Anal. Chem. 1989, 6 1 , 1427-1430
may occur in the sample cell; photodecomposition of the laser dyes is another possibility. Since the active layer of the laser diode is extremely small, minor changes in the refractive index of the solution can result in drifting of the reflected laser beam. Further studies are planned to determine the extent of this effect. The described extended intracavity single-beam laser diode spectrophotometer can detect absorbencies as well as or better than many commercial double-beam absorption spectrophotometers. The commercial double-beam spectrophotometer was unable to distinguish between the 1 nmol/L HDITC solution and a 20 nmol/L solution of HDITC from the blank. It was observed that the major noise contributing factor is the optical noise generated by the mechanical instability of the optical mounts. When compared to commercial instruments, the laser diode system proved to be superior. Moreover, it should be noted that this simple arrangement is a single-beam instrument, using only a $160 laser diode as a light source and detector simultaneously. The detectable absorbencies can be lowered by a number of improvements to the instrumentation. Less reflectivity of the front facet mirror of the laser diode will enhance the multipass effect and therefore increase the effective sample cell path length. If we can decrease the front facet’s reflectivity to 170,it would result in a much greater enhancement factor. Studies using special antireflective coated (AR) laser diodes are currently under development. Output stability can be improved by locking the optical components together. This will allow use of a more sensitive scale on the output multimeter, resulting in a lowering of the detection limits. Further improved detection limits can be expected from using an AR coated laser diode pair where one of the lasers can serve as a reference source. The use of a double-beam instrument as opposed to our present single-beam instrument will compensate for source and detector fluctuations and lower de-
1427
tection limits. The use of batteries instead of a regulated power supply can improve signal stability while reducing the size. The drive circuit can be put on a chip, further reducing the size. The instrument can be tuned to different wavelengths by changing the laser diode temperature or by subjecting it to an adjustable magnetic field. Using a 5- or 10-cm cell will allow for smaller detectable absorbencies. New laser diodes with lower emission wavelengths (a 680-nm device is now available) will extend the analytical utility of the instrument.
LITERATURE CITED Harris, T.; Mitchell, J.; Shirk, J. S. Anal. Chem. 1980, 52, 170 1-1 705.
Peterson, N. C.; Kurylo, M. J.; Braun, W.; Bass, A. M.;Keller, R. A. J. Opt. SOC.Am. 1971, 61, 746-750. Klein, M. B. Opt. Commun. 1972, 5 , 114-116. Thrash, R. J.; Weyssenhoff, H.; Shirk, J. S. J. Chem. fhys. 1971, 55, 4659-4660.
Childs, W. J.; Fred, M. S.; Goodman, L. S. Appl. Opt. 1974, 13, 2297-2299.
Green, R. B.; Latz, H. W. Spectrosc. Lett. 1974, 7, 419-430. Camparo, J.; Klimak, C. Am. J. fhys. 1983, 51(12), 1077-1081. Kim, K., et ai. f r o c . Tech. Program, I n t . Laser Expo. Chicago, IL, 1975. 191-5. Ishibashi, N.; Imasaka, T.; Kamikubo, T.; Kawabata, Y. Anal. Chlm. Acta 1983. 153. 261-263. Ishibashi, N.; Imasaka, T.; Yoshitake, A. Anal. Chem. 1984, 5 6 , 1077-1079. Ishibashi, N.; Imasaka, T.; Yoshitake, A. Anal. Chem. 1988, 58, 2649-2653. Demtroder, W. Laser Spectroscopy, 2nd ed.; Springer: Berlin, 1982; pp 369-390. Standard Methods for the Examlnatbn of Water and Wastewater, 16th ed.; American Public Health Association: Washington, DC, 1985; Method 425D, 513.
RECEIVED for review December 2,1988. Accepted March 30, 1989. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research.
Atomic Absorption Determination of Copper in Silicate Rocks by Continuous Precipitation Preconcentration Ricardo E. Santelli,’ Mercedes Gallego, and Miguel Valcbrcel* Department of Analytical Chemistry, Faculty of Sciences, University of Cdrdoba, Cbrdoba-14004, Spain
A selective atomic absorption spectrophotometric method has been developed for the preconcentratlon and determlnation of copper after continuous precipltatlon with rubeanlc acld. The preclpltate Is separated by continuous filtration and dissolved In potasslum dichromate. A concentration factor of up to 500 Is achieved. Several calibration graphs are used for the determination of copper( I I ) in the range 0.3-200 ng/mL with a sampilng frequency between 1 and 20 h-’ and a reiatlve standard deviation between 1.4 and 3.0%. The proposed method has been successfully applied to determination of copper at the pg/g level in siilcate rocks.
INTRODUCTION The precise determination of trace elements in rocks is usually hindered by the fact that they occur at levels beyond Permanent address: D e p a r t m e n t of Geochemistry, U n i v e r s i t y Federal Fluminense, Niter6i-24020, Brazil.
the typical detection limits of currently available instrumentation. Even with sensitive methods, constituents other than those of interest (whether major or minor elements) may hinder the detection of an element through secondary effects such as broadband absorption in electrothermal at,omization atomic absorption or background radiation in neutron activation. Separation techniques have been used to overcome these problems, both by increasing concentrations above detection limits and by removing matrix constituents. Precipitation is one of the separation techniques most frequently used for enrichment purposes in inorganic analysis (1, 2 ) . Coprecipitation of trace elements with an unselective organic reagent is used quite frequently in the enrichment of trace elements ( 3 ) ,mainly in waters. Dithizone (4), 8-hydroxyquinoline ( 5 ) ,thionalide (6),a-benzildioxime (7), diethyldithiocarbamate (8), ammonium pyrrolidinecarbodithioate (9), and 8-hydroxyquinoline in combination with thionalide and tannic acid (IO)are typical precipiate collectors and precipitants for copper. After collection of the desired trace elements, the precipitate collector is isolated from the sample
0003-2700/69/0361-1427$01.50/00 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
solution by filtration or centrifugation, followed by washing, drying, or dissolution for trace determination by one of various techniques available. Carrier elements and organic matter are sometimes removed by different procedures prior to the determination. These operations result in low sampling frequencies and irreproducible recoveries; however, they afford enrichment factors of about 10,. The interest roused by automatic methods of analysis is ostensibly shown by the increased number of papers and monographs published on this topic in the last few years ( 11-13). The advantages of continuous automatic techniques become particularly evident when extensive manipulation is involved (e.g. when a separation technique is required). Continuous precipitation systems were recently approached by the authors. Our earliest endeavors in this respect were aimed a t the study of continuous automatic precipitationdissolution systems, coupled on-line with conventional atomic absorption instruments in implementing different methodologies such as indirect determinations of organic and inorganic anions and preconcentration of metal traces ( 1 4 , 1 5 ) . In this work, continuous precipitation-dissolution is used in conjunction with a flame atomic absorption spectrophotometer for the preconcentration and determination of pg/g quantities of copper in silicate rocks. Dithiooxamide (rubeanic acid) is used as an organic precipitating reagent, and no collector is required. The precipitate dissolution is effected by a solution of potassium dichromate in nitric acid. This concentration method allows the determination of copper in the range 0.3-200 ng/mL.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer 380 atomic absorption spectrometer equipped with a hollow-cathodecopper lamp was used. The instrument was set at a wavelength of 324.7 nm, and the air/acetylene flame was adjusted according to standard recommendations. The peristaltic pump was a Gilson-Minipuls-2, furnished with poly(viny1 chloride) and Solvaflex tubing for aqueous and ethanol solutions, respectively. Two Rheodyne 5041 four-way valves connected to two channels were also used. A Scientific System 0.5-105 column with a removable screen-type stainless steel filter (pore size 0.5 pm, chamber inner volume 580 pL, and filtration area 3 cm2),which was originally designed as a cleaning device for high-performance liquid chromatography, was employed for filtration purposes. Reagents. A 1000 mg/L copper solution was prepared by dissolving 1.000 g of metal copper in a small volume of concentrated nitric acid and diluted to 1 L with 1%(v/v) nitric acid. A 0.1% (w/v) rubeanic acid solution was made in ethanol/water (6040 (v/v));this solution was stable for at least 1week. A 0.083 M potassium dichromate solution was prepared in 1 N nitric acid. A 1 M acetic acid/l M ammonium acetate buffer (pH 4.8) was also used. All other reagents were of analytical reagsnt grade. Preparation of Standards and Samples. Copp ?I standards in the range 0.1-2.0 pg in a 0.2 M acetic/acetate buffer (pH 4.8) were prepared in 10-250-mL calibrated flasks. The silicate rock material was decomposed by the recommended method (16). An accurately weighed portion of finely powdered silicate rock was placed in a platinum crucible and moistened with some drops of water; then 10 mL of 48% hydrofluoric acid and 1 mL of 70% perchloric acid were added. After evaporation to perchloric acid fumes in a sand bath, the mass was allowed to cool, and a new portion of 5 mL of 48% hydrofluoric acid was added before evaporating next to dryness again. Once cool, the residue was extracted with several small portions of 1 M acetic acid/ ammonium acetate, transferred to a volumetric flask, and diluted with water to a concentration of 0.2 M in the buffer. The remaining residue (e.g. iron, aluminum or silica) was removed by continuously filtering the solution through a microfilter. The solution was stored in polyethylene bottles. The amount of rock sample weighed ranged between 0.25 and 2.0 g, and the total volume of the rock solution depended on the copper content. The different solution volumes to be used in the determination (10-100 mL) contained about 1 pg of copper.
RUBEANIC - IO ACID SAMPLE
LO
RECORDER
Figure 1. Schematic diagram of flow system for preconcentration of
copper.
Procedure. The manifold used is illustrated in Figure 1. In the preconcentration step, 1C-250 mL of sample containing 0.1-2.0 pg of Cu(I1) in 0.2 M acetic acid/ammonium acetate buffer (pH 4.8) was continuously pumped into the system and mixed throughly with a 0.1% rubeanic acid solution. Precipitation was instantaneous and was followed by continuous filtration. In the dissolution step, the selecting valve was switched to pass a stream of 0.083 M potassium dichromate in 1 N nitric acid through the precipitate, which, once dissolved, gave a positive flow injection analysis (FIA)peak proportional to the amount of copper present in the sample volume introduced. No blank or precipitate washing was required. A second selecting valve was incorporated to directly aspirate a water stream intended to flush the nebulizer after each measurement. All reagents and instrumentation were kept at room temperature throughout the experiments. RESULTS AND DISCUSSION Continuous systems allow the precipitation of trace elements without the need for another precipitate collector as they permit one to deal with minute amounts of precipitates, which are not handled directly (17). Rubeanic acid can be used for the quantitative precipitation of copper, nickel, and cobalt in conventional methods (18). When a continuous preconcentration system (asshown in Figure 1)is used, neither nickel nor cobalt precipitates. The copper precipitate is dark green. There are no references to the use of this reagent for the preconcentration of metal traces, probably because of the insolubility of its precipitates. Selection and Foundation of the Method. Precipitating Reagent. We assayed various organic reagents precipitating with dilute Cu(I1) in acetic/acetate buffer. We also checked the potential precipitation of these reagents with other metal ions occurring as major or minor constituents in silicate rocks. The organic reagents in question were cupron (a-benzoinoxime), a-nitroso-@-naphthol,and rubeanic acid. Cupron and a-nitroso-@-naphtholwere discarded, as they precipitated with Fe(II1) a t concentrations identical with that of Cu(I1). Rubeanic acid was therefore chosen as the Cu(I1) precipitant on account of its increased sensitivity and selectivity (it did not precipitate with Fe(II1) or Al(II1) a t concentrations as high as 1 g/L). Dissoluing Reagent. Small amounts of the Cu(II)/rubeanate precipitate were subjected to the potential dissolving action of various solvents in test tubes. The solvents assayed were as follows: (1)acids (HCl, "OB) and bases (NaOH, NH,) a t different concentrations and organic solvents (acetone, acetonitrile, and dimethylformamide) (results dissatisfactory in every case); (2) complexing reagents such as 0.5 M ethylenediaminetetraacetic acid in 2 M ammonia and 5% (w/v) thioglycolic acid in 2 M ammonia (results also poor in this case); (3) oxidants such as H z 0 2in NH, and NaI04, KBr03, KIO,, K2Cr207,KMn04, and (NH4),Ce(NO3), a t different concentrations in acid media (results satisfactory in all cases, but some oxidants dissolved the precipitate more rapidly than others). With an automated configuration similar to that depicted in Figure 1 and a IO-mL sample containing 0.2 pg/mL of Cu(I1) in 0.2 M acetic acid/ammonium acetate buffer, the above oxidants were assayed at different concentrations in an acid medium in order to select the fastest. The
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
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Table I. Characteristic Parameters of the Calibration Graphs and Analytical Features of the Determination of Copper in Different Sample Volumes aliquot taken/mL 10 25 50 100 250
regression eq (absorbance vs 4 m L ) A A A A A
= 0.003 = 0.001 = 0.001 = 0.002
= 0.003
+ 1.023(Cu2*) + 2.519(Cu2+) + 5.204(Cu2+) + 10.510(Cu2+)
+ 26.060(Cu2+)
corr coeff
detection lim, ng/mL
RSD, %
sampling, freq/h-'
0.999 0.999 0.998
5 2.5
20
0.997
0.5 0.3
1.4 1.2 1.6 2.0 3.0
0.998
Flgure 2. Influence of the nitric acid concentration on the precipitate dissolution process. A, B, and C: 0.1, 0.5, and 1-2 N nitric acid, respectively; oxidant concentration, 0.5 N.
Figure 3. F I A peaks obtained with different oxidants in the dissolving solution. The concentrations of oxidants were 0.5 N in 1 N nitric acid; peak a, H,Op in 2 M ammonia.
results obtained in these experiments are summarized in Figures 2 and 3. As can be seen, the best results were provided by Crz072-,Ce4+,and Mn04- in 1 N nitric acid. Permanganate, which yielded the best results of all three oxidants, was discarded as it was unstable in the acid medium and gave rise to MnOz, which clogged the filter and the nebulizer, after the redox reaction. The final choice was 0.083 M (0.5 N) potassium dichromate in 1 N nitric acid, which was stable and more economical than Ce4+and dissolved the precipitate very rapidly. Precipitate Dissolution Mechanism. Copper forms an uncharged chelate (CuL) with rubeanic acid. Polymeric chelates are probably formed through bonding of the metal to S a t each end of the ligand (19). The mechanism of the redox reaction involved in the dissolution step could be as follows:
reagent
L precipitate
SO:-. NO .;
COlfI
Cu2+
The presence of S042- and NO3- was confirmed by the Ba2+ and Griess reaction; gas bubbles (COz)were also detected in the dissolution step. Optimization of Chemical Variables. This study was performed by continuously introducing into the system 10 mL of a solution containing 0.1 kg/mL of copper. As the organic reagent occurs in tautomeric forms, pH changes were bound to affect its reactivity. The maximum absorbances were ob-
1
10 5 2
1
tained in the pH range 4.3-6.0. The absorbance decrease observed above pH 6.0 was probably a result of copper precipitating and being partially absorbed on the walls of the flask containing the sample. The effect of the concentration of the acetic acid/ammonium acetate buffer (pH 4.8) was examined up to 1 M; this variable did not affect the analytical signal over a wide range (0.1-1.0 M). A 0.2 M concentration of buffer was chosen as optimum. The influence of the ionic strength (adjusted with KNOBor NH4Cl) was also tested; it had no effect, a t least up to 3.5 M. When various concentrations of rubeanic acid solutions were used for a fixed concentration of copper, 0.002% of organic reagent solution was found to be sufficient for maximum response. Above this concentration, the absorbance remained constant up to 0.5%. A 0.1% concentration of rubeanic acid in (60:40 (v/v)) ethanol/water was used. Precipitate dissolution must be instantaneous in order to obtain a transient signal rather than a plateau. Therefore, the precipitated copper rubeanate was dissolved by using a stream of potassium dichromate in nitric acid. Nitric acid concentrations below 1 N resulted in slow dissolution (Figure 2); above 1 N, the signal was virtually constant. Dissolution was complete a t potassium dichromate concentrations above 0.35 N in 1 N nitric acid. According to the above considerations, we chose 0.5 N (0.083 M) potassium dichromate in 1 N nitric acid in order to achieve the best possible results in the dissolution step. Optimization of FIA Variables. The variables studied were the length of the precipitation coil and flow rates of the sample, organic reagent, and dissolving reagent solutions. The influence of the precipitation coil length was investigated at constant flow rates in the range 20-300 cm (0.5-mm i.d.). This variable had no effect as precipitation was instantaneous. A coil length of 60 cm was chosen for further experiments. The tube length between the filter and the nebulizer (dissolution coil) did not influence the peak height below 50 cm, whereas longer lengths increased the dispersion of dissolved copper. The sample flow rate (10 mL of a solution containing 0.1 pg/mL copper in the buffer) caused the smallest variations in the range 1-5 mL/min. As the preconcentration method required the aspiration of large sample volumes (up to 250 mL), the sampling time was rather long. A high flow rate (4.0 mL/min) was therefore selected in order to increase the sampling rate. The absorbance was maximum for rubeanic acid flow rates included within the interval studied (0.4-1.6 mL/min), so that a flow rate of 1.0 mL/min was chosen. The influence of the flow rate of the dissolving solution on the peak height was examined in the range 1-7 mL/min. The latter increased with increasing flow rate of the potassium dichromate solution and was constant above 3.5 mL/min. The peak height decreased below 3.5 mL/min; however, the peak width increased, and in all instances the area under the peak remained constant (a similar effect to that observed upon decreasing the dissolving reagent concentration). A flow rate of 5.0 mL/min was chosen in order to increase the sample throughput. Determination of Copper. Figures of Merit. Under the optimum chemical conditions and by use of the manifold
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
Table 11. Analysis for Copper in Silicate Rocks composition/ % Fe,OBa
sample
aliquot taken/mL
SiO,
A1203
marine mud, MAG-1 (USGS) mica schist, SDC-1 (USGS) granite, JG-2 (GSJ) granodiorite, JG-3 (GSJ) andesite, JA-3 (GSJ)
10 10 100 25 10
51.19 66.15 76.95 67.10 62.26
16.46 15.75 12.41 15.52 15.67
a
6.98 6.85 0.92 3.73 6.59
copper content, &g/g
MnO
certfd
foundb
0.10 0.12 0.015 0.072 0.106
30 f 3 30 =k 2 0.4 6.0 45.3
27.3 f 0.8 30.1 f 0.8 0.40 rt 0.05 5.6 f 0.3 44 =k 1
Percentage of total Fe203. Average of three separate determinations.
depicted in Figure 1, several linear calibration graphs were run for copper with different sample volumes (between 10 and 250 mL). Table I lists the characteristic parameters of these graphs and the analytical features of the determination of copper(I1) in the range 0.3-200 ng/mL. The detection limit was calculated as %fold the standard deviation of the peak height for 10 determinations of the same sample (14). The precision of the method (expressed as the relative standard deviation) was checked on 11 samples containing 1 kg each in different sample volumes. Concentration factors of up to 500, calculated as the ratio between the slopes of the calibration graphs obtained by this method and by direct aspiration (A = 0.002 + 0.0517(Cu2+)), were readily achieved. The effect of some components of silicate rocks was examined in order to detect potential interferences in the determination of copper. The cations assayed in this study of interferences can be classified in two groups: (a) major and minor elements commonly found in silicate rocks (e.g. Fe3+, A13+,and Mn2+),which can precipitate in the acid medium (pH 4.8) or with rubeanic acid and clog the fiiter; and (b) trace elements such as Co2+,Ni2+,and Zn2+,which compete with copper for the reagent and give rise to poor results. A13+,Fe3+, and Mn2+caused no interference with the determination of 1 kg of copper (sample volumes of 50 mL) at concentrations of 15000 pg; neither did 3000 r g of Fe3+ or A13+ (sample volumes of 10 mL). Higher Fe3+ or A13+concentrations precipitated in the buffer of pH 4.8 and required the addition of tartrate (2% (w/v)) as a masking agent. As Co2+,Ni2+,and Zn2+normally occur in silicate rocks at concentrations 5-20 times higher than that of copper, we only assayed concentrations up to 100-fold,which posed no interference. Finally, we prepared a synthetic silicate rock solution containing all the typical major and minor constituents at their maximum levels of occurrence. The copper recovery ranged between 98 and 101%, and the error made was rather small. Determination of Trace Amounts of Copper in Silicate Rocks. The knowledge of the content and distribution of trace elements in rocks is of great interest to geochemical (particularly petrological and mineralogical) studies. The concentration of trace metals in i %ate rocks varies with the silica content. Acidic rocks (higher silica content) generally contain small amounts of copper and other metals. The applicability of the proposed copper/rubeanate method was studied by using silicate rocks in international reference samples from the United States Geological Survey (20, 21) (MAG-1 and SDC-1) and the Geological Survey of Japan (22)
(JG-2, JG-3, and JA-3). The materials were dissolved as described in the Experimental Section, and the copper content of the resulting solutions was determined by the recommended procedure. The results obtained are listed in Table 11. CONCLUSIONS Continuous preconcentration with rubeanic acid has proved to be an efficient and convenient method of overcoming the interference of relatively large amounts of some elements in the preconcentration and determination of copper in silicate rocks. The proposed automatic preconcentration method offers several major assets such as high sensitivity (a concentration factor of up to 500 is achieved), selectivity, and rapidity. LITERATURE CITED Minczewski, J.; Chwastowska, J.; Dybczynski, R. Separation and Preconcentration Methods in Inorganic Trace Analysis ; Ellis Horwood: Chichester, U.K., 1982. Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis ; Springer Verlag: Berlin and Heidelberg, 1983. Myasoedova, G. V. Zh. Anal. Khim. 1966, 2 1 , 598. Zharikov, V. F.; Senyavin, M. M. Tr. Gos. Okeanogr. Inst. 1970, 101, 128.
Scott, R. 0.; Mitchell, R. L. J. Soc. Chem. Ind. 1943, 62, 4. Portmann, J. E.;Riley, J. P. Anal. Chim. Acta 1984, 31,509. Jackwerth, E.; Salewski. S . Fresenius' Z . Anal. Chem. 1982, 310,
108. Watanabe, H. Talanta 1972, 19, 1363. Krishnamurty, K. V.; Reddy, M. M. Anal. Chem. 1977, 49,222. Silvey, W. D., Brennan, R. Anal. Chem. 1962, 34,784. Valdrcel, M.; Luque de Castro, M. D. Automatic Methods of Analysis ; Elsevier: Amsterdam, 1988. Valcircel, M.; Luque de Castro, M. D. Flow Injection Analysis: Principles and Applications: Ellis Horwood: Chichester, U.K., 1987. Ruzicka, J.; Hansen, E. Flow Injection Analysis, 2nd ed.; Wiley: New York, 1988. MartinezJimBnez, P.; Gallego, M.; ValcBrcel, M. Anal. Chem. 1987,
59,69. Valcircel, M.; Gallego, M. TrAC. Trends Anal. Chern. (Pers. Ed.) 1989, 8 , 34.
Jeffery, P. G. Chemical Methods of Rock Analysis, 2nd ed.: Pergamon Press: Oxford, U.K., 1975. Martinez-JimBnez, P.: Gallego. M.: Valdrcel, M. Analyst 1987. 112,
1233. Burger, K. Organic Reagents in Metal Analysis; Pergamon Press: Oxford, U.K., 1973. Sandell, E. B.; Onishi, H. Photometric Determination of Traces of Metais. General Aspects; John Wiley: New York. 1978. Gladney, E. S.;Goode, W. E. Geostand. Newsl. 1981, 5,31. Flanagan, F. J. U S . Geol. Surv. Prof. Pap. ( U . S . ) 1971, No. 840. Ando. A.; Mita, N.; Terashima, S. Geostand. Newsl. 1987, 1 1 , 159.
RECEIVED for review December 5 , 1988. Accepted March 20, 1989. CICYT (Spain) is acknowledged for financial support (Grant No. PA86-0146). R. E. Santelli is also grateful to the Brazilian Government for financial support (CNPq70.1898/ 87.9-PADCT/ SPGTM).
