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Quantitative determination of nickel and copper in zirconium fluoride using graphite furnace atomic absorption spectrophotometry
MICROCHEMICAL
JOURNAL
Quantitative
J.
41,
12 (I%‘@
Determination of Nickel and Copper in Zirconium Fluoride Using Graphite Furnace Atomic Absorption Spectrophotometry K. J. EWING, E. A. BUCKLEY, 3 L. PEITERSEN,~ AND I.D. AGGARWAL
JAGANATHAN,“~
Optical
l&l
Sciences
Division,
Code
6505, Naval
Research
Laboratory,
Washington,
DC 20375
Received September 14, 1989; accepted September 26, 1989 A graphite furnace atomic absorption spectrophotometer (GFAAS) with Zeeman-effect background correction has been used for the determination of nickel and copper at very low level concentrations in a 30% (w/v) solution of zirconium fluoride. Using palladium nitrate and nitric acid as matrix modifiers, the detection limits for nickel and copper were determined to be 6.3 and 3.2 rig/g, respectively. A specially made graphite platform was used for this study because it was found to withstand the drastic conditions of analysis for a longer period of time than a graphite tube alone. The technique has been validated by recovery studies on spiked samples and by comparing the slopes of standard addition and calibration curves. The precision of the procedure was significant and the relative standard deviation 0 1990 Academic Press, Inc. was < 10% for all the results.
INTRODUCTION Heavy metal fluoride glasses composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides, the so-called ZBLAN composition, have been proposed for use in very long, repeaterless fiber communication links (I). The intrinsic loss in such a fiber has been calculated to be 0.01 dB/km at a wavelength of 2.55 l.rrn, a full order of magnitude lower than that of silica fibers (2). However, achievement of the intrinsic loss in a fluoride fiber requires that the fiber contain extremely low levels of iron, cobalt, nickel, copper, and neodymium. These impurities exhibit absorption bands which impinge on the wavelength of lowest-loss in the ZBLAN glass system. Maximum impurity concentrations necessary for achievement of the theoretical loss of 0.01 dB/km in ZBLAN fibers have been calculated (3) using impurity absorptivities in the fluoride glass matrix (4, 5). The required concentrations of impurities in individual metal fluorides as well as the glass were determined to be on the order of tens to hundreds of picograms per gram. It is evident that quantitation of such low impurity concentrations in a difficult matrix are a significant challenge to the analyst. At the United States Naval Research Laboratory (NRL) research into the preparation of high purity metal fluorides and quantitative determination of impurities at or below nanogram per gram levels in both the individual metal fluorides and the glass is under way. Matrix modifiers are commonly used to reduce background interferences asso’ Present address: Geo-Centers, Inc., Fort Washington, MD 20744. ’ To whom correspondence should be addressed. 3 Present address: Virginia Polytechnic Institute and State University, 4 Present address: Sachs Freeman Assoc., Landover, MD 20785. 106 0026265)(/90 Copyright All lights
$1.50
0 1990 by Academic Rem. Inc. of reproductionin any form reserved.
Blacksburg,
VA 24061.
DETERMINATION
107
OF Ni AND Cu IN ZrF4
ciated with matrix volatilization in determination of trace levels of impurities via graphite furnace atomic absorption spectrophotometry (GFAAS). Use of palladium nitrate as a modifier (6) increases the atomization temperature of the analyte species, allowing for complete volatilization of the matrix component at a temperature lower than that of the analyte. The mechanism for the increased volatilization temperature has been shown to be due to the formation of an alloy between the analyte and palladium. For example, X-ray photoelectron spectrometry (7) of the surface of a graphite tube used in trace analysis of lead and bismuth showed the existence of PbPd and Bi-Pd bonds when palladium nitrate was used as a matrix modifier. Likewise, electron microprobe analysis (8) in the analysis of selenium using palladium nitrate as matrix modifier revealed that selenium and palladium exist in a 1:1 mole ratio on the surface of the graphite tube. It was postulated that palladium and selenium form an alloy which volatilizes at higher temperatures than selenium alone. Matrix modifiers have also been used for direct analysis of solids via GFAAS to remove the matrix before analysis. Good analytical results utilizing second surface atomizers, such as a graphite cup (9, IO) or a tantalum insert (II), placed inside the graphite tube have been obtained for solid samples. The present work involves the analysis of high solid content solutions of zirconium fluoride for part per billion (rig/g) levels of the transition elements nickel and copper via GFAAS with Zeeman background correction. The matrix modifiers palladium nitrate and nitric acid are used in conjunction with a second surface atomizer to attain optimum detection limits for this system. MATERIALS
AND METHODS
Instrumenration. A Perkin-Elmer Zeeman 5100 atomic absorption spectrophotometer and HGA-600 graphite furnace equipped with an AS-60 autosampler and a PR-310 printer were used for this work. The optimum instrumental parameters for both elements are given in Tables 1 and 2. Hollow cathode lamps of nickel and copper were used at the wavelengths of 232.0 and 324.8 nm, respectively. Only peak area measurements were used for this study since the precision was better. High purity argon (99.99%) was used as the purging gas during drying, ashing, and atomization. Purging with argon at 20 ml/min in the atomization step was necessary to reduce the redeposition of ashes on the platform. Choice of tube. Preliminary work was carried out with pyrocoated graphite TABLE 1 GFAAS Heating Program for Nickel Step
Temp, “C Ramp, s Hold, s Internal gas flow, mlhin
1
2
3
4
5
120 5 40 300
1400 10 50 300
2500 0 5 40
2600 2 3 300
20 1 20 300
108
JAGANATHAN
GFAAS
ET AL.
TABLE 2 Heating hogram
for Copper Step
Temp, “C Ramp, s Hold, s Internal gas flow, mlhnin
1
2
3
4
5
120 5 40 300
1000 10 50 300
2300 0 5 40
2400 2 3 300
20 1 20 300
tubes, L’vov platforms, and standard graphite tubes. Satisfactory sensitivity and precision were obtained only with the standard graphite tube. The pyrocoated tubes and the L’vov platforms rapidly deteriorated under the drastic conditions used for analysis. The uncoated graphite tube was stable for about 40 firings after which time the sensitivity began to decrease. The analyte signal may be suppressed by carbon deposits formed as a result of corrosion of the graphite tube thereby reducing the sensitivity. It became necessary to replace the tube at this stage. In order to increase the number of firings possible a second surface atomizer was employed. An uncoated graphite tube was cut lengthwise into small strips, 1.6 cm in length and 0.4 cm in width. A strip was then inserted into a standard tube and positioned carefully for the autosampler. The combination of a standard graphite tube and an uncoated graphite platform increased not only the precision but also the number of firings possible, from 40 to 200, before replacement of the platform became necessary. Preparation of samples and standards. The zirconium fluoride used for this study was prepared in our laboratory by reacting ZrOCl, with aqueous HF (45.9%) producing ZrF, * Hz0 (22). A solution of this hydrated zirconium fluoride containing 30.74% solids was prepared in aqueous HF (22.95%). The matrix modifier Pd(NO,), was dissolved in HN03 (0.8 M) to give a solution containing 1000 ppm Pd which was used as a stock solution for preparing solutions of lower concentration. For this study, a 450 ppm Pd solution was prepared by appropriate dilution with deionized water. The second modifier, HNOs (1 M) was prepared from Ultrex nitric acid (70.0%). The standards for nickel and copper were prepared from 1000 ppm stock solutions obtained from Aldrich Chemical Co. The autosampler was programmed to inject 20 pl of the sample and 10 ~1 of each of the matrix modifiers. Each solution was pipetted out separately onto the graphite platform, cut specially for this study before the HGA program started. Spiking of zirconium fluoride. Calibration curves for both elements were obtained from standard solutions containing 10,20, and 30 ppb of each analyte. For comparison, standard addition curves were obtained from spiked solutions of ZrF,. Exactly 10, 20, and 30 pl of a 1 ppm standard solution of the analyte were added to 990,980, and 970 ~1 of ZrF, resulting in analyte concentrations of 10,20, and 30 ppb, respectively. Recovery studies were carried out separately in six replicates for each concentration of the spiked solutions. The calculation of percentage recovery was based on the formula [analyte found (ppb) - analyte present (ppb)] x lOO/analyte added (ppb).
DETERMINATION
OF Ni AND Cu IN ZrF4
109
RESULTS AND DISCUSSION Calibration and detection limits. Calibration of aqueous standards of nickel and copper in 0.1 M HN03 was carried out by doing triplicate measurements on 10, 20, and 30 ppb analyte concentrations. The detection limits for both elements in aqueous standards were calculated using the formula, concentration (ppb) x 2 x standard deviation/analyte absorbance. For determination of detection limits in a zirconium fluoride matrix, spiked solutions containing 20 ppb Ni and 20 ppb Cu in 30.74% ZrF, were analyzed. The average concentrations of analytes recovered from six consecutive replicates were used to calculate the detection limits of 6.3 -+ 0.4 rig/g for nickel and 3.2 2 0.2 rig/g for copper. The precision (relative standard deviation, RSD) was
30.74% (w/v) 4 22.6 ppb Standard addition 5.8% 7.6%
30.74% (w/v) 4 21.2 ppb Aqueous calibration 4.6% 5.2%
110
JAGANATHAN
ET AL.
TABLE 4 Summary for Determination of Copper in Zirconium Fluoride Concn. of ZrF, Number of samples Average Cu concn Method of analysis RSD” of analysis RSD of six consecutive measurements
30.74% 4
16.5 ppb Standard addition 4.7% 6.3%
30.74% (w/v) 4
15.8 ppb Aqueous calibration 5.2% 5.7%
a Relative standard deviation.
combined use of Pd(N03)2 and HNO, as matrix modifiers effectively minimized the matrix problem for both nickel and copper. Evaluation ofthe method. The concentrations of nickel and copper in the ZrF, solution were determined both by standard addition and calibration methods. The standard addition lines were obtained from ZrF., solutions spiked with standard analytes. The spiked solutions were not diluted further with water as we wanted to check the effect of the matrix modifiers on ZrF, matrix as well. The results from both methods are given in Tables 3 and 4. The two curves for nickel are shown in Fig. 1. The linear regression equations for calibration and standard addition curves are y = 0.00145x + 0.0013 and y = 0.00155~ + 0.035, respectively. This results in a close agreement between the two values for the concentration of nickel, 21.2 ppb from the calibration curve and 22.6 ppb from the standard addition. A difference of 6.6% between these two values strongly indicates that the matrix modifiers are effective in the removal of the interferences. A similar effect is observed in the determination of copper. Figure 2 shows the calibration curve
p g/L Ni FIG. 1. Analytical curve for nickel in aqueous reference solutions (x) and spiked zirconium fluoride
solutions (0).
DETERMINATION
I
I
I
-30
-20
-10
111
OF Ni AND Cu IN ZrF4
0
1
I
I
10
20
30
pg/L cu FIG. 2. Analytical curves for copper in aqueous reference solutions (X) and spiked zirconium fluoride solutions (0).
withy = 0.0033x + 0.0017 and the standard addition curve withy = 0.0034x + 0.0558. The slopes of these curves differ very little and consequently the concentration values from the two methods, 15.8 ppb from the calibration and 16.5 ppb from the standard addition, differ by only 4.4%. In addition, the recovery studies for both elements (Tables 5 and 6) show that Pd(NO,), and HNO, restore the sensitivity to the same level as in aqueous standards. As mentioned previously, it was possible to raise the pyrolysis temperatures to 1000°Cfor copper and 1400°C for nickel without losing the analyte in the process. Most of the complex matrices are eliminated at these high temperatures, thus reducing the interferences. It is still not certain how palladium stabilizes the analytes during pyrolysis. As previously mentioned, palladium has been shown to form alloys with analyte species on the graphite tube thereby preventing significant loss of analyte during the pyrolysis step (7, 8). Whether this is the mechanism operative in this case is still open to investigation. Addition of nitric acid as a second modifier reduces the background absorption further than that obtained with Pd(NO,), only. A possible TABLE 5 Recovery Data for Zirconium Fluoride Spiked with Nickel Ni (&liter) Sample
Present
Added
Found
% Recovery”
1
22 22 22
10
33 42 53
103
20 30
2 3
100 102
LIEach value is an average of six replicates with a relative standard deviation of 6%.
112
JAGANATHAN
ET AL.
TABLE 6 Recovery Data for Zirconium Fluoride Spiked with Copper Cu (p&Iiter) Sample
Present
Added
Found
1 2 3
16 16 16
10 20 30
26 31 47
% Recovery” 108 103 102
o Each value is an average of six replicates with a relative standard deviation of 5%.
explanation is that nitric acid reacts with the fluoride matrix during pyrolysis forming volatile fluorine compounds, thus reducing the background absorption. CONCLUSIONS
Quantitative procedures for the determination of nickel and copper in ZrF, at trace levels have been developed. Solutions of ZrF, with concentrations as high as 30.74% (w/v) can be readily analyzed via GFAAS with Zeeman background correction. The use of Pd(N03)2 and HNO, as matrix modifiers greatly alleviates the problem due to interferences. It has been found that the use of an uncoated graphite platform cut out of the standard graphite tube is stable up to 200 firings under these conditions compared to the graphite tubes which are corroded quickly. Additional work is necessary to establish the mechanism involving the analytes and the modifiers. REFERENCES 1. Tran, D. C.; Sigel, G. H., Jr.; Bendow, B. J. &&wave Tech., 1984, LT-2(5), 566-586. 2. Shibata, S.; Horigochi, M.; Junguji, K.; Mitachi, S.; Kanamori, T.; Manabe, T. Electron. Lett., 1981, 17, 775-117. 3. Ewing, K. J.; Buckner, L.; Jaganathan, J.; Ginther, R.; Aggarwal, I. D. Mater. Res. Bull., 1989, 24, 163-168. 4. Ohishi, Y.; Mitachi, S.; Kanamori, T.; Manabe, T. Phys. Chem. Glasses, 1983,24, 135-140. 5. France, P. W.; Carter, S. F.; Moore, M. W.; Day, C. R. J. Brit. Telecom. Technol., 1987, S(2). 6. Schlemmer, G.; Wely, B. Spectrochim. Acta Part B, 1986, 41, 1157-1165. 7. Shan, X. Q.; Wang, D. X. Anal. Chim. Acta, 1985, 173, 315-319. 8. Teague-Nishimura, J. E.; Tominaga, T.; Katsura, T.; Matsumoto, K. Anal. Chem., 1987, 59, 1647-1651. 9. Price, W. J.; Dymott, T. C.; Whiteside, P. J. Spectrochim. Acta Par? B, 1980, 35, 3-10. 10. Akatsuka, K.; Atsuya, I. Anal. Chem., 1989, 61, 216-220. 11. Retberg, T. M.; Holcombe, J. A. Anal. Chem., 1986, 58, 1462-1467. 12. Sommers, J. A.; Perkins, V. Q. Mater. Sci. Forum, 1988, 32-33, 629633.
JOURNAL
Quantitative
J.
41,
12 (I%‘@
Determination of Nickel and Copper in Zirconium Fluoride Using Graphite Furnace Atomic Absorption Spectrophotometry K. J. EWING, E. A. BUCKLEY, 3 L. PEITERSEN,~ AND I.D. AGGARWAL
JAGANATHAN,“~
Optical
l&l
Sciences
Division,
Code
6505, Naval
Research
Laboratory,
Washington,
DC 20375
Received September 14, 1989; accepted September 26, 1989 A graphite furnace atomic absorption spectrophotometer (GFAAS) with Zeeman-effect background correction has been used for the determination of nickel and copper at very low level concentrations in a 30% (w/v) solution of zirconium fluoride. Using palladium nitrate and nitric acid as matrix modifiers, the detection limits for nickel and copper were determined to be 6.3 and 3.2 rig/g, respectively. A specially made graphite platform was used for this study because it was found to withstand the drastic conditions of analysis for a longer period of time than a graphite tube alone. The technique has been validated by recovery studies on spiked samples and by comparing the slopes of standard addition and calibration curves. The precision of the procedure was significant and the relative standard deviation 0 1990 Academic Press, Inc. was < 10% for all the results.
INTRODUCTION Heavy metal fluoride glasses composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides, the so-called ZBLAN composition, have been proposed for use in very long, repeaterless fiber communication links (I). The intrinsic loss in such a fiber has been calculated to be 0.01 dB/km at a wavelength of 2.55 l.rrn, a full order of magnitude lower than that of silica fibers (2). However, achievement of the intrinsic loss in a fluoride fiber requires that the fiber contain extremely low levels of iron, cobalt, nickel, copper, and neodymium. These impurities exhibit absorption bands which impinge on the wavelength of lowest-loss in the ZBLAN glass system. Maximum impurity concentrations necessary for achievement of the theoretical loss of 0.01 dB/km in ZBLAN fibers have been calculated (3) using impurity absorptivities in the fluoride glass matrix (4, 5). The required concentrations of impurities in individual metal fluorides as well as the glass were determined to be on the order of tens to hundreds of picograms per gram. It is evident that quantitation of such low impurity concentrations in a difficult matrix are a significant challenge to the analyst. At the United States Naval Research Laboratory (NRL) research into the preparation of high purity metal fluorides and quantitative determination of impurities at or below nanogram per gram levels in both the individual metal fluorides and the glass is under way. Matrix modifiers are commonly used to reduce background interferences asso’ Present address: Geo-Centers, Inc., Fort Washington, MD 20744. ’ To whom correspondence should be addressed. 3 Present address: Virginia Polytechnic Institute and State University, 4 Present address: Sachs Freeman Assoc., Landover, MD 20785. 106 0026265)(/90 Copyright All lights
$1.50
0 1990 by Academic Rem. Inc. of reproductionin any form reserved.
Blacksburg,
VA 24061.
DETERMINATION
107
OF Ni AND Cu IN ZrF4
ciated with matrix volatilization in determination of trace levels of impurities via graphite furnace atomic absorption spectrophotometry (GFAAS). Use of palladium nitrate as a modifier (6) increases the atomization temperature of the analyte species, allowing for complete volatilization of the matrix component at a temperature lower than that of the analyte. The mechanism for the increased volatilization temperature has been shown to be due to the formation of an alloy between the analyte and palladium. For example, X-ray photoelectron spectrometry (7) of the surface of a graphite tube used in trace analysis of lead and bismuth showed the existence of PbPd and Bi-Pd bonds when palladium nitrate was used as a matrix modifier. Likewise, electron microprobe analysis (8) in the analysis of selenium using palladium nitrate as matrix modifier revealed that selenium and palladium exist in a 1:1 mole ratio on the surface of the graphite tube. It was postulated that palladium and selenium form an alloy which volatilizes at higher temperatures than selenium alone. Matrix modifiers have also been used for direct analysis of solids via GFAAS to remove the matrix before analysis. Good analytical results utilizing second surface atomizers, such as a graphite cup (9, IO) or a tantalum insert (II), placed inside the graphite tube have been obtained for solid samples. The present work involves the analysis of high solid content solutions of zirconium fluoride for part per billion (rig/g) levels of the transition elements nickel and copper via GFAAS with Zeeman background correction. The matrix modifiers palladium nitrate and nitric acid are used in conjunction with a second surface atomizer to attain optimum detection limits for this system. MATERIALS
AND METHODS
Instrumenration. A Perkin-Elmer Zeeman 5100 atomic absorption spectrophotometer and HGA-600 graphite furnace equipped with an AS-60 autosampler and a PR-310 printer were used for this work. The optimum instrumental parameters for both elements are given in Tables 1 and 2. Hollow cathode lamps of nickel and copper were used at the wavelengths of 232.0 and 324.8 nm, respectively. Only peak area measurements were used for this study since the precision was better. High purity argon (99.99%) was used as the purging gas during drying, ashing, and atomization. Purging with argon at 20 ml/min in the atomization step was necessary to reduce the redeposition of ashes on the platform. Choice of tube. Preliminary work was carried out with pyrocoated graphite TABLE 1 GFAAS Heating Program for Nickel Step
Temp, “C Ramp, s Hold, s Internal gas flow, mlhin
1
2
3
4
5
120 5 40 300
1400 10 50 300
2500 0 5 40
2600 2 3 300
20 1 20 300
108
JAGANATHAN
GFAAS
ET AL.
TABLE 2 Heating hogram
for Copper Step
Temp, “C Ramp, s Hold, s Internal gas flow, mlhnin
1
2
3
4
5
120 5 40 300
1000 10 50 300
2300 0 5 40
2400 2 3 300
20 1 20 300
tubes, L’vov platforms, and standard graphite tubes. Satisfactory sensitivity and precision were obtained only with the standard graphite tube. The pyrocoated tubes and the L’vov platforms rapidly deteriorated under the drastic conditions used for analysis. The uncoated graphite tube was stable for about 40 firings after which time the sensitivity began to decrease. The analyte signal may be suppressed by carbon deposits formed as a result of corrosion of the graphite tube thereby reducing the sensitivity. It became necessary to replace the tube at this stage. In order to increase the number of firings possible a second surface atomizer was employed. An uncoated graphite tube was cut lengthwise into small strips, 1.6 cm in length and 0.4 cm in width. A strip was then inserted into a standard tube and positioned carefully for the autosampler. The combination of a standard graphite tube and an uncoated graphite platform increased not only the precision but also the number of firings possible, from 40 to 200, before replacement of the platform became necessary. Preparation of samples and standards. The zirconium fluoride used for this study was prepared in our laboratory by reacting ZrOCl, with aqueous HF (45.9%) producing ZrF, * Hz0 (22). A solution of this hydrated zirconium fluoride containing 30.74% solids was prepared in aqueous HF (22.95%). The matrix modifier Pd(NO,), was dissolved in HN03 (0.8 M) to give a solution containing 1000 ppm Pd which was used as a stock solution for preparing solutions of lower concentration. For this study, a 450 ppm Pd solution was prepared by appropriate dilution with deionized water. The second modifier, HNOs (1 M) was prepared from Ultrex nitric acid (70.0%). The standards for nickel and copper were prepared from 1000 ppm stock solutions obtained from Aldrich Chemical Co. The autosampler was programmed to inject 20 pl of the sample and 10 ~1 of each of the matrix modifiers. Each solution was pipetted out separately onto the graphite platform, cut specially for this study before the HGA program started. Spiking of zirconium fluoride. Calibration curves for both elements were obtained from standard solutions containing 10,20, and 30 ppb of each analyte. For comparison, standard addition curves were obtained from spiked solutions of ZrF,. Exactly 10, 20, and 30 pl of a 1 ppm standard solution of the analyte were added to 990,980, and 970 ~1 of ZrF, resulting in analyte concentrations of 10,20, and 30 ppb, respectively. Recovery studies were carried out separately in six replicates for each concentration of the spiked solutions. The calculation of percentage recovery was based on the formula [analyte found (ppb) - analyte present (ppb)] x lOO/analyte added (ppb).
DETERMINATION
OF Ni AND Cu IN ZrF4
109
RESULTS AND DISCUSSION Calibration and detection limits. Calibration of aqueous standards of nickel and copper in 0.1 M HN03 was carried out by doing triplicate measurements on 10, 20, and 30 ppb analyte concentrations. The detection limits for both elements in aqueous standards were calculated using the formula, concentration (ppb) x 2 x standard deviation/analyte absorbance. For determination of detection limits in a zirconium fluoride matrix, spiked solutions containing 20 ppb Ni and 20 ppb Cu in 30.74% ZrF, were analyzed. The average concentrations of analytes recovered from six consecutive replicates were used to calculate the detection limits of 6.3 -+ 0.4 rig/g for nickel and 3.2 2 0.2 rig/g for copper. The precision (relative standard deviation, RSD) was
30.74% (w/v) 4 22.6 ppb Standard addition 5.8% 7.6%
30.74% (w/v) 4 21.2 ppb Aqueous calibration 4.6% 5.2%
110
JAGANATHAN
ET AL.
TABLE 4 Summary for Determination of Copper in Zirconium Fluoride Concn. of ZrF, Number of samples Average Cu concn Method of analysis RSD” of analysis RSD of six consecutive measurements
30.74% 4
16.5 ppb Standard addition 4.7% 6.3%
30.74% (w/v) 4
15.8 ppb Aqueous calibration 5.2% 5.7%
a Relative standard deviation.
combined use of Pd(N03)2 and HNO, as matrix modifiers effectively minimized the matrix problem for both nickel and copper. Evaluation ofthe method. The concentrations of nickel and copper in the ZrF, solution were determined both by standard addition and calibration methods. The standard addition lines were obtained from ZrF., solutions spiked with standard analytes. The spiked solutions were not diluted further with water as we wanted to check the effect of the matrix modifiers on ZrF, matrix as well. The results from both methods are given in Tables 3 and 4. The two curves for nickel are shown in Fig. 1. The linear regression equations for calibration and standard addition curves are y = 0.00145x + 0.0013 and y = 0.00155~ + 0.035, respectively. This results in a close agreement between the two values for the concentration of nickel, 21.2 ppb from the calibration curve and 22.6 ppb from the standard addition. A difference of 6.6% between these two values strongly indicates that the matrix modifiers are effective in the removal of the interferences. A similar effect is observed in the determination of copper. Figure 2 shows the calibration curve
p g/L Ni FIG. 1. Analytical curve for nickel in aqueous reference solutions (x) and spiked zirconium fluoride
solutions (0).
DETERMINATION
I
I
I
-30
-20
-10
111
OF Ni AND Cu IN ZrF4
0
1
I
I
10
20
30
pg/L cu FIG. 2. Analytical curves for copper in aqueous reference solutions (X) and spiked zirconium fluoride solutions (0).
withy = 0.0033x + 0.0017 and the standard addition curve withy = 0.0034x + 0.0558. The slopes of these curves differ very little and consequently the concentration values from the two methods, 15.8 ppb from the calibration and 16.5 ppb from the standard addition, differ by only 4.4%. In addition, the recovery studies for both elements (Tables 5 and 6) show that Pd(NO,), and HNO, restore the sensitivity to the same level as in aqueous standards. As mentioned previously, it was possible to raise the pyrolysis temperatures to 1000°Cfor copper and 1400°C for nickel without losing the analyte in the process. Most of the complex matrices are eliminated at these high temperatures, thus reducing the interferences. It is still not certain how palladium stabilizes the analytes during pyrolysis. As previously mentioned, palladium has been shown to form alloys with analyte species on the graphite tube thereby preventing significant loss of analyte during the pyrolysis step (7, 8). Whether this is the mechanism operative in this case is still open to investigation. Addition of nitric acid as a second modifier reduces the background absorption further than that obtained with Pd(NO,), only. A possible TABLE 5 Recovery Data for Zirconium Fluoride Spiked with Nickel Ni (&liter) Sample
Present
Added
Found
% Recovery”
1
22 22 22
10
33 42 53
103
20 30
2 3
100 102
LIEach value is an average of six replicates with a relative standard deviation of 6%.
112
JAGANATHAN
ET AL.
TABLE 6 Recovery Data for Zirconium Fluoride Spiked with Copper Cu (p&Iiter) Sample
Present
Added
Found
1 2 3
16 16 16
10 20 30
26 31 47
% Recovery” 108 103 102
o Each value is an average of six replicates with a relative standard deviation of 5%.
explanation is that nitric acid reacts with the fluoride matrix during pyrolysis forming volatile fluorine compounds, thus reducing the background absorption. CONCLUSIONS
Quantitative procedures for the determination of nickel and copper in ZrF, at trace levels have been developed. Solutions of ZrF, with concentrations as high as 30.74% (w/v) can be readily analyzed via GFAAS with Zeeman background correction. The use of Pd(N03)2 and HNO, as matrix modifiers greatly alleviates the problem due to interferences. It has been found that the use of an uncoated graphite platform cut out of the standard graphite tube is stable up to 200 firings under these conditions compared to the graphite tubes which are corroded quickly. Additional work is necessary to establish the mechanism involving the analytes and the modifiers. REFERENCES 1. Tran, D. C.; Sigel, G. H., Jr.; Bendow, B. J. &&wave Tech., 1984, LT-2(5), 566-586. 2. Shibata, S.; Horigochi, M.; Junguji, K.; Mitachi, S.; Kanamori, T.; Manabe, T. Electron. Lett., 1981, 17, 775-117. 3. Ewing, K. J.; Buckner, L.; Jaganathan, J.; Ginther, R.; Aggarwal, I. D. Mater. Res. Bull., 1989, 24, 163-168. 4. Ohishi, Y.; Mitachi, S.; Kanamori, T.; Manabe, T. Phys. Chem. Glasses, 1983,24, 135-140. 5. France, P. W.; Carter, S. F.; Moore, M. W.; Day, C. R. J. Brit. Telecom. Technol., 1987, S(2). 6. Schlemmer, G.; Wely, B. Spectrochim. Acta Part B, 1986, 41, 1157-1165. 7. Shan, X. Q.; Wang, D. X. Anal. Chim. Acta, 1985, 173, 315-319. 8. Teague-Nishimura, J. E.; Tominaga, T.; Katsura, T.; Matsumoto, K. Anal. Chem., 1987, 59, 1647-1651. 9. Price, W. J.; Dymott, T. C.; Whiteside, P. J. Spectrochim. Acta Par? B, 1980, 35, 3-10. 10. Akatsuka, K.; Atsuya, I. Anal. Chem., 1989, 61, 216-220. 11. Retberg, T. M.; Holcombe, J. A. Anal. Chem., 1986, 58, 1462-1467. 12. Sommers, J. A.; Perkins, V. Q. Mater. Sci. Forum, 1988, 32-33, 629633.