11. Procedure
11.1 Sample Preparation
Blank matrix samples (strawberry (SB), wheat flour
(WF) and leek (LK)) used for validation experiments
were purchased in local retail stores and were homog-
enized with an Ultra-Turrax homogenizer, extracted and
cleaned-up prior to sample preparation. Matrix extracts
were used as matrix blank samples and dilution solvents
for matrix-matched calibration. Ready to use Thermo
Scientific QuEChERS extraction kits were used for
sample preparation, and contained 4 g MgSO
4
, 1 g
NaCl, 1 g trisodiumcitrate dehydrate and 0.5 g disodi-
umcitrate sesquihydrate for buffered extraction of target
compounds. Pre-prepared clean-up tubes contained
1200 mg MgSO
4
, 400 mg PSA and 400 mg C18 for
increased clean-up efficiency for more complex matrices
such as leek. The same QuEChERS protocol was applied
for all of the matrices.
11.1.1 Homogenization of Matrices
11.1.1.1 Select larger amount of strawberry (~500 g)
and bunch of leek matrices and put into an
appropriate size beaker and label it.
11.1.1.2 Attach the G25 dispergation tool to the
Ultra-Turrax homogenizer. (For better recovery
for some unstable compounds cryogenic
homogenization is advised).
11.1.1.3 Start homogenization at middle rotation speed
(speed level 2–3) and continue to form a
smooth homogenate.
11.1.2 Sample Extraction and Clean-up
11.1.2.1 Weigh 10 g sample into a 50 mL QuEChERS
extraction tube containing 4 g MgSO
4
, 1 g
NaCl, 1 g trisodiumcitrate dehydrate and 0.5 g
disodiumcitrate sesquihydrate.
11.1.2.2 Add 200 µL 5000 ng/mL internal standard
#141 to the samples.
11.1.2.3 Add 10 mL ACN to SB and LK samples. For
WF, first add 20 mL H
2
O to the samples, let it
completely wet the sample and then add 10 mL
ACN to it.
11.1.2.4 Shake samples for 10 min on a horizontal
shaker and centrifuge with 5000 rpm for 5 min.
Transfer supernatant (~8 mL) into the 15 mL
QuEChERS clean-up tubes containing 1200 mg
MgSO
4
, 400 mg PSA and 400 mg C18.
11.1.2.5 Vortex for 1 min and centrifuge samples with
5000 rpm for 5 min.
11.1.2.6 Collect supernatant and transfer 1 mL into a
GC vial for instrumental analysis.
11.1.2.7 Add 50 µL sorbitol solution (protectant) and
20 µL 5000 ng/mL injection standard (BFB) to
the GC vials prior to injection.
11.2 GC-MS/MS Analysis
Sample measurements were carried out using the
TRACE 1310 gas chromatograph coupled to the TSQ
8000 Triple Stage Quadrupole Mass Spectrometer
(Pesticide Analyzer). For instrument control, analysis,
data review and reporting TraceFinder 3.1 software
was used.
11.2.1 GC method settings
The injector settings were as follows:
Injector:
Thermo Scientific
™
TriPlus RSH Autosampler
with 10 μL injection syringe
Liner:
PTV Baffle Liner (Siltek), Deactivated,
2 mm ID
×
2.75 mm OD
×
120 mm Length
(recommended to be changed after
40 injections of matrix samples)
Injection mode:
splitless PTV, basic mode
Carrier mode:
constant flow
Inlet temp:
75 °C
Split flow:
50 mL/min
Splitless time:
1 min
Injection volume:
1 μL
Plunger strokes:
3
Air filling mode:
auto
Carrier flow:
1.2 mL/min
PTV injection time:
0.1 min
PTV transfer rate:
2.5 °C/s
PTV transfer temp:
300 °C
PTV transfer time:
3 min
PTV cleaning rate:
14.5 °C
PTV cleaning temp:
330 °C
PTV cleaning time:
20 min
PTV cleaning flow:
75 mL/min
PTV cleaning phase:
post cycle temperature cool down
The GC oven settings were as follows:
Carrier gas:
1.2 mL/min Helium (constant flow)
PTV cleaning phase:
post cycle temperature cool down
4
Table 1. GC temperature programming
#
Rate
[°C/min]
Temperature
[°C]
Hold Time
[min]
Initial
40
1.5
1
25
90
1.5
2
25
180
0
3
5
280
0
4
10
300
5
