ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
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Significance of Spectral Overlap in Atomic Absorption Spectrometry Sir: In a recent article in this journal ( I ) , Koizumi reported three cases of spectral interference in atomic absorption spectrometry that he was able to eliminate by using a Zeeman-effect instrument. The three cases that he investigated were: (1) In the determination of S b a t the primary resonance line of 217.58 nm, the presence of P b produced absorbance. T h e reason that was given was that the spectrometer also transmitted the nearby weakly absorbable Sb line a t 217.02 nm, which overlapped the P b resonance line a t 217.00 nm. Thus, any appreciable qriant,ity of P b in the sample would absorb some of the light a t the Sb line at 217.02 nm! and would therefore be mistaken by the instrument for Sb. (2) In the determination of Pb a t 217.00 nm, the presence of S b would produce absorbance, by absorbing the overlapping line a t 21i.02 nm. This is the converse of case 1. (3) When Sb was determined a t the resonance line of 231.15 nm, spectral interference from Ni was encountered, because Ni can absorb a t the overlapping line of 231.10 nm. In the present work, it was desired to test how much effect these three interferences would have on analysis with a conventional (non-Zeeman) atomic absorption spectrophotometer with furnace atomizer, and to see whether the problems have real analytical significance. It has been shown that atomic absorption lines have half-widths of about 0.002 nm (21, though the exact value depends on the element, atomization temperature, and form of atomizer. Because the lines have a gaussian shape, however, spectral overlap can occur between lines which are as much as 0.05 nm apart. T h e monochromators of commercially available atomic absorption spectrophotometers have a “best” resolution capability of 0.04 to 0.2 nm, depending on design. These monochromators are generally adequate to select the resonance lines of different elements, but obviously cannot be counted on t o remove spectral interferences.
Table I. Determination of Pb at 217.0 nma 1
2
3
4
element, gross Pb nomi- concn, absorb- content, iial pg/mL ance pg/mL Pb 0.05 0.22 0.05 Sb 5.0 0.04 0.0065 Sb 10.0 0.08 0.013 Sb 15.0 0.12 0.02 a Sample size = 1 0 p L ; 10 pg/mL Sb rJg/mL Pb.
5
6
equivanet lent Pb absorbabsorbance ance from Sb 0.22
0.00
0.029 0.057 0.088
0.011 0,023 0.032
absorbs like 0.005
EXPERIMENTAL An Instrumentation Laboratory TLi’51 dual-channel doublebeam atomic absorption spectrophotometer wa.s used for this work. This instrument consists essentially of two different AA spectrometers sharing the same sampling device, but possessing two totally separate monochromators which can be independently set to seven different resolution settings ranging from 0.04 to 2 nm. Separate lamps are used for each channel, as well as separate photomultiplier detectors and electronics (3). Deuterium arc background correction can be used in either or both channels, and was used in all these experimenh. The sampling device was the IL555 Controlled-Temperature Furnace Atomizer, used with uncoated graphite and nitrogen purge gas. Samples were introduced with an IL254 FASTAC Autosampler, which has been previously described ( 4 ) . Single-element IL lamps for Pb and Sb were employed. Unless otherwise stated, all analytical conditions were set according to the IL analytical methods books for the AA spectrophotometer and furnace atomizer ( 5 , 6). A Honeywell two-pen recorder and Ventron standards were used.
a t 217.02 nm is completely screened out and no detectable interference can be seen. A similar relationship between the magnitude of the Pb interference on the S b absorbance, as a function of spectral bandwidths, was also reported by Slavin (7). In the second experiment, a Pb lamp was installed and, after the sensitivity to P b a t the 217.00-nm line had been established, 10 pL of solutions with concentrations of 5, 10, and 15 pg/mL S b were deposited. Different resolution settings were tried, but, as expected, they had no effect on the apparent spectral interference. T h e data are shown in the first three columns of Table I. T h e degree of interference appeared severe, but it seemed possible that some of the problem was caused by the presence of P b in the Sb standard, since it is well known that it is difficult chemically t o separate S b completely from Pb. Therefore an analysis of the S b standard was carried out a t the Pb resonance line a t 283.33 nm, where there is no possibility of spectral interference from Sb. As first determined by flame and then confirmed with the furnace atomizer, the 1000 pg/mL Sb standard solution indeed contained 1.3 pg/mL Pb. T h e calculated P b content of the solutions in Table I are therefore listed in column 4, and the absorbances due to P b given in column 5. T h e net absorbance from S b (column 3 - column 5) is presented in column 6. There continues to be spectral interference from Sb, but the strength of the interference is less than had initially appeared. At the P b line a t 217.00 nm, the absorbance of 10 gg/mL Sb is equivalent to that from 0.005 pg/mL P b , a ratio of about 2000:l. This interference was mentioned by Koizumi ( I ) ,but no analytical data were presented. In the third experiment, a solution of 1000 pg/mL Ni was measured a t the secondary S b line a t 231.15 nm. A small spectral interference was found, which could not be removed by reducing the bandpass of the instrument monochromator. A simultaneous measurement a t the primary Sb line a t 217.58 nm indicated that the Ni standard contained no detectable Sb. At 231.15 nm, 1000 pg/mL Ni produced an absorbance equivalent t o that from 0.02 pg/mL Sb, a ratio of about 50 000:1.
R E S U L T S AND D I S C U S S I O N In the first experiment, S b lamps were used a t 217.58 nm in both channels of the spectrophotometer, and all analytical conditions were made identical except for the monochromator resolution settings. In channel A, 2.0-nm resolution was used, as in Reference 1,while 0.5-nm resolution was used in channel B. Twenty microliters of solutions of 1, 10, and 25 pg/mL Pb were deposited into the furnace cuvette. Figure 1 shows the results. At the wide resolution setting, spectral interference indeed occurs, but a t the narrower setting, the Sb line
CONCLUSIONS T h e results shown here indicate that the use of Zeemaneffect spectroscopy to remove these spectral interferences has relatively little analytical significance, though it may well be regarded as interesting scientifically. T h e actual effects are small, and can easily be avoided. T o summarize: (1) Spectral I n t e r f e r e n c e by Pb o n t h e Determination of Sb at 217.58 n m . At a resolution setting of 0.5 nm, well within the capabilities of most commercially available instruments, this spectral interference does not exist.
0003-2700:79/0351-1081$01 O O / O
C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1 9 7 9
( 3 ) S p e c t r a l Interference by Ni on the Determination of S b at 231.15 nm. The absorbance of 1000 pg/mL Ni is equivalent to that from 0.02 pg/mL Sb, a ratio of 50000:l. Moreover, the S b line a t 231.15 nm is rarely used, because the 217.58-nm line is generally preferable. LITERATURE CITED
Flgure 1. Lead interference at the Sb 217.58-nm line using S b lamp, 20-pL depositions of 1, 10, 25 pg/mL Pb and a 0.5-nm (top trace) and a 2-nm bandwidth
(2) Spectral Interference by S b on the Determination of P b at 217.00 nm. The absorbance of 10 pg/mL S b is equivalent to that of 0.005 pg/mL Pb, a ratio of about 2000:l. Even this small effect can be avoided by using the very good P b line a t 283.33 nm, where there are no known spectral interferences.
(1) Koizumi, H. Anal. Chern.. 1978, 50, 1101. (2) Robinson, J. W. "Atomic Absorption Spectroscopy", 2nd ed.; Marcel Dekker: New York, 1975; p 20. (3) Karasek. F. W. Res./Dev. 1977, 28(11), 50. (4) Kahn, H. L.: Schleicher. R. G.; Smith, S. B. Jr. I n d . Res. Dew. 1978, 20(2), 101. ( 5 ) "Atomic Absorption Methods Manual", Vol. 1, "Standard Conditions for Flame Operation", Instrumentation Laboratory: Wilmington, Mass. 01887, 1977. (6) "Atomic Absorption Methods Manual" Vol. 2, "Flameless Operations"; Instrumentation Laboratory: Wilmington. Mass. 01887, 1976. (7) Slavin, S.; Sattur, T. W. A t . Absorp. News/. 1968, 7, 99.
J o h n J. S o t e r a R o n a l d L. S t u x H e r b e r t L. K a h n * Instrumentation Laboratory Analytical Instrument Division, Wilmington, Massachusetts 01887
RECEIVED for review January 10, 1979. Accepted March 26, 1979.
AIDS FOR ANALYTICAL CHEMISTS Digestion of Environmental Materials for Analysis by Inductively Coupled Plasma-Atomic Emission Spectrometry Neil R. McQuaker," David F. Brown, and Paul D. Kluckner Environmental Laboratory, Ministry of the Environment, 3650 Wesbrook Crescent, Vancouver, British Columbia V6S 2L2, Canada
Recently various workers have reported on the use of an inductively coupled plasma-atomic emission spectrometer (ICP/AES) for the simultaneous multielement analysis of a variety of environmental materials. These materials have included waters ( I , 21, soils (31, airborne particulates (41, and biological tissues (5-7). In ICP/AES analyses, the digested or extracted sample is presented as a liquid, and the nebulizer used for sample transport places certain restraints on the sample, i.e., the glass construction of the nebulizer precludes the presence of hydrofluoric acid in the digests and, in order t o avoid errors introduced by fluctuations in nebulizer performance, variations in the acid content of samples (and standards) must be minimized (5, 8, 9). Only limited information on digestion procedures compatible with ICP/AES work is available. For example, a procedure for tissue samples which yields a residual acid content of 17% HC104 and a procedure for rock samples which yields a residual acid content of 3% HC104/20% HC1 have been described ( 3 , 5 ) . However, a comprehensive approach producing equivalent acid contents and applicable to a variety of sample types has evidently not appeared. In t h e present work such a n approach is discussed. Both HN03/HC104 and HN03/HF/HC104 digestion procedures are described. T h e procedures are variously applicable to waters, soils, tissues, and airborne particulates and provide for a mean residual perchloric acid content of 3.5% in the prepared digests. Data defining observed fluctuation about 0003-2700/79/0351-1082$01.00/0
this mean value of 3.5% are presented and the implications of sample acid content on analytical accuracy are discussed. The implications of digestion efficiencies on analytical accuracy are also discussed. EXPERIMENTAL Reagents and Apparatus. The acids used were reagent grade H N 0 3 (65%),HF (42%),and HC10, (72%). All digestions were carried out in open 150-mL beakers using a hot plate in a perchloric acid fume cupboard. The surface temperature of the hot plate was held at 250 "C for all digestion procedures. All beakers used, whether Pyrex or Teflon, were of uniform shape. Digestion Procedures. The procedures used for the various materials follow. Note that in all cases once the digests are taken to dense white fumes of HClO, they are removed from the hot plate, cooled, diluted to 100 mL, and then filtered using Whatman No. 42 paper. Usually the fuming step is accompanied by the digest going to a lighter color and the digests are removed when this color change is complete. Water Samples. A representative 100-mL sample is evaporated to about 5 mL in the presence of 5 mL "0,; an additional 5 mL HNO, is added and the sample is taken to near dryness; 5.5 mL HC10, is then added and the sample digested until dense white fumes of HClO, appear. Soil Samples. Soils are homogenized and ground so as to pass a 100-mesh sieve. A 1.0-g sample in the presence of 10 mL "03 is then taken to near dryness; 5.5 mL HC104 is then added and the sample digested until dense white fumes of HC104 appear. When a total analysis is required, a 0.5-g sample and Teflon 1979 American Chemical Society
