or boiling point increased the enhancement values. However, enhancement showed little or no dependence on density or surface tension. This is not to mean that densit y and surface tension do not contribute to the enhancement effect, but that viscosity and boiling point are the more important physical properties to be used as guidelines in selecting solvents for use in atomic-absorption spectrometry. The mean droplet diameter in micrometers, D, obtained when a solution is atomized in a gas jet flowing a t a rate of C i s expressed by the empirical Equation (29)
585
D=-
fi
-+597
uo
looow
where c' = gas velocity in m/sec, u = surface tension in dyn/cm, d = density in g/cm3, 7 = coefficient of viscosity in poises, w = solution consumption rate, V = gas consumption rate. Calculations of D from this equation show that the droplet diameter is smaller for organic solvents than for water. The difference in size, however, is not very great. For example, using a jet flow of 350 m/sec and a value of w / V of 0,00033, the D value for water is 19 pm while that for MIBK is 13.9 pm. Therefore, droplet size is probably not the most important factor affecting enhancement. The more important effect of organic solvents is the (19) S. Nukiyarna and Y . Tanasawa, Trans. SOC.Mech. €ng. Jap., 5, 68 (1939).
increase in consumption rate. According to the Poiseuille equation
ve = A p r 4
(3)
where V, = volume per unit time flowing through a capillary, -Ip = differential pressure, r = capillary radius, 1 = capillary length, 7 = solution viscosity. Pungor and Mahr (20) showed that the only property of the aspirated solution which affects V, is the viscosity. Therefore, the experimental data obtained in this work tend to confirm the theoretical work indicating the viscosity to be the more important physical property of organic solvents determining the solution consumption rate. The smaller droplet size formed when using organic solvents is of lesser importance because, although there is a significant difference in the droplet size between organic solvents and water, the difference in size obtained by varying the organic solvent is very small. Therefore, surface tension was found to have little effect on enhancement. Received for review July 27, 1972. Accepted February 14, 1973. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society. for support of this work.
(20) E. Pungor and M. Mahr, Ta/anta. 10,537 (1963)
Interferences in Nickel Determinations by Atomic Absorption Spectrometry L. L. Sundberg Department of Chemistry, University of California. Los Angeles. Calif. 90024
Zinc, F e ( l l l ) , Cu, Co, M n ( l l ) , and C r ( l l l ) interfere with Ni determinations by atomic absorption spectrometry in oxidizing and reducing air-acetylene flames. The interferences produced by the latter five elements are of remarkably similar nature. The interferences are greatly influenced by observation height, and careful adjustment of this parameter can effectively eliminate them. In a reducing flame, the Ni absorbance can be enhanced or depressed by the same concomitant, and the direction of the interferences is further dependent upon the concentration of interfering species.
.
Atomic absorption spectrometry (AAS) is a highly versatile method of trace analysis with sensitivies a t the ppm level for some 70 different elements. During the early years, investigators stressed the virtual absence of interferences in atomic absorption as compared to emission techniques and other methods of elemental analysis. However, further studies indicated that interferences do in fact exist, and much research of late has been devoted to the study of these interferences and their mechanisms. Nickel is very accessible to AAS determinations and is routinely analyzed by this method in many laboratories. Interferences have been observed in several matrices including silver alloys ( I ) , steels (2, 3), ores (4, 5 ) , and cop1460
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per-based materials (6, 7). Enhancement, depression, and absence of interference by transition metals are reported and some of these findings conflict with one another. This is partly explained by differences in solution matrices and by instrumental modes of operation where the critical parameters, flame characterization, and burner elevation are not adequately allowed for. Of the conventional flames that have been tested in AAS, the one of greatest utility is air-acetylene. If air pressure, fuel pressure, and air flow are held constant, and fuel flow is varied, then the flame can be grossly classified as being oxidizing (fuel-lean), stoichiometric, or reducing (fuel-rich). One can usually adjust the fuel/oxidant flow ratio to an optimum value which affords one a sensitivity maximum in his determination. However, for any given flow ratio, the flame is not uniform over its entire vertical J, M. Viebrock, A n a / . Lett.. 3, 373 (1970). K. Kinson and C. 6 .Belcher. A n a / . Chim. Acta. 30, 64 (1964) S. Sprague and W. Slavin. "Developments in Applied Spectroscopy," E. N. Davis, Ed., Vol. 4, Plenum Press, New York. N.Y.. 1965, p 433. K. Kinson, J. E. Dickeson. and C. B. Belcher, Ana/ Chim. A c t a . 41, 107 (1968). V. Endo, T. Hata, and V. Nakahara. Bunseki Kagaku, 1 8 , 833 (1969). K. Itsuki, H. Kornuro, and T. Nagasawa, Bunseki Kagaku. 1 9 , 1282 (1970). 6 .Gandrud and J. C. Marshall, Appi. Spectiosc.. 2 4 , 367 (1970).
dimension as marked differences in shape, composition, and temperature exist. From this and the sensitive complex equilibria that occur in the flame, there should be a discrete zone within the flame that contains the greatest population ofafree atoms per unit volume, all else being equal. By adjusting the burner elevation in such a manner that the light path samples this optimum zone, another sensitivity maximum will be obtained. Unfortunately, the general practice is to aspirate a standard solution for the purpose of locating the optimum zone. If standards are not matched to samples with respect to bulk matrix, the introduction of concomitant species may shift the distribution of free atoms from the optimum zone whereby inevitable sensitivity losses and deviations from a standard working curve will occur. Such observations generally result in interferences. In our laboratory iron meteorites are analyzed for Ni by atomic absorption and for Ga, Ge, and Ir by radiochemical neutron activation ~ ( 8 )Recently, . a study (9) was made to determine if the radiochemical carriers or the major constituents of iron meteorites (Fe, Co, etc.) interfere with the Ni determinations. Some transition metals produced remarkably large enhancements in Ni absorption. The present work was >undertakento evaluate these inferences more quantitatively.
Table I . Experimental Design Employed for the Study of Interferences Burner Analysis Wavelength, A elevation, mm Flame quality oxidizinga A 2320 0 oxidizing 0 3415 B C
D E
F G H I J AA
BB
cc DD EE
FF GG HH II JJ
KK LL MM “
EXPERIMENTAL Stock Solutions. Solutions containing 10,000 ppm (w/v) Ni, Zn, Co, Fe(II1). Cr(III), and Mn(I1) were prepared by dissolving 10.000 grams of reagent grade metal powder in a minimum of hydrochloric acid and diluting to one liter. A Cu solution of the same concentration was prepared by dissolving 26.830 grams of reagent grade CuC12.2H20 in one liter of water. S i t r a t e ion was avoided because of reported interferences ( 9 ) .Distilled and deionized water was used in all dilutions. All working solutions were prepared in triplicate. Apparatus. The atomic absorption spectrophotometer used was a Perkin-Elmer Model 303 with readout cia a Sargent Model MK recorder. A premix burner assembly (equipped with a standard flow spoiler) with a 10-cm single slot head was used. The flame gas was air-acetylene and the emission source was a Perkin-Elmer No. 303-6047 hollow-cathode lamp. The working conditions were: fuel pressure, 8 psig; air pressure, 30 psig; aspiration rate, 3.5 ml/min: lamp current, 25 mA; slit width, 0.3 mm. Interferences were studied in both oxidizing and reducing flames. The respective acetylene flows were 2.8 and 4.2 l./min while the air flow was maintained a t 18.7 l./min. Differences in flame velocity were negligible. Other variables were wavelength. 2320 and 3415 A, and burner elevation. All observation heights were referenced from the tangent between the light beam at its focal point and the top of the burner head. The vertical dimensions of the beam ranged from 15 mm at the edge of the burner slot to 6 mm at the focal point. Procedure. To ensure against temperature differences between standards and interference-test samples, all solutions were equilibrated for 24 hours to the temperature of the room which housed the spectrophotometer. 111 this study, the above settings were fixed and absorbance of the standards was maximized by rotational and horizontal adjustments of the burner assembly. Of the three variables !in this investigation, burner elevation, wavelength, and flame quality, the last was found to be the most difficult to reproduce with an adequate degree of certainty on a short term basis. Table I summarizes the design used in this series of experiments. The same standards and samples were run for every analysis. With this approach, one is able to obtain measurements at both Wavelengths under nearly identical conditions. The number of analyses was limited by instability in the upper regions of the flame. For each analysis, the following sequence of measurements was made: blank. triplicate standards, blank, triplicate samples, blank, standards, etc., until a minimum of 12 and 9 readings were obtained for standards and samples, respectively. Interferences are expressed as 100(A, - A,9)/A, where A , is the absorbance of (8) J. T. Wasson and R . Schaudy, /carus. 1 4 , 59 (1971) (9) S . M Gabrielian, UCLA. tunpublished work (1971)
3415 2320 2320 3415 3415 2320 2320 341 5 2320 3415 3415 2320 2320 341 5 341 5 2320 2320 3415 3415 2320 2320 341 5
6 6 12 12 18 18 24 24
0 0 6 6 12 12 18 18 24 24 30 30 36 36
oxidizing oxidizing oxidizing oxidizing oxidizing oxidizing oxidizing oxidizing reducingb reducing reducing reducing reducing reducing reducing reducing reducing reducing reducing reducing reducing reducing
flow, 18.7 I.!min; acetylene flow, 2.8 l . j m i n . ” A i r flow, 18.7 I.; acetylene flow. 4.2I./min.
OAir
min.
the sample and A , is the absorbance of the standard. Both were calculated from the average of six or more per cent transmission readings.
RESULTS AND DISCUSSION The effects of transition metal concentrations of 2000 ppm on the absorbance of 20 ppm Xi are shown in Figures la-f, where interference is plotted against burner elevation. Ordinate values are always referenced to the standard absorbance for any given flame, wavelength, and burner elevation. With the exceptions of Zn (Figure If) and Co (Figure l e ) , the plots share many common features. For all elevations in both flames the interferences (either enhancements or depressions) appear to be more serious at 3415 than a t 2320 A. Interferences in the oxidizing flame are depressions which increase with increasing observation height. In the reducing flame, serious enhancements occur a t low observation heights, but a slight increase produces a “transition” following which depressions are observed. Taking the Mn data (Figure l b ) a t 3415 A as an example, observations a t 12 mm show an 11% enhancement, but a t 18 mm the absorbance is depressed by 9%. Subsequent analyses a t 15 mm showed an enhancement of less than 1%. The most striking similarities in Figure 1 are found in the Mn, Cu, and Cr data (Figure 1.b-d). For these elements the interferences are of comparable magnitude and the transitions all occur a t 15 f 1 mm. Although the Co interferences in the reducing flame compare favorably (Figure le), the transition does not occur a t the same observation height for both wavelengths, and the depressions a t 2320 A appear to be more serious than a t 3415 A. A spectral interference a t 3415 A would accommodate these observations, but this has not been reported. For Fe (Figure l a ) , the interferences in the reducing flame are generally less pronounced and the transition occurs a t 11 mm. Zinc (Figure I f ) does not show any of these trends. This type of investigation provides a caveat for the novice, as often all else is sacrificed to obtain maximum senA N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 8, J U L Y 1973
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0
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I
I
I
I
I
I
I
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6
12
18
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30
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BURNER ELEVATION (mm)
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W
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sitivity. Figure 2 shows that such an approach can lead to a Pyrrhic victory as the greatest absorbances for the standards occur a t an observation height (6 and 18 mm for the oxidizing and reducing flames, respectively) where interferences are more pronounced. However, by accepting minor sensitivity losses through changing the elevation from the position of the absorbance maxima, interference can be appreciably reduced. Some workers ( I O , 11) have recommended that Ni analyses using AAS should employ an oxidizing flame and the 2320-11 resonance line. The present data support these findings, .and we strongly suggest that Ni analyses always be done in an oxidizing flame. Nonetheless, it appeared to (10) "Analytical Methods for Atomic Absorption Spectroscopy," PerkinElmer Corporation, Norwalk, Conn., 1971, p SC-Ni 1. (11) W . Slavin. 'Atomic Absorption Spectroscopy," John Wiiey-lnterscience, New York, N.Y., 1968, p 136.
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ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, JULY 1973
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I
I
be of interest to investigate the interferences encountered in more detail. As the previous plots had the same general features, Cu was chosen as a representative interferent in further studies. A second investigation examined absorbance differences between 10, 20, 30, 40, and 50 ppm Xi solutions, each containing 2000 ppm Cu and standards of identical concentrations. Results using the oxidizing flame showed the expected depressions. In Figures 3a and 3b, absorbance in the reducing flame is plotted against Ni concentration. The two burner elevations are those positions that bracketed the interference transition. The calibration curves now enable one to accurately express the errors (per cent interference) in terms of concentration. When this is done, the differences in interference between the two wavelengths become negligible. The discrepancies found earlier could be attributed to differing
1
Table I I. Monoxide Dissociation Energies (20)
0.4
Oxide
D O ,kcal
65
ZnO CUO
95
96
MnO NiO
CrO
97 99 101
coo
...
FeO
t
0.2
-
(100-200 ppm) depress Ni absorbance but higher concentrations give rise to enhancements. However, in the upper
"
Figure 2. Composite absorbance profiles for 20 pprn NI 3415 A , reducing flame, 0 2320 A , reducing flame, A 3415 A , oxldlzing flame, +2320 A, oxidizing flame
flame (Figure 1) rule out any classical physical interferences, and appreciable light losses due to scattering Seem unlikely (12). The absorbance of the Ni standards is
.7 .6
a . 12
c ~
b. 18 mm
/
w
0
z Q
m [r
0
m
m
Q
0
IO
20 30 40 NICKEL (ppm)
50
NICKEL (ppm)
Figure 3. Effects of 2000 pprn Cu on N i absorbance in the reducing flame. Observation height = (a) 1 2 rnrn, ( b ) 18 rnrn +Ni only, 3415 A;
Ni
+ 2000 ppm Cu, 3415 A; A Ni only, 2320 A; and 0 Ni + 2000 pprn Cu, 2320 A
slopes between the two working curves. As a first approximation, one might expect the interferences to be more pronounced a t the lower concentrations of Ni, where the Cu-to-Ni concentration ratios are larger. However, for any given analysis, the percentage interference encountered with 10 ppm Ni differs only slightly from those found using the other standards for both cases where Cu enhances or depresses Xi absorbance. It thus appeared that Cu was the critical factor in governing the magnitude of the interferences, and a third investigation was performed to confirm this speculation. This examined the effect of varying Cu concentrations ranging from 100 t o 5000 ppm on the absorbance of 20 ppm Ni. The data are shown in Figure 4, where Cu concentration as abscissa is plotted logarithmically against per cent interference in terms of absorbance for selected
greater in the oxidizing flame than in the reducing flame, which shows that Ni free-atom population is strongly dependent on flame quality. Taylor et al. (13) have pointed out that changing the fuel-to-air ratio in a given flame can disturb free-atom density of the analyte by changing the temperature, the concentration of flame oxygen, or the degree of background emission or absorption. Probably all of these changes occur to some extent. If the interferences are of a chemical nature, the transition metals disturb the processes that produce free Ni atoms in the flame. In the cooler reducing flame, where a t certain observation heights a decrease in Ni absorbance occurs (Figures 1,a-f), one might speculate that preferen(12) S. R . Koirtyohann and E. E. Pickett,Ana/. C h e m . . 38, 1087 (1966). (13) J. H. Taylor, T. T. Bartels, and N. L. Crurnp, Ana/. C h e m . . 43, 1780 (1971). ANALYTICAL
CHEMISTRY,
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1973
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hkg)
+20W
v
z
-
W
LT LT W
c
z
-
L
l
//
/
io3 COPPER (pprn) Figure 4. Effects of varying Cu concentration on the absorbance of 20 ppm N i in the reducing flame IO2
W 0 mm,
A 6 rnm. 0 12 mrn, + I 8 rnm, and V 24 m m
tial formation of X i 0 or of a mixed oxide between Ni and another transition metal occurs. Such explanations are sometimes given to account for cationic interference phenomena (14-16), but no such compounds have been reported for Ni. Ionization interferences in the hotter oxidizing flame might be of some significance. Of the transition metals investigated, Cr and Mn have first ionization potentials lower than Ni; the others have higher ionization potentials. If ionization effects are important, Mn and Cr should enhance Ni absorbance. However, Figures l b and IC show the opposite effects. Because of this and the general similarity in depression effects produced by all these transition elements, ionization interferences can be ruled out. The large enhancements a t low observation heights in the reducing flame (Figure 1,a-e) apparently cannot be explained by any simple mechanism. When the dissociated solute molecule enters the flame, any or all of the following processes can occur: absorbance of resonance radiation by the analyte; establishment of an ionization-recombination equilibrium; association by the analyte with other species and establishment of a second equilibrium. I t has been postulated that this last pathway can account for cationic enhancements as the analyte and the concomitant compete for oxygen in the flame during the association process (17, 18). This competition results in the production of the greatest number of atoms for the metal whose oxide is least stable. Thus, for the reaction Niig)
+
00,
e
NiO,,,
(1)
the equilibrium would be shifted to the left when Ni is in the presence of a metal whose monoxide dissociation energy is greater than that of NiO. It has been further suggested that this competition for flame oxygen could increase the concentration of atomic Ni in the flame by the two related reactions shown below (19) (14) M. Yanagisawa, M. Suzuki, and T. Takeuchi, A n a / . Chim. A c t a . 52, 386 (1970). (15) J. Y. Hwang and L. M. Sandonato, A n a / . Chim. A c t a , 48, 188 (1969), (16) A . Fulton and L. R. P. Butler, Spectrosc. Lett., 1, 317 (1968). (17) T. Ramakrishna, P. West, and J. Robinson, A n a / . Chim. Acta. 39, 81 (1967) (18) S. Sachdev, J. W. Robinson, and P. W. West, A n a / . C h i n . A c t a . 37, 12 (1967).
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+
NiO(g1
F=
MOW
+
Ki(g)
(2b)
where M and MO represent a metal and its respective oxide. The dissociation energies of the transition metals investigated are shown in Table 11. Zinc (ZnO with a lower dissociation energy than NiO) should depress Ni absorbance, which it does (Figure I f ) . However, the other monoxides have dissociation energies all within 5% of the value for NiO. For this reason Reactions 1 and 2a should proceed roughly a t the same rate, and Reaction 2b should not occur to any appreciable extent. Previous studies on oxygen competition had been carried out in systems where the concomitant-to-analyte concentration ratios were never greater than 1O:l. In the present study, this ratio is often greater than 100:1, which can perhaps explain the data a t low observation heights in Figure 4. At a 1 O : l ratio, most Ni might be present as the oxide where a 1OO:l Cu/Ni ratio might cause the atomic form to dominate, and result in an enhancement plateau where further Cu additions do not produce significant changes. At observation heights of 18 mm or greater, the degree of interference is independent of observation height, and for most purposes is negligible. Another important factor that could cause differences between our observations and those of others is the maximum droplet size that is permitted to reach the flame. Such differences which can be reflected in nebulizer and burner design make it difficult to compare our results with the work of Robinson and coworkers ( I 7, 18). From these data, no satisfactory explanation can be put forth to accommodate all the interferences in a systematic fashion. This shows the complex nature of cationic interferences, and many more data are required to gain a better understanding of their mechanisms. At the present time, we have several experiments in progress. With the exception of Zn, the Ni interferences seem to be independent of the concomitant transition metal, and we are testing the mutuality of these interferences-Le., where Ni serves as the concomitant. For Cu, Cr, Mn, and Fe, the interference transitions occur a t the same observation height for both wavelengths. This indicates that the concomitant metals are somehow altering the distribution of free Ni atoms in a given vertical cross-section of the flame, and that a t some height in the flame the Xi atom concentration of the standard is the same as the interference-test sample. Our future efforts will be directed toward obtaining detailed contours for the distribution of Ni atoms in the flame by the method of Rann and Hambly (21). These will be obtained in the presence and absence of the concomitant transition metals. The differences in free-atom distribution reflect differences produced by the high solute concentrations, and for this reason temperature measurements will also be made at each point of traverse.
ACKNOWLEDGMENT The author is greatly indebted to J. T. Wasson, J. F. Kaufman, and J. E. Sells for their assistance in the preparation of this manuscript. Received for review October 18, 1972. Accepted February 12, 1973. This study was supported in part by NASA Grant NGR 05-007-291and NSF Grant GA-32084. (19) J. Y . Marks and G. G. Welcher. Anal. C h e m . . 42, 1033 (1970) (20) A . G. Gaydon, "Dissociation Energies and Spectra of Diatomic Moiecules," Chapman and Hall Ltd., London, 1968. (21) C. S. Rann and A . N. Harnbly,Anal. Chem.. 37, 879 (1965).
