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Determination of Paraquat Residues in Adzuki Beans
southern Taiwan. These beans are often boiled
with sugar and consumed as a sweet bean paste
or a sweet soup. However, paraquat is toxic to
humans and has the ability to cause irreversible
pulmonary cellular damage
(2)
. Therefore, para-
quat residues in adzuki beans have become an
issue of concern. Paraquat is a highly polar pes-
ticide and, due to its distinctive physico-chem-
ical properties, it is difficult to analyze paraquat
using multi-residue methods. Furthermore, its
permanent ionic character and strong tendency
to interact with glass surfaces hampers the
analysis of paraquat residue in foods.
The QuPPe-method (quick polar pes-
ticides method) was developed to analyze
numerous highly polar pesticides, including
paraquat, in plant-based foods
(1)
. However,
we found that the QuPPe-method provided
poor recovery rates when analyzing paraquat
residues in adzuki beans. In addition, to our
knowledge, no published literature has report-
ed on the determination of paraquat in adzuki
beans. Therefore, in this study, we developed
a novel method for the determination of para-
quat in adzuki beans. Our proposed method,
which involves shaking the sample mixture,
ultrasonication in a hot water bath, centrifu-
gation, and analysis by LC-MS/MS, is simple,
sensitive, and rapid.
Material and Methods
Locally grown adzuki beans were used
in this study. Specifically, two grams of pul-
verized adzuki beans and 2 mL of water were
added into a 15-mL plastic centrifuge tube.
The mixture was then left to stand for 10 min.
Subsequently, 10 mL of 1% formic acid/metha-
nol (1/1, v/v) was added, and the mixture was
shaken at 1,000 strokes/min for 5 min using
a Geno-Grinder 2010 (SPEX SamplePrep,
Metuchen, NJ, USA). Following this, the mix-
ture was ultrasonicated for 30 min in a hot
water bath with a temperature of 80°C before
being shaken again for 1 min. The mixture was
then allowed to cool to room temperature. Af-
ter centrifugation (4,500
×
g
, 15°C) for 30 min,
a 0.9-mL aliquot of supernatant was mixed
with 0.1 mL of 1% formic acid/methanol (1/1,
v/v). Finally, this supernatant solution was
passed through a 0.22 μm PTFE syringe filter
into a plastic storage vial, and one aliquot was
analyzed by LC-MS/MS.
Chromatographic separation was per-
formed at 40°C using an Agilent 1200 series
high-performance liquid chromatography
(HPLC) system (Agilent Technologies, Palo
Alto, CA, USA) equipped with a Shiseido Cap-
cell Pak ST column (150
×
2.0 mm ID, Shisei-
do, Tokyo, Japan). For this, the solvents were 10
mM ammonium acetate in 0.1% (v/v) formic
acid (eluent A) and acetonitrile (eluent B). The
gradient program was: 70% B at 400 μL/min
(0 min), 10% B at 400 μL/min (1 min), 10% B
at 500 μL/min (3 min), 10% B at 500 μL/min
(6 min), 70% B at 400 μL/min (6.1 min), and
70% B at 400 μL/min (10 min). The injection
volume was 10 μL, and all reagents were HPLC
grade.
Data were acquired using a triple quad-
rupole mass spectrometer (Applied Biosystems
4000 QTRAP, Applied Biosystems, Warrington,
UK) with the following parameter settings:
electrospray ion-source in positive mode;
multiple reaction monitoring scan type; an
ion spray voltage of 5,500 V; an ion source
temperature of 500°C; curtain gas of 15 psi;
collision gas set to high; nebulizer gas of 50 psi;
and auxiliary gas of 60 psi. Two transitions of
paraquat, including 186/171 (with a decluster-