Chemiluminescent determination of humic substances based on the oxidation by peroxymonosulfate

Analytica Chimica Acta 552 (2005) 141–146

Chemiluminescent determination of humic substances based on the oxidation by peroxymonosulfate Giselle Baratti Magdaleno, Nina Coichev ∗ Instituto de Qu´ımica, Universidade de S˜ao Paulo, CP 26077, CEP 05513-970, S˜ao Paulo, SP, Brazil Received 18 May 2005; received in revised form 24 June 2005; accepted 13 July 2005 Available online 3 October 2005

Abstract The intensity of emission radiation produced by humic (HA) and fulvic acids (FA) in the presence of SO5 2− in basic medium was used to determine HA and FA in the range of 0.5–20.0 mg l−1 . The detection limit was 0.24 mg l−1 . A comparative study was carried out using H2 O2 in the presence of CH2 O as oxidizing agent. Humic substances (HS) from several soil sources, different extraction and purifying procedures led to different calibration sensitivities and selectivity. Cations and anions such as Cu(II), Cr(III), Ca(II), Cl− , EDTA2− , NO3 − , PO4 3− and CO3 2− , did not interfere with the determination of HA. Although it was not possible to confirm the accuracy of the chemiluminescent method, low concentrations of HS in natural waters can be detected. © 2005 Elsevier B.V. All rights reserved. Keywords: Humic acid; Fulvic acid; Peroxymonosulfate; Chemiluminescence

1. Introduction Humic substances (HS) could be defined as a general class of biogenic organic, polyelectrolytes and colloidal compounds that is present in water, soil and sediments, constituting a natural product widely spread on earth [1,2]. HS are generally classified according to its solubility as: humic acids, fulvic acids and humin. Humic acids (HA) are defined as the fraction that are not soluble in water under acid conditions (pH below 2), but become soluble at higher pH values. They constitute the higher molecular weight fraction from 1500 to 5000 and 50,000 to 500,000 Da in streams and soils, respectively. Fulvic acids (FA) are the fraction of HS that are soluble under all pH conditions and are referred to as moderate molecular weight substances (from 600 to 1000 and 1000 to 5000 Da in streams and soils, respectively). Humins are the fraction not soluble in water [3,4].

∗

Corresponding author. Fax: +55 11 38155579. E-mail address: [email protected] (N. Coichev).

0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.07.007

Several studies have shown that different methods of isolating and purifying may lead to HS with different characteristics [5,6]. Due to humic and fulvic acids heterogeneity and low concentrations in water samples, techniques with high sensitivity and refined analytical methods are required to analyze them [7]. Analytical methods for humic acid determination based on absorption at 430 nm [8] and fluorescence (with excitation and emission wavelengths of 270 and 460 nm, respectively) [9] are not accurate by direct analysis or by using “standard” humic acid from different sources (with structural differences) because their molar absorptivities differ up to 270%. Both methods have detection limits of 0.25 mg l−1 with linear range to 50 mg l−1 [10,11]. Other methods used for the determination of humic acids are electrochemical [12] and immunochemical [13]. The determination of HS by chemiluminescence (CL) has been investigated in some extent by using oxidizing agents such as: MnO4 − , Br2 , ClO− , Cr2 O7 2− , H2 O2 , H2 O2 in the presence of formaldehyde, and N-bromosuccinimide (BrO− as oxidizing agent) [10,14,15].

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Earlier investigations suggested that the CL is due to the oxidation of hydroxyl group attached to the aromatic rings of the humic substances [16]. This article describes the CL reaction, with analytical potentialities, of humic and fulvic acids based on their oxidation with SO5 2− , peroxymonosulfate (PMS), in alkaline medium. The triple salt Oxone (2KHSO5 ·KHSO4 ·K2 SO4 , Aldrich) was the compound providing the strong oxidant PMS. A great advantage of using this salt is the good stability of the reagent, compared to other oxidizing agents such as ClO− , BrO− , Br2 , and H2 O2 . Chemiluminescent reaction of oxidation of some fluorescent polycyclic aromatic compounds (such as pyrene, naphthalene, anthracene dyes and dansyllated amino acids) with PMS has been studied with analytical purposes. These methods were based on the Co(II) catalyzed decomposition of peroxymonosulfate, in which the strong oxidants SO5 •− , SO4 •− and HO• are formed [17–19].

2. Experimental 2.1. Reagents All reagents analytical grade (Merck or Aldrich) were used as received. Solutions were daily prepared by using deionized water purified with Milli-Q Plus Water System (Millipore). Stock and diluted solutions of humic substances from Aldrich and extracted from different soils were prepared (Table 1). The solution 0.6 mol l−1 SO5 2− was daily prepared by dissolving 9.2217 g of 2KHSO5 ·KHSO4 ·K2 SO4 , in 50 ml. A solution 0.58 mol l−1 H2 O2 and 0.44 mol l−1 CH2 O was prepared by mixing the fresh stock solutions of 1.16 mol l−1 H2 O2 (Perhidrol 30%, Merck) and 0.88 mol l−1 CH2 O (37%, Merck). Stock solutions of 5000 mg l−1 Mn(II)EDTA, Co(II) EDTA, Fe(III)EDTA, Cu(II)EDTA, Cr(III)EDTA, and

Ca(II)EDTA were prepared by dissolving the nitrate salts and Na2 EDTA (1:1). Stock solutions of 0.16 and 1.04 mol l−1 NaOH and 10,000 mg l−1 Na2 EDTA, NaCl, NaNO3 , Na2 CO3 , and Na3 PO4 were also prepared to investigate the effect of some anions. The water sample from Miranda River, Mato Grosso do Sul State, Brazil, was collected in a clean polyethylene flask and stored at −15 ◦ C. It was filtered using a Millipore HA membrane (pore size of 0.45 ␮m). 2.2. Procedure Routine procedure involved the mixture of the reagents into a 5 ml polystyrene tube, placed into the luminometer (Lumat LB 9507, Berthold), positioned across a photomultiplier tube. 2.2.1. PMS method The sequence of solution addition was: 100 ␮l of water (blank) or HS solution was introduced in the polystyrene tube, followed by automatic jet-injection and immediate mixing of 100 ␮l of 0.6 mol l−1 SO5 2− (PMS) and 100 ␮l of 1.04 mol l−1 NaOH solutions. The CL intensity versus time profile was recorded. The data represented are the average of three series of experiments. The influence of some anions and cations on this CL reaction of HA + PMS was also studied. The sequence of solution addition was: 100 ␮l of water (or HA Aldrich + interfering species) introduced in the polystyrene tube, followed by automatic jet-injection of 100 ␮l of 0.6 mol l−1 PMS and 100 ␮l of 1.04 mol l−1 NaOH solutions. 2.2.2. H2 O2 /CH2 O method The sequence of solution addition was: 200 ␮l of water (blank) (or HS solution) was introduced in the polystyrene tube, followed by automatic jet-injection of 100 ␮l of

Table 1 Characteristics of humic acids samples HA sample

Origin

Extraction

ALD PA PB1 PB2 VA1 VA2 VA3b VA4 VBH VBFc HS1 HS2 HS3

Aldrich Peat A Peat B Peat B Vermicompost A Vermicompost A Vermicompost A Vermicompost A Vermicompost B Vermicompost B Humic substance 1 Humic substance 2 Humic substance 3

– 0.5 mol l−1 0.1 mol l−1 0.1 mol l−1 0.1 mol l−1 0.1 mol l−1 0.1 mol l−1 0.1 mol l−1 0.5 mol l−1 0.5 mol l−1 0.1 mol l−1 0.1 mol l−1 0.1 mol l−1

a b c

NH4 OH NaOH Na4 P2 O7 NaOH NaOH NaOH Na4 P2 O7 NaOH NaOH NaOH NaOH NaOH

Cl content can be used as an indicative of purification degree. Extracted after 6 months. Sample of fulvic acid.

Purifying procedure

C (%)

H (%)

Cl (%)a

– Ion exchange Dialysis Dialysis Dialysis None Dialysis Dialysis Ion exchange Ion exchange Dialysis None None

51.5 52.2 53.7 54.6 50.5 43.9 44.1 47.2 57.7 46.8 36.9 28.5 24.4

4.7 4.0 3.9 4.2 5.5 5.7 5.9 5.1 4.4 4.0 3.4 2.8 2.2

0.6 0.8 1.2 0.8 0.4 0.8 3.2 2.0 0.2 0.2 1.0 12.4 14.9

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(0.58 mol l−1 hydrogen peroxide + 0.44 mol l−1 formaldehyde) and 100 ␮l of 0.16 mol l−1 NaOH solutions. The CL intensity versus time profile was recorded.

3. Results and discussion In the present work the application of CL reaction of PMS with humic substance (HS) was studied. HS from different soils extracted by several procedures were used for determination of HA and FA in water (Table 1). In order to evaluate the best experimental conditions for analytical purposes, the tests were initially carried out using HA Aldrich solutions. The best chemiluminescent signal was observed with the following order of reagents mixture: HA solution followed by addition of PMS and NaOH solutions (see Section 2). Just after the later addition of NaOH solution the relative radiation emission intensity versus time profile was recorded. The total reaction time was about 2 s. The maximum intensity of emission (Imax ) and peak area were related to HA and FA concentrations. With all samples employed in this work, Imax and area values increased with HS concentration. Thus, just Imax versus HA concentration calibration curves were represented (Figs. 2 and 3). The integrated area under the intensity–time curves is an indicative of relative chemiluminescence efficiency, while the peak height (Imax ) generally increases with increasing rate of CL reactions. The signal variation with PMS concentration is represented in Fig. 1. The best Imax and area values (data not shown) were obtained when the solution of PMS was around 0.6 mol l−1 . Concentrations higher than 0.8 mol l−1 were not suitable, since PMS salt was not soluble and a white precipitate was observed. The reaction occurred only in basic medium. When experiments were carried out adding water instead of NaOH solution, the CL signal was not observed. When the added solutions of NaOH in the range of 0.90–1.20 mol l−1 were used, Imax and area values were not significantly affected. It must be pointed out that high concentration of hydroxide must be added to neutralize the H+ (from PMS salt) to keep the medium basic.

Fig. 1. Effect of PMS concentration. Imax obtained by mixing: 100 ␮l of water (or 10 mg l−1 HA Aldrich) + 100 ␮l of 2.5 × 10−2 to 0.8 mol l−1 PMS + 100 ␮l of 1.04 mol l−1 NaOH solutions.

Since PMS is not very stable in basic medium [20], it is recommended to add NaOH solution as the last reagent. For comparative studies, the analytical method for determination of HS based on the radiation emission in the presence of hydrogen peroxide and formaldehyde was used in the present work. H2 O2 /CH2 O system is suitable for the determination of HA because of its high sensitivity and low interference from anions (Cl− , NO3 − , PO4 3− , and CO3 2− ) and cations (Fe(III), Mg (II), and Cu(II)). The analytical procedure was adapted from Kitano et al. [14] as described in Section 2. 3.1. Calibration curves The parameters of the linear regression are represented in Table 2. Imax and area values are linear functions with humic acid concentration (Aldrich), for both methods (PMS and H2 O2 /CH2 O), over the range of 0.5–20 mg l−1 , curving slightly toward the concentration axis over the range of 40–200 mg l−1 . The detection limit (DL) is defined as three times the standard deviation of the linear coefficient divided by angular coefficient value. The calibration sensitivity (M) is the slope of the calibration curve. PMS method showed better DL and higher M values than H2 O2 /CH2 O. For both methods the calibration curves Imax

Table 2 Parameters of the linear regression of the calibration curves (Imax or area = A + M × [HA]) (HA Aldrich) Method

[HA] range (mg l−1 )

A (×103 )

M (×103 )

N

r2

DL (mg l−1 )

R.S.D.

H2 O2 /CH2 O Imax Area

0.5–20 0.5–20

0.10 ± 0.06 0.10 ± 0.03

0.40 ± 0.01 0.21 ± 0.01

9 9

0.999 0.999

0.44 0.43

6.4 8.1

PMS Imax Area

0.5–20 0.5–20

2.4 ± 0.4 1.8 ± 0.3

5.5 ± 0.2 3.7 ± 0.1

9 9

0.993 0.998

0.24 0.24

3.9 1.2

N = point numbers, r2 = correlation coefficient, DL = detection limit, R.S.D. = relative standard deviation for 10 mg l−1 (n = 5).

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Table 3 Effect of some cations (as EDTA complexes) and anions (as sodium salts) added to 1 or 10 mg l−1 HA Aldrich solutions [Cation or anion] (mg l−1 ) HA = 1 mg l−1

Relative signal intensitya HA = 10 mg l−1

HA = 1 mg l−1

HA = 10 mg l−1

Co(II)

0.1 1.0

0.1 1.0

1.0 2.8

1.0 2.2

Mn(II)

0.1 1.0

0.01 1.0

1.0 1.6

1.0 1.3

Fe(III)

10 50

10 50

1.0 1.4

1.0 1.3

Cu(II)

200 500 500 200 1000

200 500 50 100 5000

1.0 0.9 1.0 1.0 1.0

1.0 0.9 1.0 1.0 1.0

50 5000 500 9000

50 5000 5000 9000

1.0 1.2 1.0 1.0

1.0 1.3 1.0 1.0

5000 9000

7500 9000

1.0 0.7

1.0 0.9

Cr(III) Ca(II) EDTA2− Cl− NO3 − PO4 3− CO3 2− a

The signal intensity (Imax ) for 1 or 10 mg l−1 HA in absence of cation and anion is normalized to 1.0.

versus HS concentration had higher sensitivities (M values) than the area curves (Table 2). H2 O2 /CH2 O method had higher relative standard deviation (R.S.D.) for 10 mg l−1 HA solutions than the PMS method (Table 2). 3.2. Influence of anions and cations on PMS method The influence of some anions (Cl− , NO3 − , PO4 3− and CO3 2− ) and cations (Co(II), Mn(II), Fe(III), Cu(II), Cr(III) and Ca(II)) is represented in Table 3. The cations were added as EDTA complexes to avoid hydroxide precipitation. These anions and cations were added to 1 and 10 mg l−1 HA solutions. Co(II) and Mn(II), even at low concentrations (as 1.0 mg l−1 ) enhanced the Imax signal. As previously reported [18,19,21] these cations have a catalytic effect on PMS decomposition, producing strong oxidant species (such as SO4 •− and HO• ). Some experiments (data not shown) were carried out adding Co(II) and Mn(II) as EDTA complexes or chloride salts to the PMS solution in basic medium. In absence of humic substance, the Imax and area were enhanced and the EDTA complexes had higher effect than chloride salts, showing that the catalytic activity of Co(II) and Mn(II) on the PMS decomposition depends on the ligand. On the other hand, in the presence of 2–20 mg l−1 HA Aldrich, the signal of Imax and area is much higher (about 12 times, for a certain HA concentration, when Co(II)EDTA was added). These results showed that in the presence of these metal ions the oxidation of humic substances and the catalytic decomposition of PMS occur simultaneously, and the metal ion ligand (EDTA or HS) will influence on the reaction efficiency.

Other metal ions (Fe(III), Cu(II), Cr(III) and Ca(II)) and anions had no marked effect on the Imax and area values up to concentrations indicated in Table 3 (first lines). None of this species is normally present in sufficient concentration in surface waters to interfere with determination of HA in a significant extent, considering the detection limit and standard deviation. 3.3. Influence of sample soil source, its extraction and purifying procedure Solutions prepared with HS from several soil sources with different extraction and purifying procedures were also used in the present work (Table 1). For comparison, same peat and vermicompost soil samples (named as A and B) with different extraction and purifying procedure were used in order to evaluate the influence on the radiation emission intensity since these treatment may lead to final substances with different characteristics. HS2 and HS3 samples, which were not purified, had high chlorine content (Table 1). Fig. 2 showed that, as will be further discussed, the calibration curves depend on the sample nature and treatment procedure. For example, the analytical curves obtained using the samples PB1 and PB2 (same soil source, but different treatment) extracted with NaOH or Na2 P2 O7 showed different sensitivities (slopes). It seems that the sample treatment with Na2 P2 O7 might change the chemical groups responsible for the CL emission. Samples PA and ALD showed different calibration curves since its sources are distinct. HA and FA separated and extracted from the same vermicompost sample (VBH and VBF) were analyzed by both methods (Fig. 3). It is interesting to note that the detection of

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Table 4 Imax and area values for a 10 mg l−1 HA solutions (see Table 1) by PMS and H2 O2 /CH2 O methods HA

Fig. 2. Imax after mixing 100 ␮l of HA + 100 ␮l of 0.6 mol l−1 PMS + 100 ␮l of 1.04 mol l−1 NaOH solutions. HA were extracted from different peat samples: PA with NH4 OH (*); PB1 with NaOH (䊉); PB2 with Na4 P2 O7 (
); and Aldrich ().

VBF (FA sample) by H2 O2 /CH2 O method was not suitable (Fig. 3B) due to the low sensitivity of this method to fulvic acid. However the PMS method was more sensitive to fulvic acid (VBF) than to humic acid (VBH) (Fig. 3A). The calibration curve of the same sample of humic acid (VBH) showed slightly better sensitivity (higher slope) with H2 O2 /CH2 O. Table 4 represents the Imax and area obtained for the same concentration (10 mg l−1 ) obtained using all samples listed in Table 1. Imax and area values for 10 mg l−1 HA solutions may differ up to 83 and 99% for PMS and H2 O2 /CH2 O methods, respectively. Peat soils samples have high carbon content. Samples of HA with high organic content (Table 1) usually showed

ALD PA PB1 PB2 VA1 VA2 VA3 VA4 VBH VBFa HS1b HS2b HS3b

PMS

H2 O2 /CH2 O

Imax

(×103 )

53 71 26.2 61 14.8 21.3 12.5 25.8 73 118 65 57 45

± ± ± ± ± ± ± ± ± ± ± ± ±

4 3 0.5 3 0.2 0.5 0.6 0.9 3 3 4 5 3

Area

(×103 )

Imax (×103 )

61 60 35.2 62 20.8 26 19.0 33 59.4 101 55 52 37

± ± ± ± ± ± ± ± ± ± ± ± ±

6.0 12.6 5.1 9.4 0.86 1.2 0.6 1.9 92 6.3 5.7 12.6 9.2

2 2 0.9 3 0.4 1 0.8 2 0.7 3 2 3 2

± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.3 0.1 0.1 0.01 0.1 0.1 0.1 2 0.2 0.1 0.2 0.3

Area (×103 ) 3.7 7.5 3.3 6.1 0.55 0.86 0.44 1.4 27.0 2.80 3.4 6.9 5.2

± ± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.3 0.1 0.1 0.03 0.02 0.04 0.1 0.8 0.04 0.1 0.1 0.1

Comparison of HA from different soils and treatment procedures. a Sample of fulvic acid. b Samples of humic substance.

high CL signals (Imax and area), although it was not always true as indicated for vermicompost sample VA1 (Table 4, %C = 50.5). As confirmed by the calibration curve (Fig. 3), the PMS method was more sensitive to FA (sample VBF), while the response of H2 O2 /CH2 O method was very small. For the PMS method the signal obtained for samples of humic substances (HS1, HS2 and HS3) showed high signal, although their carbon content was smaller (Table 1). 3.4. Measurement of dissolved HS in natural water The determination of HA in Miranda River water, Brazil, was carried out by both methods using calibration curves with and without standard addition. Both calibration curves (Imax or area versus HS concentration) with standard addition were obtained adding six increments of a standard solution (HA Aldrich) to sample aliquots of the same size, each solution was then diluted to a fixed volume before measurement. The data in Table 5 represent the average of three measurements. The values found with and without standard addition for the same method (PMS or H2 O2 /CH2 O) were different. The PMS method led to higher value than H2 O2 /CH2 O method since the oxidation of HA and FA gave rise to CL emission Table 5 Determination of humic acid from Miranda River, Brazil Calibration curves

Fig. 3. Imax vs. VBH (䊉) and VBF () concentrations: (A) PMS and (B) H2 O2 /CH2 O methods. HA and FA extracted from the same vermicompost sample.

[HA] found (mg l−1 ) H2 O2 /CH2 O

PMS

With standard addition Imax Area

9.1 ± 0.7 9.0 ± 0.5

12.3 ± 0.6 12.0 ± 0.4

Without standard addition Imax Area

5.6 ± 0.3 6.2 ± 0.2

6.6 ± 0.1 8.05 ± 0.04

Comparison of results for HA determinated by two CL methods.

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(see Fig. 3). The standard addition procedure led to higher values. As the signal depends on the humic substance source, it was expected a different value, once HA Aldrich (used as standard) might not have the same characteristics of the HA present in river water. Although some authors [10,14,15] showed virtually identical values obtained by CL calibration curve and standard addition methods, it might not always occur, since the matrix effect can be observed. The calibration curve must be done with prepared HA standard with very similar characteristics to the sample. Thus, Michalowski et al. [15] proposed a calibration using HA extracted from the river water to be analyzed. Although it was not possible to confirm the accuracy of the CL method, low concentrations of HS in natural waters can be detected. 4. Conclusions The methods based on the production of CL in the oxidation of HA with MnO4 − , Br2 , ClO− , Cr2 O7 2− , H2 O2 , H2 O2 /CH2 O, and SO5 2− (present work) were very sensitive (detection limit in the range of 0.05–10 mg l−1 ) [10,14]. In the present work, both methods (H2 O2 /CH2 O and PMS) showed that calibration curves, obtained by using HS extracted from different soil source, depend on the sample nature and purification. It means that the concentration of HA determined by direct and standard addition measurements may not be compared, unless the standard and the humic acid in the sample have the identical calibration sensitivity. The most important contribution was the comparison of HS samples from different soils in order to show the effect of extraction and purification procedures on CL signals. The device of Lumat LB 9507-2, lumicount, did not allow an FIA system alternative arrangement; on the other hand it has one measuring chamber with optimized light collection and jet-injection of small volumes of the reagents with high accuracy and precision for intensive and immediate mixing with low consumption of reagent.

Acknowledgements This work was supported by FAPESP (Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo) and CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico). To professors Julio C. Rocha, Maria O. de Oliveira Rezende and Jorge C. Masini for providing the samples used in this work.

References [1] E.A. Neves, E. Benedetti, D.C. Souza, R. Javaroni, J. Coord. Chem. 56 (7) (2003) 623. [2] R. Ikan, T. Dorsey, I.R. Kaplan, Anal. Chim. Acta 232 (1990) 11. [3] S. McDonald, A.G. Bishop, P.D. Prenzler, K. Robards, Anal. Chim. Acta 527 (2004) 105. [4] R.L. Malcolm, Anal. Chim. Acta 232 (1990) 19. [5] S. Findlay, R.L. Sinsabaugh, Mar. Freshw. Res. 50 (1999) 781. [6] P. Janos, J. Chromatogr. A 983 (2003) 1. [7] J.C.A. de Wuilloud, R.G. Wuilloud, B.B.M. Sadi, J.A. Caruso, Analyst 128 (2003) 453. [8] E.T. Gjessing, Physical and Chemical Characteristics of Aquatic Humus, Ann Arbor Science, Michigan, 1976, pp. 27, 96. [9] G.L. Brun, D.L.D. Milburn, Anal. Lett. 10 (1977) 1209. [10] D.F. Marino, J.D. Ingle Jr., Anal. Chim. Acta 124 (1981) 23. [11] M. Schnitzer, S.U. Khan, Humic Substances in the Environment, Marcel Dekker, New York, 1972, pp. 55–67. [12] A. Cominoli, J. Buffle, W. Haerdi, J. Electroanal. Chem. 110 (1980) 259. [13] P. Ulrich, M.G. Weller, D. Knopp, R. Niessner, Anal. Sci. 9 (1993) 795. [14] M. Kitano, Y. Ogasawara, H. Xincheng, N. Takenaka, H. Bandow, Y. Maeda, Microchem. J. 49 (1994) 265. [15] J. Michalowski, P. Halaburda, A. Kojlo, Anal. Chim. Acta 438 (2001) 143. [16] D. Slawinska, J. Slawinski, Nature (1967) 902. [17] Y. Makita, H. Umebayashi, T. Suzuki, A. Masuda, M. Yamada, T. Hobo, Chem. Lett. (1993) 1575. [18] S. Tsukada, H. Miki, J.M. Lin, T. Suzuki, M. Yamada, Anal. Chim. Acta 371 (1998) 163. [19] J.M. Lin, M. Yamada, Anal. Chem. 72 (2000) 1148. [20] R.J. Kennedy, A.M. Stock, J. Org. Chem. 25 (1960) 1901. [21] G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 37 (2003) 4790.