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Anal. Chem. 1991, 63,2542-2543
concentration medium was a membrane disk containing the AGl-X8 anion-exchange resin within a polypropylene housing. Preconcentration was coupled with chemical conversion of Ito 1 2 to enhance analyte transport to the plasma. The combined technique improved the limit of detection of iodine as I- and 10,-in aqueous solution by a fador of 207 (0.75 ng/mL) and 15 (31 ng/mL), respectively. Improvement factors for on-line oxidation of I- were 33 and 100, respectively, for Ar and He ICP discharges. The limit of detection should be further improved by a factor of 8 to match the current spectrophotometric technique (IO) used in the FDA Total Diet Study. With a suitable digestion procedure, preconcentration combined with oxidation would satisfy FDA needs.
ACKNOWLEDGMENT We thank W. F. Syner of the Food and Drug Administration for his assistance in constructing the data acquisition system for this work.
LITERATURE CITED (1) Lombardo, P. €nvkonmental€p&mlobgy; Kopier, F. C., Craun, G. F., Eds.: Lewis: Chelsea, MI, 1986; Chapter 11. (2) Pennington, J. A. T. J. Assoc. Off. Anal. Chem. 1087, 70, 772-782. (3) Pennington, J. A. T. J. Am. Diet. Assoc. 1083, 82, 166-173. (4) Pennington, J. A. T. Documentation for the Revised Total Diet Study: F w d List and Diets; National Technical Information Service: Springfield, VA, 1981; Accession Number PB 82-192154. (5) Pennington, J. A. T.; Young, 8. E.; Wilson, D. B. J. Am. Diet. Assoc. 1080, 89, 859-884. (6) Gunderson. E. L. J. Assoc. Off. Anal. Chem. 1088, 71, 1200-1209. (7) Capar, S. G. Analytical Method Aspects of Assessing Dietaly Intake of Trace Elements. I n Blokglcal Trace €iements Research; Subramanian, K. S.; Iyengar, G. V.; Okamoto, K., Eds., ACS Symposium Series No. 445; American Chemical Society: Washington, DC, 1991; Chapter 13. (8) Luchtefeld, R. 0. FDA Laboratory Informatlon Bulieth 1678; Food and Drug Administration, Division of Field Science: Rockviiie, MD 20857, 1974. (9) Sandeii, E. B.; Koithoff, I.M. J. Am. Chem. Soc. 1034, 56, 1426. (10) Fisher, P. W. F.; L'Abbb, M. R.; Giroux, A. J. Assoc. Off. Anal. Chem. 1088, 69, 687-889. (1 1) Blaha, J. J. FDA Laboratory Infwmation Bulletin 3045; Food and Drug Administration, Division of Field Science: Rockviiie, MD 20857, 1986. ( 12) Inductlvely Coupled Pksma in Analytfcel Atomic Specfromeby ; Montaser, A.. Goiightty. D. W., Eds.; VCH: New York, 1987; 860 pages.
(13) Sheppard, 8. S.; Caruso, J. A.; Woinik, K. A.; Fricke, F. L. Appl. SpeCtrOSC. 1090, 44, 712-715. (14) Nakahara, T.; Wasa, T. Appl, Spectrosc. 1087. 41, 1238-1242. (15) Cave, R. M.; Green, K. A. J. Anal. At. Spechom. 1080, 4 , 223-225. (16) Gustavsson, A. I n Inductively Coupled plasma in Anal)rNcel Atomic Spectrometry; Montaser, A., Golightiy, D. W.. Eds.; VCH: New York, 1987; Chapter 11. (17) Browner, R. F. I n Inductlvely Coupledplasma Emission Spectroscopy; Boumans, P. W. J. M., Ed., Wiiey: New York. 1987; Part 11, Chapter 8. (18) Christian, G. D.; Ruzicka, J. Spectrochim. Acta 1087, 428, 157-167. (19) McLeod, C. W. J. Anal. At. Spectrom. 1087, 2 , 549-552. (20) McLeod, C. W.; Zhang. Y.; Cook, I.; Cox, A.; Date, A. R.; Cheung, Y. Y. J. Res. Natl. Bur. Stand. 1088, 93, 462-465. (21) Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1975. 78. 145-157. (22) Ruzicka, J. Now Iniection Analysis (Chemical Analysis Series); 2nd ed., Wiiey: New York, 1908. (23) Hartenstein, S. D.; Ruzicka, J.; Christian, G. D. Anal. Chem. 1085, 57, 21-25. (24) Hirata, S.; Umezaki, Y.; Ikeda, M. Anal. Chem. 1986, 5 8 , 2502-2606. (25) Dean, J. A. Chemical Separation Methods; Van Nostrand Reinhold: New York. 1969; Chapter 7. (26) McLeod, C. W.; Cook, I . G.; Worsfold, P. J.; Davies, J. E.; Queay, J. Spectrochim. Acta 1085, 408. 57-62. (27) Cox, A. G.; Cook, I. G.; McLeod. C. W. Analyst 1085, 110, 331-333. (28) Cox, A. 0.; McLeod, C. W. J. Anal. At. Spectrom. 1087, 2 , 553-555. (29) Chan, S.;Montaser. A. Spectrochim. Acta 1085, 408, 1467-1472. (30) Chan, S.;Van Hoven. R. L.; Montaser, A. Anal. Chem. 1086. 58, 2342-2343. (31) Chan, S.; Montaser, A. Spectrochim. Acta 1087, 428, 591-597. (32) Montaser, A.; Chan, S.; Koppenaai, D. W. Anal. Chem. 1087. 59, 1240-1242. ~. -~ (33) Montaser, A.; Van Hoven, R. L. CRC Crit. Rev. Anal. Chem. 1087. 18, 45-103. (34) Chan, S.; Tan, H.; Montaser, A. Appl. Spectrosc. 1080, 43, 92-95. (35) Chan, S.; Montaser, A. Spectrochim. Acta 1080, 448, 175-184. (36) Montaser, A.; Ishii, 1.; Clifford. R. H.; Sinex, S. A,; Capar, S. G. Anal. Chem. 108% 61, 2589-2592. (37) Montaser, A.; Clifford. R. H.; Sinex, S. A.; Capar, S. G. J. Anal. At. Spectrom. 1080, 4 , 499-503. (38) Personal communication, Anne Stevens, Bic-Rad Laboratories, 1414 Harbour Way South, Richmond, CA 94804.
RECEIVED for review May 7,1991. Accepted August 12,1991. This research was sponsored in part by the U.S. Department of Energy under Grant No. DE-FG05-87-ER-13659. Partial finanical support for A.M. and R.H.C. and use of certain facilities was provided by the FDA.
Determination of Small Amounts of Water in Dimethyiformamide and Dimethylsulfoxide Using Luminescence Lifetime Measurements of Europium( I I I ) Stefan Lid and Gregory R. Choppin* Department of Chemistry, The Florida State University, Tallahassee, Florida 32306-3006
INTRODUCTION A linear dependence of the rate of luminescence decay on the degree of cation hydration has been observed for several trivalent lanthanides (I, 2). As a result, the luminescence decay rates have become valuable data in evaluating hydration in the primary coordination sphere of complexes of these cations (1-6). This correlation has been applied for Eu(II1) and Tb(II1) to the calculation of residual hydration numbers in complexes of these cations. Generally, the hydration numbers have been obtained by using the difference in the decay rate constants in H 2 0 and in D20 solutions. However, kHIO >> kDPO generally and kDIO = constant. As a result, a relationship was proposed in which the hydration number could be related more simply to the decay rate constant in water (7): 'On leave f r o m t h e F a c u l t y o f Chemistry, A d a m Mickiewicz University, Poznan, Poland. 0003-2700/91/0363-2542$02.50/0
nH20
= AkH20 + B
(1)
Knowledge of the residual hydration has been useful in interpreting the binding mode in biological systems as well the number of ligating functions in interactions with polydentate ligands (2-9). We have measured the luminescence lifetimes of Eu(II1) in mixtures of water (or deuterated water) and methanol (MeOH), ethanol (EtOH), tertiary butanol (t-BuOH), N,Ndimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Our results agreed well with those of Tanaka et al. (IO) for some of the same mixed solvent systems. In the alcohol-water solvents, Eu(II1) was preferentially solvated by water whereas in the DMF and DMSO solutions, the organic solvents were favored over water. However, even at very low mole fractions of water, water molecules were present in the primary solvation sphere of Eu(II1). The implications of the variation in the preferential solvation by the different organic solvents vs water is to be discussed in a subsequent publication. In the DMF 0 1991 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 63, NO. 21, NOVEMBER 1, 1991
H 2 0 in DMSO, mole Yo
Table I. Water Content and the Luminescence Lifetime for Eu(II1) mol % water added measd
lifetime," ms
0.046 0.10 0.20 0.50 1.00
1.579 1.528 1.457 1.286 1.023
B. DMSO Solution 0.070 0.10 0.20
0.50 1.00 2.00
5.00
0.063 0.11 0.21 0.48 0.98 2.04 4.98
0
I
2
3
4
I
I
I
I
I
E
5 I
n '
A. DMF Solution 0.050 0.10 0.20 0.50 1.00
2543
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1.601 1.596 1.584 1.554 1.489 1.380 1.050
Mean of at least seven measurements in each DMF sample and five measurements in each DMSO samDle. and DMSO mixtures, the luminescence lifetimes were found to be linearly dependent on the water concentration over ranges from 0.1 to 1(DMF) and 0.1 to 5 (DMSO) mol %. This paper describes a method for the determination of small amounts of water in these solvents using Eu(II1) luminescence lifetime measurements.
EXPERIMENTAL SECTION Europium perchlorate stock solutions were prepared by dissolving the oxide (Aldrich Chemical co., Inc.) in perchloric acid. This solution was evaportated to dryness and the residue dissolved in either DMF or DMSO followed by evaporation to dryness again. The preparation can be done equally well with hydrochloric acid to avoid concerns over evaporation to dryness of perchlorate solutions. The process of solution and evaporation was repeated four times to eliminate water completely (CO.01 mol %). The dry samples of Eu(C104)3were dissolved in reagent grade DMF or DMSO (Fisher Scientific). The initial content of water in these solutions was determined to be 0.05 mol % in DMF and 0.07 mol 70 in DMSO from the dependence of the luminescence lifetimes on known amounts of water added to the original 'dry" solution. The linearity of response indicated that this was as reliable as direct measurements of these low levels of HzO by conventional techniques such as Karl Fischer titrations. Moreover, these amounts of residual water are below the recommended ranges for the luminescence technique. Working solutions were prepared by introducing measured amounts of water into the Eu(II1)-DMF or Eu(II1)-DMSO solutions. All tubes containing the samples were stoppered and wrapped tightly with parafilm to avoid the contents absorbing moisture by contact with the air. The ku(II1) in the samples was excited to the 5L6electronic level by a pulsed laser beam at 395 nm, and the subsequent emission from the 6Doluminescent excited state to the ground 'F manifold ('F2) was measured. The 395-nm beam was obtained with the pulsed (10 kHz) 532-nm second-harmonic output of a Quanta Ray DCR 2A Nd-YAG laser pumping Rhodamine 640 (Exciton Chemical) in methanolic solution in Quanta Ray PDL2 equipment. The beam from the dye laser was converted to 395 nm by frequency doubling and mixing with the 1064-nm fundamental in a Quanta Ray WEX-1wavelength extender. The pulse energy was typically 2-3 mJ, and the pulse width was in the nanosecond range. The details for conducting the experiments and collecting the data have been described earlier (7). The luminescence decay curves observed in this work could be analyzed by a single exponential relation; i.e., semilogarithmic plots of the luminescence intensity versus time were linear. These plots provided the decay constants reported in Table I. In some systems where the concentration of water in DMF exceeded 1mol
I
."0.0
0.2
0.4
0.6
0.8
i .o
H20 in DMF, mole YO Figure 1. Dependence of the Eu(II1) luminescence lifetime on the H 2 0 concentration in DMF and DMSO.
% and in 5 mol % DMSO, the luminescence lifetimes for Eu(I1I) did not show a simple linear dependence on the HzO concentration. These concentrations, as a consequence, are used as the limits of applicability of the method for water analysis.
RESULTS AND DISCUSSION Normally, the degree of hydration of Eu(II1) is calculated from the luminescence decay rate constants. However, the linearity of response to water concentration was equally good for the rate constants or the directly measured lifetimes, so we report the method in terms of the latter. The luminescence lifetimes measured for Eu(III) in samples containing water in DMF and DMSO solutions are listed in Table I. The relative standard deviation for eight repeated measurements of a solution of DMF is 2.9% and, for six such measurements of a solution of DMSO, 1.7%. The plot of a calibration curve for Eu(II1) versus water concentration in DMF and DMSO systems is shown in Figure 1. The data had a linear least-squares fit with a correlation coefficient of 0.999 for both solvents. The correlation gives the following equations for the calculation of the mole percent water in each solvent:
DMF: mol % H20= [1.591 - t ] / 0 . 5 8 8
(2)
DMSO: mol % H20= [1.605 - t]/O.l12
(3)
where t is the fluorescence lifetime in milliseconds. The mole percent water content calculated from the measured lifetime values is listed in Table I along with the amount (in mole percent) known to be present from the added water.
LITERATURE CITED (1) (a) Kropp, J. L.; Windsor, M. W. J . Chem. M y s . 1065. 42, 1599; (b) 1066, 4 5 , 761. (2) Horrocks, W. De W.; Studnick, D. R. J . Am. Chem. SOC. 1070, 101, 334. (3) Breen. P. J.; Horrocks, W. De W. Inorg. Chem. 1089, 22, 536. (4) Albin, M.; Farber, G. K.; Horrocks, W. De W. Inorg. Chem. 1084, 23, 1648. (5) Bryden, C. C.; Reilley, C. N. Anal. Chem. 1082, 54, 610. (6) Okamoto, Y.; Kdo, J.; Brittain, H. G.; Paoietti, S. J . Macromol. Sc/.Chem. 1088, A25(10&11), 1385. (7) Bartheiemy, P. P.; Choppin, G. R. Inorg. Chem. 1960, 28, 3354. (8) Soini, E.; Lovgren, T. CTC Crit. Rev. Anal. Chem. 1987, 78 (2). 105. (9) Richardson, F. R. Chem. Rev. 1082, 82, 541. (10) Tanaka, F.; Kawasaki, Y.; Yamashita, S. J . Chem. Soc.,Faredy Trans. 1 , 1088, 84, 1083.
RECEIVED for review May 21,1991. Accepted August 22,1991. This research was supported by a grant from the USDOEOBES, Division of Chemical Sciences.
