Elucidating the Reaction Mechanisms between Triazine and Hydrogen Sulfide with pH Variation Using Mass Spectrometry

Article abstract of DOI:10.1021/acs.analchem.8b03107

Triazine is one of the most economical and effective scavengers for hydrogen sulfide (H₂S) removal, but the reaction mechanisms between triazine and H₂S with pH variation in solution are still poorly understood. Herein, we show that the reaction process can be directly probed by means of paper spray mass spectrometry, in which an aprotic solvent (e.g., acetonitrile) is more favorable to the observation of reaction intermediates than a protic solvent (e.g., methanol), because of hydrogen bond interaction. Varying the pH of the reaction leads to completely different reaction pathways. With the pH in the range of 5.58 to 7.73, the major product was thiadiazine. With a pH of 3.02-3.69, thiadiazine is converted to 2-(5-(2-hydroxyethyl)-1,3,5-thiadiazinan-3-yl)acetaldehyde, which differs from the traditional pathway of analogous reactions. However, as ammonia was added into the reaction and the pH was adjusted to the range 8.45-9.43, triazine readily undergoes hydrolysis, and the formed intermediate reacts with ammonia and formaldehyde generated in situ from triazine to produce 1-(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo [3.3.1.1^3,7]decane (HTAD). Further increasing the pH up to 10.27-11.21 leads to the decomposition of HTAD. Based on the experimental observation and evidence from high-resolution and tandem mass spectrometry, we propose the plausible reaction mechanisms between triazine and H₂S, as well as the derived reaction from triazine under different pH conditions.

Full text of DOI:10.1021/acs.analchem.8b03107

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Article
Elucidating the Reaction Mechanisms between Triazine and
Hydrogen Sulfide with pH Variation Using Mass Spectrometry
Xiaoting Wang, Yajun Zheng, Jun Shi, Xiaoyun Gong, Yue Ji, Weiwei
Han, You Jiang, Daniel E Austin, Xiang Fang, and Zhiping Zhang
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03107 • Publication Date (Web): 17 Aug 2018
Downloaded from http://pubs.acs.org on August 20, 2018
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Analytical Chemistry
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Elucidating the Reaction Mechanisms between Triazine and Hydrogen
Sulfide with pH Variation Using Mass Spectrometry
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Xiaoting Wang,a,‡ Yajun Zheng,a,b‡ Jun Shi,a Xiaoyun Gong,c Yue Ji,a Weiwei Han,a You Jiang,c Daniel E.
Austin,b,* Xiang Fang,c,* and Zhiping Zhanga,*
a School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
b Department of Chemistry and Biochemistry, Brigham Young University, Provo 84602, United States
c National Institute of Metrology, Beijing 100013, China
* To whom correspondence should be addressed. E-mail: austin@chem.byu.edu (D.A.), fangxiang@nim.ac.cn (X.F.) or
zhangzp0304@gmail.com (Z.Z.)
‡
These authors contributed equally to this work.
ABSTRACT: Triazine is one of the most economical and effective scavengers for hydrogen sulfide (H2S) removal, but the reaction
mechanisms between triazine and H2S with pH variation in solution are still
poorly understood. Herein we show that the reaction process can be di-
rectly probed by means of paper spray mass spectrometry, in which an
pH = 5.58 - 7.73
pH = 3.02 - 3.69
aprotic solvent (e.g., acetonitrile) is more favorable to the observation of
reaction intermediates than a protic solvent (e.g., methanol) owing to hy-
drogen bond interaction. Varying the pH of the reaction leads to complete-
ly different reaction pathways. With the pH in the range of 5.58 to 7.73,
pH = 8.45 - 9.43
the major product was thiadiazine. With a pH to 3.02 – 3.69, thiadiazine is converted into 2-(5-(2-hydroxyethyl)-1,3,5-
thiadiazinan-3-yl)acetaldehyde, which differs from the traditional pathway of analogous reactions. However, as ammonia was
added into the reaction and the pH was adjusted to the range 8.45 - 9.43, triazine readily undergoes hydrolysis, and the formed
intermediate reacts with ammonia and formaldehyde generated in situ from triazine to produce 1-(2-hydroxyethyl)-3,5,7-triaza-1-
azoniatricyclo [3.3.1.13,7]decane (HTAD). Further increasing the pH up to 10.27 – 11.21 leads to the decomposition of HTAD. Based
on the experimental observation and evidences from high resolution and tandem mass spectrometry, we propose the plausible
reaction mechanisms between triazine and H2S as well as the derived reaction from triazine under different pH conditions.
pathway for the reaction between triazine and H2S is depicted
in Figure 1a.1,3,6,7,10 Triazine first reacts with H2S by the gener-
1. INTRODUCTION
ation of 3,5-di-(2-hydroxyethyl)-hexahydro-1,3,5-thiadiazine
(thiadiazine), where a nitrogen atom is substituted with a
sulfur atom. The resulting thiadiazine then reacts with H2S
In the exploitation and production of oil and gas, hydrogen
sulfide (H2S) is a commonly encountered pollutant, and its
removal is of prime importance due to health, environmental
and corrosion issues. Several techniques, including adsorption
and membrane separation,1-3 have been developed to elimi-
nate H2S during processing. As H2S concentration is typically
below a few hundred parts per million (ppm), the most eco-
nomical and effective solution is the non-regenerative one,
and an economically affordable and fast rate of reaction
makes aqueous soluble 1,3,5-tris(2-hydroxyethyl)hexahydro-
s-triazine (later called triazine) one of the most attractive
option, which could reduce H2S concentrations down to
around 5 ppm in streams.3 In spite of this, the optimal condi-
tions for H2S removal are not always applied because the
reaction details are not well known. To gain insight into the
reaction, various techniques, including X-ray diffraction, ele-
mental analysis and NMR analysis,4-7 field asymmetric ion
mobility spectrometry8 and gas chromatography-mass spec-
trometry,9 have been employed. The currently accepted
(a)
Triazine
(M.W.: 219)
Thiadiazine
(M.W.: 192)
Dithiazine
(M.W.: 165)
Trithiane
(M.W.: 138)
(b)
25 µL
HV
spray plume
MS inlet
Figure 1. (a) Reaction between triazine and H2S forming thiadia-
zine, dithiazine and trithiane; (b) Schematic representation of the
procedure for analyzing the reaction solution using paper spray
mass spectrometry.
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Analytical Chemistry
forming 5-(2-hydroxyethyl)-hexa-hydro-1,3,5-dithiazine (di-
Page 2 of 9
for paper spray.29,30 In this way, a layer of reaction solution
would be floating at the surface of the filter paper. Before the
exhaustion of reaction solution in a spray event less than 1
min, the experiment would be complete. So the species in
solution had less chance to contact the paper substrate and
are readily eluted, similar to a previous report on analysis of
protein complexes.31 During the course of the reaction (see
Experimental Section for details), the solvent was found to
play a crucial role in determining the observed species. As
protic solvents such as methanol were employed (Figure 2a),
the dominant peaks were either the by-products of the scav-
enging reaction (m/z 62 and 86, Figures S1 and S2)10 or the
species derived from acetic acid (m/z 105, 165, 187, 197, 229
and 269). Under these conditions, it was difficult to observe
the reaction products such as thiadiazine (m/z 193) and
trithiane (m/z 139). Although a peak at m/z 166 was present
(Figure 2a), further experiments based on tandem mass anal-
ysis and high resolution mass spectrometry (Figures S3 and
S4) indicated that it was a mixture composed of C4H8O4NS+,
1
2
3
4
5
6
7
8
thiazine), which further interacts with H2S, finally producing s-
trithiane (trithiane). However, the effectiveness of triazine on
H2S removal is strongly dependent on temperature, pH, con-
tact time, and natural gas composition.11,12 Among them, the
pH value of oil and gas fields varies significantly with the
depth of the well.13 This would have a great effect on triazine
reactivity with H2S because the hydrolysis rate of triazine is
strongly dependent on the pH of the reaction solution.7 How-
ever, the reaction mechanisms between triazine and H2S un-
der various pH values are still poorly understood.
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Mass spectrometry has been demonstrated to be a power-
ful technique for chemical analysis based on its sensitivity,
specificity, and precision. The development of ambient ioniza-
tion simplifies this technique by eliminating sample pretreat-
ment prior to the final analysis.14 This greatly facilities appli-
cation of MS analysis to reaction monitoring and mechanism
determination.15-20 In the case of the reaction between tria-
zine and H2S, some attempts have been made to elucidate
the reaction mechanism using mass spectrometry. Madsen et
al.10,21 investigated the reaction products and their fragmen-
tation products from triazine and H2S using electrospray ioni-
zation mass spectrometry (ESI-MS), and possible pathways
and structures were suggested to describe the observed
products. They also studied the precipitate by use of triazines
for H2S scavenging, and found the generated polymer, result-
ing from the decomposition of dithiazine to me-
C5H12ONS2 (dithiazine) and C4H10O3NNa+. When aprotic sol-
+
vents such as acetonitrile were used (Figure 2b), it was inter-
esting to observe abundant reaction products, thiadiazine
([M+H]+, m/z 193 as Figure 2c and [M+Na]+, m/z 215), and
additional species resulting from sodiated triazine adduct
([M+Na]+, m/z 242 as Figure 2d) and side reaction (m/z 62, 74,
86 and 120) (Figures S1 and S2) as well as the species derived
(a)
(b)
thanedithiol followed by reacting between dithiazine
and other methanedithiol molecules, had an empirical
formula of (C7H15S4.5NO)n after identification by ESI-
MS in tandem with infrared spectroscopy and X-ray
62
193
100
100
diffraction.5 Those investigations are of significance to
better understand the reaction pathway and addition-
al products10,21 as well as the fouling formation5 in-
volved in H2S scavenging with triazine.
84
187
269
105
74
50
Herein, we use paper spray mass spectrometry (PS-
MS),22 owing to its unique features (e.g., simplicity,
50
low-cost and without sample pretreatment) and suc-
cess in chemical reactivity assessment,23-28 to charac-
terize the reaction mechanisms between triazine and
H2S as a function of the pH of the reaction solution.
165
229
166
197
62
157
150
m/z
120
S
269
250
Evidences from experimental observations using high
0
0
50
100
200
250
300
50
100
96
150
200
300
resolution and tandem mass spectrometry identify the
ionic species involved in the reaction procedures, and
plausible mechanisms are proposed to elucidate the
complex reaction inside the process.
m/z
(c)
100
(d)
100
m/z 193
m/z 242
OHNa
OH
N
NH
HO
N
OH
OHNa
N
N
2. RESULTS AND DISCUSSION
242
50
0
50
0
S
3.1 Selection of reaction solvent. To detect the ionic
and/or ionizable species in the reaction between tria-
zine and H2S, paper spray mass spectrometry (PS-
MS)22 was employed as shown in Figure 1b, in which
25 µL reaction solution was directly dropped on a
piece of triangle filter paper without any pretreatment
OH
OH
OH
N
NH
193
OHNa
86
N
N
∆m = 73
84
169
74
88
∆m = 32
∆m = 73
∆m = 73
5
50 75 100 125 150 175 200 225
m/z
50 75 100 125 150 175 200 225 250 275
m/z
followed by applying a +3.5 kV DC voltage to generate Figure 2. Mass spectra of the reaction between triazine and Na2S in the pres-
ence of 0.1% acetic acid using (a) methanol and (b) acetonitrile as solvents,
respectively; MS/MS spectra of the species occurred at (c) m/z 242 and (d)
m/z 193 by reaction between triazine and Na2S with a molar ratio of 1:1 in
the presence of 0.1% HOAc in acetonitrile.
the spray and MS measurement. The current method
contrasted with prior procedures that first dropped
several microliters of sample solution on the paper
substrate, dried the solution, and then applied solvent
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Analytical Chemistry
from acetic acid (m/z 105, 187 and 269) and acetonitrile (m/z
7.7E6
100
6.0E7
100
193
242
(a) 0% HOAc
(d) 1% HOAc
1
146). Under those conditions, the product occurring at m/z
166 was absent. The results from different molar ratios be-
tween triazine and Na2S in the presence of 0.1% acetic acid
(HOAc) suggest that this phenomenon could be ascribed to
the lower amount of Na2S involved in the reaction (Figures
S3). However, in both methanol and acetonitrile reaction
systems, the reaction product, trithiane (m/z 139), was barely
detectable, in agreement with the previous reports,6,7,10 pre-
sumably due to the lower reactivity of dithiazine than triazine
and thiadiazine towards the generated H2S.7 More important-
ly, the peak intensity of the reaction product, thiadiazine (m/z
193), in an aprotic solvent (e.g., acetonitrile) is much more
abundant than that in a protic solvent (e.g., methanol), and
similar results were obtained from other solvents (e.g., water,
ethanol, propanol, acetone, ethyl ether and ethyl acetate as
Figure S5). This reveals that an aprotic solvent is more favor-
able to the observation of the reaction between triazine and
H2S than a protic solvent attributable to the strong hydrogen
bond interaction between the solvent used and the reactants
or products. Based on this observation, an aprotic solvent,
acetonitrile, was used to explore the reaction mechanisms
between triazine and H2S at various pH values in reaction
solutions.
3.2 Effect of acidity on the reaction between triazine and H2S.
As reported in the literature,7,11,12 the pH of the reaction solu-
tion has a significant effect on the efficiency of H2S scaveng-
ing using triazine. To explore the effect of solution pH on the
reaction between triazine and Na2S, we adjusted its acidity by
using HOAc. With increase in the acidity of reaction solution,
abundant and stable signals were obtained for all conditions
(Figures 3 and S6), suggesting that the variation of pH had
little effect on the ionization of paper spray. When there was
no HOAc involved in the reaction (pH 9.49, Figure 3a), the
dominant peaks in the mass spectrum were the sodiated tria-
zine ([M+Na]+, m/z 242) and the hydrolyzed product of tria-
2
3
4
5
6
7
8
9
74
62
50
0
50
0
74
218
50
100 150 200 250 300
m/z
50
100 150 200 250 300
m/z
3.2E7
100
6.6E6
(b) 0.1% HOAc193
(e) 5% HOAc
191
100
50
0
10
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62
74
74
50
0
120
242
218
86
62
50
159
274
100 150 200 250 300
m/z
50
100 150 200 250 300
m/z
2.1E7
100
1.1E7
100
191
(c) 0.5% HOAc193
(f) 10% HOAc
74
62
50
0
50
0
242
50
100 150 200 250 300
50
100 150 200 250 300
m/z
Figure 3. Mass smpe/zctra of the solutions from the reaction be-
tween triazine and Na2S in the presence of different percentages
of acetic acid: (a) 0% (v/v, pH 9.49), (b) 0.1% (v/v, pH 7.73), (c)
0.5% (v/v, pH 6.40), (d) 1% (v/v, pH 5.58), (e) 5% (v/v, pH 3.69)
and (f) 10% (v/v, pH 3.02) using acetonitrile as solvent.
was above 5% (pH below than 3.69), the peak at m/z 193 was
not the most dominant, and another peak at m/z 191
emerged and became dominant (Figure 3e). Furthermore,
increasing the acidity in solution led to a gradually increased
intensity of the peak at m/z 191, whereas the peak at m/z 193
zine (m/z 74).7,10 As 0.1%
2x107
145
74
100
m/z 218
100
m/z 191
(a)
(b)
(c)
(v/v) HOAc was added into
the reaction system (Figure
3b), the peak at m/z 193
became dominant, indicat-
ing the transformation of
sodiated triazine (m/z 242)
into thiadiazine (m/z 193).
In addition, several peaks
(m/z 62, 86, 120 and 218)
resulting from the hydro-
m/z 191
1x107
5x106
50
50
145
m/z 218
191
74
86
218
m/z 193
40
56
0
0
0
0
20
60
80 100
60 90 120 150 180 210 240
m/z
50 75 100 125 150 175 200
m/z
Time (h)
120
120
m/z 193
120
m/z 193
m/z 193
100
100
100
(d) 0.1%
(e) 1.0 %
(f) 5.0%
lyzed
triazine
or
thiadiazine10 appeared be-
sides the one at m/z 74.
Further
increasing
the
86
50
50
50
amount of HOAc from 0.1%
to 1% led to the decrease in
the total signal intensity,
and species derived from
HOAc (m/z 105, 187, 229
and 269) occurred besides
those above (Figure 3c and
d). When the amount of
HOAc in the reaction system
86
74
193
86
193
74
193
74
88
88
88
145
56
56
56
145
145
0
0
0
50 75 100 125 150 175 200
50 75 100 125 150 175 200
m/z
50 75 100 125 150 175 200
m/z
m/z
Figure 4. (a) Variation in the peak intensity of the species at m/z 193, m/z 191 and m/z 218 with time
from the reaction between triazine and Na2S in the presence of 5% HOAc; MS/MS spectra of (b) m/z 218
and (c) 191 in 5% HOAc; and MS/MS spectra of m/z 193 in the presence of (d) 0.1%, (e) 1.0% and (f) 5.0%
HOAc using acetonitrile as solvent.
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Analytical Chemistry
became weaker (Figure 3f). These results demonstrate that
Page 4 of 9
1
2
3
4
5
6
7
8
the acidity in solution has a pronounced influence on the re-
action of triazine and Na2S. When the pH was in the range of
5.58 - 7.73 (Figure 3b-d), thiadiazine (m/z 193) was the domi-
nant reaction product. However, as the pH in solution was
below 3.69 (Figure 3e and f), the species at m/z 191 over-
whelmed the others. To determine the evolution of the peak
at m/z 191, we systematically investigated its change in peak
abundance with reaction time for the reaction system with 5%
HOAc. As shown in Figure S7, with extension in the reaction
time, the peak at m/z 193 initially increased, but then de-
creased, whereas the species at m/z 191 emerged after a
reaction time of 10 min. Extending the reaction time led to its
gradual increase in abundance until it became the dominant
peak after 6 h. For reaction periods above 48 h, its intensity
kept almost constant. Interestingly, a peak at m/z 218 be-
haved analogously to that at m/z 191 (Figures S6 and 4a).
Further experiments indicated that the species at m/z 218
pH = 5.58 – 9.49
9
pH = 3.02 – 3.69
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Scheme 1. Proposed Reaction Mechanisms between Triazine and
H2S with Variation in Solution Acidity.
(m/z 242) followed by reaction with H2S to generate proto-
nated thiadiazine (m/z 193). If the pH is decreased to the
range of 3.02 to 3.69, there are two pathways leading to the
conversion of the thiadiazine (II) or sodiated triazine (V, m/z
+
and 191 had compositions of C9H20O3N3 and C7H15O2N2S+
(Figure S8), which were just 2H less than protonated triazine
+
(m/z 220, C9H22O3N3 ) and thiadiazine (m/z 193, C7H17O2N2S+),
3.6E7
100
1.6E7
100
193
193
(a) 0%
(e) 5.0%
74
respectively. Such dehydrogenation is a common phenome-
non in the process of electron ionization, but is not expected
in paper spray due to soft ionization. Based on the above
discussion and the results from MS/MS spectra of m/z 218,
220, 191 and 193 (Figures 4b-d and S9), it could be speculat-
ed that the species at m/z 191 was derived from the reaction
between the one at m/z 218 and H2S, similar to thiadiazine
(m/z 193) from the reaction of triazine (m/z 220) with H2S
(Figure 1a).
74
50
0
50
0
242
215
185
120
215
242
279
159
50
100 150 200 250 300
m/z
50
100 150 200 250 300
m/z
6.3E7
100 (b) 0.1%
1.5E7
100 (f) 10%
185
193
We also observed that the composition of the species at
m/z 193 changed both with increase in the HOAc content in
the reaction solution (Figure 4d-f) and with extension in the
reaction time as the HOAc content was 5% (Figure S10). As
shown in their corresponding MS/MS spectra, there was a
tiny peak at m/z 145 steadily appeared and demonstrated an
increasing trend with variation in the experimental conditions,
and, more importantly, the peak intensity of m/z 120 gradual-
ly became weaker under the same energy during the collision
induced dissociation (CID, Figure S10). This suggested that the
composition of the species at m/z 193 varied with experi-
mental conditions. The results from a high resolution Orbitrap
mass spectrometer (Figures S2d and d’, S8c and c’ and S11)
revealed that the species was a mixture of C7H15O2N234S+ (m/z
193.0805) and C7H17O2N232S+ (193.1004). More importantly,
with changes in the experimental conditions, the level of
74
50
215
242
50
193
215
185
279
0
0
50
7.2E7
100 150 200 250 300
m/z
50
100 150 200 250 300
m/z
1.4E7
185
193
(c) 0.5%
(g) 15%
100
50
0
100
50
0
215
193
74
279
242
215
50
100 150 200 250 300
m/z
50
100 150 200 250 300
m/z
5.1E7
100
1.7E7
100
193
185
(d) 1.0%
(h) 20%
C7H15O2N234S+
progressively
overwhelmed
that
of
C7H17O2N232S+. As discussed above, the latter had a structure
of 3,5-di-(2-hydroxyethyl)-hexahydro-1,3,5-thiadiazine, name-
ly thiadiazine, which was the dominant reaction product be-
tween triazine and H2S as the associated HOAc content was
below 1% (Figure 3b-d). Taking into account the sharp de-
crease in the content of thiadiazine and the progressive in-
50
0
50
0
193
215
74
215
242
279
269
50
100 150 200 250 300
50
100 150 200 250 300
m/z
m/z
+
Figure 5. After the reaction between triazine and Na2S in the
presence of 0.1% HOAc for 5 h, add different percentages of
ammonia solution into the above solution followed by reaction
for 30 min: (a) 0% (v/v, pH 7.73), (b) 0.1% (v/v, pH 8.45), (c) 0.5%
(v/v, pH 8.91), (d) 1.0% (v/v, pH 9.43), (e) 5.0% (v/v, pH 10.27), (f)
10% (v/v, pH 10.60), (g) 15% (v/v, pH 11.01)and (h) 20% (v/v, pH
11.21) using acetonitrile as solvent (note: the concentration
means the final value in the reaction solution).
crease in the levels of C7H15O2N2S+ (m/z 191) and C9H20O3N3
(m/z 218) as Figures 3, 4a and S7, the plausible reaction
mechanisms during H2S scavenging using triazine with varia-
tion in solution acidity are proposed as Scheme 1. When the
pH of the reaction solution is in the range of 5.58 –9.49, tria-
zine (I) will either directly react with H2S by production of
thiadiazine (II) or be first sodiated to form sodiated triazine
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Analytical Chemistry
2-(5-(2-hydroxyethyl)-1,3,5-
242)
into
protonated
185, the peak at m/z 141 showed the same pattern, but no
direct correlation was noticed between their abundances
(Figure 5b-g). In addition, when the peak at m/z 185 over-
whelmed others in the mass spectrum, sodiated triazine at
m/z 242 would disappear (Figure 5b-d). When the intensity of
protonated thiadiazine (m/z 193) overwhelmed others, sodi-
ated triazine could be seen again, and an extra peak at 279
appeared (Figure 5e-h). These results illustrate that the oc-
currence of the species at m/z 185 might be closely related to
the ones at m/z 141, 242 and 269.
1
2
3
4
5
6
7
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thiadiazinan-3-yl)acetaldehyde (IV, m/z 191). One pathway is
that thiadiazine (II) first becomes sodiated thiadiazine (III, m/z
215) followed by loss of NaH with generation of IV. In another
route, one molecule of NaH is first lost from sodiated triazine
(V) by formation of protonated 2-(3,5-bis(2-hydroxyethyl)-
1,3,5-triazinan-1-yl)acetaldehyde (VI, m/z 218). Then VI will
further react with H2S by production of IV. In this process, the
pH in solution plays a central role in determining the reaction
pathway. At a higher pH, triazine is prone to become into
thiadiazine (II, m/z 193). On the contrary, 2-(5-(2-
hydroxyethyl)-1,3,5-thiadiazinan-3-yl)acetaldehyde (IV, m/z
191) is the favorable product (Figure 3).
9
To gain an insight into the correlation between those spe-
cies, it is necessary to validate their origins and corresponding
structures. As the species at m/z 185 was the central one and
originated from the reaction system involved with triazine,
Na2S, HOAc and ammonia in acetonitrile, we studied the pos-
sible systems for its generation (Figure S12), and found that
when triazine was mixed with ammonia, it could produce the
species at m/z 185 (Figure 6a), suggesting that it stemmed
from triazine and ammonia rather than other possibilities.
Further investigation using high resolution mass spectrometry
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3.3 Effect of base on the reaction between triazine and H2S.
These experiments were carried out by adding different
amounts of ammonia into the solution after the reaction be-
tween triazine and Na2S in the presence of 0.1% HOAc for 5 h.
When there was no ammonia in the reaction, protonated
thiadiazine (m/z 193) was the dominant product (Figure 5a)
similar to the above results (Figure 3). However, when 0.1%
(v/v) ammonia was introduced into the reaction system (pH
8.45), a noticeable and intensive peak at m/z 185 emerged
and overwhelmed the others (Figure 5b). Increasing the
amount of ammonia in solution led initially to an increase in
its intensity followed by a decreasing trend (Figure 5b-f).
When the percentage of ammonia was 0.5% in the solution
(pH 8.91), the species at m/z 185 showed the strongest signal
(Figure 5c), while the product was barely observed for the
system with ammonia above 20% (pH > 11.21, Figure 5h).
These results indicated this species was more stable in the
solution with pH ranging from 8.45 to 9.43. Further en-
hancement of the pH (10.27 – 11.21) resulted in its gradual
disappearance, presumably owing to decomposition.32 Along
with the emergence and disappearance of the species at m/z
+
indicated that it had a composition of C8H17ON4 (m/z
185.1398, Figure 6b). According to the documented struc-
tures33 of C8H17ON4+ and possibility in the current system, 1-
(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7
] dec-
ane (abbreviated as HTAD, Figure 6c) was speculated as the
potential candidate. To confirm the chemical structure of the
+
product C8H17ON4 (m/z 185) generated from triazine and
ammonia, MS/MS analysis was carried out and the fragmen-
tation pattern was compared with that from 1-(2-
hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]decane
bromide (Y11, standard sample). As demonstrated in Figure
+
6d and e, both cations C8H17ON4 (m/z 185) have identical
fragmentation patterns, and all generated fragments have
comparable relative intensity. This further validates that the
86
185
185.1398
m/z 185
100
100
50
0
(a)
(b)
(c)
(d)
100
50
0
N
C8H17ON4
OH
50
N
N
N
0
120 150 180 210 240 270 185.04 185.10 185.16 185.22
35
70
105
m/z
140
175
m/z
m/z
188
86
m/z 188
185
m/z 185
86
100 (e)
100 (f)
100 (g)
100 (h)
50
50
50
50
0
0
0
0
35
70
105
m/z
140
175
120 150 180 210 240 270
m/z
35
70
105 140 175
m/z
120 150 180 210 240 270
m/z
Figure 6. (a) Mass spectrum of the solution from the reaction between 46 µmol mL-1 triazine and 1.0% ammonia using acetonitrile as
solvent; (b) Mass spectrum of m/z 185.1398 with a composition of C8H17ON4 using a high resolution Orbitrap mass spectrometer; (c)
Structure of 1-(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]decane; MS/MS spectrum of the peak at m/z 185 (d) from the
reaction between triazine and ammonia in acetonitrile and (e) from the standard sample 1-(2-hydroxyethyl)-3,5,7-triaza-1-
azoniatricyclo[3.3.1.13,7]decane bromide (named as Y11) with a concentration of 1.0 µg mL-1; (f) Mass spectrum of the solution from the
reaction between 46 µmol mL-1 triazine and 0.5% 15N-labelled ammonia (15NH3·H2O) from 15NH4Cl and NaOH using acetonitrile as solvent;
(g) MS/MS spectrum of the peak m/z 188; (h) Mass spectrum of the solution from the reaction between 2.3 µmol mL-1 triazine and 1.0%
ammonia using 13C-labelled acetonitrile (13CH313CN) as solvent.
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Page 6 of 9
product (m/z 185) has a structure of HTAD.
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7
8
In an attempt to identify the sources of nitrogen atoms and
carbon atoms involved in the reaction, isotopically-labelled
standard samples, including 15N-labelled ammonium chloride
(15NH3Cl) and 13C-labelled acetonitrile (13CH313CN), were used.
After a reaction period of 5 h, the peak at m/z 188 became
dominant rather than the peak at m/z 185 when ammonia
was produced by 15N-labelled ammonium chloride and sodi-
um hydroxide (Figure 6f and g). This revealed that three ni-
pH = 8.45 – 9.43
pH = 10.27 – 11.21
+
trogen atoms in the structure of C8H17ON4 (m/z 185) were
9
from ammonia, and one nitrogen atom originated from tria-
zine because no other nitrogen sources were involved in the
reaction system. As 13C-labelled acetonitrile was used as sol-
vent, the primary peak was still m/z 185 (Figure 5h), suggest-
ing that the carbon atoms in produced C8H17ON4+ were all
from triazine and that acetonitrile did not participate in the
reaction.
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Scheme 2. Proposed Reaction Mechanism for the Production of
1-(2-Hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]decane
(XV, m/z 185) and Hexamethylenetetramine (XI, m/z 141).
Based on the above, the formation of C8H17ON4+ appears to
involve the rearrangement or decomposition of triazine fol-
lowed by a reaction with ammonia. Due to the similar struc-
+
duction of HTAD (m/z 185, C8H17ON4 ) and hexamethylenetet-
+
+
ture between HTAD (m/z 185, C8H17ON4 ) and hexamethyl-
ramine (m/z 141, C6H13N4 ) would be feasible.
+
enetetramine (m/z 141, C6H13N4 ) as generated from formal-
According to the above results, possible reaction mecha-
nisms for the generation of 1-(2-hydroxyethyl)-3,5,7-triaza-1-
azoniatricyclo[3.3.1.13,7] decane (HTAD, m/z 185) and hexa-
methylenetetramine (m/z 141) are proposed as Scheme 2.
The generation of formaldehyde plays a vital role in this pro-
cess. Because the abundances of the species at m/z 86 and
m/z 74 are more dominant than those of others (Figure S14a),
herein we only describe the pathways from them to the pro-
duction of formaldehyde followed by reaction with 2-
hydroxy-N-methyleneethanaminium (m/z 74) and/or ammo-
nia. For the pathway from the species at m/z 86 (namely
Pathway A), sodium (I) would first attack triazine (I) with the
generation of V (m/z 242), which turns into VI (m/z 218) by
loss of NaH and isomerization. V would produce VIII (m/z 86)
with a loss of VII through a structural rearrangement. Further
neutral loss leads to the formation of IX (m/z 56) and formal-
dehyde (X). For the Pathway B, triazine (I) or protonated tria-
zine (XII, m/z 220) would first undergo hydrolysis by genera-
tion of XIII (m/z 74). After rearrangement, XIII would lead to
the formation of XIV (m/z 44) and formaldehyde (X). After the
generation of formaldehyde, on the one hand, XIII (m/z)
would react with it and ammonia by production of HTAD (m/z
185) under the pH range of 8.45 – 9.43. Further increasing the
pH value up to 10.27 would result in the decomposition of
HTAD, which is not favorable to its observation (Figure 5). On
the other hand, formaldehyde would react with ammonia by
formation of hexamethylenetetramine (XI, m/z 141) after a
series of addition and dehydration reactions as reported by
Butlerow.35 Despite of the similar structure between HTAD
and hexamethylenetetramine, further investigation through
15N-labled experiments indicated that hexamethylenetetra-
mine was produced by the reaction between ammonia and
generated formaldehyde (Figure S15), and did not originate
from HTAD by loss of hydroxyethyl group.
dehyde and ammonia,34 we speculate that both species might
experience a similar reaction mechanism, undergoing an ad-
dition and dehydration reaction. In the process, formalde-
hyde plays a crucial role in determining the reaction, and it
could be produced in the current reaction system. If this de-
duction is correct, hexamethylenetetramine (m/z 141) should
be produced along with HTAD (m/z 185). After careful exami-
nation on Figure 5b-g, it could be found that with the emer-
gence and disappearance of the species at m/z 185, another
species at m/z 141 behaved the same pattern although no
correlation exists between their abundances. Further evi-
dence from high resolution mass spectrometry, MS/MS spec-
tra, and standard sample comparison (Figure S13) indicates
that the species at m/z 141 has a possible structure of hexa-
methylenetetramine. This suggests that formaldehyde (mo-
lecular weight: 30 Da) was indeed produced in the current
reaction system. To obtain direct evidence on its production,
we compared the mass spectra for the sample from the reac-
tion between 0.23 µmol mL-1 triazine and 0.5% ammonia us-
ing a full scanning mode (Figure S14a) and a neutral loss
scanning mode with a loss of 30 Da (Figure S14b). The species
at m/z 186, 116, 86 and 74, if they undergo a neutral loss of
30 Da, produce peaks at m/z 156, 86, 56 and 44, and all those
species were observed in the mass spectrum with a full scan-
ning mode (Figure S14a). Because of the close proximity be-
tween the species at m/z 186 and the HTAD (m/z 185) as well
as the low intensity of the species at m/z 156 (Figure S14a),
we only studied the compositions of the peaks at m/z 116, 86,
74, 56 and 44. The results from their corresponding high reso-
lution mass spectra (Figures S2b and b’ and S14c-e) and
MS/MS spectra (Figures S14f-g) indicated that the neutral
loss unit (30 Da) from the species at m/z 116 (C5H10NO2, Fig-
ure S14e), 86 (C4H8NO, Figure S14c) and 74 (C3H8NO, Figure
S2b and b’) indeed had a structure of CH2O, matching the
composition of formaldehyde. This further suggests that for-
maldehyde is produced from the present reaction systems,
and, more importantly, there are several routes to generate it.
Because formaldehyde and ammonia were available, the pro-
3. CONCLUSIONS
In summary, we performed a systematic investigation on the
reaction mechanism between triazine and H2S with pH varia-
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Analytical Chemistry
tion in solution using paper spray MS. We show that the reac-
Preparation of Reaction Solution with Variation in Acidity.
Quantitative transfer of 5 µL of 46 µmol mL-1 triazine aqueous
solution into a series of acetonitrile solutions (990, 989, 985,
980, 940 and 890 µL) was followed by adding 5 µL of 46 µmol
mL-1 Na2S aqueous solution. Then different volumes of acetic
acid solution (0, 1, 5, 10, 50 and 100 µL) were added into the
above solution, giving volume percentage of acetic acid of 0%,
0.1%, 0.5%, 1%, 5% and 10% (v/v), respectively. After mixing
for 1 min with a QL-901 vortex mixer (Haimen Kylin-Bell Lab
Instruments Co., Ltd, Haimen, China), the above solution was
maintained at room temperature for 5 h under static condi-
tions, in preparation for later analysis.
1
2
3
4
5
6
7
8
tion solvent plays a crucial role in determining the MS meas-
urement, in which an aprotic solvent (e.g., acetonitrile, ace-
tone, ethyl ether and ethyl acetate) is more advantageous to
the observation of reaction intermediates than a protic sol-
vent (e.g., methanol, water, ethanol and propanol) owing to
hydrogen bond interaction. With variation in the pH value of
reaction solution, completely different pathways were ob-
served. As the pH ranged from 5.58 to 7.73, triazine reacted
with H2S by generation of thiadiazine, where a nitrogen atom
was substituted with a sulfur atom. With a decrease in the pH
to 3.02 – 3.69, the reaction pathways differed from the tradi-
tional ones, and the major product was 2-(5-(2-hydroxyethyl)-
1,3,5-thiadiazinan-3-yl)acetaldehyde converted from thiadia-
zine. When ammonia was added into the reaction system and
the pH was adjusted to 8.45 - 9.43, the formed intermediate
from the hydrolysis of triazine would favorably react with
ammonia and formaldehyde in situ from triazine to generate
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Preparation of Reaction Solution with Variation in Basicity.
Analogous to the above procedure for preparing the reaction
solution with various acidities, we first prepared a reaction
solution with 0.23 µmol mL-1 triazine and 0.23 µmol mL-1 Na2S
followed by adding 1 µL acetic acid into it. After mixing thor-
oughly and then maintaining at room temperature for 5 h, we
quantitatively transferred different volumes of reaction solu-
tions (999, 995, 990, 950, 900, 850 and 800 µL) into 1.5 mL
centrifugal tubes. Subsequently, adding various volumes of 28%
ammonia solution (1, 5, 10, 50, 100, 150 and 200 µL) into the
above solutions, respectively, gave percentages of ammonia
in solutions of 0.1%, 0.5%, 1%, 5%, 10%, 15% and 20% (v/v),
respectively. After mixing for 1 min with a vortex mixer, the
above solutions were maintained at room temperature for 30
min under static conditions followed by MS analysis.
1-(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo
[3.3.1.13,7
]
decane (HTAD) along with the formation of hexamethylene-
tetramine. Further increasing the pH up to 10.27 – 11.21 led
to their disappearances due to decomposition. According to
the experimental observation and evidence from high resolu-
tion and tandem mass spectrometry, isotopically-labelled
experiments and standard sample comparison, plausible
mechanisms were proposed for the first time to elucidate the
reaction pathways between triazine and H2S and the derived
reaction from triazine with pH variation in solution. In con-
trast to the previous reports,1,3,6,7,10 the current study not only
gives a more comprehensive view on understanding the
pathways between triazine and H2S with change in the pH of
the reaction solution, but also provides an experimental basis
on how to control the pH value in order to prevent triazine
from hydrolysis and enhance the reaction efficiency between
it and H2S.
Preparation of the Reaction Solution of Triazine and Ammo-
nia from Ammonium Chloride and Sodium Hydroxide. We
first dissolved 0.25 g ammonium chloride (14NH4Cl or 15NH4Cl)
into 1 mL deionized water followed by adding 0.2 g sodium
hydroxide. After mixing thoroughly, 5 µL of the prepared so-
lution and 5 µL of 0.23 µmol mL-1 triazine were transferred
into 990 µL acetonitrile. Afterwards, the above solution was
mixed and maintained at room temperature for 5 h under
static conditions followed by analysis.
MS Analysis. All experiments on paper spray MS were carried
out either with a TSQ Quantum Access Max mass spectrome-
ter or with an Orbitrap Elite™ Hybrid Ion Trap-Orbitrap Mass
Spectrometer (Thermo Fisher Scientific, San Jose, CA). For
paper spray, filter paper was directly cut into a triangle
(around 13 mm height and 9 mm base width). The distance
between the tip of paper triangle and the MS inlet capillary
was about 15 mm. Mass spectra were recorded in the posi-
tive ion mode with a capillary temperature of 270 °C. The
identification of analyte ions was confirmed by tandem mass
spectrometry (MS/MS) using collision-induced dissociation
(CID). Argon gas (99.995% purity) was used as the collision
gas.
EXPERIMENTAL SECTION
Materials. All the necessary chemicals were from commer-
cially available sources. Triazine, sodium sulfide (Na2S·9H2O),
ammonia solution (28% in water), sodium hydroxide and
ammonium chloride were purchased from Beijing InnoChem
Science & Technology Co. Ltd. (Beijing, China), Tianjin Fuchen
Chemical Reagents Factory (Tianjin, China), Zhengzhou Paini
Chemical Reagent Factory (Zhengzhou, China) and Tianjin
Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), respec-
tively. 15N-labled ammonium chloride (15NH4Cl, ≥98.5%), 1,2-
13C2-acetonitrile (13CH313CN, 99%) and 1-(2-hydroxyethyl)-
3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]decane bromide (Y11,
≥98%) and hexamethylenetetramine were purchased from
Shanghai Research Institute of Chemical Industry (Shanghai,
China), Cambridge Isotope Laboratories, Inc. (Andover, MA,
USA), Sigma-Aldrich Inc. (Milwaukee, WI, USA) and Shaanxi
Pioneer Biotech Co,. Ltd. (Xi’an, China), respectively. The sol-
vents, including methanol, acetonitrile, acetic acid, ethyl
ether, ethyl acetate, ethanol and propanol, were purchased
from Beijing J&K Scientific Ltd. (Beijing, China). Filter paper
was purchased from Hangzhou Special Paper Co. (Fuyang,
China).
ASSOCIATED CONTENT
Supporting Information
Supporting Information Available: mass spectra of the reaction
solutions under various experimental conditions. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
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A. M. Simulation for Estimation of Hydrogen Sulfide
Page 8 of 9
*austin@chem.byu.edu (D.A.)
*fangxiang@nim.ac.cn (X.F.)
*zhangzp0304@gmail.com (Z.Z.)
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7
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Scavenger Injection Dose Rate for Treatment of Crude
Oil. Egyptian J. Petrol. 2015, 24, 469-474.
(13) Garber, J. D.; Farshad, F.; Reinhardt, J. R.; Chang, W.;
Winters, B.; Tadepally, V. Internal Corrosion Rate
Prediction In Pipelines And Flowlines Using A Computer
Model. In CORROSION 2004; NACE International: New
Orleans, Louisiana, 2004, p 04155.
(14) Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M.
Ambient Mass Spectrometry. Science 2006, 311, 1566-
1570.
Author Contributions
‡ These authors contributed equally.
ORCID
Zhiping Zhang: 0000-0002-2733-6976
Notes
9
The authors declare no competing financial interest.
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(15) Wang, Y.; Sun, M.; Qiao, J.; Ouyang, J.; Na, N. FAD Roles
in Glucose Catalytic Oxidation Studied by Multiphase
Flow of Extractive Electrospray Ionization (MF-EESI)
Mass Spectrometry. Chem. Sci. 2018, 9, 594-599.
ACKNOWLEDGMENT
We are grateful for funding from the National Natural Science
Foundation of China (No. 21575112, 21777128 and 21705125).
(16) Banerjee, S.; Liu, F.; Sanchez, D. M.; Martínez, T. J.; Zare,
R. N. Pomeranz-Fritsch Synthesis of Isoquinoline: Gas-
Phase Collisional Activation Opens Additional Reaction
Pathways. J. Am. Chem. Soc. 2017, 139, 14352–14355.
(17) Banerjee, S.; Yang, Y.-F.; Jenkins, I. D.; Liang, Y.; Toutov, A.
A.; Liu, W.-B.; Schuman, D. P.; Grubbs, R. H.; Stoltz, B. M.;
Krenske, E. H.; Houk, K. N.; Zare, R. N. Ionic and Neutral
Mechanisms for C–H Bond Silylation of Aromatic
Heterocycles Catalyzed by Potassium tert-Butoxide. J.
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For TOC Only
pH = 5.58 - 7.73
pH = 3.02 - 3.69
pH = 8.45 - 9.43
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