Agriculture Reference
In-Depth Information
water
ers of the
mobile phase have been employed to enhance ion abundance, diminish the formation
of sodium adducts, and improve chromatographic peak shape, such as formic
acid [61
-
methanol or water
-
acetonitrile. In addition, some additives or modi
63], ammonium formate [64], and ammonium acetate [65,66]. Mobile
phases containing nonvolatile compounds such as phosphate buffers should be
avoided because they can clog the interface and produce buildup of deposits in
the ion source. Other additives have been required for the chromatographic separation
of some families, such as tetracyclines, aminoglycosides, or ivermectins. For instance,
for the elution of tetracyclines [67], oxalic acid is usually used to minimize the effect
of residual silanols on the stationary phase and to avoid the formation of complexes
with traces of metals. Furthermore, triethylamine (TEA) has also been employed to
block silanol groups of the chromatographic column [67]. Aminoglycosides comprise
a family of compounds that are not easily separated under generic chromatographic
conditions due to their high polarity. Therefore, poor chromatographic retention on
classical RP-C18 columns is observed, hindering their inclusion in multiresidue
methods developed for VDs [61]. However, the use of ion pairing agents, such as
hepta
-
uorobutyric acid (HFBA) added to the LC mobile phase, improves the
chromatographic performance by increasing the retention of these compounds on
RPLC columns [68]. On the other hand, traces of sodium hydroxide (NaOH) have
been used in the mobile phase to convert ivermectin to the sodium adduct in order to
enhance the sensitivity of this compound [59]. In other analyses, the addition of
ammonium formate in the mobile phase has been used to favor the formation of
ammonium adduct [64].
6.3.2 High-Resolution Mass Spectrometers
6.3.2.1 TOF-MS Analyzer
The TOF
MS analyzer is an attractive instrument to carry out multiresidue analyses
due to its potentially unlimited
m
/
z
range and high-speed acquisition capabilities
with high sensitivity and mass accuracy. It has been widely used during the past
years to carry out multiresidue analysis of pesticides [69], pharmaceuticals [70],
or toxins [71]. Nevertheless, its application in the
-
field of VDs has been very
recent [57].
This analyzer is the simplest mass spectrometer, where ions that have same
kinetic energy but different
m
/
z
values are separated in a
flight tube and
reach the detector at different times [72]. In this way, a complete mass spectrum is
obtained simply by allowing suf
field-free
cient time for all of the ions of interest to reach the
detector. This allows fast full spectral acquisition rates and full spectral sensitivity
at high mass resolution (around 10,000 expressed at full peak width at one-half
maximum, FWHM, and de
m
is
the width of the peak at the half height of the peak) with high mass accuracy.
According to these characteristics, TOF is an interesting choice for posttarget and
nontarget analyses because it is possible to monitor every potential contaminant
ned as
m
/
Δ
m
,where
m
is the mass of the ion and
Δ
