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The atomic absorption spectrometric determination of arsenic, bismuth, lead, antimony, selenium and tin with a flame-heated silica t-tube after hydride generation
Analytica Chimica Acta. 115 (1980) 355-359 o Elsevier Scientific Publishing Company, Amsterdam -
Short
Printed in The Netherlands
Communication
TI3E ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF ARSENIC, BISMUTH, LEAD, ANTIMONY, SELENIUM AND TIN WITH A FLAME-HEATED SILICA T-TUBE AFTER HYDRIDE GENERATION
P. K. HON, 0. W. LAU*,
W. C. CHEUNG and M. C. WONG
Department of Chemistry, The Chinese University of Hong Kong, Shatin. N. T. (Hong Kong) (Received 10th July 1979) Summary. A modified technique with a simple T-tube for atomisation held above conventional atomic absorption burners is described_ The absolute sensitivities are in fhe range 0.3-11 ng and the r.s.d. is usually below 5%. It is confirmed that prior oxidation of lead markedly increases the sensitivity_
Chemical generation of volatile hydrides with subsequent atomisation offers sensitivity greatly superior to that obtained with conventional flame atomic absorption methods [l-4]. Thompson and Thomerson [4] passed the hydrides directly into a sihca tube mounted over a wide-path airacetylene burner, 15.5 cm long. This method offers high sensitivity and no background correction is required. It can be simplified by using a simple Ttube with a conventional burner, to replace their complicated atomising tube. This communication reports the optimum conditions for determining six hydride-forming elements with the use of this device and compares with those reported by Thompson and Thomerson 141. Experimental
Apparatus. A Per-kin-Elmer Model 360 atomic absorption spectrometer and arsenic electrodeless discharge lamp, and Varian-Techtron hollowcathode lamps were used. The T-shaped atomising tube (15 cm long, 4-13 mm i-d.) with a sidearm (8 cm long, 4 mm i-d.) was made of transparent vitreosil fused quartz. The tube was supported, 2 cm above the burner grid of a single or three-slot burner and aligned in the light path. The generated hydride in a stream of nitrogen was introduced through the side-arm. The nitrogen flow-rate was controlled by a Matheson Model 603 gas flow meter fitted with a needle valve. The hydride generation vessel was a round-bottom tube (7 cm high, 3 cm i.d.) with an inlet near the bottom and fitted with a rubber bung carrying a septum and an outlet tube (4 mm i.d.). Reagents. Al.l reagents were of analytical-reagent grade. The sodium tetrahydroborate solution (1%) contained potassium hydroxide (2 pellets per 100 ml).
356 Stock soIutions containing 1000 ppm of arsenic, lead and antimony were prepared from the oxide, nitrate and tartrate, respectively, and the rest were prepared from the metals by standard procedures. Standard solutions were prepared by appropriate dilution with 1.5 M hydrochloric acid or 0.5% nitric acid (lead only). Procedure. The optimal conditions for each element are given in Table 1. The acetylene flow-rate was set at 3.0 1 mir-‘, tbe air flow-rate at 17.5 1 mine1 and the nitrogen fIow-rate at 1.0 lIm.i~-_~(except for tin, when 1.8 1 ruin-’ was better)_ SampIe, standard or blauk solution (1 ml) was pipetted into the generation tube, which was immediately stoppered. The magnetic stirrer was turned on and nitrogen was passed for 10 s. Tetrahydroborate solution (2 ml) was injected into the tube through the septum (for lead, 2 ml of 1% hydrogen peroxide was injected and stirred for 20 s before injection of the tetrahychoborate solution). The peak height was then recorded. The generation tube was emptied and rinsed twice with distilled water before the procedure was repeated. Steel and Orchard Leaves were dissolved by standard procedures. For arsenic and antimony, the sample solutions were reduced with 1 ml of 1 M potassium iodid*lO% sodium ascorbate solution before addition of tetrahydrobomte.
Results and discussion Optirnisation of conditions_ The atomising tube used was a simple T-tube, which is easy to make and convenient to use. It was not necessary to cool the sidearm of the tube, which was connected to the generation flask with PVC tubing. The hydrogen liberated duringthereduction was allowed to burn at the ends of the tube, but the background absorption was still toIerable. This disposed of the need for transverse tubes for burning hydrogen. Unlike the determination of mercury by the cold-vapour technique [5], the absorbance was found to increase with increasing diameter of the atomising tube, provided that the whole tube was in the optical path. For example, the absorbauces of a 0.03 ppm arsenic solution, when a three-slot burner was used were: 0.163 (4 mm i-d.), 0.361 (7 mm i-d.), 0.420 (10 mm i.d.) and 0.324 (13 mm i-d.). The absorbance did not increase further for the last tube, part of which was observed to be outside the optical path. The response to all elements except lead did not depend greatly on the concentration of acid in the range of 14 M. Nitric acid allowed a greater response for lead than hydrochloric acid, but its concentration markedIy affected the absorbance. For example, the absorbances of 7 ppm of lead at 217 run (three-slot burner, 4-mm atomising tube) ranged from 0.343 with 0.5% nitric acid to 0.059 with 0.1% acid. The tetrahydroborate concentration was found not critical. A 1% solution is more stable than the more concentrated solutions and can be kept for two days in a refrigerator. Addition of hydrogen peroxide increased the lead signal, as reported by Vijan and Wood [S] _ Maximum absorbance was found with a 3% peroxide
357 TABLE
1
Recommended experimental condition@ Element Wavelength (nm) Bandwidth (nm) Lamp current (mA)
AS 193.7 0.7 SC
Bi 306.8b 0.7 20
Pb
Sb
217.0
217.6
0.7
0.2
15
20
Sll
Se
196.0 2.0
15
286.3 0.7
25
a1.5 M HCI was the optimal acidity, except for lead, where 0.5% MO,
was preferable. ki!hii less sensitive but more intense line is recommended; the 223.1 m line can be used under the same conditions. =The power to the electrodeless lamp was 8 W.
solution. Despite the addition of hydrogen peroxide and nitric acid, the absorbance showed little change when the tetrahydroborate concentration was changed from 1% to 4%. It appears that hydrogen peroxide oxidises lead(II) to a met&stable lead(W) compound before the tetrahydroborate converts it to plumbane. It is interesting to note that when hydrogen peroxide was added, hydrogen was no longer evolved during tetrahydroborate reduction. For the analysis of steel and Orchard Leaves, it was necessary to reduce arsenic and antimony before tetrahydroborate treatment, otherwise the results obtained were 30-40% low. A similar observation has been reported by other workers [73 _ Sensitivities, detection limits, linear calibration ranges and precision. The results given in Table 2 indicate that there is no significant difference in precision between the three-slot and single-slot burners. The sensitivities and detection limits obtained are also similar, although the three-slot burner usually gives better results. The precisions obtained with the lo-mm and 4mm tubes are similar except for antimony, The poor precision for antimony can be ascribed to the low intensity of the atomic line used, which was further decreased by the absorption of the smaller tube. The sensitivity depends markedly on the internal diameter of the atomising tube although the extent varies with the element. The detection limit is also improved in the larger tube, especially for antimony and selenium. The linear calibration range is only slightly affected by the tube diameter, except for antimony, where the range was double when a 4-mm tube instead of a lo-mm tube was used, For comparison, the calibration graphs found for the six elements are illustrated in Fig. 1. In principle the most sensitive atomic lines should be used. Unfortunately, for tin and lead, the most sensitive lines have lower intensities, and hence higher noise, and for tin. the 224.6-nm line could not be used. For bismuth, the sensitivity at the 223.1~nm line was 1.4 times better, but the detection limit was poorer because of noise problems. Comparison with the method of Thompson and Thomerson [4/. For comparison, the data obtained with the three-slot burner and the IO-mm atomising tube are used. In all cases except tin, the sensitivities obtained
358 TABLE 2 Absolutesensitmities,detectionlimits,linearcalibrationrangesandprecisiondata Element x (=)
As
193.7 223.1
Bi 306.8
Pb
217.0
Sb
217.6
No-of I.d.of Precision SensitivityDetection Linear burner atomking (ng) limit(ng) calibration(%)b slot tube(=) =nge(ppbY 10 4 10
0.3 0.8 0.4
1.4 2.0 2.0
O-70 O-50
3.9 (20) 1.4 (30) 5.6 (20)
3 3 3 1
10 4 4 4
0.4 0.9 1.3 2.0
1.5 4.8 2.8 4.3
O-70 o-55 O-70 O-70
4.4 5.3 4.2 3.1
3 3 1
10 4 10
3
10 4
0.33 1.2
1.8 11
O-50 O-100
3 3 1
10
1.4 6.9 6.4
2.6 26 33
O-70 O-120 O-120
3 3
10
1
8
3 Se
196.0
Sn
286.3
O-50
3 3 1
4 4
11 23 12
24 50 73
O-2100 O-2100 300-2100
O-70 O-85 O-70
samples. bRs.d.of lO-replicatedeterminations of standard solutions whose concentration are indicatedin parentheses.
(20) (60) (60) (60)
7.7 (900) 7.0 (1500) 6.7 (1500) 4.6 (20) 14.2 (60) 5.1 (20) 5.1 (120) 4.9 (120 5.0 (20) 4.9 (60) 4.9 (60)
=1-mI
in
ppb
with the T-tube are better than those obtained by Thompson and Thomerson, the ratios of sensitivities being 1.73 (As),1.1 (Bi), 7.3 (Pb), 1.85 (Sb), 1.5 (Se) and 0.44 (Sn). The improvement accomplished by the present method probably arises from the use of a slightly larger atomising tube and, for lead, by using a more sensitive line and prior oxidation. The detection limits obtained by the present method are worse than those obtained by Thompson and Thomerson except for lead, probably because of the greater noise levels of the instrument used in these studies. Applications_ The method was applied to the determination of arsenic, bismuth, antimony, selenium and tin in a steel sample (NBS SRM 661, AISI 4340 Steel) and to arsenic, antimony and selenium in Orchard Leaves (NBS SR.M 1571). The results are shown in Table 3. The measured values fall within the certified ranges, demonstrating the accuracy of the method. The authors express their gratitude to Professor T. S. West and Dr. A. Townshend for their valuable suggestions in the preparation of the manuscript.
359
Cmcentration,
ppm (Pb only 1 1.5
1.0
0.6
0
lo
20
30
40
50
60
70
Concentration,ppb Fig. 1. Calibration graphs obtained with a 3-slot burner and lo-mm l-ml aliquots of standard solutions.
i.d. atomising tube for
TABLE 3 The determination of some hydride-forming elements in standard reference samples Sample
Element
Certified value (rg g-l)=
Observed value (U&zg-l)
Steel (SRM 661)
As Bi
170 f 10
166
Orchard Leaves (SRM 1571)
Sb
421 40+-z
Se Sn
40210 100 + 10
As Sb Se
10 f 2 2.9 f 0.3
0.08 +O.Ol
3.5 38 9”: 10.2 2.8
0.09
aOriginally expressed in percentage for SRM 661. REFERENCES 1 2 3 4 5 6 7
W. Holak, Anal. Chem., 41 (1969) 1712. E. F. Dalton and A. J. Malanski, At. Absorpt. Newsletter, 10 (1971) 92. R. C. Chu, G. P. Barron andP. A. W. Bamngamer, Anal. Chem., 44 (1972) K. C. Thompson md D. R. Thomerson, Analyst, 99 (1974) 595. J. E. Hawley and J. D. Ingle, Jr., Anal. Chem., 47 (1975) 721. P. N. Vijan and G. R. Wood, Analyst, 101 (1976) 966. D. D. Siemer, P. Koteel and V. Jariwala, Anal. Chem., 48 (1976) 836.
1476.
Short
Printed in The Netherlands
Communication
TI3E ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF ARSENIC, BISMUTH, LEAD, ANTIMONY, SELENIUM AND TIN WITH A FLAME-HEATED SILICA T-TUBE AFTER HYDRIDE GENERATION
P. K. HON, 0. W. LAU*,
W. C. CHEUNG and M. C. WONG
Department of Chemistry, The Chinese University of Hong Kong, Shatin. N. T. (Hong Kong) (Received 10th July 1979) Summary. A modified technique with a simple T-tube for atomisation held above conventional atomic absorption burners is described_ The absolute sensitivities are in fhe range 0.3-11 ng and the r.s.d. is usually below 5%. It is confirmed that prior oxidation of lead markedly increases the sensitivity_
Chemical generation of volatile hydrides with subsequent atomisation offers sensitivity greatly superior to that obtained with conventional flame atomic absorption methods [l-4]. Thompson and Thomerson [4] passed the hydrides directly into a sihca tube mounted over a wide-path airacetylene burner, 15.5 cm long. This method offers high sensitivity and no background correction is required. It can be simplified by using a simple Ttube with a conventional burner, to replace their complicated atomising tube. This communication reports the optimum conditions for determining six hydride-forming elements with the use of this device and compares with those reported by Thompson and Thomerson 141. Experimental
Apparatus. A Per-kin-Elmer Model 360 atomic absorption spectrometer and arsenic electrodeless discharge lamp, and Varian-Techtron hollowcathode lamps were used. The T-shaped atomising tube (15 cm long, 4-13 mm i-d.) with a sidearm (8 cm long, 4 mm i-d.) was made of transparent vitreosil fused quartz. The tube was supported, 2 cm above the burner grid of a single or three-slot burner and aligned in the light path. The generated hydride in a stream of nitrogen was introduced through the side-arm. The nitrogen flow-rate was controlled by a Matheson Model 603 gas flow meter fitted with a needle valve. The hydride generation vessel was a round-bottom tube (7 cm high, 3 cm i.d.) with an inlet near the bottom and fitted with a rubber bung carrying a septum and an outlet tube (4 mm i.d.). Reagents. Al.l reagents were of analytical-reagent grade. The sodium tetrahydroborate solution (1%) contained potassium hydroxide (2 pellets per 100 ml).
356 Stock soIutions containing 1000 ppm of arsenic, lead and antimony were prepared from the oxide, nitrate and tartrate, respectively, and the rest were prepared from the metals by standard procedures. Standard solutions were prepared by appropriate dilution with 1.5 M hydrochloric acid or 0.5% nitric acid (lead only). Procedure. The optimal conditions for each element are given in Table 1. The acetylene flow-rate was set at 3.0 1 mir-‘, tbe air flow-rate at 17.5 1 mine1 and the nitrogen fIow-rate at 1.0 lIm.i~-_~(except for tin, when 1.8 1 ruin-’ was better)_ SampIe, standard or blauk solution (1 ml) was pipetted into the generation tube, which was immediately stoppered. The magnetic stirrer was turned on and nitrogen was passed for 10 s. Tetrahydroborate solution (2 ml) was injected into the tube through the septum (for lead, 2 ml of 1% hydrogen peroxide was injected and stirred for 20 s before injection of the tetrahychoborate solution). The peak height was then recorded. The generation tube was emptied and rinsed twice with distilled water before the procedure was repeated. Steel and Orchard Leaves were dissolved by standard procedures. For arsenic and antimony, the sample solutions were reduced with 1 ml of 1 M potassium iodid*lO% sodium ascorbate solution before addition of tetrahydrobomte.
Results and discussion Optirnisation of conditions_ The atomising tube used was a simple T-tube, which is easy to make and convenient to use. It was not necessary to cool the sidearm of the tube, which was connected to the generation flask with PVC tubing. The hydrogen liberated duringthereduction was allowed to burn at the ends of the tube, but the background absorption was still toIerable. This disposed of the need for transverse tubes for burning hydrogen. Unlike the determination of mercury by the cold-vapour technique [5], the absorbance was found to increase with increasing diameter of the atomising tube, provided that the whole tube was in the optical path. For example, the absorbauces of a 0.03 ppm arsenic solution, when a three-slot burner was used were: 0.163 (4 mm i-d.), 0.361 (7 mm i-d.), 0.420 (10 mm i.d.) and 0.324 (13 mm i-d.). The absorbance did not increase further for the last tube, part of which was observed to be outside the optical path. The response to all elements except lead did not depend greatly on the concentration of acid in the range of 14 M. Nitric acid allowed a greater response for lead than hydrochloric acid, but its concentration markedIy affected the absorbance. For example, the absorbances of 7 ppm of lead at 217 run (three-slot burner, 4-mm atomising tube) ranged from 0.343 with 0.5% nitric acid to 0.059 with 0.1% acid. The tetrahydroborate concentration was found not critical. A 1% solution is more stable than the more concentrated solutions and can be kept for two days in a refrigerator. Addition of hydrogen peroxide increased the lead signal, as reported by Vijan and Wood [S] _ Maximum absorbance was found with a 3% peroxide
357 TABLE
1
Recommended experimental condition@ Element Wavelength (nm) Bandwidth (nm) Lamp current (mA)
AS 193.7 0.7 SC
Bi 306.8b 0.7 20
Pb
Sb
217.0
217.6
0.7
0.2
15
20
Sll
Se
196.0 2.0
15
286.3 0.7
25
a1.5 M HCI was the optimal acidity, except for lead, where 0.5% MO,
was preferable. ki!hii less sensitive but more intense line is recommended; the 223.1 m line can be used under the same conditions. =The power to the electrodeless lamp was 8 W.
solution. Despite the addition of hydrogen peroxide and nitric acid, the absorbance showed little change when the tetrahydroborate concentration was changed from 1% to 4%. It appears that hydrogen peroxide oxidises lead(II) to a met&stable lead(W) compound before the tetrahydroborate converts it to plumbane. It is interesting to note that when hydrogen peroxide was added, hydrogen was no longer evolved during tetrahydroborate reduction. For the analysis of steel and Orchard Leaves, it was necessary to reduce arsenic and antimony before tetrahydroborate treatment, otherwise the results obtained were 30-40% low. A similar observation has been reported by other workers [73 _ Sensitivities, detection limits, linear calibration ranges and precision. The results given in Table 2 indicate that there is no significant difference in precision between the three-slot and single-slot burners. The sensitivities and detection limits obtained are also similar, although the three-slot burner usually gives better results. The precisions obtained with the lo-mm and 4mm tubes are similar except for antimony, The poor precision for antimony can be ascribed to the low intensity of the atomic line used, which was further decreased by the absorption of the smaller tube. The sensitivity depends markedly on the internal diameter of the atomising tube although the extent varies with the element. The detection limit is also improved in the larger tube, especially for antimony and selenium. The linear calibration range is only slightly affected by the tube diameter, except for antimony, where the range was double when a 4-mm tube instead of a lo-mm tube was used, For comparison, the calibration graphs found for the six elements are illustrated in Fig. 1. In principle the most sensitive atomic lines should be used. Unfortunately, for tin and lead, the most sensitive lines have lower intensities, and hence higher noise, and for tin. the 224.6-nm line could not be used. For bismuth, the sensitivity at the 223.1~nm line was 1.4 times better, but the detection limit was poorer because of noise problems. Comparison with the method of Thompson and Thomerson [4/. For comparison, the data obtained with the three-slot burner and the IO-mm atomising tube are used. In all cases except tin, the sensitivities obtained
358 TABLE 2 Absolutesensitmities,detectionlimits,linearcalibrationrangesandprecisiondata Element x (=)
As
193.7 223.1
Bi 306.8
Pb
217.0
Sb
217.6
No-of I.d.of Precision SensitivityDetection Linear burner atomking (ng) limit(ng) calibration(%)b slot tube(=) =nge(ppbY 10 4 10
0.3 0.8 0.4
1.4 2.0 2.0
O-70 O-50
3.9 (20) 1.4 (30) 5.6 (20)
3 3 3 1
10 4 4 4
0.4 0.9 1.3 2.0
1.5 4.8 2.8 4.3
O-70 o-55 O-70 O-70
4.4 5.3 4.2 3.1
3 3 1
10 4 10
3
10 4
0.33 1.2
1.8 11
O-50 O-100
3 3 1
10
1.4 6.9 6.4
2.6 26 33
O-70 O-120 O-120
3 3
10
1
8
3 Se
196.0
Sn
286.3
O-50
3 3 1
4 4
11 23 12
24 50 73
O-2100 O-2100 300-2100
O-70 O-85 O-70
samples. bRs.d.of lO-replicatedeterminations of standard solutions whose concentration are indicatedin parentheses.
(20) (60) (60) (60)
7.7 (900) 7.0 (1500) 6.7 (1500) 4.6 (20) 14.2 (60) 5.1 (20) 5.1 (120) 4.9 (120 5.0 (20) 4.9 (60) 4.9 (60)
=1-mI
in
ppb
with the T-tube are better than those obtained by Thompson and Thomerson, the ratios of sensitivities being 1.73 (As),1.1 (Bi), 7.3 (Pb), 1.85 (Sb), 1.5 (Se) and 0.44 (Sn). The improvement accomplished by the present method probably arises from the use of a slightly larger atomising tube and, for lead, by using a more sensitive line and prior oxidation. The detection limits obtained by the present method are worse than those obtained by Thompson and Thomerson except for lead, probably because of the greater noise levels of the instrument used in these studies. Applications_ The method was applied to the determination of arsenic, bismuth, antimony, selenium and tin in a steel sample (NBS SRM 661, AISI 4340 Steel) and to arsenic, antimony and selenium in Orchard Leaves (NBS SR.M 1571). The results are shown in Table 3. The measured values fall within the certified ranges, demonstrating the accuracy of the method. The authors express their gratitude to Professor T. S. West and Dr. A. Townshend for their valuable suggestions in the preparation of the manuscript.
359
Cmcentration,
ppm (Pb only 1 1.5
1.0
0.6
0
lo
20
30
40
50
60
70
Concentration,ppb Fig. 1. Calibration graphs obtained with a 3-slot burner and lo-mm l-ml aliquots of standard solutions.
i.d. atomising tube for
TABLE 3 The determination of some hydride-forming elements in standard reference samples Sample
Element
Certified value (rg g-l)=
Observed value (U&zg-l)
Steel (SRM 661)
As Bi
170 f 10
166
Orchard Leaves (SRM 1571)
Sb
421 40+-z
Se Sn
40210 100 + 10
As Sb Se
10 f 2 2.9 f 0.3
0.08 +O.Ol
3.5 38 9”: 10.2 2.8
0.09
aOriginally expressed in percentage for SRM 661. REFERENCES 1 2 3 4 5 6 7
W. Holak, Anal. Chem., 41 (1969) 1712. E. F. Dalton and A. J. Malanski, At. Absorpt. Newsletter, 10 (1971) 92. R. C. Chu, G. P. Barron andP. A. W. Bamngamer, Anal. Chem., 44 (1972) K. C. Thompson md D. R. Thomerson, Analyst, 99 (1974) 595. J. E. Hawley and J. D. Ingle, Jr., Anal. Chem., 47 (1975) 721. P. N. Vijan and G. R. Wood, Analyst, 101 (1976) 966. D. D. Siemer, P. Koteel and V. Jariwala, Anal. Chem., 48 (1976) 836.
1476.