Technical Notes Anal. Chem. 1996, 68, 4072-4075
Separation of Lanthanide β-Diketonates via Organophosphorus Adduct Formation by Supercritical Fluid Chromatography Hong Wu,† Yuehe Lin,† N. G. Smart,‡ and C. M. Wai*,†
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, and Company Research Laboratory, BNFL, Springfields Site, Salwick, Preston PR4 OXJ, U.K.
This paper describes a novel method for separation and detection of lanthanide β-diketonates by adduct formation/supercritical fluid chromatography (SFC) with an open-tubular capillary column and a FID detector. Separation of lanthanide β-diketonates cannot be achieved by SFC due to the hydration of the complexes, which exhibit strong intermolecular interactions and decomposition in the system. Formation of adducts of lanthanide β-diketonates with a neutral donor, tributylphosphine oxide (TBPO) or trioctylphosphine oxide (TOPO), alters their SFC behavior. The adduct formation approach leads to the first successful separation of lanthanide complexes of the same β-diketone ligand by SFC using neat CO2 as the mobile phase. Supercritical fluid chromatography (SFC) is generally applied to organic compounds, even though the first reported SFC separation, in 1962, was that of nickel porphyrin complexes.1 Since then, there have been fewer than 30 published papers on SFC of organometallics and metal chelates.2 Using gas chromatographic (GC) methods for separation of metal chelates, difficulties often arise due to the low vapor pressure of many metallic compounds. The use of high temperature to increase analyte volatility often causes decomposition during GC separation. High-performance liquid chromatography (HPLC) has also been used for metal chelates separation.3,4 In HPLC, problems arise from limited resolution, degradation, and irreversible adsorption of the analytes to the stationary phase. SFC, which combines the solubility property of LC and the high diffusivity of GC, can be operated at a lower temperature than GC. It appears to provide a better choice for chromatographic separation of metal chelates. So far, most SFC separations of metal chelates have been performed on packed column and have proven difficult when pure carbon dioxide is †
University of Idaho. BNFL. (1) Klesper, E.; Corwin, A. H.; Turner, D. A. J. Org. Chem. 1962, 27, 700. (2) Lin, Y.; Smart, N. G.; Wai, C. M. Trends Anal. Chem. 1995, 14, 123. (3) Willeford, B. R.; Veening, H. J. Chromatogr. 1982, 251, 61. (4) Wang, S.; Wai, C. M. J. Chromatogr. Sci. 1994, 32, 506. ‡
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the mobile phase.5-8 Usually, a modifier such as methanol is added to the carbon dioxide to increase solubility and decrease retention times and column activity. Because of the modifier, flame ionization detection (FID), a sensitive and commonly used detector for SFC, cannot be used. Problems associated with packed columns in SFC may be minimized by using deactivated fused silica open-tubular capillary columns. It is interesting to study the feasibility of separating the β-diketone complexes formed by different lanthanides with a specific ligand. Separation of lanthanide series complexes formed by the same ligand has not been accomplished using SFC. β-Diketones form the most volatile metal chelates known in the literature. For that reason, they are the most commonly studied metal chelate system for GC9,10 and SFC.5-8 Recently, various β-diketones have been used for in situ chelation/supercritical fluid extraction (SFE) of lanthanides and actinides from liquid and solid matrices.11-13 SFC is one method of analyzing the extracted lanthanide β-diketonates. This study attempts to develop a separation and detection method for lanthanide β-diketonates using open-tubular capillary SFC with neat CO2 as the mobile phase and FID as the detector. The main objective of this study is to investigate the solvation behavior and stability of lanthanide β-diketonates, as well as their chromatographic behavior under supercritical fluid conditions. Successful separation of lanthanide chelates of FOD (2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5,-octanedione) via adduct formation with an organophosphorus reagent is described for the first time in this paper. (5) Ashhraf-Khorassani, M.; Hellgeth, J. W.; Taylor, L. T. Anal. Chem. 1987, 59, 2077. (6) Laintz, K. E.; Iso, S.; Meguro, Y.; Yoshida, Z. J. High Resolut. Chromatogr. 1994, 17, 603. (7) Fujimoto, C.; Yashida, H.; Jinno, K. J. Microcolumn Sep. 1989, 1, 19. (8) Jinno, K.; Mae, H.; Fujimoto, C. J. High Resolut. Chromatogr. 1990, 13, 13. (9) Moshier, R. W.; Sievers, R. E. Gas Chromatography of Metal Chelates; Pergamon: Oxford, 1965. (10) Guiochon, G.; Pommier, C. Gas Chromatography in Inorganics and Organometallics; Ann Arbor Science Publishers: Ann Arbor, MI, 1973; pp 217218. (11) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem. 1993, 65, 2549. (12) Lin, Y.; Wai, C. M. Anal. Chem. 1994, 66, 1971. (13) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1994, 28, 1190. S0003-2700(96)00500-8 CCC: $12.00
© 1996 American Chemical Society
EXPERIMENTAL SECTION All chromatographic separations were performed using a Lee Scientific Series 600 SFC/GC (Dionex, Sunnyvale, CA) coupled with a FID detector and the AI-450 chromatography work station controlled by an IBM486 computer. The column employed in this study was an open-tubular capillary column (100 µm i.d., 200 µm o.d., 5 m length) coated with a 0.25 µm film of methyl silicone (Dionex). The flow was controlled by a 100 µm frit restrictor, trimmed to give a solvent peak of ∼5 min retention time at 100 °C and 100 atm. All separations were performed at a constant oven temperature of 80 °C, FID temperature 390 °C, and pressure gradients at various ramp rates as indicated. The injection time was 0.025 s, time-splitting on a 200 nL loop. Lanthanide-FOD complexes and organophosphorus reagents, tributylphosphine oxide (TBPO) and trioctylphosphine oxide (TOPO), used in this study were obtained from Aldrich Chemical Co. Solutions of the lanthanide-FOD complexes were prepared with HPLC-grade chloroform at a concentration of 5 × 10-3 M. The adducts of lanthanide-FOD complexes with TBPO or TOPO were prepared by the addition of an excess amount of the organophosphorus reagent to the chloroform solution containing lanthanide FOD complexes. The carbon dioxide used was SFC/SFE grade (Air Products, Allentown, PA). Hydrogen, air, nitrogen for FID were all from Liquid Carbonic (Chicago, IL) with 99.99% purity. RESULTS AND DISCUSSION 1. SFC of Lanthanide-FOD Adducts with TBPO and TOPO. When lanthanide chelates of FOD are injected into the SFC system, no peak is observed in the SFC chromatogram. The result suggests that lanthanide chelates of FOD may have strong solute interactions and probably are thermally unstable. The interactions may be the result of incomplete shielding of the center metal ion by the ligand or due to the hydration of the lanthanide metal chelates. Hydration of lanthanide complexes is common because the trivalent lanthanide ions usually have large coordination numbers, 8 or 9. Because of these interactions, the lanthanide β-diketonates probably decompose in supercritical CO2 and are irreversibly adsorbed inside the SFC column. Thermal decomposition of the hydrated lanthanide β-diketonates was studied by thermogravimetric analysis and GC and was shown to proceed by the following reaction:14
LnB3‚2H2O f LnB2OH‚H2O + H-B
where Ln represents a trivalent lanthanide ion and B a deprotonated β-diketone. If this reaction takes place in the SFC system, lanthanide-FOD may lose one ligand and precipitate onto the column walls after loss of one ligand. It is known that, when an organophosphorus ligand such as TBPO or TOPO is added to a chloroform solution of lanthanide β-diketonate, adduct formation will take place with the neutral organophosphorus ligand, replacing the water molecules coordinated with the metal complex. The resulting adduct complex should be better shielded and may reduce its intermolecular interactions and become thermally stable in SFC. Adduct formation thus may provide a method of stabilizing lanthanide β-diketonates and making them more soluble in supercritical CO2 and consequently separable in SFC. Our experimental results support (14) Khalmurzaev, V. I. Zh. Neorg. Khim. 1976, 21, 1635.
Figure 1. SFC chromatograms of lanthanide FOD adducts with (A) TBPO and (B) TOPO. Peaks: A, (1) chloroform, (2) TBPO, (3) Er(FOD)3‚2TBPO, (4) Eu(FOD)3‚2TBPO, (5) Pr(FOD)3‚2TBPO, (6) La(FOD)3‚2TBPO; B, (1) chloroform, (2) TOPO, (3) Er(FOD)3‚2TOPO, (4) Eu(FOD)3‚2TOPO, (5) Pr(FOD)3‚2TOPO, (6) La(FOD)3‚2TOPO. SFC conditions: pressure, (A) initial 100 atm, 3 atm/min increase, (B) initial 80 atm, 5 atm/min increase; oven temperature, 80 °C; FID, 390 °C.
the adduct formation approach. Figure 1 shows the complete separation of La-, Pr-, Eu-, and Er-FOD organophosphorus adducts with TBPO and TOPO by SFC at 80 °C and the specified pressure ramp conditions. The retention times of these lanthanide adduct complexes increase with increasing ionic radius of the trivalent lanthanide ions. It is known that compounds with high solubility in supercritical CO2 usually show short retention times in SFC.15,16 The SFC results suggest that the solubility of these lanthanide-FOD adduct complexes should increase from La to Lu. This trend seems to correlate with the reported SFE data, which indicate that heavy lanthanides are extracted with higher efficiencies relative to the light ones by FOD in supercritical CO2.2,12 The general trend observed in SFC retention times of the lanthanide-FOD adducts can be attributed to a combination of steric and electronic effects imposed by the lanthanide contraction. With decreasing ionic radius in the Ln3+ series, the size of the complex decreases and the shielding of the lanthanide ion improves. In addition, the increase in ionic potential of the lanthanide ion probably lowers the polarizability of the β-diketonate rings, reducing their capacity for intermolecular interactions. The overall effect is an increase in volatility and solubility as the atomic number of the lanthanide ion increases. The formation of the lanthanide adduct complex in chloroform can be represented by the following reaction:
Ln(FOD)3‚xH2O + xL f Ln(FOD)3‚xL + xH2O where L represents an organophosphorus ligand. To evaluate the number of L in the adduct complex, chloroform solutions containing various molar ratios of lanthanide FOD and L (TOPO or TBPO) were prepared by adding the former to a fixed amount of L and separated by SFC. The SFC chromatograms showed that the height of the TOPO peak decreased with increasing amount of Eu(FOD)3 in solution. When the molar ratio of Eu(FOD)3: TOPO equaled 1:2, the TOPO peak disappeared, indicating that TOPO was completely incorporated into the adduct. Therefore, (15) Bartle, K. D.; Clifford, A. A.; Jafar, S. A. J. Chem. Eng. Data 1990, 35, 355. (16) Cowey, C. M.; Bartle, K. D.; Burford, M. D.; Clifford, A. A.; Zhu, S.; Smart, N. G.; Tinker, N. D. J. Chem. Eng. Data 1995, 40, 1217.
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Figure 2. Effect of temperature on capacity factors of TBPO, Er(FOD)3‚2TBPO, Eu(FOD)3‚2TBPO, Pr(FOD)3‚2TBPO, and La(FOD)3‚2TBPO. SFC conditions are the same as in Figure 1A.
the adduct complex of Eu(FOD)3-TOPO must contain two TOPO molecules. The results obtained from other lanthanide FOD adducts with TBPO or TOPO also indicate that the number of TOPO or TBPO contained in each adduct is 2. In solvent extraction, the organophosphorus adduct complexes of lanthanide β-diketonates were found to contain one or two neutral donors.17,18 The decomposition of metal β-diketonates in an SFC system was reported previously by Jinno et al.7,8 According to their study, many metal β-diketonates are unstable in supercritical CO2. However, if methanol and β-diketone ligand were premixed with supercritical CO2 and used as the mobile phase, the stability of metal β-diketonates in SFC could be improved. Because methanol and the ligand can cause a high background in both FID and UVvisible detectors, a more selective detector, ICP-AES, was used by Jinno et al. in their studies. ICP-AES is an expensive and complicated detector which is not often available in many laboratories. The adduct formation, as demonstrated in this study, can greatly improve the stability of lanthanide β-diketonates without addition of methanol and ligand in supercritical CO2, and consequently FID can be utilized in SFC 2. Effect of Temperature on Chromatographic Retention. The effect of temperature on SFC separation of lanthanide-FODTBPO adducts was investigated and the capacity factor calculated. The changes in capacity factor as a function of temperature are shown in Figure 2. The capacity factor is defined as k ) (tr t0)/t0, where t0 is the retention time of solvent and tr is the retention time of the lanthanide-FOD-TBPO adduct. In SFC, the capacity factor is a function of a compound’s solubilities in the mobile and the stationary phases. The solubility of a solute in supercritical CO2 is a function of two competing factors, solvation and (17) Mitchell, J. W.; Banks, C. V. Talanta 1972, 19, 1157. (18) Sekine, T.; Dyrssen,D. J. Inorg. Nucl. Chem. 1967, 29, 1481.
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volatility.19 As temperature increases, the density of supercritical CO2 for a given pressure decreases; therefore, the solvating power of supercritical CO2 decreases. At the same time, the volatility of a compound increases with temperature, and the contribution of volatility to the solubility increases. The overall change of solubility in supercritical CO2 depends on which factor is the dominating one. If the solvation is the dominating factor, then the solubility of the solute should decrease, i.e., its capacity factor should increase with temperature. The SFC behavior of the solute should be LC-like. If the volatility is the dominating factor, then the solubility of the solute should increase, and its capacity factor should decrease with temperature. In this case, the SFC behavior of the solute should be GC-like. It has been demonstrated that the SFC capacity factor correlates well with the SFE recovery data of many organic compounds.20-23 Knowledge of the GC-like or LC-like behavior of the solute under various conditions is very useful for rapid optimization of supercritical fluid extraction conditions. The capacity factors of TBPO and the lanthanide-FOD-TBPO adducts increase with temperature under the specified conditions (60-120 °C), as shown in Figure 2. In this temperature range, the solvating power of the supercritical CO2 should dominate the solubility of TBPO and the lanthanide adducts in supercritical CO2, and the SFC behavior of these compounds is LC-like. From Figure 2, it can also be seen that the change in the capacity factor of TBPO, which has a much higher volatility than lanthanideFOD-TBPO adducts, is less significant with respect to temperature compared with the lanthanide complexes. At 60 °C, the capacity factors of the four lanthanide adducts are almost the (19) Chester, T. L.; Innis, D. P. J. High Resolut. Chromatogr. Chromatogr. Commun. 1985, 8, 561. (20) McNally, M. E.; Wheeler, J. R. J. Chromatogr. 1988, 447, 53. (21) Wheeler, J. R.; McNally, M. E. J. Chromatogr. Sci. 1989, 27, 534. (22) Furton, K. G.; Rein, J. Anal. Chim. Acta 1991, 248, 263. (23) Rein, J.; Cork, C. M.; Furton, K. G. J. Chromatogr. 1991, 545, 149.
same; therefore, separation of these adducts would be difficult at this temperature. According to Figure 2, the best separation of the lanthanide-FOD adducts is around 80 °C, as demonstrated by the chromatogram shown in Figure 1A. CONCLUSIONS The hydrated lanthanide-FOD complexes exhibit strong intermolecular interactions and decomposition in SFC. Formation of adducts of the lanthanide β-diketonates with a neutral donor such as TBPO or TPPO can greatly improve their SFC behavior. This adduct formation approach leads to the first successful separation of lanthanide complexes of the same ligand using neat CO2 as the mobile phase in SFC. Adduct formation has also been shown to improve the extraction efficiencies of lanthanides from different matrices by supercritical carbon dioxide containing a
fluorinated β-diketone and an organophosphorus reagent.2,12 A hyphenated technique involving adduct formation/SFE/SFC described in this paper may provide a novel method for analysis of lanthanide β-diketonates and for possible large-scale separation of the rare earth elements. ACKNOWLEDGMENT The authors thank Dr. R. E. Sievers and Dr. K. Jinno for helpful discussion. This work was supported by NSF-Idaho EPSCoR Program under NSF Cooperative Agreement OSR-9350539. Received for review May 21, 1996. Accepted August 30, 1996.X AC9605004 X
Abstract published in Advance ACS Abstracts, October 15, 1996.
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