inner filter effect make a separation necessary. Salicylamide is not too soluble in aqueous acid so that some of it is eliminated in the dissolution step. The remainder is removed by an ether extraction. Sominex tablets were analyzed in triplicate by the procedure outlined in the Experimental section. Samples spiked with standard methapyrilene hydrochloride solu-
tion were analyzed to determine the recovery. The mean of the results (Table VI) for methapyrilene was 0.9 mg higher than the stated amount of 25 mg on the label, a reasonable deviation. Received for review August 25, 1972. Accepted February 21,1973.
Correlation of Enhancement of Atomic Absorption Sensitivity for Selected Metal Ions with Physical Properties of Organic Solvents Alice J. Lemonds' and B. E. McClellan2 Department
of C h e m i s t r y , M u r r a y S t a t e U n i v e r s i t y , M u r r a y , K y . 42071
Various alcohols, ketones, esters, and other organic compounds, as solvents for Ag, Cd, Co, Ni, and Zn ions, were studied to determine the enhancement values for each metal-solvent system and to correlate the enhancement values with the physical properties of the solvents. Optimum instrumental conditions were determined for each metal-solvent system employing two different burner-aspirator systems. Absorbance values for the metal-organic solvent systems were measured and compared with the absorbance values for the aqueous system of'the same concentration in order to calculate an enhancement value. Plots of enhancement vs. log viscosity and enhancement YS. log boiling point for each ion resulted in lines with negative slopes. Various plots involving density and surface tension showed little or no dependence of enhancement on either of the constants. However, a linear relationship existed between log (viscosity X boiling point) and enhancement.
Since the introduction of atomic-absorption spectrometry in 1955, improved detection limits have been sought for this already sensitive analytical technique. Many workers have observed that the addition of water-miscible organic solvents leads to increased sensitivity (1-6). Gains in sensitivity of up to sevenfold have been reported with water-immiscible organic solvents ( I ) . Immiscible organic solvents can serve not only as enhancing agents for a given metal ion, but they can serve to separate and/or concentrate the metal ions. Many papers have appeared describing extraction procedures for separation and/or concentration of metal ions by solvent extraction prior to determination by atomic-absorption spectrometry. Several of Present address, U n i o n C a r b i d e Corp., N u c l e a r Division, P a ducah, Ky. 42001. 2 A u t h o r t o w h o m inquiries should be addressed. (1) J. E. Allan, Spectrochim. Acta, 17, 467 (1961). (2) J. W. Robinson, Ana/. Chim. Acta, 23, 479 (1960). (3) W. T. Eiwell and J. A. F. Gidley, "Atomic Absorption Spectrophotometry," Macmiilan, New York, N . Y . , 1962, pp 26-27. (4) R . Lockyer. J. E. Scott, and S. Slade, Nature (London). 189, 830 (1961 ) . ( 5 ) I . Atsuya, Sci. Rep. Inst., Tohoku Univ.. Ser. A , 18, 65 (1966). (6) I . Atsuya, J . Chem. SOC. Jap., Pure Chem. Sect., 86, 1145 (1965); Anal. Abstr.. 14, 4434 (1967).
these describe the extraction of Ag, Cd, Ni, Co, and Zn ( I , 7-13). The increased sensitivity using organic solvents partially results from an increase in the amount of solution reaching the flame. Physical properties of the solvent such as viscosity, density, and droplet size have been reported as factors which may contribute to enhancement. Allan (I) concluded that increased sensitivity is due primarily to an increase in the amount of solution reaching the flame and suggested that this increase might be the result of an increased uptake rate, the formation of a finer aerosol, and the more rapid evaporation of solvent to produce smaller droplets. Allan also suggested that viscosity, surface tension, and vapor pressure might be controlling factors in the physical processes of atomization. Other studies ( 1 1 , 24-1 7 ) have indicated similar results. The only report of an attempt to correlate the enhancement observed for organic solvents with their physical properties came with the work of Feldman, Christian, and Bosshart ( 1 8 ) . In the study of manganese with various water-miscible solvents, they reported a linear relationship between absorbance and the product of viscosity and density. The purpose of the present work was to determine enhancement factors for a number of metal-solvent systems and to determine if any correlation existed between the enhancement factors obtained and the physical properties (7) J. B. Willis. Anal. C h e m . , 34, 614 (1962). ( 8 ) J. E. Allan, Analyst (London). 86, 530 (1961). (9) T. T. Chao, M. J. Fishman. and J. W. Bali, Anal. Chim. Acta, 47, 189 (1969). (10) B. Fleet, K. V. Liberty, and T. S. West, Anaiyst (London), 93, 701 (1968). (11) T. Takeuchi, M. Suzuki, and M . Yanagisawa, Ana/. Chim. Acta, 36, 258 (1966) (12) I . Atsuya, J. Chem. SOC.Jap.. Pure Chem. Sect.. 88, 179 (1967); Anal. Abstr., 15, 2513 (1968). (13) R. E. Barringer, H. G . King, and A. D. Condrey, U.S. At. Energy Comm.. 23, Y 1661, (1969); Chem. Abstr.. 71,357249 (1969). (14) T. P . Taskaeva and E. E. Vainshtein, I z v . Sib. Otd. Akad Nauk SSSR. Ser. Khim. Nauk, 4, 110 (1967); Chem. Abstr., 69, 15930b (1968). (15) T. Takada and K . Nakano. Nippon Kagaku Zasshf. 90, 487 (1969); Chem. Abstr.. 71, 356892 (1969) (16) K . Nakano and T. Takada, Nippon Kagaku Zasshi. 88, 575 (1967); Chem. Abstr.. 67, 50029v (1967) (17) D. W. Kohlenberger, At. Absorption Newslett.. 8 ( 5 ) , 108 (1969). (18) F. J. Feldman, R. E. Bosshart, and G. D. Christian, Anal. Chem., 39, 1175 (1967) A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
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of the organic solvent. The results of this study might enable the analyst to select the organic solvent giving greatest enhancement for the ions studied by simply referring to the appropriate physical constants of the solvents.
EXPERIMENTAL Apparatus. All absorbance measurements were determined with either a Beckman D U spectrophotometer adapted with a Beckman atomic absorption accessory or a Jarrell-Ash atomic a b sorption spectrophotometer, Model 82-500, adapted with a Perkin-Elmer concentration readout unit, DCR-2B. T h e Jarrell-Ash system was further modified with a Varian-Techtron nebulizerburner system and Brooks “Sho-Rate 250” Model 1357 flow meters. A Honeywell Electronik 194 recorder was also used in conjunction with t h e Jarrell-Ash system. T h e appropriate hollowcathode discharge tube and a premixed air-acetylene flame were used in connection with each system for all measurements. To remove interfering ions from distilled water, two Illco-Way (Illinois Water Treatment Company) ion-exchange columns were used in series. An Ostwald viscometer was used in determining viscosities. Reagents. Pure metals (99.99+%) obtained from Research Organic/Inorganic Chemical Co., S u n Valley, Calif., were used to prepare aqueous solutions for each ion. T h e perchlorate salts of Ni, Co, Ag, and C d were used as t h e metal ion sources in preparation of the organic-solvent solutions. Organic solutions of zinc were prepared from t h e zinc salt of cyclohexanebutyric acid. All other reagents were Analytical Reagent Grade or better. Procedure. Standard aqueous solutions of Ag, Cd, Ni, Co, and Zn were prepared by dissolving a n accurately weighed quantity of the pure metal in a minimum amount of nitric or perchloric acid and diluting t o a given volume in a volumetric flask with deionized water so as t o yield a metal ion concentration of 1000 ppm. Organic solutions of each metal were prepared by dissolving a weighed quantity of the perchlorate salt of Ag, Cd, Co, and Ni in the appropriate organic solvent and diluting to a given volume with the solvent. Organic solutions of zinc were prepared from the zinc salt of cyclohexanebutyric acid in t h e same manner as the other organic solutions. Concentrations for the standard organic solutions were 100 ppm in t h e metal ion. Appropriate aliquots were taken from these standards and diluted to the desired concentration in a volumetric flask. The perchlorate salts were standardized against the pure metals by atomic-absorption spectrometry. T h e resonance lines for each metal utilized during the instrument operation were as follows: cadmium, 2288.0 A; cobalt, 2407.3 A; nickel, 2320.0 A: silver, 3280.7 A: and zinc, 2138.6 A. The conditions which gave the maximum absorbance for each metal-solvent system were determined. Prior t o optimization procedures, the wavelength was peaked a t t h e resonance line t o achieve a maximum energy output. Gauge settings used in conjunction with the Varian-Techtron system were 40 and 15 psi for air and acetylene, respectively. T h e corresponding solvent for each metal-solvent system was used as a blank. Optimum conditions of air, fuel, burner height, and l a m p current for t h e organic solutions were determined. T o determine the optimum conditions for t h e aqueous solutions, the optimum air setting as determined for the organic system was used and the optimum fuel, burner height, and l a m p current settings were obtained as previously described. The absorbance values were measured a t optimum conditions for each metal-solvent system. Three values were obtained and averaged a t different time intervals. Viscosity coefficients for t h e solvents were determined by applying the Poiseuille equation with water as a reference liquid. T h e surface tension of each organic solvent was obtained by the capillary method where the radius of the capillary was determined by knowing t h e volume occupied by a given weight of mercury.
RESULTS AND DISCUSSION With emphasis placed on the use of solvent extraction with atomic-absorption spectrometry to increase sensitivity in metal-ion detection, the careful selection of the solvent is importarit. The solvent must have a high distribution coefficient for the metal complex and be relatively insoluble in water. At the same time, it must possess desirable flame characteristics and enhancing properties if it is to be a successful solvent for atomic absorption. Solvents such as aromatic hydrocarbons and chlorinated hydrocar1456
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
bons, while excellent extracting solvents, generally prove unsuitable for atomic absorption because of the production of soot and toxic vapors. Oxygen-containing compounds generally produce the most desirable flames. As a result, alcohols, ketones, and esters were given prime consideration. The choice as to specific ketones, alcohols, and esters was based primarily on their future use in extractions, keeping in mind that a wide range of density, viscosity, boiling point, and surface tension was desirable for the correlation study. Solvents for the Varian-Techtron were chosen from the ones studied with the Beckman system on the basis of their enhancing characteristics, maintaining representatives from each class of compounds and a range for the physical properties. Since a maximum absorbance value for each metal-solvent system was the ultimate goal, it was necessary to optimize instrumental parameters. Optimum fuel-air ratios arise from the necessity to establish a reducing medium for maintaining a large population of free atoms. Some metals require a fuel-rich flame, whereas others only require a stoichiometric ratio. Flames are not homogeneous and the chemical environment changes from area to area. The combustion pattern varies such that different solvents liberate metal atoms at different rates in different parts of the flame, making it a necessity to locate the position a t which the concentration is the greatest. The desirable lamp current is an amperage that will provide an adequate intensity of emission from the source and suitable analytical sensitivity, yet sufficient stability to yield the required accuracy and precision. For the Beckman study, a 1-ppm concentration of zinc in each solvent and a 10-ppm concentration of nickel in each solvent were utilized in the determination of optimum conditions for each ion. Fuel, burner height, and lamp current u’ere the instrumental variables under consideration. It was generally observed that lower fuel settings were required for the more volatile solvents, and the greatest concentration of atoms was located relatively high in the flame. The optimum conditions for a given solvent usually change with a change in the metal ion. A greater variation in lamp current was observed for zinc than for nickel. The lamp current setting which gave optimum absorbance readings for zinc and nickel ranged between 4 and 7 mA. The burner height was usually a t about 2.4 for optimum readings, which corresponds to a position in which the light beam passes just above the burner head. A setting of 16 psi (7 l./min) was maintained for the air pressure. Fuel pressure varied from 0.33 (0.22 l./min) to 4.5 psi (3 l./min) to obtain maximum absorbance readings. The lower fuel ( C Z H ~flow ) rates were necessarv for ketones and esters. Optimum conditions for each metal-solvent system were once again determined employing the Varian-Techtron burner system. Concentrations of 3, 5 , 1, 0.6, and 0.5 ppm were used for Co, Ni, Ag, Zn, and Cd, respectively, in determining optimum conditions for each solvent. Air, fuel, burner height, and lamp current were the parameters investigated. Optimum air and fuel-flow rates varied from metal to metal and solvent to solvent. Moderately lean flames produced the best results for cobalt and nickel, whereas very lean flames for silver, zinc, and cadmium yielded the highest absorbance values. The greatest population of atoms generally occurred within 1 or 2 mm from the top of the burner head for all metal-solvent systems. The best results were obtained with the operating current of the hollow-cathode tube at low amperage (4-6 mA). Analysis of the data indicated the necessity for complete specification of optimum conditions for the metal as
Table I . Physical Constants for Organic Solvents and Enhancement Values for Zn and Ni Enhancement Solvent
Boiling point, " C
Density, g/ml
Surface tension, dyn cm -
Acetone Acetylacetone Cyclohexanone 2-Heptanone Methyl ethyl k e t o n e Methyl isobutyl k e t o n e 2-Octanone 1-Butanol 2,2-Dimethyl-l -pentanol 1-Hexanol 1-Pentanol n-Butyl acetate Ethyl acetoacetate Isopropyl acetate Carbon tetrachloride Cyclohexane p-Dioxane Water
56.2 139 155.6 151 79.6 11 6.85 173 11 7.5 153 158 137.3 126.5 180.4 93 76.75 81 101 100
0.7908 0.9721 0.9978 0.81 11 0.8054 0.801 0 0.8185 0.8098 0.8260 0.81 3 6 0.81 1 0 0.8324 1.0250 0.8732 1.5942 0.7791 1.0336 0.9962
28.0 31 .O 34.8 29.4 29.6 26.9 28.3 28.3 26.6 30.5 28.3 34.2 39.0 31.6 25.5 27.4 32.1 72.0
well as the solvent and burner used, if a maximum absorbance value is to be obtained. With organic solvents, an increase in the absorbance value for the metal is generally observed in comparison to the absorbance value for an aqueous system of the same concentration. On this basis, the enhancement value was defined as the ratio of the absorbance value for a metalorganic solvent system to the absorbance value for the aqueous system a t the same concentration, both measured a t optimum conditions. The enhancement value for a given metal changed with a change in the solvent. This effect is due to a change in the physical properties of the solvent. The flame temperature can be lowered by as much as 400 to 500 "C on aspirating an aqueous solution into an air-CZHz flame with some burners. For elements whose salts are difficult to dissociate, this can lead to a reduction in the ground-state atom concentration, thus reducing the sensitivity. Flammable organic solvents, on the other hand, do not change the flame temperature much, and increased detection limits would be expected. Likewise, for a given solvent, the enhancement values varied with a change in the metal. Hence, the enhancement must also be dependent on the metal and the ease with which its salts are dissociated. Also for the nickel and zinc systems, the enhancement was generally greater for the Beckman burner system than for the Varian-Techtron burner. This indicates that the burner and atomizer construction contributes to the enhancement effect. Enhancement generally increased with increased complexity of the spray chambers. The Beckman burner spray chamber, which has a number of baffles, showed the highest degree of enhancement. The Varian-Techtron spray chamber, which is just a conical chamber, showed intermediate enhancement, while the Jarrell-Ash "Tri-Flame" burner-nebulizer system sometimes gave lower readings with organic solvents than with aqueous solutions. The spray chamber on this burner is very simple and of short length. The probable explanation of these observations is that the burner-nebulizer systems operate with varying degrees of efficiency. Efficiency is defined here as the ratio of the amount of solution actually reaching the flame to the total volume aspirated. Burners with simple spray chambers are more efficient and tend to give higher readings with aqueous solutions, thus reducing the enhancement which is observed using organic solvents. Since the Varian-Techtron burner assembly showed intermediate enhancement values and is more
Viscosity, CP
Zn
Ni
0.31 0.67 1.95 0.69 0.39 0.52 0.56 2.24 5.78 3.86 3.00 0.66 1.43 0.47 0.91 0.76 1.1 1 0.89
3.0 8.2 2.4 1.7 6.6 4.6
... 4.2 2.8 3.2
...
...
6.4 1.9
1.6
...
...
1.3
...
1.6 2.5 2.6 1.1 6.2 6.7 4.4 2.2 1 .o
2.4 4.4 1.6 7.7
... ...
... 1 .o
Table II. Enhancement of Metals in Various Solvents Enhancement Solvent Acetylacetone Cyclohexanone 2-Heptanone Methyl ethyl ketone Methyl isobutyl ketone 2-Octanone 1-Butanol 1-Hexanol 1-Pentanol n-Butyl acetate Isopropyl acetate Water
Viscos- Cadity, CP mium 0.67 1.95 0.69 0.39 0.52 0.56 2.24 3.86 3.00 0.66 0.47 0.89
Cobalt Nickel
Silver
Zinc
...
..,
1.5
...
1.4 2.5
3.1
5.0
4.1
1.6
1.8
2.9
4.1
4.2
1.8
0.54
1.8 1.3 1.1 1.1 1.3 2.1 1.0
2.8 1.3 0.83 0.99 2.9 5.4 1.0
2.6 1.8 1.2 1.5 3.4 4.9 1.0
1.4 1.1 0.68 1.1 1.1 1.4 1.0
0.41 0.84
2.6 2.3
...
1.0
...
1.4
0.57
...
...
.,, 0.31 0.92 0.72 1.0
representative of most commercial burner types, it was chosen for most of the detailed study of the correlation of physical properties of organic solvents with enhancement. In an attempt to establish some relationship between the enhancement values obtained for the metal-solvent systems and the physical properties of the solvents, various plots of enhancement against the physical properties were obtained. Physical properties studied were boiling point, density, surface tension, and viscosity. Table I summarizes the physical properties of the solvents and the enhancement values for the zinc and nickel systems as obtained with the Beckman burner. The plot of enhancement us. log density for the zincsolvent systems as obtained with the Beckman burner showed no dependence of enhancement on density. The results obtained from a similar plot for the nickel systems were the same. Enhancement values acquired with the Varian-Techtron burner system for cobalt, cadmium, nickel, silver, and zinc generally varied between 3 and 5 depending on the solvent employed. The enhancement values along with the viscosities for several solvents are shown in Table 11. Graphs of enhancement vs. log density for each ion were obtained, with the same results as previously observed for the Beckman laminar flow-burner system. ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
1457
I
Table Ill. Aspiration Rate and Efficiency for Organic Solvents Solvent
Rate. rnl/rnin
Efficiency, %
Acetone Acetylacetone Cyclohexanone 2-Heptanone Methyl ethyl ketone Methyl isobutyl ketone 2-Octanone 1-Butanol 2,2-Dimethyl-1-pentanol 1-Hexanol 1-Pentanol n-Butyl acetate Ethyl acetoacetate Isopropyl acetate Carbon tetrachloride Cyclohexane p-Dioxane Water
5.4 2.4 1.6
100 56 56 42 95 69
2.5 3.7
5.1 3.5
42
1.4 0.77
61
0.92
39 46 43
1.3
3.2 1.9
...
4.1 2.6 2.6 2.5
100 100 97 82 23
3.1
VISCOSITY
Figure 3. Enhancement vs. log viscosity for cobalt ( 0 )Ketones, ( U ) esters, ( A )alcohols, ( 0 )water
0
I
01
I
I
I
I
I I I I I
I
2
3
I
4
1
1
1
1
1
5
1
1
6
V I S COSlTY
Figure 2. Aspiration time vs. viscosity for organic solvents ( 0 )Ketones, ( U ) esters, ( A )alcohols, (0)water, ( V )others
The next physical property studied was the surface tension. I t was expected that the enhancement would increase with a decrease in surface tension since this property influences the droplet size. For a n effective transfer of the solvent from the spray chamber to the flame, it is necessary for the solvent reaching the burner t o be in the form of small droplets. As the surface tension increases, 1458
* ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, JULY 1973
the cohesive forces between the molecules are reduced, and smaller droplets result. Enhancement values were plotted against log surface tension for each ion. Graphs of the results for zinc (Beckman burner system) and cobalt (Varian-Techtron burner system) were prepared. Contrary to the anticipated results, the enhancement is roughly independent of surface tension when only the organic solvents are considered. However, the sharp decrease in surface tension of the solvent in comparison with the surface tension of water can explain the production of a finer aerosol when organic solvents are used, resulting in more solution reaching the flame and a n increase in sensitivity. The correlation study was also extended to include viscosity. For the nickel systems (Figure l ) , a decrease in viscosity results in a n increase in absorbance using the Beckman burner. As the viscosity of the solvent decreases, more of the solution reaches the flame resulting in a higher population of atoms in the flame. Aspiration rates as determined for the solvent by measuring the time required to aspirate 5 ml of the solution using the Beckman burner are listed in Table 111. Figure 2 shows the dependence of aspiration time on viscosity. As the viscosity decreases, the time required to aspirate a given volume decreases, with the volume of solvent reaching the flame per unit of time becoming greater. Consequently, the metal ion concentration present in the flame increases, and the greatest enhancement value is observed with solvents possessing a low-aspiration time. A plot of aspiration time us. log density indicated that the aspiration time is not dependent on the density. Hence, density would not be expected to influence the enhancement values, and this is in agreement with the experimental data. Plots of enhancement us. log viscosity for each ion were prepared using the Varian-Techtron burner system. Generally, as the viscosity of the solvent decreased, the enhancement for a given metal increased. An example of the type of plot obtained is shown in Figure 3 for cobalt. Similar results were obtained for the other ions. Boiling point was the last of the four physical constants studied. No definite correlation was established between enhancement and boiling point; however, graphs of enhancement us. log boiling point, for all ions except silver, suggested that the enhancement value for a given ion in-
creased with a decrease in the boiling point of the solvent. The best relationship (Figure 4) was obtained for nickel using the Varian-Techtron burner. In the sequence of events occurring in the flame prior to absorption, the preliminary step is the vaporization of the solvent to leave the free-metal salt. As a result, it is easier for the salt to be stripped of the solvent if the solvent has a low-boiling point. For the Beckman laminar flow burner, efficiency values for the organic solvents (Table 111) were also determined by measuring the number of milliliters consumed (volume of solution reaching the flame) on aspiration of a given volume of the solvent. Efficiency was then defined as the ratio of the milliliters consumed to the total number of milliliters aspirated multiplied by 100. This measurement was made by aspirating a large measured volume of the solvent into a carefully dried burner-aspirator system and accurately measuring the volume of the solvent running off as waste. Figure 5 shows an increase in enhancement with an increase in effxiency. As the efficiency of the solvent increases, the volume reaching the flame becomes greater, resulting in an increase in the absorbance. Since the absorbance is proportional to the number of atoms in the flame per unit volume in the light path, the enhancement values become greater. Combinations of the physical constants such as the product of viscosity and density and the'product of viscosity and surface tension were plotted against enhancement values obtained with the Beckman burner for nickel and zinc as to the possibility of some correlation existing. However, the resulting graphs for both ions were very similar to the graphs obtained for enhancement cs log viscosity. Close evaluation indicated that the density and surface tension merely represented constants, since enhancement showed little dependence on either density or surface tension. Another combination of variables investigated in an effort t o obtain a simple relationship between the enhancement values and the physical properties of the solvent was that of viscosity and boiling point. Plots of log (boiling point times viscosity) L'S enhancement were made for the values obtained with the Varian-Techtron burner system for cadmium, cobalt, nickel, silver, and zinc. A representative plot is shown in Figure 6. All of these graphs better approximated a straight line than any of the other graphs previously considered. The line in each plot represents a least-squares fit of the data. The experimental results were in fair agreement with the calculated values of enhancement. In the plot of log (boiling point times viscosity), it was assumed that the viscosity and boiling point contributed equally in all instances. Should they not contribute equally, it was thought that a better straight line might be obtained if the physical constants were weighted. A leastsquare fit was applied t o the following equation
E = log (viscosity"
X
boiling pointP)
t
BOlLlNO POINT
Figure 4. Enhancement vs. log boiling point for nickel ( 0 )Ketones,
( m ) esters, ( A )alcohols, ( 0 )water
EFFICIENCY
Figure 5. Enhancement vs. log efficiency for nickel
( 0 )Ketones,
( m ) esters, ( A )alcohols, ( 0 )water
I
(1)
where cy and 3 represent weighting coefficients for viscosity and boiling point, in an attempt to calculate cy and /3. Attempts to do this failed for two possible reasons, either 01 and 3 were not being calculated properly or the equation was a very poor fit, because the deviation between the experimental value of enhancement and the calculated value of enhancement was approximately equal to the experimental value. Correlation studies of enhancement values and physical properties of the solvent indicated that the enhancement effect is very complex and cannot be attributed t o one factor alone. Trends suggested that a decrease in viscosity
A
01 10.0
I
100 0
I
1ooo.o
BOILING POINT X VISCOSITY
Figure 6. Enhancement vs. log (boiling point X viscosity) for cobalt ( 0 )Ketones,
( m ) esters, ( A )alcohols, ( 0 )water
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8 , J U L Y 1973
1459
or boiling point increased the enhancement values. However, enhancement showed little or no dependence on density or surface tension. This is not to mean that densit y and surface tension do not contribute to the enhancement effect, but that viscosity and boiling point are the more important physical properties to be used as guidelines in selecting solvents for use in atomic-absorption spectrometry. The mean droplet diameter in micrometers, D, obtained when a solution is atomized in a gas jet flowing a t a rate of C i s expressed by the empirical Equation (29)
585
D=-
fi
-+597
uo
looow
where c' = gas velocity in m/sec, u = surface tension in dyn/cm, d = density in g/cm3, 7 = coefficient of viscosity in poises, w = solution consumption rate, V = gas consumption rate. Calculations of D from this equation show that the droplet diameter is smaller for organic solvents than for water. The difference in size, however, is not very great. For example, using a jet flow of 350 m/sec and a value of w / V of 0,00033, the D value for water is 19 pm while that for MIBK is 13.9 pm. Therefore, droplet size is probably not the most important factor affecting enhancement. The more important effect of organic solvents is the (19) S. Nukiyarna and Y . Tanasawa, Trans. SOC.Mech. €ng. Jap., 5, 68 (1939).
increase in consumption rate. According to the Poiseuille equation
ve = A p r 4
(3)
where V, = volume per unit time flowing through a capillary, -Ip = differential pressure, r = capillary radius, 1 = capillary length, 7 = solution viscosity. Pungor and Mahr (20) showed that the only property of the aspirated solution which affects V, is the viscosity. Therefore, the experimental data obtained in this work tend to confirm the theoretical work indicating the viscosity to be the more important physical property of organic solvents determining the solution consumption rate. The smaller droplet size formed when using organic solvents is of lesser importance because, although there is a significant difference in the droplet size between organic solvents and water, the difference in size obtained by varying the organic solvent is very small. Therefore, surface tension was found to have little effect on enhancement. Received for review July 27, 1972. Accepted February 14, 1973. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society. for support of this work.
(20) E. Pungor and M. Mahr, Ta/anta. 10,537 (1963)
Interferences in Nickel Determinations by Atomic Absorption Spectrometry L. L. Sundberg D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of C a l i f o r n i a .
Los A n g e l e s .
C a l i f . 90024
Zinc, F e ( l l l ) , Cu, Co, M n ( l l ) , and Cr(lll) interfere with Ni determinations by atomic absorption spectrometry in oxidizing and reducing air-acetylene flames. The interferences produced by the latter five elements are of remarkably similar nature. The interferences are greatly influenced by observation height, and careful adjustment of this parameter can effectively eliminate them. In a reducing flame, the Ni absorbance can be enhanced or depressed by the same concomitant, and the direction of the interferences is further dependent upon the concentration of interfering species.
.
Atomic absorption spectrometry (AAS) is a highly versatile method of trace analysis with sensitivies a t the ppm level for some 70 different elements. During the early years, investigators stressed the virtual absence of interferences in atomic absorption as compared to emission techniques and other methods of elemental analysis. However, further studies indicated that interferences do in fact exist, and much research of late has been devoted to the study of these interferences and their mechanisms. Nickel is very accessible to AAS determinations and is routinely analyzed by this method in many laboratories. Interferences have been observed in several matrices including silver alloys ( I ) , steels (2, 3), ores (4, 5 ) , and cop1460
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 8,
JULY 1973
per-based materials (6, 7). Enhancement, depression, and absence of interference by transition metals are reported and some of these findings conflict with one another. This is partly explained by differences in solution matrices and by instrumental modes of operation where the critical parameters, flame characterization, and burner elevation are not adequately allowed for. Of the conventional flames that have been tested in AAS, the one of greatest utility is air-acetylene. If air pressure, fuel pressure, and air flow are held constant, and fuel flow is varied, then the flame can be grossly classified as being oxidizing (fuel-lean), stoichiometric, or reducing (fuel-rich). One can usually adjust the fuel/oxidant flow ratio to an optimum value which affords one a sensitivity maximum in his determination. However, for any given flow ratio, the flame is not uniform over its entire vertical J, M. Viebrock, Ana/. Lett.. 3, 373 (1970). K. Kinson and C. 6 .Belcher. Ana/. Chim. Acta. 30, 64 (1964) S. Sprague and W. Slavin. "Developments in Applied Spectroscopy," E. N. Davis, Ed., Vol. 4, Plenum Press, New York. N.Y.. 1965, p 433. K. Kinson, J. E. Dickeson. and C. B. Belcher, Ana/ Chim. A c t a . 41, 107 (1968). V. Endo, T. Hata, and V. Nakahara. Bunseki Kagaku, 1 8 , 833 (1969). K. Itsuki, H. Kornuro, and T. Nagasawa, Bunseki Kagaku. 1 9 , 1282 (1970). 6 .Gandrud and J. C. Marshall, Appi. Spectiosc.. 2 4 , 367 (1970).
