(56) Johnstone, H. F . , I n d . Eng. Chem., 23,559 (1931). (57) Hewson, G. W., Pearce, S. L., Pillitt, A., Rees, R. L., Engineering, 136,459 (1934). (58) Weast, R. C., “CRC Handbook of Chemistry and Physics”, 50th ed., Chemical Rubber Publishing Co., Cleveland, 1970. (59) Lamb, A. B., Jacques, A. G., J . Am. Chem. Soc.. 60. 1215 (1938). (60) Kovach, J . J., unpublished observations, 1969.
(50) Segeler, C. G., “Gas Engineer’s Handbook”, American Gas Association, Industrial Press, New York, 1965. (51) Simon, A., Lang, M., 2. Anorg. Allg. Chem., 286, 20 (1956). (52) Simon,A., Reichelt, D., Chem. Zuest., 13, 731 (1959). (53) Yannepoulos, J. C., Agarwal, J. C., “Extractive Metallurgy of Copper, an International Symposium”, Vol. 11, The Metallurgical Society, New York, 1976, Chapters 31,32, 37, 39, and 40. (54) Peters, E., Metall. Trans., 7B, 505 (1976). (55) Orozco, H. H., M.S. Thesis, University of California, Berkeley, 1976.
Received for review April 19, 1979. Accepted April 1, 1980
Enhancement of Atomic Absorption Sensitivity for Cadmium, Manganese, Nickel, and Silver and Determination of Submicrogram Quantities of Cadmium and Nickel in Environmental Samples Vincent B. Stein and Bobby E. McClellan’ Department of Chemistry, Murray State University, Murray, Ky. 42071 articles on flame emission and atomic absorption spectroscopy by Winefordner and Vickers ( I I ) , Hieftje, Copeland, and de Olivares (IZ),and Hieftje and Copeland ( 1 3 ) describe extraction procedures for several metal ions. The purpose of this study was to survey a number of complexing agent-solvent systems in order to determine the most efficient systems to use for Ni, Mn, Ag, and Cd and to determine the extent to which the sensitivity can be enhanced for the metal ions using chelate extraction and atomic absorption. System efficiency is based upon the criteria of completeness of extraction, broad pH range of extraction, solution uptake rate, combustibility aspects of the solvent, and enhancement of the absorption signal. Several environmental samples were analyzed for cadmium and nickel using the extraction-atomic absorption technique.
Several organic solvents were evaluated for their enhancement effects on nickel, manganese, cadmium, and silver using atomic absorption spectrometry. Solvents giving the greatest enhancement, usually ketones and acetate esters, were chosen for solvent extraction studies with various complexing agents. The system exhibiting the highest absorbance and broadest pH range was used to prepare standard curves for each element to determine the extent to which the detection limit can be lowered. By extracting 200 mL of aqueous solution with 2 mL of organic solvent and aspirating the organic phase, it was possible to detect nickel a t 1 ng/mL, manganese a t 0.10 ng/mL, cadmium a t 0.10 ng/mL, and silver a t 0.10 ng/mL. Cadmium and nickel were determined in several environmental samples using the extraction system of choice. Tobacco, canned oysters, and paint chips showed the highest cadmium content of the samples tested and were, respectively, 5.23, 1.84, and 0.71 pg/g. The environment of western Kentucky and Tennessee seems to be quite clean with respect to cadmium and nickel.
Experimental
A number of workers have observed enhancement of sensitivity in atomic absorption upon dissolving metal ions, in the form of complexes, in certain organic solvents (1-4). The most generally used, although not necessarily the most efficient extraction system for metal ions, has been ammonium pyrrolidinedithiocarbamate dissolved in methyl isobutyl ketone (MIBK). Takeuchi, Suzuki, and Yanagisawa ( 5 )examined the role of the solvent and complexing agent in extraction systems when atomic absorption was used for nickel, silver, and manganese. Montford (6) used a 100:5 aqueous to organic volume ratio to extract Fe, Cu, Co, Cr, Mn, and Ni with sodium diethyldithiocarbamate (NaDDC) and MIBK. A 100-fold gain in sensitivity was obtained. Belcher, Dagnall, and West ( 7 ) determined silver in the range of 0.10 to 0.01 pg/mL in aqueous solution following an extraction of silver as its butylamine salicylate into MIBK. Sachdev and West (8)studied dithizone in n-amyl acetate, n-butyl acetate, isobutyl acetate, and ethyl propionate for the extraction of Ag, Cd, Co, Cu, Ni, Pb, and Zn a t a pH of 7.5. Detectability limits of 2 ng/mL for silver and 4 ng/mL for nickel were reported. Hannaker and Hughes (9) utilized a NaDDC-MIBK extraction system followed by atomic absorption determination for Bi, Cd, Co, Cu, Cr, Pb, Ni, Ag, and Zn in geological materials. Kuz-Min et al. (10) recently published a comprehensive review of extraction techniques used for atomic absorption and emission spectroscopy. In addition, the recent review 872
Environmental Science & Technology
Apparatus. Absorbance measurements were made using either a modified Jarrell-Ash atomic absorption flame emission spectrophotometer, Model 82-500, or a Perkin-Elmer Model 603 atomic absorption unit. The Jarrell-Ash instrument was equipped with a Varian-Techtron laminar flow burner and the appropriate hollow cathode discharge tube. A Honeywell Electronik 194 recorder was also connected t o the instrument. Air and acetylene flow rates were monitored with Brooks Sho-Rate “250” Model 1357 flow meters. Extractions were performed with the aid of an Eberbach box type shaker and pH measurements were made using a Sargent Model LS pH meter. Deionized water was prepared by passing distilled water through two Illco-Way deionizers in series. Weighings were made on a Sartorius Model 2404 digital analytical balance. Reagents. Standard aqueous solutions of 1000pg/mL Ni, Mn, Ag, and Cd were prepared from the pure (99.999%)metals obtained from Research Organic/Inorganic Chemical Company. Nitric acid dissolution of the accurately weighed metal, followed by dilution to volume with deionized water, was used in all cases. Working solutions of less than 1000 wg/mL were prepared daily by appropriate dilution of the stock solution. Standard organic phase solutions were prepared in order to determine enhancement values. Organic solutions (100 pg/mL) of the metals were prepared from either the perchlorates, the cyclohexanebutyric acids, or acetylacetonate organometallic compounds. Appropriate dilutions of these stock solutions were made, as necessary. An acid mixture of 3:l:l
0013-936X/80/0914-0872$01.00/0
@ 1980 American Chemical Society
Table I. Optimum Instrumental Conditions acetylene flow rate, metal
nickel
manganese
silver
solvent
Llmln
butyl acetate isopropyl acetate butyraldehyde 2-octanone 2,4-pentanedione methyl isobutyl ketone amyl acetate butyl acetate isopropyl acetate 3-heptanone 2-octanone methyl isobutyl ketone butyl acetate 3-heptanone 2,4-pentanedione methyl isobutyl ketone
1.61 1.61 1.61 2.32 2.68 1.61 3.64 3.96 1.61 2.70 3.04 1.61 2.70 2.70 2.70 2.70
nitric, sulfuric, and perchloric acids was used for digestion of all solid samples in which cadmium or nickel was determined. All other solvents and reagents not described were of at least A.R. grade. Procedure. Instrumental variables of fuel and oxidant flow rates, burner height, and lamp current were optimized for aqueous systems as well as for organic solvent systems. Enhancement values were determined by dividing the optimum reading for each metal ion in the organic phase by the optimum reading of an aqueous solution of equal concentration. Following determination of the solvents giving the largest enhancement values, several combinations of complexing agent-solvent systems were employed for extraction of the metals. A 5-mL aliquot of the appropriate buffer system was pipetted into a 125-mL separatory funnel, followed by 5 mL of the appropriate standard solution. Ten milliliters of the given solvent containing the chelating agent was added, and the mixture was mechanically shaken for 10 min. Both aqueous and organic solvent systems were presaturated with each other prior to extraction of the metal ions in order to eliminate mutual solubility effects. After we physically separated the two layers, the organic phase was analyzed for metal content and the percent extraction was determined by reference to a calibration curve prepared from an organometallic compound. The pH of the aqueous phase was then measured, and a percent extraction vs. p H plot was made. After the most suitable extraction system was determined, a standard curve' in the low nanograms/milliliter range was prepared for each metal ion by extracting a 200-mL volume of the aqueous phase with a 2-mL volume of the organic system to determine linearity and the extent to which the limit of detection could be lowered. The environmental samples analyzed were accurately weighed in either a 50- or a 125-mL round-bottomed flask. The size of the samples varied from 0.5 to 5 g depending on the type of sample analyzed. Ten milliliters of acid mixture (3:1:1, "03, HZS04, H[C104) was added to the sample, and it was digested under reflux until dissolution was complete. After digestion, the sample was neutralized with ammonium hydroxide, the p H adjusted, and the analyte determined using chelate extraction-atomic absorption.
Results and Discussion It is necessary to optimize instrumental parameters for each metal-solvent pair in order to obtain maximum sensitivity.
air flow rate, Llrnln
6.39 6.39 6.39 6.39 6.39 7.29 6.39 6.39 6.39 6.39 6.39 6.39 5.23 6.39 6.39 4.05
burner helght, mm
lamp current,
2.00 10.00 10.00 3.00 3.00 5.00 3.00 6.00 5.00 3.00 3.00 3.00 3.00 3.00 3.00 2.00
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 6.0 6.0 6.0 6.0
mA
Table II. Complexing Agents and Concentrations metal
nickel
manganese
silver
complexing agent
8-hydroxyquinoline (oxine) dimethylglyoxime (DMG) P-isopropyltropolone (P-IPT) dithizone (HDz) thenoyltrifluoroacetone (TTA) sodium diethyldithiocarbamate (NaDDC) 1-(2-pyridylazo)-2-naphthol(PAN) 8-hydroxyquinoline 8-hydroxyquinaldine 8-hydroxyquinoline sodium diethyldithiocarbamate dithizone
concn
0.1 M 0.1 % 0.1% 0.1O h 0.1 M 1.O % 0.1 % 0.1 M 1.0% 0.1 M 1.0% 0.1%
In general, the optimum air flow rate does not vary a great deal (4-7 L/min) from one solvent to another, and a broad range may be used for the optimum setting. Combustible solvents normally require somewhat lower fuel flow rates. Generally, the fuel flow rate was found to give optimum sensitivity within a narrow range (2-4 L/min). Burner heights of from 2 to 10 mm were found to be optimum for the elements investigated, as shown in Table I. Table I lists the optimum instrumental conditions for each metal studied. The complexing agents used for each metal ion and their concentrations are listed in Table 11. These complexing agents were selected from those listed in Stary (14)and Morrison and Freiser ( 1 5 ) .These workers report the conditions for extraction of metal complexes largely from carbon tetrachloride and chloroform. Less information is available regarding the conditions for extraction into oxygen-containing solvents. Extraction preconcentration is an extensively used method for increasing the sensitivity of atomic absorption spectroscopy. Enhancements are obtained due to an increase in absorbance obtained in organic solvents and concentration of the metal ion in a small volume of the organic phase. An organic solvent should have the following characteristics for use in atomic absorption: (1)the solvent should be combustible so as not to lower flame temperature; (2) the solvent should have a favorable aspiration rate, i.e., low viscosity; (3) the solvent should not absorb radiation from the hollow cathode lamp; and (4) the solvent should have a low surface tension allowing the production of a smaller droplet size distribution. Oxygen-containing solvents and aromatic hydroVolume 14, Number 7, July 1980
873
Table Ill. Enhancement Values with Various Solvents solvent
butyl acetate isopropyl acetate ethyl acetoacetate methyl benzoate butyraldehyde methyl isobutyl ketone cyclohexanone 2-heptanone 2-octanone 2,4-pentanedione 1-butanol 1-hexanol 1-octanol 1-pentanol cyclohexanol propylene carbonate n-amyl acetate 3-heptanone acetonitrile tributyl phosphate isoamyl acetate isoamyl alcohol
enhancement value, AO/Aaq NI Mn Ag
2.08 2.59 1.41 0.88 1.91 2.54 1.33 1.71 1.84 1.95 1.18 0.61 0.45 0.90 0.23 0.84
2.06 2.15 1.50
1.65 1.10 1.14 0.85
2.34 1.44 1.72 1.72 1.94
1.85 0.96
Environmental Science & Technology
solvent
butyl acetate
isopropyl acetate
1.46
2-octanone
1.60 1.61 1.94 0.86
1.68 2,4-pentanedione 1.14 1.10
carbons usually show an enhancement of sensitivity, and behave well in an air-CsHz flame. Table I11 lists the solvents investigated along with their enhancement values. Enhancement values for cadmium have been previously reported ( 1 6 ) .The solvents which gave the greatest enhancement and were insoluble in water were used for extraction studies. Enhancement of sensitivity for nickel was obtained with acetate esters, aldehydes, and ketones. Methyl benzoate, propylene carbonate, and all alcohols used, except 1-butanol, showed depression of absorbance readings. Solvents such as p-dioxane, methyl ethyl ketone, and dimethylformamide were not used because of their water solubility. Butyl acetate, butyraldehyde, 2-heptanone, isopropyl acetate, MIBK, 2octanone, and 2,4-pentanedione were chosen for extraction studies since they exhibit enhancement and are well behaved in the flame. Some of the solvents used in the nickel enhancement studies were excluded from further study. The alcohols were excluded because of their poor aspiration characteristics and nitrobenzene because of its poor flame characteristics. Enhancement of manganese sensitivity was observed for acetate esters, ketones, and acetonitrile. Good enhancement of silver sensitivity is obtained using acetate esters, ketones, and isoamyl alcohol. Cyclohexanone and methyl benzoate showed depression of absorbance readings. The conditions for extraction of a metal complex are dependent on the solvent employed; Le., p H vs. percent extraction curves shift dependent on the solvent. Therefore, it is necessary to determine the optimum p H of extraction for each metal complex-organic solvent system studied. An example of the kind of data obtained for Ni, Mn, Ag, and Cd is shown in Table IV for nickel. A plot of percent extraction vs. p H was made to determine the optimum p H of extraction for each system studied. Six complexing agents and six solvents were used to extract nickel, for a total of 36 extraction systems. Table IV shows the results of these extraction studies. The concentration of nickel was 5 pg/mL in all cases. All of the complexing agents show quantitative extraction in a t least one of the solvents studied except 0-isopropyltropolone (0-IPT). Nickel extractions were 874
Table IV. Sensitivity and Extraction Efficiency for Nickel
methyl isobutyl ketone
complexlng agent
oxine DMG 0-IPT dithizone TTA NaDDC oxine DMG P-IPT dithizone TTA NaDDC oxine DMG P-IPT dithizone TTA NaDDC oxine DMG 0-IPT dithizone TTA NaDDC oxine DMG 0-IPT dithizone TTA NaDDC
extraction, %
100 100 80 17 100 100 84 40 86 100 82 100 100 99 a7 34 99 99 a2 4 4 60 36 100 98 94 88 100
100 100
pH range
5.5-6.5 7.4-8.5 7.4-9.1 9.0-10.0 5.0-10.0 5.0-6.0 5.0-5.5 6.5-9.0 7.5-9.0 7.5-9.0 5.0-9.0 4.5-6.0 4.2-6.5 7.0-8.0 8.5-9.0 7.2-8.5 6.5-9.0 5.9-7.0 5.0-6.0 5.0-6.0 5.0-6.0 5.0-6.0 6.0-6.5 5.0-7.5 5.0-7.0 7.0-9.0 7.7-10.0 6.7-9.4 5.6-9.0 5.6-7.0
attempted using butyraldehyde with all six of the complexing agents listed in Table 11. Each attempt failed due to the oxidation of enough butyraldehyde to butyric acid to lower the pH to a value of about 5.0. Nickel is extracted more efficiently a t a pH greater than 5.0. Since the p H of extraction with butyraldehyde was difficult to control, it was eliminated from further study. Sodium diethyldithiocarbamate (NaDDC) was the most efficient complexing agent studied, although the pH range is narrow and the complexing agent is somewhat unstable in acid solution. Essentially quantitative extraction is obtained with a number of systems. However, sensitivity is best in those systems utilizing either isopropyl acetate or MIBK. The extraction systems giving the greatest sensitivity for atomic absorption determination of nickel are the dithizone-isopropyl acetate, the TTA-MIBK, or TTA-n-butyl acetate systems. Figure 1 illustrates a typical extraction curve obtained for nickel with TTA-n -butyl acetate. The extraction of manganese consisted of six solvents and four complexing agents. The solvents employed were amyl acetate, n-butyl acetate, isopropyl acetate, 3-heptanone, 2octanone, and MIBK. The complexing agents studied were 1-(2-pyridylazo)-2-naphthol (PAN), oxine, sodium diethyldithiocarbamate (NaDDC), and 8-hydroxyquinaldine. Each complexing agent, except PAN, showed quantitative extraction in a t least one or more of the organic solvents studied. The most efficient complexing agents were NaDDC and oxine. Oxine showed quantitative extraction in each organic solvent except isopropyl acetate, while NaDDC showed quantitative extraction in all the organic solvents studied except 2-octanone and 3-heptanone. The KaDDC in the 3-heptanone system could not be separated efficiently due to emulsion formation between the organic and aqueous layers. Manga-
Table V. Detectability Limits and Linear Ranges by Solvent Extraction-Atomic Absorption Based on Aqueous Phase Concentration extraction system
metal
nickel
pH range
5-9 6.5-9.0 8-10 3.8-8.1 4.4-6.0
TTA-butyl acetate dithizone-isopropyl acetate oxine-butyl acetate dithizone-MIBK dithizone-MIBK
manganese silver cadmium
I
sample
I
' W
40
0
1.0
2.0
3.0 4.0
5.0
6.0 7.0 8.0
9.0 10.0
PH
Figure 1. Percent extraction vs. pH curve for nickel using a lTA-rrbutyl acetate system
60
w
'
40
30
0
1.0
2.0
3.0 4.0
5.0
6.0
1.0-10.0 1.0-10.0 0.1-1.0 0.1-1.0 0.1-16.0
detection limit, ng/mL
1 .o 1.o 0.10 0.10 0.10
Table VI. Determination of Cadmium in Various Samples by Extraction-Flame Atomic Absorption
I
I
llnear range, nglmL
7.0 8.0
90
10.0
PH
Figure 2. Percent extraction vs. pH curve for manganese using an oxine-n-butyl acetate! system
nese-dithiocarbam.ate complexes have been found to be unstable in organic solvents ( I 7) and, therefore, NaDDC systems were not considered for further study. The oxine-butyl acetate system was used for the preparation of a standard curve, since it gave the best sensitivity and had excellent extraction characteristics. The percent extraction vs. pH curve is shown in Figure 2 for this extraction system. Extraction data for a 1pg/mL silver solution was obtained. Four solvents and three complexing agents were used to extract silver, for a total of 1 2 extraction systems. Solvents
cigarette tobacco corn flakes rice canned tuna canned oysters potatoes celery canned pineapple cannedpeas oranges beef liver paint chips natural surface watersb
av, a M94
5.3
