| dc.description.abstract | Due to the complexity of grains like soybean, corn and rice, and the very low
pesticide concentration expected for these samples, it is really necessary to develop
efficient and reliable analytical methods to identify and quantify their residues.
The possibility to perform the determination of a large number of pesticides in
just a single chromatographic analysis is the major advantage of the multi-residue
methods. However, there are important compounds that are not possible to be
analyzed together, like the dithiocarbamates (DTC). For that, is so important to
develop and validate single methods.
Due to low acute toxicity, combined with strong action, low-cost production and
low environmental persistence, the dithiocarbamates are still using worldwide. The
first part of this study reports a sensitive analytical single method suitable for the
quantitative analysis of DTCs in soybeans, rice and corn, using GC-PFPD and GCITD-
MS. For the method validation studies, the following parameters were assessed:
detection limit (LOD), quantification limit (LOQ), precision (repeatability and
intermediate precision) and accuracy, via recovery experiments at 0.05, 0.1 and 0.5
mg CS2 kg-1 spiking levels. For soybeans is necessary a clean-up step due the high
fat content in that crop, because of it, same sorbents (Aluminum oxide, Florisil®,
Silica, PSA, C18 and C18OH) were tested and the best results were obtained applying
silica gel. For the extraction procedure, an amount of sample was weighed and water
added. The spike solution (thiram) was added to the sample, 25 mL of isoctane and
150 mL of tin (II) chloride solution in hydrochloric acid were added. The bottles were
closed immediately and heated for 1 h in a water bath at 80 °C, under shaking. After
cooling, 2 mL aliquot of the upper organic layer of each bottle was pipetted into a
tube containing 50 mg of silica gel, for the clean-up step. The analytical curves were
linear from 0.02 to 1.0 mg CS2 L-1 with determination coefficients (r2) higher than 0.99
for both detection systems and to all crops. Method LOD and LOQ values were 20
and 50 μg CS2 kg-1, respectively, for soybeans and corn. For rice LOQ was just 40 μg
CS2 kg-1. Very good precision was obtained with RSD between 1.8 and 7.9% and
recoveries from 67 to 103%, for all matrices. The results obtained in the validation
step allow us to conclude that both chromatographic detection systems are
appropriate to determine residues of DTCs in grains.
The second step was to develop and validate a method for the analysis of 75
pesticides in corn grain and breakfast cereals using LC-MS/MS, ESI positive mode.
The extraction procedure was the modified QuEChERS method, however, excluding
the dispersive SPE clean-up step. To that end, milled and homogenized corn grain
and breakfast cereals with water were spiked with 75 pesticides, at 3 spiking levels
(10, 20 and 50 μg kg-1, n=6), plus the procedure internal standard (quinalfos). A
volume of 10 mL of acetonitrile with 1% acetic acid was added and the tubes were
vigorously shaken for 1 minute. The tubes were uncapped, anhydrous magnesium
sulfate were added, the shaking procedure was repeated and the extract was
centrifuged at 4000 rpm, for 4 min. The upper layer of the extracts was transferred to
chromatographic vial, containing 0.5 mL of the instrumental internal standard
(propoxur) in methanol.
The linear range wasevaluated and validated between 0.1 and 50 μg L-1 of the
analytical curves (7 concentrations, n=6). Also LOD, LOQ, matrix effect, as well as
precision (as RSD%) and accuracy (as recovery), for 75 pesticides, were studied
The LOD were calculated based on practically realized repeatability RSD. To
determine the precision, accuracy and LOQ, blank samples were spiked at 10, 20
and 50 μg kg-1. The results showed recoveries between 70 and 120% and RSDs
<20% for the majority of the pesticides studied, even at the lowest level. As expected,
the matrix effect did not have influence on the results, and the values of LOD and r2
were considered as satisfactory for the purpose of this study. | |
