Table IV. Cadmium in Fish Exposed to 3 ppm CdClz for One Week Sample 1-1I.1,311
\%
Heart
mq
ppm C d a ( c a c h fish)
Absolute btd dcv
Re1 std di\, '
1.52 6.34 5.62 1.61 1.07 1.26 0.20
0.18 0.57 1.01 0.24 0.13 0.38 0.02
12 9 18 15 12 30 10
0.075 5.25 0.61 5.02
0.02 0.11 0.02 0.10 0.33 0.45 0.56 0.37 0.79 0.33 0.20 0.21 0.81 0.34 0.60 0.04
27 2 3 2 5 7 60 9 35 11 4 4 12 15 23 12
1.8 2.1 2.4 22.3 48.1 17.8 45.1 85.0 29 .O 34.3 26.6 20.6 10.6 15.1 9.6 17.8 9.7 10.8 19.9 38.0 17.7 8.9 4.9 4.8
Skin Muscle Gut
Gill
Kidney Liver Bone
a
LL(jllt,
...
6.67 6.49 0.94 4.13 2.26 3.04 4.91 5.18 6.71 2.28 2.59 0.32
...
...
4vLragr ppm Cd
Re1 std d e \ ,
4.5
58
1.3
21
0.14
51
3.6
72
4.7
69
3.1
30
5.6
17
1.7
71
L
ppm = p g Cd per g of wet tissue
tion in cadmium levels is attributed primarily to physiological differences between fish. Similar variations in the cadmium content of human liver from different portions of the same liver have been attributed to the heterogeneous distribution within the organ (11). In conclusion, flameless atomic absorption with background correction has been shown to have adequate sensitivity and reproducibility for the analysis of eight fish organs to permit detailed studies of uptake and distribution of cadmium in fish exposed to elevated levels in aquatic systems. LITERATURE CITED (1) D. Lee, Ed.. "Metallic Contaminants and Human Health", Academic Press, New York, 1972. (2) L. Friberg, M. Piscator, and G. Norberg, "Cadmium in the Environment", CRC Press, Cleveland, 1971. (3) J. McCauil, Environment, 13, 3 (1971).
(4) D. B. Louria. M. M. Joselow, and A. A. Browder, Ann. intern. Med.. 78, 307 (1972). (5) J. F. De L. G. Solbe. Water Res., 8, 389 (1974). (6)J. G. Eaton, Water Res., 7, 1723 (1973). (7) G. K. Pagenkopf, D. R. Neuman, and R. Woodriff. Anal. Chem., 44, 2248 (1972). (8)J. Y. Hwang. P. A. Uiiucci, and C. J. Mokeler, Anal. Chem., 45, 795 (1973). (9) J. Y. Hwang, C. J. Mokeler, and P. A. Ullucci. Anal. Chem., 44, 2018 (1972). (IO) A. W. Struempler, Anal. Chem., 45, 2251 (1973). (11) P. A. Ullucci and J. Y. Hwang, Talanta, 21, 745 (1974).
RECEIVEDfor review December 26, 1974. Accepted March 18, 1975. Part of this work was supported by NSF Grant GY-9610 under the Student Originated Studies program. Part of this paper was presented a t the 7th Materials Research Symposium on Accuracy in Trace Analysis, National Bureau of Standards, Gaithersburg, Md., in October 1974.
Determination of Trace Bismuth in Copper by Hydride Evolution Atomic Absorption Spectrophotometry Michel Bedard and J.
D. Kerbyson
Noranda Research Centre, Pointe Claire, Quebec, Canada
The determination of bismuth has importance in commercial electro-refined copper, because of the harmful effects of this impurity on some physical and electrical properties. Typical levels are 0.1-0.4 pprn Bi, and existing analytical methods comprise spectrography, extraction-spec-
trophotometry, and extraction-atomic absorption techniques, Spectrography provides one of the most widely used methods for the rapid and sensitive determination of bismuth in copper ( I ) . The detection limit is 0.1 ppm, and ANALYTICALCHEMISTRY. VOL. 47, NO. 8, JULY 1975
1441
these methods are especially attractive where large numbers of samples must be analyzed routinely. Other methods for the sensitive determination of bismuth depend upon chemical concentration from a copper sample, followed by colorimetric or atomic absorption measurements (2-4). The chemical concentration step used widely to date involves the dissolution of the copper sample, complexation of the copper with alkaline potassium cyanide solution, and extraction of the sodium diethyldithiocarbamate complex of bismuth into chloroform solution (2, 3). The contained bismuth is measured spectrophotometrically and the detection limit is 0.1 ppm. Alternatively, bismuth has been determined by atomic absorption spectrometry after an ammonia precipitation from the copper sample solution, adding a fixed amount of lanthanum which acts as an effective collecting agent for several impurity elements including bismuth ( 4 ) .The detection limit thus attained is 0.04 ppm. I t is noteworthy that each of these existing methods has a detection limit near 0.1 ppm, so that the accuracy of measurement falls off a t this, the lower level a t which bismuth occurs usually in refined copper. For precise measurements a t this and lower levels of bismuth in studies of high-purity copper, a bismuth analysis method of significantly improved detection limit was desired. Information on the evolution of bismuth hydride and its measurement by atomic absorption appeared to promise a substantial sensitivity improvement, relative simplicity, and speed of operation. The hydride evolution AA method has been described by several authors (5-8). AA measurements for bismuth evolved from aqueous standard solutions have been reported using magnesium metal and Tic13 ( 5 , 6 ) as the reductant and source of nascent hydrogen. Schmidt and Royer (7) and Fernandez (8) have described recently the conditions of measurement for bismuth hydride, utilizing reduction with sodium borohydride, NaBH4. Applications of the hydride evolution method to actual analysis samples have dealt mainly with arsenic in natural waters and marine sludge (91, and in biological material and petroleum products ( 1 0 ) .In these materials, detection limits of 1.0 ng As were reported. The requirement to determine bismuth sensitivity in copper metal directed our attention to the hydride evolution method used either directly, or following a pre-separation of impurity elements from the copper matrix using lanthanum hydroxide as a collector ( 4 ) . The objectives were to improve upon the existing sensitivity and detection limits for bismuth in copper, and to demonstrate analytical practicality in terms of speed, reliability, and freedom from interferences. This paper describes the sensitive AA determination of bismuth in copper. A pre-separation from copper is made for bismuth plus certain other trace constituents, by double precipitation with lanthanum hydroxide as a collector. The precipitate is dissolved, bismuth is converted to bismuth hydride, and is determined sensitively by AA in an argonhydrogen-entrained air flame. Suitable operating conditions and the absence of interferences are reported. The method provides a detection limit of 0.002 ppm for bismuth in copper, a relative deviation of AS%, and is relatively simple and rapid compared to traditional methods.
EXPERIMENTAL Apparatus. Atomic absorption instrumentation consisted of a Jarrell-Ash Model 82-526 spectrophotometer, equipped with a 10X 0.10-cm slot Techtron burner, Westinghouse hollow cathode lamp, and Leeds and Northrup chart recorder. The instrument operating conditions were: wavelength 2230.6 A, slit-width 0.20 nm, and lamp current 7 mA. 1442
* ANALYTICAL CHEMISTRY, VOL.
47, NO. 8, JULY 1975
A Perkin-Elmer As/& Sampling System (10) was utilized for the generation and collection of the bismuth hydride. Probably alternative equipment of similar type would be satisfactory. The particular hydride generator (10) utilizes a dosing stopcock which maintains a gas-tight seal while reagents are introduced, and has a balloon reservoir for the generated gases. The collected gases (hydrogen plus gaseous metal hydrides) are introduced into the burner via the auxiliary oxidant connection. An argon-hydrogen-entrained air flame was used, having a low background absorption in the far UV wavelength region (11). The following gas flow settings were found to be optimum: argon (15 I./min) at a pressure of 25 psi, auxiliary argon (5 l./min) a t a pressure of 20 psi, and hydrogen (10 l./min) at a pressure of 20 psi. Reagents. All solutions were prepared from analytical reagent grade chemicals. A standard stock solution was prepared by dissolving 1.1148 g of Biz03 in 20% hydrochloric acid, then diluting to 1000 ml with 20% hydrochloric acid. Working standards containing 0.25,0.50, 1.00, and 2.00 pg of bismuth per 20 ml of solution (20% hydrochloric acid) were prepared from the stock solution. NaBH4 pellets ('O13~ inch) were available from Alfa Inorganics, Beverly, MA. The lanthanum nitrate referred to throughout is the hexahydrate La(N03)3*6HzO. Sample T r e a t m e n t . The copper samples were dissolved in nitric acid. Trace quantities of bismuth were collected from these solutions using the lanthanum hydroxide precipitation procedure ( 4 ) ,as follows: Transfer 10.0 g of copper, accurately weighed, to a 500-ml beaker. Add 25 ml of distilled water and dissolve the copper by adding slowly 45 ml of concentrated nitric acid. Add 10 ml of 5% lanthanum nitrate solution, dilute to 300 ml with distilled water, and add 100 ml of concentrated ammonium hydroxide. Let stand for 1minute, then filter the solution through a Whatman No. 41 filter paper. Wash the beaker and paper with 5% ammonium hydroxide solution and finally with distilled water. Redissolve the residue into the original beaker with 40 ml of 50% hydrochloric acid, washing the paper well with distilled water. Dilute the solution to 100 ml. Re-precipitate by adding 30 ml of concentrated ammonium hydroxide. Let stand for 1 minute and filter and wash as before. Dissolve the residue with 40 ml of 50% hydrochloric acid and wash with water. Transfer to a 100-ml volumetric flask, and dilute to volume with distilled water. A reagent blank and the working standard solutions, for calibration purposes, are processed in the same way. Hydride Evolution Procedure. The procedure used for generating the gaseous hydrides has been fully described by Fernandez (81,and was as follows. Pipet 20 ml of the sample solution into the generation flask. Acidify the sample to 4N (see discussion) by adding 20 ml of 5.6N hydrochloric acid. Connect the flask to the gas flow system, with the hydrogen flame operating. Open the 4-way stopcock for about 20 seconds to admit argon, which flushes the air out of the system. After flushing, close the 4-way stopcock and add a single NaBH4 pellet via the dosing stopcock. Allow the reaction to continue for 20 seconds, using a stopwatch (see discussion). Open the 4-way stopcock, which admits argon and sweeps the generated gases into the burner. Record the absorption signal on a recorder. The prepared reagent blank and standard solutions (0.25 to 2.00 pg Bi) were measured using the same procedure, to produce the calibration curve.
RESULTS AND DISCUSSION Collection Time and Acidity. The effect of gas collection time (time period between adding the NaBH4 pellet and sweeping the generated gases into the flame) was investigated on standard solutions of bismuth. The best sensitivity was obtained using a 20-second collection period. Longer collection times resulted in a loss of sensitivity up to 2-fold as stated by Fernandez (8). Probably, this reflects the limited stability of bismuth hydride a t room temperature. It should be pointed out that there is little direct evidence for the existence of bismuth hydride, and the forma-
03
w z 0
3ar 0 2 2m a
01
05
0
Figure 1. Comparative AA signals for 1 p g Bi, and 1 p g Bi plus 1 mg Cu, before and after ( A ) lanthanum hydroxide collection
tion of a different species may account for the behavior of bismuth (12). The effect of hydrochloric acid concentration in the final solution was also investigated. As described elsewhere (81, varying the acid strength from 1N to 5N had no noticeable effect on the sensitivity obtained for bismuth. An acid concentration of 4N was adopted for routine use. Interferences. A major concern was the application of the hydride evolution AA technique to real samples. Therefore, possible interferences had to be studied and eliminated. I t was confirmed experimentally that the reduction of bismuth to the volatile hydride must he made from either hydrochloric ( 4 N ) or sulfuric acid solution, and that nitrate ion interferes and must be absent. Possible interferences by other elements which form volatile hydrides (As, Sb, Te, Se, and Sn) were also examined on appropriate test solutions. Results showed that up to 100 pg of these elements, separately and together, can be tolerated without significant effect on the measurement of 1 pg Bi. In regard to other concomitants, no interference was noted from 50 mg of Ni, Pb, and Fe. However, the principal constituent, copper, when present a t levels of 1 mg or higher in the final solution, greatly reduced the signal for bismuth. This interference was thought to arise mainly from co-precipitation on metallic copper, which precipitates visibly during the reduction stage when test solutions containing copper, or copper samples without pre-separation were processed. Increasing the acid content from 4N to SN, both for solutions of bismuth alone and those containing copper, eliminated the visible precipitation of copper metal. I t also reduced, but did not eliminate, the depressive interference by copper. There remained a decrease of 50% in the signal for Bi (1 pg) in the presence of Cu (1 mg), as shown in Figure 1. Routes to avoid the interference by copper were explored by a single or double pre-separation with ammonia, using iron or lanthanum hydroxides to collect the bismuth. In no case did a single precipitation reduce copper for a 10-g sample to the required low level, so that double precipitation was necessary. Of the alternative collecting agents, lanthanum hydroxide proved the more efficient. T h a t is, the absorbance ohtained for a given quantity of bismuth following the double lanthanum precipitation was essentially the same as for the bismuth standard directly. As shown in Figure 2, the corresponding curves A and B are sufficiently close that either precipitated or non-precipitated standards can be used interchangeably for practical calibration purposes. Sensitivity, Detection Limit, and Precision. The re-
10
I5
20
pg BI per 20 rnl
Figure 2. Calibration curves for bismuth. ( A ) Direct measurement without pre-separation. ( B ) After lanthanum hydroxide separation. (C) After iron hydroxide separation
Table I. Analysis of Electrolytic Copper Samples B1,
PPm
Sample
Spectro ra hic met2oB
Hydride evolution AA method
1
0.32
2 3
0.37 0.39
4
0.45 1.06 0.65
5
1.oo
0.95 0.46 1.09
peatability of flame AA measurements by the pre-separation and hydride evolution method was established by seven replicate measurements on a bismuth standard solution (0.5 pg Bi). The standard deviation was calculated from the range and the median, using the method of Dean and Dixon ( 1 3 ) .A very satisfactory relative standard deviation of f 8 . O % a t the 0.5-pg level was obtained. The calibration curve (Figure 2) also was repeatable, and enabled the mass sensitivity, mass detection limit, and the concentration detection limit (all based on a solution volume of 20 ml) to be calculated. The mass sensitivity was 30 ng per 1%absorption, indicating that the hydride evolution AA method is 20 times more sensitive than the usual colorimetric method. The mass detection limit for a signal twice that of the flame background was 10 ng. In a 20-ml solution volume, this corresponds to a solution detection limit of 0.5 ppb Bi, which is approximately 100 times lower than by direct spraying AA measurements for bismuth. For copper samples of 10 g, using a 2-g aliquot portion, the corresponding detection limit is 0.01 ppm Bi. This detection limit is 5 to 10 times lower than those obtained with existing pre-separation AA or spectrophotometric method or spectrography. When required, the detection limit has been extended downwards fivefold to 0.002 ppm, by using the whole lanthanum precipitate from the 10 g of sample, in a 20-ml final volume. The working range of the method, in terms of bismuth in copper samples is 0.002-1.0 ppm. The accuracy of the method was evaluated by applying it to electrolytic copper samples, previously analyzed in a cooperating laboratory by spectrography supported by extraction-spectrophotometric analysis. Results which were obtained by the method described herein are shown in Table I, and are in good agreement with the spectrographic determinations. ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1443
Applications. The method described has been applied routinely to the determination of bismuth in over one hundred samples, with concentrations ranging from 0.1 to 10 ppm of bismuth. The convenient operating range is from the detection limit to 1.0 ppm. For higher concentrations of bismuth, conventional direct spraying AA may be used. The hydride evolution AA technique described has proved useful as an independent method of analysis for low bismuth levels existing in commercial copper. Moreover, its high sensitivity is of interest for measuring bismuth in ultra-pure copper a t levels not easily measured previously. The method described has since been extended to determine arsenic, selenium, tellurium, and tin a t low ppm levels in electrolytic copper samples ( 1 4 ) .
LITERATURE CITED "Methods for Emission Spectrochemical Analysis", 6th ed., American Society for Testing and Materials, Philadelphia, PA, 1971, p 299. (2) R. J . Lacoste, M. H. Earing, and S. E. Wiberley, Anal. Chern., 23, 871 (1)
(1951). (3) E. B. Sandell, (4)
(5) (6) (7) (8) (9)
"Colorimetric Determination of Traces of Metals", 3rd ed., lnterscience Publishers, New York, 1959, p 332. W. Reichel and E. G. Bleakley. Anal. Chern., 46, 59 (1974). E. N. Pollock and S. J. West, At. Absorpt. Newsl., 11, 104 (1972). E. N. Pollock and S. J . West, At. Absorpt. News/., 12, 6 (1973). F. J. Schmidt and J . L. Royer, Anal. Lett., 6, 17 (1973). F. J . Fernandez, A t . Absorpt. News/., 12, 93 (1973). R. S. Braman, L. J . Justen, and C. C. Foreback. Anal. Chem., 44, 2195 (1972).
10) 11) 12) 13) 14)
D. C. Manning, At. Absorpt. Newsl., 10, 123 (1971). H. L. Kahn and J. E. Schalis, At. Absorpt. Newsl., 7, 5 (1968). E. J. Knudson and G. D. Christian, Anal. Lett., 6, 1039 (1973). R. E. Dean and W. J . Dixon, Anal. Chern., 23, 636 (1951). M. Bedard and J. D. Kerbyson, to be published.
15) M. Bedard, M.Sc. Thesis, Universite de Sherbrooke, November 1974.
ACKNOWLEDGMENT The advice and encouragement of F. M. Kimmerle, Universit6 de Sherbrooke, and the facilities provided by Noranda Research Centre, are gratefully acknowledged.
RECEIVEDfor review November 20, 1974. Accepted February 27, 1975. The work described formed part of a university-industry cooperative research project and has been partly described in an internal M.Sc. thesis (15).
Phosphorus-31 Fourier Transform Nuclear Magnetic Resonance Spectrometry as a Trace Analysis Tool for the Determination of Inorganic Phosphates Thomas W. Gurley and William M. Ritchey Department of Chemistry, Case Western Reserve University, Cleveland, OH 44 106
31PNMR is well established as a powerful tool of structure elucidation of phosphorus compounds. The quantitative capabilities have been employed mainly to determine phosphorus a t relatively moderate concentrations 1%and above ( I ) . Recent studies employing signal averaging capabilities report that the low ppm (milligrams/liter) level can be detected for orthophosphates, phosphonates, and cyclic phosphates but the experimental time is of the order of days and no quantitative data are given ( 2 , 3 ) . Therefore, it is the goal of this work to determine the capability of the NMR technique a t the lower pprn concentration range in a relatively short period of time. This research interest has developed from the need for better methods of analysis of environmental pollutants in wastewater and related aqueous systems. Our studies of the most abundant phosphorus compounds in the aqueous environment, namely, ortho-, pyro-, and tripolyphosphate, are reported in this paper. The advantages of using 3lP NMR include specificity based on the chemical shift (phosphorus nuclei resonate over a range of 500 ppm). There are also no interfering ion problems as is the case with a colorimetric determination. The major drawback of NMR is the inherent low sensitivity as phosphorus is only about 6% as sensitive as hydrogen. Therefore, sensitivity enhancement is necessary to enable one to observe phosphorus a t low concentrations. Among the signal enhancement techniques employed are pulsed Fourier transform capabilities, 12-mm sample tube, and the addition of a paramagnetic "relaxagent". The relaxagent can be considered effectively a signal-noise-time enhancer since its function is to reduce the relaxation time of the 31Pnuclei. The unpaired electrons of a paramagnetic species augment the normal spin-lattice relaxation ( T I ) 1444
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
pathway which results in a more rapid and efficient relaxation process. This allows more pulses to be applied per unit time without saturating the nuclei and, hence, greater signal-to-noise (S/N).
EXPERIMENTAL Apparatus. NMR spectra were taken a t ambient probe temperature (30 "C) on a Varian XL-100-15 spectrometer. An external 19F lock was employed and proton decoupling was not used. An external reference standard of approximately 0.3M tetraethylammonium phosphate (TEAP04) was placed in a precision capillary tube and used in the quantitative work. The chemical shift of TEAP04 is -2.1 ppm with respect t o 85% H3P04 (downfield from H3PO.d. Reagents. All reagents were reagent grade. No special handling was required. The tetraethylammonium phosphate [(Et)4N]2HP04 was prepared by titrating a given amount of tetraethylammonium hydroxide with phosphoric acid to a pH of 10. The solution is jellylike and was diluted before being placed in a precision capillary. Procedure. Standard solutions of 100 to 500 ppm phosphorus (a mixture of each specified phosphate) were prepared and incrementally diluted to obtain lower concentrations. The iron(II1) was added as the hydrated nitrate salt to a known volume of a phosphate mixture for the HC1 and EDTA systems. Then, after the addition of the iron, known amounts of HC1 or excess molar quantities of EDTA were added and thoroughly mixed to ensure equilibrium. Iron(II1) was added directly to a known volume of acetylacetone (acac), thoroughly mixed, and then given volumes of the chelate solution were added to the phosphate solutions. Fresh samples were prepared to ensure little or no hydrolysis of the condensed phosphates was occurring. The tripolyphosphate NMR signal used for analysis was the doublet due to the end phosphate groups and therefore was representative of only two-thirds of the phosphorus in the actual compound. The triplet due to the central phosphate group was too weak to be useful.
