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Flow injection spectrophotometric determination of europium using chlortetracycline
Talanta 59 (2003) 9 /18 www.elsevier.com/locate/talanta
Flow injection spectrophotometric determination of europium using chlortetracycline Saisunee Liawruangrath a, , Singto Sakulkhaemaruethai b a
Department of Chemistry, Faculty of Science, Water Research Center (WRC), Chiang Mai University, Chiang Mai 50200, Thailand b Chemical Research Institute, Rajamangala Institute of Technology, Patumthani 12120, Thailand Received 1 May 2002; accepted 7 August 2002
Abstract A flow injection (FI) spectrophotometric determination of europium (III) is described, based on the complexation between europium (III), and chlortetracycline (CTC) in a Tris /buffer pH 8.0 medium. The resulting yellow-coloured complex is measured at its absorption maximum of 400 nm after 100 ml of sample or standard solution containing europium (III) are injected into the merged streams of CTC and Tris /buffer solutions. Optimum conditions for determining mg amounts of europium (III) are achieved by univariate method. Various types of reactors are also investigated. It is shown that the use of a single bead string reactor gives rise to the enhancement of peak height. A linear calibration curve over the range of 0.10 /0.60 mg ml 1 europium (III) is established with the regression equation (n /6) Y /34.93X/0.01 and the correlation coefficient of 0.9994 is obtained. A detection limit (3s) of 0.01 mg ml 1 of europium (III) and the relative standard deviation (R.S.D.) of 4.32% for determining 1.0 mg ml 1 of europium (III) (n /7) are obtained. The recommended method has been applied to the quantitation of europium (III) in spiked water and stream sediment samples with average recoveries of 99.9 and 97.5%, respectively. The sampling rate is found to be 85 h 1. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Spectrophotometric determination; Chlortetracycline; Flow injection; Eu(III)
1. Introduction
Corresponding author. Tel.: /66-53-943341; fax: /66-53892277 E-mail address: [email protected] (S. Liawruangrath).
Europium, a soft silvery white metal which is one of the reactive rare earth metals,. It was discovered and isolated by Eugene-Antole Demarcay in 1896 and 1901, respectively [1], its atomic number is 63 and atomic weight is 151.96. Europium can exist in two oxidation states, Eu(II) and
0039-9140/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 4 4 3 - 5
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S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
Eu(III), but the most stable oxidation state is Eu(III). Most europium is obtained from monazite sand, which is a mixture of phosphates of calcium, thorium, cerium and most of the other rare earth metals. Europium can be separated from the rare earth metals by ion-exchange displacement process. The result is an europium ion that reacts with oxygen ions to form europium oxide, Eu2O3. Europium is reduced form europium oxide by mixing it with powdered lanthanum metal in a tantalum crucible. Various methods have been developed for the determination of rare earth metals including europium. They include spectrophotometry [2], spectrofluorimetry [3,4], polarography [5], voltammetry [6], inductively coupled plasma atomic emission spectroscopy (ICP-AES) [7], inductively coupled plasma mass spectrometry (ICPMS) [8]. Most methods for determinating rare earth metals such as Eu(III) in real samples are based on fluorimetry [3,4]. Selectivity and sensitivity can be achieved by using selective organic reagent as a ligand together with preconcentration and separation by ion exchange chromatography. For examples. Taketatsu and Sato [3] described a fluorimetric method for the determination of europium (III) using TTA-trioctylphosphine (TOPO) or Phen / Triton X-100 which is sensitive and reliable,and requires relatively simple equipment.They reported that the detection limit is 0.02 mg kg 1 Eu. Yamada et al. [4] described the laser fluorimetry combined with nitrogen laser and a pulse-gated photon counting method for determining europium (III) with 4,4,4-trifluoro /1-(2-thienyl)-1,3butadione. The detection limit for europium (III) is 0.3 ng l 1 . The europium (III) complex can be determined with high selectivity and sensitivity in a large excess of the samarium(III) complex. Lyle and Za’tar [2] presented the spectrophotometric method for the determination of 0.04 /5 mg of europium in rare earth mixtures. In this method, europium (III) is reduced on a Jones reductor to europium (II) which in turn reduces molybdophosphoric acid to molybdenum blue. the absorbance is measured at 810 nm in aqueous
solution or at 790 nm after extraction into n -amyl alcohol. Fu et al. [5] described the polarographic study of the Eu(III)-triethylenetetra-aminehexaacetic acid complex. They reported that this method was convenient to determine trace Eu(III) concentrations in the range 2.5 /10 6 /5.0 /10 4 mol l 1 in 0.1 mol l 1 NH4Cl and 0.1 mol l 1 NH3 solution. Mlakar and Branica [6] described a voltammetric method for the determination of europium (III) in the presence of 2-thenoyltrifluoroacetone (TTA). In this method, the Eu-TTA complex is strongly adsorbed at the surface of the mercury drop electrode and provides the possibility of measuring low concentration levels of europium (III). The detection limit in sodium chloride, with an accumulation time of 10 min at a potential of / 0.2 V, was 5 /10 9 mol l 1 Eu(III). However, all these procedures are troublesome or the instruments are not readily available.Therefore, there is the need for suitable laboratories equipped with inexpensive instrumentation, which allows the determination of europium to be carried out in a fast and cheap way without sacrificing precision. Flow injection spectrophotometric method is a method that satisfies these requirements and can be afforded by most laboratories. Al-Sowdani and Townshend [9] proposed the flow-injection procedures with spectrophotometric and spectrofluorimetic detections for Eu(III) determination. A zinc reductor mini column was used in the flow-injection system for the reduction of Eu(III) to Eu(II). Eu(III) was indirectly determined either spectrophotometrically by oxidation with Fe(III) and reaction of the resulting Fe(II) with 1,10-phenanthroline (Phen), or spectrofluorimetrically by reaction with Ce(IV) and the Ce(II) produced was measured. Linear calibration curves were established for 10 /200 and 0.5 /4 mg ml 1with detection limits of 2.5 and 0.25 mg ml 1 by spectrophotometry and spectrofluorimetry, respectively. Chlortetracycline (CTC), an important member of the tetracycline group of antibiotics reacts with Eu(III) resulting in Eu(III)/CTC complex which provides a sensitive means for the assay of CTC in pharmaceutical preparations. This method seems
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
promising to be adopted for the development of the procedure for Eu(III) determination. This paper describes an inexpensive flow injection (FI) spectrophotometric procedure for Eu(III) determination based on the complexation of Eu(III) with CTC in Tris /buffer pH 8.0 medium.
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appropriate dilutions. Tris /buffer pH 8.0 solution was prepared by dissolution of 1.21 g Tris(hydroxymethyl) aminomethane in 1000 ml of deionized water and adjusting the pH to 8.0 with 1 mol l 1 hydrochloric acid (J.T. Baker Inc., Phillipsbers, USA). 2.2. Apparatus
2. Experimental 2.1. Reagents All chemicals were of analytical-reagent grade and were used with further purification. Deionized water was used throughout the whole experiment. The standard stock solution of europium (III) chloride hexahydrate (100 mg l 1) was prepared by dissolving europium (III) chloride hexahydrate (0.2417 g obtained from Fluka Switzerland) in 1000 ml water. Further with water were made for appropriate concentrations. A 1.0 / 10 3 M chlortetracycline hydrochloride (Fluka, Switzerland) in 1000 ml water. The more dilute solutions of this solution were made up by
The FI manifold is shown in Fig. 1, an EYELA peristaltic pump MP-3A (Tokyo Rikakikai Co., Ltd.) was used for propelling CTC solution and Tris /buffer solution pH 8. Tygon tubing (1.14 mm i.d.) was used for the flow lines. The sample and/or standard solution containing europium (III) chloride was injected by a 1.0 ml disposable plastic syringe (Nissho Nipre Corporation Ltd., Thailand) into a merged streams of the CTC solution and Tris /buffer solution via a laboratory-made, low-cost injection valve. The merged streams were passed through a single bead (3.0 mm diameter) string reactor (s.b.s.r.) [10]. The coloured complex formed was passed through a flow through cell (Hellma, 1 cm, Suprasil I window, type 178, 711GS) in a spectrophotometer model spectronic 21 (Milton Roy Company, USA) connected to a Servograph REC 51 chart recorder (Radiometer Copenhegen, Denmark). Measurements were made at 400 nm. An UV-Vis spectrophotometer Model UV-265 Shimadzu, Kyoto, Japan was used to scan the spectra of CTC and Cu(II) /CTC complex. A Cole /Parmer (Model 5986-25) pH-meter with a combined glass and calomel electrode was used to adjust the pH values. 2.3. Procedure
Fig. 1. The FI manifold designed for Eu(III) determinaion Injection of the sample and for standard solution into the buffer stream before merging with the CTC system. Injection of the sample and/or standard solution into the merged stream of the CTC and the buffer stream. P, peristaltic pump; I, injection valve; S, sample; MR, mixing reactor; D, detector; Rec., recorder and W, waste.
2.3.1. Sample pretreatments [11] The stream sediment samples were collected from water resources around Doi Inthanon Mountain and let it air dried at room temperature. The dried samples were sieved through a sieve with the particle size of 80 mesh, dried at 100 8C for 1 h and allowed to cool in the desiccator. Approximately 1 g of the dried stream sediment sample was accurately weighed; transferred into a teflon beaker; 10 ml each of HNO3, HClO4 and HF was
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S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
added, respectively, into the beaker containing the sample. The beaker was heated on a hot plate at 100 8C covered with the lid and the temperature was raised to 225 8C and heated until the fume was completely removed. The beaker was removed from the hot plate and stood to cool to room temperature followed by adding HNO3, HClO4 and HF (10 ml each). Then the beaker was heated at 225 8C until no fume evolved. After cooling the beaker to room temperature 10 ml of 6 mol l 1 HCl was added and heated until the black residue was obtained. Then 10 ml 30% H2O2 was added to dissolve the residue, the excess H2O2 was removed by heating and the sample solutions were filtered through No 42 Whatman filtered paper. The incomplete digested residue was transferred into a platinum crucible, 3 g of lithium metaborate was added and digested by heating at 1050 8C for 20 min and the residue obtained was dissolved with 40 ml of 1 mol l 1 HNO3. This solution was combined with the filtrate and evaporated to dryness. The resulting residue was dissolved in 10 ml 1 mol l 1 HNO3, mixed well and the sample solution was for submitted to separation technique. The rare earth elements were separated from other elements by means of an ion exchange chromatographic technique based on stepwise elution of nitric and hydrochloric acids. Eu(III) in the sample solutions was determined by the proposed FI method and conventional spectrometric method based on complexation with bromopyrogallol red [12].
2.3.2. Procedure for Eu(III) determination by FIA A double channel FI manifold was designed and fabricated as shown in Fig. 1 and used for all FI runs. A 100 ml sample loop was used for injecting the sample and/or standard solutions into the merged streams of CTC reagent (4.0 /10 3 mol l 1) and the Tris /buffer (pH 8.0) solution streams which were pumped at the same flow rate of 3.0 ml min 1. Europium (III) formed the Eu(III)/CTC complex with the reagent and the absorbance as peak height was monitored at 400 nm.
Fig. 2. Absorption spectra of CTC and Eu(III) /CTC complex.
3. Results and discussion 3.1. Absorption spectra of CTC and its Eu(II) complex Europium (III) reacts with CTC resulting in a yellow complex in Tris /buffer pH 8.0 medium. The complex presents an absorption maximum at 400 nm whereas the CTC exhibits its absorption maximum at 368 nm (Fig. 2) under the same experimental conditions. It was found that the stiochiometry (Eu(III): CTC) of complex examined by both mole ratio and continuous variation methods was found to be 1:1 which was in good agreement with the previously reported value [13]. The molar absorptivity at 400 nm was 1.00 /104 l mol 1 cm 1 for europium (III) /CTC complex in Tris /buffer pH 8.0 medium. In the present work, a FI spectrophotometric procedure was proposed for determining europium (III) using CTC as chromogenic reagent. The Tris /buffer was used because it is one of the few buffer materials that does not interfere with the europium (III) /CTC complexation. 3.2. Optimization of the flow injection and the chemical conditions Two arrangements of the FI manifold were designed and fabricated (Fig. 1). Optimization of
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
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Table 1 Optimization of the FI and chemical conditions for Eu(III) determination Variable
Studied range
Optimum conditions
Wavelength (nm) pH of buffer solution CTC concentration (mol l 1) Flow rate (ml min 1) Type of mixing reactor Length of mixing reactor (cm) Internal diameter of tubing (mm) Tubing length (cm) Injection volume (ml)
380 /440 7.0 /9.0 1.0 /10 5 /1.0 /10 4 2.0 /4.0 6 types (see Table 3) 3.0 /11.0 0.76 /1.52 25 /100 50 /200
400 8.0 4.0 /10 5 3.0 s.b.s.r. 5.0 1.14 75 100
experimental conditions for determining europium (III) were carried out by means of univariate method [14], in which a variable was modified while maintaining the other variables at their constant values (selected by random). Then by maintaining that variable at its optimum value, another was modified and this procedure was repeated for all variables. The ranges over which the variables involved in the FI system studied and their optimium conditions were listed in Table 1. It was found to be more satisfactory to inject the standard or sample solutions into the merged streams (Fig. 1a) rather than injecting into the buffer stream before merging (Fig. 1b). Since the FIA peaks obtained by the former manifold provides reproducible results with the higher sensitivity than those obtained by the latter manifold. The proposed design of the manifold was shown in Fig. 1a. 3.2.1. Effect of pH on the formation of Eu(III) / CTC complex The complexation between Eu(III) and CTC in Tris /buffer medium was studied over the pH range 7.0 /9.0. The peak heights of the Eu(III)/ CTC complex were measured over the pH range 7.0 /9.0, by using Tris /buffer together with a sufficient volume of 1 mol l 1 HCl to give the required pH. The peak height increased with pH up to 8.0 and decreased when the pH values were
greater than 8.0. Hence, pH 8.0 was chosen as optimum, because at this pH value the peaks were relatively high with a good reproducibility and the linearity of the calibration curve was better (r / 0.9998) than those obtained by other pH values as shown in Table 2A. 3.2.2. Effect of CTC concentration The effect of various concentrations of CTC solutions (1.0 /10 5 /1.0 /10 4 mol l 1) on the absorption of the Eu(III)/CTC complex (as peak height) at 400 nm was examined. In any colorimetric procedure, an amount of reagent grater than required by stoichiometry is needed for colour development (Eu(III): CTC /1:1). The CTC concentrations in the studied range were sufficient for complete colour development. At higher CTC concentrations, the slope of standard calibration curve increases. The CTC concentration which exhibited the largest slope increment with reasonable sample throughput was found to be 4.0 /10 5 mol l 1 (Table 2B) and was, therefore, chosen as optimum CTC concentration. 3.2.3. Effect of flow rate The flow rate of each reagent or carrier solution is very important and should be regulated. Different flow rates ranging from 2.0 to 4.0 ml min 1 were studied. High flow rates shorten the reaction times, increase dispersion and a lower the ratio of
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S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
Table 2 Effect of pH, CTC concentration, flow rate, glass bead column length and injection volume on peak height Parameters studied
(A) Effect of pH
(B) Effect of CTC concentration (mol l 1)
(C) Effect of buffer, CTC carrier flow rate (mol min 1)
(D) Effect of glass bead column length (cm)
(E) Effect of injection volume (ml)
Regression equation
Remarks
7.0 7.5 8.0 8.5 9.0 1.0 /10 5
Y /2.5X/0.3 Y /1.64X/7.24 Y /4.04X/1.96 Y /4.08X/1.24 Y /3.62X/3.22 Y /0.44X/8.44
Optimum pH 8.0 r /0.9995 tbase /43 Sampling rate /84
2.0 /10 5 4.0 /10 5 5.0 /10 5 1.0 /10 4 2.0
Y /0.5X/1.078 Y /4.48X/3.6 Y /4.92X/2.48 Y /4.10X/10.52 Y /2.86X/1.98
2.5 3.0 3.5 4.0 3.0 5.0 7.0 9.0 11.0 50 100 150 200
Y /4.18X/4.74 Y /5X/3.32 Y /3.36X/11.4 Y /3.96X/6.88 Y /3.74X/29.58 Y /4.58X/22.54 Y /3.68X/21.52 Y /4.32X/26.68 Y /3.1X/22.66 Y /2.86X/12.06 Y /5.24X/8.28 Y /4.4X/9.88 Y /2.94X/12.5
Optimum CTC concentration /4.00 /10 5 mol l 1 r /0.9942 tbase /44 s Sampling rate /82 h 1 Optimum flow rate /3.0 ml min 1 r /0.9995 tbase /44 s Sampling rate 82 h 1 Optimum s.b.s.r. length /5.0 cm r /0.9574 tbase /42 s Sampling rate 86 h 1 Optimum injection volume /100 ml r /0.9726 tbase /42.5 s Sampling rate 85 h 1
Table 3 Effect of mixing reactor types on peak height, sensitivity and speed Eu(III) (ppm)
Reactor type Peak height (mV)
1.0 2.0 3.0 4.0 5.0 Slope (mV ppm 1) Correlation coefficient tbase (s) Sample per hour Average of triplicate results.
Serpentine 1 /1
Serpentine 1 /1
Coiled
Knitted
Straight line
S.b.s.r.
6.60 8.00 13.60 14.60 19.40 3.22 0.9786 48 75
14.40 20.80 25.40 29.40 32.00 4.36 0.9630 54 66
17.40 25.00 30.40 33.40 35.00 4.36 0.9630 48 75
24.00 31.00 38.60 36.00 38.00 3.30 0.8549 48 75
17.20 23.60 28.00 35.40 39.60 5.66 0.9968 42 85
17.00 23.20 29.60 35.20 41.60 6.12 0.9997 42 85
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Table 4 Summary of the interference effect of some foreign ions on the peak height obtained from 1.0 mg l 1 Eu(III) Interferet ion Tolerable concentration ratio (mg l 1 X of ion/ Eu(III) when ion/ Eu(III) /100a
Relative error (%)
2 /SO 4 ;/ /NO ; 3
200
/0.62, /4.38, /6.25
100 80 40 25
/9.75 /12.50, /13.75 /14.10 /21.79
Cl /
K Al3 , Na Zn2 Mg2
/ and / Represent enhancement and supression n peak heights, respectively. a The concentration of an ion is considered to be interfering when causing a relative error of more than 9/10% with respect to the peakheight of Eu(III) alone.
sample peak to blank peak. Hence the reaction is not allowed to reach completion as indicated by the decrease in absorbance of the complex formed [15]. On the other hand, low flow rates lead to a decrease in sample throughput. A flow rate of 3.0 ml min 1 for both buffer and CTC streams was
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chosen in order to obtain a reasonable sample throughput (63 h 1), the highest sensitivity (defined as slope of calibration curve) and the better linearity of standard calibration curve (r /0.9997) as shown in Table 2C. 3.2.4. Effect of using glass bead column as a mixing reactor The effect of using glass bead (3.0 mm diameter) column as a mixing reactor was examined by varying the length of the s.b.s.r. reactor from 3.0 to 11.0 cm. The results obtained are summarized in Table 2D, which show that good sensitivity and reasonable sampling rate could be obtained when the glass bead column length was 5.0 cm. Therefore, as a compromise among the sensitivity, sample throughput and a reasonable linearity of calibration curve, the glass bead column length of 5.0 cm was chosen for all experiments. 3.2.5. Effect of injection volume The effect of injection volume was studied by changing the sample loop to give an required injection volumes in the range 50/200 ml in order to find out the optimum injection volume. The results obtained (Table 2E) show that a 100 ml was found to be the optimum injection volume as a compromise between good sensitivity and a sampling frequency of sample per hour.
Table 5 Determination of Eu(III) in spiked water samples Sample code
Concentration of Eu (mg l 1) Spiked
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 Mean (X) S.D. R.S.D. (%)
0.005 0.010 0.050 0.100 0.300 0.500 0.700 1.000
Mean for five determinations.
Recovery (%) Found FIA
Spectro.
0.005 0.010 0.053 0.103 0.302 0.495 0.690 0.980
0.0049 0.010 0.051 0.103 0.301 0.510 0.690 0.975
FIA
Spectro.
100.0 100.0 106.0 103.0 100.7 99.0 98.6 98.0 100.66 2.64 2.62
98.0 100.0 102.0 103.0 100.3 102.0 98.7 97.5 100.19 2.02 2.02
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
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Table 6 Comparative determination of Eu(III) in stream sediment sample by the proposed FI method and conventional spectrometry Experiment number
Eu(III) found (mg g 1) FI method
1 2 3 4 5 Mean (X) S.D. R.S.D. (%)
Conventional spectrometry
In1 a
In2 b
In3 c
In1 d
In2 e
In4 f
43.75 45.00 45.50 42.50 43.00 43.95 1.28 2.91
44.25 45.50 44.25 43.75 43.75 44.30 0.72 1.63
42.50 41.50 40.50 42.50 43.75 42.15 1.22 2.89
43.50 45.20 45.70 42.45 43.20 44.01 1.38 3.14
44.15 45.45 44.50 43.80 43.90 44.36 0.67 1.51
42.60 41.40 40.40 42.60 43.55 42.11 1.22 2.90
3.2.6. Effect of various mixing reactor types The effect of varying the mixing reactor types (serpentine 1 cm /1 cm, serpentine 1.5 /1.5 cm, coiled, knitted, straight line, s.b.s.r.) was tested under similar conditions as previously described. The results were shown in Table 3. It was seen that the use of s.b.s.r. as a mixing reactor for the FIA system should be satisfactory because it provided a suitably high sensitivity combined with a suitably short tbase and the higher sample throughput. 3.3. Calibration curve The calibration curve for the determination of europium (III) using CTC was linear over the range up to 0.6 ppm europium (III). Over this concentration range, linear regression analysis of peak height (y) versus Eu(III) concentration (x ) (n /7) yielded the equation y /34.93x/0.01.
Linear regression analysis gave correlation coefficient of 0.9994. The calibration curve showed good correlation coefficient values which indicated excellent agreement.
3.4. Detection limit and quantitation limit The lower detection limit of europium (III) was also investigated. The detection limit (defined as three times of the standard deviation, 3s) was determined, calculated and found to be 0.010 mg l 1of europium (III). The quantitation limit is defined as analyte that concentration of the analyte producing the signal which is at least ten times of the standard deviation of the blank signal which was found to be 0.04 mg l 1 Eu(III).
3.5. Reproducibility and accuracy Table 7 Calculation for t -test Comparison
t -Value
A /d B /e C /f
0.07 0.14 0.05
2
95% Confidence t -value /2.306, n /n1/n2 /2.
The relative standard deviation (R.S.D.) for replicate injections was found to be 4.32% of europium (III) (n /12). The accuracy of the proposed method was verified by analysing spiked water samples containing europium (III). The mean percentage recovery of the added europium (III) of 99.7% was obtained.
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
3.6. Interferences The effects of some possible interfering cations and anions on the determination of Eu(III) were investigated for the maximum w/w ratio of foreign ions to Eu(III) 200: 1. The tolerence is defined as the foreign-ion concentration causing an error smaller than 9/10% for determining the analyte of interest. The tolerable values for the ions studied are summarised in Table 4. It is evident that SO2 4 , NO3 and Cl cause slight effects on the determination of Eu(III). The most serious interference is caused by Mg2 followed by Zn2, Al3 and Na , respectively. The enhancement in the FI signal of the cations probably owing to the formation of complexes with CTC which absorb at the same or very near to the working wavelength whereas the suppression of the FI signals may be due to the formation of complexes which absorb at different wavelengths. 3.7. Applications The proposed method has been applied to the determination of Eu(III) in spiked water samples and stream sediment samples after appropriate sample pretreatments [14]. A 100 ml aliquot of spiked water sample or stream sediment extract was injected into the merged streams of CTC and the Tris /buffer pH 8.0 solutions flowing with the same flow rate of 3.0 ml min 1 and the absorbance (as a peak) was continuously monitored at 400 nm. The peak heights are directly proportional to the concentrations of Eu(III). With respect to analyse Eu(III) in stream sediment samples collected from a selected sites along the water resources from Doi Inthanon Mountain to Mae Klang River at Chiang Doa District, Chiang Mai Province with the sample codes In1 /In9. It was found that Eu(III) contents were found in the stream sediment samples collected from In1,In2 and In4 but Eu(III) content in the stream sediment samples collected from In3 and In5 to In9 were not detected. The results obtained compared favourably with those obtained by spectrophotometric method [16] as shown in 5 for spiked water samples and stream sediment samples, respectively. The accuracies of both methods were found
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to be not different significantly because the calculated Student t-values were less than the theoretical values at a confident level of 95% (Table 7).
4. Conclusions Flow injection spectrophotometric procedure has been developed for determining Eu(III) using CTC as chromogenic reagent. The FI method, a double-chanel manifold equipped with a single bead string reactor was found to be appropriate FI manifold for Eu(III) determination. A calibration curve was linear over the range up to 0.6 ppm Eu(III). A detection limit of 0.01 mg g 1 Eu(III), with a precision of 4.32% could be obtained. The presence of Na, Zn(II), SO2 and NO3 cause 4 slightly effect. Al(III), Cl , Mg(II) and K were not interfere only if the analyte to interfering ion ratios were less than 1:100 but these ratios were as high as 1000 fold interference effects would be observed. The proposed FI-spectrophotometric method proposed for the determination of europium (III) was found to be simple, sensitive and rapid. It offer several advantages such as low cost, good accuracy (recovery /90%) and reproducibility (R.S.D. B/5%), and high sample throughput (85 h 1).
Acknowledgements The authors would like to express their sincere thanks to the National Research Council of Thiland, Institute for Science and Technology Research and Development, Chiang Mai University for financial support and also to the consortium of Postgraduate Education and Research Program in Chemistry (PERCH) for partial support.
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[10] K. Grudpan, S. Liawruangrath, P. Sooksamiti, Analyst 120 (1995) 2107. [11] J.G. Crock, F.E. Lichite, T.R. Wildeman, Chem. Geol. 45 (1984) 149. [12] J. Gano, J. He, X. Wang, Z. Wang, G. Bai, Analyst 112 (1987) 1081. [13] T.A. Kelly, G.D. Christian, Anal. Chem. 53 (1981) 2110. [14] A.S. Vincente, A. Arranz, J.M. Moreda, J.F. Arranz, Anal. Chim. Acta 298 (1994) 87. [15] B.W. Renoe, K.K. Stewart, G.R. Beecher, M.R. Willis, J. Savory, Clin. Chem. 26 (1980) 331.
Flow injection spectrophotometric determination of europium using chlortetracycline Saisunee Liawruangrath a, , Singto Sakulkhaemaruethai b a
Department of Chemistry, Faculty of Science, Water Research Center (WRC), Chiang Mai University, Chiang Mai 50200, Thailand b Chemical Research Institute, Rajamangala Institute of Technology, Patumthani 12120, Thailand Received 1 May 2002; accepted 7 August 2002
Abstract A flow injection (FI) spectrophotometric determination of europium (III) is described, based on the complexation between europium (III), and chlortetracycline (CTC) in a Tris /buffer pH 8.0 medium. The resulting yellow-coloured complex is measured at its absorption maximum of 400 nm after 100 ml of sample or standard solution containing europium (III) are injected into the merged streams of CTC and Tris /buffer solutions. Optimum conditions for determining mg amounts of europium (III) are achieved by univariate method. Various types of reactors are also investigated. It is shown that the use of a single bead string reactor gives rise to the enhancement of peak height. A linear calibration curve over the range of 0.10 /0.60 mg ml 1 europium (III) is established with the regression equation (n /6) Y /34.93X/0.01 and the correlation coefficient of 0.9994 is obtained. A detection limit (3s) of 0.01 mg ml 1 of europium (III) and the relative standard deviation (R.S.D.) of 4.32% for determining 1.0 mg ml 1 of europium (III) (n /7) are obtained. The recommended method has been applied to the quantitation of europium (III) in spiked water and stream sediment samples with average recoveries of 99.9 and 97.5%, respectively. The sampling rate is found to be 85 h 1. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Spectrophotometric determination; Chlortetracycline; Flow injection; Eu(III)
1. Introduction
Corresponding author. Tel.: /66-53-943341; fax: /66-53892277 E-mail address: [email protected] (S. Liawruangrath).
Europium, a soft silvery white metal which is one of the reactive rare earth metals,. It was discovered and isolated by Eugene-Antole Demarcay in 1896 and 1901, respectively [1], its atomic number is 63 and atomic weight is 151.96. Europium can exist in two oxidation states, Eu(II) and
0039-9140/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 4 4 3 - 5
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Eu(III), but the most stable oxidation state is Eu(III). Most europium is obtained from monazite sand, which is a mixture of phosphates of calcium, thorium, cerium and most of the other rare earth metals. Europium can be separated from the rare earth metals by ion-exchange displacement process. The result is an europium ion that reacts with oxygen ions to form europium oxide, Eu2O3. Europium is reduced form europium oxide by mixing it with powdered lanthanum metal in a tantalum crucible. Various methods have been developed for the determination of rare earth metals including europium. They include spectrophotometry [2], spectrofluorimetry [3,4], polarography [5], voltammetry [6], inductively coupled plasma atomic emission spectroscopy (ICP-AES) [7], inductively coupled plasma mass spectrometry (ICPMS) [8]. Most methods for determinating rare earth metals such as Eu(III) in real samples are based on fluorimetry [3,4]. Selectivity and sensitivity can be achieved by using selective organic reagent as a ligand together with preconcentration and separation by ion exchange chromatography. For examples. Taketatsu and Sato [3] described a fluorimetric method for the determination of europium (III) using TTA-trioctylphosphine (TOPO) or Phen / Triton X-100 which is sensitive and reliable,and requires relatively simple equipment.They reported that the detection limit is 0.02 mg kg 1 Eu. Yamada et al. [4] described the laser fluorimetry combined with nitrogen laser and a pulse-gated photon counting method for determining europium (III) with 4,4,4-trifluoro /1-(2-thienyl)-1,3butadione. The detection limit for europium (III) is 0.3 ng l 1 . The europium (III) complex can be determined with high selectivity and sensitivity in a large excess of the samarium(III) complex. Lyle and Za’tar [2] presented the spectrophotometric method for the determination of 0.04 /5 mg of europium in rare earth mixtures. In this method, europium (III) is reduced on a Jones reductor to europium (II) which in turn reduces molybdophosphoric acid to molybdenum blue. the absorbance is measured at 810 nm in aqueous
solution or at 790 nm after extraction into n -amyl alcohol. Fu et al. [5] described the polarographic study of the Eu(III)-triethylenetetra-aminehexaacetic acid complex. They reported that this method was convenient to determine trace Eu(III) concentrations in the range 2.5 /10 6 /5.0 /10 4 mol l 1 in 0.1 mol l 1 NH4Cl and 0.1 mol l 1 NH3 solution. Mlakar and Branica [6] described a voltammetric method for the determination of europium (III) in the presence of 2-thenoyltrifluoroacetone (TTA). In this method, the Eu-TTA complex is strongly adsorbed at the surface of the mercury drop electrode and provides the possibility of measuring low concentration levels of europium (III). The detection limit in sodium chloride, with an accumulation time of 10 min at a potential of / 0.2 V, was 5 /10 9 mol l 1 Eu(III). However, all these procedures are troublesome or the instruments are not readily available.Therefore, there is the need for suitable laboratories equipped with inexpensive instrumentation, which allows the determination of europium to be carried out in a fast and cheap way without sacrificing precision. Flow injection spectrophotometric method is a method that satisfies these requirements and can be afforded by most laboratories. Al-Sowdani and Townshend [9] proposed the flow-injection procedures with spectrophotometric and spectrofluorimetic detections for Eu(III) determination. A zinc reductor mini column was used in the flow-injection system for the reduction of Eu(III) to Eu(II). Eu(III) was indirectly determined either spectrophotometrically by oxidation with Fe(III) and reaction of the resulting Fe(II) with 1,10-phenanthroline (Phen), or spectrofluorimetrically by reaction with Ce(IV) and the Ce(II) produced was measured. Linear calibration curves were established for 10 /200 and 0.5 /4 mg ml 1with detection limits of 2.5 and 0.25 mg ml 1 by spectrophotometry and spectrofluorimetry, respectively. Chlortetracycline (CTC), an important member of the tetracycline group of antibiotics reacts with Eu(III) resulting in Eu(III)/CTC complex which provides a sensitive means for the assay of CTC in pharmaceutical preparations. This method seems
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
promising to be adopted for the development of the procedure for Eu(III) determination. This paper describes an inexpensive flow injection (FI) spectrophotometric procedure for Eu(III) determination based on the complexation of Eu(III) with CTC in Tris /buffer pH 8.0 medium.
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appropriate dilutions. Tris /buffer pH 8.0 solution was prepared by dissolution of 1.21 g Tris(hydroxymethyl) aminomethane in 1000 ml of deionized water and adjusting the pH to 8.0 with 1 mol l 1 hydrochloric acid (J.T. Baker Inc., Phillipsbers, USA). 2.2. Apparatus
2. Experimental 2.1. Reagents All chemicals were of analytical-reagent grade and were used with further purification. Deionized water was used throughout the whole experiment. The standard stock solution of europium (III) chloride hexahydrate (100 mg l 1) was prepared by dissolving europium (III) chloride hexahydrate (0.2417 g obtained from Fluka Switzerland) in 1000 ml water. Further with water were made for appropriate concentrations. A 1.0 / 10 3 M chlortetracycline hydrochloride (Fluka, Switzerland) in 1000 ml water. The more dilute solutions of this solution were made up by
The FI manifold is shown in Fig. 1, an EYELA peristaltic pump MP-3A (Tokyo Rikakikai Co., Ltd.) was used for propelling CTC solution and Tris /buffer solution pH 8. Tygon tubing (1.14 mm i.d.) was used for the flow lines. The sample and/or standard solution containing europium (III) chloride was injected by a 1.0 ml disposable plastic syringe (Nissho Nipre Corporation Ltd., Thailand) into a merged streams of the CTC solution and Tris /buffer solution via a laboratory-made, low-cost injection valve. The merged streams were passed through a single bead (3.0 mm diameter) string reactor (s.b.s.r.) [10]. The coloured complex formed was passed through a flow through cell (Hellma, 1 cm, Suprasil I window, type 178, 711GS) in a spectrophotometer model spectronic 21 (Milton Roy Company, USA) connected to a Servograph REC 51 chart recorder (Radiometer Copenhegen, Denmark). Measurements were made at 400 nm. An UV-Vis spectrophotometer Model UV-265 Shimadzu, Kyoto, Japan was used to scan the spectra of CTC and Cu(II) /CTC complex. A Cole /Parmer (Model 5986-25) pH-meter with a combined glass and calomel electrode was used to adjust the pH values. 2.3. Procedure
Fig. 1. The FI manifold designed for Eu(III) determinaion Injection of the sample and for standard solution into the buffer stream before merging with the CTC system. Injection of the sample and/or standard solution into the merged stream of the CTC and the buffer stream. P, peristaltic pump; I, injection valve; S, sample; MR, mixing reactor; D, detector; Rec., recorder and W, waste.
2.3.1. Sample pretreatments [11] The stream sediment samples were collected from water resources around Doi Inthanon Mountain and let it air dried at room temperature. The dried samples were sieved through a sieve with the particle size of 80 mesh, dried at 100 8C for 1 h and allowed to cool in the desiccator. Approximately 1 g of the dried stream sediment sample was accurately weighed; transferred into a teflon beaker; 10 ml each of HNO3, HClO4 and HF was
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S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
added, respectively, into the beaker containing the sample. The beaker was heated on a hot plate at 100 8C covered with the lid and the temperature was raised to 225 8C and heated until the fume was completely removed. The beaker was removed from the hot plate and stood to cool to room temperature followed by adding HNO3, HClO4 and HF (10 ml each). Then the beaker was heated at 225 8C until no fume evolved. After cooling the beaker to room temperature 10 ml of 6 mol l 1 HCl was added and heated until the black residue was obtained. Then 10 ml 30% H2O2 was added to dissolve the residue, the excess H2O2 was removed by heating and the sample solutions were filtered through No 42 Whatman filtered paper. The incomplete digested residue was transferred into a platinum crucible, 3 g of lithium metaborate was added and digested by heating at 1050 8C for 20 min and the residue obtained was dissolved with 40 ml of 1 mol l 1 HNO3. This solution was combined with the filtrate and evaporated to dryness. The resulting residue was dissolved in 10 ml 1 mol l 1 HNO3, mixed well and the sample solution was for submitted to separation technique. The rare earth elements were separated from other elements by means of an ion exchange chromatographic technique based on stepwise elution of nitric and hydrochloric acids. Eu(III) in the sample solutions was determined by the proposed FI method and conventional spectrometric method based on complexation with bromopyrogallol red [12].
2.3.2. Procedure for Eu(III) determination by FIA A double channel FI manifold was designed and fabricated as shown in Fig. 1 and used for all FI runs. A 100 ml sample loop was used for injecting the sample and/or standard solutions into the merged streams of CTC reagent (4.0 /10 3 mol l 1) and the Tris /buffer (pH 8.0) solution streams which were pumped at the same flow rate of 3.0 ml min 1. Europium (III) formed the Eu(III)/CTC complex with the reagent and the absorbance as peak height was monitored at 400 nm.
Fig. 2. Absorption spectra of CTC and Eu(III) /CTC complex.
3. Results and discussion 3.1. Absorption spectra of CTC and its Eu(II) complex Europium (III) reacts with CTC resulting in a yellow complex in Tris /buffer pH 8.0 medium. The complex presents an absorption maximum at 400 nm whereas the CTC exhibits its absorption maximum at 368 nm (Fig. 2) under the same experimental conditions. It was found that the stiochiometry (Eu(III): CTC) of complex examined by both mole ratio and continuous variation methods was found to be 1:1 which was in good agreement with the previously reported value [13]. The molar absorptivity at 400 nm was 1.00 /104 l mol 1 cm 1 for europium (III) /CTC complex in Tris /buffer pH 8.0 medium. In the present work, a FI spectrophotometric procedure was proposed for determining europium (III) using CTC as chromogenic reagent. The Tris /buffer was used because it is one of the few buffer materials that does not interfere with the europium (III) /CTC complexation. 3.2. Optimization of the flow injection and the chemical conditions Two arrangements of the FI manifold were designed and fabricated (Fig. 1). Optimization of
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
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Table 1 Optimization of the FI and chemical conditions for Eu(III) determination Variable
Studied range
Optimum conditions
Wavelength (nm) pH of buffer solution CTC concentration (mol l 1) Flow rate (ml min 1) Type of mixing reactor Length of mixing reactor (cm) Internal diameter of tubing (mm) Tubing length (cm) Injection volume (ml)
380 /440 7.0 /9.0 1.0 /10 5 /1.0 /10 4 2.0 /4.0 6 types (see Table 3) 3.0 /11.0 0.76 /1.52 25 /100 50 /200
400 8.0 4.0 /10 5 3.0 s.b.s.r. 5.0 1.14 75 100
experimental conditions for determining europium (III) were carried out by means of univariate method [14], in which a variable was modified while maintaining the other variables at their constant values (selected by random). Then by maintaining that variable at its optimum value, another was modified and this procedure was repeated for all variables. The ranges over which the variables involved in the FI system studied and their optimium conditions were listed in Table 1. It was found to be more satisfactory to inject the standard or sample solutions into the merged streams (Fig. 1a) rather than injecting into the buffer stream before merging (Fig. 1b). Since the FIA peaks obtained by the former manifold provides reproducible results with the higher sensitivity than those obtained by the latter manifold. The proposed design of the manifold was shown in Fig. 1a. 3.2.1. Effect of pH on the formation of Eu(III) / CTC complex The complexation between Eu(III) and CTC in Tris /buffer medium was studied over the pH range 7.0 /9.0. The peak heights of the Eu(III)/ CTC complex were measured over the pH range 7.0 /9.0, by using Tris /buffer together with a sufficient volume of 1 mol l 1 HCl to give the required pH. The peak height increased with pH up to 8.0 and decreased when the pH values were
greater than 8.0. Hence, pH 8.0 was chosen as optimum, because at this pH value the peaks were relatively high with a good reproducibility and the linearity of the calibration curve was better (r / 0.9998) than those obtained by other pH values as shown in Table 2A. 3.2.2. Effect of CTC concentration The effect of various concentrations of CTC solutions (1.0 /10 5 /1.0 /10 4 mol l 1) on the absorption of the Eu(III)/CTC complex (as peak height) at 400 nm was examined. In any colorimetric procedure, an amount of reagent grater than required by stoichiometry is needed for colour development (Eu(III): CTC /1:1). The CTC concentrations in the studied range were sufficient for complete colour development. At higher CTC concentrations, the slope of standard calibration curve increases. The CTC concentration which exhibited the largest slope increment with reasonable sample throughput was found to be 4.0 /10 5 mol l 1 (Table 2B) and was, therefore, chosen as optimum CTC concentration. 3.2.3. Effect of flow rate The flow rate of each reagent or carrier solution is very important and should be regulated. Different flow rates ranging from 2.0 to 4.0 ml min 1 were studied. High flow rates shorten the reaction times, increase dispersion and a lower the ratio of
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Table 2 Effect of pH, CTC concentration, flow rate, glass bead column length and injection volume on peak height Parameters studied
(A) Effect of pH
(B) Effect of CTC concentration (mol l 1)
(C) Effect of buffer, CTC carrier flow rate (mol min 1)
(D) Effect of glass bead column length (cm)
(E) Effect of injection volume (ml)
Regression equation
Remarks
7.0 7.5 8.0 8.5 9.0 1.0 /10 5
Y /2.5X/0.3 Y /1.64X/7.24 Y /4.04X/1.96 Y /4.08X/1.24 Y /3.62X/3.22 Y /0.44X/8.44
Optimum pH 8.0 r /0.9995 tbase /43 Sampling rate /84
2.0 /10 5 4.0 /10 5 5.0 /10 5 1.0 /10 4 2.0
Y /0.5X/1.078 Y /4.48X/3.6 Y /4.92X/2.48 Y /4.10X/10.52 Y /2.86X/1.98
2.5 3.0 3.5 4.0 3.0 5.0 7.0 9.0 11.0 50 100 150 200
Y /4.18X/4.74 Y /5X/3.32 Y /3.36X/11.4 Y /3.96X/6.88 Y /3.74X/29.58 Y /4.58X/22.54 Y /3.68X/21.52 Y /4.32X/26.68 Y /3.1X/22.66 Y /2.86X/12.06 Y /5.24X/8.28 Y /4.4X/9.88 Y /2.94X/12.5
Optimum CTC concentration /4.00 /10 5 mol l 1 r /0.9942 tbase /44 s Sampling rate /82 h 1 Optimum flow rate /3.0 ml min 1 r /0.9995 tbase /44 s Sampling rate 82 h 1 Optimum s.b.s.r. length /5.0 cm r /0.9574 tbase /42 s Sampling rate 86 h 1 Optimum injection volume /100 ml r /0.9726 tbase /42.5 s Sampling rate 85 h 1
Table 3 Effect of mixing reactor types on peak height, sensitivity and speed Eu(III) (ppm)
Reactor type Peak height (mV)
1.0 2.0 3.0 4.0 5.0 Slope (mV ppm 1) Correlation coefficient tbase (s) Sample per hour Average of triplicate results.
Serpentine 1 /1
Serpentine 1 /1
Coiled
Knitted
Straight line
S.b.s.r.
6.60 8.00 13.60 14.60 19.40 3.22 0.9786 48 75
14.40 20.80 25.40 29.40 32.00 4.36 0.9630 54 66
17.40 25.00 30.40 33.40 35.00 4.36 0.9630 48 75
24.00 31.00 38.60 36.00 38.00 3.30 0.8549 48 75
17.20 23.60 28.00 35.40 39.60 5.66 0.9968 42 85
17.00 23.20 29.60 35.20 41.60 6.12 0.9997 42 85
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Table 4 Summary of the interference effect of some foreign ions on the peak height obtained from 1.0 mg l 1 Eu(III) Interferet ion Tolerable concentration ratio (mg l 1 X of ion/ Eu(III) when ion/ Eu(III) /100a
Relative error (%)
2 /SO 4 ;/ /NO ; 3
200
/0.62, /4.38, /6.25
100 80 40 25
/9.75 /12.50, /13.75 /14.10 /21.79
Cl /
K Al3 , Na Zn2 Mg2
/ and / Represent enhancement and supression n peak heights, respectively. a The concentration of an ion is considered to be interfering when causing a relative error of more than 9/10% with respect to the peakheight of Eu(III) alone.
sample peak to blank peak. Hence the reaction is not allowed to reach completion as indicated by the decrease in absorbance of the complex formed [15]. On the other hand, low flow rates lead to a decrease in sample throughput. A flow rate of 3.0 ml min 1 for both buffer and CTC streams was
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chosen in order to obtain a reasonable sample throughput (63 h 1), the highest sensitivity (defined as slope of calibration curve) and the better linearity of standard calibration curve (r /0.9997) as shown in Table 2C. 3.2.4. Effect of using glass bead column as a mixing reactor The effect of using glass bead (3.0 mm diameter) column as a mixing reactor was examined by varying the length of the s.b.s.r. reactor from 3.0 to 11.0 cm. The results obtained are summarized in Table 2D, which show that good sensitivity and reasonable sampling rate could be obtained when the glass bead column length was 5.0 cm. Therefore, as a compromise among the sensitivity, sample throughput and a reasonable linearity of calibration curve, the glass bead column length of 5.0 cm was chosen for all experiments. 3.2.5. Effect of injection volume The effect of injection volume was studied by changing the sample loop to give an required injection volumes in the range 50/200 ml in order to find out the optimum injection volume. The results obtained (Table 2E) show that a 100 ml was found to be the optimum injection volume as a compromise between good sensitivity and a sampling frequency of sample per hour.
Table 5 Determination of Eu(III) in spiked water samples Sample code
Concentration of Eu (mg l 1) Spiked
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 Mean (X) S.D. R.S.D. (%)
0.005 0.010 0.050 0.100 0.300 0.500 0.700 1.000
Mean for five determinations.
Recovery (%) Found FIA
Spectro.
0.005 0.010 0.053 0.103 0.302 0.495 0.690 0.980
0.0049 0.010 0.051 0.103 0.301 0.510 0.690 0.975
FIA
Spectro.
100.0 100.0 106.0 103.0 100.7 99.0 98.6 98.0 100.66 2.64 2.62
98.0 100.0 102.0 103.0 100.3 102.0 98.7 97.5 100.19 2.02 2.02
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Table 6 Comparative determination of Eu(III) in stream sediment sample by the proposed FI method and conventional spectrometry Experiment number
Eu(III) found (mg g 1) FI method
1 2 3 4 5 Mean (X) S.D. R.S.D. (%)
Conventional spectrometry
In1 a
In2 b
In3 c
In1 d
In2 e
In4 f
43.75 45.00 45.50 42.50 43.00 43.95 1.28 2.91
44.25 45.50 44.25 43.75 43.75 44.30 0.72 1.63
42.50 41.50 40.50 42.50 43.75 42.15 1.22 2.89
43.50 45.20 45.70 42.45 43.20 44.01 1.38 3.14
44.15 45.45 44.50 43.80 43.90 44.36 0.67 1.51
42.60 41.40 40.40 42.60 43.55 42.11 1.22 2.90
3.2.6. Effect of various mixing reactor types The effect of varying the mixing reactor types (serpentine 1 cm /1 cm, serpentine 1.5 /1.5 cm, coiled, knitted, straight line, s.b.s.r.) was tested under similar conditions as previously described. The results were shown in Table 3. It was seen that the use of s.b.s.r. as a mixing reactor for the FIA system should be satisfactory because it provided a suitably high sensitivity combined with a suitably short tbase and the higher sample throughput. 3.3. Calibration curve The calibration curve for the determination of europium (III) using CTC was linear over the range up to 0.6 ppm europium (III). Over this concentration range, linear regression analysis of peak height (y) versus Eu(III) concentration (x ) (n /7) yielded the equation y /34.93x/0.01.
Linear regression analysis gave correlation coefficient of 0.9994. The calibration curve showed good correlation coefficient values which indicated excellent agreement.
3.4. Detection limit and quantitation limit The lower detection limit of europium (III) was also investigated. The detection limit (defined as three times of the standard deviation, 3s) was determined, calculated and found to be 0.010 mg l 1of europium (III). The quantitation limit is defined as analyte that concentration of the analyte producing the signal which is at least ten times of the standard deviation of the blank signal which was found to be 0.04 mg l 1 Eu(III).
3.5. Reproducibility and accuracy Table 7 Calculation for t -test Comparison
t -Value
A /d B /e C /f
0.07 0.14 0.05
2
95% Confidence t -value /2.306, n /n1/n2 /2.
The relative standard deviation (R.S.D.) for replicate injections was found to be 4.32% of europium (III) (n /12). The accuracy of the proposed method was verified by analysing spiked water samples containing europium (III). The mean percentage recovery of the added europium (III) of 99.7% was obtained.
S. Liawruangrath, S. Sakulkhaemaruethai / Talanta 59 (2003) 9 /18
3.6. Interferences The effects of some possible interfering cations and anions on the determination of Eu(III) were investigated for the maximum w/w ratio of foreign ions to Eu(III) 200: 1. The tolerence is defined as the foreign-ion concentration causing an error smaller than 9/10% for determining the analyte of interest. The tolerable values for the ions studied are summarised in Table 4. It is evident that SO2 4 , NO3 and Cl cause slight effects on the determination of Eu(III). The most serious interference is caused by Mg2 followed by Zn2, Al3 and Na , respectively. The enhancement in the FI signal of the cations probably owing to the formation of complexes with CTC which absorb at the same or very near to the working wavelength whereas the suppression of the FI signals may be due to the formation of complexes which absorb at different wavelengths. 3.7. Applications The proposed method has been applied to the determination of Eu(III) in spiked water samples and stream sediment samples after appropriate sample pretreatments [14]. A 100 ml aliquot of spiked water sample or stream sediment extract was injected into the merged streams of CTC and the Tris /buffer pH 8.0 solutions flowing with the same flow rate of 3.0 ml min 1 and the absorbance (as a peak) was continuously monitored at 400 nm. The peak heights are directly proportional to the concentrations of Eu(III). With respect to analyse Eu(III) in stream sediment samples collected from a selected sites along the water resources from Doi Inthanon Mountain to Mae Klang River at Chiang Doa District, Chiang Mai Province with the sample codes In1 /In9. It was found that Eu(III) contents were found in the stream sediment samples collected from In1,In2 and In4 but Eu(III) content in the stream sediment samples collected from In3 and In5 to In9 were not detected. The results obtained compared favourably with those obtained by spectrophotometric method [16] as shown in 5 for spiked water samples and stream sediment samples, respectively. The accuracies of both methods were found
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to be not different significantly because the calculated Student t-values were less than the theoretical values at a confident level of 95% (Table 7).
4. Conclusions Flow injection spectrophotometric procedure has been developed for determining Eu(III) using CTC as chromogenic reagent. The FI method, a double-chanel manifold equipped with a single bead string reactor was found to be appropriate FI manifold for Eu(III) determination. A calibration curve was linear over the range up to 0.6 ppm Eu(III). A detection limit of 0.01 mg g 1 Eu(III), with a precision of 4.32% could be obtained. The presence of Na, Zn(II), SO2 and NO3 cause 4 slightly effect. Al(III), Cl , Mg(II) and K were not interfere only if the analyte to interfering ion ratios were less than 1:100 but these ratios were as high as 1000 fold interference effects would be observed. The proposed FI-spectrophotometric method proposed for the determination of europium (III) was found to be simple, sensitive and rapid. It offer several advantages such as low cost, good accuracy (recovery /90%) and reproducibility (R.S.D. B/5%), and high sample throughput (85 h 1).
Acknowledgements The authors would like to express their sincere thanks to the National Research Council of Thiland, Institute for Science and Technology Research and Development, Chiang Mai University for financial support and also to the consortium of Postgraduate Education and Research Program in Chemistry (PERCH) for partial support.
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