Anal. Chem. 1984, 56,2709-2714
2709
Surface Tension Effects on Aerosol Properties in Atomic Spectrometry John F a r i n o a n d R i c h a r d F.Browner* School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
Experlmental drop size dlstrlbutlons for two organlc solvents (light machlne oil and dloctyi phthalate) and aqueous soiutlons of two surfactants (sodlum dodecyl sulfate and Tween 20) show that the organic solvents generally cause a shlR In the tertiary aerosol to smaller drop slre, whereas the surfactants cause no such shift. There Is no conslstent pattern of lncrease In analyte mass transport to the atomizer, with eRher organic solvents or surfactants. Inductively coupled plasma emlsslon signals are essentially free from surfactant effects, but some enhancements are found for copper and chromium wlth atomic absorption. Enhancements are greatly dependent on the operating conditions. Nebullzer gas and llquid flows, the presence or absence of mixer paddles and impact beads, and the deslgn of the nebullrer and spray chamber all Influence the slgnal. Wlth a nebulizer/spray chamber combination whlch removes most droplets below 4 pm from the aerosol, enhancements observed are all below 10%.
The most common means of sample introduction in atomic spectrometry involves the pneumatic generation of aerosols from liquid samples. However, in spite of the wealth of studies available in the literature describing the use of aerosols for sample introduction, there are relatively few studies which systematically examine the relationships between aerosol properties and analytical performance. As a consequence, there remain many ill-defined areas, where conflicting reports leave the practicing atomic spectroscopist without a working model for the prediction of experimental trends. The relationship between the surface tension of a sample and the resultant analytical signal is one such area. A number of groups (2-6) have studed the subject, especially where surface tension is varied by the addition of surfactants to aqueous solutions, but no cohesive model has yet been developed. Furthermore, published studies are not always transferable between laboratories. This is generally an indication that one or more important experimental variable has been omitted from consideration. In AAS surfactant studies, reports range from claiming no detectable enhancement for a number of elements (I, 2) to claiming absorbance enhancements of up to 3.5X for Cr (5) and 1.6X for Cu (6). Recently, Kodama and Miyagawa (5) have attempted to relate variations in the analytical signal to differences in drop size distribution, caused by changes in solution surface tension. Cook and Kornahrens (6) have also reported enhancements for a number of elements but attempt to explain the behavior in terms of aerosol ionic redistribution (AIR) (7). Recent studies from our laboratories have shown (8,9) that the mean particle size of an aerosol reaching the atomizer may have a significant effect on both signal magnitude and interference severity for many elements. Additionally, spray chamber transport properties have been shown to have a direct bearing on analytical signal magnitude in AAS (10). Our objective in this study was to attempt to clarify the mechanisms active when surfactant solutions, or organic solvents,
are nebulized in atomic spectrometry and to determine the basis for the conflicting data in the literature. In order to accomplish this goal, we have attempted to make careful measurements of selected aerosol properties, namely particle size distribution and analyte mass transport, for solvents with a wide range of surface tension values. Both atomic absorption and inductively coupled plasma systems have been considered, and an effort has been made to explain the analytical signals for some selected elements in terms of aerosol properties. EXPERIMENTAL SECTION In this study, atomic absorption and inductively coupled plasma measurements were made by using a Perkin-Elmer Model 5500 AA/ICP system, except as otherwise noted. For enhancement studies with the Varian nebulizer and spray chamber, the entire burner/spray chamber assembly was removed from the PerkinElmer system and the Varian components were put in its place. For surface tension measurements, a du Nuoy-type instrument employing a platinum-iridium ring was used. Aerosol Collection, AAS. The drop size distributions were obtained by using procedures described in earlier work (IO, 11). Aerosols were collected on the plates of a cascade impactor (Anderson Model 2000). The aqueous solutions used to determine drop size distributions were all 5000 ppm in copper. The aerosols were drawn through the impactor, and each plate was washed into an individual volumetric flask. The resulting solutions were analyzed by atomic absorption to determine the concentration of copper. To obtain drop size distributions of aerosols of organic solvents, the aerosols were drawn through the impactor and the individual plates were weighed to determine the mass of the aerosol directly. For the transport efficiency measurements on aqueous solutions of chromium and copper, with and without surfactant, the aerosol was drawn through the impactor and all plates were washed into a single 200-mL volumetric flask. The resultant solution was analyzed for chromium or copper by conventional AAS. ICP. Drop size distributions were obtained as described above for the AAS systems. Transport efficiency measurements for the aqueous solutions of copper were carried out by drawing the aerosol through a Gelman Spectro Grade glass-fiber filter and treating the filter as described in previous work (12). Transport efficiency values for the organic solvents were obtained as described above for the AAS systems. Aerosol Generation. The inductively coupled plasma (ICP) nebulizer used in this study was a concentric, all-glass Meinhard type, Model TR-30-A3(J.Meinhard Associates, Tustin, CA). The spray chamber used with this nebulizer was a conventional dual-concentric Scott-type chamber (13). Two atomic absorption nebulizers were used. They were (1)a Perkin-Elmer concentric type (Model 0303-0358) with an adjustable sample capillary, used with either a paddle (flow spoiler)-or an impact bead in a Perkin-Elmer AAS premix unit (Model 037-0988), and (2) a Varian adjustable-capillaryconcentric-typenebulizer,used with an impact bead in a Varian spray chamber (Model AA-875). Data Presentation. The format for drop size distribution plots drawn in this study has been described in an earlier publication (IO). The utility of such plots is discussed below. RESULTS AND DISCUSSION Drop Size Distributions of Selected Organic Solvents a n d S u r f a c t a n t Solutions i n AA a n d ICP Systems. The equation most commonly used to describe the production of
0003-2700/84/0356-2709$01.50/0 0 1984 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table I. Physical Properties of Solvents
solvent
density, g/cm3
surface tension, dyn/cm
light machine oil dioctyl phthalate aqueous Cu (5000 ppm) Cu soln + 1000 ppm Tween 20 Cu soln + 5000 ppm Tween 20 Cu soln + 1000 ppm SDS Cu soln + 5000 rmm SDS
0.87 0.97 1.00 1.00 1.00 1.00 1.00
31.4 32.8 74.1 37.6 37.1 41.9 31.0
viscosity, P 0.44 1.10 0.01
0.01 0.01 0.01 0.01
aerosols by pneumatic nebulizers is that developed by Nukiyama and Tanasawa (14). Some of the applications, and limitations, of this equation have been discussed recently by Browner, Smith, and Boorn (10) and by Gustavsson (15).In spite of its restricted boundary conditions, the Nukiyama and Tanasawa equation appears to have general validity (10) and in the absence of any superior model will form the basis for discussion in this work. The equation relates the mean droplet size produced by a pneumatic nebulizer to the operating parameters of the nebulizer. The equation is
where d, is the Sauter mean diameter (pm), V is the velocity difference between gas and liquid flows to the nebulizer (m/s), u is the surface tension of the liquid (dyn/cm), p is the liquid density (g/cm3), 1 is the liquid viscosity (P),and Q1and Q, are the volume flow rates of liquid and gas (cm3/s), respectively. This equation was developed for nebulizers using subsonic gas velocities and solvents with surface tensions between 30 and 73 dynlcm and viscosities between 1and 30 CP. In this study, drop size distributions were measured for aerosols produced from both selected organic solvents and aqueous solutions containing surfactants. Sodium dodecyl sulfate (SDS), an anionic surfactant, was chosen for two reasons: It has been frequently studied by previous workers (1-6), and it is one of the most widely used industrial surfactants. Tween 20, a nonionic surfactant, was also tested in order to determine whether the ionic character of the surfactant might play a role in any of the effects observed. The concentrations of surfactants selected (1000 and 5000 ppm) ensured that all surfactant solutions would contain in excess of the critical micelle concentration (CMC) (16). Solutions with these surfactant concentrations also have surface tensions typical of organic solvents. The organic solvents selected, dioctyl phthalate (DOP) and light machine oil, were chosen principally for their low volatility, which allowed direct cascade impactor runs to be made without the need to take evaporation effects into account (17,18). Table I shows the results obtained experimentally for surface tension, viscosity, and density measurements of both aqueous solutions and organic solvents. Inductively Coupled Plasma Systems. The drop size distributions of light machine oil and dioctyl phthalate leaving the spray chamber of a typical ICP system, with a Meinhard nebulizer and Scott spray chamber, are shown in Figure 1. Drop size distributions for three aqueous solutions of 5000 ppm copper (1)with no surfactant, (2) with 5000 ppm SDS, and (3) with 5000 ppm Tween 20 are shown in Figure 2. It should be noted that these curves all refer to tertiary aerosol (i.e., those reaching the atomizer), compared to the primary aerosols described by the Nukiyama and Tanasawa equation. Solvent uptake rates for all experiments were set a t 1.0 mL/min, using a syringe pump. This was done in order to
0
2
4 DROPLET DIAMETER
6
10
8
(rm)
Figure 1. Drop size distributions for organic solvents with ICP nebulizer. (V)Light machine oil; (W) dioctyl phthalate. Solvent uptake rate: 1.0 mL/min. Ar flow: 1.0 L/min. Glass concentric nebulizer; dual concentric spray chamber.
0
2
4 DROPLET DIAMETER
6
8
1
(rm)
Figure 2. Drop size distributlons for aqueous surfactant solutions with ICP nebulizer. (m) 5000 ppm Cu, no surfactant; (V)5000 ppm Cu -45000 ppm Tween 20; (0)5000 ppm Cu 5000 ppm SDS. Solvent uptake rate: 1.0 mL/min. Ar flow: 1.0 L/min. Glass concentric nebulizer; dual concentric spray chamber.
+
Table 11. ICP Transport Efficiency (e,) and Mass Median Droplet Diameter Values (a,)'' d,,
w
solvent
e", ?&
light machine oil dioctyl phthalate 100 ppm Cu soln 100 ppm Cu soln + 1000 ppm Tween 20 100 ppm Cu soln + 5000 ppm Tween 20
1.73 1.54 1.61 1.21
2.30 2.21 2.77
1.43
2.97
100 ppm Cu soln + 1000 ppm SDS 100 ppm Cu soln + 5000 ppm SDS
1.14
1.26
2.88
Concentric all-glass nebulizer; 1.0 mL/min liquid uptake rate; 1.0 L/min nebulizer Ar flow.
eliminate natural uptake rate variations resulting from widely different solvent viscosities (see Table I). The figures show three clear trends: (1)Both of the organic solvents tested give very similar drop size distributions, (2) neither the presence of Tween 20 nor SDS causes a detectable shift in drop size distribution compared to pure aqueous copper solution, and (3) the mass median droplet diameters for both organic solvents (2.30 km for light machine oil, 2.21 pm for dioctyl phthalate) are significantly lower than the average for the three aqueous solutions (2.87 pm).
ANALYTICAL CHEMISTRY, VOL. 56,NO. 14, DECEMBER 1984
Transport efficiency values for the organic solvents and surfactant solutions were obtained by using the direct method of Smith and Browner (12). The results, shown in Table 11, indicate that the overall shift to smaller median drop size found with the organic solvents may sometimes result in a slight increase in transport efficiency compared to the aqueous solvents. However, for dioctyl phthalate there is actually a slight decrease in transport efficiency. The aqueous solutions E, averages 1.33%,compared to the organic solvents average value of 1.64%. As the liquid uptake rate is fixed a t 1.0 mL/min in all these experiments, the 6, values directly reflect the relative analyte mass transport rates (Wtot) (IO) to the plasma to be expected from these solvents. I t is very interesting to note that the presence of surfactants in the aqueous solutions apparently leads to a net reduction in the analyte transport efficiencies, compared to pure aqueous solvent. This indicates that the surfactant may actually inhibit the formation of droplets in the critical size range below approximately 4 pm, which agrees with the pattern of the drop size distribution curves shown in Figure 1. The drop size distribution plots and transport efficiency data can readily be understood in terms of the model for aerosol transport proposed in an earlier study (IO). This is based on the premise that when aerosol evaporation in the spray chamber is not significant, the tertiary drop size distribution (i.e., that reaching the flame or plasma) is largely determined by the spray chamber configuration. The spray chamber imposes a cutoff diameter (d,) on the primary aerosol (i.e., that produced at the nebulizer) and effectively acts on the aerosol as a filter, eliminating most droplets with diameters larger than d,. The value of d, for ICP spray chambers is typically found to be 2.5 pm. The cutoff diameter is determined by the spray chamber design and dimensions and their interaction with the aerosol and gas flow patterns. When this model is applied to the present situation, a shift in the primary aerosol to smaller median drop diameter can therefore also result in a corresponding shift to smaller median droplet size in the tertiary aerosol. By contrast, when all other factors are kept constant, an increase in primary median drop size does not generally give rise to a corresponding increase in tertiary median drop size. The additional large drops are collected by the spray chamber and pass to waste. This effect is clearly illustrated by the insensitivity of ICP systems to solution uptake rate changes (IO). As liquid flow rate is increased above about 0.5 mL/min, the additional solution goes almost exclusively to form droplets with diameters larger than the cutoff diameter, d,, and so passes directly to waste. The organic solvents appear to give rise to a primary aerosol slightly shifted to smaller droplets, taking pure aqueous solutions as a reference point. This results in a tertiary aerosol also shifted to smaller droplets. Consequently, additional aerosol reaches the atomizer. The surfactant solutions, by contrast, appear to cause a slight shift in the primary aerosol drop size distribution to larger median drop size. However, this change in the primary aerosol does not result in a corresponding shift in the tertiary aerosol distribution to larger values, for reasons explained above. Instead, it shows only as a slight net decrease in analyte mass transport to the atomizer. It is important to recognize that the discussion above refers specifically to aerosol droplets in the approximate size range 0-10 pm. This is the particle size region over which aerosol generation and loss processes interact in our system, to produce the tertiary aerosol which finally reaches the atomizer. It is possible that surface tension forces could act to reduce the proportion of very large droplets in the aerosol, say above 75-c~mdiameter, and produce in their place a large number of intermediate sized droplets, say 25-pm diameter. However,
I
2711
I
L
0 0
2
4
6
6
10
DROPLET DIAMETER (vm)
Flgure 3. Drop size distributions for organic solvents with AAS nebulizer. (V)Light machine oil;).( dioctyl phthalate. Solvent uptake rate: 1.O mL/min. Nebulizer air flow: 6.3 L/min. Perkin-Elmer nebulizer and spray chamber with mixer paddle.
with the approximately 2.5 pm cutoff diameter imposed by the present system, this shift in drop size distribution would not lead to any detectable change in the drop size distribution of the tertiary aerosol or to a change in aerosol mass transport. If, on the other hand, the system had a cutoff diameter, d,, of say 30 pm, this would allow most 25 pm diameter droplets to pass. Consequently, a distribution shift from 75- to 25-pm droplets would result in a significant downward shift in the tertiary drop size distribution and a corresponding increase in aerosol mass transport to the atomizer. The range of surface tensions of the surfactant solutions and organic solvents (31.4-41.9 dyn/cm) is relatively small, and all are much lower than that of surfactant-free 5000 ppm copper solution (74.1 dyn/cm). However, the different behavior of the organic solvents as one group, and the aqueous surfactant solutions as another, indicates that there may be different modes of action of surface tension forces in droplet formation, dependent on the physical processes involved. For organic solvents, surface tension forces are an expression of existing bulk properties of the liquid, whereas for surfactant solutions, they are due to migration and orientation of additive molecules in the liquid surface. In other words, the very rapid time scale in which droplets are formed in pneumatic nebulization, probably of the order of microseconds, is not adequate for the relatively bulky surfactant molecules to reorient themselves in the new surface sufficiently fast to exert a major influence on droplet formation. This conclusion fits with the dynamic behavior of surfactant solutions noted by Thomas and Potter (19),where the surface tension of a newly formed liquid surface was found to take several milliseconds to reach an equilibrium value. The work of Dean (2)and Venable (4) also supports this conclusion, but it is in conflict with the differing mechanisms proposed by Kodama and Miyagawa (5) and Kornahrens et al. (6). Atomic Absorption Systems. Drop size distributions of aerosols were obtained for a Perkin-Elmer nebulizer and spray chamber, using the same organic solvents and surfactant solutions as for the ICP system. Two configurations were used (1)with a mixer paddle and (2) with an impact bead. Representative distributions obtained with the mixer paddle are shown in Figures 3 and 4. The trends found with the ICP system are repeated in these studies, although the actual values for median droplet diameter and cutoff diameter differ from the ICP system, as expected. Experimental values for median droplet diameters are shown in Table 111. A wide range of liquid uptake rates, nebulizer gas flows, and auxiliary gas flow was tested, to ensure that all reasonable practical measurement conditions were covered. The liquid uptake
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table 111. AA Median Drop Diameters, fima
solvent light machine oil dioctyl phthalate aq Cu (5000 ppm) Cu soln + 5000 ppm Tween Cu soln + 5000 ppm SDS
nebulizer gas flow (0 L/min aux; 1mL/min soln) 7.9 L/min 6.9 L/min 6.3 L/min calcdb exptlc calcd exptl calcd exptl 19.8 23.5 16.9 12.6 11.8
2.14 2.01 2.14 2.51 2.17
21.8 26.4 17.2 13.0 13.6
2.01 2.00 1.81 2.14 1.95
23.4 28.7 17.4 13.2 13.9
nebulizer gas flow (8 L/min aux; 4.7 mL/min soln) 7.9 L/min 6.3 L/min calcd exptl calcd exptl
1.98 1.97
22.4
2.56
22.5
2.40
1.77
27.6 25.0 25.3
3.17 3.35 2.83
32.4 30.6 30.8
2.53 3.13 2.56
2.01 1.90
Perkin-Elmer AA nebulizer and spraychamber with mixer paddle. Calculated values for primary distribution (dc),obtained from eq 1. Experimental values for tertiary distribution (dm),obtained from cascade impactor data. (I
Table IV. AA Transport Efficiency (en) Values"
solution 10 ppm Cu
+ Tween 20 10 ppm Cu + 10 ppm Cu 5000 ppm 5000 ppm 10 ppm Cr
2
4 6 DROPLET DIAMETER (pm)
8
10
Flgure 4. Drop size distributions for aqueous surfactant solutions with AAS nebulizer. (V)5000 ppm Cu, no surfactant; (m) 5000 ppm Cu 5000 ppm Tween 20; (0)5000 ppm Cu 5000 ppm SDS. Solvent uptake rate: 4.7 mL/min. Nebulizer air flow: 6.3 L/min. Auxiliary air flow: 8.0 L/min. Perkin-Elmer nebulizer and spray chamber with mixer paddle.
+
+
rates of 1 and 4.7 mL/min were set with a syringe pump, to avoid uptake rate changes resulting from the widely differing solvent viscosities. The higher flow rate is close to the natural aspiration rate for aqueous solutions found when the nebulizer is operated a t 6.9 L/min gas flow. An uptake rate of 4.7 mL/min was not possible with the DOP,due to its high viscosity; consequently this flow rate was not tested with the organic solvents. An attempt was made to compare the qualitative accuracy of the current aerosol transport model (IO), taken in conjunction with the Nukiyama and Tanasawa equation, for predicting the influence of surface tension forces on liquid sample introduction properties. Table I11 shows d, values calculated by using the Nukiyama and Tanasawa equation for the various solutions and solvents, together with experimental mass median diameters (d,) obtained in this study. A wide range of experimentd conditions of gas and liquid flow was studied. The nebulizer parameters used in the calculations were taken from previously published data (IO). The mass median diameter is the droplet diameter corresponding to a cumulative mass of 50% of the collected aerosol. The d, and d, values may be validly compared, as they refer to similar physical properties (IO). That the values for d, and d, shown in Table I11 differ greatly is to be expected, because they refer to aerosols with different distributions. Results calculated from the Nukiyama and Tanasawa equation (d,) are for primary aerosols, whereas the experimental mass median diameters (d,) are for tertiary aerosols. However, when the general aerosol transport model (10) is used, it is still possible to make useful comparisons between these values, as a means of seeing if experimental and predicted trends match.
bead
2.0 1.9
3.8 3.6
2.0
4.0
1.9 1.9
3.6 3.5
1.8
4.0
SDS
10 ppm Cr + 5000 ppm Tween 20 10 ppm Cr + 5000
paddle
ppm SDS
" Perkin-Elmer nebulizer; 4.7 mL/min solution uptake rate; 6.3 L/min nebulizer gas flow; 8 L/min auxiliary gas flow. The results shown in Table I11 demonstrate several interesting points, taking aqueous copper solution data as a reference. The gas and liquid flows used for one set of experiments were selected as fairly typical of normal analytical use. These conditions are shown in the last two columns of Table 111. For these conditions, the following trends are observed: (1)for light machine oil, both calculated and experimental median drop diameters are lower than for aqueous copper. (2) For the surfactant solutions, calculated drop diameters are all lower than for aqueous solutions. Experimental values, however, are either higher than (Tween 20) or the same as (SDS) the aqueous solution values. For other experimental conditions examined, the following points apply: (1)The organic solvents generally give both higher calculated and experimental median droplet diameters than the aqueous solution values, although not in all instances. (2) The surfactant solutions, by contrast, generally give lower calculated but higher actual median droplet diameters than the aqueous copper solution. The lack of correspondence between trends in the shift of actual drop diameters, and those predicted from the Nukiyama and Tanasawa equation, reinforces the conclusions reached with regard to ICP nebulizers and spray chambers, e.g., that, at least when considering small droplet formation, surface tension forces active with organic solvents and with surfactant solutions are different in kind and that only bulk-active properties are applicable to the Nukiyama and Tanasawa equation. Transport efficiency data for the AA system are shown in Table IV. The results for the paddle may be directly compared with the data in the last two columns of Table I11 and with the distribution curves of Figure 4. Data for both copper and chromium are shown, because it has been previously observed that AAS enhancements for these elements in the presence of surfactants may differ significantly (5, 6). As aerosol ionic redistribution (AIR) has been suggested as a possible mechanism for surfactant-induced enhancements in AAS, the possibility of different transport properties for the two ions could not be ruled out a priori. Transport efficiency values with an impact bead attached to the nebulizer are also
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table V. AA Absorbance Enhancements with Surfactantsa
sample 1 ppm Cu
+
1000 ppm Tween 20 1 ppm Cu + 5000 ppm Tween 20 1 ppm Cu + 1000 ppm SDS 1 ppm Cu + 5000 ppm SDS 1 ppm Cr + 1000 ppm Tween 20 1 ppm Cr + 5000 ppm Tween 20 1 ppm Cr + 1000 ppm SDS 1 ppm Cr + 5000 ppm SDS
Perkin-Elmer system 4.7 mL/min 8.0 mL/min Varian bead paddle bead paddle system 5
0
0
0
0
15
0
0
0
-5
15
5
9
5
5
18
5
5
7
2
8
8
0
0
0
14
8
5
0
0
21
8
13
9
18
21
8
13
9
9
‘Values are percentage enhancements, with respect to either 1 Cu solution or 1 pprn Cr solution, as appropriate.
pprn
presented. The effect of the impact bead is to increase the transport efficiency for all solvents (8,20), as it shifts the cutoff diameter to a larger value. The possibility that such a shift could alter the observed trends for surfactants was considered to be worthy of examination. Transport efficiency values with the paddle were uniformly lower than with the impact bead, as expected. For each configuration of the nebulizer and spray chamber, however, there was less than a 13% variation in transport efficiency for all combinations of cations and surfactants examined. Effect of Surfactants on Analytical Signals. ICP. Copper and chromium solutions containing 1 ppm of the element, together with (a) 1000 ppm Tween 20, (b) 5000 ppm Tween 20, (c) 1000 ppm SDS, and (d) 5000 ppm SDS were examined by ICP spectrometry. The test solutions were a t pH 2, in order to avoid possible precipitation of copper as the hydroxide. For all solutions examined, the surfactants had no detectable effect on the emission intensities of the elements, when compared to pure aqueous solutions. AAS. All signal studies were performed a t a natural aspiration rate of 4.7 mL/min. The solutions examined were identical with those used in the ICP studies. The flame conditions were fuel rich, just short of luminous. Table V shows the absorbance enhancements observed, compared to pure aqueous solutions of the elements. Data obtained with both mixer paddle and bead configurations are shown. With the mixer paddle, relatively little signal change was noted for either element. Enhancements ranged from 0% for Cu + Tween 20 to 8% for Cr + both Tween 20 and SDS. No noticeable influence of surfactant concentration was observed for either analyte. With the impact bead, on the other hand, substantial enhancements were observed for Cu and Cr solutions in the presence of both surfactant types. Enhancements due to SDS (anionic) were slightly greater than those observed for Tween 20 (nonionic). All surfactant effects were found to be independent of measurement height in the flame and altered only slightly by flame conditions. This indicates strongly that particle size variations in the analyte reaching the flame cannot be responsible for the observed enhancements. This contrasts with the proposed enhancement mechanism of Kodama and Miyagawa (5). Effect of Increased Aspiration Rate on Signal Enhancement in AAS. The nebulizer was adjusted to give a higher natural aspiration rate (from 4.7 to 8.0 mL/min). Such nebulizer adjustments cause a shift in the tertiary drop size distribution to larger median diameters, due to the change
2713
in gas and liquid flows. If the effects observed are in any way determined by the drop size of the primary aerosol, then this shift to larger droplets should cause a noticeable change in the magnitude of the enhancements observed. In fact, smaller enhancements were found a t the higher aspiration rate compared to the lower value but followed the same trends observed at lower aspiration rates. These results are also presented in the Table V. Comparisons with Other Studies. Previous attempts to evaluate the characteristics of aerosols encountered in atomic spectroscopy have yielded contradictory results. Other workers have reported surfactant-induced enhancements of much greater magnitude than those observed in this study, particularly in the case of anionic surfactants like SDS (3-6). We believe that these results are mainly due to differences in the characteristics of the aerosol produced, caused by differences both in spray chamber configurations, and in the operating conditions of the system (such as uptake rate, gas flows, measurement height, and fuel-oxidant ratio). In table V, results are shown for the enhancement observed with a Varian AA system, operated with an impact bead. It can be seen that the enhancements observed with this system are not as great as those observed with the Perkin-Elmer apparatus operated with a bead. When the Perkin-Elmer spray chamber is operated with a mixer paddle, the enhancements become minimal. The plots shown in Figures 3 and 4 and the general aerosol transport model of Browner, Smith, and Boorn conflict with the approach of Kodama and Miyagawa ( 5 ) . The aerosols measured by Kodama and Miyagawa were primary aerosols (e.g., those present a t the nebulizer), and no allowance was made for the aerosol-modifying influence of the spray chamber. These latter authors considered that solutions with lower surface tensions produced finer aerosols, which in turn gave rise to smaller particles in the atomizer. The smaller particles reaching the flame were considered to be the cause of enhanced signals observed in flame AAS for several elements. As can be seen in the present study, the spray chambers of modern instruments generally exert a leveling effect on the aerosol drop size distribution, such that variations in the upper end of the primary aerosol size distribution do not appear as significant variations in the tertiary aerosol size distribution, for reasons discussed before. The nebulizer and spray chambers used by the other workers were of early design and produce a much coarser aerosol than any current device that we have studied. The relatively small influence of surfactants found with the use of a mixer paddle indicates that under controlled nebulizer and spray chamber conditions they should not be considered a major source of potential interference. The dependence of the signal enhancement on the ionic character of the surfactant has led some investigators to postulate than an ionic redistribution (AIR) mechanism is responsible. AIR involves the migration of “spectator” ions to the droplet surface (6, 7). Kornahrens and Cook (6) have proposed an extension of this theory in which the “spectator” ions are surfactant molecules. If the ionic (hydrophilic) ends of the surfactant molecules are opposite in charge to the analyte atoms, then analyte atoms will tend to asssociate with surfactant molecules and the surface of the droplets will become analyte enriched. When these larger droplets shatter, the smallest droplets, formed (i.e., the ones most likely to reach the flame) will be analyte enriched. Whether or not this is an accurate picture of the enhancement effect is not clear from this study, as no attempts were made to analyze cascade impactor data in the manner used in earlier studies (7). Certainly signal enhancements observed cannot be accounted for by corresponding changes in analyte mass transport (e,, or Wbt) values. It appears clear that the mechanism for
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Anal. Chem. 1984, 56,2714-2720
enhancements observed in this study is drop size related, as the mixer paddle largely eliminates any enhancement. Such behavior is compatible with the AIR enhancement model. However, enhancements observed with nonionic surfactants are not explicable using an AIR model. This anomaly indicates that some mechanism other than AIR may in fact responsible for enhancements observed with both anionic and nonionic surfactants.
ACKNOWLEDGMENT The loan of spray chambers and nebulizers from PerkinElmer Corp. and Varian Techtron Pty. is gratefully acknowledged. Registry No. DOP, 14117-96-5;SDS, 151-21-3;Tween 20, 9005-64-5;Cu, 7440-50-8;Cr, 7440-47-3. LITERATURE CITED Pungor, E.; Mahr, M. Talanfa 1983, 10, 537. Dean, J. A. "Flame Emission and Atomic Absorption Spectrometry"; Dean, J. A., Rains, T. C., Eds.; Marcel Dekker: New York, 1969; p 306. Venable, R. L.; Ballad, R V. Anal. Chem. 1974, 46, 131. Venable, R. L. Report PB-264 184; U. S.Department of Commerce: Springfieldi VA, 1976. Kodama, M.; Miyagawa, S. Anal. Chem. 1980, 52,2358.
(6) Korhnahrens, H.;Cook, K. D.; Armstrong, D. W. Anal. Chem. 1982, 54, 1325. (7) Borowiec, J. A,; Boorn, A. W.; Dillard, J. M.; Cresser, M. S . ; Browner, R. F.;Matteson, M. J. Anal. Chem. 1980, 52, 1054. (8) Smith, D. D. Doctoral Thesis, Georgia Institute of Technology, Atlanta, GA, 1983. (9) Smith, D. D.; Browner, R. F. Anal. Chem. 1984, 56, 2702. (IO) Browner, R. F.; Boorn, A. W.; Smith, D. D. Anal. Chem. 1982, 5 4 , 141 1. (11) Browner, R. F.; Cresser, M. S. Spectrocblm. Acta, Part B 1980, 358, 73. (12) Smith, D. D.; Browner, R. F. Anal. Chem. 1982. 5 4 , 533. (13) Scott, R. H.;Fassei, V. A,; Kniseiey, R. N.; Dixon, D. E. Anal. Chem. 1974, 46,75. (14) Nukiyama, S.;Tanasawa, Y. "Experiments on the Atomization of Liquids in an Air Stream": Hope, E.;Transl.; Defense Research Board, Department of National Defense: Ottawa, Canada, 1950. (15) Gustavsson, A. Anal. Chem. 1983, 55,94. (16) Rosen, M. J. "Surfactants and Interfacial Phenomena"; Wiiey: New York, 1978: Chapter 3. (17) Boorn, A. W.; Cresser, M. S.; Browner, R. F. Spectrochlm. Acta, Part B 1980, 355, 832. (18) Boorn, A. W.; Browner, R. F. Anal. Chem. 1982, 54, 1402. (19) Thomas, W. D. E.; Potter, L. J . Colloid Inferface Sci. 1975, 50,397. (20) Cresser, M. S.;Browner, R. F. Appl. Spectrosc. 1980, 34,364.
RECEIVED for review April 2, 1984. Accepted July 12, 1984. This material is based on work supported by the National Science Foundation under Grant CHE80-19947.
Atomization Mechanism with Arrhenius Plots Taking the Dissipation Function into Account in Graphite Furnace Atomic Absorption Spectrometry Chan-Huan Chung'
Department of Chemistry, Faculty Hiroshima 730, Japan
of
Science, Hiroshima University, Higashisenda-machi, Naka-ku,
The mechanism of atom formatlon and dissipation for seven elements in graphlte furnace atomic absorptlon spectrometry has been studied through a combined ihermodynamlc and klnetic approach In which the rate of atom formatlon ( k ) at a given temperature was calculated by using the corresponding absorbance ( A , ) and the maximum absorbance a,) where kd IS the dlssipatlon ( A m a x ) :k = k&,/(A,,, constant. The Arrhenlus plot obtained for all the cases was linear over the temperature range from the appearance to the maximum absorbance. The activation energy E , of atom formation obtained showed that four atomlzation mechanisms are operative as the rate-determiningprocess: thermal dlssoclatlon of the oxide for Zn, AI, and Mg and of the hallde for Sb; vaporlzatlon of metal for Ag; carbon reduction for Fe; dissociation of the dimer for Cu.
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The mechanism for atom formation in graphite furnace atomizers has been discussed by many workers (1-17). In one popular hypothesis for atomization, the metal oxide is reduced by the carbon of the furnace to the free metal. On the basis of this theory, Campbell et al. ( 1 ) found agreement between these theoretically predicted temperatures and the experiPresent address: Laboratory of Anhui Geological Bureau, Hefei, Anhui, People's Republic of China.
mentally observed temperatures in the reduction process for 27 elements, but there were a few exceptions. Quite recently, the problem of atomization was studied by considering effects of the partial pressure of oxygen (16,17),and the easiness of reduction on graphite for some metals such as copper and silver was explained. Fuller (2-4) has first described a kinetic approach to the atomization process in graphite furnace atomic absorption spectrometry under isothermal conditions. Sturgeon et al. ( 5 ) have studied the mechanism of atom formation in a graphite furnace through a combined thermodynamic and kinetic approach. They derived eq 1 and used it for obtaining E, values for the early stages of signal production of 15 elements. In A , = -E,/RT + A. (1) Two sequential atomization energies have been obtained for 12 elements such as copper with 340 and 187 kJ mol-'. In eq 1 A , is the absorbance a t time t ( s ) and A. is the constant. Using an equation by L'vov (8), Smets (12) derived the expression 1
A,
where /3 is the atomization efficiency and k the first-order rate constant for the formation of analyte atoms a t any temperAture. L'vov used eq 2 for macrokinetic studies (9) and an
0003-2700/84/0356-2714$01.50/00 1984 American Chemical Society
