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Applications of a slotted tube atom trap and flame atomic absorption spectrometry: determination of bismuth in copper-based alloys with and without hydride generation
ANALYTICA
CHIMICA ACEi ELSEVIER
Analytica
Chimica Acta 311 (1995) 93-97
Applications of a slotted tube atom trap and flame atomic absorption spectrometry: determination of bismuth in copper-based alloys with and without hydride generation D. Thorburn Burns
*, Narong
Chimpalee
‘, Michael Harriott
Department ofAnalytical Chemistry, The Queen’s University of Belfast, Belfast Bi’9 5AG, UK
Received 9 September 1994; revised 17 February 1995; accepted 16 March 1995
Abstract Improved systems are reported for the determination of bismuth (O-0.1%, w/w) in copper-based alloys by flame atomic absorption spectrometry. The use of a slotted quartz tube atom trap (STAT) with nebulised solutions or coupled with hydride generation decreases the characteristic concentrations (nebulized solution, 0.44/0.25, without/with STAT; hydride, 0.022/0.014, without/with STAT) and improved precisions by factors of two compared with those obtained without the use of the STAT. Determination of bismuth in a series of reference materials demonstrates the usefulness of STAT systems for the analysis of metallurgical samples. Keywords: Hydride
generation;
Slotted tube atom trap (STAT); Bismuth;
1. Introduction Bismuth commonly occurs at trace levels in copper-based alloys. Methods reported for the determination of trace levels of bismuth in such alloys include spectrophotometry [l-3], D.C. polarography [4], anodic stripping voltammetry [5] and atomic absorption spectrometry @AS) [6-151. The characteristic concentration of bismuth by AAS is high and various methods have been examined in efforts to increase sensitivity. These include the use of solvent extraction [6], graphite [7-91 and
* Corresponding author.
’ Present address: Department sity, Thailand
of Chemistry,
Silkaporn
Univer-
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SD1 0003.2670(95)00164-6
Copper-based
alloys
induction furnaces [lo], and particularly hydride generation methods [ll-151. Hydride methods for many elements including bismuth are prone to interference from the presence of concomitants such as copper [ 111. These interferences may be overcome by co-precipitation separation of bismuth on lanthanum hydroxide [9,12] or on hydrated iron(II1) oxide [13], by masking copper with thiosemicarbazide [14] or by prior removal of copper by electrolysis [15]. For the determination of bismuth by slotted quartz tube atom trap (STAT) flame AAS using a nebulised solution, the presence of copper was found to be beneficial, suppressing the interference from most other cations. However. for hydride generation with STAT flame AAS, it was necessary to mask copper. Thiourea [16], previously used in inductively coupled plasma AES, proved
94
D.T. Burns et al. /Analytica
Chimica Acta 311 (1995) 93-97
Copper stock solution, 50 mg ml - I Dissolve 5.00 g of copper wire (99.99% Cu, BDH) in 15 ml of cont. nitric acid and dilute to 1 1 with water.
satisfactory in masking copper. In both cases the use of the STAT decreased the characteristic concentrations and improved precisions compared with those obtained by the use of normal premixed laminar flames.
Sodium tetrahydroborate(III), 2% w / u Dissolve 2 g of NaBH, (98%, Aldrich) in 100 ml of distilled water. After stabilising with addition of sodium hydroxide (1 pellet, ca. 1 g) the solution is filtered through a Whatman No. 540 filter paper.
2. Experimental
2.1. Apparatus
Thiourea solution Dissolve 2 g of thiourea (99%, Aldrich) in 100 ml of distilled water. All acids and salts used in this study were of analytical grade. Doubly distilled water was used throughout.
A Perkin-Elmer Model 403 atomic absorption spectrometer equipped with a bismuth hollow cathode lamp was used and signals were recorded using a Perkin-Elmer Model 56 chart recorder set at the 10 mV range. The spectrometer conditions for bismuth were: wavelength 223.1 nm, band pass 0.7 nm, lamp current 10 mA; flame air 11.6 1 min-‘; acetylene 3.8 1 min-‘; aspiration rate 6.2 ml min-‘; chart recorder full scale deflection 0.25 absorbance. The slotted quartz tube (satin finish) was 140 mm long with a centralised longitudinal base slot 10 X 3 mm, and an upper row of six holes of 6 mm diameter, 15 mm apart, centrally drilled above the base slot. The cradle [17] and hydride generation system [18] were as described earlier.
The improvements possible in characteristic concentrations and precisions for the determination of bismuth in copper based alloys by flame-AAS have been studied by comparing data from two sets of parallel pairs of experiments. In the first set bismuth was introduced as bismuthane and in the second set in nebulized solutions, AAS measurements being made on both sets, both with and without the use of a STAT.
2.2. Reagents 2.3. Hydride generation Bismuth stock solution 1.0 mg ml-’ as bismuth(II1) grade (BDH).
nitrate,
procedure
(181
Pipette 3 ml of the sample solution (containing 20%, v/v, hydrochloric acid and 2%, v/v, nitric
SpectrosoL
to extraction
hood
Three way stopcock to AAS -
distilled water -
25 ml flask
to waste
Fig. 1. Hydride generation
system.
-N2 flow meter
inlet
D.T. Burns et al. /Analytica
acid) into the generation flask (Fig. 1). Make up to 10 ml with distilled water. Connect the generation flask (via a 3-way stopcock) directly with the nebulizer (N, flow rate 600 ml min-‘). Inject 3 ml of 2% NaBH, solution and record the absorbance signal. Finally, turn the 3-way stopcock so as to aspirate distilled water into the flame. Drain and wash out the generation flask prior to addition of the next sample. 2.4. Analysis of copper-based alloys by hydride-generation flame AAS Dissolve accurately weighed samples, containing about 0.02 mg bismuth, in 20 ml of cont. hydrochloric acid, 2 ml of cont. nitric acid and 5 ml of distilled water in 150 ml conical flasks. Complete dissolution by gentle warming on a hot plate. Cool the solutions and transfer quantitatively to 100 ml volumetric flasks and make up to volume with distilled water. Place 3 ml of these sample volumes in the generation flask, add 3 ml of 2% thiourea and make up to 10 ml with distilled water. Pairs of analyses were completed as described under the hydride generation procedure above, making measurements both with and without the STAT in position. Evaluate the amount of bismuth from appropriate calibration graphs, with and without STAT, covering the calibration range O-O.5 pg ml-’ Bi, using standards made up in 20%, v/v, hydrochloric acid and 2%, v/v, nitric acid. 2.5. Analysis of copper-based solution-flame AAS
alloys by nebulized
2.6. Examination of the main experimental variables The optimum flame and STAT conditions were found, by univariate search as described earlier [17] using 0.5 and 4 pg ml-’ Bi for the with and the without hydride generation procedures, respectively, to be in both cases, flame: air 11.6 1 min-‘, acetylene 3.8 1 mini’, the STAT positioned with an 8 mm gap between the burner head and base of the STAT and with the spectrometer optical axis central to the STAT. Since copper based alloys are conveniently dissolved in a mixture of hydrochloric acid and nitric acid the effects of variation in concentration of both acids were examined. For hydride generation (0.5 pg mll’ Bi in 2%, v/v, nitric acid) the absorbance signals were found to increase with increasing hydrochloric acid concentrations up to lo%, v/v, and remain constant up to 30%, v/v. A hydrochloric acid concentration of 20%, v/v, was used thereafter. Absorbances (0.5 pg Bi ml-’ in 20%, v/v, hydrochloric acid) were inde-
Table 1 Effect of diverse bismuth following STAT-AAS
ions on the determination of 0.5 pg ml-’ bismuthane generation using flame and flame
Ion added
Amount (I*gml-‘)
PMII), S&V), Fe(III) Ni(II), Z&I), (XIII) A&II) Sb(Ill)
Dissolve weighed samples (I 0.5 g>, to contain about 0.4 mg of bismuth, in 25 ml of cont. hydrochloric acid, 5 ml of cont. nitric acid and 5 ml of distilled water in 150 ml conical flasks. Gently warm if necessary, to complete dissolution. Cool the solutions, transfer to 100 ml volumetric flasks and make up to volume with distilled water. Examine the sample solutions by nebulized solution-flame AAS taking measurements both with and without the STAT in position. Evaluate the amounts of bismuth present from appropriate calibration graphs, with and without the STAT in position, covering the range O-8 pg mll’ Bi using standards containing 5000 pg ml-’ copper(B).
95
Chimica Acta 311 (1995) 93-97
cow Al(III) M&I) cuw
A&) Se(W)
a Less than 2% variation. b Add 3 ml of 2% (w/v)
500 500 20 500 250 500 100 500 20 500 SO 100 10 10000 h 100 10 1000 2
thiourea solution.
Variation peak height (%) _ “
+46 _ f21 _ -62 _ -77 _ - 15 _ - 100 -77 _ -67 _ - 100 _
96
D.T. Burns et al./Analytica
Table 2 Effect of diverse ions (% change in signal) on the determination absence of 5000 pg ml-’ copper Ion added
Sn(IV1
Amount
Flame
(kgml-‘1
No Cu added
A&II)
of 4 pg ml-
STAT 5000 pg ml- ’ Cu added
-5 +5 _
+2 +2 +2 -
+5 -5 -5 -5
+2 +2 -
pendent of nitric acid concentration up to 5%, v/v. A nitric acid concentration of 2%, v/v, was selected. For the STAT system without hydride generation, absorbances (4 pg Bi ml-’ in 5% nitric acid) increased by 3% over the range 5 to 30%, v/v, hydrochloric acid (25%, v/v, was used in subsequent experiments), and to be independent of nitric acid concentration over the range O-15%, v/v. For hydride generation (1 pg ml-’ Bi in 20%, v/v, hydrochloric acid and 2.0%, v/v, nitric acid), absorbances were found to increase rapidly up to l%, w/v, NaBH, and to decrease slowly thereafter. l%, w/v, NaBH, was used in subsequent experiments. Peak heights were independent of nitrogen flow rate over the range 200-1000 ml min-‘; a flow rate of 600 ml min-’ was found to be convenient for routine use.
Table 3 Characteristic
concentrations,
detection
limits and precision
Method
Characteristic
Nebulized solution flame without STAT flame with STAT Hydride jlame without STAT with STAT a Concentration b Concentration ’ n = 10.
’ bismuth by flame AAS and STAT-AAS in the presence or
No Cu added
5000 pg ml- r Cu added
-
1000 1000 1000 500 500 500 500 100 100 100
PM111 Zn(II) Ni(II) Fe(III) Cr(III) M&I) Al(III) Sb(III)
Chimica Acta 311 (1995) 93-97
concentration
_ _
+27 +27 +7 +23 +20 +20 +20
+3 +3 _ _ _
The possible interferences on the generation/ atomisation of bismuthane were examined. The results (Table 1) were the same with or without the STAT confirming the interference effects to be solution based. The only interference at levels of interest in the analysis of copper based alloys was that of copper, which was overcome by masking with addition of 3 ml of 2% thiourea solution. During initial studies of methods using the direct aspiration of solutions it was noted that absorbance values were increased by 5% for flame and 13% for the STAT after 10 min aspiration of solutions containing above 400 pg ml-’ copper, the enhancement effects in both cases being constant up to 10,000 pg ml-‘. The addition of copper to sample solutions decreased the interference of many diverse ions both by normal premixed laminar flame and by STAT
data for AAS determination a
+10
of bismuth at 223.1 nm
Detection limit b
R.S.D. ’ (%)
0.44 0.23
0.6 0.3
4.6 2.5
at4FgmI-’
0.022 0.014
0.02 0.01
3.5 1.8
at 0.2 pg ml-’
( pg ml-‘) giving an absorbance of 0.0044 (i.e., 99% transmission). ( pg ml- ’ ) giving 3 X baseline noise.
Bi
Bi
91
D.T. Burns et al. /Analytica Chimica Acta 311 (199.5) 93-97 Table 4 Analysis of bismuth in BNF copper-based BNF No.
c50.01 c50.04 c71.31 C71.32 c71.34 a Mean f
Description
Leaded bronze Leaded bronze Gunmetal Gunmetal Gunmetal standard
deviation
alloys
Reported value
Found (o/o, w/w)
(%, w/v)
Solution nebulized
0.03 0.08 0.02 0.04 0.03
a Hydride flame
Flame without STAT
Flame with STAT
Without STAT
With STAT
0.031 0.077 0.019 0.039 0.025
0.030 0.077 0.017 0.039 0.023
0.030 0.084 0.018 0.038 0.024
0.030 0.078 0.023 0.038 0.024
+ jr * f +
0.001 0.004 0.001 0.001 0.002
+ + + + f
0.001 0.004 0.0005 0.001 0.0004
+ f + + f
0.0008 0.0016 0.0005 0.001 0.0001
f rt + + +
0.0005 0.001 0.0004 0.0005 0.001
for 5 analyses.
(Table 2). The effect of copper in modifying interferences is similar to that found earlier for tin [18] and antimony [19]; its origin is partly in the gas phase but with a significant surface contribution. The characteristic concentrations, detection limits and precision data are summarised in Table 3.
3. Results and conclusions All the samples were analysed by hydride generation and by direct nebulisation with a premixed laminar flame with and without a STAT in place. The change from normal aspiration to the hydride technique provides a considerable decrease in characteristic concentrations and can be effected rapidly if the hydrides are passed to the flame via the gas mixing chamber. Further statistically significant (P = 0.025 by F test) improvements in precision and decreases in characteristic concentration are gained by the use of the STAT. The results summarised in Table 4 show good agreement between the results of all four methods examined and confirm further the advantageous use of a STAT in the analysis of metallurgical samples.
Acknowledgements One of us (N.C.) wishes to thank Silkaporn University for leave of absence and also for financial support from the DuPont Science Grant 1988/89/90 made to DTB. Both acknowledge helpful assistance
from E. Todd, Bureau of Analytical dlesborough.
Standards,
Mid-
References [l] [2] [3] [4] [5]
A.V. Grunin, Zavod. Lab., 39 (1973) 1070. G. Norwitz and M. Galan, Anal. Chim. Acta, 83 (1976) 289. E.M. Donaldson, Talanta, 25 (1978) 131. C.H. McMaster, Can. J. Chem., 43 (1965) 405. G. van Dyck and F. Verbeek, Z. Anal. Chem., 249 (1970) 89. [6] 1. Tsukahara and T. Yamamoto, Anal. Chim. Acta, 63 (1973) 464. [7] W.B. Barnett and E.A. McLaughlin Jr., Anal. Chim. Acta, 80 (1975) 285. [8] J.D. Mullen, Talanta, 23 (1976) 846. [9] British Standards Institution, British Standards Methods for Analysis of high purity copper cathode, Cu-Cath-1. BS7317. Part 4. Method for the determination of antimony, arsenic bismuth, lead, selenium tellurium and tin by electrothermal atomisation atomic absorption spectrophotometry. BSl, London, 1990. 1101 A.A. Baker and J.D. Headridge, Anal. Chim. Acta, 125 (1981) 93. [Ill A.E. Smith, Analyst, 100 (1975) 300. [121 M. Bedard and J.B. Kerbyson, Anal. Chem., 47 (1975) 1441. 031 L. Wang, Y. Li and F. Zhang, Fenxi Shiyanshi, 8 (1989) 73. 1141 T. Takada and K. Fujita, Talanta, 32 (1985) 571. 1151 J.R. Castilo, J.M. Mir, M.L. Vela and C. Martinez, At. Spectrosc., 7 (1986) 85. tt61 T. Nakahara, K. Nakanishi and T. Wasa, Spectrochim. Acta, 42B (1987) 119. 1171 D.T. Bums, G.D. Atkinson, N. Chimpalee and M. Harriott, Fresenius’ J. Anal. Chem., 331 (1988) 814. 1181 D.T. Burns, N. Chimpalee and M. Harriott, Fresenius’ J. Anal. Chem., 348 (1994) 248. 1191 D. T. Burns, N. Chimpalee and M. Harriott, Fresenius’ J. Anal. Chem., 348 (1994) 527, 530.
CHIMICA ACEi ELSEVIER
Analytica
Chimica Acta 311 (1995) 93-97
Applications of a slotted tube atom trap and flame atomic absorption spectrometry: determination of bismuth in copper-based alloys with and without hydride generation D. Thorburn Burns
*, Narong
Chimpalee
‘, Michael Harriott
Department ofAnalytical Chemistry, The Queen’s University of Belfast, Belfast Bi’9 5AG, UK
Received 9 September 1994; revised 17 February 1995; accepted 16 March 1995
Abstract Improved systems are reported for the determination of bismuth (O-0.1%, w/w) in copper-based alloys by flame atomic absorption spectrometry. The use of a slotted quartz tube atom trap (STAT) with nebulised solutions or coupled with hydride generation decreases the characteristic concentrations (nebulized solution, 0.44/0.25, without/with STAT; hydride, 0.022/0.014, without/with STAT) and improved precisions by factors of two compared with those obtained without the use of the STAT. Determination of bismuth in a series of reference materials demonstrates the usefulness of STAT systems for the analysis of metallurgical samples. Keywords: Hydride
generation;
Slotted tube atom trap (STAT); Bismuth;
1. Introduction Bismuth commonly occurs at trace levels in copper-based alloys. Methods reported for the determination of trace levels of bismuth in such alloys include spectrophotometry [l-3], D.C. polarography [4], anodic stripping voltammetry [5] and atomic absorption spectrometry @AS) [6-151. The characteristic concentration of bismuth by AAS is high and various methods have been examined in efforts to increase sensitivity. These include the use of solvent extraction [6], graphite [7-91 and
* Corresponding author.
’ Present address: Department sity, Thailand
of Chemistry,
Silkaporn
Univer-
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SD1 0003.2670(95)00164-6
Copper-based
alloys
induction furnaces [lo], and particularly hydride generation methods [ll-151. Hydride methods for many elements including bismuth are prone to interference from the presence of concomitants such as copper [ 111. These interferences may be overcome by co-precipitation separation of bismuth on lanthanum hydroxide [9,12] or on hydrated iron(II1) oxide [13], by masking copper with thiosemicarbazide [14] or by prior removal of copper by electrolysis [15]. For the determination of bismuth by slotted quartz tube atom trap (STAT) flame AAS using a nebulised solution, the presence of copper was found to be beneficial, suppressing the interference from most other cations. However. for hydride generation with STAT flame AAS, it was necessary to mask copper. Thiourea [16], previously used in inductively coupled plasma AES, proved
94
D.T. Burns et al. /Analytica
Chimica Acta 311 (1995) 93-97
Copper stock solution, 50 mg ml - I Dissolve 5.00 g of copper wire (99.99% Cu, BDH) in 15 ml of cont. nitric acid and dilute to 1 1 with water.
satisfactory in masking copper. In both cases the use of the STAT decreased the characteristic concentrations and improved precisions compared with those obtained by the use of normal premixed laminar flames.
Sodium tetrahydroborate(III), 2% w / u Dissolve 2 g of NaBH, (98%, Aldrich) in 100 ml of distilled water. After stabilising with addition of sodium hydroxide (1 pellet, ca. 1 g) the solution is filtered through a Whatman No. 540 filter paper.
2. Experimental
2.1. Apparatus
Thiourea solution Dissolve 2 g of thiourea (99%, Aldrich) in 100 ml of distilled water. All acids and salts used in this study were of analytical grade. Doubly distilled water was used throughout.
A Perkin-Elmer Model 403 atomic absorption spectrometer equipped with a bismuth hollow cathode lamp was used and signals were recorded using a Perkin-Elmer Model 56 chart recorder set at the 10 mV range. The spectrometer conditions for bismuth were: wavelength 223.1 nm, band pass 0.7 nm, lamp current 10 mA; flame air 11.6 1 min-‘; acetylene 3.8 1 min-‘; aspiration rate 6.2 ml min-‘; chart recorder full scale deflection 0.25 absorbance. The slotted quartz tube (satin finish) was 140 mm long with a centralised longitudinal base slot 10 X 3 mm, and an upper row of six holes of 6 mm diameter, 15 mm apart, centrally drilled above the base slot. The cradle [17] and hydride generation system [18] were as described earlier.
The improvements possible in characteristic concentrations and precisions for the determination of bismuth in copper based alloys by flame-AAS have been studied by comparing data from two sets of parallel pairs of experiments. In the first set bismuth was introduced as bismuthane and in the second set in nebulized solutions, AAS measurements being made on both sets, both with and without the use of a STAT.
2.2. Reagents 2.3. Hydride generation Bismuth stock solution 1.0 mg ml-’ as bismuth(II1) grade (BDH).
nitrate,
procedure
(181
Pipette 3 ml of the sample solution (containing 20%, v/v, hydrochloric acid and 2%, v/v, nitric
SpectrosoL
to extraction
hood
Three way stopcock to AAS -
distilled water -
25 ml flask
to waste
Fig. 1. Hydride generation
system.
-N2 flow meter
inlet
D.T. Burns et al. /Analytica
acid) into the generation flask (Fig. 1). Make up to 10 ml with distilled water. Connect the generation flask (via a 3-way stopcock) directly with the nebulizer (N, flow rate 600 ml min-‘). Inject 3 ml of 2% NaBH, solution and record the absorbance signal. Finally, turn the 3-way stopcock so as to aspirate distilled water into the flame. Drain and wash out the generation flask prior to addition of the next sample. 2.4. Analysis of copper-based alloys by hydride-generation flame AAS Dissolve accurately weighed samples, containing about 0.02 mg bismuth, in 20 ml of cont. hydrochloric acid, 2 ml of cont. nitric acid and 5 ml of distilled water in 150 ml conical flasks. Complete dissolution by gentle warming on a hot plate. Cool the solutions and transfer quantitatively to 100 ml volumetric flasks and make up to volume with distilled water. Place 3 ml of these sample volumes in the generation flask, add 3 ml of 2% thiourea and make up to 10 ml with distilled water. Pairs of analyses were completed as described under the hydride generation procedure above, making measurements both with and without the STAT in position. Evaluate the amount of bismuth from appropriate calibration graphs, with and without STAT, covering the calibration range O-O.5 pg ml-’ Bi, using standards made up in 20%, v/v, hydrochloric acid and 2%, v/v, nitric acid. 2.5. Analysis of copper-based solution-flame AAS
alloys by nebulized
2.6. Examination of the main experimental variables The optimum flame and STAT conditions were found, by univariate search as described earlier [17] using 0.5 and 4 pg ml-’ Bi for the with and the without hydride generation procedures, respectively, to be in both cases, flame: air 11.6 1 min-‘, acetylene 3.8 1 mini’, the STAT positioned with an 8 mm gap between the burner head and base of the STAT and with the spectrometer optical axis central to the STAT. Since copper based alloys are conveniently dissolved in a mixture of hydrochloric acid and nitric acid the effects of variation in concentration of both acids were examined. For hydride generation (0.5 pg mll’ Bi in 2%, v/v, nitric acid) the absorbance signals were found to increase with increasing hydrochloric acid concentrations up to lo%, v/v, and remain constant up to 30%, v/v. A hydrochloric acid concentration of 20%, v/v, was used thereafter. Absorbances (0.5 pg Bi ml-’ in 20%, v/v, hydrochloric acid) were inde-
Table 1 Effect of diverse bismuth following STAT-AAS
ions on the determination of 0.5 pg ml-’ bismuthane generation using flame and flame
Ion added
Amount (I*gml-‘)
PMII), S&V), Fe(III) Ni(II), Z&I), (XIII) A&II) Sb(Ill)
Dissolve weighed samples (I 0.5 g>, to contain about 0.4 mg of bismuth, in 25 ml of cont. hydrochloric acid, 5 ml of cont. nitric acid and 5 ml of distilled water in 150 ml conical flasks. Gently warm if necessary, to complete dissolution. Cool the solutions, transfer to 100 ml volumetric flasks and make up to volume with distilled water. Examine the sample solutions by nebulized solution-flame AAS taking measurements both with and without the STAT in position. Evaluate the amounts of bismuth present from appropriate calibration graphs, with and without the STAT in position, covering the range O-8 pg mll’ Bi using standards containing 5000 pg ml-’ copper(B).
95
Chimica Acta 311 (1995) 93-97
cow Al(III) M&I) cuw
A&) Se(W)
a Less than 2% variation. b Add 3 ml of 2% (w/v)
500 500 20 500 250 500 100 500 20 500 SO 100 10 10000 h 100 10 1000 2
thiourea solution.
Variation peak height (%) _ “
+46 _ f21 _ -62 _ -77 _ - 15 _ - 100 -77 _ -67 _ - 100 _
96
D.T. Burns et al./Analytica
Table 2 Effect of diverse ions (% change in signal) on the determination absence of 5000 pg ml-’ copper Ion added
Sn(IV1
Amount
Flame
(kgml-‘1
No Cu added
A&II)
of 4 pg ml-
STAT 5000 pg ml- ’ Cu added
-5 +5 _
+2 +2 +2 -
+5 -5 -5 -5
+2 +2 -
pendent of nitric acid concentration up to 5%, v/v. A nitric acid concentration of 2%, v/v, was selected. For the STAT system without hydride generation, absorbances (4 pg Bi ml-’ in 5% nitric acid) increased by 3% over the range 5 to 30%, v/v, hydrochloric acid (25%, v/v, was used in subsequent experiments), and to be independent of nitric acid concentration over the range O-15%, v/v. For hydride generation (1 pg ml-’ Bi in 20%, v/v, hydrochloric acid and 2.0%, v/v, nitric acid), absorbances were found to increase rapidly up to l%, w/v, NaBH, and to decrease slowly thereafter. l%, w/v, NaBH, was used in subsequent experiments. Peak heights were independent of nitrogen flow rate over the range 200-1000 ml min-‘; a flow rate of 600 ml min-’ was found to be convenient for routine use.
Table 3 Characteristic
concentrations,
detection
limits and precision
Method
Characteristic
Nebulized solution flame without STAT flame with STAT Hydride jlame without STAT with STAT a Concentration b Concentration ’ n = 10.
’ bismuth by flame AAS and STAT-AAS in the presence or
No Cu added
5000 pg ml- r Cu added
-
1000 1000 1000 500 500 500 500 100 100 100
PM111 Zn(II) Ni(II) Fe(III) Cr(III) M&I) Al(III) Sb(III)
Chimica Acta 311 (1995) 93-97
concentration
_ _
+27 +27 +7 +23 +20 +20 +20
+3 +3 _ _ _
The possible interferences on the generation/ atomisation of bismuthane were examined. The results (Table 1) were the same with or without the STAT confirming the interference effects to be solution based. The only interference at levels of interest in the analysis of copper based alloys was that of copper, which was overcome by masking with addition of 3 ml of 2% thiourea solution. During initial studies of methods using the direct aspiration of solutions it was noted that absorbance values were increased by 5% for flame and 13% for the STAT after 10 min aspiration of solutions containing above 400 pg ml-’ copper, the enhancement effects in both cases being constant up to 10,000 pg ml-‘. The addition of copper to sample solutions decreased the interference of many diverse ions both by normal premixed laminar flame and by STAT
data for AAS determination a
+10
of bismuth at 223.1 nm
Detection limit b
R.S.D. ’ (%)
0.44 0.23
0.6 0.3
4.6 2.5
at4FgmI-’
0.022 0.014
0.02 0.01
3.5 1.8
at 0.2 pg ml-’
( pg ml-‘) giving an absorbance of 0.0044 (i.e., 99% transmission). ( pg ml- ’ ) giving 3 X baseline noise.
Bi
Bi
91
D.T. Burns et al. /Analytica Chimica Acta 311 (199.5) 93-97 Table 4 Analysis of bismuth in BNF copper-based BNF No.
c50.01 c50.04 c71.31 C71.32 c71.34 a Mean f
Description
Leaded bronze Leaded bronze Gunmetal Gunmetal Gunmetal standard
deviation
alloys
Reported value
Found (o/o, w/w)
(%, w/v)
Solution nebulized
0.03 0.08 0.02 0.04 0.03
a Hydride flame
Flame without STAT
Flame with STAT
Without STAT
With STAT
0.031 0.077 0.019 0.039 0.025
0.030 0.077 0.017 0.039 0.023
0.030 0.084 0.018 0.038 0.024
0.030 0.078 0.023 0.038 0.024
+ jr * f +
0.001 0.004 0.001 0.001 0.002
+ + + + f
0.001 0.004 0.0005 0.001 0.0004
+ f + + f
0.0008 0.0016 0.0005 0.001 0.0001
f rt + + +
0.0005 0.001 0.0004 0.0005 0.001
for 5 analyses.
(Table 2). The effect of copper in modifying interferences is similar to that found earlier for tin [18] and antimony [19]; its origin is partly in the gas phase but with a significant surface contribution. The characteristic concentrations, detection limits and precision data are summarised in Table 3.
3. Results and conclusions All the samples were analysed by hydride generation and by direct nebulisation with a premixed laminar flame with and without a STAT in place. The change from normal aspiration to the hydride technique provides a considerable decrease in characteristic concentrations and can be effected rapidly if the hydrides are passed to the flame via the gas mixing chamber. Further statistically significant (P = 0.025 by F test) improvements in precision and decreases in characteristic concentration are gained by the use of the STAT. The results summarised in Table 4 show good agreement between the results of all four methods examined and confirm further the advantageous use of a STAT in the analysis of metallurgical samples.
Acknowledgements One of us (N.C.) wishes to thank Silkaporn University for leave of absence and also for financial support from the DuPont Science Grant 1988/89/90 made to DTB. Both acknowledge helpful assistance
from E. Todd, Bureau of Analytical dlesborough.
Standards,
Mid-
References [l] [2] [3] [4] [5]
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