Acid Interferences in Atomic Absorption Spectrometry William B. Barnett Perkin-Elmer Corp., Norwalk, Conn. 06852 A study was made of the interferences caused by five mineral acids with the determination of copper, chromium, manganese, and nickel by atomic absorption. Variations in acid concentration, flame type, and flame stoichiometry were examined. Significantly smaller interferences were found than had been previously reported, probably because of differences in burner design. The use of a lean air-acetylene flame on a single-slot head produced the fewest and smallest interferences. Shifting to a nitrous oxide-acetylene flame, often recommended as a means of reducing interferences, frequently increased them.
FLAME SPECTROMETRY offers a rapid, accurate method for the determination of metallic elements. One potential problem, widely discussed in the literature, is the extent and severity of analytical interferences. Recent reports ( I , 2) attracted our attention because they reported interferences which were larger than our experience indicated should exist. The elements involved were comm o n - C u , Cr, Mn, and Ni-and the interfering substances were mineral acids at frequently encountered concentrations. The original objective of the study, thus, was to see if the reported observations applied to the burner we use. Subsequently, it was decided to extend the study to include the nitrous oxide-acetylene flame. It is well known (3) that use of the nitrous oxide-acetylene flame eliminates many interferences which are present in a n air-acetylene flame. Some workers (4-6), however, have noted interferences peculiar to the nitrous oxide-acetylene flame. The work reported in this study, while not purporting to explore all the possible permutations, has attempted to examine some of the more obvious variables. The intent was to ascertain what experimental conditions would provide the most interference-free environment for determining these four elements in the presence of five commonly employed mineral acids.
Table I. Flame Description and Approximate Gas Flows for Flames Employed in This Study Height of red Gas flows feather, (liter/min) Description Burner head mm Oxidant Fuel 50 X 0 . 5 mm Lean Nz0-C2Hz 1 14. 6.5 Medium N20-C2H2 50 X 0 . 5 mm 10 12. 6.5 Rich Nz0-C2Hz 50 X 0 . 5 mm 25 11. 6.5 Lean air-C2Hz 100 X 0 . 6 5 m m ... 21. 3.0 100 mm 3-slot ... 22. 3.7
BURNER HEAD
FLOW SPOILER
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SAMPLE CAPILLARY
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‘-
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Figure 1. Schematic diagram of the burner system employed in this study
EXPERIMENTAL
The instrumentation employed for this study consisted of a Perkin-Elmer Model 403 atomic absorption spectrophotometer equipped with its standard burner, single-element Intensitron hollow cathode lamps, and a TR-1 typewriter readout. This combination is especially suited to experiments of this type where a large amount of data must be recorded. The instrument produces data in digital form eliminating the need to interpret strip chart recordings and the TR-1 can be programmed to log these data directly in convenient, tabular form. (1) J. Y. Hwang and F. J. Feldman, Appl. Spectrosc., 24, 371 (1970). (2) B. Gandrud and J. C. Marshall, ibid., p 367. (3) W. Slavin, “Atomic Absorption Spectroscopy,” Wiley-Interscience, New York, N.Y., 1968, p 41. (4) S. R. Koirtyohann and E. E. Pickett, ANAL.CHEM.,40, 2068 (1968). (5) J. D. Kerbyson and C. Ratzkowski, Can. Spectrosc., 15, 43 (1970). (6) J. Y. Marks and G . G. Welcher, ANAL.CHEM.,42, 1033 (1970).
A cross-sectional sketch of the burner is shown in Figure 1. It is constructed of stainless steel and coated internally with Penton plastic (Hercules Powder Co., Wilmington, Del.) for corrosion resistance. The burner heads are all constructed of solid titanium. The burner chamber contains a flow spoiler which only permits the smallest, most easily dried sample droplets to reach the flame. Sensitivity is reduced slightly, but also, hopefully, are interferences. This unit is based on an earlier model (7) which was specifically designed to minimize matrix effects. The burner was adjusted in all cases so that the burner slot was just below the optical path of the instrument. Table I contains gas flow rates and descriptive data on the various flames employed. All solutions were prepared from commercially available standards (Hartman-Leddon Co., Philadelphia, Pa.) and reagent grade, concentrated acids. The concentrated acids employed contained the following approximate percentages : ”OB, 70; HCl, 37; HzS04,97; H3P04,86; and HC1o4, 61. (7) W. Slavin, A t . Absorption Newslett., 2, 1 (1963). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
695
Table 11. Comparison of Acid Interferences for Copper, Chromium, Manganese, and Nickel Signal change from pure aqueous solutions Copper Chromium Manganese This This This This study This study This study This study single- study single- study single- study singleRef l a slot 3-slot slot slot 3-slot Ref In 3-slot Ref 10 slot +0.3 +4.5 -0.3 -1.4 +9.0 +0.9 -9.4 0 +1.0 +3.5 $0.6 +0.3 -1.7 ... +2.1 -17.1 ,.. 0 +1.0 ,.. +0.3 t5.0 +0.5 -0.1 -2.5 -5.0 O +1.0 +3.4 -0.3 +0.3 -1.5 ... +0.6 +2.9 -2.9 ... +1.7 ,.. -1.1 -0.2 +3.3 $10.0 -3.9 -2.0 0 -0.5 +0.7 +5.0 +0.6
Acid concn, vol.
Acid HClOa HzSO4
H3P04
HCl
0.2 2.0 0.06 0.6 0,076 0.76 0.1 1.0 0.075 0.75
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+1.7
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0
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0
+0.4
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-6.0 -0.9
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-0.1 -0.1 -0.4 +o. 1 -0.1
0
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,
Nickel This study 3-slot
Ref l a +2.0
+0.9 +2.0 +2.6 +1.0
-16.0
+3.5
+14.0
+0.8
+13.0
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I
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+0.6
0
-0.3
+2.1
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...
... ... -2.5 ...
+0.3 +0.3 +1.0 -0.3 -1.0 ... -2.7 -0.3 -0.5 a Estimated from a figure in Reference I . The values for a lean flame at a height of 5 mm were chosen from the reference as most closely approximating the conditions employed for this study. The work in Reference I was performed on an IL 153 equipped with its standard air/acetylene burner head. HNOa
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ACID CONCENTRATION ( %,V/V I
Figure 2. Interferences for 20 ,ug/ml copper as a function of acid concentration in four different flame systems
All acid concentrations specified in this paper are expressed in (VjV) percentage, milliliters of concentrated acid per 100 ml of final solution. The metal concentration was 20 pg/ml in all four cases. RESULTS A N D DISCUSSION
The initial experiments attempted to duplicate data reported in the literature (I) where various degrees of interference by five acids on copper, manganese, nickel, and chromium were 696
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
-30t
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1
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10
10 0
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ACID CONCENTRATION ( % , V / V 1
Figure 3. Interferences for 20 pg/ml chromium as a function of acid concentration in four different flame systems
reported. A set of solutions duplicating those employed for the literature study, and a set with ten times as much acid, were prepared. These were then analyzed with air-acetylene flames burning on both a 10-cm single-slot head (which is standard on the Model 403) and a 10-cm three-slot head. The gas flows shown in Table I were employed except for chromium where the fuel flow was increased by about
10
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Figure 4. Interferences for 20 kg/ml manganese as a function of acid concentration in four different flame systems -30 -20[
10%. All solutions were analyzed in triplicate using the “100 Average” mode of the instrument for maximum precision. The resulting data, expressed as per cent signal change caused by the presence of the acid compared with aqueous solutions are summarized in Table 11. These data show that the single-slot air-acetylene flame exhibited interferences of about 1% or less for copper, manganese, and nickel, even when the acid concentrations were ten times greater than those employed in the previous study (I). Use of the 3-slot head increased the interferences significantly, but they remained below 4 %. Chromium is well known to be more interference-prone than the other three elements (8, 9). Even here, when the single-slot head was used, only phosphoric acid interfered by more than 1% at the lower acid concentration. The three-slot head gave considerably higher interferences, rendering it a dubious choice for the determination of chromium in acid media. In contrast to these data, Hwang and Feldman (1) reported interferences which ranged up to 10% for copper, 9 % for chromium, 5 % for manganese, and 16% for nickel. It is believed that differences in the burner designs used in the two investigations account for the discrepant results. In an extension of the investigation, a second series of solutions was prepared for each element. This time, six different concentrations of perchloric, sulfuric, and phosphoric acids were employed. The studies with hydrochloric and nitric acid were discontinued because few interferences were (8) F. L. Vogliotti, At. Absorption Newslett., 9, 123 (1970). (9) M. Yanagisawa, M. Suzuki, and T. Takeuchi, Anal Ckim. Acta, 52, 386 (1970).
-501
I 01
I
I
I
10
10 0
100 0
ACID CONCENTRATION ( % , V / V
Figure 5. Interferences for 20 pg/ml nickel as a function of acid concentration in four different flame systems
observed for them. In addition to the air-acetylene flames, the study was extended to the nitrous oxide-acetylene flame operated in both the normal position and at right angles to the optical path. The data obtained from this phase of the experiments are shown in Figures 2 through 5. Here the per cent change from a pure aqueous solution as a function of acid concentration has been plotted. Again, it is possible to draw a number of general conclusions from the data which should aid the analyst in avoiding interference problems. Fewer interferences were observed in the air-acetylene flames than in the nitrous oxide-acetylene flame operated in either position. In particular, the air-acetylene flame operated on the singleslot burner head was the most interference-free. It was also observed that as the acid concentration was increased to the 5-10Z region, a decrease in signal was observed, due to a bulk matrix effect. It is interesting to note that the data obtained in the nitrous oxide-acetylene flame resemble those reported by Koirtyohann and Pickett (4) for other elements in the presence of high acid concentrations. If the data from Table I1 are compared to Figures 2-5, some minor inconsistencies become apparent. It is almost certain that the differences arise from small changes in the ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
697
stoichiometry of the air-acetylene flames between the two portions of the experiment. Interferences in the nitrous oxide-acetylene flames were measured at three stoichiometries: lean, medium, and rich. The curves varied considerably from each other, but not in any identifiable pattern; one stoichiometry did not appear to give “better” results than another. The data in the Figures 2-5 were taken from the medium flame. CONCLUSIONS
The primary conclusion emerging from these experiments is that use of the air-acetylene flame burning on the single slot, flat-top burner produces the fewest interferences when determining copper, manganese, and nickel in acid media. Both the air-acetylene flame burning on a 3-slot head and the nitrous oxide-acetylene flame showed a greater tendency to exhibit interference effects, suggesting that these flames should be chosen with caution. For chromium the choice is not so
clear. Both air-acetylene and nitrous oxide-acetylene flames produced roughly equivalent sensitivities and acid interferences. In this case the choice would be dependent on the presence of other possible interferences. The interferences from hydrochloric and nitric acids were smallest indicating that these acids should be used if there is a choice of dissolution methods. However, if high accuracy is required, and the acid medium is other than dilute, standards should be made up in approximately matching concentrations of the same acid or acids. Finally, it is clear that the design of the burner system has an important influence on analytical interferences, even within the general category of premix burners. The interferences found in this work are far smaller and less important than those reported in Reference I , in which a different premix burner system was employed. RECEIVED for review August 6 , 1971. Accepted November 23,1971.
Double Modulation-Optical Scanning and Mechanical Chopping-in Atomic Absorption Spectrometry Using a Continuum Source R. C. Elser and J. D. Winefordnerl Department of Chemistry, University of Florida, Gainesvilie, Fla. 32601 An atomic absorption flame spectrometer employing chopper modulated continuum radiation and wavelength modulation of radiation transmitted through the flame cell is described. The instrumental system does not respond either to Rayleigh scattering or to molecular absorption and emission processes occurring within theflamegases. Limits of detection of the order of 0.1 bg/ml and analytical curves linear over at least three orders of magnitude in concentration for Ag, Ca, Cd, Cr, Cu, Fe, Mg, and Ni are obtained. Expressions are derived to predict the response of the instrumental system to variation in experimental parameters and are verified experimentally.
ATOMIC ABSORPTION SPECTROMETRY has proved its utility as an analytical tool in the years since Walsh (I) introduced it. To the present time, the development of the technique has largely excluded the use of continua as sources. Several groups of workers (2-6) have realized the advantages of using a continuum source and have shown that it is practical. However, limits of detection have been poorer by a factor of 10 to 100 Author to whom reprint requests should be sent. (1) A. Walsh, Spectrochim. Acta, 7, 108 (1955). (2) W. W. McGee and J. D. Winefordner, Anal. Chim. Acta, 31,429 ( 1967). (3) L. de Galan, W. W. McGee, and J. D. Winefordner, ibid., p 436. (4) J. D. Winefordner, V. Svoboda, and L. J. Cline, CRC Crit. Rev. Anal. Chem., 1, 233 (1970). (5) V. A. Fassel, V. G. Mossotti, W. E. L. Grossman, and R. N. Kniseley, Spectrochim. Acta, 22, 347(1966). (6) J. H. Gibson, W. E. L. Grossman, and W. 0. Cooke, Proc. Feigl Annio. Symp., 287 (1962).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
compared to those obtained using high intensity line sources. With a continuum source-medium resolution monochromator and low concentrations of absorber, the fraction of radiation absorbed by the absorption line is small as compared to the fraction absorbed with a line source. As a result, the signal due to the absorption line becomes buried in the noise of the continuum background and while the line may be discerned, its signal-to-noise ratio is low. It was thought that the weak absorption signal could be extracted from the noise and enhanced by using a derivative technique which has been employed successfullyin other areas of spectrometry. The technique of derivative spectrometry-i.e., taking the derivative of the transmitted spectrum with respect to time or wavelength-was introduced in 1952 by French and discussed more fully in 1955 by Giese and French (7); they demonstrated its theoretical utility in resolving overlapping absorption bands having as much as 90% overlap. Collier and Singleton (8) applied the technique to infrared absorption spectra by taking the second derivative of the spectrum electronically ; however, as Bonfiglioli and Brovetto (9) and Perregaux and Ascarelli (10) point out, analog differentiation of the detector output may actually degrade the signal-tonoise ratio of the derivative signal as compared to the original signal. Bonfiglioli and Brovetto (9) constructed a self-modulating derivative optical spectrometer which employed a vibrating mirror to modulate the image of the spectrum. (7) A. T. Giese and C. S. French, Appl. Spectros., 9,78 (1955). (8) G. L. Collier and F. Singleton, J . Appl. Chem., 6,495 (1956). (9) G. Bonfiglioli and P. Brovetto, Appl. Opt., 3, 1417 (1964). (10) A. Perregaux and G. Ascarelli, ibid., 7, 2031 (1968).
