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Effect of atomizer surface type on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry
Analytica Chimica Acta, 167 (1985) 317-324 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
EFFECT OF ATOMIZER SURFACE TYPE ON THE QUANTITATION OF MOLYBDENUM AND YTTERBIUM BY ELECTROTHERMAL ATOMIZATION ATOMIC ABSORPTION SPECTROMETRY
JOSEPH SNEDDON* Department
and VILI A. FUAVAOs
of Chemistry,
New Mexico State
University,
Las Cruces, NM 88003
(U.S.A.)
(Received 9th July 1984)
SUMMARY The effect of pyrocoated graphite, uncoated graphite, metal-carbide, and metal atomization surfaces on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry was investigated. The peak shape was affected by heating rate and the different surfaces gave different shapes. Except for the case of uncoated graphite, the sensitivities and detection limits were similar for all surfaces. In a sodium chloride matrix it is preferable to use uncoated graphite for molybdenum because an ashing stage greater than the boiling point of sodium chloride can be used without loss of molybdenum. Tube lifetime depended on atomization temperature, atomization time and the matrix.
Atomic absorption spectrometry (a.a.s.) with electrothermal atomization has become widely used and accepted in trace and ultratrace metal determinations because of the low detection limits that can be achieved. This can be attributed to complete use of the sample at relatively low temperatures, ca. 2500-3000” C, and long residence time, but these conditions also generate a high matrix concentration. Thus, the technique is prone to matrix interferences, particularly from alkali and alkaline earth metal chlorides. Complicated chemical reactions involving the atomizer surface and its reactions with sheath gas, analyte and matrix will play an important part in sensitivity and atom formation processes. Several workers have tried to explain these matrix effects in terms of the formation of molecular chlorides on the atomizer surface and in the vapor phase [ 1,2] , covolatilization of the analyte with a more volatile matrix [3, 41, and the trapping of aggregates of matrix molecules which can be expelled from the atomizer by the expanding gases on rapid heating [ 5,6] . The introduction of the sample by an aerosol deposition technique [ 71 or use of a matrix modifier [8] reduces interferences associated with the formation of crystals on the atomizer surface. The effects of pyrocoated and uncoated graphite on lead and nickel in a magnesium chloride matrix [9], glassy carbon [lo], totally pyrolytic carbon [ll] , and aPresent address: Department of Chemistry, University of the South Pacific, Suva, Fiji. 0003-2670/85/$03,30
o 1985 Elsevier Science Publishers B.V.
318
the determination of gold, silver and manganese with pyrocoated and uncoated graphite surfaces [ 121 have been investigated. A recent review describes atomic absorption spectrometry from metal atomizers [ 131. This paper reports the effects of different atomization surfaces on the measurement of ytterbium and molybdenum with particular interest on peak shape, precision and lifetime of the surface. Molybdenum and ytterbium were chosen to evaluate these parameters because they are two of the elements which are more difficult to quantify by electrothermal atomization a.a.s. because of their refractory nature. EXPERIMENTAL
An Instrumentation Laboratory 457 atomic absorption/atomic emission spectrometer and 655 temperature controlled furnace was used. The spectrometer was equipped with a deuterium arc background corrector that was used for molybdenum. The temperature readout was used as described previously [14]. The molybdenum and ytterbium peak-height and peak-area absorbances were monitored at 313.3 and 398.8 nm, respectively. Unless otherwise specified, the temperature program involved drying at 120°C for 30 s, optimized ashing for 30 s and atomizing at 2800°C for 10 s. A clean stage was used for each sample to ensure a return to background levels before a further injection. Electron micrographs were taken with a Philips SEM 501B scanning electron microscope with attached Steinheil camera with Polaroid back and Polaroid 107C film. Stock solutions of 1029 c(g ml-’ Yb as ytterbium trichloride and 1000 pg ml-’ MO as ammonium heptamolybdate (Alfa Products) were diluted with deionized water as required. Analytical-grade 70.7% (w/v) nitric acid (Alfa Products) was used and diluted as required. High-purity deionized water was used for the preparation of all solutions. Coating of the graphite tube was done similarly to the procedure of Wahab and Chakrabarti [ 151 and involved dissolving the maximum permissible weight of solid zirconium sulfate tetrahydrate (Aldrich Chemicals), and lanthanum chloride hexahydrate (Alfa Products) in water/acid solution. The tube was immersed vertically in the zirconium or lanthanum solution for 24 h, dried at room temperature and baked at 120°C for 60 min. The tube was then conditioned by being mounted in the electrothermal atomizer, the temperature being gradually raised from room temperature to 2700°C over a period of 60 s. This process was further repeated before use. Linings of tungsten and tantalum were prepared from foils (0.05 mm thick) of pure metal (A. D. Mackay, Dar&n, CT) which were inserted inside the graphite tube in such a way to ensure complete coverage of the tube. A 1.5-mm hole was drilled over the injection port to allow introduction of the sample. A lo-111 Eppendorf micropipet with disposable plastic tips was used to introduce samples to the electrothermal atomizer.
319 RESULTS
AND DISCUSSION
Characteristics of the peak shape The effect of heating rate on the peak shape of ytterbium atomized from a pyrocoated graphite surface is shown in Fig. 1. At the fastest heating rate of 600°C s-l [14] the peak height is sharpest and highest. As the heating rate is reduced, the peak height becomes smaller and broader. Peak area was not affected by heating rate. For peak-height measurements, maximum sensitivity will be achieved at fastest heating rates. In Fig. 2, the peak shapes of molybdenum atomized from a pyrocoated graphite, uncoated graphite, and metal-carbide (zirconium-pyrocoated) surface are shown. It is obvious that the peak shapes are rather different for the three atomization surfaces. The release of molybdenum atoms from the pyrocoated graphite and metal-carbide surface occurs at approximately the same atomization time and approximately 0.5 s before molybdenum atoms from the uncoated graphite. This reflects the nonadhesive or less permeable behavior of the metal-carbide and pyrocoated graphite surfaces. The peak from uncoated graphite has a large tail which is not found from the other two surfaces and will lead to a memory effect if not removed. There is a small tailing effect with the pyrocoated surface. Different matrices gave slightly different peak shapes. The sensitivities are considerably poorer when an uncoated graphite surface is used, whereas the pyrocoated graphite, metal and metal-carbide surfaces provide comparable sensitivities as shown in Table 1. A similar trend was found in detection limits. Precision with the different atomization surfaces Figure 3 shows how imprecisions of absorbance and concentration values vary with concentrations of ytterbium with a metal-carbide (Zr-coated)
25 TIME (s)
50 ATOMIZATION
TIME
(s)
Fig. 1. The absorbance signals for 1 ng of ytterbium at various heating rates from a pyrocoated graphite atomizer surface. Heating rates are from 0 (fastest) to 9 (slowest) as described previously [ 14 1. Fig. 2. The peak shapes of molybdenum atomized from different surfaces: (1) metal carbide; (2) pyrocoated graphite; (3) uncoated graphite.
320 TABLE 1 Comparison of inverse sensitivity, detection limits and imprecision for the quantitation of molybdenum and ytterbium from pyrocoated graphite, uncoated graphite, metal and metal-carbide atomization surfaces Element
Molybdenum
Ytterbium
Atomizer surface
Uncoated graphite Pyrocoated graphite Zr-coated pyrocoated graphite La-coated pyrocoated graphite Tantalum Uncoated graphite Pyrocoated graphite Zr-coated pyrocoated graphite La-coated pyrocoated graphite Tungsten
No. of trials
Inverse sensitivitya
Detection limitb ( ng ml-’ )
Useful rangeC (ng ml-‘)
(pg)
(ng ml-‘)
10
70.0
7.00
45
100-500
10
6.4
0.64
16
30-350
7
5.8
0.58
21
50400
10
2.1
0.21
11
50-400
7 10
17.4 52.6
1.74 5.3
20 27
40-480 150-600
10
3.6
0.36
5
40-480
10
2.2
0.22
8
40-500
10
2.6
0.26
8
40-600
7
2.7
0.27
15
60-470
aConcentration or mass giving 1% absorption (0.0044 absorbance). bConcentration giving a signal-to-noise ratio of 2. CRange for which relative standard deviation is <3.5% (lo-p1 sample).
surface. Despite the curvature in the calibration plot [ 161, a precision of 3% is possible at 400 ng ml-’ at which the absorbance is 2.50. As expected, precision is very poor at or near the detection limit. Other information related to imprecision is given in Table 1. The uncoated graphite surfaces have a less sensitive range but have similar precision with respect to the other surfaces. Optimization of ashingand atomization from a sodium chloride matrix For an aqueous molybdenum solution, a maximum ashing temperature of 1200” C is recommended (Pye-Unicam, and Instrumentation Laboratories). The optimum ashing and atomization temperature for molybdenum in a sodium chloride matrix on a pyrocoated and uncoated graphite surface was obtained as described previously [14] and is shown in Fig. 4. For both
321
2 l5 5 z u it
10
u
-
3.17 2.62yj z 1.87 s 5
5_
4 0
200
CONCENTRATION
3 0 400 (ngml-‘1
f300°
0
1000
F-4 2000
TEMPERATURE
’
3000 (‘C)
Fig. 3. Variation of imprecision of the concentration and absorbance of ytterbium atomized from a metal-carbide (zirconium) surface: (A) calibration curve; (e ) precision of concentration; (o ) precision of absorbance. Fig. 4. Optimization of ashing and atomization for 1 ng of molybdenum: (0) ashing from an uncoated graphite surface; (0) ashing from a pyrocoated graphite surface; (a) atomization from a pyrocoated graphite surface; (0) atomization from an uncoated graphite surface.
surfaces, an optimum ashing temperature was found with loss of molybdenum occurring above the optimum. The optimum ashing temperature for the pyrocoated graphite surface (1350°C) was 150°C below that found for the uncoated graphite surface. The boiling point of sodium chloride is 1413°C and it is probable that most of the matrix originally present in the pyrocoated graphite surface will be present at the start of the atomization whereas the matrix has been removed from the uncoated surface. Despite the reduced sensitivity and memory effect found with an uncoated graphite surface, it may be desirable to determine molybdenum in a sodium chloride matrix with this surface because the sodium chloride can be removed from the sample during ashing. Both surfaces require high atomization temperatures in order to ensure complete atomization. Lifetime of the atomization surfaces The useful lifetime of the atomization surfaces used here obviously varies with the temperature program applied. During this study, several atomizer surfaces were tested to destruction and the peak-height absorbance was periodically monitored during the complete sequence of firings. Normally, the signal from a surface exhibits a slow but steady decrease, the rate of which increases more rapidly as the surface deteriorates. The useful lifetime of the surface was judged to be over, when the tube broke or the relative standard deviation exceeded 10%. In Table 2, the lifetimes of different surfaces in different matrices are shown. The commercially available uncoated and pyrocoated surfaces give about 180 measurement cycles when used at an atomization temperature of 2800°C for 10 s; this number is approximately trebled when an atomization temperature of 1400°C and atomization time of 5 s is used. A matrix of nitric acid decreases the useful lifetime, with the lifetime decreasing as the
322 TABLE 2 Lifetime of atomization surfaces Matrix
Average no. of cycles
No. of trials
10
Aqueous
162
2
3.09
2800
10
Aqueous
190
3
7.46
MO
2800
10
1% HNO,
112
2
4.41
MO
2800
10
5% HNO,
78
4
11.22
Ag
1400
5
Aqueous
530
&
1400
5
Aqueous
590
Yh Yb Yb Yb Yb
2800 2800 2800 2800 2800
10 10 10 10 10
Aqueous 5% HNO, Aqueous 5% HNO, Aqueous
98 62 107 51 181
Yb
2800
10
5% HNO,
179
1400
5
Aqueous
503
Atomizer surface
Element
Uncoated graphite Pyrocoated graphite Pyrocoated graphite Pyrocoated graphite Uncoated graphite Pyrocoated graphite Tungsten Tungsten Tantalum Tantalum Zr-coated uncoated graphite Zr-coated pyrocoated graphite Zr-coated pyrocoated graphite
Atomization Temperature CC)
Time (8)
MO
2800
MO
R.s.d. (%I
4.29
17.32 0.92 9.23 4.41
2
4.41
-
concentration increases. The zirconium-coated graphite tubes follow a pattern similar to those of the uncoated and pyrocoated graphite tubes. The lifetimes of the tungsten and tantalum surfaces were about half those of the other surfaces. After about thirty cycles, the metal inserts started to split and curl at the edges and eventually broke up completely. Figure 5 is an electron micrograph of the surface of a pyrocoated graphite tube after approximately 200 firings at 2600°C for 10 s. The light area is where lo-p1 aliquots of aqueous solution were in direct contact with the surface and shows holes and cracks appearing which will eventually lead to poor precision. Figure 6 is an electron micrograph of a zirconium-coated pyrocoated graphite tube after approximately 200 firings at 28OO’C for 10 s. The cracks are severe and several pieces of the surface layer have been removed. Some weight loss with pyrocoated, uncoated, and precoated surfaces was found after the useful lifetime of the surface was over but never exceeded 5% of the initial weight of the tube. The nature of the different atomization surfaces may play a role
Fig 5. Electron micrograph about 200 firmgs at 2600°C
(X 320) of the surface of a pyrocoated graphite for 10 s for lo-p1 aliquots of aqueous solutions.
Fig. 6. Electron micrograph (X 320) of the surface of a zirconium-coated 200 firmgs at 2800°C for 10 s for lo-p1 aliquots of aqueous solutions.
tube
after
tube after about
in assessing the possible mechanisms of atom formation. Using x-ray diffraction studies, Sneddon et al. [17] have shown that two intermediate carbides are formed in the atomization of molybdenum. Atomization from the metalcarbide and metal surfaces would exclude these intermediate carbides. Examination by electron microscopy of metal surfaces after several determinations, particularly in acidic medium, shows that the surface is not well defined and it is probable that a metal oxide or metal vapor is continuously deposited on the surface. The authors acknowledge the support of Grant RR-08136 from the Minorities Biomedical Research Support Program, Division of Research Resources, National Institute of Health. This paper was presented in part at FACSS X, Philadelphia, PA, in September, 1983. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13
W. Frech and A. Cedergren, Anal. Chim. Acta, 82 (1976) 93. B. V. L’Vov, Spectrochim. Acta, Part B, 33 (1978) 153. E. J. Czobik and J. P. Matousek, Anal. Chim. Acta, 90 (1978) 2. D. A. Segar and J. G. Gonzalez, Anal Chim. Acta, 58 (1972) 7. D. J. Churella and T. R. Copeland, Anal. Chem., 50 (1978) 309. J. A. Krasowskl and T. R. Copeland, Anal. Chem., 51(1979) 1843. M. K. Conley, J. J. Sotera and H. L. Kahn, Instrumentation Laboratory Report No. 149, Reduction of Interferences in Furnace Atomizer Atomic Absorption Spectroscopy Instrumentation Laboratory, Wilmington, MA, 1981. L. Ebdon, A. T. Ellis and R. W. Ward, Talanta, 28 (1982) 297. J. P. Erspamer and T. M. Niemczyk, Anal. Chem., 54 (1982) 2150. L. de Galan, M. T. C. de Loos-Vollebregt and R. A. M. Oosterling, Analyst (London), 108 (1983) 138. D. Littlelohn, I. Duncan, J. Marshall and J. M. Ottaway, Anal. Chim. Acta, 157 (1984) 291. J. Fazakas, Anal. Lett., 15(A7) (1982) 573. M. Suzuki and K. Ohta, Prog. Anal. At. Spectrosc., 6 (1982) 49.
324 14 15 16 17
V. A. Fuavao and J. Sneddon, At. Spectrosc., 4 (1983) 179. H. S. Wahab and C. L. Chakrabarti, Spectrochim Acta, Part B, 36 (1981) 482. H. P. J. Van Dalen and L. de Galan, Analyst (London), 106 (1981) 695. J. Sneddon, W. B. Rowsten and J. M. Ottaway, Analyst (London), 103 (1978) 776.
in The Netherlands
EFFECT OF ATOMIZER SURFACE TYPE ON THE QUANTITATION OF MOLYBDENUM AND YTTERBIUM BY ELECTROTHERMAL ATOMIZATION ATOMIC ABSORPTION SPECTROMETRY
JOSEPH SNEDDON* Department
and VILI A. FUAVAOs
of Chemistry,
New Mexico State
University,
Las Cruces, NM 88003
(U.S.A.)
(Received 9th July 1984)
SUMMARY The effect of pyrocoated graphite, uncoated graphite, metal-carbide, and metal atomization surfaces on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry was investigated. The peak shape was affected by heating rate and the different surfaces gave different shapes. Except for the case of uncoated graphite, the sensitivities and detection limits were similar for all surfaces. In a sodium chloride matrix it is preferable to use uncoated graphite for molybdenum because an ashing stage greater than the boiling point of sodium chloride can be used without loss of molybdenum. Tube lifetime depended on atomization temperature, atomization time and the matrix.
Atomic absorption spectrometry (a.a.s.) with electrothermal atomization has become widely used and accepted in trace and ultratrace metal determinations because of the low detection limits that can be achieved. This can be attributed to complete use of the sample at relatively low temperatures, ca. 2500-3000” C, and long residence time, but these conditions also generate a high matrix concentration. Thus, the technique is prone to matrix interferences, particularly from alkali and alkaline earth metal chlorides. Complicated chemical reactions involving the atomizer surface and its reactions with sheath gas, analyte and matrix will play an important part in sensitivity and atom formation processes. Several workers have tried to explain these matrix effects in terms of the formation of molecular chlorides on the atomizer surface and in the vapor phase [ 1,2] , covolatilization of the analyte with a more volatile matrix [3, 41, and the trapping of aggregates of matrix molecules which can be expelled from the atomizer by the expanding gases on rapid heating [ 5,6] . The introduction of the sample by an aerosol deposition technique [ 71 or use of a matrix modifier [8] reduces interferences associated with the formation of crystals on the atomizer surface. The effects of pyrocoated and uncoated graphite on lead and nickel in a magnesium chloride matrix [9], glassy carbon [lo], totally pyrolytic carbon [ll] , and aPresent address: Department of Chemistry, University of the South Pacific, Suva, Fiji. 0003-2670/85/$03,30
o 1985 Elsevier Science Publishers B.V.
318
the determination of gold, silver and manganese with pyrocoated and uncoated graphite surfaces [ 121 have been investigated. A recent review describes atomic absorption spectrometry from metal atomizers [ 131. This paper reports the effects of different atomization surfaces on the measurement of ytterbium and molybdenum with particular interest on peak shape, precision and lifetime of the surface. Molybdenum and ytterbium were chosen to evaluate these parameters because they are two of the elements which are more difficult to quantify by electrothermal atomization a.a.s. because of their refractory nature. EXPERIMENTAL
An Instrumentation Laboratory 457 atomic absorption/atomic emission spectrometer and 655 temperature controlled furnace was used. The spectrometer was equipped with a deuterium arc background corrector that was used for molybdenum. The temperature readout was used as described previously [14]. The molybdenum and ytterbium peak-height and peak-area absorbances were monitored at 313.3 and 398.8 nm, respectively. Unless otherwise specified, the temperature program involved drying at 120°C for 30 s, optimized ashing for 30 s and atomizing at 2800°C for 10 s. A clean stage was used for each sample to ensure a return to background levels before a further injection. Electron micrographs were taken with a Philips SEM 501B scanning electron microscope with attached Steinheil camera with Polaroid back and Polaroid 107C film. Stock solutions of 1029 c(g ml-’ Yb as ytterbium trichloride and 1000 pg ml-’ MO as ammonium heptamolybdate (Alfa Products) were diluted with deionized water as required. Analytical-grade 70.7% (w/v) nitric acid (Alfa Products) was used and diluted as required. High-purity deionized water was used for the preparation of all solutions. Coating of the graphite tube was done similarly to the procedure of Wahab and Chakrabarti [ 151 and involved dissolving the maximum permissible weight of solid zirconium sulfate tetrahydrate (Aldrich Chemicals), and lanthanum chloride hexahydrate (Alfa Products) in water/acid solution. The tube was immersed vertically in the zirconium or lanthanum solution for 24 h, dried at room temperature and baked at 120°C for 60 min. The tube was then conditioned by being mounted in the electrothermal atomizer, the temperature being gradually raised from room temperature to 2700°C over a period of 60 s. This process was further repeated before use. Linings of tungsten and tantalum were prepared from foils (0.05 mm thick) of pure metal (A. D. Mackay, Dar&n, CT) which were inserted inside the graphite tube in such a way to ensure complete coverage of the tube. A 1.5-mm hole was drilled over the injection port to allow introduction of the sample. A lo-111 Eppendorf micropipet with disposable plastic tips was used to introduce samples to the electrothermal atomizer.
319 RESULTS
AND DISCUSSION
Characteristics of the peak shape The effect of heating rate on the peak shape of ytterbium atomized from a pyrocoated graphite surface is shown in Fig. 1. At the fastest heating rate of 600°C s-l [14] the peak height is sharpest and highest. As the heating rate is reduced, the peak height becomes smaller and broader. Peak area was not affected by heating rate. For peak-height measurements, maximum sensitivity will be achieved at fastest heating rates. In Fig. 2, the peak shapes of molybdenum atomized from a pyrocoated graphite, uncoated graphite, and metal-carbide (zirconium-pyrocoated) surface are shown. It is obvious that the peak shapes are rather different for the three atomization surfaces. The release of molybdenum atoms from the pyrocoated graphite and metal-carbide surface occurs at approximately the same atomization time and approximately 0.5 s before molybdenum atoms from the uncoated graphite. This reflects the nonadhesive or less permeable behavior of the metal-carbide and pyrocoated graphite surfaces. The peak from uncoated graphite has a large tail which is not found from the other two surfaces and will lead to a memory effect if not removed. There is a small tailing effect with the pyrocoated surface. Different matrices gave slightly different peak shapes. The sensitivities are considerably poorer when an uncoated graphite surface is used, whereas the pyrocoated graphite, metal and metal-carbide surfaces provide comparable sensitivities as shown in Table 1. A similar trend was found in detection limits. Precision with the different atomization surfaces Figure 3 shows how imprecisions of absorbance and concentration values vary with concentrations of ytterbium with a metal-carbide (Zr-coated)
25 TIME (s)
50 ATOMIZATION
TIME
(s)
Fig. 1. The absorbance signals for 1 ng of ytterbium at various heating rates from a pyrocoated graphite atomizer surface. Heating rates are from 0 (fastest) to 9 (slowest) as described previously [ 14 1. Fig. 2. The peak shapes of molybdenum atomized from different surfaces: (1) metal carbide; (2) pyrocoated graphite; (3) uncoated graphite.
320 TABLE 1 Comparison of inverse sensitivity, detection limits and imprecision for the quantitation of molybdenum and ytterbium from pyrocoated graphite, uncoated graphite, metal and metal-carbide atomization surfaces Element
Molybdenum
Ytterbium
Atomizer surface
Uncoated graphite Pyrocoated graphite Zr-coated pyrocoated graphite La-coated pyrocoated graphite Tantalum Uncoated graphite Pyrocoated graphite Zr-coated pyrocoated graphite La-coated pyrocoated graphite Tungsten
No. of trials
Inverse sensitivitya
Detection limitb ( ng ml-’ )
Useful rangeC (ng ml-‘)
(pg)
(ng ml-‘)
10
70.0
7.00
45
100-500
10
6.4
0.64
16
30-350
7
5.8
0.58
21
50400
10
2.1
0.21
11
50-400
7 10
17.4 52.6
1.74 5.3
20 27
40-480 150-600
10
3.6
0.36
5
40-480
10
2.2
0.22
8
40-500
10
2.6
0.26
8
40-600
7
2.7
0.27
15
60-470
aConcentration or mass giving 1% absorption (0.0044 absorbance). bConcentration giving a signal-to-noise ratio of 2. CRange for which relative standard deviation is <3.5% (lo-p1 sample).
surface. Despite the curvature in the calibration plot [ 161, a precision of 3% is possible at 400 ng ml-’ at which the absorbance is 2.50. As expected, precision is very poor at or near the detection limit. Other information related to imprecision is given in Table 1. The uncoated graphite surfaces have a less sensitive range but have similar precision with respect to the other surfaces. Optimization of ashingand atomization from a sodium chloride matrix For an aqueous molybdenum solution, a maximum ashing temperature of 1200” C is recommended (Pye-Unicam, and Instrumentation Laboratories). The optimum ashing and atomization temperature for molybdenum in a sodium chloride matrix on a pyrocoated and uncoated graphite surface was obtained as described previously [14] and is shown in Fig. 4. For both
321
2 l5 5 z u it
10
u
-
3.17 2.62yj z 1.87 s 5
5_
4 0
200
CONCENTRATION
3 0 400 (ngml-‘1
f300°
0
1000
F-4 2000
TEMPERATURE
’
3000 (‘C)
Fig. 3. Variation of imprecision of the concentration and absorbance of ytterbium atomized from a metal-carbide (zirconium) surface: (A) calibration curve; (e ) precision of concentration; (o ) precision of absorbance. Fig. 4. Optimization of ashing and atomization for 1 ng of molybdenum: (0) ashing from an uncoated graphite surface; (0) ashing from a pyrocoated graphite surface; (a) atomization from a pyrocoated graphite surface; (0) atomization from an uncoated graphite surface.
surfaces, an optimum ashing temperature was found with loss of molybdenum occurring above the optimum. The optimum ashing temperature for the pyrocoated graphite surface (1350°C) was 150°C below that found for the uncoated graphite surface. The boiling point of sodium chloride is 1413°C and it is probable that most of the matrix originally present in the pyrocoated graphite surface will be present at the start of the atomization whereas the matrix has been removed from the uncoated surface. Despite the reduced sensitivity and memory effect found with an uncoated graphite surface, it may be desirable to determine molybdenum in a sodium chloride matrix with this surface because the sodium chloride can be removed from the sample during ashing. Both surfaces require high atomization temperatures in order to ensure complete atomization. Lifetime of the atomization surfaces The useful lifetime of the atomization surfaces used here obviously varies with the temperature program applied. During this study, several atomizer surfaces were tested to destruction and the peak-height absorbance was periodically monitored during the complete sequence of firings. Normally, the signal from a surface exhibits a slow but steady decrease, the rate of which increases more rapidly as the surface deteriorates. The useful lifetime of the surface was judged to be over, when the tube broke or the relative standard deviation exceeded 10%. In Table 2, the lifetimes of different surfaces in different matrices are shown. The commercially available uncoated and pyrocoated surfaces give about 180 measurement cycles when used at an atomization temperature of 2800°C for 10 s; this number is approximately trebled when an atomization temperature of 1400°C and atomization time of 5 s is used. A matrix of nitric acid decreases the useful lifetime, with the lifetime decreasing as the
322 TABLE 2 Lifetime of atomization surfaces Matrix
Average no. of cycles
No. of trials
10
Aqueous
162
2
3.09
2800
10
Aqueous
190
3
7.46
MO
2800
10
1% HNO,
112
2
4.41
MO
2800
10
5% HNO,
78
4
11.22
Ag
1400
5
Aqueous
530
&
1400
5
Aqueous
590
Yh Yb Yb Yb Yb
2800 2800 2800 2800 2800
10 10 10 10 10
Aqueous 5% HNO, Aqueous 5% HNO, Aqueous
98 62 107 51 181
Yb
2800
10
5% HNO,
179
1400
5
Aqueous
503
Atomizer surface
Element
Uncoated graphite Pyrocoated graphite Pyrocoated graphite Pyrocoated graphite Uncoated graphite Pyrocoated graphite Tungsten Tungsten Tantalum Tantalum Zr-coated uncoated graphite Zr-coated pyrocoated graphite Zr-coated pyrocoated graphite
Atomization Temperature CC)
Time (8)
MO
2800
MO
R.s.d. (%I
4.29
17.32 0.92 9.23 4.41
2
4.41
-
concentration increases. The zirconium-coated graphite tubes follow a pattern similar to those of the uncoated and pyrocoated graphite tubes. The lifetimes of the tungsten and tantalum surfaces were about half those of the other surfaces. After about thirty cycles, the metal inserts started to split and curl at the edges and eventually broke up completely. Figure 5 is an electron micrograph of the surface of a pyrocoated graphite tube after approximately 200 firings at 2600°C for 10 s. The light area is where lo-p1 aliquots of aqueous solution were in direct contact with the surface and shows holes and cracks appearing which will eventually lead to poor precision. Figure 6 is an electron micrograph of a zirconium-coated pyrocoated graphite tube after approximately 200 firings at 28OO’C for 10 s. The cracks are severe and several pieces of the surface layer have been removed. Some weight loss with pyrocoated, uncoated, and precoated surfaces was found after the useful lifetime of the surface was over but never exceeded 5% of the initial weight of the tube. The nature of the different atomization surfaces may play a role
Fig 5. Electron micrograph about 200 firmgs at 2600°C
(X 320) of the surface of a pyrocoated graphite for 10 s for lo-p1 aliquots of aqueous solutions.
Fig. 6. Electron micrograph (X 320) of the surface of a zirconium-coated 200 firmgs at 2800°C for 10 s for lo-p1 aliquots of aqueous solutions.
tube
after
tube after about
in assessing the possible mechanisms of atom formation. Using x-ray diffraction studies, Sneddon et al. [17] have shown that two intermediate carbides are formed in the atomization of molybdenum. Atomization from the metalcarbide and metal surfaces would exclude these intermediate carbides. Examination by electron microscopy of metal surfaces after several determinations, particularly in acidic medium, shows that the surface is not well defined and it is probable that a metal oxide or metal vapor is continuously deposited on the surface. The authors acknowledge the support of Grant RR-08136 from the Minorities Biomedical Research Support Program, Division of Research Resources, National Institute of Health. This paper was presented in part at FACSS X, Philadelphia, PA, in September, 1983. REFERENCES 1 2 3 4 5 6 7
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