21 / 302
3
Table 2. HPLC Gradient. Mobile phase A is water with ammonium
formate (5 mM) and formic acid (2 mM), and mobile phase B is
methanol with ammonium formate (5 mM) and formic acid (2 mM).
Mass Spectrometry
MS analysis was carried out on a Thermo Scientific TSQ
Quantum Access MAX triple stage quadrupole mass
spectrometer with an electrospray ionization (ESI) probe.
The MS conditions were as follows:
The divert valve was connected to the front of the TSQ
Quantum Access MAX
™
and was fully controlled from
the data system software.
Results and Discussion
The comparison of peak shapes between the acetonitrile
and methanol / water sample solutions demonstrated that
only early eluting analytes were altered by the mobile
phase composition (Figure 3). Without the divert valve,
the peak shape of omethoate, which elutes earlier than
methomyl, was unacceptable in acetonitrile solution;
whereas the peak shape of methomyl was better but not
optimum (Figure 3a). The peak shape of metamitron,
which elutes later than methomyl, was good in both
acetonitrile and methanol / water sample solutions
(Figures 3a, 3b). With the divert valve switched to the waste
position for 1.30 minutes in the beginning of the run, the
peak shapes of both omethoate and methomyl resembled
those in the methanol / water sample solutions (Figure 3c).
The amount of time the valve was in the waste position
affected the combination of peak shape and S/N ratio. As
shown in Figure 4, the optimum combination of peak
shape and RMS S/N ratio was achieved with a divert
valve time of 1.30 minutes. Longer duration times were
avoided, since the column equilibration was disturbed.
Figure 5 shows the range of injection volumes used. To
assess the dependence between each compound peak area
and the corresponding injection volume, eight injection
volumes (1–8 µL) at a level of 10 µg/L were run three
times each. The linear correlation coefficients (R
2
values)
of the curve plots for all analytes studied were >0.99, and
relative standard deviations were <20% (range 1%–14%).
A S/N ratio greater than 10 for acephate and omethoate
could not be achieved for injection volumes of 1 µL and 2 µL.
Figure 6 shows the curve of each compound’s peak area
versus concentration for a 5 µL injection volume. Seven
different concentration levels (5, 10, 25, 50, 70, 100,
200 µg/L) with 5 µL injection volumes were run three
times. The linear correlation coefficients (R
2
values) of the
curve plots for all analytes studied were >0.99 and relative
standard deviations were <20% (range 2%–16%). Using
5 µL injections of 5 µg/L acetonitrile solutions, RMS S/N
ranged between 75 and 263,000.
No.
Time
A% B% μL/min
0
0.00
90.0
10.0
450.0
1
2.40
90.0
10.0
450.0
2
7.00
40.0
60.0
450.0
3
14.00
10.0
90.0
450.0
4
21.00
10.0
90.0
450.0
5
21.10
90.0
10.0
560.0
6
25.00
90.0
10.0
560.0
7
25.10
90.0
10.0
450.0
8
31.00
90.0
10.0
450.0
Ion polarity
Positive
Q1 Resolution
0.7 Da
Spray Voltage
4000 V
Sheath/Auxiliary Gas
Nitrogen
Sheath Gas Pressure
40 (arbitrary units)
Auxiliary Gas Pressure
25 (arbitrary units)
Ion Transfer Tube Temperature 325 °C
Scan Type
Selected-Reaction Monitoring (SRM)
Collision Gas
Argon
Collision Gas Pressure
1.5 mTorr
Divert Valve
Rheodyne
®
model 7750E-185