Anal. Chern. 1981, 53,2345-2347
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CORRESPONDENCE Isotopic Abundance Determinations by Inductively Coupled Plasma Atorhc Emission Spectrometry Sir: Inductively coupled plasma atomic emission spectrometry (ICP-AES) is rapidly becoming the method of choice for elemental determinations at the trace, minor, and major constituent level especially when simultaneous or rapid sequential multielement capabilities, powers of detection, large dynamic range, and freedom from interelement effects are important considerations (1). Many atomic spectrrtl lines exhibit isotopic structures that have been exploited for quantitative determination of isotopic abundance ratios via atomic emission spectrometry (2). Usually, this exploitation has involved observation of the atomic emission from relatively low-pressure and low-temperature vaporization--excitation cells. Although these experimental conditions have led to reduced collisional and Doppler broadening effiects, the atomization-excitation cells have usually not been amenable to convenient sample exchange and to the direct analysis of samples in solution. Because ICP atomization-excitation cells are not burdened by these limitations, it io of interest to inquire whether isotopic structures can be adequately resolved in the lines emitted from this source. THEORETICAL CONSIDERATIONS The temperature broadening contribution to the overall line profile may be calculated from the Doppler effect equation (2). The resulting line shape is Gaussian, and for a heavy atom (M2 100) at 8O00 K, a Doppler width of 0.0025 nm fwhm (full width at half maximum intensity) or less at 400 nm would be expected. Pressure broadening effects are not easily calculated, although it is recognized that (a) a Lorentzian line shape results from this broadening process (3), (b) the convolution of the Gaussian (temperature broadening) and Lorentzian (pressure broadening) profiles may be fit to a Voigt profile (4), and (c) the fwhm of a Voigt profile is chiefly determined by the Gaussian component whereas the Lorentzian compsnent is predominant in the wings. A high-resolution study of ICP emission lines has demonstrated that ICP line shapes can be fit with a Voigt analysis (5) and that the Voigt parameter required to successfully fit observed emission lines is dominated by its Gaussian component (5). Therefore the Doppler width calculations can serve as a useful guide for selecting appropriate lines and for determining the necesswy instrumental resolution required for their observation.
EXPERIMENTAL SECTION ICP. The appmatus used for this study is illustrated in Figure 1. The ICP is of conventional design (6) and was used without modification. The operating conditions employed are noted in Table I. Spectrometer. For the reduction of instrumental broadening contributions, a spectral resolution of 0,001 nm at 400 nm (resolving power 2400 OOO) iu desirable. To achieve this resolution, we constructed a tunable, high-resolution spectrometerby coupling a medium-resolution monochromator (Hilger-Engis,Model 1OOO) and a piezoelectrically t u n d Fabry-Perot interferometer (Burleigh, RC-150). The monochromator band-pass was set approximately equal to the free spectral range of the interferometer (usually 0.04-0.07 nm) and was used to filter the ICP emission output prior
Table I. Inductively Coupled Plasma Operating Conditions Plasma Generator 1000-W forward power, < 5 W Model HFS 3000D, PlasmaTherm, Inc., reflected power, 27.12 MHz Kresson, N J quartz, Ames Laboratory construction ( 7 )
Plasma Torch 16.5 L/min plasma Ar flow, 1 L/min aerosol carrier Ar flow
Pneumatic Nebulizer 1.7 mL/min sample uptake rate Teflon and glass, for aerosol Ar flow rate of 1 Ames Laboratory construction (8) L/min to analysis by the interferometer. The Fabry-Perot interferomebr employed one set of mirrors to cover the 400-800 nm spectral range (Burleigh, RC-620) and was aligned with a helium-neon laser (Spectra-Physics,Model 145);the laser back-reflection was sent through the monochromator to establish the source position. The effective “finesse” of the interferometer was determined to be 25-50 by measuring the hyperfine structure of the 546.1-nm emission of a Hg lamp (American Ultraviolet Co., Model UF4-13). The transmitted light was detected by a cooled, high-gain photomultiplier tube (EMI, 9789QA). A low bandwidth photon counting apparatus consisting of (a) preamplifier (Ames Laboratory), (b) amplifier (RIDL, 27001), (c) discriminator (Tennelec, Model TC-405), (d) scaler (RIDL, 27301), and (e) pulse counting card (ADAC, Model 1604/0P1) was used to analyze the photomultiplier output. The high sensitivity of photon counting detection allowed us to insert an iris diaphragm immediately before the Fabry-Perot interferometer to reduce the plate area irradiated by the source and thereby to improve the instrument resolution. The measured resolving power of our monochromator-interferometer was >250000 at the above indicated free spectral range.
RESULTS AND DISCUSSION Selection of Candidate Elements and Lines. The atomic emissions of heavy elements, such as platinum, mercury, lead, and uranium, will exhibit relatively small Doppler widths even at ICP temperatures. Heavy elements possess complex electronic structure that often results in several electronic states with substantial isotopic splitting. These considerations led us to choose lead and uranium for our first observations on the feasibility of utilizing the ICP for isotopic analysis. Pb Isotopic Analysis. The P b I1 537.2-nm line of lead was selected from among the P b lines showing isotopic structure (!+11), because of the pattern and extent of its isotopic structure and because, in the first stage of this study, we were using Fabry-Perot plates that were coated for operation in the 550-650 nm spectral range. This line was emitted weakly and 1000-10 000 ppm solutions were required for our experiments. The isotopic spectrum of this line, as recorded for naturally occurring Pb, is shown in the upper curve in Figure 2. A computed simulation of the expected isotopic pattern is shown in the lower portion of the figure. This simulated spectrum was constructed by summing the individual isotopic contri0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981 I
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High-resolution monochromator-interferometer. The 1. O m monochromator employed a 1200 lines/mm gratlng (Bausch & Lomb) that was interferometrically ruled and blazed at 500 nm (S= slit, FPI = Fabry-Perot interferometer). Flgure 1.
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Flgure 3. Isotopic structure of the U I1 424.4 nm emission llne excited by the ICP. This sample is a 1:5 235U:238U mixture (125 ppm in 238U). The 235U-238U splitting is 0.0249 nm and the 238U line wldth (fwhm) is 0.0037 nm.
Isotopic structure of the Pb I1 537.2-nm emission line. The upper curve is the ICP-AES spectrum of a 10 000 ppm solution of Pb in dilute nitric acid. The lower spectrum is a computed slmulation assuming a Voiat Darameter of 0.18, line width [fwhm) = 0.0046 nm, Figure 2.
235
I
butions, where (i) the individual isotope line positions were taken from ref 9, (ii) a common Voigt parameter and line width were assumed for each isotopic component, and (iii) the Voigt line shape was approximated by the method of Kielkopf (4). The resolution of the emission from the P b 206,207, and 208 isotopes is clearly apparent. U Isotopic Analysis. The isotopic analysis of uranium, using a dc arc source, was reported by Burkhart et al. in 1949 (12). Uranium has a very rich optical spectrum offering several promising candidate lines for isotopic analysis. The 424.4-nm line has been used by others for isotopic analyses of uranium (13,14) where the spectra were excited in hollow cathode discharge lamps or in dc arcs. Figures 3 and 4 present recordings of the isotopic patterns of this line for a 235U*U mixture and a 235U-U-238v mixture that were excited by the ICP. The 235U-238Uresolution is excellent and the 235U-236Usplitting (0.01 nm) is very well resolved. Analytical Calibration. A set of 235U/238U mixtures was prepared by adding measured amounts of a stock solution to a 238Usolution. The 238Uconcentration was maintained a t 4000 ppm and the added 235Uvaried from 40 to 200 ppm (i.e,, concentration range of 1% to 5% 235U/238U); more concentrated solutions were prepared at lo%, 50%, and 100% 235U/238U.An analytical calibration curve that related the measured 235U/238U line intensity ratio to the known concentration ratio was prepared. This curve was linear and had no detectable bias. The average relative deviation of the plotted points, representing the analysis of several isotopic mixtures ranging in 23sU/mUrelative concentration from 1% to loo%, from an ideal, unit slope, linear calibration curve with zero intercept, was ca. 5.5%. The more dilute isotopic mixtures exhibited a complex, incompletely resolved line pattern in the immediate vicinity of the 235Ucomponent of the 424.4-nm line that was not due to the presence of 235U.This spectral structure may have been caused by weak 238Uemission lines or by instrumental effects such as grating “ghosts”. Additionally, these lines may represent the remnants of strong 238Uemission lines near the
236
/
1
FI ure 4. Isotoplc structure of the U I1 424.4 nm emission line from a ‘U, ‘%J, mixture (ca.100 ppm 23%J and ‘%U and 90 ppm %). The 235U-236U splitting is 0.010 nm.
424.4-nm line that were insufficiently blocked by the monochromator and were subsequently folded into the spectral window observed with the interferometer. Future studies will be directed to the positive identification of this weak structure. Regardless of the cause of this interference, it could be removed by a relatively simple and consistent method to arrive a t a corrected 236Uintensity. To accomplish this background correction we (i) measured the total area of the multiplet a t the 236Uline position (the emission line intensity was integrated over a wavelength range ca. 0.012 nm wide that was approximately centered about the 424.4-nm 235Ucomponent line position-no other isotopic component of the 424.4 nm line falls within this small wavelength domain), (ii) normalized the measured area relative to the area of a weak line in the vicinity of the 424.4-nm emission line that was assumed to be due to z38U emission (if one assumes that this line is an unresolvable uranium emission line and represents all uranium species in solution, the data are slightly improved), and (iii)
Anal. Chem. 1981. 53. 2347-2349
corrected the 235Ucorr~ponentintensity by subtracting from it an empirically derived percentage (>85%) of the structure component position in the spectrum of the found in the 235U 238Ustock solution used to prepare the 235U/23sU mixtures. The percentage was chosen to minimize the average relative deviation of the plotted points from the ideal calibration curve-the difference between the percentage chosen and the total normalized area of the 23sUstock solution spectrum may reflect the difficulty of choosing the base line for the peak integrations performed in the analysis. Although a high-resolution instrument incorporating a Fabry-Perot interferometer was used in this feasibility study, commercial scanning monochromators of equivalent or even superior resolving power are currently available. Thus, with the availability of commercial instrumentation to achieve comparable results, we anticipate application of the obssrvations on the U isotopic shift to several analytical tasks involved in the nuclear fuel cycle and nuclear safeguards.
ACKNOWLEDGMENT The authors wish to acknowledge the many helpful discussions they have had with Joseph Goleb, Office of Safeguards and Security, U .S. Department of Energy, during this work. We also wish to thank Gerald Small, Iowa State University-Ames Laboratory, for letting us use his FabryPerot interferometer and Glenn Waterbury, Los Alamos National Laboratory, for performing mass spectral analyses of our uranium isotope stock solutions.
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LITERATURE CITED Fassel, V. A. Science 1978, 202, 183-191. Kuhn, H. G. "Atomic Spectra", 2nd ed.; Academic Press: New York, 1969; pp 369-385, 412-417. Peach, G. Contemp. Phys. 1975, 16, 17-34. Kielkopf, J. F. J. Opt. SOC.Am. 1973, 63, 987-995. Human, H. G. C.; Scott, R. H. Spectrochim. Acta, Part 8 1978, 318, 459-473. Scott, R. H.; Fassel, V. A.; Knlseley, R. N.; Nixon, D. E. Anal. Chem. 1974, 46, 75-80. Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 1155 A-1164 A. Kniseley, R. N.; Amenson, H.; Butler, C. C.; Fassel, V. A. Appl. SpectrOSC. 1974, 28, 285-286. Murakawa, K.; Suwa, S. J. Phys. SOC. Jpn. 1950, 5, 382-383. Murakawa, K. J. Phys. SOC.Jpn. 1953, 8, 382-387. Geiger, F. E., Jr. J. Opt. SOC. Am. 1958, 48, 302-303. Burkhart, L. E.; Stukenbroeker, G. L.; Adams, S. Phys. Rev. 1949, 75, 85. Mullln, H. R.; Goldbeck, C. G. "Analyses of Essential Nuclear Reactor Materials"; Hodden, C. J., Ed.; U S . Atomlc Energy Commlslon: Washington, DC, 1964; pp 946-953. Mainka, E.; vonBaeckmann, A. I n Proc. XVI "Colloqulum Spectroscoplcum Internatlonale"; Heidelberg, Oct 1971; Adam Hllger: London, 1972; VOI. I, pp 247-252.
Martin C. Edelson* Velmer A. Fassel Ames Laboratory-USDOE Iowa State University Ames, Iowa 50011
RECEIVED for review June 22,1981.
Accepted September 14, 1981. This investigation was supported by the Office of Safeguards and Security, U.S.D.O.E. under Contract No. W-7405-Eng-82.
Effect of Mollecular Size, Ionic Strength, and pH on Retentions of Aromatic Acids on XAD-8 Resins Sir: We recently fractionated humic acids adsorbed on XAD-8 resin (1) by diluting a universal buffer (2)with strong base to produce a nearly linear pH gradient. This technique was used to take advantage of the fact that stronger acids should be ionized and desorbed from the resin a t lower pH values than weaker acids. Continuous elution of humic acid components over a wide pH range was observed as indicated by the absorbance of 254 nm. Positive deviations from linearity in the pH gradient caused humic acid components to be desorbed at a faster rate. The overall pattern of elution was consistent with the wide range of acidities observed in humic acids for the different carboxylic and phenolic groups (3). In our previous study, model aromatic acids eluted in the order of decreasing acid strength. However, since those compounds were all benzene derivatives, the effect of the size of the aromatic system on the retention was unknown. Because a wide range of inolecular weights are encountered in humic acids, this factor may also influence the retention of individual humic acid Eipecies. Pietrzyk ( 4 ) has shown that, in addition to the pK, value, the hydrophobicity of am organic acid influences its retention behavior on a resin a t a given pH. Also, the ionic strength of the eluent affects the retention of an acid on both ion exchange (5) and neutral (6) resins. In both cases, adsorption increased with an increase in the ionic strength (salting-out effect). In the present study, aromatic acids having known pK, values and different molecular sizes were chromatographed
by using a pH gradient. In addition, different ionic strengths of the eluents were used to evaluate salting-out effects.
EXPERIMENTAL SECTION Apparatus. The minicomputer-controlled liquid chromatographic system has been described previously ( I ) . Chemicals. Phosphoric acid, glacial acetic acid, and boric acid (all J. T. Baker, Phillipsburg, NJ) were used to prepare the universal buffer. Water, used to prepare the eluents and dissolve samples,was doubly deionized, passed through activated charcoal, and, finally, distilled in glass. Sodium hydroxide solution, 50% (J.T. Baker) was used to prepare the basic titrant. Sodium titrate (reagent grade, 3. T. Baker) was used to adjust the ionic strength of an eluent. The model compounds: benzoic acid, l-naphthoic acid, 9-anthranoic acid, 4-biphenylcarboxylicacid, diphenic acid, phenol, and l-naphthol (all from Aldrich Chemical Co., Milwaukee, WI) were used as received. The XAD-8 resin (Supelco Inc., Bellefonte, PA) was cleaned, sized, and packed into 25-cm columns as described previously ( I ) . Procedures. Samples were prepared at a concentration of 1 mg/mL except for 9-anthranoicacid and diphenic acid which were prepared at a concentration of 0.1 mg/mL due to their high molar absortivities. The sample solvent was 0.1 M NaOH except for 4-biphenylcarboxylicacid which was dissolved in 0.1 M NaOH 1:l in ethanol due to its low solubility in aqueous base. Since phenols are sensitive to light and air, especially in basic solution, the samples of phenol and l-naphthol were deaerated with and stored under nitrogen in darkness. The universal buffer, 0.025 M each in phosphoric, acetic, and boric acids, was diluted using 0.05 M NaOH from a second pump to generate the pH gradient. To study ionic strength effects, we prepared the buffer and the base at three levels of ionic strength:
0003-2700/81/0353-2347$01.25/00 1981 American Chemical Society
