Agriculture Reference
In-Depth Information
effect of the resolution of an instrument and its ability to discriminate between
two nearly isobaric compounds. Regarding mass accuracy, mass accuracies ranging
from 2 to 5 ppm can be achieved by ToF instruments with the external calibration
method. It can even improve to
<
2 ppm with new generations of orbital
trap
instruments.
These high-resolution mass spectrometers allow nontargeted, full scan analyses. In
such studies, retrospective analysis of the acquired data in full scan mode facilitates
identi
cation of suspect compounds from a theoretically unlimited number of
analytes. Such methodologies can improve throughput by allowing the screening
of multiple analytes simultaneously, eliminating the need for separate analyses for
every analyte of interest. Because these HRMS instruments are becoming more
common, it is worth discussing the factors that need to be considered when using them
for targeted and nontargeted analyses.
7.3.1 Targeted Analysis Using HRMS
In this approach, development of chromatographic methods and optimization of mass
spectrometer parameters for detection (source conditions: temperature, gas, voltage,
etc.) in positive or negative ion modes (mode depends on ionization ef
ciency of the
compounds) for different classes of standard veterinary medicinal products is
necessary prior to real sample analysis. However, compromise is generally necessary
to get voltages, temperatures, and gas values that allow the ionization of the maximum
number of target compounds.
Extracts of biological matrices from foods, using previously developed methods,
are subjected to LC
MS analysis under full scan high-resolution conditions. From full
scan mass chromatograms, the
-
nd the target
compounds is to extract the ion chromatogram (EIC) trace of the exact masses. In this
analysis, the accurate masses of targeted [M
first and most simple approach to
H]
ions of the
veterinary drugs are extracted from their MS raw chromatograms using a mass
tolerance window of
H]
+
ions or [M
+
5 ppm. This narrow window avoids isobaric ion interfer-
ence. The resulting extracted ion chromatogram shows a chromatographic peak of the
requested exact mass, without interference from the other matrix components. The
information in the mass spectrum associated with this peak (i.e., fragmentation and/or
isotopic distribution) can be useful to con
±
3
-
rm the presence of targeted analytes. In
addition to mass spectral data, chromatographic retention times also support in the
identi
cation of veterinary drugs. For further con
rmation, MS/MS analysis of
H]
ions can be performed. This, however, has
obvious implications on the throughput of the overall methodology. These two events
(full scan and MS/MS) can be implemented within a single run by using data-
dependent acquisition (DDA). This combination allows the screening of analytes and
their full con
H]
+
ions or [M
suspected [M
+
rmation at the same time. Indeed, using DDA allows acquiring MS/MS
data extracted from ions selected from a full survey scan. The choice of the selected
ions is made automatically either from several prede
ned criteria (signal intensity,
etc.) or from a list of targeted masses. The
first event, full scan, provides the screening
step and the second event, that is, MS/MS analysis, gives the con
rmatory step as the
