Agriculture Reference
In-Depth Information
ion intensities increased approximately by one order of magnitude compared with
[M
H]
+
and detection of minor triacylglycerols was enabled. In another study,
formation of [M
+
Cl]
ions of some poorly ionizing
Fusarium
mycotoxins in cereal
extracts was achieved in negative DART ionization mode. The characteristic isotope
pro
+
le of the chlorine-containing ion could be used for con
rmation of identity [128].
Mass Spectrometric Detection
Both DESI and DART ionization sources can be
relatively easily coupled to any of the currently available LC
MS systems. These novel
techniques represent versatile and attractive alternatives to conventional ESI andAPCI,
which are typically used with separation techniques. Considering the fact that in
ambientMS all components (both analytes andmatrix) present in the sample are ionized
almost simultaneously, high requirements are laid on the MS detection to provide
desired selectivity and sensitivity that
-
fit the purpose for a particular application.
Different types of mass analyzers have speci
c features that make them more or less
suitable to deal with diverse tasks faced in analysis of food by ambient MS. The triple
quadrupole and (linear) ion trap analyzers operated in MRM mode provide superb
sensitivity and selectivity in cases of targeted measurements of relatively low
numbers of analytes. When full spectral information and rapid data acquisition are
required (e.g., in food pro
fingerprinting), the use of high-resolution
instruments using TOF or Orbitrap mass analyzers is preferred. Additionally, the
identity of analytes can be estimated based on accurate mass measurements and
elemental formula estimation. In this respect, hybrid instruments capable of high-
resolution tandemspectra (MS/MS) acquisition provide higher degrees of con
ling and
dence
in identi
cation of unknowns. It is worth noting that in practice, either low- or high-
resolution systems are applied to similar applications, largely due to limited access to
the other instrumentation.
The selectivity of measurement often plays a critical role in ambient MS of
complex samples. The presence of isobaric interferences in food samples or extracts
can complicate both qualitative and quantitative analyses, potentially resulting in false
positive results. While MS/MS or MS
n
can overcome this drawback only in targeted
analysis, the use of (ultra)high mass resolving power instruments that mitigate the loss
in spectral peak capacity represents a more generally applicable option [95,96]. An
example of the bene
t of high resolving power was provided in a study by Cajka et al.,
who compared medium high-resolution TOF and Orbitrap mass analyzers in DART-
based analysis of dithiocarbamate fungicides in fruit extracts [129]. The mass
resolving power of the TOF analyzer (
cient to entirely
separate the signal of the analyte (thiram) from matrix interferences. On the other
hand, an Orbitrap mass analyzer operated at 25,000 FWHM allowed complete
spectral separation even if the intensity of the analyte was lower than that of the
interferences. One should be aware that the mass resolving power provided by the
Orbitrap is linked to the acquisition speed, which might not be suf
∼
5000 FWHM) was not suf
cient for good
desorption peak characterization at ultrahigh mass resolving power settings.
A major drawback often encountered during MS-based analysis of complex
samples is the suppression of analyte signals caused by the sample matrix. While
in LC
-
MS this phenomenon can be diminished to some extent by chromatographic
