Analysis of Diafenthiuron in Tea – Strategies to
Overcome the Degradation Problem
Stefan Kittlaus, Astrid Dylda, Thomas Anspach
Eurofins | Dr. Specht Laboratorien, Großmoorbogen 25, 21079 Hamburg
Introduction Approaches to Prevent Degradation
Diafenthiuron is a selective thiourea insecticide and acaricide,
widely used in cotton, fruits, vegetables and tea. It is acting
selectively on different groups of insects and mites. Currently,
Japan, Australia and other countries set MRLs at the total value of
20 mg/kg for diafenthiuron and its two metabolites in tea. [1]
In the European Union diafenthiuron is not approved. Thus, the
default MRL is 0.01 mg/kg according to Art. 18(1)(b) Reg. 396/2005.
The analytical determination of diafenthiuron often is complicated by
its low stability in different matrices. In the European Union
Proficiency Test for Screening Methods in 2015 (EUPT-SM07) a lack
in the detection of diafenthiuron was observed for broccoli. Only 9%
of the laboratories reported this compound. The authors of the PT
report also supposed an oxidative degradation of diafenthiuron in
this matrix. On their poster at RAFA 2015 they demonstrated that
the presumed degradation is related to the presence of the matrix
broccoli and cannot be observed in pure water. [2]
[1] L. Wang, P. Zhao, F. Zhang, F. Du, C. Pan; Ecotoxicology and Environmental Safety 79 (2012) 75–79
[2] S. Uclés, A. Lonzano, S. Herrera López. A. Uclés, A.R. Fernández-Alba; Poster P13 on RAFA 2015, Prague
Depending on the matrix diafenthiuron tends to rapid degradation. In
this study this was demonstrated for black tea. Different approaches
were tested to overcome this problem. The best results were obsorved
after extraction with pure acetonitrile.
For future studies it would be helpfull to also analyse the metabolites
or to correct the results with the an isotopically labeled internal
standard. But, to our knowlege both are not available until now.
Conclusion
30%
40%
50%
60%
0 5 10 15 20
rec
ove
ry
time in h
0%
20%
40%
60%
80%
100%
0 20 40 60 80 100 120
rec
ove
ry
shaking time in minutes
only acetonitrile (ACN)
H2O / ACN
+ 200 mg Na2SO3
H2O / ACN
+ 200 mg ascorbic acid
H2O / ACN
(classic QuEChERS extraction)
iso-propanol / ACN EDTA / ACN
addition of 10 mL
acetonitrile
shake
spiking with diafenthiuron
(0,5 mg/kg)
addition of 10 mL
…
LC-MS/MS
2 g black tea
centrifuge
H2O (classic QuEChERS)
acetonitrile
iso-propanol
H2O + 200 mg ascorbic acid
H2O + 200 mg Na2SO3
EDTA-buffer
1 min
5 min
10 min
30 min
60 min
120 min
240 min
QuEChERS Degradation Study in Tea diafenthiuron
DP1 C23H32N2OS
385.2308
sulfomonoxide
DP2 C23H32N2O2S
401.2257
carbodiimide
DP3 C23H30N2O
351.2430
urea
DP4 C23H32N2O2
369.2536
Degradation of
diafenthiuron
according to [2]
0%
20%
40%
60%
80%
100%
58%
46%
13%
rec
ove
ry
immediately
after spiking
10 minutes
after spiking
60 minutes
after spiking
sample preparation
addition of 10 mL
acetonitrile
shake 5 min
spiking with diafenthiuron
(0,5 mg/kg)
addition of 10 mL
H2O
LC-MS/MS
2 g black tea
centrifuge
0 min
10 min
60 min
wait
Dependance of the recovery of diafenthiuron from the incubation time after spiking
quantification and recovery calculation was done with solvent calibration
Degradation in the final extract
each point quantified with coresponding
solvent standard to correct for
instrument sensitivity variations
Impact of different approaches on the degradation
quantification and recovery calculation with solvent calibration
The addition of iso-propanol and EDTA-buffer
supported the degradation process. Significant
improvements of compound recovery were
observed after the addition of sodium sulfite. But,
even after short extraction times recovery
decreased dramatically. The addition of ascorbic
acid had no impact.
The best results were obtained with pure aceto-
nitrile. But, also here a significant degradation of
the diafenthiuron during extraction process was
measured.
To overcome the presumed
degradation problem different
approaches were checked. The
impact of water addition and
different solvents was tested by
replacing water with acetonitrile or
iso-propanol. The authors of the
EUPT also proposed the use of
antioxidants. [2] For this reason,
we tested the addition of ascorbic
acid and sodium sulfite. Finally,
we also tried the addition of an
EDTA/Cystein buffer (pH 11).
The results demonstrated the high
degradation rate of diafenthiuron.
Increased waiting times lead to
significant lower recoveries.
Even in final extract (vial was
placed in a cooled autosampler) a
rapid degradation was detected.
As opposed to that, this was not
observed in pure solvent.
In the past we also observed very low recoveries for diafenthiuron in
spiking experiments. Not in all types of matrices but specifically in
tea samples. One possible assumption is that some compounds of
tea promote this degradation pathway.
In the first experiment the correlation between different waiting times
after spiking and the resulting recovery was determined. Moreover,
the degradation in the final extract was observed.
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