Method Development for Quantification of Bromochloramine Using

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Environmental Measurements Methods

Method development for quantification of bromochloramine using membrane introduction mass spectrometry Sebastien Allard, Wei Hu, Jean-Baptiste Le Menn, Keith Cadee, Herve Gallard, and Jean-Philippe Croue Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00889 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Environmental Science & Technology

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Method development for quantification of bromochloramine using

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membrane introduction mass spectrometry

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4

Sébastien Allarda*, Wei Hua, Jean-Baptiste Le Menna, Keith Cadeea, Hervé Gallardb** and Jean-

5

Philippe Crouéa

6

a

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U1987, Perth WA 6845, Australia

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b

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ENSIP, 1 rue Marcel Doré TSA 41105, 86 073 Poitiers Cedex 9, France

Curtin Water Quality Research Centre, Department of Chemistry, Curtin University, GPO Box

Institut de Chimie des Milieux et des Matériaux IC2MP UMR 7285 CNRS Université de Poitiers,

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*Corresponding author phone: +61 8 9266 7949; email: [email protected]

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** Corresponding author phone: +33 5 49 45 44 31; email: [email protected]

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ACS Paragon Plus Environment

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TOC art

NHBrCl

[NHBrCl] M

UV

MIMS

Ion current (A)

HPLC-UV

DPD

15

Time

16


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Page 3 of 19


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Abstract

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During chloramination of bromide-containing waters, the main brominated-amine formed is

19

bromochloramine (NHBrCl). To date, there is no analytical method, free of interference, allowing its

20

accurate quantification. The major reason is that it is not possible to produce a pure NHBrCl solution.

21

In this study, we report a method allowing the accurate quantification of NHBrCl with membrane

22

introduction mass spectrometry (MIMS). Firstly, the molar absorption coefficient for NHBrCl was

23

determined by quantifying NHBrCl as 2,4,6-tribromophenol by HPLC-UV and comparing the results

24

with the direct UV response at 320 nm. A molar absorption coefficient of 304 M-1cm-1 was obtained.

25

The results obtained by direct UV measurements were compared to the MIMS signal recorded at m/z

26

131 corresponding to the mass of the molecular ion, and used to establish a calibration curve. A limit

27

of detection of 2.9 µM (378 µg/L) was determined. MIMS is the only method enabling the

28

unambiguous quantification of NHBrCl, as it is based on m/z 131, while with other analytical

29

techniques, other halamines can interfere, i.e. overlapping peaks with direct UV measurements,

30

reaction of several halamines with colorimetric reagents or phenols. While the detection limit is not

31

quite low enough to measure NHBrCl in actual drinking water, this analytical method will benefit the

32

scientific community by allowing further mechanistic studies on the contribution of NHBrCl to the

33

formation of toxic disinfection by products.

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Introduction

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Monochloramine (NH2Cl) is an alternative to the use of chlorine for the disinfection of drinking

37

water.1 Monochloramine is a more stable compound than chlorine in presence of natural organic

38

matter (NOM) which enables the residual disinfectant to persist over longer distances.

39

Monochloramine is produced by the reaction between ammonia and chlorine.2,

40

bromide, many halamines can form from reactions between chlorine, bromide and ammonia i.e.,

41

chloramines (NHCl2, NH2Cl), bromamines (NH2Br, NHBr2) and bromochloramine (NHBrCl).4-8

42

In the literature, different models have been developed to predict the formation and stability of

43

inorganic chloramines depending on the main water quality parameters (pH, temperature, ionic

44

strength, concentration of inorganic compounds).2,

45

simulate

the

kinetics 9-11

of

formation

and

9

3

In presence of

Kinetic models have also been developed to

decomposition

of

inorganic

bromamines

and

46

bromochloramine.

47

formed (injection of NH2Cl) and in-line (pre-chlorination followed by ammonia) chloramination of

48

iodide/bromide containing waters in absence of organic matter.12 During in–line chloramination,

49

chlorine reacts with bromide to form hypobromous acid (HOBr) which in turn may react with natural

50

organic matter by electrophilic substitution or redox-reactions.13 Once ammonia is added, bromine is

51

either already trapped as total organic bromine (TOBr)14, 15 or reacts with ammonia and NH2Cl to form

52

brominated-amines.4, 6, 8 It was demonstrated that at pH 7-8 commonly found in drinking water, and in

53

the absence of NOM, NHBrCl accumulated during the first day and was the main brominated-amine

54

present.12

55

It has been shown that bromamines are more reactive than chloramines with acetic acid, phenolic

56

compounds and NOM.16-18 As an undesired side effect, the reaction of HOBr and brominated-amines

57

with NOM may lead to the formation of disinfection by–products (DBPs).14, 15, 19, 20 Moreover, the

58

relative abundance of chloramines, bromamines and bromochloramine can strongly affect the stability

59

of the oxidant residual required to deliver safe and healthy drinking water.12 Despite its potential role

60

in DBPs formation, the quantification of NHBrCl is challenging and this impedes mechanistic studies.

61

The presence of bromochloramine was discovered by Trofe et al.21 from the absorbance spectrum of

62

an ether extract of a bromide-containing monochloramine solution. NHBrCl exhibit a strong

63

absorption peak at 220 nm and a weak peak at 320 nm. In this work the molar absorptivity of NHBrCl

64

was estimated to be 2100 M-1cm-1 at 220 nm based on comparison with the aqueous spectrum of

65

NHCl2 and NHBr2, which exhibits a peak at 206 nm with a molar absorptivity of 2100 M-1cm-1 and a

66

peak near 232 nm with a molar absorptivity of approximately 1900 M-1cm-1, respectively. As NH2Cl

67

also absorb at 220 nm, the weak peak (320 nm) of NHBrCl is commonly used for quantification.

68

Based on the molar absorptivity (εNHBrCl = 2100 M-1cm-1 at 220 nm) and an aqueous HPLC/diode array

69

spectrum of NHBrCl, Gazda22 determined εNHBrCl = 170 M-1cm-1 at 300 and 340 nm for the weak peak

A recent study modeled the behavior of the different halamines species for pre-


4

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70

of NHBrCl. A value of 195 M-1cm-1 at 320 nm was derived from this study and used by Luh and

71

Mariñas11 to develop their kinetic model. Valentine23 estimated a different value of εNHBrCl = 300 M-

72

1

73

another weak peak at 294 nm with a molar absorptivity of 276 M-1cm-1 for NHCl2 and a weak peak

74

near 350 nm with a molar absorptivity of approximately 325 M-1cm-1 for NHBr2. This difference may

75

lead to discrepancies in terms of kinetic modelling and the further study of the impact of NHBrCl on

76

DBP formation.

77

The main issue for its quantification is that it is not possible to synthesize a pure NHBrCl solution.

78

NHBrCl is always present in a mixture with some other halamine species and it is not stable.

79

Therefore, most research on NHBrCl has been performed by subtracting the NH2Cl concentration

80

from the total oxidant concentration24 or by solving simultaneous equation of Beer’s law since other

81

halamines are absorbing in the same range of wavelengths.11 Moreover, UV spectrometry is not

82

suitable for low concentration as the molar extinction coefficient is low and can’t be used for real

83

samples due to interferences with the water matrix and other oxidants. Membrane Introduction Mass

84

Spectrometry (MIMS) has previously been used to confirm the presence of NHBrCl.5,

85

composed of a semi-permeable membrane that act as an interface between a liquid or gas sample and a

86

mass spectrometer. This analytical device enables unstable analytes to be identified by their mass-to-

87

charge ratio. In this study, different analytical methods were used and compared to quantify NHBrCl

88

and a novel method allowing the quantification of NHBrCl by MIMS was developed. The three main

89

research objectives were to: 1) find the optimal conditions for preparation of NHBrCl such that

90

interferences from HOBr and other halamines formed are minimized 2) determine the molar

91

absorptivity of NHBrCl by comparing direct UV measurements with an HPLC-UV method based on

92

the derivatization of NHBrCl into 2,4,6-tribromophenol 3) compare the direct UV measurements to

93

the MIMS signal at m/z 131 to establish a calibration curve for MIMS.

cm-1 at 320 nm also based on comparison with the aqueous spectrum of NHCl2 and NHBr2, but using

18

MIMS is

94 95


5


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Material and methods

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Reagents

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All solutions were prepared in ultrapure water (18.2 MΩ.cm) supplied by an Elga water purification

99

system and all chemicals were of the highest purity grade (AR grade ≥ 99%).

100

NH2Cl was prepared by dropwise addition of an equal volume of a sodium hypochlorite solution to an

101

ammonium sulphate solution at pH 8 (Cl2:N weight ratio = 4:1, molar ratio 0.79:1). The solutions were

102

buffered with 10 mM phosphate and chilled in an ice bath under continuous stirring to avoid the

103

formation of dichloramine. NHBrCl solutions were prepared under various experimental conditions as

104

described in the results and discussion section. The variables studied included initial monochloramine

105

concentration (0.2-2 mM), bromide ion concentration (0.5-5 mM), pH (5.0-6.2) and the total

106

concentration of phosphate (10-50 mM).

107

Analytical methods

108

Four analytical methods were evaluated in parallel (direct UV measurements, DPD (N, N-diethyl-p-

109

phenylenediamine), HPLC-UV, MIMS) for the determination of NHBrCl and NH2Cl.

110

Direct UV measurements: NH2Cl and NHBrCl concentrations were measured by UV

111

spectrophotometry using a Shimadzu UV Pharmaspec 1700 spectrophotometer with a 1-cm path

112

length cuvette. The concentrations of monochloramine and bromochloramine were determined by

113

solving simultaneous equations of Beer’s law using four molar extinction coefficients at 243 nm and

114

320 nm (εNH2Cl,243 = 461 M-1cm-1 25, εNHBrCl,243 = 500 M-1cm-1 11, εNH2Cl,320 = 7 M-1cm-1 and εNHBrCl,320 =

115

195 M-1cm-1 11 or 300 M-1cm-1 23).

116

DPD method: The concentrations of NH2Cl and NHBrCl were measured with the DPD colorimetric

117

method with or without addition of KI, respectively.23, 26

118

HPLC-UV: For NHBrCl measurements, solutions were quenched with 2,4-dibromophenol and

119

analyzed as 2,4,6-tribromophenol by HPLC-UV. 2,4-dibromophenol (100 mM) was added in a large

120

excess (at least 100 times the initial concentration of NH2Cl) to the sample to form 2,4,6-

121

tribromophenol by reaction with NHBrCl. The samples were vigorously shaken for 10 seconds before

122

the oxidant residual was quenched by a slight excess of sodium thiosulfate (1.5 [NH2Cl]0). NHBrCl

123

was then analyzed as 2,4,6-tribromophenol by HPLC-UV assuming a 100% conversion yield, as has

124

been demonstrated for the reactions of HOBr and HOI with phenolic compounds.27-29 The HPLC

125

separation was undertaken on an Agilent 1100 series with an AlltimaTM C18 5 µm column and an

126

eluent consisting of 55% acetonitrile, 44.9% water, and 0.1 % acetic acid. The UV detection at 280 nm

127

yielded a detection limit of 0.2 µg Br/L with a precision of 2%.


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128

Membrane introduction mass spectrometry: MIMS measurements were performed with a MIMS 2000

129

(Microlab, Aarhus, Denmark). For NHBrCl and NH2Cl analysis with MIMS, the membrane inlet

130

temperature was set to 65oC and the sample flow rate to 2.8 mL min-1. The cathode voltage was set to -

131

70 V and the emission current to 1.5 mA. The mass to charge ratio m/z of the molecular ion m/z 131

132

and m/z 53 were used to quantify NHBrCl and NH2Cl, respectively. m/z 131 corresponds to the

133

molecular ion of two isotopes (NH79Br37Cl and NH81Br35Cl) which gave the highest MS signal

134

compared to m/z 129 (NH79Br35Cl) and m/z 133 (NH81Br37Cl) (Figure S1). The signals corresponding

135

to the fragmentation of NHBrCl were not used to avoid interferences from other halamines present in

136

solution. The membrane of the MIMS was first conditioned using a NH2Cl solution at the desired

137

concentration for 10 min, thereafter a concentrated bromide solution was spiked to the 250 mL beaker

138

containing the NH2Cl solution.

139

Kinetic Modelling

140

Modelling was performed using the Kintecus software.30 The model developed by Luh and Marinas11

141

and modified by Allard et al.,12 was used in this study. The model was used to simulate different

142

experimental conditions including bromine concentration (50-300 µM), bromide ion concentration

143

(0.5-5 mM), initial monochloramine concentration (0.05-5 mM) and pH (5-8).

144


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145

Results and discussion

146

Optimisation of experimental conditions for minimising/controlling the presence of other

147

halogenated oxidants by kinetic modelling.

148

The kinetic model developed by Luh and Marinas11 was first used to determine the optimal

149

experimental conditions to prepare a solution containing NHBrCl with low concentrations of other

150

oxidants i.e. the different halamines and HOBr. The presence of any halogenated oxidants might

151

interfere with DPD and/or direct UV measurements, while the presence of brominated oxidants might

152

interfere with HPLC-UV measurements. Different parameters were tested i.e., pH, concentration of

153

reactants and buffer, ionic strength and temperature to determine the optimum conditions. A

154

preliminary study was conducted where the formation of NHBrCl from the reaction of equimolar

155

concentration of HOBr and NH2Cl was modelled (Figure S2, S3). This reaction leads to an immediate

156

formation of NHBrCl with 100% conversion. However, the degradation of NHBrCl leads to a

157

recycling of a considerable amount of HOBr back into solution (10% of the initial NHBrCl

158

concentration for pH 7 (Figure S2)). This approach is therefore not suitable for the measurement of

159

NHBrCl by the HPLC-UV method which involved quenching with 2,4-dibromophenol and formation

160

of 2,4,6-tribromophenol since it is not possible to differentiate between 2,4,6-tribromophenol formed

161

from reaction with HOBr or NHBrCl or other brominated-amines. HOBr is very reactive towards

162

phenolic groups compared to HOCl, chloramines, bromamines and bromochloramine.16, 31 Therefore,

163

the synthesis of NHBrCl was carried out with NH2Cl and Br-. Small differences in the oxidant species

164

were observed for different ionic strength and temperature. However, the pH and the initial

165

concentration of NH2Cl ([NH2Cl]0) and bromide ion ([Br]0) have significant effects on the formation

166

and decomposition of bromochloramine (Figure S4). For all the data presented in this paper, i.e.

167

experiments carried out with [NH2Cl]0 0.2 to 5mM, [Br]0 0.5 to 5 mM and pH 5.0-6.2, the kinetic

168

model was used to ensure that the concentrations of HOBr, NH2Br and NHBr2 were negligible and

169

would not lead to interferences during HPLC-UV measurements (an example is given in Figure 1, and

170

Figure S5 shows all the experimental conditions). Since the HOBr, NH2Br and NHBr2 concentrations

171

are several orders of magnitude lower than the NHBrCl concentration (Figure 1 and S5), the

172

contribution of these oxidants to the formation of 2,4,6-tribromophenol during HPLC-UV

173

measurement and to the DPD measurement will be negligible. NH2Cl will not affect HPLC-UV

174

measurements but might affect direct UV measurements if NH2Cl is present at high concentration

175

compared to NHBrCl by overlapping the signal of NHBrCl at 320 nm. Considering that NH2Cl

176

slightly absorbed at 320nm (εNH2Cl,320 = 7 M-1cm-1), a small interference is expected only at the very

177

beginning of the experiment when virtually no NHBrCl is formed and high amount of NH2Cl is

178

present. Actually, the first minutes of reaction could not be used for MIMS calibration of NHBrCl

179

because it corresponds to the time response of our MIMS set-up i.e. the time for the solution to reach


8

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180

the membrane and therefore the mass spectrometer from the beaker where the reaction between NH2Cl

181

and Br- was initiated.

6e-3

Concentration M

5e-3 HOBr NH2Cl NH2Br NHBr2 NHBrCl

4e-3

3e-3

2e-3

1e-3

0 0

10

182

20

30

40

50

Time (min)

183

Figure 1. Variation of NHBrCl, NH2Cl, NH2Br, NHBr2 and HOBr concentrations. Model of Luh and

184

Marinas11 was used with [Br-]0 = [NH2Cl]0 = 5 mM at pH 6 and phosphate buffer 10 mM.

185 186

The model was then used to investigate if it was possible to stabilise NHBrCl by changing the pH once

187

NHBrCl reached its maximum concentration at pH 6. Figure S6 shows the comparison of the NHBrCl

188

remaining at different pHs after 30 min compared to the maximum bromochloramine concentration.

189

The optimal pH to stabilize the solution of NHBrCl is 7-8 (Figure S6). However, only 8% of the initial

190

concentration of NHBrCl remains after 30 minutes at pH 7 (Figure S7). Moreover, the

191

bromochloramine decomposition is significant during the first few minutes even at the optimal pH

192

(Figure S7). After 5 minutes, only 25% of the initial concentration was still present in the solution,

193

which highlights the high unstability of this oxidant. Therefore, we concluded that stabilising the

194

NHBrCl solution through pH of the solution was not effective.

195 Comparison of direct UV measurements, MIMS signal and modelling results for NH2Cl and

196 197

NHBrCl

198

Figure 2 shows a comparison of direct UV measurements and MIMS experimental results with output

199

from the model for monochloramine and bromochloramine kinetics. This experiment was conducted at

200

pH 6 using experimental conditions similar to Luh and Marinas11 to enable comparison with their

201

results.


9


2.5e-4

Page 10 of 19

-

[Br ]0 = 2mM UV 1e-9

-

[Br ]0 = 4mM UV -

[Br ]0 = 2mM Modelling

2.0e-4

-

[Br ]0 = 4mM Modelling

[NH2Cl] M

1.5e-4 6e-10

1.0e-4 4e-10

5.0e-5

2e-10

0.0

0 8e-11

Ion current (A)

8e-10

-

[Br ]0 = 2mM MIMS

8e-5

6e-5 4e-11 4e-5

Ion current (A)

[NHBrCl] M

6e-11

2e-11 2e-5

0

0 0

10

20

30

40

50

60

202 203

Figure 2. Comparison of monochloramine (top) and bromochloramine (bottom) kinetics based on

204

modelling with direct UV measurements and MIMS signal. Two experimental conditions were tested

205

[NH2Cl]0 = 0.2 mM, phosphate buffer 10 mM and T = 23.7°C, pH 6, [Br-]0 = 2 mM and 4 mM.

206

Symbols represent direct UV experimental data; lines represent MIMS data and dotted lines represent

207

the model predicted concentration profiles. εNHBrCl,320 = 195 M-1cm-1 similar to Luh and Marinas11 was

208

used for quantification of NHBrCl. MIMS signal is presented in ampere which represent the intensity

209

of ion current for m/z = 131. For information about NH2Cl and NHBrCl quantification by direct UV

210

please refer to the material and methods section.

Time (min)

211 212 213

As predicted by the model, results from direct UV measurements for 2 and 4 mM bromide showed that

214

NH2Cl concentration rapidly decreased while bromochloramine is formed in the first 10 to 20 min and

215

then decreased. The discrepancies observed for the concentration of bromochloramine between the

216

model and the direct UV measurements are similar to results published by Luh and Marinas.11 The

217

trends of NHBrCl formation and decomposition obtained with the MIMS were comparable to the

218

direct UV measurements even though these results are not quantitative. However, the monochloramine

219

profile obtained with the MIMS was different from the UV signal and the predicted concentration by

220

the model. The “bump” observed for the MIMS response at m/z=53 (Figure 2 (top)) was identified as

221

an NHBrCl interference. We hypothesized that NHBrCl was fragmented to NHCl, protonated in the

222

mass spectrometer and generates a signal at m/z ratio corresponding to NH2Cl. This result highlights


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223

the need to use the mass of the molecular ion and not a fragment for detection of NHBrCl to avoid

224

interferences coming from other halamines with similar molecular structures or surface catalysed

225

reaction in the vacuum system.32

226

Comparison of direct UV measurements, DPD and HPLC-UV method for quantification of

227

NHBrCl

228

According to Valentine23, the reaction of NHBrCl with the DPD is instantaneous. Therefore, it is

229

possible to discriminate NHBrCl from NH2Cl since the later only slowly reacts with DPD in the

230

absence of iodide. Figure 3 shows a good agreement between the direct UV measurements and the

231

DPD measurements for NH2Cl. However, even though a similar pattern is observed, a large difference

232

is observed for NHBrCl with a much higher concentration measured by using direct UV measurements

233

compared to the DPD method.

234 2e-4

[NH2Cl] M

2e-4

1e-4

5e-5

0 1.2e-4

Direct UV DPD

1.0e-4

[NHBrCl] M

8.0e-5

6.0e-5

4.0e-5

2.0e-5

0.0 0

10

20

30

40

50

Time (min)

235 236

Figure 3. Comparison of direct UV measurements and DPD methods for monochloramine and

237

bromochloramine measurements. Crossed hexagons are DPD measurements and black circles are UV

238

measurements. [NH2Cl]0 = 0.2 mM, [Br-]0 = 2 mM, phosphate buffer 10 mM, pH 6. εNHBrCl,320 = 195

239

M-1cm-1 similar to Luh and Marinas11 was used for quantification of NHBrCl.


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Page 12 of 19

240

Therefore, another analytical method was tested to compare the direct UV measurements method to

241

the DPD method. Valentine23 and Gazda et al.5 showed that the bromine atom of NHBrCl is very

242

labile and reactive. Considering this, another method was developed to indirectly quantify

243

bromochloramine by reacting NHBrCl with 2,4-dibromophenol which leads to the formation of 2,4,6-

244

tribromophenol. It was verified that the chlorine atom was not reacting by electrophilic substitution as

245

no peaks corresponding to the 2,4-dibromo-6-chlorophenol was detected by HPLC-UV. The

246

experimental procedure for quantification of NHBrCl by HPLC-UV was used for various experimental

247

conditions (Br- and NH2Cl concentrations).

248

Figure 4 shows the comparison of results obtained with direct UV measurements and HPLC-UV to

249

quantify NHBrCl.

250 251 252 253


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Page 13 of 19


1e-4

1e-4

[NH2Cl]0=0.2mM; [Br ]0=2mM; CT,PO4=10mM; pH6

8e-5

[NHBrCl] M

8e-5

[NHBrCl] M


6e-5

4e-5

2e-5

6e-5

4e-5

2e-5 -1

-1

UV (εε = 195 L mol cm ) UV (εε = 300 L mol-1 cm-1) HPLC-UV as 2,4,6-tribromophenol 0

0 0

10

20

30

40

50

0

10

Time (min)

254

20

30

40

Time (min) 2e-4



1e-4

2e-4

[NHBrCl] M

[NHBrCl] M

8e-5

6e-5

4e-5

1e-4

5e-5 2e-5

0

0 0

255

10

20

30

40

0

10

Time (min)

20

30

40

Time (min)

256

Figure 4. Comparison between 2,4,6-tribromophenol

formation and NHBrCl concentration

257

determined by direct UV measurements (using εNHBrCl,320 = 195 M-1cm-1 and 300 M-1cm-1) for various

258

conditions at pH 6 (CT,PO4 = phosphate buffer concentration).

259

The shape of 2,4,6-tribromophenol formation kinetic is similar to the direct UV response, but a

260

significant difference in concentration is observed for εNHBrCl,320 = 195 M-1cm-1. As previously stated,

261

two different molar absorptivity coefficients at 320 nm are reported for NHBrCl in the literature i.e.,

262

300 and 195 M-1cm-1. If the concentration of NHBrCl is calculated using εNHBrCl,320 = 300 M-1cm-1 both

263

direct UV measurements and HPLC-UV concentrations are almost identical. The concentrations are

264

much higher when calculated with εNHBrCl,320 = 195 M-1cm-1. The HPLC-UV method seems to be

265

accurate for measurements of NHBrCl assuming a 100% conversion of NHBrCl to 2,4,6-

266

tribromophenol and negligible concentrations of bromamines and HOBr as predicted by the kinetic

267

model. For contact times > 20 min the differences observed between the 2,4,6-tribromophenol and the

268

direct UV results are due to the reactivity of the other brominated oxidants (e.g. NH2Br, NHBr2,


13


Page 14 of 19

269

HOBr) formed from degradation of NHBrCl leading to formation of additional 2,4,6-tribromophenol.

270

In order to estimate the molar extinction coefficient based on our data, the direct UV absorbance at

271

320 nm was plotted against the concentration of NHBrCl determined by HPLC-UV for reaction time

Method Development for Quantification of Bromochloramine Using

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