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1050
C
HANG
ET AL
.:
J
OURNAL OF
AOAC I
NTERNATIONAL
V
OL
.
99, N
O
.
4, 2016
natural degradation dynamics of bifenthrin, fenpropathrin,
cypermethrin, and buprofezin on new shoots of the Oolong
tea plant using GC. Chen et al. (17) developed a GC-MS method
for the analysis of bifenthrin, cyhalothrin, teÀubenzuron,
Àufenoxuron, and chlorÀuazuron in dried Oolong tea leaf
samples, and then studied the natural degradation of these
pesticides in the leaves of Oolong tea trees and the effect of
processing steps on the residue.
Hitherto, less attention has been given to the degradation of
pesticides in aged tea. To research the degradation regularity
of pesticides in aged tea samples, on the basis of our
previous studies (18–20), the developed GC-MS/MS method
was used to determine the multiresidue of 271 pesticides,
including organonitrogen, organophosphorus, organochlorine,
organosulfur, carbamates, and pyrethroids, in aged Oolong tea
over 3 to 4months. Meanwhile, the regularity of 271 pesticides in
aged Oolong tea determined over 40 and 120 days was discussed
in different aspects according to ¿tting curves. Subsequently, 20
representative pesticides from different classes were optimized
for further study. At a higher spray concentration, the residues of
the selected 20 pesticides in aged Oolong tea were studied over
90 days to investigate the degradation regularity at different
concentrations. The degradation values of target pesticides
on a speci¿c day could be predicted by the logarithmic
function obtained from plotting the determination time (day) on
the
x
-axis and the difference between each determined value
and the ¿rst-time-determined value of target pesticides on the
y
-axis, according to the degradation results of the 20 pesticides
at the higher concentration over 90 days.
Lastly, the proposed procedure was validated by predicting
the pesticide residue at one of the Youden pair concentrations
according to the logarithmic function fromanother concentration.
The predicted values were compared to the measured results,
and they were evaluated by their deviation ratios.
Experimental
Reagents
(a)
Solvents
.—Acetonitrile, dichloromethane, isooctane,
and methanol (HPLC grade) were purchased from Dikma Co.
(Beijing, China).
(b)
Anhydrous sodium sulfate
.—Analytically pure. Baked at
650°C for 4 h and stored in a desiccator.
(c)
Pesticide standards and internal standard (ISTD;
heptachlor epoxide)
.—Purity 95 (LGC Promochem, Wesel,
Germany).
(d)
Stock standard solutions
.—Weigh 5–10 mg individual
pesticide and chemical pollutant standards (accurate to 0.1 mg)
into a 10 mL volumetric Àask. Dissolve and dilute to volume
with methanol, toluene, acetone, acetonitrile, isooctane, etc.,
depending on each individual compound’s solubility. Store all
standard stock solutions in the dark at 0–4°C.
(e)
Mixed standard solutions
.—Depending on properties and
retention time of each pesticide, all 271 pesticides for GC-MS/
MS analysis are divided into three groups. The concentration of
each mixed standard solution depends on the sensitivity of each
compound for the instrument used for analysis. Mixed standard
solutions should be stored in the dark below 4°C.
Material
(a)
SPE cartridge
.—Cleanert
®
Triple Phase of Tea SPE
(Cleanert TPT; 10 mL, 2000 mg; Agela, Tianjin, China).
(b)
Homogenizer
.—Rotational speed higher than 13 500 rpm
(report also in
g
-force units; T-25B; IKA-Labortechnik, Staufen,
Germany), or equivalent.
(c)
Rotary evaporator
.—Buchi EL131 (Flawil, Switzerland),
or equivalent.
(d)
Centrifuge
.—Centrifugal force higher than 2879 ×
g
(Z320; B. HermLe AG, Gosheim, Germany), or equivalent.
(e)
Nitrogen evaporator
.—EVAP 112 (Organomation
Associates, Inc., New Berlin, MA), or equivalent.
Apparatus and Conditions
(a)
GC-MS/MS system.—
Model 7890A gas chromatograph
connected to aModel 7000B triple quadrupolemass spectrometer
with electron ionization (EI) source, and equipped with a Model
7693 autosampler with tMass Hunter data processing software
system (Agilent Technologies, Wilmington, DE). GC separation
was achieved on a DB-1701 capillary column (30 m × 0.25 mm
× 0.25 ȝm; Agilent - W Scienti¿c, Folsom, CA).
(b)
Conditions—
The oven temperature was programmed as
follows: 40°C hold for 1 min, increase to 130°C at 30°C/min,
increase to 250°C at 5°C/min, increase to 300°C at 10°C/min,
and hold for 5 min. The carrier gas was helium, purity 99.999 ;
the Àow rate, 1.2 mL/min; the injection port temperature, 290°C;
the injection volume, 1 ȝL; the injection mode, splitless, purge
on after 1.5 min; the ionization voltage, 70 eV; the ion source
temperature, 230°C; the GC/MS interface temperature, 280°C;
and the ion monitoring mode was multireaction monitor mode.
Each compound is monitored by one quantifying precursor/
product ion transition and one qualifying precursor/product ion
transition.
Preparation Procedures for Aged Tea Samples
Pass Oolong tea leaves (free from the target pesticide after
testing) through 10-mesh and then 16-mesh sieves after initial
blending in a blender. Spread 500 g sieved Oolong tea leaves
uniformly over the bottom of a stainless steel vessel 40 cm in
diameter to await spraying. Accurately transfer a certain amount
of pesticide mixed standard solution into the full-glass sprayer
and spray the tea leaves. Spray while stirring the tea leaves with
a glass rod for uniform coverage. After spraying, continue to
stir the tea leaves for 30 min to dissipate the volatile solvents
from the tea leaves. Place the sprayed tea leaves in a 4 L brown
bottle to avoid exposure to light. Store at room temperature and
continue oscillation blending for 12 h.
Spread the aged tea on the bottom of a Àat-bottomed vessel,
draw an ; and weigh a total of ¿ve portions of aged tea samples
collected from the symmetrical four points of the X and from the
central area. Submit the samples for GC-MS/MS determination,
and calculate the average value of the pesticide content of the
aged tea samples and RSD. When the RSD is 4 for GC-MS/
MS, it can be judged that tea samples have been sprayed and
mixed homogeneously.