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2
Experimental Conditions
Sample Preparation
Individual stock solutions of pesticides were prepared
at concentrations that were sufficient to evaluate the
linearity of peak area versus injection volume at the same
concentration e.g. 10 µg/L, but different injection volumes
(e.g. 1, 2, 3, 4, 5, 6, 7 µL, etc.). Additional solutions with
different concentrations (5, 10, 25, 50, 70, 100, 200 µg/L)
were prepared to study the linearity of peak area versus
compound concentration. Finally, solutions with different
solvents (acetonitrile or methanol / water [1:1 v/v]) were
prepared to study the solvent effect on the methanol /
water gradient mobile phase during the injection.
HPLC
HPLC analysis was performed using a Thermo Scientific
Accela UHPLC system. The chromatographic conditions
were as follows:
The trap column was used to trap the analytes, while the
divert valve was switched to the waste position. A tee
union between the trap column and the analytical column
was connected to the divert valve. The two positions of
the divert valve are shown in Figure 2.
Figure 2. Divert valve positions
The gradient used is detailed in Table 2. The duration of
the gradient was 21 minutes and the column equilibration
time was 10 minutes. The flow rate increased at
21.10 min and decreased at 25.10 min to increase the
speed of column equilibration for the next run (larger
column volumes in less time). The maximum backpressure
was 9,500 psi.
Name
Pesticide Class
Chemical
Formula
Water Solubility [mg/L]
/ pKow
Vapor Pressure
[Pa]
Molecular Weight
[g/mol]
Acephate
Organophosphorous
C
4
H
10
NO
3
PS
790,000 / -0.85
2.26 x 10
-4
(24 °C)
183.165862
Aldicarb sulfone
Oxime carbamate
C
7
H
14
N
2
O
4
S
10,000 (25 °C) / -0.57
(calculated)
0.012 (25 °C)
222.26206
Metamitron
Triazinone
C
10
H
10
N
4
O
1770 (25 °C; pH 5) / 0.85
(21 °C, not pH dependent)
7.44 x 10
-7
(25 °C)
202.2126
Methomyl
Oxime carbamate
C
5
H
10
N
2
O
2
S
55,000 (25 °C, pH 7) /
0.09 (25 °C, pH 4-10)
7.2 x 10
-4
(25 °C)
162.210100
Monocrotophos Organophosphorous
C
7
H
14
NO
5
P
water miscible
2.9 x 10
-4
(20 °C)
223.163522
Omethoate
Organophosphorous
C
5
H
12
NO
4
PS
water-miscible / -0.74
(20 °C)
3.3 x 10
-3
(20 °C)
213.191842
Oxamyl
Oxime carbamate
C
7
H
13
N
3
O
3
S
148,100 (20 °C, pH 5) /
-0.44 (25 °C, pH 5)
5.12 x 10
-5
(25 °C)
219.26142
Table 1. List of studied pesticides and their physicochemical properties
HPLC Column
Thermo Scientific Hypersil GOLD,
100 mm x 2.1 mm, 1.9 µm particle size
Trap Column
Hypersil™ GOLD, 10 mm x 2.1 mm,
5 µm particle size
Column Temperature 40 °C
Mobile Phase A
Water with ammonium formate (5 mM) and
formic acid (2 mM)
Mobile Phase B
Methanol with ammonium formate (5 mM)
and formic acid (2 mM)