Anal. Chem. 1985, 57, 561-563
and consequently 3
3
2
8Ka
n=Ka--+-
(
1 1 + Ka -+
...)
(19)
In the limit of large KU values, n becomes equal to KU - 3/2. The relative velocity profiles given by eq 1 and 4 are plotted on Figure 1 for the case where KU = 10 and n = 8.5413, according to eq 19. The partially flat, partially parabolic flow profile, giving the same u / u o value is also plotted on Figure 1. It corresponds to a p value of 0.3083, where p represents the fractional distance from the wall where a radial velocity gradient exists. On Figure 2, the relative errors in these approximate flow profiles are plotted as a function of radial position. It is obvious from Figures 1 and 2 that the flow profile given by eq 4 better approximates the true electroosmotic flow than the partially flat, partially parabolic velocity profile. I t should be noted that the differences between the true and approximative flow profiles decrease with increasing Ka values. It is therefore expected that the C, value given by eq 17 will be in closer agreement with the true C, value than the one obtained with the previously studied velocity profile (7). The C, values for the true electroosmotic profile have been numerically computed according to the h i s dispersion theory for the two cases KU = 10 and KU = 50. They are compared with C, values given by eq 17 and by the previous study (eq 10, 14, and 15 of ref 7) in Tables I and 11, respectively, for various k'values. It is seen that the C, values given by eq 17 are all better estimates of the exact values for the electroosmotic flow profile than the values previously obtained with a partially flat, partially parabolic velocity profile. In addition, the relative errors are decreasing with increasing retention and, for a given k ' value, they are, as expected, smaller a t larger K U . In all cases studied here, they are less than 570,which is quite satisfying, as precise values of K a are difficult to obtain. The conclusions which can be derived from these results are the same as those previously discussed (7). Electroosmotic flow provides a lower contribution to peak broadening than
561
pressure-driven flow, at any given value of k'. However, while this contribution is very small for unretained solutes, it increases significantly with increasing k '. consequently, the potential of open tubular liquid chromatography with electroosmotic flow lies essentially in the low k'region. It should lead to better separations of weakly retained compounds than laminar flow, while it should offer a small advantage in separations of more strongly retained compounds. The eq 4, for the flow profile, and 17, for the mobile phase nonequilibrium contribution to plate height, are simple and accurate descriptions of the electroosmotic flow processes which can be used for fully estimating the potential of electroosmotic flow, optimization of separations and comparison with other techniques or types of flow.
LITERATURE CITED (1) Tsuda, T.; Nomura, K.; Nakagawa, G. J. Chromatogr. 1982, 248, 241. (2) Tsuda, T.; Nomura, K.; Nakagawa. G. J . Chromatogr. 1983, 264, 385. (3) Jorgenson, J. W.; Lukacs. K. D. Anal. Chem. 1981, 53, 1298. (4) Pretorius. V.; Hopkins, B. J.; Schieke, J. D. J. Chromatogr. 1974, 99, nn LJ.
Jorgenson, J. W.; Lukacs, K. D. J. Chromatogr. 1981, 278, 209. Stevens. T. S.;Cortes, H. J. Anal. Chem. 1983, 55, 1365. Martin, M.;Guiochon, G. Anal. Chem. 1984, 56, 614. , Rice, C. L.; Whitehead, R. J. fhys. Chem 1965, 69,4017. (9) Aris. R. R o c . R . SOC.London, Ser. A 1959, A252. 538. (IO) Golay, M. J. E. "Gas Chromatography"; Desty. D. H., Ed.; Butterworths: London, 1958; p 36. ~
Michel Martin* Georges Guiochon Ecole Polytechnique Laboratoire de Chimie Analytique Physique 91128 Palaiseau, France Yvonne Walbroehl James W. Jorgenson Department of Chemistry University of North Carolina Chapel Hill, North Carolina 27514
RECEIVED for review September 4,1984. Accepted October 30, 1984.
Determination of Uranium by Reversed-Phase High-Performance Liquid Chromatography Sir: In spite of the large number of methods already reported ( I ) , the determination of the uranyl ion is still a relevant analytical problem because the usual techniques of atomic absorption and emission spectroscopy are not suitable for the trace analysis of U02'-+,owing to their high detection limits. Also recently spectrophotometric methods have been proposed (2-4); in some cases laser sources (5, 6) or flow injection systems (7, 8) were used. In this paper a new chromatographic procedure is proposed; this procedure is based on the separation and determination of U02'+ as a neutral complex, using reversed-phase chromatography and spectrophotometric detection. Examples of chromatographic separation of UOZ2+as a complex have been already reported, in most cases using TLC and paper chromatography. Besides the widely used organophosphorus ligands, other complexing agents were used in the mobile phase such as TTA (9),formic acid (IO),p(diethy1amino)aniline of phenylgliossal (11, 12) and in the stationary phase such as EDTA (13),NTA ( 1 4 ) , and anilines (15).
In this work the ligand 2,6-diacetylpyridine bis(benzoy1hydrazone) was used for the solvent extraction and chromatographic determination of uranium. The compounds of this
H,DIB
ciass of bis(aroy1hydrazone) derivatives are potential pentadentate chelating agents which can behave as neutral or
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
bisdeprotonated ligands (16-18). Because of its steric and electronic features, H2DIB is particularly suitable for complexing the uranyl ion (17). High extraction yields of U022+ from aqueous t o dichloromethane solutions have been obtained, copper being the only metal with comparable extraction yield (19). The behavior of this ligand in the extraction of the uranyl ion, in addition to the high molar absorption of the complex in the UV region, suggested its use for the chromatographic determination of uranium, using a conirnercial apparatus with spectrophotometric detection.
EXPERIMENTAL SECTION The ligand was synthesized from 2,6-diacetylpyridine and benzoylhydrazide, as already reported (18). Metal complexes of HzDIB were obtained by mixing methanol solutions of the metal salts and ethanol solutions of the ligand (molar ratio 1:l)at room temperature following the procedure described in ref 18; yellow crystals of the complexes are immediately formed. The compounds were characterized by melting point, elemental analysis, and IR spectra. The chromatographic separations were obtained on a PerkinElmer Series 3B chromatograph, equipped with a Rheodyne 1705 injection valve, a variable wavelength spectrophotometricdetector I,C75, and a Shimadzu CR2A integrator. A LiChrosorb RP2 column, 250 mm X 4 mm i.d., 10-Fm mean particle size (Merck, Darmstadt, FR G), was used. Solvents were HPLC grade (Carlo Erba, Milan, Italy). Dichloromethane solutions of the complexes, typically 5-10 pLywere injected. The flow rate was usually 2.0 cm3 m i d . In some cases column eluate was checked with IR spectroscopy, using a Perkin-Elmer 283B spectrophotometer and mass spectrometry (Finningan 1020),collecting fractions of column eluate. NhlIR spectra of the ligand of the UO?+ complex were run by using a spectrometer Brucker CPX-200 with F T facility. The solution of the reference ore (SY-3, GRS from Geological Survey of Canada) was obtained by adding to the sample (1 g) 10 cm3 of water, 1 cm3 of concentrated sulfuric acid, and 15 cm3 of 48% hydrofluoric acid and evaporating to dryness. The procedure was replicated 3 times, and then the residue was redissolved in 10 cm3of 5 M nitric acid. The solution, whose pH was adjusted to 2.0 with NaOH, was diluted to 100 cm3 with distilled water. RESULTS AND DISCUSSION The uranyl ion in aqueous solutions a t the concentration levels 10-5-10-3 M is completely extracted by HzDIB into dichloromethane, when an excess of 2 t o 1of the ligand with respect to the cation is reached. The extracted complex is in the neutral form U02DIB, the ligand is bisdeprotonated and neutralizes the charge of the uranyl ion (19). Among the other metals considered, copper shows a similar behavior with a complete extraction, with a ligand-to-metal ratio of about 2. A stoichiometric ratio 101is required to obtain appreciable extraction of some other metals such as Ni2+,Zn2+,Ca2+,and SnZ+.Ter- and tetravalent metals are not extracted with any metal-to-ligand ratio (19). The high efficiency of the extraction of U02'+, which derives, among other factors, from the stability of the complex and from its solubility in low polarity phases, suggested the possibility of using the complex U02DIB for the separation of uranium by partition chromatography. First, a LiChrosorb RP18 column and methanol-water and methanol-dichloromethane mobile phases were used, but better results, with respect to efficiency and resolution, were obtained on a LiChrosorb RP2 column using methanol or methanol-water mixtures as mobile phases. On this column, using a mobile phase of lower polarity (e.g., methanol-dichloromethane 80/20), the dichloromethane solutions of H2DIB give rise to two peaks. The mass spectra run on the fractions of column eluates corresponding to the peaks, indicating that both peaks are to be attributed to H2DIB. The two peaks could be attributed to the two conformational isomers E,E and E,Z, which derive from the ro-
--
0
2
4
t/min
Figure 1. Separation of H,DIB (peak 1) and U0,DIB (peak 2); column LiChrosorb RP2, mobile phase methanol-water 60/40; flow rate 2 cm3/min;detection UV, A = 265 nm.
0
2
4
6
8
t/min
Figure 2. Separation of H,DIB (peak l), U0,DIB (peak 2), and CuDIB (peak 3) using a gradient elution: mobile phase methanol-water
60/40-90/10.
Other conditions as in Figure 1.
tation of the benzoylhydrazone moieties around the iminic double bond, the former corresponding to the major peak. The existence of these isomers in CDC1, is demonstrated by the presence of two separated resonances a t 9.3 and 14.5 ppm for the NH protons in the 'H NMR spectra, which correspond to the E,E and E,Z forms, respectively. The difference in the chemical shifts, which derives from the formation of an intramolecular hydrogen bond in the E,Z form, has been observed and discussed for similar compounds (19,20). The behavior of the complexes was examined using dichloromethane solutions of synthesized compounds. When methanol is used as the mobile phase, the UOzDIB and CuDIB complexes are not retained, while the complexes of Ni'+, Zn2+, Mn2+,and Co2+are eluted with high retention volumes: with a methanol-H20 60/40 (v/v) mixture, the separation between UOzDIB and the free ligand is achieved (Figure 1). Under these conditions only one peak for the ligand is observed, while the copper complex is not eluted. T o obtain the elution in the same run of the ligand and both complexes, a gradient of elution is necessary. Figure 2 shows the separation of HzDIB, U02DIB, and CuDIB obtained with a gradient of elution between 60% and 90% methanol in the mobile phase. When the double effect of the selectivity of the extraction, which excludes the tervalent and many of the divalent metals, and of the chromatographic separation is used, a highly selective determination of uranium appears to be possible. Moreover, while dichloromethane solutions of H2DIB show an absorption maximum a t 265 nm, the dichloromethane solutions of the complexes show strong absorption also a t about 350 nm. In particular for U02DIB,,,e, a t 345 nm is 3.5 x IO4 mol-' L cm-I. At this wavelength the absorption of the ligand is almost negligible. T o evaluate the analytical potentiality of this method in the uranium determination and to state the concentration range in which it could be applied, the dependence of the peak area on the quantity of uranium injected was obtained, injecting dichloromethane solution of the synthesized complex. The dependence is linear in the range 5-100 ng of uranium injected. In this range the experimental data fit the equation y = 2.87 X 10% - 1.5 X lo2, where y is the peak area in
Anal. Chem. 1985,
563
57. 563-564
of the uranyl complex are present. From the comparison of the mean value of the peak area, obtained from five extractions from the same nitric solution, with those of the calibration curve, a concentration of 650 30 ppm in the original sample is deduced. These preliminary results, even though are to be confirmed on a wider range of real samples, indicate that the proposed method could be effective and highly selective in the determination of uranium at parts per million concentration levels even in the complex matrices. Registry No. H,DIB, 73818-26-5; UO,DIB, 74091-79-5;CuDIR, 86305-88-6;U, 7440-61-1.
*
0
2
4
:/mn
Flgure 3. Chromatogram of the solution obtained by extraction with H,DIB in dichloromethane of a nitric solution of a uranium-containing ore; column LiChrosorb RP2, mobile phase methanol-water 65/35; detection UV, A = 345 nm; 10-pL injection.
arbitrary units and x is the nanograms injected; r = 0.9996 for n = 8. The relative standard deviation of the slope is 1.55%. This curve was compared with that obtained when starting from aqueous solutions of uranium in the same range of concentration (1-10 ppm) and extracting them with the same volume of dichloromethane solutions of ligand. A stoichiometric ratio of ligand to uranium of about 5 to 1 is maintained in the extraction. This ratio does not cause interference in the chromatographic separation, because of the high separation between ligand complex and because of the low absorptivity of the ligand a t 345 nm. Under these conditions the experimental data fit the equation, expressed in the same units as the previous one, y = 2.97 X 102x- 4.52 X lo2;r = 0.9995, relative standard deviation of the slope 2.1 % for n = 8. A t test was carried out on the slopes of the two curves (22)to verify if the dependence of the detector response on the quantities injected was significantly different following the two procedures. The value of the slope of the curve obtained by starting from dichloromethane solutions of the complex was assumed as the "true" value. This test indicates that the slopes of the two curves are not significantly different a t 95% confidence level. T o verify the effects of the presence of a complex matrix, the method was applied to the determination of the uranium content in a reference ore (SY-3, GRS from Geological Survey of Canada, certified concentration 760 A 38 ppm). The nitric solution (pH 2) of the reference sample, corresponding to 1 g of sample in 100 cm3, was used for the extraction. A 2-cm3 aliquot of this solution was extracted with an equal volume of dichloromethane solution of the ligand M) by equilibrating the two phases for a few minutes using a Vortex stirrer; 10 p L of the organic phase was injected. Figure 3 shows the corresponding chromatogram obtained using a methanol-water mixture 65/35 (v/v) as the mobile phase and UV detection at 345 nm. In spite of the complexity of the matrix, under these conditions only the peaks of the free ligand and
LITERATURE C I T E D "Gmelin Handbook of Inorganic Chemistry"; Springer-VerLag: Berlin, 1982: Uranium Supplement Volume A7. Aznarez, J.; Paiacios, F.; Vidal, J. C. Analyst (London) 1983, 108, 1392. Uesugi, K.; Nagahiro, T.; Mlyawaki, M. Anal. Chin?. Acta 1983, 148.
315. Ohshita, K.; Wada. H.; Nakagawa, G. Anal. Chim. Acta 1983, 149. 269. Kenney-Wallace, G. A.; Wilson, J. P.; Farrell, J. F.; Gupta. B. K. Talanta 1981, 28, 107. Zhi-Lin, W.; Chi-Ke, C.; Xia-Nian, L.; Fu-Xin, T.: Xun-Xi, P. Anal. Chlm. Acta 1984, 160, 295. Lynch, T. P.; Taylor, A. F.; Wilson, J. N. Analyst (London) 1983, 108, 470. Sielfwerbrand-Lindh, C.; Nord, L.; Danielsson, L. G.; Ingman, F. Anal. Chlm. Acta 1984, 160, 11. Johri, K. N.; Bakshi. K. Chromatographia 1972, 5 ,309. Qureshi, M.; Sethi, 8. M.; Sharma, S. D. Sep. Sci. Technol. 1980, 15. 1685. Upadhyay, R. K., Tewari. A. P. J. Indian Chem. SOC. 1979, 56,972. Upadhyay, R. K.; Tewari, A. P. Sep. Scl. Technol. 1980, 15, 1793. Srivastava, S. P.; Dua, V. K.: Gupta, V. K. 2. Anal. Chem. 1977, 286,255. Srivastava, S. P.; Dua. V. K.; Gupta, V. K . Anal. Lett. 1979, 12,169. Srivastava, S. P.; Dua, V. K.; Gupta, V. K. Z. Anal. Chem 1978, 292,415. Mangia, A.; Pelizzi. C.; Pelizzi, G. Acta Ctystallogr., Sect. B 1974, 8 3 0 . 2146. Paolucci. G.; Marangoni, G.; Bandoli, G.; Clemente, D. A. J . Chem. SOC., Dalton Trans. 1980, 1304. Lorenzini, C.; Pelizzi, C.; Pelizzi, G.; Predieri, G. J. Chem. Soc., Dalton Trans. 1983, 721. Casoli, A.; Mangia, A,; Predieri, G. Anal. Chlm, Acta, submitted for publication. Palla. G.; Mangia. A,; Predieri, G. Ann. Chlm. (Rome) 1984, 74, 153. Marangoni, G.; Paoiucci, G. J. Chem. Soc., Datton Trans. 1981, 357. Mack, C. "Essential of Statistics"; Plenum Press: New York, 1967; p 142.
Antonella Casoli Alessandro Mangia* Giovanni Predieri Istituto di Chimica Generale ed Inorganica Universiti di Parma Via M. D'Azeglio 85 43100 Parma, Italy
RECEIVED for review August 6, 1984. Accepted October 12, 1984. This work was financially supported by CNR (Italy) Grant 83.00233.03.
Flow-Injection System for the Rapid and Sensitive Assay of Concentrated Aqueous Solutions of Strong Acids and Bases Sir: We frequently use a flow-injection system when optimizing the electrode potential wave forms for pulsed-amperometric detection (PAD) of carbohydrates and amino acids in liquid chromatography (1-3). For cases where a substantial background signal is present due to formation of surface oxide
on a Pt electrode, i.e., large positive value of detection potential, we have noted the necessity of exactly matching the concentration of NaOH in the samples and the carrier stream to eliminate blank peaks. The exact match of pH is not critical for application of PAD to liquid chromatography because the
0003-2700/85/0357-0563$01.50/00 1985 American Chemical Society
