2358
Anal. Chem. 1980, 5 2 , 2358-2361
Enhancing and Suppressing Effects of Surfactants in Atomic Absorption Flame Spectrometry Michiko Kodama' and Seinosuke Miyagawa Department of Chemistry, Faculty of Science, Kwansei Gakuin University, Nishinomiya Hyogo, 662 Japan
The role of the surfactants sodium dodecyl sulfate and dodecyltrimethylammoniumchloride in atomic absorption flame spectrometry is discussed with emphasis on droplet size in aerosols produced by the pneumatic nebulizer. An experlmental determination of droplet size distributions indicates that the marked effectiveness of the surfactants in improving the sensitivity and reducing interference in the determination of chromium is related to productionof finer aerosols which result from a depression of the surface tension of sample solutions containing surfactants.
The effect of an aerosol generated by the pneumatic nebulizer on analytical sensitivity and chemical interferences in atomic absorption flame spectrometry has been investigated by many workers, and the importance of fine aerosols for efficient atomization of analyte metals has been particularly noted (1-4). The effectiveness of organic solvents in atomic absorption spectrometry has been demonstrated ( I , 2, 5 , 6 ) while surfactants, which are known to depress the surface tension of aqueous solutions, have been reported to have no effect ( 4 , 5 ) . In our previous work (7), we found that sodium dodecyl sulfate (SDS),one of the most typical anionic surfactants, acts not only to enhance absorption but also to suppress interferences. T o elucidate the effects of SDS, we determined droplet size distributions in aerosols injected into the flame from the most widely used type of premix burner. The results are discussed in terms of the physical properties of the sample solution according to Nukiyama and Tanasawa's equation for the production of droplets (8). The effectiveness of dodecyltrimethylammonium chloride (DTAC), used as a cationic surfactant, was investigated and compared with SDS.
EXPERIMENTAL SECTION Reagents. Stock solutions were prepared by using analytical grade reagents containing 100 g/m3 of each metal (Wako Chemical Reagents Co. and Nakarai Chemical Reagents Co.) Stock solutions
of Cr(II1) and Cr(V1) were prepared by dissolving CrC13 and K2Cr207,respectively, in water. For the preparation of stock solutions of metals such as Ca(II), Mg(II), Mn(II), Fe(III),Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Al(III),nitrates and chlorides of these metals were dissolved with 1 mol/m3 of either nitric or hydrochloric acid. The surfactants SDS and DTAC were obtained from Tokyo Kasei Chemical Reagents Co. and used after adequate purification: SDS was recrystallized several times from water and butyl alcohol, respectively (9);DTAC was recrystallized several times from a mixture of acetone and alcohol at a volume ratio of 5050 (IO). Aqueous solutions (300 mol/m3) of each surfactant were prepared. Procedure and Apparatus. Stock solutions and surfactant solutions thus prepared were used to makeup the test solutions. The dilute standard solutions of chromium were made up immediately before use. To investigate the effect of the concentration of the surfactants on chromium absorption, we prepared test solutions, containing various concentrations of SDS ranging from 0 to 60 mol/m3 together with 1 g/m3 chromium. The interference-suppressing effects of the surfactants on the Cr(V1) 0003-2700/80/0352-2358$01 .OO/O
absorption were investigated with test solutions containing 1 g/m3 Cr(V1) and 20 g/m3 of each interferent in the presence of the surfactants at a concentration of 50 mol/m3. A Nippon Jarrel-Ash Model AA-1 atomic absorption spectrometer was used with a premix, 5-cm slot burner and an airacetylene flame. Hollow cathode lamps (Hamamatsu T V Co.) were used as light sources for atomic absorption of the analyte metal. The optimum flame condition giving the maximum absorption signal was determined by varying the height of the optical path above the burner and the flow rate of acetylene, maintaining the air flow rate constant at 6.5 dm3/min. Percent absorption was measured throughout this work. In solutions of surfactants, micelle formation occurs above a certain concentration, which has been termed the critical micelle concentration (CMC). The CMC of the surfactants used in this study was determined by using a Yanagimoto Model MY-7 electric conductivity measurement apparatus. Conductivitywas measured in a water-thermostat at 25 "C. Size distribution of droplets in an aerosol was determined from their photomicrographs taken with an Olympus Model FHT and EHT microscope and its accessory camera. The apparatus for catching droplets is shown in Figure 1 (11). When the piston of the syringe (B) was drawn out, the aerosol that was sprayed through the burner with the flame off is drawn in through the opening (D) and the droplets are caught on a thin immersion oil film spread on a cover glass (C). The droplets were allowed to settle down on the cover glass for at least 20 min. The photomicrographs were enlarged 350 times, and the diameter of each droplet was measured. The number of droplets per photomicrograph was about 1W-120, and a histogram of the droplet size distribution was prepared from five photomicrographs.
RESULTS A N D DISCUSSION I t is well-known that the CMC of a surfactant is greatly influenced by impurities, either electrolytic or nonelectrolytic (12). Results of conductivity measurements of the aqueous solutions of purified SDS and DTAC in the presence of 1 g/m3 of Cr(VI) are shown in Figure 2, where electrolytic conductivity ( K ) is plotted against the concentration of SDS and DTAC. A breakpoint corresponding to the CMC appears a t about 8 mol/m3 for SDS and 21 mol/m3 for DTAC (9). In our previous work (7) it was demonstrated that the greatest enhancement of Cr(V1) absorption caused by SDS was observed with the flame condition giving the highest absorption signal when using the standard containing only Cr(V1). The same phenomenon occurred when DTAC was used instead of SDS. Under the optimum flame condition, i.e., 13-mm flame height above the burner and 2.5 dm3/min acetylene flow rate, about 100% maximum enhancement of Cr(V1) absorption by the surfactants was obtained; and the optimum flame condition did not vary with the concentration of the surfactants. Figure 3 shows the effect of the concentration of the surfactants on the atomic absorption of 1 g/m3 chromium(V1). Chromium(VI) absorption gradually increases with the concentration of SDS and DTAC up to each CMC, shown in Figure 2, and then becomes nearly constant at levels which are twice as large as those without the surfactants. The effects of the surfactants on Cr(II1) were the same as on Cr(V1). Figure 4 shows photomicrographs illustrating the examples of aerosols in the absence and in the presence of SDS as a @ 1980 American Chemical Society
!/::k,I.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2359
rc
A
3Flgure 1. Device for catching droplets: (A) cylinder, (B) 5 k m 3 Syringe, (C) cover glass, (D) funnel-type opening. A cover glass is coated with a thin immersion oil film.
~
2
,
, , ,, ,
lo
0
50,
9
18
27
9
0
Id
18
27
0
!I
18
27
r e
I
t
c (mo~/rn')
Flgure 2. Electrolytic conductivities of SDS (a) and DTAC (b) solutions in the presence of Cr(V1). C is concentration of SDS and DTAC. k is electrolytic conductivity. The concentration of Cr(V1) is constant (1 g/m3). A breakpoint corresponds to the CMC of each surfactant.
dt 70
1
1
Figure 5. Droplet size distributions of aerosols: (a) in the absence of svfactant, (b) in the presence of SDS (2 mol/m3), (c) in the presence of SDS (12 mol/m3), (d) in the presence of SDS (50 moi/m3),(e) In the presence of DTAC (5 mol/m3), (f) in the presence of DTAC (50 mol/m3). d is diameter of droplet. ~
Table I. Droplet Size Mean Diameter,
i
30f
T 0 10 20 30 40 50 60
Cs:
c (rnob'rr 3 , Flgure 3. Effect of the concentrations of SDS (a) and DTAC (b) on the atomic absorption of Cr(V1). Cis concentration of SDS and DTAC. The concentration of Cr(V1) is constant (1 g/m3).
a
mo1/m3 0 2
b a
d , pm
2
in the presence of DTAC
in the presence of SDS
c D , b mol/m3
14.3 10.8
Cs = concentration of SDS.
d , pm
0 5
14.3
CD
= concentration of
9.8
DTAC.
-
100um
Flgure 4. Photomicrographs magnified 130X of aerosols in the absence (a) and in the presence (b) of SDS at a concentration of 50 mol/m3.
comparison. Typical patterns of the droplet size distribution obtained from the photomicrographs are shown in Figure 5, where the droplet diameter is grouped a t intervals of 3 pm over the range of 0-27 pm. The fraction (in percent) of droplets in each group is the ordinate. The distribution
patterns in the presence of the surfactants are represented by two types below and above the CMC, regardless of the type of surfactants used in this study, as shown in Figure 5. T h e distribution pattern, a t a considerably lower concentration than the CMC, approached that of the water aerosol, while the distribution pattern above the CMC was almost independent of the concentration of the surfactant,. It is apparent from Figure 5 that aerosols in the presence of the surfactants contain a smaller number of large-diameter droplets, compared to the water aerosol. This phenomenon becomes more prominent above the CMC than below the CMC. Thus,above the CMC, the number of droplets smaller in diameter than 6 pm exceed 70% of the total number of droplets and the droplets larger than 1 2 pm are less than 2% of the total. The mean (volume/surface) diameter was estimated from the droplet size distribution ( I , 4 , 1 3 ) and the result is shown in Table I. T h e mean diameter below the CMC of SDS decreases with an increase in the concentration of SDS and that above the CMC gives excellent agreement. T h e substitution of DTAC for SDS produced the same result. T h e mean diameter for the water aerosol, shown in Table I, agreed closely with that obtained by Stupar and Dawson (4).
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2360
Table 11. Effect of Concomitants on Chromium(V1) Absorption in the Absence and Presence of DTAC ( 5 0 m01/m3) absorption in the in the absence of presence of DTAC DTAC 30.0 59.5 42.0 59.5 51.2 59.5 31.8 59.5 30.8 59.5 11.6 57.4 31.6 59.7 14.4 58.5 33.6 59.5 16.8 58.5 55.8 59.5 42.8 59.5 36.2 59.5 42.8 57.0 %
concomitanta none 0
-
Mg Ca
0 3
10
20
30
Lo
50
6C
C(rnol/m 3)
Figure 6 . Density, viscosity, and surface tension of SDS solution: C, concentration of SDS; p , density; 7 , surface tension; 11, viscosity. All measurements were made at 25 OC.
The most representative equation for the droplet production was given by Nukiyama and Tanasawa (8)
do = 585( -Y)"~ V
+ 597
(
)"'"(
9 (YPS'2
103-
(1)
where do = mean diameter (pm), v = a difference in velocity between air and liquid (m d),p = density (g ~ m - ~y) = , surface tension (g s - ~ ) ,7 = viscosity (g cm-' s-l), Q1 = uptake rate of liquid (cm3SS'), Q, = flow rate of air (cm35 8 ) . This equation indicates that do depends on the physical properties of the liquid being aspirated and on the nebulizer construction which relates to the values of v, Q1, and Q,. For a nebulizer operating under a constant air flow rate condition, eq 1 demonstrates that a variation of the mean diameter may be attributed to changes in the solution parameters, viscosity, density, surface tension, and also uptake rate which depends upon the viscosity (4, 8). One of the distinctive features of the surfactant is a depression of the surface tension of the solution (14). As shown in Figure 6, the surface tension of the aqueous solution of SDS decreased with the concentration of SDS up to the CMC and then assumed a nearly constant value above the CMC. For investigation of the effect of the physical properties of the SDS solution on the droplet production, the mean diameters for the different concentrations of SDS a t a constant air flow rate of 6.5 dm3/min were calculated approximately from eq 1 with the viscosity, density (15),and surface tension (Figure 6) and uptake rate which varied from 5.0 to 4.4 cm3/min. T h e concentration of SDS ranged from 0 to 50 mol/m3. The calculated values of the mean diameter were as follows: 33 pm a t an SDS concentration of 0 mol/m3, 30 pm a t 5 mol/m3, 28 pm a t 12 mol/m3, 28 pm at 50 mol/m3. Thus, the calculated mean diameter initially decreased with the concentration of SDS and finally gave the expected value in agreement with the experimental result shown in Table I. Furthermore, eq 1 indicated that the variation of the calculated mean diameter was induced by the first term which reflected the surface tension behavior of the SDS solution (Figure 6), while the second term was almost constant for the different concentrations of SDS. It may be concluded that the depression of the surface tension of the solution caused by the surfactants produces the small droplets in the aerosol generated by the pneumatic nebulizer (8). In the atomization process for metal in the flame, the solvent of the droplets introduced into the flame is first evaporated. Then the vaporization of the resultant solid particles follows during their passage at high speed through the flame. It has therefore been recognized that the production of free atoms is greatly governed by the vaporization rate of the solid particles, which depends on flame temperature, flame nature, particle size, and so on (1, 3, 4 ) . In this study, the injection of the sample solution containing the
Mn(I1) Fe(111) Fe(III), NO; Co( 11)
Co(II), NO,Ni(I1) Ni(II), NO,' Cu(I1) Zn( 11) Cd(I1) Al(II1)
a Concentration of all added concomitants, 20 g/m3. All added concomitants were present as chloride salts unless otherwise specified,
surfactant into the flame might be expected to increase the reducing nature of the flame. However, the greatest enhancement of chromium absorption caused by SDS appears a t the flame condition which gives the highest absorption signal (7). The same magnitude of enhancement above the CMC is found as illustrated in Figure 3. These phenomena probably mean that the enhancing effects of the surfactants are not due to the increased reducing atmosphere of the flame. When comparing Figure 3 to Table I, it will be noted that the chromium absorption depends on droplet size. These experimental results point out the importance of small droplet size in the atomization of chromium. The smaller droplets brought about by the surfactant result in smaller solid particles and, consequently, an increased rate of vaporization in the flame. This seems to be the reason for the enhancement of chromium absorption by the surfactants. Furthermore, the effect of SDS on atomic absorption of Zn, Cu, Ni, and Fe was investigated in comparison with Cr. It was observed that the absorption signals of these metals at the optimum flame conditions are enhanced by a t least 5-40% and that the magnitude of the enhancement by SDS becomes greater in the order of Fe > Ni > Mn > Cu > Zn, that is, with an approximately decreasing order of the analytical sensitivity of these metals in atomic absorption spectrometry using the air-acetylene flame (16). These results may indicate that the atomization of the metal which forms a more stable solid particle in the flame is strongly influenced by the droplet size. About 100% enhancement of the chromium absorption obtained here may be related to the formation of more stable compounds caused by reaction of chromium atoms with oxygen in the flame (17-19). The first column of Table I1 shows the interference resulta of various concomitants at a concentration of 20 g/m3 on the absorption of 1 g/m3 Cr(V1) under the optimum flame condition. Of the ten concomitants present as chloride salts, all of them provided enhancement of the Cr(V1) absorption. Different results were obtained when the nitrate form of the concomitant was used; Fe, Ni, Co, Mn, Zn, Cu, and Cd gave a depression in Cr(V1) absorption. In particular, Fe, Ni, and Co showed significant depressing effects, as shown in Table 11. The difference in the type of interference between chloride and nitrate salts seems to be related to the intermediates which are formed after the evaporation of the water solvent in the atomization process as pointed out by Fujiwara et al. (18). The chloride is formed when the chloride salt is used,
Anal. Chem. 1980,
while the oxide is formed when the nitrate salt is used. The depressing effect of metals used in the nitrate form on Cr(V1) absorption may occur because the metals are transformed into the refractory oxides mixed with the Cr(V1). On the other hand, the metals used in the chloride form are transformed from the chlorides into atoms, some of which are subsequently converted into oxides by combining with oxygen in the flame; therefore, conversion of the Cr(V1) atom into the oxide is depressed. The enhancement of Cr(V1) absorption may occur a t the expense of the production of coexisting metal atoms. Calcium, Mg, and Al, not only in the chloride form but also in the nitrate form, enhanced Cr(V1) absorption. The result seems to be connected with the fact that these metals are remarkably stabilized by formation of the oxides, as compared with other metals. Changes in flame condition gave changes in the degree of interference, but no change in the type of interference, depression, or enhancement. Serious interferences were observed under fuel-rich flame conditions which gave a high absorption signal for Cr(V1). Under the hotter, fuel-lean flame condition a t an acetylene flow rate of 1.25 dm3/min, the interferences were almost eliminated. When the same concomitants were present with either SDS or DTAC, each interference on Cr(V1) absorption was almost eliminated. T h e interference-suppressing effects of DTAC under the optimum flame condition are summarized in the second column of Table 11, where results are similar to those obtained by use of SDS in our previous work (7). It is also apparent that the interference-suppressing effects of the surfactants are related to their enhancing effects which are not disturbed by the presence of the concomitants. As seen in Table 11, the surfactant shows a larger enhancement in Cr(V1) absorption than each concomitant. Thus, when the concomitant coexists with the surfactant, the enhancing effect of the concomitant may be masked by that of the surfactant, and consequently, only the enhancement caused by the surfactant will be observed. On the other hand, the suppressing effects of the surfactants on depressions in Cr(V1) absorption
52,2361-2365
2361
caused by the concomitants in the nitrate form may be attributed to the increase in the rapidity of vaporization of the refractory compounds, the size of which is reduced by the surfactants. The interference-suppressing effects of the surfactants were complete for all flame conditions.
ACKNOWLEDGMENT We express thanks to Professor Kenji Yamaji of Kwansei Gakuin University for his guidance in taking photomicrographs and also for the apparatus to catch droplets. We thank Professor Masaji Miura of Hiroshima University for his measurement of surface tension. We also thank Ms. Diane Lawrence of the Perkin-Elmer Corp. who edited this manuscript for this journal.
LITERATURE CITED Dean, J. A.; Carnes, W. J . Anal. Chem. 1982, 34, 192. Allan, J . E. Spectfochim. Acta 1981, 17, 467. Willis, J. B Spectfochim. Acta, PartA 1987, 23A, 811. Stupar, J.; Dawson, J . B. Appl. Opt. 1988, 7 , 1351. Lockyer, R.; Scott, J. E.: Slade, S. Nature (London) 1981, 189,830. Robinson, J . W. Anal. Chim. Acta 1960, 23, 479. (7) Kodama, M.; Shimizu, S.; %to, M.; Tominaga, T . Anal. Lett. 1977, 70, 591. (8) Nukiyama, S.;Tanasawa, Y. Nippon Kikai Gakkai Ronbunshu 1939, 5 ,
(1) (2) (3) (4) (5) (6)
68.
(9) Miura, M.: Kodarna, M. Bull. Chem. SOC.Jpn. 1972, 45,428. (IO) Ekwall, P; Mandell, L.; Fontell, K. J . CollaidInterfaceSci. 1989, 29, 639. (11) Horri, K. "Studies of Fogs"; Kozima, K., Ono, T., Yamaji, K., Eds.; (12) (13) (14)
(15) (16) (17) (18) (19)
Institute of Low Temperature Science, Hokkaldo University: Satsuporo. 1953; p 303. Shinoda, K.; Nakagawa, T.; Tamamushi. B.; Isemura, T. "Colloidal Surfactants"; Academic Press: New York, 1963; p 58. Mugele, R. A.; Evans, H. D. Ind. Eng. Chem. 1951 43, 1317. Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. "Colloidal Surfactants"; Academic Press: New York. 1963; p 181. Kodama, M.; Miura, M. Bull. Chem. SOC.Jpn. 1972, 45, 2265. Takeuchi, T.; Swuki, M. "Atomic Absorption Spectrochemical Analysis"; Nankoudo: Tokyo, 1969; p 70. Ottaway, J. M.; Pradhan, N. K. Talanta 1973, 20, 927. Fujiwara, K.; Haraguchl, H.; Fuwa, K. Anal. Chem. 1975, 47, 743. Hurlbut, J. A,; Chriswell, C. D. Anal. Chem. 1971, 43, 465.
RECEIVED for review May 17,1979. Accepted August 20,1980.
Determination of Ultratrace Ammonium, Nitrite, and Nitrate Nitrogens by Atmospheric Pressure Helium Microwave-Induced Plasma Emission Spectrometry with Gas Generation Technique Kiyoshi Tanabe, Kazuko Matsumoto, Hiroki Haraguchi,
and Keiichlro Fuwa
Department of Chemistry, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Ammonium, nitrite, and nitrate nitrogens were generated as nitrogen gas from the solutions and introduced into the plasma. This method made It possible to determine nanogram per milliliter level of these nitrogen compounds.
Ammonium, nitrite, and nitrate nitrogens are found in soils, waters, biological substances, and other materials. The determination of nitrogen in these chemical forms, especially in water, is of importance because these nitrogen compounds have great influences on biological activities and environment. The conventional methods such as titration, azotometry, and colorimetry have usually been used for the determination of the nitrogen compounds. 0003-2700/80/0352-2361$01 .OO/O
Recently, spectrometric methods for the determination of ammonium nitrogen using molecular emission or absorption have been reported (1-7). Cresser has utilized the ultraviolet molecular absorption of gaseous ammonia after its liberation from strongly alkaline sample solutions (1-3). Belcher et al. used a similar generation technique in molecular emission cavity analysis and observed the emission from NH2 species at 500 nm (4).Butcher and Kirkbright observed the emission from N H species in hydrogen-nitrogen diffusion flame by use of a similar generation technique ( 5 ) . Alder, Gunn, and Kirkbright introduced nitrogen gas which was evolved by the hypobromite oxidation reaction to an inductively coupled plasma and subsequently observed the emission from nitrogen-containing species (6). In general, these spectrometric methods are rather convenient, compared with other con1980 American Chemical Society
