Enhancement of Atomic Absorption Sensitivity for Copper, Cadmium, Antimony, Arsenic, and Selenium by Means of Solvent Extraction J. C. Chambers and B. E. McClellan* Department of Chemistry, Murray State University, Murray, Ky. 4207 1
Several organic solvents were evaluated for atomic absorption sensitivity effects on copper, cadmium, antimony, arsenic, and selenium. Those solvents giving greatest sensitivlty enhancement were used in solvent extraction studies. By extracting 200 ml of aqueous solutlon with 2 ml of organic solvent and aspirating the organic phase, it is possible to detect copper at 0.01 ppb, cadmium at 0.1 ppb, and antlmony at 10 ppb based upon the original aqueous phase concentration. Arsenic and selenium standard curves were prepared by extracting 200 ml of aqueous solution with 10 ml of organic solvent. The organic phase was back-extracted with 2 ml of aqueous solutlon and the aqueous phase aspirated. Arsenic is detectable at 20 ppb and selenium at 2 ppb by this method.
A number of chelating agents and organic solvents have been used in studying the effect of solvent extraction on atomic absorption sensitivity. Enhancement effects have been observed in most cases. Studies of the mechanism of this enhancement (1-3) have revealed that the organic solvent modifies the aspiration and combustion processes in several ways. Several workers (4-23) have studied selected extraction systems for copper, cadmium, antimony, arsenic, and selenium. Enhancement was observed in most cases, and in some instances preconcentration was used. The recent review articles by Winefordner and Vickers ( 2 4 ) and by Hieftje, Copeland, and de Olivares ( 2 5 ) describe extraction systems for many metal ions. The most generally used extraction system for preconcentration of most metal ions has been APDC dissolved in MIBK. This system is capable of extracting approximately 20 different metal ions under the same pH conditions. Although it is an extremely broad range system and quite efficient for many metal ions, it may not necessarily give the greatest sensitivity enhancement for a particular metal ion. The purpose of this study was to survey a number of complexing agent-solvent systems in order to determine the best systems to use for Cu, Cd, Sb, As, and Se and to determine the extent to which the sensitivity can be enhanced for the metal ions using combination solvent extraction preconcentration and atomic absorption spectrometry. EXPERIMENTAL Apparatus. All absorbance measurements were made using a Jarrell-Ash Atomic Absorption-Flame Emission Spectrophotometer, Model 82-500. The 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 to the instrument. Premixed air-acetylene and nitrogen (air entrained)hydrogen flames were used. Air and acetylene flow rates were monitored with Brooks Sho-Rate “250” Model 1357 flow meters. Nitrogen and hydrogen flow rates were monitored with the flow meters incorporated in the Jarrell-Ash atomic absorption instrument. Extractions were performed with the aid of an Eberbach automatic shaker. All 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 1000 ppm Cu, Cd, As, and Se were prepared from the pure (99.999%) metals obtained from Research Organic/Inorganic Chemical Company. Acid dissolution of the accurately weighed metal followed by dilution to volume with deionized water was used in all cases. Standard antimony solutions were prepared by dissolving an accurately weighed quantity of reagent grade antimony trichloride in 0.1 M tartaric acid. Working solutions of less than 1000 ppm were prepared daily by appropriate dilution of the stock solution. Standard organic phase solutions were prepared in order to determine the enhancement values. Organic solutions of copper were prepared from reagent grade bis(l-phenyl-1,3-butadieno)copperand organic solutions of antimony were prepared from reagent grade triphenylantimony. Organic cadmium solutions were prepared from carefully dried reagent grade cadmium perchlorate and selenium solutions were prepared from reagent grade selenic acid. Stock solutions of 100 ppm were prepared in all cases with appropriate dilutions made, as necessary. All other solvents and reagents not previously described were reagent grade or better. Procedure. Standard solutions of each metal ion in aqueous, as well as organic solvents, were prepared as described in the Reagents section. The complexing agents were normally dissolved in the organic solvent for extractions. However, aqueous solutions of sodium diethyldithiocarbamate (DEDC) and cupferron were prepared for extractions as they are quite insoluble in organic solvents. Table I lists the complexing agents and their concentrations used for the extraction of each metal ion. In order to optimize the instrumental parameters, a standard solution was aspirated and the fuel flow rate varied, while holding the other three variables constant, until an optimum value was obtained. A blank was aspirated and the instrument zeroed prior to reading the absorbance a t each setting. In like manner, the other three variables, oxidant gas flow rate, burner height, and lamp current, were optimized. The enhancement of sensitivity for each organometallic-solvent pair was then obtained by dividing the absorbance reading of a standard aqueous solution into the absorbance reading of a standard organic solution of equal metal concentration. A general extraction procedure was followed to determine the optimum conditions for extraction of copper, cadmium, and antimony. 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 ml of the given solvent containing the chelating agent were added, and the mixture was mechanically shaken for 10 min. After physically separating the two layers, the organic phase was analyzed for metal content and the per cent extraction determined. The pH of the aqueous phase was then measured with a pH meter and recorded as the pH of extraction. A percent extraction vs. pH plot was made. Arsenic and selenium were extracted by the above procedure and then back-extracted. The organic phase was placed in a 125-ml separatory funnel and 10 ml of the aqueous stripping solution added. The solutions were mechanically shaken for 10 min and then allowed to stand 5-10 min. The organic layer was discarded. T h e aqueous layer was analyzed for metal content and the percent back extraction and percent overall extraction were determined. After measuring the pH of initial extraction, the aqueous solution was analyzed for metal content and the percent initial extraction determined. A percent extraction vs. pH plot was made. A standard curve was prepared for each metal studied by plotting absorbance vs. concentration data obtained by extracting a series of solutions containing the metal a t various concentrations. T h e concentration ranges for each metal are given in Table 11. The extractions were performed a t the optimum pH of extraction,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14,DECEMBER 1976
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Table I. Complexing Agents and Concentrations Metal Copper
Cadmium
Antimony
Arsenic
Selenium
Complexing agent Cupferron DEDC PAN Dithizone PAN 8-Hydroxyquinoline (oxine) P-Isopropyltropolone 1-Nitroso-2-naphthol Cupferron DEDC APDC Tri-n-octylamine Tri-n-octylamine Trioctyl phosphine oxide APDC DEDC DEDC APDC
Concentration 1.0% 0.01 M 0.1%
0.01% 0.1% 0.1 M 0.1% 0.1%
1.0% 1.0% 0.2%
5.0% 5.0% 0.1 M 0.2% 1.0% 1.0%
0.2%
and absorbance readings made at the optimum instrumental settings. A concentrationof 200 ml to 2 ml was employed. In all cases, both the aqueous and the organic solvents were presaturated with each other prior to cxtraction of the metal ion. This is mandatory in order to eliminate mutual solubility effects.
RESULTS AND DISCUSSION I t is necessary to optimize the instrumental variables for each metal-solvent pair as these variables will influence the analytical sensitivity. The variables considered are fuel flow rate, oxidant gas flow rate, burner height, and lamp current. The optimization procedure is to vary only one factor at a time, while holding the others constant. This procedure is applicable as long as there are no interactions between two or more factors subject to variation. In general, the optimum oxidant flow rate does not vary a great deal from solvent to solvent, and the optimum range is fairly broad. The combustion characteristics of the solvent and the type of flame required for atomization of the element determine the optimum fuel flow rate. The optimum range, in most cases, is narrow. With a burner elevation of 0.0, the beam from the hollow cathode is just touching the burner head. As the burner elevation is decreased, there is a sharp increase in sensitivity until an optimum value is found. Then there is generally an optimum range of 3 to 5 mm before the sensitivity begins to decrease and the noise level to increase. The optimum lamp current for each metal does not vary with the solvent. Therefore, it is necessary to optimize this variable only once for each metal. Table I11 lists the optimum instrumental settings for each metal studied. Only the solvents used for extractions are included. Solvent extraction is the most extensively used method for enhancement of metal ion sensitivity in atomic absorption spectroscopy. Large enhancements may be achieved because higher absorbance readings are generally obtained in organic solvents than in aqueous media, and because of the possibility of concentrating the dissolved metal ion species in the organic phase. A suitable organic solvent for atomic absorption determinations should be combustible, exhibit a reasonable aspiration rate and the combustion products should not absorb radiation from the hollow cathode lamp. Carbon tetrachloride and chloroform are commonly used for many metal extractions but are not suitable for atomic absorption work because of 2062
Table 11. Concentration Ranges for Preparation of Calibration Curves Element
Aqueous phase concn, ppb
Copper Cadmium Antimony Arsenic Selenium
0.10-1.00 10.00-100.00
0.01-0.25 20.00-100.00 2 .OO-14.00
their unfavorable combustion characteristics. Several water-soluble solvents exhibit good enhancement of sensitivity but cannot be used for solvent extraction. I t was found that aliphatic hydrocarbons cause too much absorption of radiation to be suitable for reliable determinations. Aromatic hydrocarbons can be used if low fuel flow rates are maintained. However, they do not exhibit as much enhancement of sensitivity as do the oxygen-containing solvents. In the present study, several organic solvents have been evaluated for enhancement effects on copper, cadmium, antimony, and selenium. Table IV lists the solvents used and the enhancement values obtained for each of the metal ions. Organic solvents depress arsenic sensitivity; therefore, no enhancement studies were carried out for this metal. Those solvents giving the greatest sensitivity enhancement, which are insoluble in water, have been used in solvent extraction studies. Good enhancement of copper sensitivity is obtained using esters, ketones, aromatic hydrocarbons, aldehydes, and ethers. Isopropyl acetate, n-butyl acetate, 2-heptanone, toluene, butyraldehyde, and n-butyl ether were considered the most suitable for copper extraction studies. Alcohols depress the absorbance readings for copper because of their high viscosity. Some of the solvents used in the copper enhancement studies were excluded from further study. Cyclohexanol and n -octyl alcohol were excluded because of their poor aspiration characteristics, nitrobenzene because of its poor flame characteristics, and p-dioxane because of its solubility in water. Standard organic solutions of cadmium could not be prepared with toluene, p-xylene, and n-butyl ether because of the insolubility of the 2,4-pentanedione cadmium derivative, cyclohexane-butyric acid cadmium salt and cadmium perchlorate in these solvents. Therefore, these solvents were not studied for cadmium. Acetate esters, ketones, and aldehydes were found to give the best enhancement of cadmium sensitivity. Alcohols also depress cadmium sensitivity. Butyraldehyde, a-butyl acetate, MIBK, and cyclohexanone were chosen for cadmium extraction studies. Enhancements for antimony are highest with acetate esters and ketones. MIBK, n-butyl acetate, and 2-octanone give high enhancements of antimony sensitivity and are insoluble in water. These solvents were used in the antimony extraction studies. A suitable flame could not be obtained with aromatic hydrocarbons and n-butyl ether solutions of antimony. The atomic absorption determination of arsenic and selenium presents a problem due to their low resonance lines. The most sensitive line for selenium is at 1960.3 8,while the most sensitive line for arsenic is at 1937.0 8,.These wavelengths are in the low ultraviolet region where many gases, including oxygen, absorb over wide bands. Consequently, studies carried out in this region should be made with no air in the light path. Since nitrogen does not absorb a t wavelengths above 1850 %., measurements may be more accurately made in the vacuum ultraviolet region and by sweeping all the air from the flame
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table 111. Optimum Instrumental Conditions
Metal Copper
Cadmium
Antimony
Arsenic Se1enium
Solvent Isopropyl acetate Butyl acetate Butyraldehyde 2-Heptanone n-Butyl ether Toluene Methyl isobutyl ketone n-Butyl acetate Cyclohexanone Butyraldehyde Methyl isobutyl ketone n-Butyl acetate 2-Octanone
Water Water
Acetylene flow rate, l./min
Air flow rate, I./min
Burner height, mm
Lamp current, mA
1.61 2.32 1.94 1.94 2.68 1.61 1.94 1.61 2.32 1.94 1.94 2.32 3.04 Hydrogen flow rate 30.0 31.2
6.94 6.39 6.39 5.23 5.82 6.39 5.23 5.82 6.94 3.49 6.39 6.39 6.94 Nitrogen flow rate 10.8 10.8
3.0
1.0
8.0 8.0 8.0 8.0 8.0 8.0 4.0 4.0 4.0 4.0 15.0 15.0 15.0
5.5 4.0
18.0 12.0
region with nitrogen. Acetylene and coal-gas fuels are impractical for arsenic and selenium determinations because the flame species absorb strongly in the low ultraviolet region. Absorbance readings for aqueous arsenic solutions were found to be 2.43 times larger in a nitrogen shielded air entrained hydrogen flame than in an air-acetylene flame. Selenium sensitivity is enhanced by a factor 2.54 by using a nitrogen shielded hydrogen flame. Organic solvents give poor results with both arsenic and selenium. The absorbance readings are lower than for aqueous solutions and the noise level is considerably larger. Therefore, arsenic and selenium determinations are best made by extracting into an organic solvent such as carbon tetrachloride or chloroform, followed by back extraction into an aqueous solution for preconcentration purposes. The conditions for extraction of a metal complex change with a change in solvent. As a result, i t was necessary to prepare a percent extraction vs. pH curve for each metal for each organic solvent-complexing agent system studied. In order to determine the optimum p H of extraction for each extraction system, several extractions were carried out a t varying p H values. The absorbance values for the organic extracts were measured on the atomic absorption instrument. The concentration of metal in the organic extracts was determined by comparison of absorbance values for the extracts with those of standard solutions of the metal in the appropriate organic solvent. From these data, a percent extraction vs. p H plot was made. The extraction systems for each metal are discussed below. Three chelating agents and six solvents were used to extract copper, for a total of 18 extraction systems. Table V shows the results of these extractions. Equal volumes of aqueous solution and organic solvent were used and the copper concentration was 3 ppm. Sodium diethyldithiocarbamate (DEDC) does not extract copper as efficiently as cupferron or 1-(2-pyridylazo)2-naphthol (PAN). The suggested pH ranges are narrow, the sensitivity of the copper-dithiocarbamate complex is not as great as with the other complexes and reproducibility is poor. Cupferron and PAN exhibit comparable extraction efficiency with all six solvents. Essentially, quantitative extraction is obtained with each system. However, the sensitivity is much better when PAN dissolved in n-butyl ether or isopropyl acetate is used to extract copper. This, in part, may be attributed
1.0
4.0 1.0
1.5 1.0
1.5 1.0 0.0
2.0 2.5 1.0
Table IV. Enhancement Values with Various Solvents Enhancement (AoIA,,) -
Solvent
cu
Cd
Sb
Se
n-Butyl acetate Isopropyl acetate Butyl butyrate Ethyl acetoacetate Methyl benzoate Butyraldehyde 2,4-Pentanedione 2-Heptanone Methyl isobutyl ketone Methyl ethyl ketone Cyclohexanone 2-Octanone n-Butanol n-Hexanol n-Octyl alcohol Cyclohexanol p -Xylene Toluene Nitrobenzene p-Dioxane Propylene carbonate n-Butyl ether Amyl acetate n-Pentanol
1.87 2.00 1.58 1.51 1.09 2.09 1.35 1.77 1.70 1.80 1.29 1.58 1.25 0.45 0.58 0.29 1.80 1.83 0.83 1.38 1.12 2.32
1.32 1.44 0.77 1.05 0.44 1.77 1.16 1.35 1.35 1.27 1.05 1.07 0.81 0.65
1.58
0.21 0.29
...
...
... ... ... ... ... ...
...
1.33 1.13 0.68 0.62 1.52 1.05 1.75 1.53 1.36 1.50 1.18 0.87
...
*..
...
... ... ...
0.72
1.01
... ...
1.00
...
...
1.22
... ... ...
... ... 0.26 0.30
... ... ... ... ... ... ... ... ... ... ...
... ... ...
...
to the enhancement effects of the solvents. However, quantitative extraction of copper is obtained with both cupferron or PAN in n-butyl ether yet the sensitivity of the latter system is much greater. The difference in absorbance readings for the two systems must be due to enhancement of the absorbance reading by the chelating agent PAN. This type of enhancement of sensitivity has not been fully explained but has been reported (26). The best extraction system for atomic absorption determination of copper is the n-butyl ether-PAN system. Quantitative extraction is obtained over a wide pH range and the sensitivity is exceptionally high. Figure 1 shows the percent extraction vs. p H curve for this system.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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Table V. Sensitivity and Extraction Efficiency for Copper
Solvent Butyl acetate n-Butyl ether Butyraldehyde
Toluene
2-Heptanone
Isopropyl acetate
Chelating or complexing agent
Maximum absorbance
Extraction,
Cupferron DEDC PAN Cupferron DEDC PAN Cupferron DEDC PAN Cupferron DEDC PAN Cupferron DEDC PAN Cupferron DEDC PAN
0.478 0.465 0.450 0.515 0.555 0.640 0.500 0.442 0.500 0.480 0.520 0.510 0.500 0.435 0.520 0.620 0.620 0.630
100 100 98
100-
Suggested pH range
%
3.50-8.60 2.80-5.00 3.20-5.50 2.65-9.60 1.00-7.00 6.00-9.90 3.80-5.20 1.00-4.10 3.00-4.20 4.30-9.55 6.50-7.50 4.70-9.20 2.60-5.00 1.00-2.50 3.20-9.00 2.70-4.50 3.20-5.07 5.55-6.57
100
98 100 98 90 98 100 98 100 100 80 97 100 98 93
1001
80-
z
:
J
60-
4
c a
8 40401.
li1
_/I,
,
,
,
,
,
,
,
70
8 0
90
100
0 I0
2 0
30
4 0
50 60 PH
40
50
60
70
80
90
I00
110
PH
Figure 1. Percent extraction vs. pH curve for copper using an n-butyl ether-PAN system
Figure 2. Percent extraction vs. pH curve for cadmium using an MIBK-dithizone system
Extraction data for 1ppm cadmium solution were obtained as was done for copper. The complexing agents PAN, oxine, dithizone, /3-isopropyltropolone (P-IPT), and l-nitroso-2naphthol were studied with the solvents MIBK, n-butyl acetate, and cyclohexanone. Equal volume ratios were used for all extractions. The n-butyl acetate-oxine extraction was carried out at an oxine concentration of 0.01 M, and 93% extraction obtained. At this concentration of oxine the aspirator on the atomic absorption instrument slowly clogs, causing inconsistent absorbance readings. If the oxine concentration is lowered to the point that a good aspiration rate is maintained, less than 10% extraction of cadmium is obtained. The MIBK-oxine extraction was carried out using a 1% oxine solution and only 8%extraction resulted. This indicates a strong dependence on the oxine concentration. For this reason a cyclohexanone-oxine extraction was not tried. The extractability of cadmium complexes is poor in cyclohexanone and cadmium sensitivity is not as great as in other solvents. Only 80% to 90% extraction of cadmium is obtained. Cadmium extractions were attempted with PAN and dithizone in butyraldehyde. Each attempt failed because of oxidation of enough butyraldehyde to butyric acid to lower the
p H to a value of approximately 5.0. Cadmium extractions are generally more efficient at a p H of 6.0 or greater. (3-IPT and 1-nitroso-2-naphthol are effective complexing agents for cadmium extraction, but sensitivity is much better with dithizone. The absorbance readings are highest with a MIBK-dithizone extraction system. This system was used for preparation of the cadmium standard curve. A percent extraction vs. p H curve is shown in Figure 2 for this extraction system. Antimony extraction data were obtained on several systems. The complexing agents cupferron, DEDC, APDC, and trioctylamine (TOA) were studied with the solvents MIBK, n-butyl acetate, and 2-octanone. The extractions were carried out using a 10 ppm antimony solution and equal volume ratios. Antimony is quantitatively extracted as an ion-association complex from 0.5 M hydrochloric acid by TOA. The amine is protonated by the acid and this positively charged compound forms an ion-association complex with anions such as SbCl&, which are soluble in the organic solvent. The percent extraction was shown to be dependent upon the chloride ion concentration. The chloride ion concentration was varied, by the addition of sodium chloride, from 0.5 M to 6.0 M. Quantitative extraction is obtained with a chloride ion concentration of 3.0
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
l o Table VI. Detectability Limits by Solvent ExtractionAtomic Absorption Based on Aqueous Phase Concentration
90
Metal Copper Cadmium Antimony Arsenica I
0
1
2
3
4
5
6
7
Selenium”
8
Extraction system
Absorbance
Metal, PPb
PAN-n-butyl ether Dithizone-methyl isobutyl ketone APDC-methyl isobutyl ketone DEDC-chloroform APDC-chloroform APDC-chloroform
0.010
0.01 0.10
0.008
0.020
10.
0.011
20. 20. 2.
0.010
0.005
PH
Figure 3. Percent extraction vs. pH curve for selenium using an APDC-chloroform system
M or greater. An attempt was made to extract antimony as an ion-association complex of S ~ ( S C N ) Band ~ - TOA. The complex is only partially soluble in organic solvents and poor extraction is obtained. APDC can be used to extract antimony from hydrochloric acid. The extraction is quantitative with an acid concentration of 0.001 M to 1.00 M. MIBK or n-butyl acetate may be used as solvents, but MIBK is more sensitive for atomic absorption determinations. This system is as sensitive as the MIBK-TOA system and the extraction procedure less time consuming. For this reason it was used to prepare the antimony standard curve. The atomic absorption sensitivity of arsenic and selenium is best in aqueous solutions. Enhancement of sensitivity is obtained by concentrating the elements by solvent extraction, followed by back-extraction into an aqueous system. Attempts were made to extract arsenic as an ion-association complex with TOA. Extractions were carried out a t various chloride ion concentrations and acid concentrations. All of these extractions failed. Extractions with trioctylphosphine oxide (TOPO) from 5.0-7.0 M HC1 indicated 55-60% extraction of arsenic had been obtained. However, extraction of arsenic from 7.0 M HCl solutions containing no TOPO gave 60% extraction, indicating the extracted species in the TOPO extraction was arsenic chloride and not an arsenic complex of TOPO. Extraction of arsenic by the method of Menis and Rains (6) with DEDC followed by back-extraction with copper gives quantitative recovery of the arsenic. The same results are obtained if APDC is substituted for DEDC. Both systems were used to prepare standard curves for arsenic. Selenium is quantitatively extracted by DEDC in carbon tetrachloride. The reagent and many metal-diethyldithiocarbamate complexes are unstable in even weakly acidic media. However, back-extraction of the selenium diethyldithiocarbamate complex with 7 M hydrochloric acid yields only 18%recovery of the selenium, indicating a very stable complex. The order of stability of the metal-diethyldithiocarbamates has been established by exchange reactions to be mercury(I1) > palladium(I1) > silver(1) > copper(I1) > thallium(II1) > nickel(I1) > bismuth(1) > lead(I1) > cadmium(I1) > thallium(1) > zinc(I1) > indium(II1) > antimony(II1) > iron(II1) > tellurium(1V) > manganese(I1) (27). Back-extraction of selenium with copper failed, indicating the selenium complex is more stable than the copper complex. Mercury(I1) and silver(1) will displace selenium. However, high concentrations of mercury and silver are necessary and these ions cause some interference in the atomic absorption determination of selenium. I t was found that the cyanide ion will quantitatively back-extract selenium. It is necessary to use a concentration of CN- less than 50 ppm as a marked depression of the selenium absorbance signal is observed a t concentrations greater
a Back-extraction was performed and measurement was made in aqueous medium.
than 50 ppm CN-. Several examples of this type of interference have been reported by Gilbert (28). Selenium is also extracted by APDC in chloroform. Quantitative extraction is obtained over a p H range of 1.5 to 5.30. Figure 3 shows a percent extraction vs. p H curve for this system. Quantitative back extraction is achieved using a 50 ppm cyanide ion solution. Attempts a t back extraction using iron, copper, nickel, antimony, lead, and EDTA all failed. The solvent-complexing agent system giving the highest absorbance was used to prepare a standard curve for each element in the low ppb range in order to determine linearity and detection limit. Copper, cadmium, and antimony standard curves were prepared by extracting 200 ml of aqueous solution with 2 ml of organic solvent and aspirating the organic phase. Arsenic and selenium standard curves were prepared by extracting 200 ml of aqueous solution with 10 ml of organic solvent. The organic phase was then back-extracted with 2 ml of the appropriate aqueous solution and the aqueous solution aspirated. In all cases, the aqueous and organic phases were presaturated with each other. Table VI lists the detectability limit of each metal by this method. The detectability limit is defined as the lowest concentration which will give an absorbance reading twice the noise level. Standard curves in the low ppb range were prepared for all the systems listed in Table VI. Good linearity and reproducibility was obtained in all cases.
ACKNOWLEDGMENT The authors are grateful to Vincent B. Stein, Kenneth R. Harrison, and David A. Darnall for performing some of the experimental work included in this manuscript.
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