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The use of a slotted tube for the determination of lead, zinc, cadmium, bismuth, cobalt, manganese and silver by atomic absorption spectrometry
Anclytica 0 Elsevier
Chinzica Acts
Scientific
97 (1978)
Publishing
395-398
Company,
Amsterdam
-
Printed
in The Netherlands
Short Communication
THE
USE OF A SLOTTED
TUBE
FOR
THE DETERMINATION
OF LEAD, ZINC, CADi\lIupiiI, BISMUTH, COBALT, MANGAIQSE AND SILVER BY ATOMIC ABSORPTION SPECTROMETRY
R J. WATLING Applied Spectioscopy Division, National Physical Research Laboratory, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001 (South Africa)
(Received
21st September
1977)
Since the inception of atomic absorption spectrometry as an analytical tool [ 11, accessories such as heated graphite a’tomizers [2] , tzntalum filaments [3], Delves cups [d] and long path-length tubes [5: 61 have been developed in order to increase the detection power of the technique. All of these accessories, while substantially lowering analytical limits, suffer from being more cumbersome than a conventional flame or from an increase in time of analysis_ This communication describes the use of a slotted tube [7] which is positioned on top of a conventional atomic abso_rptionburner head. It is used in conjunction with an air-acetylene flame to give increased sensitivity for seven elements. Precision of analysis is also significantly improved at low element concentrations. The tube can be made either of quartz or stainless steel, is inexpensive and can be manufactured in most workshops. Experimental
Composite multi-element standards containing lead, zinc, cadmium, bismuth, cobalt, manganese and silver were prepared in the range 0.01-50.00 rzg ml-‘. These solutions were sprayed into a premixed air-acetylene flame, and instrument and flame conditions were optimized for each element to give maximum absorbance signals. The same solutions were then sprayed into the tube which was aligned in the optical path, using burner alignment controls. Inslrument and flame conditions were again optimized and the new absorbance signals recorded. A Varian-Techtron AA6 atomic absorption spectrometer and BC6 hydrogen background corrector were used with a Hitachi flatbed recorder. Results and discussion Sensitivity. The calibration curves which were obtained by spraying
directly into the air-acetylene flame (F) and into the slotted tube (T) are plotted for bismuth, cadmium, silver and lead in Fig_ 1.
396 l.!iOO-
Cd
0 pg
IO
20
ml-’
Fig. 1. Absorbance signal enhancement with the slotted tube.
Characteristic concentrations [8] (concentrations giving 0.004 absorbance) obtained by direct nebulization of the seven elements under the conditions of these experiments are compared with those determined by the slotted tube in Table 1. The improved sensitivity which is observed with the slotted tube is probably due to increased residence time of atoms in the optical path. This in turn is due to the slower flame speed and the longer optical path through which the atoms pass. The partial exclusion of entrained air also affects the chemical environment in the tube, as that portion of the flame which burns inside the tube probably consists largely of interconal gases and contains fewer oxidizing species. As a consequence, the chemical environment is more stable and the concentration of neutral atoms in the optical path is increased. Precision. Much better analytical precision was obtained with the slotted tube. The results for four elements are shown in Fig. 2 and data for allseven elements are summarized in Table 2. In all cases there is an improvement in precision at low concentrations when the slotted tube is used. The improve-
397 TA3LE 1 Characteristic concentrations obtained with an air-acetylene flame directly and the slotted tube
with
Eiement
Flame Gg ml-* 1
Slotted tube (rg ml-‘>
Element
Flame +g ml-’ 1
SIotted tube fflg ml-‘)
ZR
O-01 0.02 0.09 0.15
0.003 0.005 0.01 0.08
Bi Pb Mn
0.40 0.25 0.10
0.10 0.08 0.06
Cd Ag co 28 24
t: !TlSbe
Pb
Fig. 2. Relative standard deviation-concentration and silver.
graphs for lead, zinc, bismuth
ment is apparently related to the degree of thermolability of the metal and the ease with which atoms are formed. All comparative results were obtained from absorbance values in the linear portion of the calibration curve. Inferference studies. For these studies three new series of standards were prepared, one containing 1.00 ~g ml-’ lead, bismuth, cobalt and .manganese, another 1.00 pg ml -’ silver and the third 0.01 Ergml-’ zinc and cadmium. All of these standards, excepting silver, were made up in solutions containing interfering chlorides as well as ni”trates. The silver stzndard was only made up with nitrate saits of interfering ions. The solutions tested were sodium and potassium chlorides, sodium, potassium, calcium, magnesium and strontium nitrates, and the final interferent concentrations ranged from 0 to 1000 ,~g ml-‘. The effects of these salts on the absorbance signal values were investigated
TABLE 2 Comparisons of precisionsnear the analyticallimits for tbe flame and the slotted tube Element
Slotted tube
Flame Concentration (pg ml-’ )
s, (%)”
Concentration (ug ml-‘)
s, (%)a
Zn Cd
0.01 0.05
18.0 14.4
Ag
0.05
16.6
co
0.1
28.4
Bi
1.0
9.1
Pb
0.5
9.0
Mn
0.05
0.01 0.05 0.01 0.05 0.01 0.1 0.05 1.0 0.1 0.5 0.05 0.05
5.2 1.9 10.9 6.2 28.3 5.4 18.0 1.9 21.0 2.3 27.0 13.2
23.4
aFor 20 determinations.
for each element_ The study indicated that certain of the solutions
investc
igated did affect the absorbance signal values for some elements with either the flame or the slotted tube. While it was apparent that some of the effects were specific to the slotted tube, others were reduced by its use, notably calcium and magnesium enhancement of lead absorbance signal values. In general, however, interference effects were minimal. The proposed slotted tube gives a significant increase in sensitivity and precision for all the elements studied. The tube is inexpensive and easily manufactured in a laboratory workshop. REFERENCES 1 2 3 4 5 6 7 8
k Walsh, Spectrochim. Acta, 7 (1955) 108. I-L Massmann, Rev. GAMS, 4 II (1968) 193. M. R. Sensmeier, W. F. Wagner and G. D. Christian, Fresenius 2. Anal. Chem., 277 (1975) H. T. Delves, Analyst, 95 (1970) 431. K Fuwa and B. L Vallee, Anal. Chem., 35 (1963) 942. J. W. Robinson, Anal. Chim. Acta, 27 (1962)465. R. J. Watling, CSIR FIS 108 Special Report (1977). B. M. Gatehouse and J. B. Willis, Spectrochim. Acta, 17 (1961) 710.
19.
Chinzica Acts
Scientific
97 (1978)
Publishing
395-398
Company,
Amsterdam
-
Printed
in The Netherlands
Short Communication
THE
USE OF A SLOTTED
TUBE
FOR
THE DETERMINATION
OF LEAD, ZINC, CADi\lIupiiI, BISMUTH, COBALT, MANGAIQSE AND SILVER BY ATOMIC ABSORPTION SPECTROMETRY
R J. WATLING Applied Spectioscopy Division, National Physical Research Laboratory, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001 (South Africa)
(Received
21st September
1977)
Since the inception of atomic absorption spectrometry as an analytical tool [ 11, accessories such as heated graphite a’tomizers [2] , tzntalum filaments [3], Delves cups [d] and long path-length tubes [5: 61 have been developed in order to increase the detection power of the technique. All of these accessories, while substantially lowering analytical limits, suffer from being more cumbersome than a conventional flame or from an increase in time of analysis_ This communication describes the use of a slotted tube [7] which is positioned on top of a conventional atomic abso_rptionburner head. It is used in conjunction with an air-acetylene flame to give increased sensitivity for seven elements. Precision of analysis is also significantly improved at low element concentrations. The tube can be made either of quartz or stainless steel, is inexpensive and can be manufactured in most workshops. Experimental
Composite multi-element standards containing lead, zinc, cadmium, bismuth, cobalt, manganese and silver were prepared in the range 0.01-50.00 rzg ml-‘. These solutions were sprayed into a premixed air-acetylene flame, and instrument and flame conditions were optimized for each element to give maximum absorbance signals. The same solutions were then sprayed into the tube which was aligned in the optical path, using burner alignment controls. Inslrument and flame conditions were again optimized and the new absorbance signals recorded. A Varian-Techtron AA6 atomic absorption spectrometer and BC6 hydrogen background corrector were used with a Hitachi flatbed recorder. Results and discussion Sensitivity. The calibration curves which were obtained by spraying
directly into the air-acetylene flame (F) and into the slotted tube (T) are plotted for bismuth, cadmium, silver and lead in Fig_ 1.
396 l.!iOO-
Cd
0 pg
IO
20
ml-’
Fig. 1. Absorbance signal enhancement with the slotted tube.
Characteristic concentrations [8] (concentrations giving 0.004 absorbance) obtained by direct nebulization of the seven elements under the conditions of these experiments are compared with those determined by the slotted tube in Table 1. The improved sensitivity which is observed with the slotted tube is probably due to increased residence time of atoms in the optical path. This in turn is due to the slower flame speed and the longer optical path through which the atoms pass. The partial exclusion of entrained air also affects the chemical environment in the tube, as that portion of the flame which burns inside the tube probably consists largely of interconal gases and contains fewer oxidizing species. As a consequence, the chemical environment is more stable and the concentration of neutral atoms in the optical path is increased. Precision. Much better analytical precision was obtained with the slotted tube. The results for four elements are shown in Fig. 2 and data for allseven elements are summarized in Table 2. In all cases there is an improvement in precision at low concentrations when the slotted tube is used. The improve-
397 TA3LE 1 Characteristic concentrations obtained with an air-acetylene flame directly and the slotted tube
with
Eiement
Flame Gg ml-* 1
Slotted tube (rg ml-‘>
Element
Flame +g ml-’ 1
SIotted tube fflg ml-‘)
ZR
O-01 0.02 0.09 0.15
0.003 0.005 0.01 0.08
Bi Pb Mn
0.40 0.25 0.10
0.10 0.08 0.06
Cd Ag co 28 24
t: !TlSbe
Pb
Fig. 2. Relative standard deviation-concentration and silver.
graphs for lead, zinc, bismuth
ment is apparently related to the degree of thermolability of the metal and the ease with which atoms are formed. All comparative results were obtained from absorbance values in the linear portion of the calibration curve. Inferference studies. For these studies three new series of standards were prepared, one containing 1.00 ~g ml-’ lead, bismuth, cobalt and .manganese, another 1.00 pg ml -’ silver and the third 0.01 Ergml-’ zinc and cadmium. All of these standards, excepting silver, were made up in solutions containing interfering chlorides as well as ni”trates. The silver stzndard was only made up with nitrate saits of interfering ions. The solutions tested were sodium and potassium chlorides, sodium, potassium, calcium, magnesium and strontium nitrates, and the final interferent concentrations ranged from 0 to 1000 ,~g ml-‘. The effects of these salts on the absorbance signal values were investigated
TABLE 2 Comparisons of precisionsnear the analyticallimits for tbe flame and the slotted tube Element
Slotted tube
Flame Concentration (pg ml-’ )
s, (%)”
Concentration (ug ml-‘)
s, (%)a
Zn Cd
0.01 0.05
18.0 14.4
Ag
0.05
16.6
co
0.1
28.4
Bi
1.0
9.1
Pb
0.5
9.0
Mn
0.05
0.01 0.05 0.01 0.05 0.01 0.1 0.05 1.0 0.1 0.5 0.05 0.05
5.2 1.9 10.9 6.2 28.3 5.4 18.0 1.9 21.0 2.3 27.0 13.2
23.4
aFor 20 determinations.
for each element_ The study indicated that certain of the solutions
investc
igated did affect the absorbance signal values for some elements with either the flame or the slotted tube. While it was apparent that some of the effects were specific to the slotted tube, others were reduced by its use, notably calcium and magnesium enhancement of lead absorbance signal values. In general, however, interference effects were minimal. The proposed slotted tube gives a significant increase in sensitivity and precision for all the elements studied. The tube is inexpensive and easily manufactured in a laboratory workshop. REFERENCES 1 2 3 4 5 6 7 8
k Walsh, Spectrochim. Acta, 7 (1955) 108. I-L Massmann, Rev. GAMS, 4 II (1968) 193. M. R. Sensmeier, W. F. Wagner and G. D. Christian, Fresenius 2. Anal. Chem., 277 (1975) H. T. Delves, Analyst, 95 (1970) 431. K Fuwa and B. L Vallee, Anal. Chem., 35 (1963) 942. J. W. Robinson, Anal. Chim. Acta, 27 (1962)465. R. J. Watling, CSIR FIS 108 Special Report (1977). B. M. Gatehouse and J. B. Willis, Spectrochim. Acta, 17 (1961) 710.
19.