JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 139 Effect of Long-chain Surfactants on Drop Size Distribution Transport Efficiency and Sensitivity in Flame Atomic Absorption Spectrometry With Pneumatic Nebulization* Juan Mora Antonio Canals and Vicente Hernandis Division de Quimica Analitica Universidad de Alicante 030 71 Alicante Spain The variations of the droplet size distribution transport efficiency and absorbance which arise as a consequence of the addition of long-chain surfactants to aqueous solutions of Mnll were studied. The results show that the drop size distribution of the primary aerosol does not change on increasing the surfactant concentration irrespective of whether cationic or anionic long-chain surfactants are used. Concomitantly neither transport efficiency nor absorbance change.A mechanism is also suggested in order to explain these results based on the following points ( i ) surfactant molecules require a certain period of time in order to redistribute on the new surface that is being generated during the nebulization step; and (ii) this time increases as the length of the hydrophobic chain increases. Keywords Surfactant; flame atomic absorption spectrometry; drop size distribution; transport efficiency The most common method of sample introduction in atomic spectrometry is through the pneumatic nebulization of sample solutions. In the nebulization step a primary aerosol is generat- ed the characteristics of which will have a great influence on the signal intensity (absorbance) and the degree of some inter- ferences.Obviously in order to obtain a high transport efficiency and a decrease in the interferences it is very important that the primary aerosol be as fine as possible for a given set of gas and liquid flow-rates. Among the physical properties of the solution surface tension is probably the most influential on the characteristics of the primary aerosol. Owing to the high value for the surface tension of water many workers have suggested the addition of surfactants to the aqueous solutions in order to decrease their surface tension and hence. to improve transport efficiency and/or atomization efficiency. Some workers have found noticeable sensitivity im- provements,'-I however others have found little or none.I2-I4 It is clear that there is a certain controversy about this matter.Several mechanisms sometimes contradictory have been proposed in order to explain the sensitizing effect of the sur- factants in flame atomic absorption spectrometry (FAAS). Kodama and Miyagawa7 observed that an increase in surfac- tant concentration yields finer aerosols until the critical micel- lization concentration (CMC) is reached. These finer aerosols yield finer solid particles in the flame thus giving rise to an in- crease both in transport and in atomization efficiencies. Ac- cording to these workers results there is no influence from the surfactant or analyte charge [sodium dodecyl sulphate (SDS) or dodecyltrimethylammonium chloride Cr"' or CrVIJ. proposed a model which is an enlarge- ment of the aerosol ionic redistribution model previously intro- duced by Borowiec ef a1.,Is to explain the improvement of the sensitivity they observed on adding increasing amounts of SDS to solutions of Cu". They claimed to find sensitivity im- provements of up to 44% for SDS concentrations slightly below the CMC.Similar results were obtained for other anionic surfactants. The mechanism they proposed assigned a great influence to the surfactant charge. Thus with the analyte used neither cat ionic (hexadec y 1 trime t h y lammon i um bromide CTAB) nor non-ionic (Triton X- 100) surfactants modify the analytical signal although the surface tension values reached in each instance are very similar. The mechanism they pro- posed to account for this signal improvement is based on the migration of surfactant molecules to the droplet surface.If the Komahrens e f * Presented at the Fifth Biennial National Atomic Spectroscopy Sympo- sium (BNASS) Loughborough UK I8th-20th July 1990. ionic (hydrophilic) ends of the surfactant molecules are oppo- site in charge to the analyte ions then the analyte ions will tend to associate with the surfactant molecules and the surface of the large droplets will become analyte enriched. When these large droplets break apart the smallest droplets formed will be analyte enriched. As these small droplets are more effectively sampled a signal enhancement is observed. Venable and Ballads explained the signal variations in the presence of surfactants by the ability of some of them to keep the analyte in solution under conditions where otherwise it could precipitate. Recently Yan and Zhangii have proposed a mechanism based on the formation of reverse micelles during the nebuliza- tion process.If the hydrophilic chain of the surfactant is oppo- site in charge to the analyte then the latter will be attached to the hydrophilic end of the surfactant within the droplets. The micro-environment created by the surfactant molecules forming the micelle around the analyte ion will cause the atomization efficiency to increase because of the reduction processes involving the products of the decomposition of those molecules thus giving an improvement in the sensitivity. In this model the charges of the surfactant and analyte contribute to the improvement. In general cationic surfactants give rise to improvements in the signal owing to anionic analytes and \,ice wi-sa whereas non-ionic surfactants have no effect.These workers state that the surface tension decrease is of no significance in the improvement. Farino and Browner,' on comparing the behaviour of organic solutions with that of aqueous solutions of surfactants concluded that in FAAS the sensitizing effect depends on the nebulizer-spray chamber design more than on the surfactant charge. For instance using a paddle spray chamber they found signal enhancements of between 0 and 8% whereas with an impact bead spray chamber the enhancements were between 0 and 20%. From all these examples it appears that there is a certain controversy among the published results not only on the mag- nitude of the sensitizing effect but also on the significance of the surfactant and analyte charges or on the enhancement mechanism (transport or atomization). Another point to be noted is that in many of these papers some relevant informa- tion is missing and one or several important parameters have not been properly controlled.Therefore this paper aims to clarify the mechanism of the surfactant effect in FAAS. To this end the drop size distribu- tions of the primary aerosols generated in the nebulization step the transport efficiencies for analyte and solvent and the analytical signal have been measured.140 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 Manganese(l1) was chosen as the analyte because in FAAS the sensitivity of this cation is only slightly altered by the com- position of the flame.Thus minor variations in the nature of the flame caused by the presence of the surfactant molecules will not significantly affect the efficiency of the analyte atomi- zation. The surfactants employed are two typical long-chain surfac- tants one anionic (SDS) and the other cationic (CTAB). Experimental Aqueous solutions of Mn" 2 pg g-' in 1% v/v HCl were em- ployed throughout. The surfactants were SDS and CTAB both from Fluka with a degree of purity of about 98%. An adjustable concentric nebulizer (Perkin-Elmer) locked at the position of maximum interaction between gas and liquid was used in all the experiments. Paddles were used as impact surfaces within the spray chamber. Liquid flow was controlled by means of a peristaltic pump (Gilson Minipuls 2) and kept constant at 4.5 ml min-I throughout the experiments.The addition of surfactants to aqueous solutions might modify their viscosity which in turn might modify the uptake rate. The use of a peristaltic pump avoids this potential problem. It was considered that it might be the lack of control of the uptake rate that may effectively modify the experimental results. However the pumping rate chosen lies close to the natural (normal) uptake rate of the neb- ulizer operated at 5.65 1 min-' with air. When evaluating the effect on the nebulization step of increasing surfactant concen- tration it was necessary to ensure that the liquid and gas flow- rates were kept constant. The air flow for nebulization was kept constant at 5.65 I min-I by means of a precision flow meter (Cole-Pamer).Similarly total air flow and gas flow were kept constant throughout the experiments. Transport efficiencies the percentage of solvent (E,) and analyte (q,) taken up that reaches the flame were measured by an indirect method (in the batch version). A drawing of the device employed is shown in Fig. I in the drain collec- tion position. Firstly the solution K is sprayed for 2-3 min in order to condition the spray chamber A. During this time the valve C is placed in the drain waste position. In this po- J r l Fig. 1 Device employed for the measurement of the transport efticiency by the indirect method (shown in drain collection position). A. Spray chamber (front view) B. nebulizer C . drainage outlet of the spray chamber D. drainage waste outlet E.drainage collection outlet; F. tube to allow the loop to continue being the closure system of the spray chamber during the drainage collection period; G. valve; H. tube for drainage col- lection; 1. loop J . waste K. solution and L. peristaltic pump sition the drainage flows through tube C to tube D then to the loop I and finally on to waste J. After this the valve is turned through 90" in the clockwise direction to the drain col- lection position and the drainage is collected in a previously weighed tube H. When in this position the drainage flows by gravity through tubes C and E to the drain collection tube. The tube F serves to connect the spray chamber to the loop which then continues to act as the closure system of the spray chamber and thereby damps possible pressure variations in the spray chamber.With this system the spray chamber works the same way irrespective of the valve position. The drainage is collected for 10-15 min after which the valve is switched back through 90" in the counterclockwise direction tlo the waste position again. The collection tube is disconnect- ed and re-weighed. From these results the values for E and the total solvent transport rate (Stol) i.e. the amount of solvent reaching the flame (in ml min-I) are calculated. The c value and the total analyte transport rate (Wtot) value i.e. the amount of analyte reaching the flame (pg min-I) are ob- tained by comparing the absorbances of the drained with the original solutions. Drop size distributions for the primary aerosols were meas- ured 28 mm from the nebulizer tip by means of a laser Fraun- hofer diffraction system (Malvern Instruments Model 2600~) the measurement range of which was 1.9-188 pm.The calcu- lations were performed with a model-independent algorithm using software version M5.4.I6 A Perkin-Elmer 373 atomic absorption spectrometer equipped with a hollow cathode lamp was used. The instru- mental conditions used are shown in Table 1. Results and Discussion Drop Size Distributions for the Primary Aerosols 'The drop size distribution (in volume) of the primary aerosols obtained with water and with the highest surfactant concentra- tions above their respective CMC are shown in Figs. 2 and 3. From these results it is apparent that under the experimen- tal conditions employed the distributions are not significantly different from one another.This means that the surfactants em- ployed are ineffective in modifying the aerosol drop size distri- butions. The same conclusions can be drawn from Table 2 which shows the most significant distribution parameters of the primary aerosols obtained with these solutions although there are important decreases in their surface tension values as the surfactant concentration increases. Obviously the surface tension values measured under surface equilibrium conditions (as determined here) are ineffective for the generation of a greater surface area. However in Table 2 some slight though unequivocal ten- dencies can be observed. There are slow increases in droplet size and obscuration with increasing surfactant concentration for both SDS and CTAB.The explanation for these behavi- ours cannot be linked to the surface tension decrease (as then the size tendency would be just the opposite) or the small viscosity increase as a coarser distribution would give rise to lower obscuration values (the amount of light diffracted Table 1 Instrumental conditions used for the FAAS determinations Parameter Setting Wavelength Slit-width Lamp intensity Height above burner Acetylene flow-rate Air flow-rate (total) Air flow-rate (nebulizer) Integration time 279.5 nm (Mn I ) 0.2 nm 35.0 mA 8.0 mm 2.7 I min-' 19.6 I min-' 5.65 1 min-' 5.0 sJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 141 0 1 2 3 4 5 Ln ( i/p m I 0 1 2 3 4 5 Ln ( d/p m I Fig. 2 tions of variable concentration.A 0 mmol dm3; and B 1 .S mmol dm-j Drop size d distribution of the primary aerosol for CTAB solu- by a given volume of sprayed solution increases as the mean drop size of the distribution decreases). In our opinion the variations are a result of the fact that once a droplet has been generated the surfactant molecules cover its surface almost completely (depending on the surfactant concentra- tion). This causes the solvent evaporation rate to diminish as the surfactant concentration increases. This is related to the hydrodynamic effect of the surf act ant^.'^ Therefore if the be- haviour of two solutions are compared one free of surfac- tants and the other with a given surfactant concentration both would have the same distribution at the outlet of the nebulizer nozzle but then the droplets of the first aerosol will undergo a relatively rapid initial evaporationIx that leads to a small size reduction on their way towards the laser beam (placed 28 mm from the nebulizer tip) whereas the droplets of the second aerosol would hardly undergo any variation in their size because their evaporation rate is much slower.Fig. 3 of variable concentrations. A 0 mmol dm-? and B 8.5 mmol d m 3 Drop size d distribution of the primary aerosol for SDS solutions Surface Tension Effect Surface tension values for aqueous solutions of surfactants are static values when obtained under surface equilibrium condi- tions. These values might not apply in fast processes such as pneumatic nebulization which take place in milliseconds or less.For this reason it might be advisable in these instances to refer to the ‘dynamic’ surface tension values.I9 In surfactant solutions surface tension is a property in which a certain period of time is required for the surfactant molecule to migrate from the bulk of the liquid or from the previously ex- isting surface to the surface that is now being generated and to distribute and orientate on it.?() Thomas and Potter” state that SDS requires about 10 ms to migrate to the new surface and to distribute on it in order to modify the surface tension of the solution effectively. WestI9 found that an increase in Triton X- 100 concentration does not noticeably affect the emission signal of Ca when using a direct injection burner Table 2 Parameters of the drop size distributions for the primary aerosols of the surfactant solutions Surface cm*/ tension/ DW+/ D4.# D3.N Distribution mmol d d N m-’ x lo3 Pm Pm Pm span1 Obscuration I1 CTAB- 0.1880 0 70.43 14.2 17.6 8.2 I .8 0.2 0.4 0.8 1.5 SDS- 0 2.0 4.0 7.5 8.5 40.93 35.95 34.22 29.86 70.43 5 1.68 44.39 35.92 34.23 4.4 4.5 4.7 4.8 4.2 4.3 4.6 4.9 5 .o 18.4 18.6 18.4 19.5 17.6 17.3 18.0 19.2 19.8 8.3 8.2 8.3 8.4 8.2 8.I 8.3 8.4 8.5 1.9 1.9 1.8 1.9 1 .8 1.8 I .8 I .9 3.0 0. I985 0.1974 0.20 I6 0.2024 0.1880 0.2037 0.207 I 0.2245 0.2272 * (’” = Surfactant concentration. i- Dso = Droplet distribution diameter below which 50% of the cumulative aerosol volume is found. Hence. Dy( and D l o = 90 and 10%. respectively. $ DJ,3 = Statistical diameter the diameter of the droplet whose mass is equal to the average mass of the distribution (mass mean diameter).8 DJ.? = Statistical diameter. the diameter of the droplet whose surface is equal to the average surface of the distribution (surface mean diameter). also 1 Distribution span [(D% - DIo)/Ds(,]. A measurement of the distribution spread. II This value is based on the measured laser intensity and indicates the proportion of light which is being scattered out of the beam by the sample. I t will known as Sauter mean diameter. depend on the amount of sample added (see reference 16).142 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 Table 3 Values of the transpon parameters and absorbance for the surfactant solutions CTAB- 0 70.43 0.4 1 9. I 0.4 1 4.6 0.1 so 0.8 34.22 0.39 8.7 0.37 4.1 0.153 I .s 29.86 0.40 8.9 0.39 4.3 0.IS2 0.2 40.93 0.42 9.3 0.43 4.8 0.149 0.4 35.95 0.4 1 9.1 0.4 I 4.6 0.149 SDS- 0 70.43 0.4 1 9. I 0.4 1 4.6 0.1 50 2.0 5 1.68 0.43 9.5 0.45 5.0 0. I47 4.0 44.39 0.44 9.7 0.47 5.2 0. IS6 7.5 35.92 0.4 1 9.1 0.4 1 4.5 0. I53 8.5 34.23 0.39 8.7 0.37 4.1 0.149 although the surface tension values show a net decrease. The explanation being that the surfactant molecules have insufficient time to reach equilibrium on the new surface during the time that the primary aerosol is formed. Addison,’* working with Cs-Cx aliphatic alcohols found that the time necessary to reach a given equilibrium surface tension in- creases with increasing chain length and with decreasing con- centration. The process of pneumatic droplet generation in AAS is clearly too rapid for the molecules of the most common long- chain surfactants to move to the surface being generated and they will not be able to modify the droplet formation process.This is again related to the ‘hydrodynamic effect’ of the sur- factant solutions,~7 which causes a local transient increase in the surface tension when a volume of liquid in contact with the surface is replaced with liquid coming from the bulk of the so- lution. The local surface tension at this moment in time is about the same as for the solvent alone. Hence it is reasonable to assume that the drop size distribu- tion of the primary aerosols for water and for aqeuous solu- tions of long-chain surfactants should be similar except in relation to the evaporation rates as discussed above.Transport The amount of analyte that will finally reach the atomization cell is dependent on the primary aerosol characteristics for a given spray chamber configuration. Of the whole aerosol only the smallest droplets go through the entire spray chamber carried by the gas stream the remainder of the aerosol will be unable to follow the gas flow-lines and will go to waste. It seems clear that if under the same experimental conditions two solutions of equal analyte concentration give rise to similar primary aerosols they will undergo the same type and extent of liquid losses and hence they will transport the same amounts of analyte and solvent to the flame. Table 3 shows that within the poor precision limits inherent in the in- direct method employed for the transport measurement this is true.Thus it can be seen that transport rates do not improve at least to a significant extent when surface tension goes from 0.070 N m-I for water to 0.030 N m-I for 1.5 mmol dm-3 of CTAB. Signal Given that Slol and W, values do not change in the presence of long-chain surfactants (of cationic and anionic nature) at con- centrations of greater and less than their CMC and given that the characteristics of the tertiary aerosol will also remain fun- damentally unchanged,lx the absorbance is unlikely to depend on the surfactant concentration of the solutions. Table 3 shows the results of the absorbance of the solutions at 279.5 nm (,Mn I). As can be seen the absorbance values remain nearly constant irrespective of the nature or concentration of the surfactant.Although the experimental conditions are not in general strictly comparable the results seem to contradict those of other workers,G‘x.l I which have shown noticeable signal im- !provements brought about by the use of long-chain surfac- tants. However some 9.12-14 workers have claimed not to have observed any significant improvement. The results shown in this work agree with those of the second group of workers and suggest a possible explanation for this behav- iour. The use of spray chambers in which re-nebulization is significant could favour the action of surfactants given that the re-nebulization (i.e. at the impact bead surface) can be much slower than the primary nebulization. This could explain the results obtained by Farino and Browner.9 On the other hand it could also be possible for short-chain surfactants in more concentrated solutions to have sufficient time to redistribute on the surface being generated as their equilibration time is shorter; thus they could have a tensioactive effect in the nebulization step.” Although presently embarking on a systematic study on the effect of short-chain surfactants in aqueous solutions we can say that the additon of 1 % m/m of pentanoic acid to an aqueous solution of MnlI yields a net decrease in the mean drop size of the primary distribution and as expected parallel improvements (40-50%) in transport and signal. Conclusions The presence of long-chain surfactants of different charge in variable concentrations decreases the surface tension values of their aqueous solutions but in spite of this they do not modify the characteristics of the pneumatically generated aero- sols.This is attributable to the ‘hydrodynamic effect’ of the surfactants which takes place during the very short period of time required for the nebulization. Surface-active molecules require some time to migrate to the surface. This time increas- es with increasing chain length and with decreasing concentra- tion. With long-chain surfactants this time is so great that they do not influence nebulization. Similar primary aerosols should lead to similar S,, and W,, values and also to similar absor- bance values. The last effect will be true only if the surfactant does not modify the atomization efficiency which seems to be the situation for Mn”.Short-chain surfactants require a higher concentration than long-chain surfactants to reach a given surface tension value,JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991. VOL. 6 143 but their surface equilibration time seems to be shorter. Hence it is possible that short-chain surfactants do really modify primary aerosol characteristics thus influencing transport variables and signal. This point is currently under investigation in our laboratories. The CICYT (Spain) is acknowledged for financial support (Grant No. PB88-0288). J.M. expresses his appreciation to the Instituto de Estudios Alicantinos Juan Gil-Albert (Diputacion de Alicante Spain) for his scholarship. References Nukiyama. S. and Tanasawa Y.. Experiments on Atomi:arion of Liquids in an Air Stream Defence Research Board Department of National Defence Ottawa Canada 1950.Browner. R. F. 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