Anal. Chem. 1982, 54, 2377-2378
2377
Selective Laser Photoionization for Mass Spectrometry
Sir: High-precision isotope ratio mass spectrometers typically have mass resolutions in the range 102-103. Consequently, the measurement of the ratios of isotopes of a single element is often complicated by isobaric interferences resulting from incomplete chemical separations. Subtraction of the isobaric background from the signal of interest can lead to a decrease in precision or to gross inaccuracies if the isotopic distribution of the interference is not well-defined, as might be the case in nuclear enlergy or weapons materials. Selective laser photoionization (1-8) of the element of interest has the potential to reduce or eliminate the isobaric interference while at the same time accurately reflecting the isotopic distribution of the element of interest. We demonstrate here the selective ionization of Lu in the presence of Yb. This pair is particularly difficult to separate chemically, or by common physical methods, and so provides an interesting test of the capabilities of selective laser ionization. Samples were prepared by evaporating aqueous chloride solutions of the appropriate rare earth (or mixture) onto the side filamentai of a conventional triple filament thermal ionization source (9). Gentle heating (700 O C ) produced a continuous supply of atoms in the source region of a 12-in. magnetic mass spectrometer modified to permit optical access to the thermal ionization region. An unfocused flashlamp-pumped dye laser beam, supplying 1-mJ pulses at 20 Hz, was propagated through the source region parallel to the sample filaments. The laser was tuned to the Lu, 452 nm 2D3j2(5d6s2) 2D0312(5d6s6p) transition (10) via optogalvanic excitation of a commercial Lu hollow cathode lamp (Figure 1)(11,12). Excitation to the 2D03jzexcited state was followed by photoionization using photons of the same wavelength. Laser-generated ions were detected and the signals processed with a current integrating multiplier interfaced to a microcomputer. The resonance-enhanced two-photon ionization process was found to be extremely selective. Figure 2a shows the thermal mass analysis of an equimolar mixture of Lu and Yb. The thermal Yb signal is orders of magnitude larger than that from Lu due to the large difference in volatilities of the two elements. Figure 2b depicts the result of laser photoionization of the same sample undler conditions in which thermal ionization is minimized and shows a dramatic enhancement in the Lu signal. No laser-generated Yb ions were detectable, so that the selectivity, S, of the photoionization process is S I50 000 = N175/N17,1/2, where NlT6is the number of photogenerated Lu ions, N174 is the background signal at mass 174 (Yb), and the calculated iselectivity represents a rigorous lower limit. Both signals were integrated for 2 min. This high selectivity is likely due to the absence of both one and two photon resonances in Y'b (see Figure 1) and the relatively gentle laser conditions, I I30 kW/cm2. The fidelity of the laser photoionizatnon method is also excellent. The observed ratio of 175Lu/176Lu is within 1% of the thermal ionization value, 37.6, after - ~ min 2 integration a t each mass; the measured ratio could be made arbitrarily accurate by extending the length of the integration period. This lack of isotope selectivity is presumably due to the relatively large line width of the excitation laser, -1 cm-lJ coupled with Doppler broadening so that all isotopic and hyperfine components were addressed by the laser. While rigorous detection limits ( S I N 2 2) have not yet been determined, a preliminary series of experiments has shown that samples I 1 ng are detectable.
-
t I
6oooo~
I I
I I
_ _ _ -Ic _ __-_
t
I I
=+= I
40000-
-
%
-
I
I
I
i
20000 -
I I
I
I
-
I I I
I
0-
Yb
Lu
Flgure 1. Partial energy level scheme for Lu and Yb, showing the two photon ionization scheme used for Lu. Dashed lines indicate ionization limit: 50441 cm-' for Yb, arid 43762 cm-' for Lu. Note that the closest one photon transition in Yb is 1064 cm-' from the excltation frequency, while the closest allowed two photon transition Is 17 cm-' from resonance.
I
"
I
I
I
I
I
I
n
MASS (omul
Figure 2. (a) Thermal ionization of equimolar LulYb mixture. The iqnizing filament is at -2000 OC. Full scale is -5000 counts. (b) Photoiodization of the same sample with thermal ionizing filament turned off. Full scale Is -250 counts. Scan time is the same in both cases; SIN is degraded for the photoionization curve because of the low duty cycle Note that only Yb is observed in the upper trace because of Its greater volatility and susceptibility to thermal ionization, while only Lu is seen In the lower trace.
0003-2700/82/0354-2377$01.25/00 1982 American Chemical Society
Anal. Chem. 1982,5 4 , 2378-2379
2378
In summary, we have demonstrated that resonance ionization mass spectrometry can be used in the analysis of (aqueous) samples of common analytical interest. Matrix and charge exchange effects do not seriously degrade the selectivity of the process, and ionization process is isotopically nonselective.
(9) Inghram, M. G.: Chupka, W. A. Rev. Sci. Instrum. 1953, 24, 518-520. (IO) Martin, W. C.; Zalubas, R.; Hagen. L. "Atomic Energy Levels-The Rare-Earth Elements"; National Bureau of Standards: Washington, DC, 1978: DD 398-403. (11) Keller, R. A,'; Engleman, R., Jr.; Zalewski, E. F. J. Opt. SOC. Am. 1979, 69,738-742. (12) Keller, R. A.; Zalewski, E. F. Appl. Opf. 1980, 19, 3301-3305.
C. M. Miller N. S. Nogar*
ACKNOWLEDGMENT The technical assistance of D. J. Rokop and several discussions with R. A. Keller are gratefully acknowledged.
Group CNC-2, Mail Stop G738 Los Alamos National Laboratory Los Alamos, New Mexico 87545
LITERATURE CITED Whltaker, T. J.; Bushaw, B. A. Chem. Pbys. Left. 1981, 79,506-508. Bushaw, B. A.; Whitaker, T. J. J. Chem. Phys. 1981, 74,6519-6520. Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1979, 5 1 , 767-819. Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Chen, C. H. Phys. Today 1980, 3 3 , 24-29. Young, J. P.; Hurst, G. S.; Kramer, S. D.; Payne, M. G. Anal. Chem. 1979, 51, 1050A-1080A. Hurst, G. S. Anal. Chem. 1081, 53, 1448A-1456A. Beekman, D. W.: Callcott, T. A.; Kramer, S.D.; Arakawa, E. T.; Hurst, G. S. I n t . J . Mass Spectrom. Ion Phys. 1980, 34, 89-97. Mayo, S.: Lucatorto, T. B.; Luther, G. G. Anal. Chem. 1982, 5 4 , 553-556.
A. J. Gancarz W. R. Shields Group CNC-7, Mail Stop E514 Los Alamos National Laboratory Los Alamos, New Mexico 87545 RECEIVED for review June 21, 1982. Accepted July 16, 1982. The support of the Department of Energy under the auspices of the Los Alamos National Laboratory is gratefully acknowledged.
Composition Determinations of Liquid Chloroaluminate Molten Salts by Nuclear Magnetic Resonance Spectrometry Sir: Some binary mixtures of aluminum chloride and selected organic chloride salts form melts that are liquid considerably below room temperature. Two types of these mixtures have been studied in recent years: one having alkylpyridinium (I) as the cation (1-3) and another employing a dialkylimidazolium (11)cation (4). These molten salts are 0
-
i,tL R ,
R
I
/N,L;,
R
I1
aprotic, anhydrous ionic liquids that may be useful for electrochemistry, spectroscopy, and synthesis. The melts are prepared by simply mixing AlC13 and the chloride salt of I or 11, resulting in a clear liquid. Many of the physical and chemical properties of the melts depend markedly on composition, i.e., the relative proportions of AlC13 and the organic chloride salt. The composition is usually expressed as the apparent mole fraction of AlC13 (XAQ),although no molecular A1C13 or AlzCls species apparently exist in these melts (5). Important compositiondependent properties of these melts include their acid-base characteristics (4,6, 7) and melting points. For example, melts prepared from I1 (where R = methyl and R' = ethyl) exhibit melting points of 8 "C and -98 OC for XMQ= 0.50 and 0.66, respectively. Clearly it is important to know the composition of a given melt. An approximation of the melt composition may be made from the amount of the ingredients used to prepare the sample. However, impurities in the starting materials and changes in the composition during the melt preparation procedure often result in the need for a more accurate determination of composition. T o date, the most accurate method for composition determination in chloroaluminate melts is by a tedious potentiometric titration of the melt (3-5),where an equivalence point is observed at precisely XAICls = 0.5. We report here a simple, rapid and nondestructive technique for composition determinations in chlo-
roaluminate molten salts having organic cations such as I or 11. We have applied the method to chloroaluminate melts prepared from 1-(1-buty1)pyridinium (BuPy) and l-methyl3-ethylimidazolium (MeEtIm) chlorides.
EXPERIMENTAL SECTION Apparatus. Nuclear magnetic resonance spectra were recorded at 60 MHz on a Varian T-60A or a Hitachi Perkin-Elmer R-24P
spectrometer. Reagents. Chloroaluminate melts were prepared from purified AlC1, and 1-(1-buty1)pyridinium chloride (6) or l-methyl-3ethylimidazolium chloride (2) as described elsewhere. Procedure. The NMR spectra were obtained from samples contained in 5-mm tubes, which were loaded in dry argon or nitrogen atmosphere gloveboxes. Either an internal standard of chlorotrimethylsilane or an external standard of MezSO was added, and the tubes were securely capped to exclude moisture. The temperature was maintained constant during the NMR measurements (35 f 1 OC for type I melts and 34 f 1 OC for the type I1 melts).
RESULTS AND DISCUSSION Robinson et al. (8)observed that the proton chemical shifts of I (R = 1-butyl) in chloroaluminate molten salts varied as the dielectric of the melt was changed by addition of a solvent. We have performed similar experiments, but we in addition measured proton chemical shifts as a function of XMQin the A1Cl3-BuPy melt and one having a type I1 (MeEtIm) cation. The chemical shifts of the ring protons changed significantly in both types of melts, especially in the compositions where XAlcls< 0.5. Figure 1 shows the composition dependence of the chemical shifts for the two types of melts. Data for the particular proton shift undergoing the largest chemical shift change are shown in each case. For the compositions XAQ < 0.5 the NMR chemical shifts are sensitive indicators of melt composition and may be used to determine the composition of an unknown melt. This is a valuable finding, since the potentiometric methods for determining melt composition that involve measurements at an aluminum electrode are not ap-
0003-2700/82/0354-2378$01.25/0 C 1982 American Chemical Society
