Anal. Chem. 1990, 62, 1891-1893
Huckel calculations and for her comments regarding this project. Steven L. Dixon is also acknowledged for implementing the software used to generate the partial atomic charges used to develop several of the new descriptors presented in this study. LITERATURE CITED Ewing, D. F. Org. Magn. Reson. 1979, 72, 499-524. Johnels, D.; Ediund, U.; Johansson, E.; Wold, S. J. Magn. Reson. 1983, 55, 316-321. Rudolf, M.; Jordls, U. Chemom. Intell. Lab. Syst. 1989, 5, 323-327. Newmark, R. A. Comput. Chem. 1988, 70, 223-228. Nelson, 0.L.; Willlams, E. A. Prog. Phys. Org. Chem. 1978, 72,
229-342. Bloor, J. E.; Breen, D. L. J. Phys. Chem. 1988, 72. 716-722. Sardella, D. J. J. Am. Chem. Soc.1978, 98. 2100-2104. Martin, G. J.; Martln, M. L.; Odiot, S. Org. Magn. Reson. 1975, 7 , 2-17. Kaichhauser, H.; Roblen, W. J. Chem. Inf. Comput. Sci. 1985, 2 5 ,
103-108. Milne, G. W. A.; Zupan, J.; Heiler, S. R.; Miller, J. A. Org. Magn. Reson. 1979, 72,289-296. Grant, D. M.; Paul, E. G. J. Am. Chem. Soc. 1984, 8 6 , 2984-2989. Lindeman, L. P.; Adams, J. Q. Anal. Chem. 1971, 4 3 , 1245-1252. Ejchart, A. Org. Magn. Reson. 1980, 73, 368-371. Ejchart. A. Org. Magn. Reson. 1981, 75,22-24. Bernassau, J. M.; Fetizon, M.; Mala, E. A. J. Phys. Chem. 1988, 9 0 ,
6129-6134. Bastard, J.; Bernassau, J. M.; Bertranne, M.; Mala, E. R. Magn. Reson. Chem. 1988, 2 6 , 992-1002. Small, 0.W.; Jurs, P. C. Anal. Chem. 1983, 55, 1121-1127. Small, G.W.; Jurs, P. C. Anal. Chem. 1983, 55, 1128-1134. Small, 0.W.; Jurs, P. C. Anal. Chem. 1984, 56, 2307-2314. Egoif, D. S.;Jurs, P. C. Anal. Chem. 1987, 5 9 , 1586-1593. Egolf, D. S.;Brockett, E. B.; Jurs, P. C. Anal. Chem. 1988, 6 0 ,
2700-2706. Sutton, G. P.; Jurs, P. C. Anal. Chem. 1989, 67, 863-871. Ranc, M. L.;Jurs, P. C. Anal. Chem. 1989, 67,2489-2496. Egolf, D. S.Computer-Aided Carbon-13 Nuclear Magnetic Resonance Spectrum Simulation Investigations. Ph.D. Dissertation, The Pennsyivania State University, University Park, PA, 1988. McIntyre, M. K.; Small, G. W. Anal. Chem. 1987, 59, 1805-1811. Small, 0.W.; McIntyre, M. K. Anal. Chem. 1989, 67,666-674. Barber, A. S.;Small, G. W. Anal. Chem. 1989, 67,2858-2664. Breitmaier, E.; Voeiter, W. Carbon-73 NMR Spectroscopy, 3rd ed.; VCH: York. 1987 . - - . .- New . . __._, Wilson, N. K.; Stothers, J. 8. J. Magn. Reson. 1974, 75,31-39. Dailing, D. K.; Ladner, K. H.; Grant, D. M.; Woolfenden, W. R. J. Am. Chem. SOC. 1977. 99. 7142-7150. Ernst, L.; Mannschreck; A. Chem. Ber. 1977, 770,3258-3265. Kitchlng, W.; BuilpM, M.; Gartshore, D.; Adcock, W.; Khor, T. C.; Doddrell, D.; Rae, 1. D. J. Org. Chem. 1977, 42, 2411-2418.
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(33) Caspar, M. L.; Stothers, J. B.; Wilson, N. K. Can. J. Chem. 1975, 5 3 , 1958-1969. (34) Gobert, F.; Combrrisson, S.; Platzer, N.; Ricard, M. Org. Magn. Reson. 1978, 8,293-298. (35) Bullpitt, M.; Kitching, W.; Adcock, W.; Doddreii, D. J. Organomet. Chem. 1978, 776,161-185. (36) Stothers, J. 8.; Tan, C. T.; Wilson, N. K. Org. Magn. Reson. 1977, 9 , 408-413. (37) Hansen, P. E.; Poulsen, 0. K.; Berg, A. Org. Magn. Reson. 1975, 7 , 475-477. .. - . . . .
(38)Ozubko, R. S.; Buchanan, G. W.; Smith, I . C. P. Can. J. Chem. 1974, 52. 2493-2501. (39)Jones, D. Shaw, J. D. Magn. Reson. Chem. 1985, 2 3 , 787-789. (40)Johnson, L. F.; Jankowski, W. C. Carbon-73 NMR Spectra; J. Wiley and Sons: New York, 1972. (41)Hansen, P. E. Org. Magn. Reson. 1979, 72, 109-142. (42)BullpM, M.; Kitching, W.; Adcock, W.; Doddrell, D. J. Organomet. Chem. 1978, 716, 187-198. (43) Berger, S.;Zeller, K. P. Org. Megn. Reson. 1978, 7 7 , 303-307. (44) Storek, W.; Sauer, J.; Stoder, R. 2. Neturforch. 1979, 34A, 1334-1343. (45) Brugger, W. E.; Jurs, P. C. Anal. Chem. 1975, 47, 781-783. (46) Stuper, A. J.; Jurs, P. C. J. Chem. Inf. Comput. Sci. 1978, 76. 99-105. (47) Rohrbaugh. R. H.; Jurs, P. C. MJRAW;Quantum Chemistry Exchange, Program 300, 1988. (48) Stuper, A. J.; Brugger, W. E.; Jurs, P. C. Computer Assisted Studies of
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Chemical Struchve and Ebbgical Function; Wiley-Interscience: New York, 1979;pp 83-90. (49) Abraham, R . J.; Griffiths, L.; Loftus, P. J. Comput. Chem. 1982, 3 ,
407-416. (50)Abraham, R. J.; Smith, P. E. J. Comput. Chem. 1988, 9 , 288-297. (51) Small, G. W.; Jurs, P. C. Anal. Chem. 1984, 56, 1314-1323. (52) Draper, N. R.; Smith, H. Applied Regresslon Analysis, 2nd ed.;WileyInterscience: New York, 1981. (53) Neter, J.; Wasserman, W.; Kutner, M. H. Applied Linear Statistical Models, 2nd ed.; Richard D. Irwin: Homewood, IL, 1985. (54) Belsley, D. A.; Kuh, E.; Welsch, R. E. Regression Diagnosflcs: Identi-
wing Influentla1Data and Sources of Collinear/ty; Wiley-Interscience: New York, 1980. (55)Allen, D. M. Technical Report No. 23, 1971;Department of Statistics, University of Kentucky, Lexington, KY. (56)Snee, R. D. Technometrics 1977, 79, 415-427.
RECEIVED for review March 8,1990. Accepted May 17, 1990. This work was supported by the National Science Foundation under Grant CHE-8815785. The Sun 4/110 Workstation was purchased with partial financial support of the National Science Foundation. Portions of this paper were presented a t the 41st Annual Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, New York, NY, March 1990.
CORRESPONDENCE Chromatographic Detection of Interaction between Polyoxyethylene and Nitrous Acid Sir: Multidentate cyclic and acyclic complexing agents have been well-investigated from various points of view (1-4). Most of the researchers have focused on the selectivity toward cations (1-3). It is well-known that the selectivity of these complexing agents originates in a match of the cavity size with the crystalline size of a cation, the nature of donor atoms, facility in building up the coordination shell a t the optimum distance required by a cation, etc. Developing new compounds showing unique selectivity according to these bases is still a topic of fundamental importance. On the other hand, despite the requirements, only a few compounds have been known to be selective toward anionic compounds ( 4 , 5 ) . Continued efforts are made to seek anion-selective agents. Polyoxyethylenes (POE), which form the complexes with some metal cations similar to crown ethers, have been regarded 0003-2700/90/0362-1891$02.50/0
as acyclic cation-selective complexing agents (6). The reaction of POEs with anionic compounds has not been reported except for the case where a POE-metal complex forms an ion pair with a counteranion (6). In this contribution, I would like to show that POE interacts with nitrous acid. This interaction results in unique selectivity in chromatography and solvent extraction. EXPERIMENTAL SECTION The chromatographic system was composed of a computercontrolled pump, CCPM or CCPD (Tosoh Co.), a Rheodyne sample injection valve equipped with a 100-pL sample loop, a column oven (CO-SOOO, Tosoh), a conductometric detector (CM8000, Tosoh), a UV-visible detector (UV-8000, Tosoh), and a refractive index detector (830-RI, Jasco). A separation column was an Inertsil ODS 2-T (particle size, 5 pm; 4.6 mm id. X 150 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
Table I. Change in the Retention of Anions le, min POE(9)D POE(23)D POE(2O)H POE(20)O
0.500 0.250 0.286 0.289
1.32 1.33 1.35 1.43
2.12 2.24 2.30 2.40
mm; Gasukuro Kogyo). The flow rate was kept at 1 mL/min. A Shimadzu Model spectrophotometer UV-160 was used for the colorimetric determination of nitrite and fluoride. Polyoxyethylene(23)dodecyl ether (POE(23)D)of the amino acid analysis grade was purchased from Nacharai Chemicals. Other surfactants, polyoxyethylene(9)dodecyl ether (POE(9)D), polyoxyethylene(2l)dodecyl ether (POE(21)D), polyoxyethylene(25)dodecyl ether (POE(25)D), polyoxyethylene(20)hexadecyl ether (POE(20)H),and polyoxyethylene(20)octadecyl ether (POE(20)O)were gifts from Nikko Chemicals. Numbers in parentheses represent mean numbers of oxyethylene units. Surfactant solutions were deionized with a mixed-bed rain column and filtered through a Millipore membrane filter. Distilled deionized water was used for all experiments. Inorganic salts of analytical grade were dried at 110 OC under vacuum. Other reagents were also of analytical grade and used as received.
RESULTS AND DISCUSSION In chromatographic experiments with a bare octadecylsilanized silica (ODS)column and the same column coated with a POE-containing nonionic surfactant, the following phenomena were observed: (1)nitrite and fluoride were retained on the surfactant-coated column, whereas the other anions tested were not retained, (2) anions were excluded from the surfactant micelles, and (3) the surfactant-coated stationary phase showed cation-exchange ability (in purely aqueous mobile phase, cation exchange sites were occupied by H+). Experimental results and discussions are presented below in detail. The void volume of the ODS column was ca. 1.4 mL. All anions tested were not retained on the bare ODS column and were eluted a t void volume with water. However, the POEcoated ODS specifically retained NOz- and F,while C1-, Br-, I-, S042-, NO3-, and C104- remained unretained as shown in Table I. Similar results were obtained when other surfactants containing POE chains as part of the structure were used to cover the stationary phase surface. This result suggests the possibility of the interaction between POEs and NO, or F-. H N 0 2 (pK, = 3.15) and H F (pK, = 3.17) are both weak acids, whereas other analytes examined are anions of strong acids. Therefore, the present separation might be due to differences in the acidity of analytes. However, the retention of acetic (pK, = 4.56) and formic acid (pK, = 3.55) on the POE-coated ODS was weaker than that of F- or NO2- regardless of whether injected samples were acids or salts. We cannot know the pH of sample bands in an unbuffered aqueous mobile phase. However, if a difference in acidity between analytes provides the separation mode, the weaker acid should be eluted later. Thus, the interaction cannot be related to acidity of analytes. The retention of NO, and P increased with increasing the concentration of nonionic surfactant micelles in mobile phases. This is explained by the exclusion of NOz- and F- from micelles. In the case of the total exclusion, the retention of an analyte is represented by the following equation (7): l/Wr - v,, = (-OC, + l)/(V,Ksw + Vi) (1) where V,, V,, Vi, and V, are the retention volume of an analyte, the volume of an external solvent into which micelles can permeate without restriction, the volume of an inner solvent into which micelles cannot permeate, and the volume of the
1.73 1.81 1.80 1.93
1.40 1.42 1.42 1.44
0
1.38 1.50 1.40 1.49
2
6
4 time
1.40 1.40 1.40 1.50
min
Flgure 1. Chromatographic separation of CI-, NO2-,and F- on POE(23)D-coated ODS: mobile phase, 0.02 M POE(23)D; flow rate, 1 mL/min; sample concentration, 0.1 mM for NO2-, 0.5 mM for F-, and 0.05 mM for Cl-;detection, conductivity.
stationary phase; 0 is the partial molal volume of the micelle; C, is the concentration of the micelle, which is usually equal to the difference between the surfactant concentration and the critical micelle concentration: Ksw is the partition coefficient between the mobile and the stationary phases. Equation 1shows that analytes being excluded from micelles will be more strongly retained on the stationary phase in the presence of micelles than in the absence. This consideration holds for the present observation for NO, and F; these anions are excluded from the nonionic micelles, albeit the mechanism is not elucidated. Other anions may be also excluded from the micelles. However, eq 1 is not applicable to such unretained analytes, because denominators in eq 1 are zero (V, = V , and Ksw = Vi = 0). Exclusion of NO2- and F from the micelles results in better separation as predicted from eq 1. Figure 1 shows an example of the separation of NO2-, F,and C1- with 0.02 M POE(23)D as a mobile phase. Although broadening of the F- peak is unusual, these anions are well-resolved. Peak sizes, which were conductometrically monitored, obtained with the POE-coated column were larger than those obtained with the bare column. This is explained by a hypothesis that anions eluted from the POE-coated column are accompanied by H+ regardless of cations contained in a solution injected. The pH measurements of column effluents supported this consideration; pH in a sample band was lower than that of an eluent. These results indicate that cation exchange occurs in the POE-coated column and that a countercation in a sample solution is replaced by H+ during the separation. Although it is possible that cation-exchange sites originate from impurities contained in the surfactants, a salt injected into the POE-coated column is readily changed to the corresponding acid. In order to interpret the unique retention of nitrite and fluoride on the POE-coated column, the interaction between POE and H N 0 2 or H F should therefore be taken into account as well as that between POE and the anions. Effects of acidity and salinity in mobile phases were investigated to clarify which interacted with POE, neutral (HNO2and HF) or anionic (NO2- and P) species. The retention increased with increasing acidity but decreased with increasing salinity of mobile phases. These results suggest that POE interacts with the neutral species. As mentioned
Anal. Chem. 1990, 62, 1893-1895 100
s C
Z 50
h X
F0
I
0.1
1 10 conc. of HCI,
100 mM
Figwe 2. Extraction of NO,- and F- as a function of HCI concentration in aqueous phases: organic phase, dichloromethane saturated with water; aqueous phase, 0.1 M PEG 1000 with HCI; phase ratio, 1:l.
above, since this interaction was not detected in aqueous media, low permittivity circumstances may be required. The column used in this work is almost completely endcapped. However, there still remain a few silanol sites. H F is thought to interact with silanol groups. In fact, HF was retained on a silica gel column, and its retention became stronger with the acidity of mobile phases. Other anions and their acids (including NO< and HNO2) were not retained on the silica gel column. Thus, the retention of HF on the POE-coated ODS column is possibly due to the interaction with residual silanol groups. This interaction was observed only for the POE-coated ODS column but not for the bare column. This will be explained by the following inference. In the bare column, octadecyl groups prevent the contact of H F with silanol groups because wetting of such a hydrophobic surface with water does not occur. Adsorption of POEs lowers the hydrophobicity of the surface of the stationary phase and permits contact with aqueous mobile phase. Thus, the interaction of residual silanol groups with H F appeared. The retention of acidic analytes on the bare ODS column should be investigated by using a purely aqueous mobile phase to verify the effect of residual silanol groups. However, anions accompanied by H+ as a countercation were not eluted from the bare column regardless of the nature of anions. This phenomenon is probably due to the hydrogen bonding to H+ in the column packing material under the low ionic strength
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condition. A counteranion is trapped by the electrostatic attraction to H+. Solvent extraction of nitrite and fluoride was attempted to ensure the interaction chromatographically observed. Dichloromethane was selected as a solvent because it was successfully used for the extraction of metal cations with crown ethers and was not emulsified even in the presence of POEs (8). Poly(ethy1ene glycol) (PEG) lo00 was chosen as a POE. Choice of a countercation was also important. Some metal cations (K+, Rb+, Hg2+,Ba2+,etc) are possibly extracted to an organic phase by forming PEG complexes. In such cases, the extraction of a cation is accompanied by the phase transfer of desolvated anions. To avoid this ion-pair formation, Na+, the complexation of which was known to be rather weak, was chosen as a countercation (6,8).Although Cl-, Br-, NO3-, and I- were not extracted, fluoride and nitrite were extracted from acidic aqueous phases to the organic pase as the neutral species as shown in Figure 2. In particular, the extraction ratio of HNOz is very high. HNOz is extractable as N203 to some extent even in the absence of POEs (9). However, the extraction ratio of Nz03 was much lower than the results shown in Figure 2. Thus, the results presented here strongly support the interaction of POE with HNOz in low permittivity media, though the nature of the reaction has not been elucidated. Spectrometric investigation of this interaction is now in progress in our laboratory. Registry No. POE, 25322-68-3;HN02,7782-77-6; F,1698448-8.
LITERATURE CITED (1) Izatt, R. M.; Bradshaw, J. S.; Nlelsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1085. 65, 271. (2) Izatt, R. M.; Christensen, J. J. Synthesis ofhcrocyc/es-7h Design of Selective Complexing Agent; Wlley: New York, 1987. (3) Kimura, K.; Hayala, E.; Shono, T. J . Chem. Soc., Chem. Commun. 1984, 271. (4) Graf, E.; Lehn, J. M. J . Am. Chem. SOC. 1078, 97, 6403. (5) Wuthler, U.; M m , H. V.; Zund, R.; Welti, D.; Funck, R. J. J.; Bexegh, A.; Ammann, D.; Retsch, E.; Simon, W. Anal. Chem. 1084, 56, 535. (6) Cross, J. Nonionic Surfactants; Dekker: New York, 1987. (7) Okada, T. Anal. Chim. Acta 1000, 230, 9. (8) Yanagida, S.; Takahashl, K.; Okahara. M. But. Chem. Soc. Jpn. 1077. 50, 1386. (9) Beattle, I . R. Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Wiley-Interscience: New York, 1963; Voi. 5, pp 1-26.
Tetsuo Okada Faculty of Liberal Arts Shizuoka University Shizuoka 422, Japan RECEIVED for review December 18,1989. Accepted May 16, 1990.
Multiple Mode Semiconductor Diode Laser as a Spectral Line Source for Graphite Furnace Atomic Absorption Spectroscopy Sir: Since the report of atomic absorption spectroscopy
(AAS)by Walsh in 1955 (I),AAS is a principal technique for trace element analysis. An important component of an AA spectrometer is the radiation source. Glow discharges including hollow cathode lamps (HCL) and electrodeless discharge lamps have been used as the line sources (21, and continuum sources also have been employed (3). HCLs give stable, sharp spectral characteristics, high sensitivity, and ease 0003-2700/90/0362-1893$02.50/0
of operation and are the preferred line sources used in virtially all commercial instruments. L'vov in 1961 (4) introduced the technique of graphite furnace (GF) AAS, which proved to provide superior sensitivities and detection limits over the flame approach. The electrically heated graphite furnace, however, requires reproducible operation for getting an acceptable signal precision. This reproducible operating includes the heating temperatures, the position of the sample in the 0 1990 American Chemical Society
