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Internal standard method in flame atomic absorption spectrometry
0584-8547181/070735-1 Q 1981 Pergamon
Specrmchimico Acla Vol. 36B. No. 7. pp 735 to 745. 1981 Printed in Great Britain
Internal standard method in flame atomic absorption
Iw2.00/0 Press Ltd.
spectrometry
TAKEOTAKADAand KUNIO NAKANO Department
of Chemistry,
College of Science, Rikkyo (St. Paul’s) University, ku, Tokyo, 171. Japan
Nishi-Ikebukuro,
Toshima-
(Receioed 25 February 1981) Abstract-Some selection criteria and experimental considerations for selecting an internal standard in flame atomic absorption spectrometry were studied. Rational criteria for the selection of internal standards include the atomization efficiencies of the elements. An experimental study of the variation of the atomization degree due to a temperature decrease can be made with the help of plots of absorbance against the rate at which the sample solution is introduced. The temperature decrease produced by increasing the rate at which water was introduced using a heated chamber without water-cooled condenser was estimated to vary from 2420 K to 2218 K with 1.0 ml water per minute. Curves for the alkali. alkaline earth, and several other metals were obtained in the air-acetylene flame. The shapes of the curves are critically dependent on the degree of atomization of the metal itself. Elements with curves that have similar slopes can be used as mutual internal standards. Some of the other factors which affect the atomization efficiencies-i.e. changes in fuel and air flow, and matrix composition-were also studied.
1. IN~R~DLJC~~~N THE PRECISION of a measurement in atomic absorption spectrometry is mainly dependent on maintaining a constant atom production in the area of the flame traversed by the light beam. However, the absorption intensity of a flame atomizer is variable, because of the change in conditions such as flame temperature, aspiration rate-of the sample solution, flame composition, and so on. These factors have been recognized. in flame emission spectrometry, and an internal standard method is commonly used as a compensation technique [l]. In spite of the many advantages of an internal standard, reports of its applications to atomic absorption are scarce[2-6]; and indeed, no fundamental studies on the selection of an internal standard seem to have yet been made. The present paper describes some selection criteria and experimental considerations for selecting an internal standard in flame atomic absorption spectrometry. 2. THEORETICAL AND EXPERIMENTALBASIS The relation between the absorbance A, of an analyte element and the number of free atoms N, of the element in the flame can be simply formulated by the BeerLambert law
A, = K,l,N,,
(1)
where K, is a proportionality constant and 1, the path length in the flame. N, is proportional to the concentration in the sample solution of the analyte element. In this case, N, is given by N
-
‘dCl VI ’
(2)
[I] W. GERLACHand E. SCHWEIIZER, Foundations and Methods of Chemical Analysis by the Emission Spectrum, Vol. 2, p. 2. VosslAdam Hilger, London (1929). [2] L. R. P. BUTLERand A. STRASHEIM,Spectrochim. Acto 7, 1207 (1965). [3] F. J. FELDMAN,J. A. BLASI and S. B. SMITH,JR., Anal. Chem. 41, 10% (1969). [4] F. J. FELDMAN,Anol. Chem. 42,719 (1970). [5] K. NAKANO,T. TAKADAand T. SATHO,Nippon Kagnku Zasshi 91,293 (1970). [6] T. TAKADAand K. NAKANO,Anal. Chim. Acto 107. 129 (1979). .%.(B) 36/7-J
735
736
T. TAKADAand K. NAKANO
where cl is the amount of sample solution in the flame, p, the degree of atomization of the element in the flame, C,, the concentration of the analyte in the sample solution and V, the volume of the flame. When there are changes in the amount of aspirated solution introduced into the flame, or in the atomization efficiency of an element, or in the volume of the flame, the number of free atoms will also change markedly. Now, if a known amount of an internal standard element is added to the sample solution, we have (3) where AZ, K,, 12, eZ, &, C, and V2 are the corresponding quantities for the internal standard element. Since both internal standard and analyte are measured under identical conditions, the absorbance ratio of the two elements reduces to (4) It follows from this equation that the ratio Al/A2 will not be affected by a change in the flame conditions, for example, a change in the flame temperature, if the elements exhibit the same relative change in degree of atomisation. Mathematically
1 dp, dln(AllA2) =---__= dT
PidT
1 dP2 PzdT
0 ’
(5)
Elements that fulfil these conditions can be used as internal standard for each other in a procedure where the ratio Al/AZ instead of A, alone is taken as the measure of concentration. If we wish to apply this principle to atomic absorption measurements, we meet two problems. One is the simultaneous detection of the two signals. The application of an internal standard to atomic absorption has been difficult because most of the instruments are single-channel monochromators. Therefore we developed a new dualchannel monochromator system. The second problem is the establishment of general rules for internal standard selection. As is clearly seen from the previous equation, reasonable criteria for the selection of internal standards should include the consideration of the degree of atomization. and therefore it is necessary to study the factors affecting atomization efficiency. The important factors are, firstly, the temperature of the atomizer flame, and secondly, the composition of the flame, mainly the ratio of fuel to oxygen in the flame, and, thirdly, the sample composition. The degree of atomization of an element in the flame is given by a B-factor[7] which is defined by:
where N, is the concentration of free atoms of the element and N, the total concentration of the element. Using equation (6), it is possible to calculate the p-factor from the measurement of N,,, and N,. but the determination of N, is difficult because this requires an absolute measurement of N,. The following relation suffices if analyte and internal standard are compared.
dA, _ dAz -_dT
dT’
[71 L. DECALANand J. D. WINEFORDNER, J. Quant. Specfty Radiat. Transfer I. 251 (1967).
(7)
Internal standard method in flame atomic absorption spectrometry “6”T - d’d”;-p”
Ci,
131
(8)
where i = 1.2. Therefore, we measured the relative change in absorbance by running absorbance measurements on several elements as a function of the amount of sample solution introduced into the flame, while changing the in flame temperature and also of the flame composition. 3. EXPERIMENTAL 3.1 Experimental system Atomic absorption measurements were made with a dual-channel atomic absorption spectrometer which was designed in our larobatory and assembled by the Nippon Jarrell-Ash Company[6]. Two hollow cathode lamps are placed at right angles to each other, one for the analyte element, and one for the internal standard element. The light beams from the lamps are combined by a half-mirror and after having passed through the flame, they are split again into two beams by a next half-mirror. Each beam falls on a slit of either of two monochromators and is detected with the corresponding photodetector. The sample signal and internal standard signal are amplified synchronously with the pertinent lamp currents by lock-in amplifiers. The ratio of the two absorbance signals is computed by an analog computer. The burner employed was the usual premix-type with an air-acetylene flame. As an air flow is used to nebulize the sample solution in this system, varying the air flow to change the solution supply would add the complication of changing the ratio of fuel to oxidant. To avoid this, a heated laboratory built spray chamber without water-cooled condenser[8] was used to vary the amount of water introduced into the flame while maintaining a constant aspirant flow rate of water. In the present system, a 35 mm wide and 150 mm long Pyrex glass cylindrical chamber was used. The exterior of the chamber was wrapped with a 400 W ribbon heater, and the heater input and chamber. temperature were controlled by a variable transformer connected with a thermistor inserted in to the chamber and with an associated bridge circuit. The resulting aerosol particle size was of the order of one tenth to one twentieth smaller than the unheated droplets of the sprayed solution[9]; many very fine droplets were produced and large amounts of solution could be introduced into the flame. The measuring conditions and other experimental conditions in each case are given in Table 1. 4. RESULTS AND DISCUSSION 4.1 Variation of flame temperature with the amount of water introduced into the flame With an unheated chamber the cooling effect of the flame due to introduced water is generally smaller[ IO], but it seems very significant for the heated spray chamber without condenser, where a large amount of water is forced into the flame, and may be expected to lower its temperature markedly. An attempt was made to estimate the decrease in flame temperature caused by the introduction of water. The flame temperature was measured by introducing a rod of zirconium oxide into the flame and determining its brightness temperature with an optical pyrometer (Model IR-PH 100 Chino, Japan). Figure 1 shows the characteristic curve of the temperature change in the flame due to the amount of aqueous solution introduced. The cooling effect of water with the system of the heated chamber without condenser and premixed flame is clearly indicated in this figure. Using the calibration data, the temperature of the flame was estimated to vary from about 2420 to 2218 K with the introduction of 1.0 ml min of water. The present study fully utilized this effect for determining the change of atomization efficiencies of many metal elements brought about by the cooling effect. [8] T. TAKADA and K. NAKANO, Nippon Kagaku [9] T. TAKADA, unpublished data. [IO] R. AVNI and C. T. J. ALKEMADE, Mikrochim.
Zasshl Acta
90, 383 (1969). 1960. 460.
T. TAKADAand K. NAKANO
738
Table 1. Apparatus and experimental Spectrometers Mounting Gratings Slit width Lamp sources Atomizer burner Nebulizer Spray chamber
A(nm) Li Na K Rb cs Mg Ca Sr Ba
670.8 589.6 766.5 780.0 852.1 285.2 422.7 460.7 553.5
conditions used in this study
Dual-channel atomic absorption spectrometer 0.5 m Ebert 1180 lines per mm used in the first order Entrance: 100 Nrn. Exit: 150 Frn Two phase pulse discharge 100 mm slit burner, air-acetylene flame Varian nebulizer Aspirant flow rate: 3.4 ml min-’ Heated spray chamber without condenser Heating power: 400 W Amount of sample solution introduced into the flame: 0.05-1.2 ml min-’
Test elements, wavelengths and concentrations A(nm) A(nm) pg ml-’ pg ml-’ 1.00 0.25 0.25 1.00 2.00 0.20 5.00 20.0 200.0
Ag Bi Cd co Cr cu Fe Hg In
328.1 223.1 228.8 240.7 357.9 324.7 248.9 253.7 303.9
0
0.5 Sob&on anod
2.0 20.0 0.5 4.0 5.0 2.5 5.0 100 20.0
Mn MO Ni Pb Sb Tl Zn
279.5 313.3 232.0 217.0 217.6 276.8 213.9
fig ml-’ 1.0 50.0 4.0 10.0 20.0 20.0 0.6
10
enterrq the fbme.ml n-d
Fig. 1. Variation of the flame temperature with introduction of aqueous solution. Aspirant Row rate: 3.4 ml min-‘. air flow rate: 6.7 I mitt-‘, acetylene flow rate: 1.6 I mitt-‘.
Internal standard method in flame atomic absorption spectrometry
139
Measurement of the amount of sample solution introduced into the flame at each chamber temperature was carried out as follows. After spraying 5.0 ml of solution (Cu 10.0 kg ml-‘) the heated spray chamber, the interior of the burner, and the drain, were washed out. The washing solution and drain the solution were collected and made up to about 70 ml. After 10 ml of 10% ammonium citrate solution was added and adjusted to pH 9 by addition of aqueous ammonia, 10 ml of 0.1% diethyldithiocarbamate solution was added. The sample was then adjusted to 1OOml with water using a volumetric flask and transferred to a separatory funnel. The sample solution was shaken for 7 min in a separatory funnel with exactly 10 ml of carbon tetrachloride and the amount of copper was determined by spectrophotometry using a calibration curve. The amount of solution introduced into the flame was calculated from the difference of the sprayed and the determined amount of copper. 4.2 Effect of flame temperature 4.2.1 Alkali metals. The dependence of the absorbance on the amount of solution introduced into the flame varies with a particular metal and flame temperature. As seen in Fig. 2, each metal has an optimum amount at which its absorbance is maximum (all curves have been normalized to make the peak read 1.0) and also there is a small difference between the slopes of the curves (magnitudes of enhancement) for the metals. These results suggest that the absorbances of all metals are directly proportional to the amount of solution in the flame in the range where a relatively small amount of solution is introduced into the flame. So, up to each maximum, the predominant factor contributing to the enhancement of the absorbance of the metal is the increase in the amount of metal. However, when a sample solution is introduced beyond a certain rate, the large number of water molecules decreases the temperature because of power consumption and, in turn, reduces the number of free ground state atoms. This results in a decrease in absorbance of the metals. The solution amount at which the peak in the absorbance appears depends upon the degree of atomization of the particular metal. ’ The elements with high atomization efficiencies-e.g. sodium, potassium, rubidium and cesium-are fairly completely atomized even,in a low temperature flame, so their
Na K.Rb cs
Li
Soluhcm
omcunt e&n-q
the fbme.
ml
tii
Fig. 2. Dependence of the absorbances of the alkali metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions as in Fig. 1.
740
T. TAKADA and K. NAKANO
maxima appear in the region of high introduction rate. It is thought that the degree of compound formation is small. On the other hand, lithium shows a maximum at a lower rate. This suggests that the ability of compound formation of lithium is larger than for the other alkali metals; it is known that lithium hydroxide is produced in the flame [I I]. flame[ll]. When the cooling effect of water becomes predominant, the absorbance of elements with lower atomization efficiency falls off more rapidly than that of elements with higher atomization efficiency. Therefore, the characteristic shape of the curve illustrates how the degree of atomization of each element varies with a given change in flame temperature. The internal standard for a particular element must have essentially the same behavior with respect to changes in the flame conditions. Elements of which the curves can be essentially superimposed, that is, have similar slopes, can be used as internal standards for each other. For example, the curves for sodium, potassium, rubidium and cesium are almost identical, so it is expected that they are a good internal standard mutually. 4.2.2 Alkaline earth metals. As shown in Fig. 3, the difference between the curves of absorbance vs solution introduction rate for alkaline earth metals are more marked than for alkali metals. The maximum points of the curves are rather widely separated. Also, in the rising parts of the curves there are considerable differences between the slopes. The differences in the downward slopes are larger than in the rising parts. For instance, when compared with calcium and strontium, barium is rather incompletely atomized and the degree of atomization decreases more rapidly with decreasing flame temperature. The cause of the decrease in atomization of calcium, strontium, and barium is considered to be related to the strong tendency to form compounds, such as hydroxide[l2-131 and oxide[l4]. Measurements and calculations
Fig. 3. Dependence of the absorbances of the alkaline earth metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions except acetylene Row rate of barium as in Fig. I. Acetylene flow rate of barium: I .9 I min-‘. [ll] [I21 [13] [14]
C. G. JAMES and T. M. SUCDEN, Proc. Roy. Sot. 227A. 312 (1955). C. G. JAMES and T. M. SUGDEN, Nature 175. 333 (1955). T. M. SIJGDEN and K. SCOFIELD, Trans.Faraday Sot. 62. 566 (1%2). R. W. B. PEARSE and A. G. GAYWN. The Identification of Molecular (1976).
Spectra.
Chapman and Hall
741
Internal standard method in flame atomic absorption spectrometry
of the degree of atomization of calcium, strontium and barium were carried out by ZEEGERS,TOWNSENDand WINEFORDNER[~~]. They showed that the measured j3 values of calcium, strontium and barium were 6.6 x 10V2,7.5 x 10e2 and 9 x 10e4, respectively. While a direct comparison with their results is impossible, because the present experiments were carried out under different conditions and the p values themselves were not estimated, our results as to the order in which the slopes of the curves vary are similar to theirs. On the other hand, magnesium seems to be fairly completely atomized, and the effect of a temperature decrease on the degree of atomization is thought to be smaller. Although magnesium may also form an oxide and a hydroxide [14], the low dissociation energy of the magnesium compounds [15-171 results in good efficiency of atomization. 4.2.3 Other metals. For various other metals the absorbance change due to variation in the rate of solution introduction were measured, and the results are as shown in Fig. 4. Copper, silver, cadmium and zinc are fairly completely atomized even when the temperature decreases. Therefore, it can be expected that they would be good internal standards for each other. Figure 5 shows a case in which copper is used as internal standard for cadmium. As seen in this figure, the slopes of the working curves obtained with the direct method increase drastically with the amount of standard solution aspirated, but the ratio of the signals of cadmium and copper shows very little change in the slope of the working curve, demonstrating the effectiveness of the correction by the internal standard. The foregoing experimental results are summarized in Table 2, in which the order of change in atomization efficiencies with decreasing flame temperature for various metals is shown. Thus, for example, it can be expected that copper will be a good internal standard for silver, zinc, cadmium and so on, and would be a poor one for molybdenum, lead, chromium and so on. 4.3 Effect of jlame composition Another factor affecting atomization efficiency is the composition of the flame, mainly the ratio of fuel to oxygen in the flame. Figure 6 shows the effect of a irariation in flame composition on the absorbances of the alkali metals. As seen in the figure, the
0
0
05
10
Solutlonamountentermg me flame, ml mri Fig. 4. Dependence of the absorbances of various metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions, except acetylene flow rate of molybdenum and chromium. as in Fig. 1. Acetylene flow rate of both metals: I .9 I min-‘. [15] P. J. TH. ZEECERS, W. P. TOWNSEND and J. D. WINEFORDNEK, Spectrochim. Acre 24B. 243 (1969). 1161 E. M. BULEWICZ and T. M. SUGDEN. Trans. Faraday Sot. 55,720 (1959). (I71 A. G. GAYDON. Dissociation Energies and Spectra of Diatomic Molecules. Chapman and Hall (1968).
T. TAKADA and K. NAKANO
742
Cd concentration,
pgml-’
Fig. 5. Calibration curves for various aspirant flow rates of sample solution. The dashed line represents the absorbance ratio of cadmium to copper for 0.39-4.26 ml/mitt-’ of aspirant flow rate. The aspirant solution of internal standard method contains 2.5 pg ml-’ copper. Air flow rate: 6.7: acetylene flow rate: 1.6 I min-‘.
Table 2. Order of change in atomization efficiency for various metals with decreasing flame temperature Alkali metals Alkaline earth metals Other metals
Li>Cs>K,Rb>Na Ba>Sr,Ca>Mg MO* > Cr* > Rb > Sb. Ni. Co. Fe > Mn. In. TI > Zn. Cd, Cu. Ag
*Measured in fuel-rich flame
0. Oxidiztng
Stoich
Reducing
Fig. 6. Relationships between the change in absorbances of alkali metals and flame type. The aspirant solutions except cesium contain 1000 wg ml-’ CsCl as an ionization suppressor and the cesium solution contains IO00 pg ml-’ RbCI. Air flow rate: 6. I I min-‘.
Internal standard method
in flame atomic absorption spectrometry
743
metals behave similarly when the flame condition changes from lean to rich and even when the geometry and the temperature of the flame are altered. Especially, these elements are not flame sensitive under oxidizing conditions. Therefore, it can be expected that the alkali metals will be good internal standards for each other in an oxidizing flame. The results for other elements, obtained in the same way as mentioned above, are shown in Fig. 7. As can be seen, these curves are rather complex, and a systematic analysis on the basis of atomization was not conclusive. One of the reasons for the complexity is the geometry of-the flame; that is, the light absorption path will become longer or shorter with a change in the ratio of fuel to air, which alters the flame temperature and the neutral atom populations. But, generally speaking, calcium, barium, chromium and molybdenum are very sensitive to flame composition, in contrast to zinc, thallium, and copper. The order of change in the atomization efficiencies with changing flame composition for various metals is shown in Table 3. However, for practical purposes, the elements of which curves have similar slopes can be used as mutual internal standards. Figure 8 shows the effect of flame composition on the absorbance ratios of several analytes to internal standards. Although calcium and strontium are known to be very sensitive to flame composition, the ratio of the calcium to strontium absorbances remained constant over a wide range of fuel-air ratios. Therefore, strontium will be a good internal standard for calcium and vice versa. 4.4 Efect of sample composition. One of the advantages of using an internal standard alkali
Oxidizing
Stoich.
Reducing
Fig. 7. Relationshps between the changes in the absorbances of various metals and the flame type. All curves have been normalized to make the peak read 1.0. Air flow rate:6.1 I min-‘.
Table 3. Order of change in atomization efficiency due to variation in the ratio of fuel to oxygen in the flame Alkali metals Alkaline earth metals Other metals
Li>K>Rb>Cs>Na Ca>Sr>Ba>Mg MO, Cr > Fe, Co, In > Ni, Mn, Sb, Hg > Ag, Cu, Cd, Zn, Pb, Bi, Tl
T. TAKADAand K. NAKANO
1.0
0 1.0
25
2.0
15 Acetylene
flow
rate
,
I min*
Fig. 8. Effect of acetylene flow rate on the absorbance ratio for some metals. Air flow rate: 6.1 I cnin-‘. Conditions as in Table 1.
is its effectiveness in combatting chemical interferences. Although this problem is interesting, systematic experiments were ditficuit because of complex phenomena caused by numerous interfer’ences. Figure 9 is a good example for demonstrating internaf standardization. As is well known, the presence of a small amount of phosphate causes a drastic decrease in the absorbance of calcium. But as shown in this figure, the calcium and strontium ratio remained constant over a wide range of..phosphoric acid concentrations.
H3F04
.
mol i-’
Fig. 9. Effect of HIPOl concentration on the change in absorbance of calcium and absorbance ratio of calcium and strontium. Air flow rate: 7.0. acetylene flow rate: 1.8 I min-‘, Ca: 25 gg ml-‘, Sr: 25 pg ml-‘.
Internal standard method in flame atomic absorption spectrometry
745
Table 4 shows similar examples observed for several acids. The acids depress the calcium signal unevenly, but intensity ratios are fairly constant. The internal standard method may easily correct for influence of such substances, and improve the precision and accuracy of analytical results. Table 4. Effect of addition of acids Acids 100 m mol I-’
None Acetic Citric Nitric Perchloric Sulfuric
Relative change in analysis signal .Ca/Sr Ca
1.00 1.01 0.87 0.71 0.91 0.55
1.00 1.00 0.90 0.95 1.01 0.82
5. CONCLUSIONS For internal standardization in flame atomic absorption spectrometry, the choice of an internal standard element is extremely important. For this reason, we developed a dud-channel spectrometer to establish the criteria for selecting internal standards in flame atomic absorption spectrome~y. Rational criteria for selection of internal standards will consider the change in degree of atomization and the factors affecting the atomization efficiency. Therefore the change in the degree of atomization due to a temperature decrease in the flame were systematically studied using a heated spray chamber without water-cooled condenser. With this system, a large amount of very fine droplets could be introduced into the flame. The greater the amount introduced the more the temperature of the air-acetylene flame was lowered. Therefore, absorbances at various flame temperatures could be measured by exploiting the cooling effect, and the absorbances were found to be strongly dependent on flame temperature. An internal standard for an analyte element must show essentially the same or at least a closely similar change in j3 value with temperature in the flame. A curve was experimentally drawn by plotting the change in absorbance for each metal as a function of the rate at which the sample solution is introduced into the flame. The elements of which the curves have similar slopes were recognized as satisfactory internal standards for each other. The flame composition also affects the degree of atomization. As a change in the flame composition may also change the temperature and geometry of the flame, fundamen~l discussions of the selection criteria with respect to changes in game composition are necessary but troublesome. We are now studying this problem as well as the practical implementations of the conclusions drawn hitherto.
Specrmchimico Acla Vol. 36B. No. 7. pp 735 to 745. 1981 Printed in Great Britain
Internal standard method in flame atomic absorption
Iw2.00/0 Press Ltd.
spectrometry
TAKEOTAKADAand KUNIO NAKANO Department
of Chemistry,
College of Science, Rikkyo (St. Paul’s) University, ku, Tokyo, 171. Japan
Nishi-Ikebukuro,
Toshima-
(Receioed 25 February 1981) Abstract-Some selection criteria and experimental considerations for selecting an internal standard in flame atomic absorption spectrometry were studied. Rational criteria for the selection of internal standards include the atomization efficiencies of the elements. An experimental study of the variation of the atomization degree due to a temperature decrease can be made with the help of plots of absorbance against the rate at which the sample solution is introduced. The temperature decrease produced by increasing the rate at which water was introduced using a heated chamber without water-cooled condenser was estimated to vary from 2420 K to 2218 K with 1.0 ml water per minute. Curves for the alkali. alkaline earth, and several other metals were obtained in the air-acetylene flame. The shapes of the curves are critically dependent on the degree of atomization of the metal itself. Elements with curves that have similar slopes can be used as mutual internal standards. Some of the other factors which affect the atomization efficiencies-i.e. changes in fuel and air flow, and matrix composition-were also studied.
1. IN~R~DLJC~~~N THE PRECISION of a measurement in atomic absorption spectrometry is mainly dependent on maintaining a constant atom production in the area of the flame traversed by the light beam. However, the absorption intensity of a flame atomizer is variable, because of the change in conditions such as flame temperature, aspiration rate-of the sample solution, flame composition, and so on. These factors have been recognized. in flame emission spectrometry, and an internal standard method is commonly used as a compensation technique [l]. In spite of the many advantages of an internal standard, reports of its applications to atomic absorption are scarce[2-6]; and indeed, no fundamental studies on the selection of an internal standard seem to have yet been made. The present paper describes some selection criteria and experimental considerations for selecting an internal standard in flame atomic absorption spectrometry. 2. THEORETICAL AND EXPERIMENTALBASIS The relation between the absorbance A, of an analyte element and the number of free atoms N, of the element in the flame can be simply formulated by the BeerLambert law
A, = K,l,N,,
(1)
where K, is a proportionality constant and 1, the path length in the flame. N, is proportional to the concentration in the sample solution of the analyte element. In this case, N, is given by N
-
‘dCl VI ’
(2)
[I] W. GERLACHand E. SCHWEIIZER, Foundations and Methods of Chemical Analysis by the Emission Spectrum, Vol. 2, p. 2. VosslAdam Hilger, London (1929). [2] L. R. P. BUTLERand A. STRASHEIM,Spectrochim. Acto 7, 1207 (1965). [3] F. J. FELDMAN,J. A. BLASI and S. B. SMITH,JR., Anal. Chem. 41, 10% (1969). [4] F. J. FELDMAN,Anol. Chem. 42,719 (1970). [5] K. NAKANO,T. TAKADAand T. SATHO,Nippon Kagnku Zasshi 91,293 (1970). [6] T. TAKADAand K. NAKANO,Anal. Chim. Acto 107. 129 (1979). .%.(B) 36/7-J
735
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T. TAKADAand K. NAKANO
where cl is the amount of sample solution in the flame, p, the degree of atomization of the element in the flame, C,, the concentration of the analyte in the sample solution and V, the volume of the flame. When there are changes in the amount of aspirated solution introduced into the flame, or in the atomization efficiency of an element, or in the volume of the flame, the number of free atoms will also change markedly. Now, if a known amount of an internal standard element is added to the sample solution, we have (3) where AZ, K,, 12, eZ, &, C, and V2 are the corresponding quantities for the internal standard element. Since both internal standard and analyte are measured under identical conditions, the absorbance ratio of the two elements reduces to (4) It follows from this equation that the ratio Al/A2 will not be affected by a change in the flame conditions, for example, a change in the flame temperature, if the elements exhibit the same relative change in degree of atomisation. Mathematically
1 dp, dln(AllA2) =---__= dT
PidT
1 dP2 PzdT
0 ’
(5)
Elements that fulfil these conditions can be used as internal standard for each other in a procedure where the ratio Al/AZ instead of A, alone is taken as the measure of concentration. If we wish to apply this principle to atomic absorption measurements, we meet two problems. One is the simultaneous detection of the two signals. The application of an internal standard to atomic absorption has been difficult because most of the instruments are single-channel monochromators. Therefore we developed a new dualchannel monochromator system. The second problem is the establishment of general rules for internal standard selection. As is clearly seen from the previous equation, reasonable criteria for the selection of internal standards should include the consideration of the degree of atomization. and therefore it is necessary to study the factors affecting atomization efficiency. The important factors are, firstly, the temperature of the atomizer flame, and secondly, the composition of the flame, mainly the ratio of fuel to oxygen in the flame, and, thirdly, the sample composition. The degree of atomization of an element in the flame is given by a B-factor[7] which is defined by:
where N, is the concentration of free atoms of the element and N, the total concentration of the element. Using equation (6), it is possible to calculate the p-factor from the measurement of N,,, and N,. but the determination of N, is difficult because this requires an absolute measurement of N,. The following relation suffices if analyte and internal standard are compared.
dA, _ dAz -_dT
dT’
[71 L. DECALANand J. D. WINEFORDNER, J. Quant. Specfty Radiat. Transfer I. 251 (1967).
(7)
Internal standard method in flame atomic absorption spectrometry “6”T - d’d”;-p”
Ci,
131
(8)
where i = 1.2. Therefore, we measured the relative change in absorbance by running absorbance measurements on several elements as a function of the amount of sample solution introduced into the flame, while changing the in flame temperature and also of the flame composition. 3. EXPERIMENTAL 3.1 Experimental system Atomic absorption measurements were made with a dual-channel atomic absorption spectrometer which was designed in our larobatory and assembled by the Nippon Jarrell-Ash Company[6]. Two hollow cathode lamps are placed at right angles to each other, one for the analyte element, and one for the internal standard element. The light beams from the lamps are combined by a half-mirror and after having passed through the flame, they are split again into two beams by a next half-mirror. Each beam falls on a slit of either of two monochromators and is detected with the corresponding photodetector. The sample signal and internal standard signal are amplified synchronously with the pertinent lamp currents by lock-in amplifiers. The ratio of the two absorbance signals is computed by an analog computer. The burner employed was the usual premix-type with an air-acetylene flame. As an air flow is used to nebulize the sample solution in this system, varying the air flow to change the solution supply would add the complication of changing the ratio of fuel to oxidant. To avoid this, a heated laboratory built spray chamber without water-cooled condenser[8] was used to vary the amount of water introduced into the flame while maintaining a constant aspirant flow rate of water. In the present system, a 35 mm wide and 150 mm long Pyrex glass cylindrical chamber was used. The exterior of the chamber was wrapped with a 400 W ribbon heater, and the heater input and chamber. temperature were controlled by a variable transformer connected with a thermistor inserted in to the chamber and with an associated bridge circuit. The resulting aerosol particle size was of the order of one tenth to one twentieth smaller than the unheated droplets of the sprayed solution[9]; many very fine droplets were produced and large amounts of solution could be introduced into the flame. The measuring conditions and other experimental conditions in each case are given in Table 1. 4. RESULTS AND DISCUSSION 4.1 Variation of flame temperature with the amount of water introduced into the flame With an unheated chamber the cooling effect of the flame due to introduced water is generally smaller[ IO], but it seems very significant for the heated spray chamber without condenser, where a large amount of water is forced into the flame, and may be expected to lower its temperature markedly. An attempt was made to estimate the decrease in flame temperature caused by the introduction of water. The flame temperature was measured by introducing a rod of zirconium oxide into the flame and determining its brightness temperature with an optical pyrometer (Model IR-PH 100 Chino, Japan). Figure 1 shows the characteristic curve of the temperature change in the flame due to the amount of aqueous solution introduced. The cooling effect of water with the system of the heated chamber without condenser and premixed flame is clearly indicated in this figure. Using the calibration data, the temperature of the flame was estimated to vary from about 2420 to 2218 K with the introduction of 1.0 ml min of water. The present study fully utilized this effect for determining the change of atomization efficiencies of many metal elements brought about by the cooling effect. [8] T. TAKADA and K. NAKANO, Nippon Kagaku [9] T. TAKADA, unpublished data. [IO] R. AVNI and C. T. J. ALKEMADE, Mikrochim.
Zasshl Acta
90, 383 (1969). 1960. 460.
T. TAKADAand K. NAKANO
738
Table 1. Apparatus and experimental Spectrometers Mounting Gratings Slit width Lamp sources Atomizer burner Nebulizer Spray chamber
A(nm) Li Na K Rb cs Mg Ca Sr Ba
670.8 589.6 766.5 780.0 852.1 285.2 422.7 460.7 553.5
conditions used in this study
Dual-channel atomic absorption spectrometer 0.5 m Ebert 1180 lines per mm used in the first order Entrance: 100 Nrn. Exit: 150 Frn Two phase pulse discharge 100 mm slit burner, air-acetylene flame Varian nebulizer Aspirant flow rate: 3.4 ml min-’ Heated spray chamber without condenser Heating power: 400 W Amount of sample solution introduced into the flame: 0.05-1.2 ml min-’
Test elements, wavelengths and concentrations A(nm) A(nm) pg ml-’ pg ml-’ 1.00 0.25 0.25 1.00 2.00 0.20 5.00 20.0 200.0
Ag Bi Cd co Cr cu Fe Hg In
328.1 223.1 228.8 240.7 357.9 324.7 248.9 253.7 303.9
0
0.5 Sob&on anod
2.0 20.0 0.5 4.0 5.0 2.5 5.0 100 20.0
Mn MO Ni Pb Sb Tl Zn
279.5 313.3 232.0 217.0 217.6 276.8 213.9
fig ml-’ 1.0 50.0 4.0 10.0 20.0 20.0 0.6
10
enterrq the fbme.ml n-d
Fig. 1. Variation of the flame temperature with introduction of aqueous solution. Aspirant Row rate: 3.4 ml min-‘. air flow rate: 6.7 I mitt-‘, acetylene flow rate: 1.6 I mitt-‘.
Internal standard method in flame atomic absorption spectrometry
139
Measurement of the amount of sample solution introduced into the flame at each chamber temperature was carried out as follows. After spraying 5.0 ml of solution (Cu 10.0 kg ml-‘) the heated spray chamber, the interior of the burner, and the drain, were washed out. The washing solution and drain the solution were collected and made up to about 70 ml. After 10 ml of 10% ammonium citrate solution was added and adjusted to pH 9 by addition of aqueous ammonia, 10 ml of 0.1% diethyldithiocarbamate solution was added. The sample was then adjusted to 1OOml with water using a volumetric flask and transferred to a separatory funnel. The sample solution was shaken for 7 min in a separatory funnel with exactly 10 ml of carbon tetrachloride and the amount of copper was determined by spectrophotometry using a calibration curve. The amount of solution introduced into the flame was calculated from the difference of the sprayed and the determined amount of copper. 4.2 Effect of flame temperature 4.2.1 Alkali metals. The dependence of the absorbance on the amount of solution introduced into the flame varies with a particular metal and flame temperature. As seen in Fig. 2, each metal has an optimum amount at which its absorbance is maximum (all curves have been normalized to make the peak read 1.0) and also there is a small difference between the slopes of the curves (magnitudes of enhancement) for the metals. These results suggest that the absorbances of all metals are directly proportional to the amount of solution in the flame in the range where a relatively small amount of solution is introduced into the flame. So, up to each maximum, the predominant factor contributing to the enhancement of the absorbance of the metal is the increase in the amount of metal. However, when a sample solution is introduced beyond a certain rate, the large number of water molecules decreases the temperature because of power consumption and, in turn, reduces the number of free ground state atoms. This results in a decrease in absorbance of the metals. The solution amount at which the peak in the absorbance appears depends upon the degree of atomization of the particular metal. ’ The elements with high atomization efficiencies-e.g. sodium, potassium, rubidium and cesium-are fairly completely atomized even,in a low temperature flame, so their
Na K.Rb cs
Li
Soluhcm
omcunt e&n-q
the fbme.
ml
tii
Fig. 2. Dependence of the absorbances of the alkali metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions as in Fig. 1.
740
T. TAKADA and K. NAKANO
maxima appear in the region of high introduction rate. It is thought that the degree of compound formation is small. On the other hand, lithium shows a maximum at a lower rate. This suggests that the ability of compound formation of lithium is larger than for the other alkali metals; it is known that lithium hydroxide is produced in the flame [I I]. flame[ll]. When the cooling effect of water becomes predominant, the absorbance of elements with lower atomization efficiency falls off more rapidly than that of elements with higher atomization efficiency. Therefore, the characteristic shape of the curve illustrates how the degree of atomization of each element varies with a given change in flame temperature. The internal standard for a particular element must have essentially the same behavior with respect to changes in the flame conditions. Elements of which the curves can be essentially superimposed, that is, have similar slopes, can be used as internal standards for each other. For example, the curves for sodium, potassium, rubidium and cesium are almost identical, so it is expected that they are a good internal standard mutually. 4.2.2 Alkaline earth metals. As shown in Fig. 3, the difference between the curves of absorbance vs solution introduction rate for alkaline earth metals are more marked than for alkali metals. The maximum points of the curves are rather widely separated. Also, in the rising parts of the curves there are considerable differences between the slopes. The differences in the downward slopes are larger than in the rising parts. For instance, when compared with calcium and strontium, barium is rather incompletely atomized and the degree of atomization decreases more rapidly with decreasing flame temperature. The cause of the decrease in atomization of calcium, strontium, and barium is considered to be related to the strong tendency to form compounds, such as hydroxide[l2-131 and oxide[l4]. Measurements and calculations
Fig. 3. Dependence of the absorbances of the alkaline earth metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions except acetylene Row rate of barium as in Fig. I. Acetylene flow rate of barium: I .9 I min-‘. [ll] [I21 [13] [14]
C. G. JAMES and T. M. SUCDEN, Proc. Roy. Sot. 227A. 312 (1955). C. G. JAMES and T. M. SUGDEN, Nature 175. 333 (1955). T. M. SIJGDEN and K. SCOFIELD, Trans.Faraday Sot. 62. 566 (1%2). R. W. B. PEARSE and A. G. GAYWN. The Identification of Molecular (1976).
Spectra.
Chapman and Hall
741
Internal standard method in flame atomic absorption spectrometry
of the degree of atomization of calcium, strontium and barium were carried out by ZEEGERS,TOWNSENDand WINEFORDNER[~~]. They showed that the measured j3 values of calcium, strontium and barium were 6.6 x 10V2,7.5 x 10e2 and 9 x 10e4, respectively. While a direct comparison with their results is impossible, because the present experiments were carried out under different conditions and the p values themselves were not estimated, our results as to the order in which the slopes of the curves vary are similar to theirs. On the other hand, magnesium seems to be fairly completely atomized, and the effect of a temperature decrease on the degree of atomization is thought to be smaller. Although magnesium may also form an oxide and a hydroxide [14], the low dissociation energy of the magnesium compounds [15-171 results in good efficiency of atomization. 4.2.3 Other metals. For various other metals the absorbance change due to variation in the rate of solution introduction were measured, and the results are as shown in Fig. 4. Copper, silver, cadmium and zinc are fairly completely atomized even when the temperature decreases. Therefore, it can be expected that they would be good internal standards for each other. Figure 5 shows a case in which copper is used as internal standard for cadmium. As seen in this figure, the slopes of the working curves obtained with the direct method increase drastically with the amount of standard solution aspirated, but the ratio of the signals of cadmium and copper shows very little change in the slope of the working curve, demonstrating the effectiveness of the correction by the internal standard. The foregoing experimental results are summarized in Table 2, in which the order of change in atomization efficiencies with decreasing flame temperature for various metals is shown. Thus, for example, it can be expected that copper will be a good internal standard for silver, zinc, cadmium and so on, and would be a poor one for molybdenum, lead, chromium and so on. 4.3 Effect of jlame composition Another factor affecting atomization efficiency is the composition of the flame, mainly the ratio of fuel to oxygen in the flame. Figure 6 shows the effect of a irariation in flame composition on the absorbances of the alkali metals. As seen in the figure, the
0
0
05
10
Solutlonamountentermg me flame, ml mri Fig. 4. Dependence of the absorbances of various metals on the amount of solution introduced into the flame. All curves have been normalized to make the peak read 1.0. Conditions, except acetylene flow rate of molybdenum and chromium. as in Fig. 1. Acetylene flow rate of both metals: I .9 I min-‘. [15] P. J. TH. ZEECERS, W. P. TOWNSEND and J. D. WINEFORDNEK, Spectrochim. Acre 24B. 243 (1969). 1161 E. M. BULEWICZ and T. M. SUGDEN. Trans. Faraday Sot. 55,720 (1959). (I71 A. G. GAYDON. Dissociation Energies and Spectra of Diatomic Molecules. Chapman and Hall (1968).
T. TAKADA and K. NAKANO
742
Cd concentration,
pgml-’
Fig. 5. Calibration curves for various aspirant flow rates of sample solution. The dashed line represents the absorbance ratio of cadmium to copper for 0.39-4.26 ml/mitt-’ of aspirant flow rate. The aspirant solution of internal standard method contains 2.5 pg ml-’ copper. Air flow rate: 6.7: acetylene flow rate: 1.6 I min-‘.
Table 2. Order of change in atomization efficiency for various metals with decreasing flame temperature Alkali metals Alkaline earth metals Other metals
Li>Cs>K,Rb>Na Ba>Sr,Ca>Mg MO* > Cr* > Rb > Sb. Ni. Co. Fe > Mn. In. TI > Zn. Cd, Cu. Ag
*Measured in fuel-rich flame
0. Oxidiztng
Stoich
Reducing
Fig. 6. Relationships between the change in absorbances of alkali metals and flame type. The aspirant solutions except cesium contain 1000 wg ml-’ CsCl as an ionization suppressor and the cesium solution contains IO00 pg ml-’ RbCI. Air flow rate: 6. I I min-‘.
Internal standard method
in flame atomic absorption spectrometry
743
metals behave similarly when the flame condition changes from lean to rich and even when the geometry and the temperature of the flame are altered. Especially, these elements are not flame sensitive under oxidizing conditions. Therefore, it can be expected that the alkali metals will be good internal standards for each other in an oxidizing flame. The results for other elements, obtained in the same way as mentioned above, are shown in Fig. 7. As can be seen, these curves are rather complex, and a systematic analysis on the basis of atomization was not conclusive. One of the reasons for the complexity is the geometry of-the flame; that is, the light absorption path will become longer or shorter with a change in the ratio of fuel to air, which alters the flame temperature and the neutral atom populations. But, generally speaking, calcium, barium, chromium and molybdenum are very sensitive to flame composition, in contrast to zinc, thallium, and copper. The order of change in the atomization efficiencies with changing flame composition for various metals is shown in Table 3. However, for practical purposes, the elements of which curves have similar slopes can be used as mutual internal standards. Figure 8 shows the effect of flame composition on the absorbance ratios of several analytes to internal standards. Although calcium and strontium are known to be very sensitive to flame composition, the ratio of the calcium to strontium absorbances remained constant over a wide range of fuel-air ratios. Therefore, strontium will be a good internal standard for calcium and vice versa. 4.4 Efect of sample composition. One of the advantages of using an internal standard alkali
Oxidizing
Stoich.
Reducing
Fig. 7. Relationshps between the changes in the absorbances of various metals and the flame type. All curves have been normalized to make the peak read 1.0. Air flow rate:6.1 I min-‘.
Table 3. Order of change in atomization efficiency due to variation in the ratio of fuel to oxygen in the flame Alkali metals Alkaline earth metals Other metals
Li>K>Rb>Cs>Na Ca>Sr>Ba>Mg MO, Cr > Fe, Co, In > Ni, Mn, Sb, Hg > Ag, Cu, Cd, Zn, Pb, Bi, Tl
T. TAKADAand K. NAKANO
1.0
0 1.0
25
2.0
15 Acetylene
flow
rate
,
I min*
Fig. 8. Effect of acetylene flow rate on the absorbance ratio for some metals. Air flow rate: 6.1 I cnin-‘. Conditions as in Table 1.
is its effectiveness in combatting chemical interferences. Although this problem is interesting, systematic experiments were ditficuit because of complex phenomena caused by numerous interfer’ences. Figure 9 is a good example for demonstrating internaf standardization. As is well known, the presence of a small amount of phosphate causes a drastic decrease in the absorbance of calcium. But as shown in this figure, the calcium and strontium ratio remained constant over a wide range of..phosphoric acid concentrations.
H3F04
.
mol i-’
Fig. 9. Effect of HIPOl concentration on the change in absorbance of calcium and absorbance ratio of calcium and strontium. Air flow rate: 7.0. acetylene flow rate: 1.8 I min-‘, Ca: 25 gg ml-‘, Sr: 25 pg ml-‘.
Internal standard method in flame atomic absorption spectrometry
745
Table 4 shows similar examples observed for several acids. The acids depress the calcium signal unevenly, but intensity ratios are fairly constant. The internal standard method may easily correct for influence of such substances, and improve the precision and accuracy of analytical results. Table 4. Effect of addition of acids Acids 100 m mol I-’
None Acetic Citric Nitric Perchloric Sulfuric
Relative change in analysis signal .Ca/Sr Ca
1.00 1.01 0.87 0.71 0.91 0.55
1.00 1.00 0.90 0.95 1.01 0.82
5. CONCLUSIONS For internal standardization in flame atomic absorption spectrometry, the choice of an internal standard element is extremely important. For this reason, we developed a dud-channel spectrometer to establish the criteria for selecting internal standards in flame atomic absorption spectrome~y. Rational criteria for selection of internal standards will consider the change in degree of atomization and the factors affecting the atomization efficiency. Therefore the change in the degree of atomization due to a temperature decrease in the flame were systematically studied using a heated spray chamber without water-cooled condenser. With this system, a large amount of very fine droplets could be introduced into the flame. The greater the amount introduced the more the temperature of the air-acetylene flame was lowered. Therefore, absorbances at various flame temperatures could be measured by exploiting the cooling effect, and the absorbances were found to be strongly dependent on flame temperature. An internal standard for an analyte element must show essentially the same or at least a closely similar change in j3 value with temperature in the flame. A curve was experimentally drawn by plotting the change in absorbance for each metal as a function of the rate at which the sample solution is introduced into the flame. The elements of which the curves have similar slopes were recognized as satisfactory internal standards for each other. The flame composition also affects the degree of atomization. As a change in the flame composition may also change the temperature and geometry of the flame, fundamen~l discussions of the selection criteria with respect to changes in game composition are necessary but troublesome. We are now studying this problem as well as the practical implementations of the conclusions drawn hitherto.