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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1 9 7 9
( 3 ) S p e c t r a l I n t e r f e r e n c e by Ni on the D e t e r m i n a t i o n of S b at 231.15 nm. T h e absorbance of 1000 pg/mL Ni is equivalent to t h a t 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 I n t e r f e r e n c e by S b on the Determination of P b at 217.00 n m . T h e absorbance of 10 pg/mL S b is equivalent to that of 0.005 pg/mL P b , 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 t h e 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 t h e digests and, in order t o avoid errors introduced by fluctuations in nebulizer performance, variations in t h e acid content of samples (and standards) must be minimized (5, 8, 9). Only limited information on digestion procedures compatible with I C P / A E S 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 H N 0 3 / H C 1 0 4 and H N 0 3 / H F / H C 1 0 4 digestion procedures are described. T h e procedures are variously applicable t o waters, soils, tissues, a n d 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. T h e 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
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table I. Acid Content of Digested Samples: Replicates of the Same Samples
no. of replicates
samplea water particulate tissue soil tissue* soil*
acid content, % 3.57 t 3.60 i 3.33 = 3.45 i 3.48 r 3.51 +
7 7 7 7 7 n
0.09 0.07 0.11 0.10 0.12 0.09
-
Table 11. Acid (and Salt) Content of Digested Samples: Replicates of Different Samples of the Same Sample Type no. of
sam- acid content, salt content, ples % %b
water particulate tissue soil tissue * soil * composite result
10 10
10 10 10 10
60
3.59 t 0.14 3.58 k 0.22 3.44 f 0.26 3.51 i 0.21 3.46 i 0.23 3.51 t 0.15 3.5 t 0.3:
10 mL "0,; next, 5.5 mL HC104 is added and the sample digested until dense white fumes of HC104 appear. Note that high volume samplers are used to collect the airborne particulates (11). The sampling rate and sampling duration are normally 40 ft3/min and 24 h, respectively.
RESULTS AND DISCUSSION
a An asterisk * identifies a HF/HNO,/HClO, procedure, All other samples were carried through the HIiO,/HClO, procedure,
sample typea
1083
0.03 2 0.007 0.02 i 0.005 0.44 i 0.09 0.48 i 0.19 0.43 t 0.07 0.42 i 0.17
An asterisk * identifies a HF/HNO,/HClO, digestion procedure. All other samples were carried through the Percent salt content is based HNO,/HC10, procedure. on the presence of Al, Fe, Ca, Mg, Na, K, and NH, as phos. phates, sulfates, and perchlorates. (Chlorides and nitrates are not present because of volatilization as HC1 and HNO, during the digestion step.) beakers are used. The HN03/HF/HC104procedure parallels the HN03/HC104 procedures except that after the treatment with HN03 the sample is taken t o dryness in the presence of 5.0 mL HF. Tissue Samples. Tissues (plant or animal) are homogenized (10) and a 1.0-g sample is then digested in the presence of successive 15- and 10-mL aliquots of " O s ; after each addition the sample is taken to near dryness; next, 5.5 mL HC104is added and the sample digested until dense white fumes of HC104appear. When a total analysis is required, 1.0-g samples and Teflon beakers are used. The HNO3/HF/HC1O4 procedure parallels the HN03/HC104 procedure except that after the treatment with H N 0 3 the sample is taken t o dryness in the presence of 5.0 mL HF. Particulate Samples. Two 46-mm diameter disks, representing 8.12% of the exposed filter area, are digested in the presence of
Digest Acid Content. The addition of the HC104 to the samples subsequent to the HNO, and/or H N 0 3 / H F will minimize the amount of HC104 consumed in oxidizing any residual organic materials, and it was hoped that for all samples an equivalent amount of HC104 would remain after bringing the digests to fumes of HC104, thus satisfying the requirements of ICP/AES analysis. Also, during this step all residual HNO, and HF could be volatilized, thus providing for a residual acid matrix consisting of HC104. In order to check the mean HC104 content of the samples and to define the fluctuations about the mean, a number of samples were analyzed for total acid content (by titration with standard base). Table I indicates that the digestion procedure controls the residual acid content to j=O.1% HC104. When variations introduced by dissimilarities between samples within a group are introduced (Table 11), a standard deviation of between 0.15% and 0.25% HC104results. Table I1 also indicates that, regardless of sample type, the residual acid content of any digest can be expected to be 3.5 f 0.4% HC104. ICP/AES data appearing elsewhere (9) indicate that when the standards are prepared in 3.5% HCIOI this fluctuation of up to A0.47~about a mean acid content of 3.5% HC104 may introduce analytical error of up to 1 1 . 3 % . Such a degradation in accuracy is small and we may conclude that the digestion procedures allow for adequate control over the residual acid content of the samples and in so doing are compatible with ICP/AES techniques. Note that variation in total salt content can also produce the same effect as variation in acid content. Since the ability of ICP/AES analytical systems to handle dissolved salts may be limited, the total salt content of standards and samples should be kept below 0.5% (9). This criterion for typical sample digests is satisfied (see Table 11) and the low salt content together with the fact that HC104 is the major matrix component will minimize any analytical deviation. Digestion Efficiency. The acid digestion procedure must not only be compatible with ICP/AES techniques but must also provide for satisfactory extraction efficiencies. Previous workers have described the relative merit of HN03/HC104 digestion procedures in detail (12, 13). For most environmental work, a HN03/HC104 digestion is considered adequate since it will typically recover all materials of anthropogenic
Table 111. Analytical Results (pgig): Biological Samples SRM 1 5 7 1 Orchard Leaves AES~ RSD, %
element
NBS'
As Ba Ca Cr cu
102 2 (44) 2 0 9 0 0 i 300 2.6 12i 1 3 0 0 t 20 6 200 I200 91 I 4
Fe
Mg 31n
11
42i 2 1 9 8 0 0 I500 2 11 I1 267 i 6 5 700 i 60 861 1
4.8 2.5 9.1 2.2
SRM 1677 Bovine Liver
rec, % 110 95 95
1.2
92 89 92 95
2.1 2.4 2.7 3.8
90 93 100 104%
1.1
Sr Zn
2 100 t 452 37 2 25i-
100 3 1 3
1900 i 42t 37 i 26 i
40 1 1 1
mean 3.2% a
Results in parentheses are not certified.
AES~
(123)
127
1 9 3 i 10 2 7 0 i 20 (605)
10.3 i 1.0
(3.2)
MO
P Pb
NBSa
1 3 0 i 10
96%
Results for 7 replicate samples.
rec, %
0.4
103
1861 2 244 i 2 573 i 4
0.8
10 i 1
10.0
96 90 95 97
0.7
101
i
12
RSD, %
0.8
0.7
2
131 i 1
mean 3.7%
97%
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table IV. Analytical Results ( p g l g ) : Soils ELR-007
ele-
ment A1 Cd cu Fe Mg Mn P Pb
Zn
K a
ELR" 63 700 i 1 7 0 0 20f 1 95t 6 25 500 t 1 0 0 5 700 r 2 170 593+ 13 1 7 3 0 i 250 1 6 5 0 I 60 2 510 i 70 23 900 i 800
AES~ 44 600 i 2 900 19f 1 91 i 3 23 300 i 200 5 990 i 220 449 k 8 157Ok 60 1 5 2 0 i 30 2 500 2 40 21 500 t 600
Uncertified reference values (16).
ELR-008 RSD, % 6.5 5.3 3.3 0.8 3.7
1.8 3.8 2.0 1.6 2.8
ELRa
rec, %
70 95 96 91 105 76 91 92 100
90
66 500
3 700
40 i 1 26600 i 300 7 7 7 0 + 2 080 723 r 1 5 1 5 5 0 1 60 275 ?. 96 208 i 1 0 23 400 i 1 3 0 0
mean 3.2% 91% Results of 7 replicate samples.
origin (12). This is reflected in the fact t h a t regulatory agencies typically do not specify drastic recovery procedures involving HF in defining standards or guidelines for the total metal content of waters and airborne particulates (10,14, 15). However, there may be situations (particularly in soil and tissue analysis) where a total analysis is indicated. In these cases H F may be used to release materials associated with the silicate matrix (15). All t h e digestion procedures described in this work are characterized by a predigestion with HN03. T h e amount of H N 0 3 used has been individually adjusted to accommodate the destruction of maximum levels of organic materials in the samples. If destruction of the silicate matrix is required, this is accomplished by subsequent digestion in t h e presence of HF. Finally, all samples are carried through a perchloric acid digestion. Here the sample is exposed to both a powerful oxidizing system and a source of chloride ions, which aid in t h e dissolution of many metals. Selected reference materials were used to check the effectiveness of these digestion procedures. In the case of the tissue samples, standard reference materials (SRM) provided by the National Bureau of Standards were used. These were S R M 1571 Orchard Leaves and SRM 1577 Bovine Liver. In the case of soils, two reference materials used in quality control procedures within this laboratory were used. These were ELR 007 a n d E L R 008. Characterization of the reference soil materials was achieved by atomic absorption spectrometry (AAS) preceded by dissolution in a H F / H N 0 3 / H C 1 0 4 acid mixture using a pressure vessel (13). A description of these materials appears elsewhere (16). Tables 111and IV compare reference values for the soil and tissue samples with those obtained using ICP/AES analytical procedures described in another publication (9). Note that soil reference values for Ca and Na do not appear because of reagent contamination from boric acid which is used t o dissolve the precipitated metal fluorides (12, 16). Reference t o t h e tables indicates t h a t in general the results obtained using the HF/HN03/HC104 digestion procedure of this work compare favorably with the reference values. Some significantly low recoveries (
