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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
2
membrane introduction mass spectrometry
3
4
Sébastien Allarda*, Wei Hua, Jean-Baptiste Le Menna, Keith Cadeea, Hervé Gallardb** and Jean-
5
Philippe Crouéa
6
a
7
U1987, Perth WA 6845, Australia
8
b
9
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,
10 11
*Corresponding author phone: +61 8 9266 7949; email:
[email protected] 12
** Corresponding author phone: +33 5 49 45 44 31; email:
[email protected] 13
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TOC art
NHBrCl
[NHBrCl] M
UV
MIMS
Ion current (A)
HPLC-UV
DPD
15
Time
16
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Abstract
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During chloramination of bromide-containing waters, the main brominated-amine formed is
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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.
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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
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quite low enough to measure NHBrCl in actual drinking water, this analytical method will benefit the
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scientific community by allowing further mechanistic studies on the contribution of NHBrCl to the
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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
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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,
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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
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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
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iodide/bromide containing waters in absence of organic matter.12 During in–line chloramination,
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chlorine reacts with bromide to form hypobromous acid (HOBr) which in turn may react with natural
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organic matter by electrophilic substitution or redox-reactions.13 Once ammonia is added, bromine is
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either already trapped as total organic bromine (TOBr)14, 15 or reacts with ammonia and NH2Cl to form
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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
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relative abundance of chloramines, bromamines and bromochloramine can strongly affect the stability
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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.
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The presence of bromochloramine was discovered by Trofe et al.21 from the absorbance spectrum of
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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-
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of NHBrCl. A value of 195 M-1cm-1 at 320 nm was derived from this study and used by Luh and
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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
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DBP formation.
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The main issue for its quantification is that it is not possible to synthesize a pure NHBrCl solution.
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NHBrCl is always present in a mixture with some other halamine species and it is not stable.
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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
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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
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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
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absorptivity of NHBrCl by comparing direct UV measurements with an HPLC-UV method based on
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the derivatization of NHBrCl into 2,4,6-tribromophenol 3) compare the direct UV measurements to
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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
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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
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system and all chemicals were of the highest purity grade (AR grade ≥ 99%).
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NH2Cl was prepared by dropwise addition of an equal volume of a sodium hypochlorite solution to an
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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
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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.
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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
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solving simultaneous equations of Beer’s law using four molar extinction coefficients at 243 nm and
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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 =
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195 M-1cm-1 11 or 300 M-1cm-1 23).
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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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Membrane introduction mass spectrometry: MIMS measurements were performed with a MIMS 2000
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(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.
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Kinetic Modelling
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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
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(0.5-5 mM), initial monochloramine concentration (0.05-5 mM) and pH (5-8).
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Results and discussion
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Optimisation of experimental conditions for minimising/controlling the presence of other
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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
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the membrane and therefore the mass spectrometer from the beaker where the reaction between NH2Cl
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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.
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-
[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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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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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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1e-4
1e-4
[NH2Cl]0=0.2mM; [Br ]0=2mM; CT,PO4=10mM; pH6
8e-5
[NHBrCl] M
8e-5
[NHBrCl] M
[NH2Cl]0=0.2mM; [Br ]0=4mM; CT,PO4=10mM; pH6
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
[NH2Cl]0=0.22mM; [Br ]0=5mM; CT,PO4=10mM; pH6
[NH2Cl]0=0.45mM; [Br ]0=5mM; CT,PO4=10mM; pH6
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,
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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
