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Improved cesium sensitivity in electrothermal atomic absorption spectrometry
Analytica Chimica Acta, 187 (1986) 307-311 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
IMPROVED CESIUM SENSITIVITY ABSORPTION SPECTROMETRY
JAMES F. CHAPMAN* and LESLIE
IN ELECTROTHERMAL
ATOMIC
S. DALE
CSIRO Division of Energy Chemistry, Lucas Heights Research Illawarra Road, Lucas Heights, NSW 2234 (Australia)
Laboratories,
New
SHARON A. TOPHAM AAEC Materials Division, Lucas Heights Research Lucas Heights, NSW 2234 (Australia)
Laboratories,
New Illawara
Road,
(Received 23rd April 1986)
Summary, The sensitivity for cesium determination by electrothermal atomic absorption spectrometry is improved four-fold by the addition of a large excess of potassium nitrate. Zeeman background correction is used to compensate for the large non-specific absorption signal resulting from the potassium. The characteristic concentration and detection limit are 0.44 and 2 rg l-l, respectively, and the coefficient of variation IS 2% at the 50 rg 1-l level. The procedure is suitable for the rapid determination of cesium in leach solutions from nuclear waste fixation experiments.
In studies of nuclear waste fixation, radioactive waste is incorporated into inert materials such as Synroc [l, 21. To assess the stability of these materials, aqueous leach tests were applied initially on materials containing simulated inactive waste. Because cesium is one of the most chemically mobile of the radioactive waste elements, its determination at low concentrations is important in leachability studies of these materials. In this laboratory, cesium has been determined in leach solutions by flame atomic emission spectrometry with potassium as an ionization suppressor. A 20-fold increase in sensitivity was achieved, leading to a detection limit of 2 pg 1-l. Although the procedure was satisfactory for inactive samples, it could not be applied to radioactive samples leached from inert materials containing actinides because of the potential hazard of aspirating a radioactive solution into a conventional burner/nebulizer system. Electrothermal atomic absorption spectrometry offered a better procedure because, with the small volumes used, contamination of equipment would be minimal and the small volume of vapour generated could be contained. However, because the detection limit for cesium for the instrument available was 8 pg l-‘, it was insufficient to provide leach rate data comparable with that obtained in the inactive leach tests. To improve the sensitivity, the addition of potassium as an ionization suppressant was considered. Studies of ionization in electrothermal atomizers have been undertaken by 0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
308
numerous workers [3-g]. Some of them observed slight enhancements of the analyte signal in the presence of added buffer [3, 41, whereas others found no enhancement [5-71. More detailed studies suggested that ionization is negligible owing to the high population of background electrons generated by the graphite [8, 91 and small compared to that obtained in conventional flames owing to the different time- and temperature-dependence of the atom and ion populations [9]. In several of these investigations, the conclusions were drawn from the results of experiments in which either the atom excess of buffer was small, or volatilization losses may have been caused by the use of chloride media. The present investigations into the use of potassium to improve the sensitivity for cesium show that with a large excess of potassium, the atomic absorption signal for cesium can be increased by a factor of four. In view of this, and because of conclusions reached by previous workers, the enhancement effect of potassium on cesium was studied in more detail. Experimental Apparatus and reagents. All measurements were made on a Hitachi Polarized Zeeman atomic absorption spectrometer, model 7000, equipped with a pyrolytic graphite tube cuvette and autosampler. All absorbance measurements were taken in the peak-height mode with Zeeman background correction. They were displayed and recorded on a Hitachi AA data processor. The spectral bandpass was 1.3 nm. Hollow-cathode lamps were operated at their recommended currents. All chemicals and acids were of analytical-reagent grade. Water from a Millipore Mill&Q purification system was used. Procedure. To 1.00 ml of sample was added 0.10 ml of potassium nitrate modifier solution (1.000 g of potassium as its nitrate, made up to 100 ml in 1 + 1 nitric acid). Portions (40 ~1) were injected into the graphite furnace. The heating program is shown in Table 1. The argon carrier gas flow of 200 ml min-’ was decreased during the atomization cycle to 30 ml min-‘. The peak-height absorbance at 852.1 nm was measured. Results and discussion Potassium addition. Poor sensitivity was obtained from a 50 pg 1~’ cesium solution, and there was no increase in cesium absorbance when potassium TABLE 1 Conditions used for cesium determination metry Step
1. 2. 3.
Temperature (“C)
Dry Dry Ash
Start
End
80 150 500
150 500 500
Time(s)
50 10 10
by electrothermal
Temperature (“C)
Step
4. 5. 6.
atomic absorption spectro-
Atomize Clean Cool
Start
End
2000 3000 -
2600 3000 -
Time(s)
6 3 30
309
06. 06
0
2
4
6
a
0
Nitric acld (vol %I
Fig. 1. Effect of nitric acid concentration
on cesium absorbance.
Fig. 2. Effect of potassium concentration
on cesium absorbance.
1000 Fotassium (mg ~~‘1
2000
chloride was added. However, when potassium nitrate was added, the cesium signal was significantly enhanced. The use of nitrate media to minimize interferences has been recommended by Ottaway and Shaw [lo]. The effect of increasing concentrations of nitric acid on a 50 pg 1-l cesium solution was studied; the results are shown in Fig. 1. The reason for the dip in the curve before the plateau region is not clear but it may be due to different volatilization rates of compounds such as CsN03.HN03 and CsN03*2HN03 [ 111. Because the plateau region occurs above ca. 3% (v/v) nitric acid, 5% was chosen for subsequent investigations. The effect of increasing the potassium concentration in the nitric acid was then studied. Figure 2 shows the change in response of 50 r_1gl-’ cesium in 5% nitric acid with increasing potassium concentration. The plateau region occurs at 1000 mg 1-l potassium, an atom ratio (K/es) of ca. 68,000. The background absorption signal resulting from the volatilization of the added potassium was very high. However, this was adequately compensated for by Zeeman background correction. This level of potassium addition, which is similar to that used in flame emission spectrometry, was chosen as the optimum. When potassium chloride was used in the presence of nitric acid, the characteristics were similar to those produced with potassium nitrate. Sturgeon et al. [ 121 reported that, because the thermionic work function for graphite is 4.6 eV, only those elements with a lower ionization potential (i.p.) would be significantly affected by the presence of an ionization suppressor. These elements are cesium (i.p. = 3.9 eV), rubidium (4.2 eV) and potassium (4.3 eV). The thermionic work functions for other electrothermal atomizer materials are 4.2 eV for tantalum [13] and molybdenum [14] and 4.5 eV for tungsten [15]. Similar ionization behaviour would therefore be expected with devices made from these materials. Cesium absorbance was also found to be significantly enhanced by addition of rubidium. However, the high level of cesium impurity in the rubidium prevented the determination of cesium at low levels.
310
06.
2000
b
2300 Temperature
(‘c)
2600
Fig. 3. Absorbance temperature profiles for 50 rg Cs 1“ in 5% nitric acid measured at 852.1 nm: (a) alone; (b) in presence of 1000 mg K 1-l. Curve (c) is the response for solution (b) measured at 404.4 nm (K lamp).
Atomic a bsorp tionhempera ture profiles. Figure 3 compares the atomic absorption/temperature profiles for the atomization of cesium in the presence of nitric acid with and without the addition of potassium. Also included is the atomic absorption profile for the potassium. A substantial enhancement of the cesium absorbance can be seen in the presence of potassium. The Saha equation [16] predicts that at 2270°C the approximate temperature at which the enhanced cesium peak occurs, ca. 90% of the cesium should be ionized. From the heights of peaks (a) and (b), the degree of ionization is estimated by the method of Manning and Capacho-Delgado [ 171 to be 70-75%. Some of the confusion about ionization in electrothermal atomizers has arisen because their materials of construction produce a background concentration of electrons which affects the ionization of analyte elements [ 141. This is why the degree of ionization of cesium was less than that expected. In flames, the populations of ground-state atoms, excited atoms and ions are in local equilibrium because there is a constant flow of sample into a fixed temperature atomizer. In electrothermal atomizers, these conditions do not prevail and the processes of absorption, emission and ionization reach a maximum in sequence as the temperature rises. This temperature difference explains why the degree of ionization of an analyte element is considerably lower when measured by absorption [8] than by emission [9]. Ionization is known to be greatest after the atom peak has occurred [8]. The observation of the cesium absorption peak at a lower temperature in the presence of potassium may be due to the coincidence of that temperature with the maximum electron density produced by the ionization of potassium. Analytical method. A calibration graph obtained from 16 points in the range O-50 pg 1-l cesium was linear, with a slope of 0.1025 + 0.0021 absorbance/pg 1-l. The characteristic concentration (sensitivity) derived from the calibration slope was 0.44 pg I-‘, and the characteristic mass was 18 pg of cesium. The reproducibility and detection limit were evaluated for 5 and 50 pg 1-l solutions of cesium, respectively. Ten replicate measurements of each solu-
311 TABLE 2 Comparison of results on leach test solutions by flame atomic emission (a.e.s.) and electrothermal atomic absorption spectrometry Sample no. Cs found (rcg 1-l) A.e.s. This work
1
2
3
4
5
33 37
17 19
15 19
14 12
<2 <2
tion gave relative standard deviations of 20 and 2%, respectively. Each result corresponds to a detection limit of 2 pg 1-l cesium, which is adequate for the screening of leach test solutions. Table 2 compares the results obtained for several samples which had previously been analysed by flame atomic emission spectrometry. Conclusions There is substantial evidence to suggest that the improved sensitivity for cesium results from ionization suppression by the large added excess of potassium. This suppression requires a much larger excess of buffer than has been used in most previous investigations of ionization under such conditions. The very high non-specific background absorption signals which then arise can be compensated for by using Zeeman correction. The method has proved suitable for screening leach test solutions for cesium content, particularly when sample volume is a restriction. REFERENCES 1 A. E. Ringwood, Safe Disposal of High Level Nuclear Reactor Wastes; A New Strategy, Australian National University Press, Canberra, 1978. 2 K. D. Reeve, D. M. Levins, E. J. Ramm, J. L. Woolfrey, W. J. Buykx, R. K. Ryan and J. F. Chapman, At. Energy Aust., 24 (1981) 1. 3 W. G. Schrenk and R. T. Everson, Appl. Spectrosc., 29 (1975) 41. 4 M. S. Epstein, T. C. Rains and T. C. O’Haver, Appl. Spectrosc., 30 (1976) 324. 5 P. Frigieri, R. Trucco, I. Ciaccolini and G. Pampurini, Analyst, 105 (1980) 651. 6 Z. Grobenski, D. Weber, B. Welz and J. Wolff, Analyst, 108 (1983) 925. 7 J. M. Ottaway and F. Shaw, Analyst, 101 (1976) 582. 8 R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part B, 32 (1977) 231. 9 R. E. Sturgeon and S. S. Berman, Anal. Chem., 53 (1981) 632. 10 J. M. Ottaway and F. Shaw, Analyst, 100 (1975) 438. 11 J. C. Bailar and A. F. Trotman-Dickenson (Eds.), Comprehensive Inorganic Chemistry, Pergamon Press, Oxford, 1973, p. 478. 12 R. E. Sturgeon, S. S. Berman and S. Kashyap, Anal. Chem., 52 (1980) 1049. 13 M. D. Friske, Phys. Rev., 61 (1942) 513. 14 R. W. Wright, Phys. Rev., 60 (1941) 465. 15 G. F. Smith, Phys. Rev., 94 (1954) 295. 16 M. N. Saha and N. K. Saha, A Treatise on Modern Physics, Vol. 1, Indian Press, Calcutta, 1934. 17 D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36 (1966) 312.
Short Communication
IMPROVED CESIUM SENSITIVITY ABSORPTION SPECTROMETRY
JAMES F. CHAPMAN* and LESLIE
IN ELECTROTHERMAL
ATOMIC
S. DALE
CSIRO Division of Energy Chemistry, Lucas Heights Research Illawarra Road, Lucas Heights, NSW 2234 (Australia)
Laboratories,
New
SHARON A. TOPHAM AAEC Materials Division, Lucas Heights Research Lucas Heights, NSW 2234 (Australia)
Laboratories,
New Illawara
Road,
(Received 23rd April 1986)
Summary, The sensitivity for cesium determination by electrothermal atomic absorption spectrometry is improved four-fold by the addition of a large excess of potassium nitrate. Zeeman background correction is used to compensate for the large non-specific absorption signal resulting from the potassium. The characteristic concentration and detection limit are 0.44 and 2 rg l-l, respectively, and the coefficient of variation IS 2% at the 50 rg 1-l level. The procedure is suitable for the rapid determination of cesium in leach solutions from nuclear waste fixation experiments.
In studies of nuclear waste fixation, radioactive waste is incorporated into inert materials such as Synroc [l, 21. To assess the stability of these materials, aqueous leach tests were applied initially on materials containing simulated inactive waste. Because cesium is one of the most chemically mobile of the radioactive waste elements, its determination at low concentrations is important in leachability studies of these materials. In this laboratory, cesium has been determined in leach solutions by flame atomic emission spectrometry with potassium as an ionization suppressor. A 20-fold increase in sensitivity was achieved, leading to a detection limit of 2 pg 1-l. Although the procedure was satisfactory for inactive samples, it could not be applied to radioactive samples leached from inert materials containing actinides because of the potential hazard of aspirating a radioactive solution into a conventional burner/nebulizer system. Electrothermal atomic absorption spectrometry offered a better procedure because, with the small volumes used, contamination of equipment would be minimal and the small volume of vapour generated could be contained. However, because the detection limit for cesium for the instrument available was 8 pg l-‘, it was insufficient to provide leach rate data comparable with that obtained in the inactive leach tests. To improve the sensitivity, the addition of potassium as an ionization suppressant was considered. Studies of ionization in electrothermal atomizers have been undertaken by 0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
308
numerous workers [3-g]. Some of them observed slight enhancements of the analyte signal in the presence of added buffer [3, 41, whereas others found no enhancement [5-71. More detailed studies suggested that ionization is negligible owing to the high population of background electrons generated by the graphite [8, 91 and small compared to that obtained in conventional flames owing to the different time- and temperature-dependence of the atom and ion populations [9]. In several of these investigations, the conclusions were drawn from the results of experiments in which either the atom excess of buffer was small, or volatilization losses may have been caused by the use of chloride media. The present investigations into the use of potassium to improve the sensitivity for cesium show that with a large excess of potassium, the atomic absorption signal for cesium can be increased by a factor of four. In view of this, and because of conclusions reached by previous workers, the enhancement effect of potassium on cesium was studied in more detail. Experimental Apparatus and reagents. All measurements were made on a Hitachi Polarized Zeeman atomic absorption spectrometer, model 7000, equipped with a pyrolytic graphite tube cuvette and autosampler. All absorbance measurements were taken in the peak-height mode with Zeeman background correction. They were displayed and recorded on a Hitachi AA data processor. The spectral bandpass was 1.3 nm. Hollow-cathode lamps were operated at their recommended currents. All chemicals and acids were of analytical-reagent grade. Water from a Millipore Mill&Q purification system was used. Procedure. To 1.00 ml of sample was added 0.10 ml of potassium nitrate modifier solution (1.000 g of potassium as its nitrate, made up to 100 ml in 1 + 1 nitric acid). Portions (40 ~1) were injected into the graphite furnace. The heating program is shown in Table 1. The argon carrier gas flow of 200 ml min-’ was decreased during the atomization cycle to 30 ml min-‘. The peak-height absorbance at 852.1 nm was measured. Results and discussion Potassium addition. Poor sensitivity was obtained from a 50 pg 1~’ cesium solution, and there was no increase in cesium absorbance when potassium TABLE 1 Conditions used for cesium determination metry Step
1. 2. 3.
Temperature (“C)
Dry Dry Ash
Start
End
80 150 500
150 500 500
Time(s)
50 10 10
by electrothermal
Temperature (“C)
Step
4. 5. 6.
atomic absorption spectro-
Atomize Clean Cool
Start
End
2000 3000 -
2600 3000 -
Time(s)
6 3 30
309
06. 06
0
2
4
6
a
0
Nitric acld (vol %I
Fig. 1. Effect of nitric acid concentration
on cesium absorbance.
Fig. 2. Effect of potassium concentration
on cesium absorbance.
1000 Fotassium (mg ~~‘1
2000
chloride was added. However, when potassium nitrate was added, the cesium signal was significantly enhanced. The use of nitrate media to minimize interferences has been recommended by Ottaway and Shaw [lo]. The effect of increasing concentrations of nitric acid on a 50 pg 1-l cesium solution was studied; the results are shown in Fig. 1. The reason for the dip in the curve before the plateau region is not clear but it may be due to different volatilization rates of compounds such as CsN03.HN03 and CsN03*2HN03 [ 111. Because the plateau region occurs above ca. 3% (v/v) nitric acid, 5% was chosen for subsequent investigations. The effect of increasing the potassium concentration in the nitric acid was then studied. Figure 2 shows the change in response of 50 r_1gl-’ cesium in 5% nitric acid with increasing potassium concentration. The plateau region occurs at 1000 mg 1-l potassium, an atom ratio (K/es) of ca. 68,000. The background absorption signal resulting from the volatilization of the added potassium was very high. However, this was adequately compensated for by Zeeman background correction. This level of potassium addition, which is similar to that used in flame emission spectrometry, was chosen as the optimum. When potassium chloride was used in the presence of nitric acid, the characteristics were similar to those produced with potassium nitrate. Sturgeon et al. [ 121 reported that, because the thermionic work function for graphite is 4.6 eV, only those elements with a lower ionization potential (i.p.) would be significantly affected by the presence of an ionization suppressor. These elements are cesium (i.p. = 3.9 eV), rubidium (4.2 eV) and potassium (4.3 eV). The thermionic work functions for other electrothermal atomizer materials are 4.2 eV for tantalum [13] and molybdenum [14] and 4.5 eV for tungsten [15]. Similar ionization behaviour would therefore be expected with devices made from these materials. Cesium absorbance was also found to be significantly enhanced by addition of rubidium. However, the high level of cesium impurity in the rubidium prevented the determination of cesium at low levels.
310
06.
2000
b
2300 Temperature
(‘c)
2600
Fig. 3. Absorbance temperature profiles for 50 rg Cs 1“ in 5% nitric acid measured at 852.1 nm: (a) alone; (b) in presence of 1000 mg K 1-l. Curve (c) is the response for solution (b) measured at 404.4 nm (K lamp).
Atomic a bsorp tionhempera ture profiles. Figure 3 compares the atomic absorption/temperature profiles for the atomization of cesium in the presence of nitric acid with and without the addition of potassium. Also included is the atomic absorption profile for the potassium. A substantial enhancement of the cesium absorbance can be seen in the presence of potassium. The Saha equation [16] predicts that at 2270°C the approximate temperature at which the enhanced cesium peak occurs, ca. 90% of the cesium should be ionized. From the heights of peaks (a) and (b), the degree of ionization is estimated by the method of Manning and Capacho-Delgado [ 171 to be 70-75%. Some of the confusion about ionization in electrothermal atomizers has arisen because their materials of construction produce a background concentration of electrons which affects the ionization of analyte elements [ 141. This is why the degree of ionization of cesium was less than that expected. In flames, the populations of ground-state atoms, excited atoms and ions are in local equilibrium because there is a constant flow of sample into a fixed temperature atomizer. In electrothermal atomizers, these conditions do not prevail and the processes of absorption, emission and ionization reach a maximum in sequence as the temperature rises. This temperature difference explains why the degree of ionization of an analyte element is considerably lower when measured by absorption [8] than by emission [9]. Ionization is known to be greatest after the atom peak has occurred [8]. The observation of the cesium absorption peak at a lower temperature in the presence of potassium may be due to the coincidence of that temperature with the maximum electron density produced by the ionization of potassium. Analytical method. A calibration graph obtained from 16 points in the range O-50 pg 1-l cesium was linear, with a slope of 0.1025 + 0.0021 absorbance/pg 1-l. The characteristic concentration (sensitivity) derived from the calibration slope was 0.44 pg I-‘, and the characteristic mass was 18 pg of cesium. The reproducibility and detection limit were evaluated for 5 and 50 pg 1-l solutions of cesium, respectively. Ten replicate measurements of each solu-
311 TABLE 2 Comparison of results on leach test solutions by flame atomic emission (a.e.s.) and electrothermal atomic absorption spectrometry Sample no. Cs found (rcg 1-l) A.e.s. This work
1
2
3
4
5
33 37
17 19
15 19
14 12
<2 <2
tion gave relative standard deviations of 20 and 2%, respectively. Each result corresponds to a detection limit of 2 pg 1-l cesium, which is adequate for the screening of leach test solutions. Table 2 compares the results obtained for several samples which had previously been analysed by flame atomic emission spectrometry. Conclusions There is substantial evidence to suggest that the improved sensitivity for cesium results from ionization suppression by the large added excess of potassium. This suppression requires a much larger excess of buffer than has been used in most previous investigations of ionization under such conditions. The very high non-specific background absorption signals which then arise can be compensated for by using Zeeman correction. The method has proved suitable for screening leach test solutions for cesium content, particularly when sample volume is a restriction. REFERENCES 1 A. E. Ringwood, Safe Disposal of High Level Nuclear Reactor Wastes; A New Strategy, Australian National University Press, Canberra, 1978. 2 K. D. Reeve, D. M. Levins, E. J. Ramm, J. L. Woolfrey, W. J. Buykx, R. K. Ryan and J. F. Chapman, At. Energy Aust., 24 (1981) 1. 3 W. G. Schrenk and R. T. Everson, Appl. Spectrosc., 29 (1975) 41. 4 M. S. Epstein, T. C. Rains and T. C. O’Haver, Appl. Spectrosc., 30 (1976) 324. 5 P. Frigieri, R. Trucco, I. Ciaccolini and G. Pampurini, Analyst, 105 (1980) 651. 6 Z. Grobenski, D. Weber, B. Welz and J. Wolff, Analyst, 108 (1983) 925. 7 J. M. Ottaway and F. Shaw, Analyst, 101 (1976) 582. 8 R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part B, 32 (1977) 231. 9 R. E. Sturgeon and S. S. Berman, Anal. Chem., 53 (1981) 632. 10 J. M. Ottaway and F. Shaw, Analyst, 100 (1975) 438. 11 J. C. Bailar and A. F. Trotman-Dickenson (Eds.), Comprehensive Inorganic Chemistry, Pergamon Press, Oxford, 1973, p. 478. 12 R. E. Sturgeon, S. S. Berman and S. Kashyap, Anal. Chem., 52 (1980) 1049. 13 M. D. Friske, Phys. Rev., 61 (1942) 513. 14 R. W. Wright, Phys. Rev., 60 (1941) 465. 15 G. F. Smith, Phys. Rev., 94 (1954) 295. 16 M. N. Saha and N. K. Saha, A Treatise on Modern Physics, Vol. 1, Indian Press, Calcutta, 1934. 17 D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36 (1966) 312.