Agriculture Reference
In-Depth Information
Although electrospray ionization (ESI) is a common ionization method, especially
for relatively polar compounds, less polar or neutral analytes may have lower
ionization ef
ciencies and lower sensitivities and thus require atmospheric pressure
chemical ionization (APCI) or atmospheric pressure photoionization (APPI). There-
fore, a multimode ionization source (e.g., ESI and APCI or ESI and APPI) is ideal for
analyzing compounds with a wide range of chemical structures and properties. The
flexible ionization capabilities of these multimode sources also minimizes the need for
multiple LC
MS injections or repeated analyses of failed samples using an alternative
ionization mode. For example, with ESI and APCI multimode ionization source, ESI
(
-
+
), ESI(
), APCI(
+
), and APCI(
) ionization modes can be chosen and performed
in a single LC
MS run to obtain maximum ionization and sample coverage. While
perhaps not a high-throughput technique
per se
, eliminating the need for multiple
analyses of the same sample clearly increases the number of different samples that can
be analyzed.
-
Nontargeted Analysis
Nontargeted analysis includes the possibility of detecting
any compounds (i.e., compounds related or not related to contaminations such as
pesticides, mycotoxins, drugs, and plasticizers) present in a sample. It offers the
possibility of identifying unexpected contaminations, transformation products, and/or
impurities [48]. Such analyses are more complicated because they require the identifi-
-
cation of unknown compounds. Due to the complexity in identifying unknown
compounds, high-resolution MS, such as TOF and Orbitrap MS, especially QTOF/
Q-Orbitrap, are often employed for this purpose. Zhang et al. demonstrated the rapid
screening and accurate mass con
rmation of 510 pesticides at low ppb levels using
UHPLC coupled to a high-resolution benchtop Orbitrap mass spectrometer [50].
QTOF/Q-Orbitrap provides not only accurate mass measurements and high full
scan sensitivity but also additional features such as structure con
rmation. The accurate
product ion spectra can be obtained by performing MS/MS experiments using QTOF/
Q-Orbitrap MS and used to search compound libraries to con
rm the structures of
compounds. This is crucial for analytes or their metabolites when the reference
standards are not available. The number of applications of LC
QTOF/Q-Orbitrap
for targeted and nontargeted analyses in food safety has recently increased [32]. For
example, a UHPLC-QTOFMS system was used to perform targeted and nontargeted
analyses of
-
1000 organic contaminants, including residues and illicit substances,
such as mycotoxins in food samples, cocaine and several metabolites in human urine,
and pesticides, antibiotics, and drugs of abuse in urban wastewater [51].
∼
Matrix Effects
MS/MS are sensitive and selective, they
often suffer from matrix effects, especially with ESI ionization. Matrix effects are the
alteration of ionization ef
Although LC
-
MS and LC
-
ciency of target analytes in the presence of coeluting
compounds in the same matrix. They can be observed as either a loss of the signal
of the target (ion suppression) or a gain of the signal (ion enhancement). Matrix effects
alter the detection capability, precision, and/or accuracy of measurements for the
analytes of interest. Evaluating and minimizing ion suppression and enhancement are
important considerations during method development and validation [31,38]. Two
