800
Anal. Chem. 1987, 5 9 , 808-813
(5) Tarter, J. G. Anal. Chem. 1984, 5 6 , 1264-1268. (6) Dasgupta, P. K. Anal. Chem. 1984, 56, 769-772. (7) Irgum. K. Anal. Chem. 1987, 59, 358-362. (8) Riviello, J. M., Pohl, C. A,, Taylor, M. S.37th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1986; Abstract No. 447. (9) Rocklin, R. D.; Pohl, C. A. 37th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1986; Abstract No. 585. (10) Rein,; M I Van der Linden. W E , Poppe, H Anal Chjm Acta 1981, 123 229-237 (11) Dasgupta. P. K. Anal. Chem. 1984, 5 6 , 96-103. (12) Stilliin, J. R. LC Mag. 1985. 3 , 802-812. (13) Mercurio-Cason, M. A.; Dasgupta, P. K.; Blakeley, D. W.; Johnson, R. L. J . Membr. Sci. 1986. 2 7 , 31-40. (14) Tanaka, K.; Ishihara, Y.; Sunahara, H. Bunseki Kagaku 1975, 2 4 , 235-238. (15) Girard. J. E. Anal. Chem. 1979, 57, 836-839.
(16) Tarter, J. G. J . Liq. Chromatogr. 1984, 7 , 1559-1566. (17) Ruricka, J.; Hansen, E. H.; Zagatto, E. A. Anal. Chim. Acta 1977, 88, 1-36. (18) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley: New York, 1981; p 212. (19) Dasgupta, P. K.; Bligh, R . Q.; Lee, J.; D'Agostino, V. Anal. Chem. 1985, 5 7 , 253-257. (20) Hwang, H.; Dasgupta, P. K. Anal. Chem. 1986, 5 8 , 1521-1524.
RECEIVED for review September 3,1986. Accepted November 24,1986. This research was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Grant No. DE-FG05-84ER-13281. However, this report has not been subject to review by the agency and no official endorsement should be inferred.
Computer-Controlled Pneumatic Amplifier Pump for Supercritical Fluid Chromatography and Extractions Gilbert L. Pariente, Stephen L. Pentoney, Jr., Peter R. Griffiths,* and Kenneth H. Shafer
Department of Chemistry, University of California, Riverside, California 92521
A computer-cuntrdled pumping system for supercritkai fluids has been constructed based on a pneumatic amplifier pump. When CO, Is used as the fluM, It has been shown that the pump has pressure control as good as that of a syringe pump. I t must refill more often than a syringe pump with a large syringe volume, but the reflil t h e is short enough that the pump can refill during supercritical fluid extraction or chromatography without 111 effect. Additionally, the pump can attaln a set pressure much faster than syringe pumps, whlch makes it much more convenlent for extraction experiments where several dlscrete pressure intervals may be desirable.
The advantage of using supercritical fluid chromatography (SFC) for the high-resolution separation of molecules of low volatility has become widely recognized in recent years. The burgeoning popularity of SFC stems from the advantages that may be gained from the density, solvation, and diffusivity characteristics of supercritical fluids. A supercritical fluid can solvate large molecules that cannot be volatilized in a gas chromatograph (GC). The diffusivities of solutes in supercritical fluids are greater than their diffusivities in liquids. Thus the equilibrium of solutes between the mobile phase and the stationary phase is more rapid than in high-performance liquid chromatography (HPLC). In addition, the viscosity of most supercritical fluids is sufficiently low that they may be used with long open tubular columns to achieve higher resolution than HPLC using packed columns in reasonable analysis times. A further advantage of supercritical fluids over liquid solvents is that it is possible to control the solvation of solutes in the supercritical fluid by varying the density of the fluid. This control is accomplished by varying the pressure and/or the temperature of the system. This property makes supercritical fluid extraction (SFE) especially attractive in view of the potential selectivity of extraction by pressure control. Until recently no commercial instrumentation for capillary SFC has been available. Modified Varian 8500 syringe pumps (I,2) have been used by many workers for capillary SFC, and recently other syringe pumps specifically designed for SFC
are beginning to be introduced commercially ( 3 ) . Syringe pumps allow direct control of the flow rate of the mobile phase, while pressure is controlled by increasing or decreasing the flow rate through the restrictor at the end of the column. Refilling syringe pumps can be a time-consuming procedure. While it may not be necessary to refill a syringe pump often for capillary SFC because of the low flow rates required (a few microliters per minute), when supercritical fluids are used for packed column chromatography or for extractions where flow rates of several milliliters per minute may be desired, the slow speed of the refill cycle presents a severe limitation, since a reduction of pressure can occur for several minutes each time the pump refills. Reciprocating pumps also have been modified to pump compressible fluids above their critical pressure in an attempt to circumvent this limitation of syringe pumps ( 4 , 5 ) . Reciprocating pumps have enjoyed less popularity than syringe pumps in part because the pump heads must be cooled to improve pumping efficiency of fluids such as carbon dioxide ( 4 , 5 )and in part because of the large pressure fluctuations due to the reciprocation of the piston ( 5 ) . Both types of pumps are primarily designed for flow control and must be modified in order to control the pressure of the mobile phase. Conversely, pneumatic amplifier pumps are designed to contol output pressure and should therefore have appropriate properties for pumping compressible fluids. This type of pump works by applying air pressure to the lowpressure side of a two-sided pump piston. The low-pressure side of the piston has a large area relative to the side that displaces the fluid to be pumped (high-pressure side). T o refill the pump, the pressure is removed from the low-pressure side and the piston is allowed to slide back while fluid from a reservoir flows into the empty chamber. Past applications of these pumps have been successful but cumbersome. These systems controlled output pressure by regulating the flow of high-pressure nitrogen into the low pressure side of the pump by means of a pressure regulator. Pressure programming was accomplished by attaching a stepper motor to the regulator (6, 7). In this paper we describe a new method of controlling a pneumatic amplifier pump for SFC and SFE. This pump
0003-2700/87/0359-0808$01.50/0 C 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
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LABORATORY CYLINDER
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-
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allows for rapid refilling and operates either at constant pressure or under pressure or density programming.
EXPERIMENTAL SECTION i. Pumping System. The pumping system used is built around a pneumatic amplifier pump (Haskel Model DSTV-122, Burbank, CA). A block diagram of this system is shown in Figure 1. This pump has a nominal 122:l pressure amplification and a 10-mL volume. The fluid to be pumped (typically COz for this work) is fed to the pump through two filters, one of silica gel 423, 100/200 mesh (Alltech Associates), mixed with a molecular sieve (MX1583L-1, Type 4A, 8-12 mesh beads, MCB Reagents), the other Ambersorb XE-340 (Rohm and Haas), and a cooled reservoir constructed from 3/8-in.-o.d.stainless steel tubing 2 ft in length. The reservoir is cooled to maintain the COP drawn from the cylinder in the liquid state and is mounted vertically to minimize the amount of gas that is drawn into the pump. It is cooled by wrapping the tube with 1/4-in-o.d.aluminum tubing through which ice water is circulated. When C02 is used from a tank equipped with an eductor tube, it is not necessary to cool the reservoir once it is filled with liquid. To control the output pressure a control loop was implemented by using a pressure transducer (Setra 204E) on the high-pressure side of the pump. This transducer provides a signal of 0-5 V over 0-1OO00 psi. The signal was buffered by an operational amplifier, digitized by a Micronetworks MN5280 analog-to-digital converter (ADC) configured for 14 bits over 0-5 V, and monitored by the computer (Commodore 64) used for instrument control. The pressure is also displayed as a decimal number on a four-digit display. A minimum pressure was maintained by a pressure regulator on the low-pressure side. Pressure control of the high-pressure output from the pump was accomplished by controlling the pressure on the low-pressure side by supplying the pump pulses of compressed air. For large pressure changes, a two-way solenoid (labeled B, Figure 1) with a I/&. opening (Eemco 24B3L80T6Y-l2D), which opened for approximately 5 ms per pulse, was used. For small pressure changes, a much smaller solenoid valve (Vacuum General Model CV6-41)open for approximately 14 ms per pulse was used. The control program was written in the BASIC language provided by the computer. This BASIC program was sufficiently fast to control the small changes required to maintain the fluid at a given pressure and to generate a gentle pressure ramp. However, for refilling the pump or for generating large pressure changes, the speed of an assembly language program was required. For this reason the BASIC pressure control subroutine depicted in Figure 2 must call the assembly language subroutine shown in Figure 3. The desired set pressure is passed to the BASIC
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Flgure 2. Flow chart of BASIC language pressure control subroutine.
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Flgure 3. Flow chart of assembly language pressure control subroutine.
subroutine from the main program. For linear pressure ramps, this pressure was taken from a pressure/time table generated by using a simple linear equation prior to the start of the experiment.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
Table I. Estimated Standard Deviation (esd) of the Difference between Measured Pressure (Digitized at 2 Hz over 3 min) and Set Pressure set pressure,
5 pm i.d. FUSED SILICA
Sample collection apparatus for infrared spectrometry.
For linear density ramps a pressure/time table is generated by using a seventh order polynomial fit of pressure-density data. In processing step 1, the pressure is converted to a raw decimal value equivalent to the decimal value the ADC would return a t that pressure. This conversion allows comparison to the actual ADC values without changing units, which is especially convenient in the assembly language subroutine. Two parameters are adjusted by the program to control the pressure. First the length of time a valve is open may be adjusted by sending control pulses to the solenoid controller at a rate which is sufficient to retrigger the one-shot before the one-shot's pulse is over. A BASIC ForNext loop is sufficient for this purpose in view of the pulse duration of the one-shot. The duration the valve is open can be controlled by changing the limit of the loop. The second parameter controls the frequency of these pulses. Both parameters are determined by the set pressure and the difference between the set pressure and the actual pressure, AF'. In the processing step 2, the constraints on these parameters are set, on the basis of the set pressure. The ADC is then read and AP is calculated. If AF' is great enough, the assembly language subroutine is called. In this case the ADC is again read and AP calculated. The small solenoid is first pulsed to help stabilize the pressure. The large solenoid is then pulsed until the pressure is in the range predetermined by processing step 2 of the pressure control subroutine. Once the pressure is within this range, the pressure is again checked to see if it falls in the next range that was determined in processing step 2 to be within the control range of the large solenoid. The necessity of the two checks results primarily from the two byte representation of the ADC value in the computer. If the pressure is close enough for the small solenoid to be effective, the subroutine gives the small solenoid a pulse before returning. The BASIC subroutine calls the assembly language subroutine 150 times because the pressure will drop rapidly after it reaches the set point. By the repetitive calls the program can stabilize the pressure before asking the relatively slow reacting BASIC control to take over. Once the pressure is stabilized, the BASIC pressure control subroutine sets the two parameters based on AP within the constraints set in processing step 2 of the subroutine. A software counter keeps track of how often the ADC reads that the pressure is low. The counter is compared to a set value determined in processing step 8. If the pressure is read to be low the necessary number of times, the small solenoid is then pulsed. The duration of the pulse is determined by the limit of this loop, which is determined in processing step 8. Once the solenoid is actuated, the counter is reset. An IBM PC A T computer with a Kiethley 5000 14-bit data acquisition system collecting at 2 Hz was used to monitor the performance of this system. ii. Supercritical Fluid Extractions. Supercritical fluid extractions were carried out by using the apparatus shown in Figure 4. The sample was loaded into a 1-in. length of '/&-o.d.
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stainless steel tubing with a stainless steel frit mounted in a to lIl6-in. reducing union at one end. The opposite end was then capped with the same type of reducing union. This unit was connected to l/la-in.-o.d. tubing and placed in an aluminum extractor block. The block was heated with a temperature controller (Accuspec) with the heating element and transducer placed on either side of the sample. Threads were cut into the outside of the block to accommodate 1/16-in.stainless steel tubing to allow the fluid to be preheated above the critical temperature before it reached the sample. The extract was collected by passing it out of the extraction column to a heated 1/16-in.stainless steel transfer line which is terminated with a 5-cm-long, 5-pm4.d. fused silica restrictor held in a graphitized Vespel ferrule (Alltech, 10010-VG2) drilled with a 250-pm hole. The restrictor was positioned above a potassium bromide window where the extract was deposited. CO, expanding out of the restrictor results in sufficient cooling to condense atmospheric water vapor, which occasionally dripped onto the window. To eliminate this problem, the entire collection apparatus was housed in a container which was purged with dry air. The window could be positioned without disrupting the purge. To characterize the extract, the window was moved to a Digilab FTS-60 spectrometer equipped with a microscope accessory and narrow-band mercury cadmium telluride detector. Spectra were obtained by signal averaging 256 scans at 4 cm-' resolution. Base line correction was necessary because of light scattering by the solid extract collected in this manner. iii. Supercritical Fluid Chromatography. Supercritical fluid chromatography was performed by using a Hewlett-Packard Model 5890A gas chromatograph equipped with a flame ionization detector. A 19.5-m-long, 100-pm-i.d., wall coated open tubular column (J&W Scientific, Rancho Cordoba, CA) with a 0.4-pm coating of a 5% phenyl methyl siloxane stationary phase (DB5) was used. Samples were injected with a 60-nL injector (Valco) used in a direct injection manner.
RESULTS AND DISCUSSION i. Pump Performance. The primary consideration in the performance of a pump for SFE and SFC is its ability to control pressure accurately. To monitor the pressure when the pump was used in an isobaric mode, the pump was connected to a capillary GC column terminated with a 5-pm restrictor maintained at 70 "C. The pressure was increased to the set pressure and then measured for 3 min. The results are shown in Table I. The pressure was only taken t o 4300 psi t o avoid damage t o the column connections, but it may be noted that the pump itself is rated to 10000 psi. At all pressures the pump maintains the average pressure to within 2 psi, and the estimated standard deviation is less than 2 psi. One advantage of the pneumatic amplifier pumping system is its capability to attain a given pressure very rapidly. A plot of pressure vs. time during startup is shown in Figure 5 . At the start of operation, it can be seen that the pump was able to change the pressure by 1000 psi and reach its set point in about 1s. Another 5 s were required to stabilize the pressure. When operating with a fairly high flow rate, pumps must rapidly refill during an experiment with a minimal change in pressure. In Figure 6, the variation in pressure during a linear pressure ramp is shown at a point where a refill occurs. During this refill, the largest pressure change was about 50 psi and
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
811
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the set pressure was regained in 18 s. While the pressure can be raised significantly faster, pressure overshoot is possible and this condition was strenuously avoided. The times given above are the result of a compromise that allows the pressure to be raised quickly but without overshooting the set pressure. The effect of refilling will be discussed for both supercritical fluid extractions and chromatography. ii. Coffee Extractions. The use of supercritical C 0 2 to extract caffeine from coffee has been shown previously both on an experimental and commercial scale (8). T o test the effectiveness of the pneumatic amplifier pump for SFE, the extraction of dry ground coffee by using supercritical COPwas carried out at several different densities. A coffee sample (Sarks Supreme Vienna Roast) was extracted at densities of 0.6-0.9 g/mL for 10 min a t each density by using the apparatus described above. No condensable extract could be detected until a density of 0.7 g/mL was attained. The infrared spectrum of the resulting extract is shown in Figure 7A. The carbonyl bands at 1701 and 1655 cm-' are characteristic of caffeine. A rapid step change in density, to 0.8 g/mL, was then made. The spectrum of the extract at this density shows a third carbonyl band, at 1743 cm-l (see Figure 7B), assignable to a second compound (presumably a lipid) being extracted. At a density of 0.9 g/mL, the spectrum of the extract indicates that little caffeine remains, but the second compound continues to be extracted. T o determine if the above results were due to the complete extraction of caffeine by the time the density had been increased to 0.9 g/mL, or whether the extraction is exhibiting selectivity as a function of density, the extraction was repeated with a new coffee sample. In this case the density was maintained a t 0.7 g/mL for the first three 10-min collection periods and the density was then raised to 0.9 g/mL. It can be seen from the spectra in Figure 8 that only the carbonyl
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Figure 8. Infrared spectra of coffee extract with sample at 40 O C . The COPdensity is indicated on each figure and the times after the start of the extraction: (A) 0-10 min; (B) 10-20 min; (C)20-30 min; (D) 30-40 min; (E) 40-50 min.
bands of caffeine could be observed as long as the density was maintained at 0.7 g/mL. Once the density was raised to 0.9 g/mL, the carbonyl band of the extracted lipid appeared. This example not only illustrates the performance of this pump for SFE, but also demonstrates the selectivity that density programming can afford in supercritical extractions. iii. Supercritical Fluid Chromatography. Whereas refilling the pump would not be expected to cause problems for extractions, because solutes will simply cease to be solvated during the pressure drop and then redissolve, the same is not true of chromatographic applications because a continuous record is being obtained. T o test the operation of this pump for capillary SFC, a separation of a 50% phenyl methyl polysiloxane GC stationary phase (Dow Corning 710) was performed. The chromatogram shown in Figure 9A was measured with an oven temperature of 70 "C and a linear pressure ramp
812
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987 A 4 -
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programmed from 1940 to 4200 psi at 20 psi/min. The chromatograph shown in Figure 9B was measured with the same pressure ramp, but with the oven temperature held at 140 "C. The increase in resolution of the early eluting peaks achieved by operating at higher temperature is readily apparent. Presumably the increase in resolution is assignable to the increased solute diffusivities in the mobile and stationary phases at elevated temperature. Since the density of C 0 2 a t a given pressure is much lower for the 140 OC separation, the higher oligomers do not elute in reasonable times for a mobile-phase pressure of 4000 psi. A more rapid separation can be obtained at the cost of some resolution by using a density ramp from 0.45 to 0.775 g/mL at 0.003 (g/mL)/min, with an oven temperature of IO "C; see Figure 10. The beginning and ending pressures are about the same as those for the linear pressure ramp described above but the pressure-time curve is nonlinear. For all chromatograms, the refill cycle was sufficiently fast that the chromatography was hardly affected even if a refill occurred while a component of the mixture was eluting. For example two refills occurred during the chromatogram shown in Figure 10. The effect on the peaks can just be observed in the expanded plots of the two peaks shown in the offset regions of this figure. I t may be recalled from Figure 6 that the pressure drops by about 5% during a refill cycle and that refilling is essentially complete (i.e., within 5 psi of the set pressure) within 10 s. During this period the flow rate may be expected to drop by 5% at most, so that the effect of a refill would be to increase
the retention volume of subsequent peaks by about 0.5 s. Since the run-to-run reproducibility of retention times in capillary SFC is generally of the order of a few seconds, the effect of refills is negligible. The performance of this pump is equivalent to that of syringe pumps with similar pressure/flow control, and superior to that of most syringe pumps for which specifications have been published. The most detailed description of a reciprocating pump for supercritical fluids was that of Greibrokk et al. ( 5 ) ,where long-term pressure variations of *lo0 psi were reported. Unfortunately short-term reproducibility was given in terms of retention times and the effect on an ultraviolet detector, rather than in terms of pressure. Nevertheless it is apparent that the pressure control of the pneumatic amplifier pump described above is far superior to that of all reciprocating pumps for which specifications have been published. It is significant to note that reciprocating pumps have not been used routinely by any group involved in capillary SFC. For supercritical fluid extractions or packed column SFC where flow rates much greater than 1 mL/min may be required, the use of syringe pumps becomes less attractive because of the time needed to refill the syringe. The volume of typical syringe pumps may be large enough that refilling is not required for extractions of small samples a t low flow rate or for a single analytical-scale packed-column SFC run. However, this will not be the case for supercritical fluid extractions of large sample quantities or for preparation scale SFC. Reciprocating pumps are generally used for these experiments. The excellent pressure control and short refill times of the pneumatic amplifier pump described in this paper suggest that it has many of the properties desirable for these applications. Thus this pumping system could conceivably be used for all types of supercritical fluid chromatography and extractions.
ACKNOWLEDGMENT The authors gratefully acknowledge the donation of the Haskel pump by Analect Instruments and the donation of the two robotically drawn fused silica restrictors by Thomas L. Chester of the Proctor and Gamble Corp. The guidance of Jerry Sorrels in the design of the control electronics was greatly appreciated. LITERATURE CITED (1) Van Lenten, F. J.; Rothman. L. D. A n a l . Chem. 1976, 4 8 ,
1430-1432. (2) Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L.; Springston, S . R.; Novotny, M. A n a l . Chem. 1982, 5 4 , 1090-1093. (3) Chem. Eng. News 1986, 3(24), 52. (4) Gere, D. R.; Board, R.; McManigill, D. A n a l . Chem. 1982, 5 4 , 736-740. (5) Greibrokk, T.; Blllle, A. L.; Johansen, E. J.; Lundanes, E. A n a l . Chem. 1984. 5 6 , 2681-2684.
Anal. Chem. 1987, 59,813-818 (6) Nieman. J. A.; Rcdgers, L. B. Sep. Sci. 1975. 70, 517-545. (7) Jentoft, R. E.: Oouw, T. H. Anal. Chem. 1972, 4 4 , 681-686. (8) Paulaiiis, M. E.; Krukonis, V. J.; Kurnik, R. T.: Reid, R. C. Rev. Chem. .Ens. 1983, 2, 179-249.
RECEIVED for review July 11,1986. Accepted November 10,
813
1986. This work was supported by the University of California Toxic Substances Program and by Cooperative Agreement CR812258-01 between the University of California, Riverside, CA, and the U.S. Environmental Protection Agency’s Environmental Monitoring System Laboratory, Las Vegas, NV.
Solvent Extraction Studies of Europium(II I), Ytterbium(II I), and Lutetium(II I) with Ionizable Macrocyclic Ligands and Thenoyltrif luoroacetone V. K. Manchandal and C. Allen Chang*
Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968-0513
Solvent extraction behavlor of Eu( I II ) , Yb( II I), and Lu( III ) has been investigated by uslng thenoyltrtfiuoroacetone (TTA) as extractant In the presence of 1,7-dlaza-4,10,13-trloxacyclopentadecane-N,N’dlacetlc acid (DAPDA) aml 1,lO-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N’-dl~ceticacid (DACDA) as macrocyclic Ionophores. DAPDA and DACDA were chosen in this work In view of thelr unique complexation toward lanthanides. I t was observed that In the pnsence of DAPDA (L), Eu( I I I ) extracted predominantly as ternary complex [Eu(L)(lTA)], whereas Yb( 111) and Lu( 111) were extracted as mixed, binary Ln(lTA), and ternary [Ln(L)(lTA)] complexes. On the other hand, In the presence of DACDA, Eu( II I ) formed mixed binary and ternary complexes In the organic phase, whereas Yb( 111) and Lu( 111) formed predominantly binary complexes. I n contrast to the extraction In the presence of DAPDNDACDA, heavler lanthanldes, Le., Yb( II I ) and Lu( I I I),were extracted much less compared to lighter lanthanldes, Le., La( I I I ) and Nd( I I I), in the presence of ethylenediamine-N,N’-dlacetlc acid (EDDA), a structurally analogous noncyciic poiyamlnopolycarboxyllc acid.
In spite of several reported procedures (Id), separation of lanthanides as a group from trivalent actinides as well as separations of individual lanthanides from each other still offers a formidable challenge to analytical chemists (6, 7). In general, multistage extraction is carried out to achieve the desired purification of a particular lanthanide from mixtures (8,9), which is tedious and time-consuming. Thus, there is a growing interest in developing alternate procedures including the use of ion-specific compounds or crown ethers for separation of lanthanides as a group or from one another (10). In a n effort to develop lanthanide ion selective reagents, we initiated a systematic study of lanthanide complexes of macrocyclic ligands with ionizable, functional pendant arms. Two such ligands, i.e., 1,7-diaza-4,10,13-trioxacyclopentadecane-N,”-diacetic acid (K21DA or DAPDA) and 1,lO-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N’-diacetic acid (K22DA or DACDA) shown in Figure 1, have been characterized in terms of the thermodynamic complex formation stabilities (11,12),mixed ligand complex formation (13),and On leave from Radiochemistry Division, B.A.R.C., Bombay, India. 0003-2700/87/0359-0813$01.50/0
dissociation kinetics (14) of complexes of lanthanide ions. It was observed that in contrast to the open-chained EDTA, both ionizable macrocyclic ligands form stronger complexes with the lighter lanthanides. In particular, DAPDA, the 15membered ring compound, formed the strongest complex with europium(II1) among all lanthanide ions (12). In view of such unique selectivity, we have investigated the solvent extraction behavior of the lighter lanthanides, i.e., La(II1) and Nd(III), by using thenoyltrifluoroacetone (TTA) as extractant in benzene in the presence of the two macrocyclic reagents (15). It was found that the binary complex, Ln(TTA)3, was the dominant species extracted at pH 15.0 and the ternary complex, Ln(DAPDA/DACDA) (TTA) was the dominant species at pH -7.5. Extraction of the ternary complex of La(II1) was greater in the case of DAPDA and smaller in the case of DACDA as compared to the extraction of corresponding ternary complex of Nd(II1). The measured extraction cbnstants followed the order which could be explained on the basis of both metal ionic potential as well as steric effects of the resulting complexes. In the present work, we report the results of spectral as well as distribution studies carried out with heavier lanthanides, e.g., Eu, Yb, and Lu in the presence of DAPDA and DACDA as macrocyclic ionophores. TTA was employed as the organic extractant and benzene was used as the organic diluent. For comparison, studies have also been carried out in the presenoe of EDDA, a structurally analogous noncyclic polyaminbpolycarboxylic acid. These studies revealed significant differences in extraction mechanisms and thus the nature of extracted species for lighter and heavier lanthanides.
EXPERIMENTAL SECTION Reagents. DAPDA and DACDA were synthesized and purified in our laboratory by the procedure reported earlier (11). Analytical reagent grade EDDA and EDTA were purchased from Spectrum Chemical Manufacturing Corp. and Mallinckrodt, Inc., respectively. Nitrate salts of europium, ytterbium, and lutetium used were supplied by Aldrich Chemical Co. Standard metal salt solutions (0.01 M) were prepared by titrating them against
standard EDTA solution using xylenol orange as indicator. Thenoyltrifluoroacetone (TTA, laboratory reagent grade) was obtained from Aldrich and it was used after recrystallization from benzene/hexane mixture. Its purity was confirmed by melting point determination and measurement of acid dissociation constant. Spectroscopic grade benzene was used as organic diluent. Distribution Studies. The aqueous phase contained 1.8 X lo4 M metal ion and 1.8 X lo4 M DAPDA/DACDA, and the ionic strength was adjusted to 0.2 M with tris(hydroxymethy1)aminomethane (Tris) buffer and tetramethylammonium chloride. 0 1987 American Chemical Society
