267 ◦ C) used serves many purposes. Methanol and water are soluble in glycerol, whereas trimethyl borate forms polyolborate complex with it. So large pressure build up inside the MVPME assembly is prevented. Additionally, methanol and trimethyl borate ester (both toxic) vapors are not released into the environment. Multiple experimental observations indicated that solution of glycerol and methanol (2:1, 300 ml) is ideal to carry out the vapor phase matrix elimination. In open volatilization of any boron matrix as trimethyl borate ester a dehydrating agent i.e. sulfuric acid is used to facilitate the reversible esterification reaction. However, the use of sulfuric acid introduces interferences in GFAAS analysis. Additionally, higher impurity levels in sulfuric acid often results in an increase in the process blank levels. In this aspect, in the MVPME procedure, no sulfuric acid is required. When matrix separation is carried out, most of the interferences, reported in literature [3–5] are non-existent and
(4) The net effect of reactions (1), (3) and (4) is the migration of boron from B2 O3 in the PFA container (upper portion) to the glycerol solution present in the reagent chamber (lower portion) where no net methyl alcohol is consumed, it only acts as a vehicle in transporting boron from the PFA container into the reagent chamber. Formation of the monobasic acid was confirmed by titration of the glycerol–methanol solution after MVPME against standard sodium hydroxide to phenolphthalein end point. Titration values obtained corresponds exactly (within the titrimetric error of ±0.2%) to the amount of B2 O3 taken initially in the PFA containers. This implies that more than 99% of the matrix is eliminated in the described procedures. Water molecules formed in reactions (1) and (3) are simultaneously distilled off under the sub boiling temperature (≈70 ◦ C) that exists inside the MVPME assembly and thus get absorbed in the glycerol solution due to hydrogen bonding [17]. As a result, insides of the PFA sample containers were absolutely dry and empty after MVPME procedure. This matrix elimination process physically resembles to sublimation of solid carbon dioxide.
quantification can be obtained from external calibration. As the matrix is effectively separated and an almost pure solution is injected into the furnace, it was also possible to omit the matrix modifiers, which when added in large quantities to samples carry the risk of contamination. Another major advantage of the proposed procedure is the contamination free elimination of matrix, without the need for a clean room. On the other hand, matrix volatilization as the fluoride using suprapur HF requires clean room facility. 3.3. Evaluation of process blanks and analytical figures of merit Removal of boron matrix by volatilization can be achieved either as trimethyl borate ester or as boron triflouride using HF. So the respective process blanks were evaluated by evaporating required amount of reagents in a laminar flow clean bench. The average process blanks (normalized to 1 g sample) based on measurements using GFAAS by MVPME, open volatilization (methanol) and from GR and suprapur
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Table 2 Comparison of average process blanks (ng g−1 ) of MVPME, open volatilization (methanol) and BF3 volatilization (suprapur HF) Elements
MVPME (n = 3)
Open volatilizationa (methanol) (n = 2)
Open volatilizationb (GR grade HF) (n = 3)
Open volatilizationb (suprapur HF) (n = 3)
LOD’s (ng g−1 ) (MVPME)
Al Cu Cr Cd Fe Mn Mg Ni Sb Sn Zn
3±1 1.5 ± 0.3 1 ± 0.2 1.5 ± 0.3 4.3 ± 0.7 1.8 ± 0.4 2.1 ± 0.2 0.7 ± 0.2 n.d n.d 1.6 ± 0.2
18 ± 3 85 ± 7 15 ± 2 6 ± 0.5 27 ± 4 23 ± 3 27 ± 7 12 ± 2 8±2 n.d 25 ± 10
150 ± 10 8±2 15 ± 3 2±1 85 ± 6 35 ± 3 62 ± 5 18 ± 4 n.d n.d 70 ± 6
2.5 ± 0.8 3.1 ± 0.3 1.9 ± 0.3 n.d 18 ± 3 7.8 ± 0.4 11 ± 2 8±3 n.d n.d 11.0 ± 0.4
2.9 0.9 0.5 0.6 2.1 1.5 0.7 0.5 – – 0.6
Limit of detection (LOD, MVPME). a 10 ml of methanol (required for complete dissolution of 1 g of sample), was evaporated in a laminar flow clean bench and the residue was leached into 1 ml of 1% HNO3 . b 6 ml of HF (required for complete dissolution of 1 g of sample), was evaporated in a laminar flow clean bench and the residue was leached into 1 ml of 1% HNO3 .
grade HF reagents are presented in Table 2. Typical values for different elements are in the range of 0.7 ng (Ni)–4.2 ng (Fe) by MVPME. As can be seen from the Table 2 reagents blanks of Fe, Zn, Cr, etc. are significantly reduced, when compared to direct dissolution by methanol. In case of GR grade HF dissolution also, higher process blank values were obtained which hampers the analysis of high purity B2 O3 . The process blanks achieved for the ubiquitous analytes Fe, Mn and Zn were less in MVPME than those obtained through BF3 volatilization using even suprapur grade HF, despite carrying out the sample preparation in normal laboratory environment. The total process blank values (MVPME, normalized to 1 g sample) add up to ≈20 ng. These values suggest that high purity boron trioxide in the purity range of 5–6N (total impurities 10−1 g g−1 ) can be analyzed using this procedure. The main advantages of the MVPME method are the drastic reduction of process blank values and therefore the achievability of low LODs. The limit of detection based on 3 s blank criterions, are summarized in Table 2.
such suspicion is ill founded. So a boron trioxide sample (0.5 g) was dissolved in 3 ml of 40% HF (suprapur, Merck) and the resulting digest was analyzed for required trace elements by ETAAS. For most of the elements, there was good agreement among the values obtained by the two methods, except for Al. The Al value by MVPME was much higher, whereas in HF digest it was not detected at all. This could be due to the formation of stable aluminum fluoride, which leads to volatilization losses, when concentrated digests are evaporated during the temperature cycle. Similar type of observation has also been reported by Hauptkorn and Krivan [20] in the analysis of quartz. The determination of aluminum
3.4. Method validation In the absence of any reference material, alternate sample preparation approach was used to cross-validate the results. Among the possible approaches, solid sampling [18] has been quite successful for the analysis of high purity materials, because it requires no sample preparation, thereby solving the analytical blank problems. However, boron matrix reacts with carbon at temperatures around 2000 ◦ C [19] to form boron carbide via solid-state reaction, which influences the atomization conditions. Alternately, the B2 O3 matrix could be dissolved in conc. HF. However, the use of HF-digested solution prior to GFAAS measurement is rare, because these solutions are suspected of causing damage to the quartz window or to the pyrolytic material of the atomizer. However, extensive work by Lopez-Garcia et al. [12,13] suggests that
Fig. 2. (a) Absorption signals of Al (1.7 ng) obtained from boron trioxide sample (bto-2) MVPME, Aint = 0.137; (b) HF digest, Aint = 0.007; (c) HF digest + 5 l of Pd–Mg modifier; Aint = 0.009; (d) HF digest + 10 l of Pd–Mg modifier; Aint = 0.008. A, background corrected (Zeeman) signal and B, background absorption. Conventional furnace programme as described in the instrument manual was used. Aint = integrated absorbance. In MVPME: 0.5 g sample after matrix elimination was made up to 3 ml in 1% HNO3 ; HF digest: 0.5 g sample was taken in 3 ml of 40% HF.
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in conc. HF digests was further carried in platform tube using Pd–Mg modifier (10 l of the mixed solution containing 3000 mg l−1 of Pd and 2000 mg l−1 of Mg(NO3 )2 and the STPF concept. The atomization profiles are presented in Fig. 2. It can be seen from Fig. 2 that the aluminium signal was completely suppressed even in the presence of universal modifier. It was, therefore, essential to perform analyte–matrix separation, as the interference could not be handled using the universal modifier and the STPF concept. Another drawback of HF digestion is the higher dilution (0.5 g → 3 ml), when compared to MVPME process (0.5 g → 1 ml). 3.5. Recovery studies In the proposed matrix extraction procedure, more than 99.9% matrix was removed, it was imperative to carry out
recovery studies in order to test the validity of this technique. To a 0.5 g sample of highly pure grade boron trioxide (suprapur grade, Merck), 5–20 ng of the analytes were spiked, the matrix extraction procedure was carried out, trace residues were leached and analyzed as described in the procedure. The recoveries were quantitative and ranged between 97% (Cr) and 103% (Zn). 3.6. Sample analysis, accuracy and precision The proposed method was applied for the analysis of three boron trioxide samples of different purity grade and the results are presented in Table 3 along with those obtained by matrix elimination through HF volatilization (both off line and inside furnace). Quantification of analytes was carried out from calibration graphs obtained from aqueous standards,
Table 3 Analyte concentrations (ng g−1 ± ) obtained through MVPME and HF digests followed by ETAAS measurements Analytes
Sample
MVPME (n = 5)
Matrix volatilization (HF, suprapur) (n = 3)
HF (suprapur) dissolution (n = 3)
Manufacturer’s specificationa
Al
bto-1 bto-2 bto-3
32 ± 4 520 ± 20 112 ± 10
28 ± 5 495 ± 25 115 ± 12
– – –
50 – –
Cu
bto-1 bto-2 bto-3
8±1 68 ± 5 76 ± 4
7±2 70 ± 5 72 ± 5
12 ± 3 74 ± 6 80 ± 5
20 – –
Cr
bto-1 bto-2 bto-3
3±1 121 ± 9 109 ± 7
6 115 112 ± 9
n.d 132 116 ± 8
20 – –
Cd
bto-1 bto-2 bto-3
<0.4 22 ± 2 15 ± 3
<0.4 26 ± 3 18 ± 5
<0.4 18 ± 3 14 ± 3
50 – –
Fe
bto-1 bto-2 bto-3
18 ± 3 152 ± 13 160 ± 9
31 ± 3 168 ± 15 165 ± 10
27 ± 5 132 ± 17 157 ± 9
50 – –
Mg
bto-1 bto-2 bto-3
3±1 8±2 5±2
8±5 12 ± 4 7±3
4±1 10 ± 3 4±2
–
Mn
bto-1 bto-2 bto-3
3±1 19 ± 3 11 ± 3
7±2 28 ± 3 10 ± 4
4±2 15 ± 3 14 ± 4
20 – –
Ni
bto-1 bto-2 bto-3
7±2 25 ± 4 14 ± 2
9±2 26 ± 5 12 ± 3
n.d 32 ± 5 16 ± 2
20 – –
Sb
bto-1 bto-2 bto-3
n.d 14 ± 2 23 ± 3
n.d 12 ± 3 28 ± 5
– – –
–
Sn
bto-1 bto-2 bto-3
n.d 29 ± 3 35 ± 3
n.d 33 ± 4 32 ± 3
n.d <25 36 ± 5
– –
Znb
bto-1 bto-2 bto-3
9±2 35 ± 6 18 ± 3
8±2 43 ± 8 22 ± 4
– – –
50 – –
Manufacturer’s specification, where available are also provided. bto-1, suprapur grade boron trioxide (E-Merck, Germany); bto-2, AR grade boron trioxide (Loba Chemie, India); bto-3, AR grade boron trioxide (SD fine Chem, India). n.d; not detected. a Maximum limit. b Magnetic field strength ratio (H med /Hmax = 0.7/1.0) obtained by the three-field measurement mode [21] was used to extend the linearity of calibration range.
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which were linear (r2 > 0.991). The results for most trace analytes showed no significant differences indicating that all these digestion procedures give fairly representative sample solution. The t-test with multiple samples (paired by difference) was applied to examine whether the results by VPD digestion (methanol) and BF3 volatilization (HF) differed significantly at the 95% confidence level. As the calculated values of t were less than the critical t value of 2.776 (degree of freedom = 4), it follows that there is no statistically significant difference between the results. However, for Fe, Cr and Mn (suprapur grade B2 O3 ) the results between MVPME and BF3 volatilization (HF, off line) deviate significantly. In this instance, more reliance can be placed on the values obtained from MVPME, because it provides comparatively lower process blank values. The elements Cd, Sb and Sn could not be determined in suprapur grade sample. The precision (MVPME) of replicate determinations was calculated from R.S.D. (%) of the mean of five replicate measurements of samples. Precision was in the range of 4–15% in most of the cases. This reflects the cumulative imprecision of all of the sample handling, matrix extraction and detection steps. For Cr, Ni and Zn (bto-1, suprapur grade) the R.S.D. values were in the range of 25–46%. However, in these cases, the concentrations in question are at the lower nanogram per gram level and are therefore influenced from small variation in the analytical blanks. 4. Conclusions The proposed MVPME method allows accurate determination of impurities in boron trioxide at trace levels by ETAAS. The matrix elimination effected in closed assembly, results in drastic reduction of the process blank values and eliminates matrix-induced interferences in ETAAS. Matrix elimination carried out in the described assembly, avoids any fume-hood emission and thereby prevents contamination of the laboratory with boron. Matrix elimination carried out via in situ generated methanol vapors is much inexpensive than the BF3 volatilization that requires costly suprapur grade HF. Additionally, handling methanol is convenient, compared to the highly corrosive nature of HF. Further, the low volume of (l) of sample required for quantification in ETAAS, allows one to achieve only a limited dilution factor 2 after matrix elimination by MVPME, so that detection limits refereed to the solid sample remains low. The drawbacks of the proposed method are that, it takes more time (8 h) than the HF dissolution and the use of large quantity of methanol. However, methanol can be distilled and reused.
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Acknowledgements Grateful acknowledgement is made to Dr. T. Mukherjee, Associate director, Chemistry group, BARC for his keen interest and encouragement throughout this work. We sincerely thank the anonymous referees for their insightful comments.
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Vapor phase matrix extraction of high purity di-boron trioxide and trace analysis using electrothermal AAS K. Dash, S. Thangavel, S.M. Dhavile, S.V. Rao, S.C. Chaurasia, J. Arunachalam ∗ National Centre for Compositional Characterisation of Materials (CCCM), Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, India Received 28 January 2005; received in revised form 30 April 2005; accepted 4 May 2005 Available online 24 June 2005
Abstract A method has been developed for the determination of Al, Cd, Cr, Cu, Fe, Mg, Mn, Ni, Sb, Sn and Zn at trace levels in high purity di-boron trioxide using ETAAS. The boron trioxide matrix was eliminated as trimethyl borate ester in a multiplex vapor phase matrix extraction (MVPME) device using a mixture of glycerol and methanol. In this MVPME device, in situ reagent purification, sample digestion and simultaneous matrix elimination were achieved by a single step in closed condition, which in combined effect reduce the process blanks. The matrix extraction procedure allows determination of trace elemental impurities by electrothermal atomic absorption spectrometry (ETAAS) with fast furnace analysis (without an ashing step and modifier) and calibration against aqueous standards. The performance and accuracy of the vapor phase matrix elimination technique are compared to those of suprapur grade hydrofluoric acid solution in two ways; (i) matrix separation as BF3 over hot plate and (ii) in situ matrix elimination inside graphite furnaces. The method detection limits calculated from blank samples are in the range of 0.5 (Ni) and 2.9 (Al) ng g−1 . Thus the MVPME-based sample preparation approach is well suited for the trace analysis of high purity di-boron trioxide used in microelectronics applications. © 2005 Elsevier B.V. All rights reserved. Keywords: Di-boron trioxide; Multiplex vapor phase matrix extraction; ETAAS; Trace analysis
1. Introduction Di-boron trioxide (B2 O3 ) is used as a network former [1] in optical fiber, where stringent purity levels with particular reference to transition metal impurities, which act as trap centers are required. Further, highly pure grade B2 O3 is required in the electronics industry as a liquid encapsulent [2] for the Czochralski type growth of group iii–v semiconductor crystals. Electrothermal atomic absorption spectrometry (ETAAS) is one of the most sensitive techniques for the determination of trace impurities and its power of detection extends to the low picogram range. In boron materials, however, the matrix at higher concentrations reduces the atomic absorption signal for the transition metal impurities [3–5] and thus the optimal power of detection and best accuracy may not be achieved in ∗
Corresponding author. Fax: +91 40 27125463. E-mail address: [email protected] (J. Arunachalam).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.05.009
presence of this matrix. In spite of the remarkable progress achieved by applying stabilized temperature platform furnace (STPF) concept, chemical modifiers and powerful Zeeman background correction, the presence of matrix continues to be a fundamental analytical problem for trace and ultra trace analysis. One approach to circumvent such problems is to perform some form of analyte–matrix separation prior to ETAAS measurements, which may also permit the use of lower dilution factor, and hence improve the limit of detection of the method. Some reports describing the boron matrix removal prior to instrumental detection have been published. Tompuri and Tummavuori [3] reported in situ matrix (H3 BO3 ) removal during the pyrolysis step using hydrofluoric acid as a chemical modifier. Smolander et al. [5] removed boric acid by ion exchange and esterification with ethanol. Esterification with ethanol produced too strong a background that required the sample to be diluted to 1:10. Hence, this method requires extensive sample preparation and may not be suitable for ultra trace analysis. Further, Yu et al. [6] reported that the
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reaction of boric acid with methanol gives highly volatile borate ester, which readily evaporates. However, handling trimethyl borate ester in an open laboratory is hazardous [7] and storage of methanol digest in autosampler cups was problematic because the analyte concentration increased during the sequence of automatic sample injection, owing to solvent evaporation. Alternately, off line volatilization of trimethyl borate ester could degrade the detection limits by contamination encountered in a normal laboratory or from reagents used. In such circumstances, the accurate determination of analytes at ng g−1 levels in high purity B2 O3 is mostly limited by contamination encountered in sample preparation steps rather than by the sensitivity of ETAAS. So, the reliability and validity of the analytical data strongly depend on the degree to which contamination can be minimized. The high and/or random procedural blank problems from reagents or from laboratory environment can be surmounted by vapor phase digestion (VPD) [8,9]. In an earlier study [10] methanol was used for boric acid matrix volatilization, where the non-ionic nature of the solvent (methanol) and lower process blanks were advantageous for the conductivity detection of trace ions by suppressed ion chromatography. In the present work, multiplex vapor phase matrix extraction (MVPME) is combined with fast furnace analysis [11–15] with the aim of determining trace impurities in boron trioxide. In this closed vapor phase digestion assembly in situ reagent (methanol) purification, sample digestion and simultaneous matrix (B2 O3 ) elimination are achieved in a single step. The results obtained by MVPME are compared to those of suprapur grade hydrofluoric acid solution in two ways: (i) matrix separation as BF3 over hot plate and (ii) in situ matrix elimination inside graphite furnace. In the described digestion device eight samples (including three blanks) can be handled in a batch, thus improving the sample throughput.
2. Experimental 2.1. Reagents and chemicals Ultrapure water with a specific resistance of 18 M cm or greater was obtained through a Milli-Q water purification system. Hydrofluoric acid (GR and suprapur grade) and nitric acid (suprapur grade) from Merck, Germany were used. Glycerol and methanol (both AR grade) were from SD Fine Chemicals, India. Standard solutions of 1000 mg l−1 were prepared from respective nitrate salts (GR grade, Merck, India) for Al, Cd, Cr, Cu, Fe, Mg, Mn, Ni, and Zn, whereas for Sb and Sn, respective chloride salts (Titrisol grade, Merck, Germany) were used. The multi-element calibration standard contained 50 g l−1 each of Al, Ni, Sn and Sb, 20 g l−1 (Fe, Ni, Cr and Cu), 5 g l−1 (Cd, Mg and Zn) and 10 g l−1 of Mn. Five point calibration curves were prepared for each element with the autosampler by dilution in tube. A sample volume of 20 l was used.
2.2. VPD apparatus The high purity boron trioxide samples were digested in the MVPME assembly (Fig. 1). The polypropylene (PP) vessel is partitioned into sample compartment (upper portion) and reagent compartment (lower portion) by placing a perforated sample rack made out of PP sheet. Through wholes (8 mm) were drilled to channelise the methanol vapors into the sample compartment. The vessel and the lid have air-locking arrangement through a rib in the lid and corresponding recess in the vessel. To ensure that the assembly is completely leak-proof, it was covered with a 0.2 mm thick circular Teflon sheet with the lid pressed into the vessel. Polyflouroalkoxy (PFA) vials (7 ml, Cole Parmer, NY, USA) were used as sample containers. To clean the digestion assembly a nitric acid solution (1:1) was taken in the reagent compartment, capped and kept on IR hot plate (temperature ≈90 ◦ C) for 2 h. After cooling, it was thoroughly rinsed with DI water. PFA containers were soaked for 24 h in 2% nitric acid rinsed several times and were dried in class 10 clean bench before use. The micropipette tips and autosampler cups were kept in warm 20% (v/v) suprapur nitric acid for 24 h and washed with ultrapure water for several times. 2.3. Instrumentation All the measurements were carried out using an Analytikjena model ZEEnit 65 (Germany) graphite furnace atomic absorption spectrometer equipped with an MPE 60 autosampler. The spectrometer was provided with both Zeeman (transverse variable magnetic field) and deuterium–HClbased background correction systems. The deuterium device was used through out this work, the Zeeman corrector being used only for comparison purpose. Hollow cathode lamps of various origins were used. For volatile elements (Cd, Mg, Sb, Sn, Zn) pyrolytically PIN platform (part no. 407-152.011) tubes were used. The other elements (Al, Fe, Cr, Ni, Cu, Mn) were atomized directly off the wall of the pyrolytically coated tubes (part no. 407-152.013). Details of other instrumental parameters and fast furnace programme are given in Table 1. 2.4. Sample preparation 2.4.1. Sample digestion in MVPME assembly Sample vials of PFA (perfluoroalkoxy, Cole parmer, USA) were cleaned with DI water and dried to ensure that there was no residue. Boron trioxide samples (0.5 g each) were weighed into the PFA vials in a class 1000 clean room and were placed in the grooves (22 mm) of the sample rack. Thoroughly mixed glycerol–methanol (2:1, 300 ml) was poured into the reagent compartment. The vessel was capped and the assembly was kept on IR hot plate (surface temperature ≈90 ◦ C) for 8 h. The assembly was allowed to cool, PFA vials were removed from the sample rack. The trace metals were leached into 1 ml of 1% nitric acid (by an eppendorf micropipette) prior to their determination by ETAAS.
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Fig. 1. Matrix extraction device.
2.4.2. Sample dissolution in HF 0.5 g of boron trioxide in a PFA vial was dissolved in 3 ml of suprapur grade HF and the resulting solution was analyzed for trace elements.
Table 1 Instrumental and fast furnace programme conditionsa Element
Al Cu Cr Cd Fe Mg Mn Ni Sb Sn Zn
Wavelength (nm)
309.3 324.8 357.9 228.8 248.3 285.2 279.5 232.0 217.6 224.6 213.9
Temperature (◦ C) Drying (ramp: ◦ C s−1 )
Atomization (ramp: ◦ C s−1 )
240 (15) 300 (15) 300 (20) 120 (25) 240 (20) 240 (20) 240 (20) 240 (20) 140 (25) 240 (20) 120 (15)
2400 (1600) 2300 (1600) 2500 (1600) 1600 (1200) 2200 (1600) 2200 (1500) 2400 (1600) 2400 (1600) 1900 (1000) 2300 (1500) 1800 (1000)
a Thermal pretreatment: drying, hold 15 s, for all elements. Atomization: hold 4 s, read 4 s, for all elements. Cleaning: 100 ◦ C more than the respective atomization temperature, ramp 1 s, hold 3 s, for all elements.
2.4.3. Matrix volatilization by HF In order to compare the results, conventional matrix volatilization was used. One gram sample was accurately weighed into PFA vial and to this 6 ml of suprapur grade HF was added. The resulting boron matrix BF3 was volatilized in a laminar clean flow bench over an IR hot plate. After drying up, the residues were leached into 1 ml of 1% nitric acid and analyzed by ETAAS. 2.4.4. Open volatilization with CH3 OH Methanol (10 ml) was taken in a PFA vial and was evaporated in a laminar flow clean bench (class 10) and the residue was leached into 1 ml of 1% (v/v) nitric acid and analyzed to find out the process blank values.
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The completely dry and empty condition inside the PFA container makes it possible to leach the trace impurities accurately into 1 ml of 1% nitric acid solution.
3. Results and discussions 3.1. Principles of matrix extraction in closed MVPME assembly
3.2. Advantages of MVPME The polypropylene vessel is divided into reagents compartment and sample compartment (Fig. 1). On heating the MVPME assembly, vapors of methanol from the glycerol–methanol mixture are channeled into the sample compartment, which reacts with B2 O3 in the PFA vial forming trimethyl borate as follows: B2 O3 + 6CH3 OH ↔ 2B(OCH3 )3 ↑ + 3H2 O
(1)
Further, the three water molecules produced in reaction (1) react with B2 O3 as follows: B2 O3 + 3H2 O → 2H3 BO3
(2)
Again the boric acid formed in reaction (2) reacts with methanol vapors as: H3 BO3 + 3CH3 OH ↔ B(OCH3 )3 ↑ + 3H2 O
(3)
Reactions (1) and (3), where trimethyl borate is formed are reversible reactions. So the progress of those reactions depends upon the facility with which water or the trimethyl borate or both can be removed from the reaction zone. Vapors of trimethyl borate (bp = 68.7 ◦ C) formed in reactions (1) and (3) react with glycerol (present in the reagent compartment) to form a polyolborate complex [16] which is a monobasic acid and again three molecules of CH3 OH are released as shown in the following reaction:
The use of MVPME results in a controlled transfer of methanol vapors (on heating) from the reagent compartment to the sample compartment, which effects the dissolution and thus the impurities from the reagent are not added into the sample. Additionally the PFA containers are separated by a distance from the glycerol–methanol solution, which ensures that creeping of methanol is completely eliminated. The glycerol (bp 267 ◦ C) used serves many purposes. Methanol and water are soluble in glycerol, whereas trimethyl borate forms polyolborate complex with it. So large pressure build up inside the MVPME assembly is prevented. Additionally, methanol and trimethyl borate ester (both toxic) vapors are not released into the environment. Multiple experimental observations indicated that solution of glycerol and methanol (2:1, 300 ml) is ideal to carry out the vapor phase matrix elimination. In open volatilization of any boron matrix as trimethyl borate ester a dehydrating agent i.e. sulfuric acid is used to facilitate the reversible esterification reaction. However, the use of sulfuric acid introduces interferences in GFAAS analysis. Additionally, higher impurity levels in sulfuric acid often results in an increase in the process blank levels. In this aspect, in the MVPME procedure, no sulfuric acid is required. When matrix separation is carried out, most of the interferences, reported in literature [3–5] are non-existent and
(4) The net effect of reactions (1), (3) and (4) is the migration of boron from B2 O3 in the PFA container (upper portion) to the glycerol solution present in the reagent chamber (lower portion) where no net methyl alcohol is consumed, it only acts as a vehicle in transporting boron from the PFA container into the reagent chamber. Formation of the monobasic acid was confirmed by titration of the glycerol–methanol solution after MVPME against standard sodium hydroxide to phenolphthalein end point. Titration values obtained corresponds exactly (within the titrimetric error of ±0.2%) to the amount of B2 O3 taken initially in the PFA containers. This implies that more than 99% of the matrix is eliminated in the described procedures. Water molecules formed in reactions (1) and (3) are simultaneously distilled off under the sub boiling temperature (≈70 ◦ C) that exists inside the MVPME assembly and thus get absorbed in the glycerol solution due to hydrogen bonding [17]. As a result, insides of the PFA sample containers were absolutely dry and empty after MVPME procedure. This matrix elimination process physically resembles to sublimation of solid carbon dioxide.
quantification can be obtained from external calibration. As the matrix is effectively separated and an almost pure solution is injected into the furnace, it was also possible to omit the matrix modifiers, which when added in large quantities to samples carry the risk of contamination. Another major advantage of the proposed procedure is the contamination free elimination of matrix, without the need for a clean room. On the other hand, matrix volatilization as the fluoride using suprapur HF requires clean room facility. 3.3. Evaluation of process blanks and analytical figures of merit Removal of boron matrix by volatilization can be achieved either as trimethyl borate ester or as boron triflouride using HF. So the respective process blanks were evaluated by evaporating required amount of reagents in a laminar flow clean bench. The average process blanks (normalized to 1 g sample) based on measurements using GFAAS by MVPME, open volatilization (methanol) and from GR and suprapur
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Table 2 Comparison of average process blanks (ng g−1 ) of MVPME, open volatilization (methanol) and BF3 volatilization (suprapur HF) Elements
MVPME (n = 3)
Open volatilizationa (methanol) (n = 2)
Open volatilizationb (GR grade HF) (n = 3)
Open volatilizationb (suprapur HF) (n = 3)
LOD’s (ng g−1 ) (MVPME)
Al Cu Cr Cd Fe Mn Mg Ni Sb Sn Zn
3±1 1.5 ± 0.3 1 ± 0.2 1.5 ± 0.3 4.3 ± 0.7 1.8 ± 0.4 2.1 ± 0.2 0.7 ± 0.2 n.d n.d 1.6 ± 0.2
18 ± 3 85 ± 7 15 ± 2 6 ± 0.5 27 ± 4 23 ± 3 27 ± 7 12 ± 2 8±2 n.d 25 ± 10
150 ± 10 8±2 15 ± 3 2±1 85 ± 6 35 ± 3 62 ± 5 18 ± 4 n.d n.d 70 ± 6
2.5 ± 0.8 3.1 ± 0.3 1.9 ± 0.3 n.d 18 ± 3 7.8 ± 0.4 11 ± 2 8±3 n.d n.d 11.0 ± 0.4
2.9 0.9 0.5 0.6 2.1 1.5 0.7 0.5 – – 0.6
Limit of detection (LOD, MVPME). a 10 ml of methanol (required for complete dissolution of 1 g of sample), was evaporated in a laminar flow clean bench and the residue was leached into 1 ml of 1% HNO3 . b 6 ml of HF (required for complete dissolution of 1 g of sample), was evaporated in a laminar flow clean bench and the residue was leached into 1 ml of 1% HNO3 .
grade HF reagents are presented in Table 2. Typical values for different elements are in the range of 0.7 ng (Ni)–4.2 ng (Fe) by MVPME. As can be seen from the Table 2 reagents blanks of Fe, Zn, Cr, etc. are significantly reduced, when compared to direct dissolution by methanol. In case of GR grade HF dissolution also, higher process blank values were obtained which hampers the analysis of high purity B2 O3 . The process blanks achieved for the ubiquitous analytes Fe, Mn and Zn were less in MVPME than those obtained through BF3 volatilization using even suprapur grade HF, despite carrying out the sample preparation in normal laboratory environment. The total process blank values (MVPME, normalized to 1 g sample) add up to ≈20 ng. These values suggest that high purity boron trioxide in the purity range of 5–6N (total impurities 10−1 g g−1 ) can be analyzed using this procedure. The main advantages of the MVPME method are the drastic reduction of process blank values and therefore the achievability of low LODs. The limit of detection based on 3 s blank criterions, are summarized in Table 2.
such suspicion is ill founded. So a boron trioxide sample (0.5 g) was dissolved in 3 ml of 40% HF (suprapur, Merck) and the resulting digest was analyzed for required trace elements by ETAAS. For most of the elements, there was good agreement among the values obtained by the two methods, except for Al. The Al value by MVPME was much higher, whereas in HF digest it was not detected at all. This could be due to the formation of stable aluminum fluoride, which leads to volatilization losses, when concentrated digests are evaporated during the temperature cycle. Similar type of observation has also been reported by Hauptkorn and Krivan [20] in the analysis of quartz. The determination of aluminum
3.4. Method validation In the absence of any reference material, alternate sample preparation approach was used to cross-validate the results. Among the possible approaches, solid sampling [18] has been quite successful for the analysis of high purity materials, because it requires no sample preparation, thereby solving the analytical blank problems. However, boron matrix reacts with carbon at temperatures around 2000 ◦ C [19] to form boron carbide via solid-state reaction, which influences the atomization conditions. Alternately, the B2 O3 matrix could be dissolved in conc. HF. However, the use of HF-digested solution prior to GFAAS measurement is rare, because these solutions are suspected of causing damage to the quartz window or to the pyrolytic material of the atomizer. However, extensive work by Lopez-Garcia et al. [12,13] suggests that
Fig. 2. (a) Absorption signals of Al (1.7 ng) obtained from boron trioxide sample (bto-2) MVPME, Aint = 0.137; (b) HF digest, Aint = 0.007; (c) HF digest + 5 l of Pd–Mg modifier; Aint = 0.009; (d) HF digest + 10 l of Pd–Mg modifier; Aint = 0.008. A, background corrected (Zeeman) signal and B, background absorption. Conventional furnace programme as described in the instrument manual was used. Aint = integrated absorbance. In MVPME: 0.5 g sample after matrix elimination was made up to 3 ml in 1% HNO3 ; HF digest: 0.5 g sample was taken in 3 ml of 40% HF.
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in conc. HF digests was further carried in platform tube using Pd–Mg modifier (10 l of the mixed solution containing 3000 mg l−1 of Pd and 2000 mg l−1 of Mg(NO3 )2 and the STPF concept. The atomization profiles are presented in Fig. 2. It can be seen from Fig. 2 that the aluminium signal was completely suppressed even in the presence of universal modifier. It was, therefore, essential to perform analyte–matrix separation, as the interference could not be handled using the universal modifier and the STPF concept. Another drawback of HF digestion is the higher dilution (0.5 g → 3 ml), when compared to MVPME process (0.5 g → 1 ml). 3.5. Recovery studies In the proposed matrix extraction procedure, more than 99.9% matrix was removed, it was imperative to carry out
recovery studies in order to test the validity of this technique. To a 0.5 g sample of highly pure grade boron trioxide (suprapur grade, Merck), 5–20 ng of the analytes were spiked, the matrix extraction procedure was carried out, trace residues were leached and analyzed as described in the procedure. The recoveries were quantitative and ranged between 97% (Cr) and 103% (Zn). 3.6. Sample analysis, accuracy and precision The proposed method was applied for the analysis of three boron trioxide samples of different purity grade and the results are presented in Table 3 along with those obtained by matrix elimination through HF volatilization (both off line and inside furnace). Quantification of analytes was carried out from calibration graphs obtained from aqueous standards,
Table 3 Analyte concentrations (ng g−1 ± ) obtained through MVPME and HF digests followed by ETAAS measurements Analytes
Sample
MVPME (n = 5)
Matrix volatilization (HF, suprapur) (n = 3)
HF (suprapur) dissolution (n = 3)
Manufacturer’s specificationa
Al
bto-1 bto-2 bto-3
32 ± 4 520 ± 20 112 ± 10
28 ± 5 495 ± 25 115 ± 12
– – –
50 – –
Cu
bto-1 bto-2 bto-3
8±1 68 ± 5 76 ± 4
7±2 70 ± 5 72 ± 5
12 ± 3 74 ± 6 80 ± 5
20 – –
Cr
bto-1 bto-2 bto-3
3±1 121 ± 9 109 ± 7
6 115 112 ± 9
n.d 132 116 ± 8
20 – –
Cd
bto-1 bto-2 bto-3
<0.4 22 ± 2 15 ± 3
<0.4 26 ± 3 18 ± 5
<0.4 18 ± 3 14 ± 3
50 – –
Fe
bto-1 bto-2 bto-3
18 ± 3 152 ± 13 160 ± 9
31 ± 3 168 ± 15 165 ± 10
27 ± 5 132 ± 17 157 ± 9
50 – –
Mg
bto-1 bto-2 bto-3
3±1 8±2 5±2
8±5 12 ± 4 7±3
4±1 10 ± 3 4±2
–
Mn
bto-1 bto-2 bto-3
3±1 19 ± 3 11 ± 3
7±2 28 ± 3 10 ± 4
4±2 15 ± 3 14 ± 4
20 – –
Ni
bto-1 bto-2 bto-3
7±2 25 ± 4 14 ± 2
9±2 26 ± 5 12 ± 3
n.d 32 ± 5 16 ± 2
20 – –
Sb
bto-1 bto-2 bto-3
n.d 14 ± 2 23 ± 3
n.d 12 ± 3 28 ± 5
– – –
–
Sn
bto-1 bto-2 bto-3
n.d 29 ± 3 35 ± 3
n.d 33 ± 4 32 ± 3
n.d <25 36 ± 5
– –
Znb
bto-1 bto-2 bto-3
9±2 35 ± 6 18 ± 3
8±2 43 ± 8 22 ± 4
– – –
50 – –
Manufacturer’s specification, where available are also provided. bto-1, suprapur grade boron trioxide (E-Merck, Germany); bto-2, AR grade boron trioxide (Loba Chemie, India); bto-3, AR grade boron trioxide (SD fine Chem, India). n.d; not detected. a Maximum limit. b Magnetic field strength ratio (H med /Hmax = 0.7/1.0) obtained by the three-field measurement mode [21] was used to extend the linearity of calibration range.
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which were linear (r2 > 0.991). The results for most trace analytes showed no significant differences indicating that all these digestion procedures give fairly representative sample solution. The t-test with multiple samples (paired by difference) was applied to examine whether the results by VPD digestion (methanol) and BF3 volatilization (HF) differed significantly at the 95% confidence level. As the calculated values of t were less than the critical t value of 2.776 (degree of freedom = 4), it follows that there is no statistically significant difference between the results. However, for Fe, Cr and Mn (suprapur grade B2 O3 ) the results between MVPME and BF3 volatilization (HF, off line) deviate significantly. In this instance, more reliance can be placed on the values obtained from MVPME, because it provides comparatively lower process blank values. The elements Cd, Sb and Sn could not be determined in suprapur grade sample. The precision (MVPME) of replicate determinations was calculated from R.S.D. (%) of the mean of five replicate measurements of samples. Precision was in the range of 4–15% in most of the cases. This reflects the cumulative imprecision of all of the sample handling, matrix extraction and detection steps. For Cr, Ni and Zn (bto-1, suprapur grade) the R.S.D. values were in the range of 25–46%. However, in these cases, the concentrations in question are at the lower nanogram per gram level and are therefore influenced from small variation in the analytical blanks. 4. Conclusions The proposed MVPME method allows accurate determination of impurities in boron trioxide at trace levels by ETAAS. The matrix elimination effected in closed assembly, results in drastic reduction of the process blank values and eliminates matrix-induced interferences in ETAAS. Matrix elimination carried out in the described assembly, avoids any fume-hood emission and thereby prevents contamination of the laboratory with boron. Matrix elimination carried out via in situ generated methanol vapors is much inexpensive than the BF3 volatilization that requires costly suprapur grade HF. Additionally, handling methanol is convenient, compared to the highly corrosive nature of HF. Further, the low volume of (l) of sample required for quantification in ETAAS, allows one to achieve only a limited dilution factor 2 after matrix elimination by MVPME, so that detection limits refereed to the solid sample remains low. The drawbacks of the proposed method are that, it takes more time (8 h) than the HF dissolution and the use of large quantity of methanol. However, methanol can be distilled and reused.
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Acknowledgements Grateful acknowledgement is made to Dr. T. Mukherjee, Associate director, Chemistry group, BARC for his keen interest and encouragement throughout this work. We sincerely thank the anonymous referees for their insightful comments.
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