Journal of Agricultural and Food Chemistry
Article
RESULTS AND DISCUSSION
Table 2. OH Radical Reaction Rate Constants of MITC and
MIC
■
OH Radical Reaction Rate Constants of MITC and MIC.
Samples taken and analyzed using GC-MS every 15 min for 1 h
in the dark indicated that there was no significant loss of either
the test compound or the reference chemicals in the Tedlar bag
during this period.
When the lamps were turned on, about 50% of the initial
concentration of m-xylene was observed to be lost within 10
min, consistent with an observation of Biermann and co-
workers.11 As discussed in this previous study, a series of
complex photoreactions induced by a small amount of impurity
in CH3ONO added could be one possible reason accounting
for this phenomenon.11 Thus, for these experiments, the
samples taken at 10 min after turning on the lamps were used
as the time 0 samples, since samples taken earlier showed a high
level of inconsistency.
test
reference
chemical
k1 (×10−12 cm3
k1 (×10−12 cm3
a
b
compd
k1/k2
molecule−1 s−1
)
molecule−1 s−1
15.4
)
MITC
MIC
toluene
m-xylene
toluene
m-xylene
2.71
14.9 2.66
15.8 2.98
3.52 0.152
0.688
0.641
0.162
3.62
3.73 0.0823
a
OH radical rate constant ratio, mean value from three replications
b
using each reference chemical. Average OH radical rate constants,
mean value from three replications and for each of the two reference
chemicals.
Table 3. Estimated Half-lives of MITC and MIC
a
test compd
half-life (h)
MITC
MIC
15.6
66.5
Concentrations of the test compound and the reference
chemicals obtained from each sampling time point during the 1
h irradiation period were plotted based on eq 5 with standard
deviations from three replications (Figures 1 and 2). Relevant
index of the plots is shown in Table 1.
a
Half-lives calculated by using the average OH radical rate constants in
Table 2 and an average OH radical concentration of 8.0 × 105
molecules cm−3.
as important as direct photolysis for transformation of MITC.
MIC was also observed during the photolysis reactions,
although it presumably went through a photochemically
produced intermediate, methyl isocyanide, which was then
oxidized to MIC.14 The possibility that the OH radical was
created during the direct photolysis studies also cannot be
excluded.
Molar Conversion Ratio of MITC to MIC. At the same
weight per volume concentration, the GC peak area ratio of
MITC to MIC was determined to be 1.98 to 1. The calculated
disappearance rate of MIC in the first 30 min of the irradiation
of the MIC−OH reaction was approximately 14%. Since the
MIC formed in the MITC−OH reaction also reacts with OH
radicals to degrade at the same time, a correction of 14% should
be made to the increased peak area of MIC measured during
the first 30 min irradiation period of MITC−OH reaction.
Thus, molar conversion ratio of MITC to MIC can be
calculated by combining their peak area ratio, molecular
weights (Mr) and the changed peak area (ΔApeak) of MITC
and MIC obtained in the first 30 min of the irradiation,
respectively:
Table 1. Index of MITC and MIC OH Radical Reaction Data
Plots
plots of eq 5
reference chemical
equation
R2
plot 1
plot 2
plot 3
plot 4
toluene
m-xylene
toluene
m-xylene
y = 2.71x
y = 0.688x
y = 0.641x
y = 0.162x
0.998
0.943
0.996
0.996
A consistent, but small curvilinear response was observed for
MITC, but not for MIC, when m-xylene was used as the
reference chemical (see Figure 1), suggesting that some
additional mechanism for loss of MITC exists, in addition to
reaction with OH radicals. Previous research has indicated that
MITC does undergo direct photolysis, although the photo-
chemical reaction is slower than the OH radical reaction.12 The
reason for the observed lack of linearity may be due to a small
contribution from direct photolysis, since while the lamps have
a maximum emission at 351 nm, this is a broad band emission
and emits light of shorter wavelengths. The relatively greater
loss of MITC in those plots is consistent with this suggestion.
OH radical rate constants of 5.48 × 10−12 cm3 molecule−1 s−1
for toluene and 23.0 × 10−12 cm3 molecule−1 s−1 for m-xylene
reported by Atkinson were used in this study.13 Thus, according
to eq 5, multiplying the slope of a linear plot by the rate
constant of its corresponding reference chemical gives the OH
radical rate constant of MITC and/or MIC (Table 2).
For determining the rate of loss of chemicals in the
atmosphere from OH radical reactions, the concentration of
OH radicals is required. Since the OH radical concentration
varies at different times in a day, an average concentration of
8.0 × 105 molecules cm−3 is commonly utilized for estimating
the rate of OH radical reactions.5 The calculated half-lives of
MITC and MIC using this average concentration are shown in
Table 3.
Molar conversion ratio
ΔApeak(MIC) × (1 + 14%) × 1.98 Mr(MITC)
=
·
ΔApeak(MITC)
Mr(MIC)
(6)
Finally, a molar conversion ratio of MITC to MIC during the
first 30 min irradiation period of the MITC−OH reaction is
estimated to be 67% 8% based on eq 6, indicating that MIC
is a primary product of MITC in the OH radical reaction.
OH radical reaction rate constants measured in this work are
15.36 × 10−12 cm3 molecule−1 s−1 for MITC and 3.62 × 10−12
cm3 molecule−1 s−1 for MIC. The result of MITC is comparable
to that of the former work done by Sommerlade, employing a
smog chamber−mass spectrometer system.5 Results obtained in
this work demonstrate that MIC is a primary transformation
product of MITC and it disappears approximately 4 times
slower than MITC in the presence of OH radicals. Due to the
large consumption of metam sodium in U.S. agriculture1 and
the important health concern of MIC,9 this study will provide
useful information for the exposure assessments for use of the
Photolysis is also observed when MITC is exposed to
midsummer sunlight, with a half-life on the order of 30 h of
continuous exposure (including nighttime).12 While direct
comparisons using these results are only approximate, the
results of the previous study on photolysis of MITC and the
present study indicate that OH radical reactions are about twice
1794
dx.doi.org/10.1021/jf404526t | J. Agric. Food Chem. 2014, 62, 1792−1795