Anal. Chem. 1982, 5 4 , 1325-1329
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Mechanism of Enhancement of Analyte Sens Surfactants in Flame Atomic Spectrometry Heather Kornahrens and Kelsey D. Cook* School of Chemical Sciences, Universlty of Illinois, 44 Roger Adams Laboratory, Box 49, 7209 W. California Si., Urbana, Illinois 6 180 7
Danlel W. Armstrong” Department of Chemisity, Georgetown University, Washington, D. C. 20057
A model Is proposed wherein sensitivlty enhancements by surfactants in flame atomic spectrometry using a premix burner result primarily from ionic redistributlon upon the generation of aqueous aeramols by pneumatic nebulization. Enhancement Is predicted and experimentally observed with surfactants of charge olpposlte to that of the anaiyte Ion and is greatest at surfactant concentrations just below the crltlcal micelle concentration (CMC). The degree of enhancement Is dependent on oxldaiit flow rate and can be as hlgh as 150 %.
Solvent matrices can greatly affect the absorption by a given species in flame atomic spectrometry (FAS) (1). Use of organic cosolvents, for example, can enhance the sensitivity of aqueous atomic absorption (AA) determinations by a factor of 2 or more. This phenomenon has been attributed to the low surface tension of organic solvents (e.g., less that 25 dyn cm-2 for ethanol and methanol, two of the most commonly used solvents, as compared to 72 dyn cm-2 for water), which can promote generation of iimall droplets during the aspiration and nebulization processes, resulting in greater efficiency in the flame processes and hence greater sensitivity. However, addition of water-miscible solvents also results in dilution of the sample, offsetting the gain in sensitivity. If water-immiscible solvents are used, chelating agents and time-consuming extraction procedures must be used. Because of their water solubility and ability to reduce the surface tension of aqueous systems, it has been suggested (1-7) that surfactants might improve the sensitivity of FAS determinations. However, relatively few successful applications have been reported. For example, Foster and Hume (5) found that addition of the nonionic surfactant tergitol NPX lowered the surface tension of aqueous solutions to 32-38 dyn cm-2 (depending on the concentration of surfactant). This is comparable to surface tensions obtained with commonly used water-miscible organic solvents (e.g., 25-32 dyn cm-2 for 50-80’70 methanol solutions). Nevertheless, no change in sodium emission intensity accompanied the addition of the surfactant to solutions aspirated with a total consumption burner-nebulizer. By contrast, Dean (7) observed that “a trace of surface-active agent” dlid enhance emission signals obtained in experiments using a premix burner, suggesting a burnerselective effect (no experimental details were given, and analyte and surfactant were not identified). However, subsequent reports (6, 8) contradicted this observation, leading Dean to discount his eairlier data and conclude that even in the premix burner, detergent surface activity is impeded by the inability of detergents to accumulate at droplet surfaces during the vigorous agitation process which accompanies aspiration and nebulization (1). This conclusion has not resolved the issue. More recent studies by Kodama et i d . (2) report use of the surfactant 0003-2700/82/0354-1325$0 1.25/0
sodium dodecyl sulfate (SDS) in an improved method for chromium determinations by AA using a premix burner. Absorption of chromium from dichromate more than doubled as SDS concentration increased up to the critical micelle concentration (CMC). No further increase was noted above the CMC, reflecting the surface tension of the aqueous surfactant system (surface tension decreases with added surfactant until the CMC is reached). In a later study of droplet size distribution, Kodama et al. (3) confirmed that surfactants promoted production of aerosols finer than those attainable in simple aqueous systems. Finally, there has been one other report of sensitivity enhancement by surfactants in FAS. Venable and Ballad ( 4 ) (employing a premix burner) found that SDS improved the sensitivity of the AA analysis of Cu and Ni. Tetradecylpyridinium bromide (TPB), however, did not enhance the sensitivity of the determination. It is evident from all of these studies that a complete understanding of the role of surfactants in FAS is not available. While Venable and Ballad ( 4 ) indicated that the type of surfactant used is important in determining whether an effect is observed, they offered no model explaining their data. Kodama (3)attributed the absorption enhancement by surfactants to a surface tension effect while others (5-7) observed no enhancement by surfactants in either AA or AE despite lowered surface tensions, and Dean ( I ) predicted that bulk surface tension may not be reflected in the properties of agitated droplets during formation (although, as will be shown below using simple diffusion calculations, surfactants do have ample time to migrate to the surface during droplet lifetimes in the premix burner). Clearly, surface tension is only one of the parameters involved in the mechanism of surfactant enhancement of signals in FAS, if it is involved at all. The present study aims to determine the parameters which influence the incidence and extent of surfactant enhancement of sensitivity in FAS.
EXPERIMENTAL SECTION Reagents. Stock metal solutions of 1000 ppm were prepared with analytical grade reagents. Ni(N03)2.6H20,KMnO,, and MnC12.4H20(Fisher Scientific Co.), Ca(N03)2.4H20and K2Cr207 (Mallinckrodt), CrC13.6H20and MgC12.6H20(Baker), and Cu(N03)2.6H20(Allied Chemical Co.) were used as received. ‘The surfactants SDS (BDH Chemicals), ammonium dodecyl sulfate (NH4DS; Richardson Co.), Aerosol OT (AOT; Fisher Scientific Co.), cetyltrimethylammoniumbromide (CTAB),Triton X-100 (TX),and tetradecyltrimethylammonium bromide (TTAB; Sigma Chemical Co.) were also used without purification. All glassware was cleaned with 10% HN03 and triply rinsed with distilled deionized water prior to use. Solutions were prepared within 24 h of use. Procedure and Apparatus. A Varian Model 475 atomic absorption spectrophotometer with a Varian Techtron long path, premix burner was used for all determinations. Because of the internal flash prevention trap in the Varian nebulizer-burner, 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
a
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SDS CONCENTRATION ( mM 1 Figure 1. Cu absorbance and surface tension vs. [SDS]: (0)absorbance; (A)surface tension. Air flow rate = 12.6 L/min; C2H, flow
rate = 2.5 L/min. it was not possible to examine waste from this burner. A GCA McPherson Model EU-703-70 AA, AE, AF (flame) module with a similar (but trapless) burner was therefore used to generate wastes, which were subsequently examined with the Varian AA spectrometer. An air-acetylene flame was used for all determinations except that of Ca, for which a nitrous oxide-acetylene flame was employed. Surface tension measurements were made by using a du Nuoy tensiometer with a platinum ring. Relative kinematic viscosities were measured with an Ostwald viscometer.
RESULTS AND DISCUSSION Results with Copper. Figure 1shows the dependence of the absorbance of a 10 ppm Cu solution (1.57 X loe4M) on the concentration of SDS. Also shown is the surface tension of the solutions as a function of SDS concentration. The break in the latter curve is an accepted indication of the onset of micellization (the CMC). At SDS concentrations below the CMC, the absorbance is enhanced by increasing the concentration of surfactant. Higher SDS concentrations (above the CMC) suppress the enhancement, although the absorbance remains higher than that in the absence of surfactant. The absorbance levels off a t SDS concentrations above 20 mM. Similar profiles were obtained for two additional anionic surfactants (NH,DS and AOT). The maximum degree of enhancement for each is dependent upon air flow rate (see below). Under similar conditions, the cationic surfactant CTAB and the nonionic surfactant T X had no effect on the absorbance, although their solutions had surface tensions comparable to those of the anionic surfactant systems. These results indicate that the charge of the surfactant is important in determining whether an enhancement is observed, in agreement with results reported by Venable and Ballad ( 4 ) . Results with Other Metals. In a further investigation of this apparent charge effect, the absorance of several metals was measured in the presence of cationic, anionic, and nonionic surfactants. SDS enhanced the absorbance of CuP+,Cr3+, Mn2+, Ni2+, Ca2+, and Mg2+,while CTAB and T X either slightly depressed the signal or had no effect at all (except for Cr3+;see below). By contrast to the results with atomic metal cations, MnO, was not enhanced by either SDS or TX. It was not possible to test the effect of CTAB on permanganate absorbance due to the evident insolubility of CTA-permanganate. CTAB did enhance Cr20,2- absorbance, as did SDS. Model for Enhancement. A model for the interactions responsible for these observations can be derived from an extension of the theory of aerosol ionic redistribution (AIR), as proposed by Borowiec et al. (9). These authors suggest that “spectator” ions in a FAS sample can promote enrichment of the analyte in the double layer at the air-water interface on the surface of large drops. As these drops subdivide in the
c) Figure 2. Aerosol ionic redistribution: (a) migration of surfactant to interface; (b) diffusion of analyte to interface; (c)stripping action; (d) enrichment of smaller droplets in analyte. Shaded regions denote analyte (and surfactant). nebulization process, smaller droplets form from a “stripping” of the drop surface, while larger droplets arise from the analyte-depleted bulk drop center (see Figure 2c,d). Because smaller droplets are more efficiently sampled by the premix burner, a net enrichment results and enhancement of the analytical signal is observed. In the systems under study here, the “spectator” ions are surfactant molecules, known to accumulate at air-water interfaces due to hydrophobic interactions (Figure 2a, in which the air-water interface lies near the surfactant head group (IO)). These ions may therefore be expected to establish a double layer more readily than simple hydrophilic ions, enhancing analyte surface concentrations significantly, provided that analyte and surfactant head groups are oppositely charged (Figure 2b). This provision accounts for charge effects observed here and elsewhere ( 4 ) . At surfactant concentrations above the CMC, micelles distributed throughout the bulk solution provide competitive interaction sites away from the surface, reducing the enrichment effect and accounting for the observed decrease in sensitivity beyond the CMC. This hypothesis suggests that decreasing absorbance beyond the CMC results from depletion of analyte at the surface because of binding to micelles in bulk. Most of the metals studied are relatively tightly bound to SDS micelles (notably Cu2+,Mn2+,and Ni2+) ( I I , 1 2 ) . By way of contrast, Cr3+is not tightly bound ( I I ) , and Cr absorbance did not decrease significantly above the CMC. It is important to note that the model proposed here predicts absorption enhancement due to analyte enrichment in the smaller droplets which are more effectively sampled by the premix burner. Significantly, no net enrichment is predicted for a total consumption burner, consistent with the absence of surfactant effects in Foster and Hume’s FAS study with that burner type (5). However, these authors employed a nonionic surfactant, which would not be predicted to affect FAS sensitivity according to the model proposed here, regardless of burner type. Thus, further study is needed to test the predictions of this model with relation to total consumption burners, i.e., to confirm the long-held belief of no surfactant effect with these burners (1,7). Feasibility of Surfactant Diffusion to the Droplet Surface. The AIR mechanism for surfactant-induced enrichment of analyte in the aerosol described above requires that a surfactant-analyte double layer be established a t the surface of drops formed in the aspiration process before these
ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982 0'32
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Flgure 3. Cu absorbance vs. [SDS] for source and waste solutions: (0)source; (A)waste. Air flow rate = 12.6 L/min; C,H, flow rate = 2.8 L/min.
drops subdivide into smaller droplets in the premix burner chamber (due, for example, to impaction with the glass bead). Dean ( I ) suggested that time may not be available for diffusion of surfactant and analyte during the vigorous agitation which accompanies aspiration and nebulization (see discussion above). If this were the case, AIR could not account for the FAS signal enhancements observed here and elsewhere. A simple test of Dean's hypothesis employs Einstein's relationship (13) for the mean square displacement (( r 2 ) )of a particle undergoing diffusion in three dimensions due to Brownian motion
( r 2 )= 6Dt
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(1)
where D is the diffusion coefficient (cm2/s) and r is the displacement (in cm) of the particle from its position a t time t = 0 s. Assuming that the diffusion coefficient for surfactant in solution is about 5.7 x lo4 cm2/s (14), and that maximum drop radius is roughly 6 pm (6 X cm) (15), it may be estimated that a dodecyl sulfate anion at the center of a drop would require about 110ms to diffuse to the surface. By comparison, droplet life times in a premix nebulizing chamber are generally from 100 to 400 ms, depending on flow rate (16). While stripping and droplet formation may not occur throughout this time, the fact that estimated diffusion time is an order of magnitude shorter suggests that diffusion requirements can be met. This hypothesis gains further support from the facts that (1)[surfactant is distributed throughout the drop and so is, on average, closer than 6 pm from the surface and (2) most concentrations. are smaller than 6 pm and are shrinking due to evaporation. Examination of Nelbulization Waste. This model suggests that the waste produced during nebulization (which consists of large droplets caught by the baffles within the premix burner and which can account for as much as 90% of the sample) should be depleted in analyte. Thus, the model was tested by collecting and examining the waste from a surfactant-metal system. The liquid trap within the Varian burner-nebulizer precluded collection of waste on this instrument. Instead, a GCA McPherson Model EU-703-70 AA, AE,AF (flame) module was used to generate and collect waste from aspiration of a series of 10 ppm Cu solutions with varying SDS concentrations. Absorbance data from each of these waste solutions were compared with corresponding data from the solutions aspirated to generate the waste. Figure 3 shows that surfactant does in fact induce depletion of copper in the waste. Only for the solution with no surfactant are the absorbances of waste and source solutions equal. The fact that the waste absorbance profile otherwise parallels that of the source solutions suggesb that surfactant concentration is not greatly affected by the aspiration process. Specifically, the
15 SDS CONCENTRATION (mM) IO
25 20
Figure 4. Aspiration rate and viscosity vs. [SDS]: (0)aspiration rate; (A)viscosity. Air flow rate = 11.9 Llmin; C2H, flow rate = 2.6 L/min.
Table I. Least-Squares Data for Working Curves for CU*+ (0-12ppm) With and Without SDS ( 2 mM) slope, abs units/ intercept, corr solution ppm abs units coeff with surfactant without surfactant
0.025 0.018
-0,001
0.001
0.9993 0.9996
coincident position of the maxima in the curves of Figure 3 suggests that the surfactant concentration is near the CMC for both this source solution and the waste generated from it. Dependence of Absorbance on Air Flow Rate. Aerosol ionic redistribution is extremely dependent upon droplet size (9),which in turn is highly sensitive to flow rate of oxidant. When the flow rate is increased, the aspirated solution strikes the glass bead impactor in the nebulizer with a higher velocity, resulting in smaller droplets formed during nebulization. High flow rates further reduce droplet size by hydrodynamic compaction and by increasing the rate of evaporation during formation and transport ( I ) . Thus, according to the AIR model, increased flow rate should result in greater enrichment and corresponding signal enhancement. As a test of this prediction, the enhancement of Cu absorption by SDS was measured at several flow rates. The fuel flow rate was kept constant at 2.7 L/min. The percent enhancement, %E, was evaluated as follows:
where (Abs), = the absorbance of a 10 ppm Cu solution containing 1 mM SDS (corresponding to the maximum of Figure 1) and (Abs), = the absorbance of 10 ppm Cu in the absence of surfactant. As expected, % E increased with flow rate, and in fact a near-linear relation was obtained for flow rates (u) attainable ( % E = 2 5 . 4 ~- 277, correlation coefficient = 0.937 for u between 11.8 and 12.8 L/min). Dependence of Absorbance on Aspiration Rate and Viscosity. Venable and Ballad ( 4 ) suggested that the fall-off in Cu absorbance at high SDS concentrations might be due to the increase in viscosity with increasing surfactant concentration. To investigate this possibility, we measured both aspiration rate and kinematic viscosity of 10 ppm Cu solutions as a function of SDS concentration (Figure 4). As expected, aspiration rate decreased with increasing surfactant concentration as a consequence of increasing viscosity. However, even a t 20 mM SDS, the aspiration rate decreased by only 10% from its value in the absence of surfactant (and by only 7% from its value at the CMC), whereas the absorbance was
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
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Figure 5. Cu absorbance vs. burner height in the presence of SDS: (V) 0 mM SDS; (0) 1 mM SDS; (A) 5 mM SDS; (0)20 mM SDS.
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Figure 6. Cu absorbance vs. burner height in the presence of CTAB: (V) 0.0 mM CTAB; (0) 0.1 mM CTAB; (A)0.5 mM CTAB; (0)2.0 mM CTAB.
reduced by 30% from its maximum value. Furthermore, the nonionic surfactant T X had no effect on the absorbance of Cu, even though it also increased solution viscosity and decreased aspiration rate. Clearly, these factors have at most a minor effect on the depression of absorbance beyond the CMC. Dependence of Absorbance on Burner Height. Figures 5-7 show the dependence of absorbance of a 10 ppm Cu solution on burner height (or height of the optical path above the burner) in the presence of varying amounts of SDS, CTAB, and TX, respectively. From these results it is evident that the shape of the profile is independent of surfactant concentration. In fact, for CTAB and TX, the profiles are identical with and without surfactant. These profiles indicate that little if any of the signal enhancement by surfactants in FAS is the result of a change of the profile of analyte in the flame; the surfactant does not induce transport of the analyte to a hotter portion of the flame as part of the enhancement mechanism. These results further support the hypothesis that the surfactant effect is one that occurs during nebulization, before the sample reaches the flame. Results with Chromium. Kodama (2,3)found that the absorbance of Crz072-and Cr3+was enhanced by both SDS and dodecyltrimethylammonium chloride (DTAC). Furthermore, beyond the CMC the absorbance leveled off a t roughly twice its value in the absence of surfactant. These results were confirmed here for SDS and TTAB. The nonionic surfactant T X had no effect on the absorbance of either Cr2072-or Cr3+,as expected. However, results with the ionic surfactants were inconsistent with the aerosol redistribution model, in that the enhancement is independent of the charge of the ionic surfactant. By the hypothesis proposed above, Cr2072- absorbance should be enhanced only by TTAB, while
Flgure 7. Cu absorbance vs. burner height in the presence of TX: (V) 0.000 w/v% TX; (0) 0.001 w/v% TX; (A) 0.010 w/v% TX; (0) 0.050 w/v% TX.
SDS should enhance only Cr3+absorbance. This anomaly may be partially attributable to the complex solution chemistry of the chromic and dichromate ions. For example, because Cr3+was prepared from the chloride salt, and because cationic surfactants include bromide counterions, anionic complexes of the form CrC1,Br,(HzO),(Z+y-3)-may have been present in these solutions, favored by the singularly slow kinetics of Cr3+ ligand exchange reactions. At the pHs employed (near pH 5 unbuffered), dichromate could have existed at equilibrium with c1-0~~or the less negatively charged form HCr04-. Clearly, additional experimental tests are needed to ascertain whether the behavior of chromium can be reconciled with the ionic redistribution model or if the model itself needs refinement. Practical Implications. Working curves for Cu2+were found to be linear with and without 2 mM SDS (Table I). The surfactant increased the sensitivity (the slope of the working curve) of the determination by 44%. Higher flow rates can increase the enhancement further. While surfactant concentrations just below the CMC give maximum enhancement, it must be emphasized that the absorbance dependence on surfactant concentration is also greatest near the CMC. Rather than trying to avoid surfactants altogether (as suggested by Venable and Bellad ( 4 ) )or to optimize the absorbance by using surfactant concentrations near the CMC, it may be preferable to swamp out the effect by adding large quantities of the surfactant such that the concentration is much larger than the CMC. Some sensitivity enhancement may still be observed (up to 20%), but precise control of surfactant concentration is not critical.
ACKNOWLEDGMENT The authors thank R. E. Boehm of Georgetown University for useful discussions and for assistance with the diffusion calculations. LITERATURE CITED (1) Dean, J. A., Ralns, T. C., Eds. "Flame Emission and Atomic Absorption Spectrometry";Marcel Dekker: New York, 1969; Vol. 1 (Theory). (2) Kodama, M.; Shimizu, S.; Sato, M.; Tominaga, T. Anal. Left. 1977, 10, 591. (3) Kodama, M.; Miyagawa, S. Anal. Chem. 1980, 52, 2358. (4) Venable, R. L.; Ballad, R. V. Anal. Chern. 1974, 4 6 , 131. (5) Foster, W. H.; Hume, D. N. Anal. Chem. 1959, 3 1 , 2028. ( 6 ) Lockyer, R.; Scott, J. E.; Slade, S. Nature (London) 1981, 189, 830. (7) Dean, J. A. "Flame Photometry"; McGraw-Hill: New York, 1960. (8) Pungor, E.; Mahr, M. Talanta 1983, 10, 537. (9) Boroweic, J. A,; Boorn, A. W.; Dillard, J. H.; Cresser, M. S.; Browner, R. F. Anal. Chern. 1980, 52, 1054. (10) Tanford, C. "The Hydrophobic Effect", 2nd ed.; Wiley: New York, 1980. (11) Ziemicki, H.; Cherry, W. R. J . Am. Chern. SOC. 1981, 103, 4479. (12) Newberry, J. E. J . Colloid Interface Sci. 1980, 7 4 , 483. (13) Einstein, A. Ann. fhys. (Leiprig) 1805, 17, 549. (14) Kamenka, N.; Lindman, B.; Brun, B. Colloid folym. Scl. 1974, 252, 144.
Anal. Chem. 1982, 5 4 , 1329-1332 (15) Novak, J. W.; Browner, IR. F. Anal. Chem. 1980, 52, 287. (16) Browner, R. F., Georgia [nstltute of Technology, Atlanta, GA, communicatlon, 1981.
personal
RECEIVED for review December 24, 1981. Accepted April 8,
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1982. Acknowledgment is made to the donors of the Petrolem Research Fund, administered by the b e r i c a n Chemical Society, for partial support of this research. Support from the Research Corporation and from the University of Illinois Campus Research Board is also gratefully acknowledged.
Determination of Rare Earth Elements in Geological Materials by Inductively Coupled Argon Plasma/Atomic Emission Spectrometry J.
G. Crock” and F. E. Llchte
U.S.Geological Survey, Analytical Laboratories, M.S.928, Denver Federal Center, Denver, Colorado 80225
Inductlvely coupled arlgon plasma/optlcal emlsslon spectrometery (ICAP/OES) Is useful as a slmultaneous, multlelement analytlcal technlque for the determlnation of trace elements In geologlcal mateslals. A method for the determlnatlon of trace-level rare earth elements (REE) In geologlcal materials uslng an ICAP 68-channel emlsslon spectrometer is descrlbed. Separation aind preconcentratlon of the REE and yttrium from a sample tilgest are achleved by a nltrlc acld gradlent cation exchange and hydrochlorlc acld anlon exchange. Preclsion of 1-4 % relative standard devlatlon and comparable accuracy
