Anal. Chem. 1989, 61, 1789-1791
reduction to I&- on gold in 0.1 M TBAP in dry acetonitrile. The UV-visible absorption spectrum of IA'-was not observed with the 560-pm cell, but was observed with the 160-pm cell. When stationary IA solution was reduced in the 100-Mm cell, the 1A'- spectrum grew to a maximum over 10 s and disappeared over a totalperiod of 35 s. Decreasing the cell thickness ( I ) from 560 to 160 pm produced a 12-fold decrease in the time taken ( t ) to achieve exhaustive electrolysis for electroactive species with similar diffusion coefficients ( D ) . This agrees with the approximate relation
t
a
12/D
The compound under observation must have a molar extinction coefficient above a certain limit, with the minimum requirements being c > 200 M-' cm-' for the 560-pm cell and e > 600 M-' cm-' for the 160-rm cell. The flow operation of this system permits cleaning and O2 removal so that the cell need not be removed from the spectrometer between experiments on different solutions. Thus the cell has an application for photoelectrochemical detection in high-performance liquid chromatography and for flow injection analysis (10, 11). To conclude, we have shown that a well controlled spectroelectrochemical system may be constructed without in-
1789
strument workshop facilities, thus making the technique much more widely available.
ACKNOWLEDGMENT We thank D. Bethel1 for providing samples of l-bromo9,lO-anthracenedione and l-iodo-9,lO-anthracenedione. Registry No. BA, 632-83-7; Baa-, 121176-25-8;IA, 3485-80-1; IA*-, 121176-26-9;Au, 7440-57-5.
LITERATURE CITED (1) Murray, R. W.; Heineman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 3 9 , 1666. (2) Yildiz, A.; Kissinger, P. T.; Reiiley, C. N. Anal. Chem. 1968, 4 0 , 1018. (3) Heineman, W. R.;Burnett, J. N.; Murray, W. M. Anal. Chem. 1968, 40, 1974. (4) Muth, E. P.; Fuller, J. E.; Doane, L. M.; Blubaugh, E. A. Anal. Chem. 1982, 5 4 , 604. (5) Porter, M. D.; Dong, S.; Gui, Y.-P.; Kuwana, T. Anal. Chem. 1984, 56, 2263. (6) Sanderson, D. G.; Anderson, L. 6 . Anal. Chem. 1985, 5 7 , 2388. (7) Nevin, W. A.; Lever, A. B. P. Anal. Chem. 1988, 6 0 , 727. (8) Zhang, C.; Park, S A . Anal. Chem. 1988, 6 0 , 1639-1642. (9) Compton, R. G.; Pilkington, M. B. G.; Bethell, D., unpublished work, Physical Chemistry Laboratory, Oxford University, November 1988. (IO) Dewald, H. D.; Wang, J. Anal. Chem. 1984, 766, 163. (11) Lacourse, W. R.; Krull, I. S. Anal. Chern. 1985, 5 7 , 1810.
RECEIVED for review December 19, 1988. Accepted March 27, 1989. M.B.G.P. thanks the SERC for a studentship.
Application of a Nested-Loop System for the Simultaneous Determination of Thorium and Uranium by Flow Injection Analysis Jose Luis PBrez Pavbn, Bernard0 Moreno Cordero,* Jesiis Herniindez MBndez, and Rosa Maria Isidro Agudo Department of Analytical Chemistry, Bromatology and Food Sciences, University Numerous methods have been described for the determination of uranium (1-6) and thorium (7-11), most of them colorimetric. However, owing to the low concentrations in samples of interest and the presence of interferents, direct determinations are difficult, and separation or preconcentration techniques such as liquid-liquid extraction, ion-exchange chromatography, and extraction columns are always employed prior to analytical measurement. Continuous automated or semiautomated analytical techniques are to be preferred over wet methods when one is dealing with hazardous materials or when large numbers of samples have to be analzyed. Several flow injection analysis (FIA) procedures have been proposed for the determination of uranium (12-15) and thorium (16);in this work we propose for the first time a sensitive and selective FIA method for the simultaneous determination of thorium and uranium without previous separation from the matrix; Arsenazo 111 is used as the reagent, with monitoring of the systems a t X = 665 nm. The proposed FIA system is a two-channel manifold with a two-valve nested-loop injection system, the loop of one valve being a lead powder reducing column (Figure 1). The injected sample is split into two sections, one of them passing through the reducing column.
EXPERIMENTAL SECTION Reagents. Stock solutions of uranium and thorium at a M were prepared by dissolving apconcentration of 2.0 X propriate amounts of uranyl nitrate hexahydrate (Merck) and thorium nitrate pentahydrate (Merck) in water. Stock solutions of Arsenazo I11 were prepared by dissolving 0.4100 g of the solid product (Fluka) in 250 mL of water. Aqueous 10% (w/v) solutions were of Triton X-100 (Analema). The reagent solution was prepared by mixing 25.0 mL of the stock Arsenazo I11 solution, 25.0 mL of the 10% (w/v) solution, and 75 mL of concentrated HC1 and then diluting with water up to 250 mL. All chemicals
Qf
Salamanca, Salamanca, Spain
reagents were of analytical grade. Apparatus. The flow system comprised a peristaltic pump (Gilson Minipuls 2 HP-4) and a Perkin-Elmer Coleman 55 with a l-cm flowthrough cell (18 pL, Hellma 178 12-QS). All connections were 0.5 mm i.d. Teflon tubing. Injection System (Figure I ) . Dasgupta and Hwang (17)report a configuration of a six-port injection valve installed within the loop of another six-port injection valve for the determination of aqueous peroxides; the loop of the nested (inner) valve contains an immobilized packed reactor with differentiating action on the analyte components. We have adapted this system for the simultaneous determination of thorium and uranium; the loop of the inner valve contains a lead reductor minicolumn. The minicolumn was a 5-cm length of 2 mm i.d. glass tubing; it was packed with lead powder (0.1-0.3 mm) with small glass wool beds at each end to prevent the escape of the material. How the nested-loop device should be handled is described in the original paper (17)and summarized in our procedure. The resulting detector output is two separate sequential signals. When a uranium solution is injected into the system, the first peak (section L1, nonreduced sample) is much lower than the second peak due to the low molar absorptivity of the U(V1)-Arsenazo I11 complex; the second peak (section L2, reduced sample) corresponds to the U(1V)-Arsenazo I11 complex and is similar to the peaks obtained when a thorium solution is injected into the system. Procedure. The system is started with both valves in the inject mode; this allows the filling of the column and R1with carrier solution. Later, both valves are switched to the loading mode and the sample (thorium and/or uranium, 3.6 M HC1) is pumped to fill L1 and L2. Then, V2 is switched first, followed by VI, and the carrier solution (3.6 M HCl) directs the sample toward the reagent stream (2.0 X 10"' M Arsenazo 111, 3.6 M HCl, 1% (w/v) Triton X-100); the signal is recorded at X = 665 nm. The conditions under which these determinations were carried M out were as follows: carrier, 3.6 M HCl; reagent, 2.0 X Arsenazo I11 (3.6 M HC1, TX-100 1%); L , = 143 pL; L2 = 254 pL;
0003-2700/89/0361-1789$01.50/0 0 1989 American Chemical Society
1790 * ANALYTICAL CHEMISTRY, VOL. 61, NO. 15. AUGUST 1. 1989
R1
Table I. Simultaneous Determination of Thorium and Uranium in Synthetic Mixtures
Ls
Reducing column
w
0.95
0.94 4.01
0.23 1.39 0.09
0.47 0.95 1.43 0.24 1.43
0.92 0.45 0.22 1.36 0.09
1.39 1.39
0.09 0.47
1.36 1.36
0.93
0.93 0.46
Carrier
Flgure 1. Injection system
R, = 100 em; R, = 50 cm; lead column length = 5.5 cm; Q, = 3.1 mL/min, with flow rates kept the same in both branches of the setup. RESULTS AND DISCUSSION Arsenazo 111 is only sparingly soluble in concentrated acid media, but its solubility increases upon addition of Triton X-100 to the solutions. A 1% (w/v) concentration of the surfactant is sufficient to stabilize 2.0 X 10’’ M Anenazo III; for more concentrated solutions 2% Triton X-100 is recommended. Preliminary studies showed that the presence of surfactant does not cause any modifications in the complex or spectra of the reagents. As a previous reduction process was necessary to obtain U(IV) from the uranyl cation, lead was chosen as the reducing agent (powder packed in a minicolumn) since it quantitatively reduces U(V1) to U(1V) without the production of hydrogen bubbles up to proton concentrations close to 4 M. A 3.6 M concentration of HCI was used for all the experiments. Influence of Variables. The effect, of chemical and hydrodynamic variables was studied by using four different column lengths (4.0,5.5,9.0, and 15.5 cm); the signals did not vary appreciably. Regarding the influence of Arsenazo I11 concentration, the results show that the signal reaches a constant value when the reagent concentration was about 10-fold that of the cation. For the rest of the variables studied (L,, L2, R,, R2. and QJ the results were just as predictable. Final optimized values are reported in the Experimental Section. Analytical Characteristics of t h e Determination. For cation concentrations ranging between 1.0 X 10.’ and 2.0 X M, when two columns of different lengths (5.5 and 9.0 em) and two concentrations of Arsenazo I11 (2.0 X 10’’ and 4.0 X 10’’ M) were used, the signal was found to be proportional to the cation Concentration. The increase in column length or in the concentration of Arsenazo I11 did not lead to significant increases in linearity or in the intensity of the signal, although it did cause an increase in the stability of the base line, such that it was preferable to use short columns and the minimum concentration of reagent that would ensure linearity within the concentration range studied. Under the optimum experimental conditions straight lines were obtained for the following calibrations: uranium h, = 1832[U] 0.002 ( r = 0.9973); h2 = 32507[U] 0.001 (r = 0.9997)
+
+
thorium
h, = 42553[Th] + 0.005 (r = 0.9997); h2 = 37157[Th] + 0.0002 ( r = 0.9997) (h,: nonreduced sample; h2: reduced sample) In order to determine the precision of the method, triplicate injections of 10 identical solutions of each of the cations were
4.03 0.97 0.47
0.96 1.37 0.26 1.43 0.10 0.46
made (2.0 X 10-6 M) for all the experimental conditions under which the straight lines for the calibrations were obtained. For the column of 5.5cm length and Arsenazo I11 at 2.0 X lo-‘ M, the relative standard deviation for the second peak was 2.1% for U and 0.9% for Th. No significant differences were found under the other experimental conditions. Calculation of the detection limits was carried out from the measurements of noise (width of the base line), considering as the minimum amount detectable that which yielded a signal double the width of the base line. With this criterion, the values obtained were 1.23 X liT’ M for uranium and 9.4 X 1@ M for thorium. Simultaneous Determination of Thorium a n d Uranium. Nine synthetic mixtures of varying composition were used as unknown samples for the determination. In order to check the validity of the procedure for ores, analysis was performed on a standard ore (pitchblende S-12) supplied by the Atomic Energy Agency and prepared by the Junta de Energia Nuclear (Madrid, Spain)for contrast analysis in which only the content of uranium was specified (0.014%). The procedure followed by attacking the rock was as follows: 0.5 g of rock was treated with 5 mL of HN03 and heated until dry. The residue obtained was treated with HC1 and brought to dryness again. The new residue was suspended in 0.1 M HCI, filtered, washed, and brought up to volume in a 50-mL flask after addition of HC1 to adjust the acid concentration to 3.6 M. After determination of the straight lines of the corresponding calibrations, a system of equations was postulated as follows:
hi = Xmijcj where h; is the height of the two peaks obtained on injecting mixtures and mij are the slopes of the straight lines of the corresponding calibrations. With this system of equations it was possible to calculate the concentration of each of the components of the sample. From the results obtained for the synthetic mixtures (Table I), the proposed method may be used for the determination of small amounts of thorium (0.09 ppm) even in the presence of amounts uranium up to 16-fold higher. I t is possible to determine uranium in the presence of thorium for ratios of the same order. The results obtained in the case of the standard ore were as follows: found, % U30s = 0.0147 and % T h = 0.0005; reference, % U30s = 0.014; % Th, not certified. Registry No. U, 7440-61-1; Th, 7440-29-1. LITERATURE C I T E D (1) (2) (3) (4) (5) (6)
John. F. W.: Black. R. A. Anal. Chem. 1953. 25, 1200-1204. Clinch. J.: Guy. M. J. Analyst 1957, 82. 800-807. Foreman. J. K.: Riley. C. J.: Smith, T. D. Analyst 1957, 82, 89-95. Banisberger. R. J. Anal. Chem. 1964, 36. 2369-2370. Borak, J.: Slovak. 2.:Fisher. J. Taknta 1970, 17. 215-229. Kadam, 8. V.; Mahi. 0.: Sathe. R. M. Analyst 1981, 106. 724-726.
Anal. Chem. 1989, 61, 1791-1792 (7) Sarma, D. V.; Raghava Rao, 0. S. Anal. Chlm. Acta 1955, 73, 142- 149. (8) Purushottam, A. 2. Anal. Chem. 1955, 745, 245-248. (9) Fletcher, M. H.; Miikey, R. G. Anal. Chem. 1958, 28, 1402-1407. (10) Arnfelt, A. L.; Edmundsson, I. Talenta lW1, 8 , 473-476. (11) Onishi, H.; Sekine, K. Talanta 1972, 19, 473-478. (12) Lynch, T. P.; Taylor, A. F.; Wilson, J. N. Analyst 1983, 708, 470-475. (13) Sllfwerbrand-Lindh, C.; Nord, L.; Danielsson, L. G.; Ingman, F. Anal. Chim. Acta 1984, 760, 11-19.
1791
(14) Jones, E. A. Anal. Chlm. Acta 1985, 769, 109-115. (15) Atallah, R. H.; Christian, G. D.; Hartenstein, S.D. Analyst 1988, 773. 463-469. (16) Baban, S. B. Anal. Proc. 1980, 77, 535-537. (17) Dasgupta. P. K.; Hwang, H. Anal. Chem. 1985, 57, 1009-1012.
RECEIVEDfor review January 3,1989. Accepted April 3,1989.
On-Line Dilution Scheme for Liquid Chromatography Javier N. Oquendo*J and Joseph A. Leone* ARC0 Oil a n d Gas Company, Research and Technical Services, Plano, Texas 75075
INTRODUCTION In our laboratories, dilution is the most frequent pretreatment for the ion chromatographic analysis of samples. A commercial dilutor, whose operation was independent of that of an automated ion chromatograph, was used to handle a large number of samples. The advantages of operating both pieces of equipment as a single unit were obvious and prompted an investigation into an on-line dilution scheme. On-line dilution is the subject of several publications dealing with spectroscopic techniques ( I d ) . However, the idea does not appear to have been exploited in liquid chromatography. The on-line chromatographic dilution scheme utilized in this work combines a sample stream, provided by a stepper motor driven syringe, with that of the eluent after it has passed through the column or the detector.
EXPERIMENTAL SECTION Apparatus. A schematic diagram of the ion chromatographic system used in this work is shown in Figure 1. The equipment consisted of a Model 2120 dual channel ion chromatograph equipped with autoion 300 software (Dionex, Sunnyvale, CA), a Model 401 stepper motor driven syringe (Gilson Medical Electronics Inc., Middleton, WI), a Model 212B syringe controller (Gilson Medical Electronics Inc., Middleton, WI), and a Model 32297 autosampler (Dionex, Sunnyvale, CA). Modifications and optional parts for the ion chromatograph, all Dionex brand unless otherwise indicated, were as follows. Two injection valves (P/N 35913) placed in series and referred to here as valves A and B, an in-line filter (P/N 35331), a 250 X 3 mm anion separator (HPIC-AS4A, P / N 037041), an anion micromembrane suppressor (P/N 038019), and a mixing tee (P/N 24313). Valve C was a two-stack four-way valve (P/N 35914) with plugs as shown in Figure 1. The volumes of injection of valves A and B were 25 and 10 pL, respectively. The mixing coil was a 50 cm long piece of tubing packed with glass beads that was prepared by cutting a reaction coil (P/N 37556) used for postcolumn derivatizations. Tubing connections between the autosampler and the sample waste were made with 0.5 mm i.d. Teflon tubing (P/N 35519). All other liquid connections utilized 0.3 mm i.d. Teflon tubing (P/N 35548). A computer program was written for the syringe controller that allows the user to select among several modes of operation. The operation of the syringe controller was synchronized to that of the ion chromatograph by means of a contact closure relay. All work was performed with a 1.0-mL glass syringe. The analytical pump was operated in the constant flow/constant pressure mode. All ion separations were carried out with a solution of 2.2 mM NaHC03 and 0.8 mM Na2C03as eluent at a flow rate of 2.0 mL/min. The regenerant solution was 0.025 N HZS04. Present address: South American Petrolite of Venezuela, Apartado 5685, Caracas 1010A, Venezuela.
Procedures. Unless otherwise stated, the syringe operates at 250 pL/s. With valves A and B in the load position, and valve C positioned so as not to allow any mixing, the syringe pumps 2.0 mL of sample in four aliquots of 0.5 mL. For a given total volume of sample, carryover decreases with the volume of these aliquots. At this stage, nondiluted samples can be injected via valve A. If the mode of operation calls for sample dilution, the syringe draws a volume of sample that equals 70 pL plus the sample dilution flow rate times a constant (55). Next, valve C is positioned so as to allow the combination of the sample and eluent streams while the syringe pumps 70 p L of sample at 7 pL/s, and the remaining volume at the rate selected by the user. Carryover in the lines between valve C and the mixing tee is largely reduced by pumping sample at an initial flow rate much higher than that required for the on-line dilution of the sample. One minute later, valve C is switched to the nonmixing position and the diluted sample injected via valve B. Eluent dilution flow rates different from that used for the ion separation can be utilized after allowing sufficient time for equilibration (-30 8 ) . RESULTS AND DISCUSSION The performance of the instrument was evaluated with brines since brine characterization is of considerable interest in the oil industry. Dilution factors (DF) were calculated from
DF = (Qs + Qe)/Qs
(1)
where Qs and &e are the sample and eluent dilution flow rates, respectively. A plot of peak area of the on-line diluted samples of a 20000 ppm chloride solution versus the inverse of the dilution factors obtained from all combinations of sample dilution flow rates (6,12, 24,48,96, 192, and 384 rL/min) and eluent dilution flow rates (1.00, 2.00, and 3.00 mL/min) was linear. The resulting linear equation was peak area = (396 f 0.3)DF-1 - (0.7 0.3), with a correlation coefficient ( r ) of 0.999. Peak-area relative standard deviations (RSDs) of six consecutive injections of each of these samples ranged from 0.4% to 3.0% and were under 2.1% for dilution factors of up to 250. RSDs of similar samples diluted manually with an adjustable pipet were in the range from 0.6% to 2.2%. The system is capable of handling the dilution of highly concentrated samples. A plot of peak area versus the concentration of the solutions placed in the autosampler was linear for concentrations of chloride of 0.5, 1.0, 2.0,5.0, and 10.0% when operating with fixed eluent and sample dilution flow rates of 2.00 and 0.06 mL/min, respectively. The corresponding linear equation was peak area = (64.8 & 0.4)C1(%) - (0.31 & 0.08) with a correlation coefficient ( r ) of 1.000. "Water dips" for samples diluted with the nonsuppressed eluent were much smaller than those of samples diluted with the suppressed eluent. This observation is consistent with
0003-2700/89/0361-1791$01.50/00 1989 American Chemical Society
