Agriculture Reference
In-Depth Information
A set of methods was developed to analyze pesticide residues that could not be
covered in large groups of multiresidue analysis [29]. An example is the analysis of
polar pesticides such as paraquat and mepiquat. In the method, stable isotopically
labeled internal standards were added to samples before extraction. For dry samples,
water was added to the sample
first and then methanol with 1% formic acid was used
to extract the samples. After centrifugation and
filtration, the extracted solutions were
injected into LC
cation. For the analysis of paraquat and diquat,
H
2
O:MeOH (1:1) with. 0.05M HCl was used as the extraction solution.
An ef
-
MS/MS for quanti
cient acetonitrile extraction method followed by using a C-18 SPE cartridge
for cleaning up the extracted solution was developed and fully validated to detect
tetracycline and seven other groups of veterinary drug residues in eggs by LC
-
MS/
MS [30]. The method can detect 1
-
2 ng/g of 40 drugs from eight different classes.
1.2.4 Turbulent Flow Chromatography
TFC was introduced in the late 1990s as a technique for the direct injection of
biological
fluids into a small-diameter column packed with 30
μ
m spherical porous
particles [31]. A high
flow rate mobile phase runs through the column to form a
turbulent
flow. Then, the eluents are directed to an analytical column or waste
controlled by a switch valve. The
flow column) runs SPE, which
can be reversed phase, hydrophilic interaction liquid chromatography (HILIC), size
exclusion, or some other modes. The second column runs regular HPLC separation.
Today, TFC has been developed as an automated online high-throughput sample
preparation technique that makes use of high
first column (turbo
flow rates in 0.5 or 1.0 mm internal
diameter columns packed with particles of size 30
-
60
m. These large particle
μ
columns allow much higher
flow rate with lower backpressure. The smaller analytes
diffuse more extensively than larger molecules (e.g., proteins, lipids, and sugars from
the matrix) into the pores of the sorbent. The larger molecules do not diffuse into the
particle pores because of high
flow rate and are washed to waste. The trapped analytes
are desorbed from the TFC column by back-
ushing it with an organic solvent and the
eluate can be transferred with a switching valve onto the analytical LC
-
MS/MS
system for further separation and detection.
Compared with traditional SPE, TFC reduces the time required for off-line sample
preparation from hours to minutes because it uses reusable extraction columns in a
closed system. It also allows automatic removal of proteins and larger molecules in
complex mixtures by combining turbulence, diffusion, and chemistry. TFC technol-
ogy also allows a broad selection of stationary phases for different matrices. For
example, melamine and eight veterinary drugs, belonging to seven different classes,
were detected by TFC
-
LC
-
MS/MS in milk [31,32].
1.2.5 Pressurized Liquid Extraction
PLE is a rapid extraction of solid/semisolid matrices using organic solvents or water
by applying high temperatures (up to 200
C) and high pressures (up to 1500 psi) to
keep solvents in a liquid state above their atmospheric boiling points to increase
°
