Anal. Chem. 1992, 64, 2201-2205
2201
AC RESEARCH
Determination of Boron in Light- and Heavy-Water Samples by Flow Injection Analysis with Indirect UV-Visible Spectrophotometric Detection Thbrdse Lussier’ and Roland Gilbert Institut de recherche d’Hydro-Qukbec, 1800 montke Sainte-Julie, Varennes, Qukbec, Canada J3X 1Sl
Joseph Hubert’ Dkpartement de Chimie, Uniuersitk de Montrkal, P.O. Box 6128, Montrkal, Qukbec, Canada H3C 3J7
A flow lnjectlon analyds (FIA) system was bullt to determlne boron In aqueous samples Wing lndlrect UV-vlslble detectlon. The analyzer condsts of a two-line manlfold In which the sample (300 FL) Is Introduced Into a water carrier stream and the bufferedchromotroplc acldreagent constltutes the second channel. The merglng polnt of the two streams appears Just In front of the reactlon coll, thus ensurlng a constant reagent dlutlon. The reactor Is made of 1 0 4 packed Teflon tublng followed by 3m open Teflon tublng, both In serles. Wlth a flow rate of 0.3 mL/(mln/channel), the sample dlsperdon Is about 2 and the reddence tlme Is 7 mln, allowing a sample throughput of 15 samples/h. The response Is llnear over the concentration range of 40-6000 pg/L, and at levels up to 10000 pg/L a thlrdorder polynomlal can be used. The precidon Is 3% In the low end of the concentratlon range (JOmass spectrometry,” inductively coupled plasma (ICP) atomic emission ~pectrometry,’~J3 and ICP mass ~pectrometry’~ generallyprovide appropriate precision and detection capabilities, although they rely on expensive instrumentation. Atomic absorption spectrometry16 and potentiometric methodsI6have also been used, but they are often not sensitive or precise enough and are prone to severe interferences. A liquid chromatography technique using a nonpolar bonded phase was proposed by Motomizu et al.17 (4)Association of Official Analytical Chemists. Methods of Analysis of the Association of Official Analytical Chemists, 2nd ed.; Washington, DC, 1975. (5)Horwitz, W. Official Methods of Analysis of the Association of Official Agricultural Chemists, Washington, DC, 1955,p 38. (6)Cohen,P. Water Coolant Technology ofPowerReactors;American Nuclear Society Publishers: USA, 1980. (7)Green, L. W.; Davey, E. C.; Gulens, J.; Longhurst, T. H.; Mislan, J. P. Can. J. Chem. 1984,62,1452-1464. (8)Walker, C. S.Nucl. Saf. 1965,7 (l),45. (9)Garbrah, B. W.; Whitley, J. E. Anal. Chem. 1967,39,346. (10)Isenhour, T.L.; Morrison, G. H. Anal. Chem. 1966,38, 167. (11)Catanzaro, E.J.; Champion, C. E.; Garner, E. L.; Marinenko, G.; Sappenfield, K. J.; Shields, W. R. NBS Special Publication;Washington, DC, 1970,No. 260. (12)Nygaard, D. D.; Leighty, D. A. Appl. Spectrosc. 1985,39(6),968. (13)Ball, J. W.; Thompson, J. M.; Jenne, A. E. Anal. Chim.Acta 1978, 98,67. (14)Gregoire, D. C.J. Anal. At. Spectrom. 1990,5,623. (15)Goyal, N.;Dhobale, A. R.; Patel, B. M.; Sastry, M. D. Anal. Chim. Acta 1986,182,225. (16)Savel’yanova, R. T.; Savel’yanov, V. P.; Mendeleev, D. I. Z . An. Chim.1976,31 (lo),2056. (17)Motomizu, S.:Sawatani, I.; Oshima, M.: T&i, K. Anal. Chem. 1983,55 (9),1629. 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 18I92
Under such conditions, the dynamic range extends from 1to lo00 pg/L, but no indication is given on the precision or detection limit. An anion exchange mode was later proposed by the same group.18 A dynamic range with an upper limit of 100 pg/L was reported. The detection limit was not specified but probably is lower than 1pg/L. Other methods rely on the boron complex formation and spectrophotometric measurements.2J"26 Although sensitivity was adequate for boron determination in CANDU nuclear reactors, the number of samples that could be analyzed per day was limited and precision either varied or was unknown. One of the oldest and most common boron determination methods involvesthe reaction between boric and chromotropic (4,5-dihydroxy-2,7-naphthalenedisulfonic acid referred to as CTA)acids using UV-visible absorption spectrophotometry as a detection mode. This reaction was first described by Andress and T ~ p fwho , ~showed ~ that absorbance of the CTA reagent decreases when boric acid is added. Kuemmel and MellonZs further proposed a manual analytical procedure based on these spectral changes. Boron complexes are indirectly detected at a wavelength (361.5 nm) where the UV radiation is only absorbed by the reagent since the borateCTA complexes spectrum is completely overlapped by the bands associated to the reagent at lower wavelengths. Maximum sensitivity is obtained at a pH equal to or higher than 7 due to the formation of larger amounts of complexes for a given amount of boric acid as well as a larger absorption change for a given amount of complexes. Direct UV-visible detection of boron complexes requires that they be extracted from the reaction mixture using an organic s01vent.l~ This approach was not generalized due to its complexity compared to the method previously described and currently used at many CANDU plants. The automation of the Kuemmel and Mellon method through a segmented flow system was first attempted by James and King29 in 1967. Their approach for improving the performance of this time- and labor-consuming technique was of interest, but their experiments revealed considerable baseline drift associated to the instability of the reagent which impeded the accurate determination of small amounts of boron. More recently, Motomizu et aL30s31 proposed a continuous flow system using direct detection of the same boron complexes by fluorimetry. Although the sensitivity is adequate for application in CANDU nuclear reactors, the dynamic range presented does not allow the required concentrations to be covered. Moreover, this technique involves a detection system not usually available in field laboratories, and its implementation would require an extended laboratory training program. A simple and lowcost approach for automation would consist of an FIA (flow injection analysis) chemical manifold equipped with a standard UV flow-through detector widely used in the field for high-performance ion chromatography. (18)Jun, Z.;Oshima, M.; Motomizu, S. Analyst 1988,113, 1631. (19)Korenaga, T.; Motomizu, S.; T b i , K. Analyst 1978,103, 745. (20)Sato, S.;Uchikawa, S. Anal. Chim. Acta 1982,143, 283. (21)Oshima, M.; Fujimoto, K.; Motomizu, S.; T&i, K. Anal. Chin. Acta 1982,134, 73. (22)Korenaga, T.; Motomizu, S.; T&i, K. Anal. Chim. Acta 1980,120, ""1
Jll.
(23)Bassett, J.; Mattewe, P. Analyst 1974,99, 1174. (24)Oshima, M.; Shibata, K.; Gyouten, T.; Motomizu, S.; T&i, K. Talanta 1988,35 (5),351. (25)Fletcher, W.D.Nucl. Sci. Abstr. 1968,22 (lo),20825. (26)Sato, S. Anal. Chim. Acta 1983,151, 465. (27)Andress, K.; Topf, W. Z. Anorg. Allg. Chem. 1947,254, 52. (28)Kuemmel, D.F.;Mellon, M. G. Anal. Chem. 1967,29(3),378. (29)James, H.; King, G. H. Automation in Analytical Chemistry, Technicon Symposia, 1966; Mediad Inc.: New York, 1967;Vol. 1,p 123. (30)Motomizu, S.;Oshima, M.; T b i , K. Bunseki Kagaku 1983,32, 458-463. (31)Motomizu, S.;Oshima, M.; Jun, Z. Anal. Chim. Acta 1991,251, 269-274.
As part of this study, a successful FIA system was built to determine boron in aqueous samples using indirect UV-visible detection of complexes formed with the CTA reagent. Optimum conditions for the FIA system were studied. Its precision and accuracy were validated by comparing interlaboratory analytical performances. In addition, several interference tests were carried out with ions potentially found in the moderator and primary heat transport system (PHTS) of CANDU nuclear reactors. Results on fields samples are presented.
EXPERIMENTAL SECTION Apparatus. All the experiments were performed using a oneor two-line FIA analyzer assembled from standard chromate graphic and spectrophotometricequipment. With the one-line configuration,the sample is introduceddirectly into the buffered CTA reagent stream, whereas with the two-line arrangement, it is injected into a water carrier stream and the buffered reagent constitutes the second channel. The same six-way rotary valve (Model 5041, Rheodyne) was used for both systems, allowing a well-reproducible volume to be injected. With the two-line arrangement, a Kel-F polymer T-shaped confluence point (PN 5-8750, Supelco) was installed just in front of the reaction coil to mix both streams together with a constant reagent dilution. Each stream was propelled by a double plunger pump (Model 590, Waters Assoc.) to ensure constant flow. The different types of reaction coils tested were made of suitable lengths of open Teflon tubing of 0.3- or 0.5-mm i.d. (PN 035548 and 035519, Dionex) and packed Teflon tubing (chemically inactive pearl string reactor) of 0.96-mm i.d. filled with Teflon microbeads (PN 36036, Dionex). Two-way couplings (PN 20052, Dionex and PN YB-0647355,Cole Parmer) were used to attach the coils, manifold tubes, and various inlet and outlet ports together. Detection is performed using a UV-visible spectrophotometer (Model LC95, Perkin-Elmer) equipped with a 4.5-~Lflow cell under the following conditions: wavelength, 361.6 nm; spectral bandwidth, 5 nm; response time, 0.5 8. The recording system was either a strip chart recorder (Model585, Omega) or an integrator (Model 3390A, Hewlett-Packard). Chemicals. Purified water (Milli-QWater System,Millipore) was used as a sample carrier and to prepare the reagent and standard solutions. The reagent stream was a 2.9 X lo-' M solution of CTA (sodium or disodium salt, Assured, BDH Chemicals) buffered at pH 7.4 with a 0.5 M sodium acetate (Certified ACS, Fisher) solution. Maximum reagent stability was obtained by separately dissolving the weighting amount of each salt (CTA and sodium acetate) followed by mixing these aqueous solutions to obtain the final concentration. Details on the optimal preparation procedure are given elsewhere.32 Standards were prepared by dilution from a 1g/L boron stocksolution (Certified ACS, Fisher) or a 5 g/L boron standard reference material solution (SRM 3107, NIST), both as boric acid. The concentration levels ranged from 40 to 10 OOO pg of boron per liter. Heavy water (99.86% DzO, Atomic Energy of Canada Limited) was used to prepare some of the analytical standards. A solution of tartrazine (Schwartz) was used to optimize the reaction coil, flow rate, and sample volume of the FIA analyzer. Procedure. The reagent and the water used as carrier were filtered through a 0.45-pm filter (Type HA, Millipore). After degassing,they were kept in two 2-LHPLC reservoirs (Kontes). The reagent is sensitive to light and should be protected by covering the reservoir with an aluminum foil. For light- and heavy-water calibration curves, a triplicate injection was performed at each concentration level. RESULTS AND DISCUSSION Optimization. The FIA system developed uses indirect UV-visible absorption spectrophotometry (FIA-W)to detect the borate-CTA complexes formed in a stream. After initial experiments with a single-line manifold, it was noted that injecting the sample into the reagent flow produces a high (32)Lussier, T.; Gilbert, R.; Hubert, J. Unpublished work.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992 2209
t11
A
A
2.5
DI
A
A
A A A
e
AA
1.5
I)
I
I
so
IlM
1
1511
I
I
I
,IN1
2511
31M1
I 350
Time (s)
Flgure 1. Influence of reactor conflguratlon on sample dlsperslon:(0) open tube, 0.3-mm i.d. 1-9-m length; (A)open tube, 0.5-mm Ld., 1-18-m length; and (0)packed tube, 0.96-mm lad.,2-14-m length. reagent dilution which in turn induces a significant decrease in reagent stream absorbance with regard to the vacancy peak associated with the analyte. A two-line manifold was therefore adapted where purified water was used as sample carrier and the reagent as second stream. This set-up ensures the constant volumetric ratio of the two streams at the merging point in front of the reaction coil. In order to maximize sensitivity, the flow rate, reactor configuration, and sample volume were optimized by injecting a tartrazine solution into the carrier stream and measuring the sample dispersion. Flow Rate Effect. The total flow rate was varied from 0.1 to 2 mL/min with an equal value in each channel. This study was done using a T-shaped confluence point connected to the detector by an open tube of approximately 1m. For a given FIA manifold, the dispersion due to the flow can be expressed by
D = Co/Cma = ho/h where Coand Cmaare the sample concentrations before and after dispersion respectively, hoand h being the corresponding peak height signals. The best flow rate conditions to increase the sample residence time and limit the dispersion were found to be 0.3 mL/(min/channel); below this value, the signal becomes noisy. Under these conditions, the sample dispersion was 1.35 with a residence time of 27 s. To accommodate the slow borate-CTA reaction, a longer reactor length should be used. Reactor Configuration. At a flow rate of 0.3 mL/(min/ channel), the dispersion was studied as a function of the reactor length by connecting several 1-,2-, or 3-m sections of (1) 0.3-mm-i.d. open Teflon tubing, (2) 0.5-mm-i.d. open Teflon tubing, or (3) 0.96-mm4.d. packed Teflon tubing. The results are shown in Figure 1 where the reactor length is expressed in terms of sample residence time. With the packed tube, the increase in sample dispersion in relation to the residence time obtained is about equivalent to that observed with the smallest open tubing. Although the effective apparent tube diameter of the packed tube was estimated at 0.58 mm, which is very close to the widest open tube i.d., the residence time for equivalent dispersion is about half as long with an open tube of 0.5-mm i.d. This phenomenon was observed by Poppe et al.33who showed that for a given tube i.d., longer residence times can be obtained with a pearlstring reactor without any significant loss of sensitivity due to decreased broadening of the band. Under these conditions, the packed tube seemed to be the most interesting type of reaction coil to accommodate the slow borate-CTA reaction since it allows, for a given reactor length, the longest residence (33) Reijn, R. M.; van der Linden, W. E.; Poppe, H. Anal. Chirn. Acta 1981,123, 229.
0
0
2000
4000
6000
8000
10000
Boron concentration (pg/L) Flgure 2. Calibrationcurve for boron determination uslng Indirect UVvisible spectrophotometric detectlon. For concentratlon levels up to 6000 pg/L, the regression llne of the peak height o a t the concentration of boron (x) Is described as follows: y = 0.1060~- 0.7750 (r = 0.9993), and for concentratlon levels up to 10 000 pg/L, the cwespondlng thlrd order polynomlal 1sy = - 0 . O O O W 0.00509 0.0930~-0.0046 (r = 0.9999)where rls the coefflclent of correlatlon.
+
+
time with low dispersion compared to both of the open tubes tested. When the reactor comprised only sections of packed tubing, the signal was pulsed but the pulsations could be minimized by adding a 3-m section of open tubing (i.d. = 0.5 mm) after the pearl-string reactor coil. The change in the tube's internal diameter (0.58-mm effective i.d. vs 0.5-mm i.d.) did not introduce any significant perturbation into the sample plug. The final reactor was made of 10 m of packed tubing connected in series with a 3-m open tube allowing a sample residence time of 7 min with a dispersion of about 2. Since the borate-CTA reaction is slow, this residence time allows enough complexesto be formed to produce at low boron concentration a useful signal for the present application. Sample Volume. With the optimized flow rate and reactor configuration, the sample volume was varied from 30 to 530 pL, and it was found that a 38-pL sample volume is necessary to reach 50% of the steady-state signal and that maximum sensitivity was obtained for a sample volume of 300 pL. Under such conditions, the sample dispersion remains around 2, allowing a sample throughput of 15 samples/h. Analytical Performances. When using the optimized FIA-UV analyzer, boron can be quantified over 3 orders of magnitude whereas it was previously limited to 1W2400 pg/L with the manual method.28 As seen in Figure 2, the calibration curve established over a concentration range of 40-10 OOO pg/L shows a linear response at concentration levels up to 6000 pg/L, and for concentrations up to 10 000 pg/L a thirdorder polynomial can be used. This change in the shape of the curve could probably be attributed to the presence of different complexesin the reaction mixture. From our resulta complex type 1:2 (borate-CTA) would be favored over complex type 1:lwhenever the boron concentration becomes almost equal to or greater than that of the CTA. Under these latter conditions,both complexeswould coexistand contribute to the observed absorbancy changes. The formation of polyborate is not expected as the boron concentration is quite low in the range of interest. The polynomial calibration curve presented by Kuemmel and Mellon does not agree with our results probably due to the fact that with the manual method, a steady state of chemical equilibrium had to be reached in a reaction mixture always kept homogeneous in terms of concentration and pH, and under such conditions, both complexeswould be present even a t low boron concentrations. However, the high reproducibility of the analyzer allowsboron to be determined without completing the borate-CTA re-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992
Table 11. Precision and Relative Accuracy
reference measured precision relative sample boron value boron valuea RSD accuracy identification (pg/L) (rg/L) (%) (%) COGIS 9-7 100 96 2 -4 COGIS 11-9 210 219 0.6 4 COGIS 12-N25 350 343 0.8 -2 COGIS 9-8 800 795 -0.6 0.8 825 -2 COGIS 12-N27 806 0.3 1210 1161 -4 COGIS 11-8 0.3 COGIS 9.9 1650 1653 0.8 0.2 1750 COGIS 12-N26' 1665 0.2 -5 COGIS 8-9 1900 1830 0.3 -4 COGIS 11-7 2200 2074 0.1 -6 COGIS 8-8 5300 5258 0.4 -0.8 COGIS 8-7 8700 8351 0.5 -4 pg
Sc'ln
-
Flgure 3. Typical FIA signals for the determination of boron using indirect UV-visible spectrophotometric detection of the borate-CTA complexes (concentrations expressed in micrograms per liter).
Table I. Effect of Other Ions in the Determination of Boron.
effect of other ions, pg/L of boron, at a boron concentration of
other ions Gd3+ K+
added as Gd(NOd3 KC1
F
LiCl NaCl NH&l CaCl~2H20 NaF
c1-
KCl
Lit Na+
NH,+ Ca2+
Ob (MIL)
63 0 6 0 0 0 0
NaNOz
lo00 pg/L), a 1% ' precision is obtained with the FIA-UV analyzer. A slightly better precision was obtained by Motomizu et aL30using fluorimetric detection of the borateCTA complexes; however, five sets of experimental conditions are required to cover a less extensive working range (0.2-5000 pg/L). The FIA-UV analyzer shows excellent agreement between the reference and measured values on the entire dynamic range. This includes the sample containing 3000 pg of gadolinium per liter (COGIS 12-N26)for which the positive signal previously noted at high Gd3+ concentration levels was not detectable for boron concentrations as high as 1750pg/L. Some of these
.
(34)John, M. K.; Chuah, H. H.; Neufeld, J. H. Aml. Lett. 1975,8 (8) 559-568.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1902
Table IV. Comparison with Other Methods for Field Samples. sample method measured value (w/L) RSD (%) Moderator UV manual 180 8
90
1900
60
n
I/
100 h
5
88-08-21
Moderator
30
89-06-07
x
G:
Moderator 89-11-29
: o
0
.2
-
Moderator 90-01-30
) I
-30
PHTS 90-02-02
-60
Flguro 4. Accuracy study. For each sample, the expected boron concentrationis IndicatedIn microgramsper liter overthe corresponding results: filled bars, FIA-UV method; open bars, other methods or
laboratories. Table 111. Comparison with Other Methods for Reference or Spiked Samples. reference measured relative value value RSD accuracy samde method (rca/L) (rcn/L) (%) (%) light water UV manual
IDMS FIA-UV FIA-F Moderator IDMS 90-01-30 FIA-W FIA-F PHTS IDMS
0
IDMS
FIA-UV FIA-F IDMS FIA-UV FIA-F IDMS FIA-UV FIA-F IDMS FIA-UV FIA-F IDMS FIA-UV FIA-F
232.9 209.4 215.6 268.8 338 283.0 276.6 276 250.0 383 391.0 380.0 55 35 36
0.6 0.4 0.2
0.8 1 0.7 2 1 0.7 1 0.3 0.2 20 9 3
Average of three replicates.
- 90
90-02-02
2205
FIA-UV
FIA-F
50b
(spike) 25 (spike) 25
55 54.9 47 46.2 21.2 26 20 22 24 28
7 0.5 3 0.6 0.54 0.4 0.5 14 4 0.9
10 10 -6
-8 -15 4 -20 -11 -4 12
Average of three replicates. b 50 i 4 p g of boron per liter.
results were compared in Figure 4 to those produced by the other participants in the round-robin. The comparative study shows that the relative accuracy of all FIA-UV results was as good as or better than the others, especially at low boron concentration levels. The precision for boron determination for each participant was not reported. Performances of the FIA technique were assessed on field samples through a comparative study using isotope dilution mass specrometry (IDMS), a manual spectrophotometric method, and another FIA method involving the fluorescence detection mode developed in our laboratory. The following sampleswere analyzed four heavy-water moderator samples, one heavy-water PHTS sample,and one sample in light water supplied by the Gentilly 2 laboratory. Two of the heavywater sampleswere spikedwith25 pg of boron per liter. Results are summarized in Table I11 for known and spiked samples and in Table IV for the remainder. IDMS data were obtained at the Chalk River Nuclear Laboratories where the boron determination procedure1' recommended by the National Bureau of Standards is applied. The manual spectrophotometric data were determined at the Gentilly 2 laboratory using a modified version of the original Kuemmel and Mellon method.31 Other data refer to both FIA methods for which the same manifold was used except for the detection mode. The spectrofluorimeter (ModelLS-4, Perkin-Elmer)equipped with a 3-pL flow cell serving as a substitute for the UV flow-
through detector previously described was used under the followingconditions: excitation wavelength, 313 nm; emission wavelength, 350 nm; spectral bandwidths, 10 nm; response time, 8 s. These conditions do not correspond to the maximum sensitivity that can be obtained by fluorimetry but are adjusted to provide an equivalent working range as for Wvisible detection. For both FIA analyzers, the boron content of each sample was evaluated against appropriate standards. Calibration curves performed with heavy-water standards have a positive intercept whereas the regressions generally pass through zero when light-water standards were used. This discrepancy can be associated to the difference in the absorbance of the DzO and HzO medium. When using the UV-visible detection mode, precision is at least 3% and relative accuracy better than 6% ' . All of our results agreed with those obtained by other methods or groups.
CONCLUSIONS The FIA-UV system allows fast analysis and requires only small amounts of reagent and sample solutions to determine total boron in light- and heavy-water samples. In both matrices, very good precision and relative accuracy are obtained when applied on synthetic and field samples over the entire 4&10 OOO pg/L boron range. Those performances make this new analyzer adequate for field operation at CANDU nuclear power stations since the moderator and heat transport systems can be analyzed without any interference from ions potentially found in those systems. In addition, inexpensive equipment and a restricted laboratory training program make the FIA-UV system attractive for field operation.
ACKNOWLEDGMENT We thank the National Science and EngineeringResearch Council of Canada (NSERCC),the Fonds pour la Formation des Chercheurs et d'Aide 8. la Recherche du Quebec (FCAR), the CANDU Owners Group (Working Party No. 15), and the Institut de recherche #Hydro-Qubbec (IREQ) for their financial support. We are also grateful to Yiicel Dfhdar and Richard Laporte of Hydro-Qubbec's Gentilly 2 power plant for their collaboration in the validation of the present technique.
RECEIVED for review February 19, 1992. Accepted June 10, 1992.
