AOAC-RI ERP Book - FAOM METHOD.pdf

HANG

ET AL

OURNAL OF

AOAC I

NTERNATIONAL

99, N

4, 2016

1051

Extraction

Weigh 5 g dry tea powder (accurate to 0.01 g) into a 80 mL

centrifuge tube, add 15 mL acetonitrile, and homogenize at

13500 rpm for 1 min. Centrifuge the mixture at 2879 ×

for

5 min, and transfer the supernatant into a pear-shaped Àask. Re-

extract the residue with 15 mL acetonitrile and centrifuge the

mixture. Combine the two extracts, and rotary evaporate in a

water bath at 40°C to about 1 mL for cleanup.

Cleanup

Place a pear-shaped Àask under the ¿ve-port Àask vacuum

manifold, and mount a Cleanert TPT cartridge onto the

manifold. Add about 2 cm anhydrous sodium sulfate onto the

Cleanert TPT cartridge packing material, prewash with 10 mL

acetonitrile–toluene (3 1, v/v) and discard the efÀuents to

activate the cartridge. Stop the Àow through the cartridge when

the liquid level in the cartridge barrel has just reached the top of

the sodium sulfate packing. Discard the waste solution collected

in the pear-shaped Àask and replace with a clean pear-shaped

Àask.

Transfer the concentrated sample extract (

see Extraction

section) into the SPE cartridge, rinse the sample solution bottle

with 2 mL acetonitrile–toluene (3 + 1, v/v), and repeat this step

thrice, transferring the rinsing liquids to the cartridge. Attach

a 50 mL storage device onto the cartridge, and then elute with

25 mL acetonitrile–toluene (3 + 1, v/v), collecting the efÀuent

into the pear-shaped Àask by gravity feed. Rotary evaporate the

efÀuent in a water bath at 40°C to about 0.5 mL. Add 40 ȝL

heptachlor epoxide ISTD to the sample. Evaporate to dryness

under a stream of nitrogen in a 35°C water bath. Dissolve the

dried residue in 1.5 mL hexane, ultrasonicate the sample to mix,

and ¿lter through a 0.2 ȝm membrane ¿lter. The sample is ready

for GC-MS/MS analysis.

Results and Discussion

Degradation of 271 Pesticides in Aged Oolong Tea

The residues of 271 pesticides in aged Oolong tea were

determined 25 times by GC-MS/MS over 120 days (every

5 days) to monitor their degradation behavior. To study the

degradation regularity of the 271 pesticides in aged Oolong tea,

scatter diagrams at

and

spray concentrations (

and

concns)

over 40 and 120 days were prepared, using determination days

as horizontal ordinates and concentrations of pesticide residues

as vertical ordinates. The degradation equations are summarized

in supplemental Table 1 (at

concn) and supplemental Table 2

(at

concn).

By comparing the degradation equations of pesticides at

and

concns over 40 and 120 days, it was found that their degradation

trends were different. The trends included the following

aspects: A—the residues of pesticide dropped exponentially

over both 40 and 120 days; B—the residues of pesticide

dropped exponentially over 40 days and logarithmically over

120 days; C—the residues of pesticide dropped logarithmically

over 40 days and logarithmically/polynomially over 120 days;

D—no trend over 40 days, showing only as scatter points

<0.4), although a dropping trend was seen over 120 days;

E—some dropping trend over 40 days, whereas no trend over

120 days, showing only as scatter points (R

<0.4); and F—no

trend over either 40 or 120 days, showing only as scatter points

<0.4). In addition, ¿ve pesticides were not detected at either

concn.

The degradation regularity of 271 pesticides was studied

according to the A–F trends. The ratios of pesticides associated

with degradation trends A–F in the 271 pesticides are shown in

Figure 1. It was observed that most of the pesticides followed

the A, B, or E degradation trends. At

concn, the ratios for A, B,

and E were 21.8, 35.1, and 24.4 , respectively, and their total

number was 220, accounting for 81.2 of the 271 pesticides. At

concn, the ratios for A, B, and E were 25.5, 33.9, and 14.8 ,

respectively, and their total number was 201, accounting for

74.2 of the 271 pesticides. These results demonstrate that A,

B, and E degradation trends could represent the main aspects

of the 271 pesticides. That is, most of the pesticides dropped

exponentially over 40 days, and they presented dropping trends

exponentially or logarithmically over 120 days. Although the

ratios of pesticides with other degradation trends were small,

they did represent a certain degree of degradation regularity.

Therefore, all trends (A–F) are discussed below.

Degradation trend A.—

Taking propachlor as an example, trend

A degradation rates over 40 and 120 days are shown in Figure 2a

and b, respectively. It is clearly observed that the concentration of

propachlor in aged Oolong tea exponentially decreased with the

increase of intervals. It is indicated that the degradation kinetics of

trend A is a ¿rst-order reaction by Equation 1:

e 1

C C

)

(

where

is the concentration of each pesticide in aged

Oolong tea,

is the initial concentration of each pesticide

in aged Oolong tea,

is degradation rate constant, and

is the

determination time (day).

It can be concluded that without the inÀuence of other factors,

the degradation rate of pesticides in trend A has a direct ratio to

the initial concentration of pesticides in aged Oolong tea. Based

on the ¿rst-order reaction model, the half-life of pesticides in

trend A could be calculated by Equation 2:

ln2 2

1 2

)

(

where

1/2

is the half-life of the determined pesticide.

Half-lives were calculated according to the degradation

equations of pesticides in trend A listed in supplemental Table 1

Figure 1. Percentage of pesticides in accordance with degradation

trends A–F in 271 pesticides at

and

concentrations. Values

represent the number of pesticides in accordance with each trend

multiplied by 100 and divided by the total number (271) of pesticides

(% =

× 100/