49
3.2.2 Polar Analytes
The real revolution in ionization methods for polar compounds is represented by
Fast Atom Bombardment (FAB) introduced in 1981 by Barber and Bordoli [7]. FAB
overcame the previous methods as it was easy, sensitive, reproducible, fast and able
to ionize relatively big molecules.
But few years after its introduction, other ionization techniques, and in particular
electrospray (ESI) and matrix-assisted laser desorption (MALDI), owing to their
higher versatility, replaced FAB. Soon other atmospheric pressure ionizations were
introduced, i.e. atmospheric pressure chemical ionization (APCI), atmospheric
pressure photo ionization (APPI). More recently an increasing plethora of ambient
mass spectrometry methods [8], such as desorption electrospray ionization (DESI)
[9], direct analysis in real time (DART) [10], low temperature plasma (LTP) [11],
rapid evaporative ionization mass spectrometry (REIMS) [12], paper spray ionization (PSI) [13], electrospray laser desorption ionization (ELDI) [14] and many others, which ionize analytes in their ambient, reducing or eliminating extraction and
purification steps, have been introduced. As an example, ambient conditions using
DART ionization coupled to high resolution mass spectrometry have been used in
the analysis of multiple mycotoxins in cereals [15], while ELDI has been used for a
rapid identification of herbal toxins emergency care [16].
3.3 Analyzers
Once the ions are formed in the ion source, they are accelerated towards the mass
analyzer where their separation occurs according to their m/z ratios.
Analyzers can be divided into two main groups: those based on ion separation in
space (or ion-beam analyzers: sectors, quadrupole, time of flight) and those separating ions in time (ion traps, Orbitrap, FT-ICR) (Table 3.1).
For ion separation, analyzers can use a magnetic field (B), an electric field (E)
and a radiofrequency. As an example, quadrupoles use an electric field and a radiofrequency while with Orbitrap only an electric field is used.
Analyzers differ each other for some features, such as coverage of m/z range and
resolving power. While time-of-flight has not restriction on m/z range, routinely all
the others can analyze ions up to m/z 4000÷6000.
Actually, analyzers with high resolving power are: sectors (EB, BE), time of
flight, FT-ICR and Orbitrap. A high resolving power allows to obtain some advantages, such as an increase of selectivity due to isobaric ion differentiation, elimination of interfering species and measurement of accurate mass from which the
elemental composition of an ion can be obtained. On the other hand, an increase of
resolving power implies a decrease of sensitivity and an increase of scan time.
Analyzers are involved not only in obtaining a full scan mass spectrum but also
in tandem mass spectrometry experiments. Briefly, tandem mass spectrometry uses
3 Mass Spectrometry Methods for Food Safety/Detection of Toxins in Food
3.2.2 Polar Analytes
The real revolution in ionization methods for polar compounds is represented by
Fast Atom Bombardment (FAB) introduced in 1981 by Barber and Bordoli [7]. FAB
overcame the previous methods as it was easy, sensitive, reproducible, fast and able
to ionize relatively big molecules.
But few years after its introduction, other ionization techniques, and in particular
electrospray (ESI) and matrix-assisted laser desorption (MALDI), owing to their
higher versatility, replaced FAB. Soon other atmospheric pressure ionizations were
introduced, i.e. atmospheric pressure chemical ionization (APCI), atmospheric
pressure photo ionization (APPI). More recently an increasing plethora of ambient
mass spectrometry methods [8], such as desorption electrospray ionization (DESI)
[9], direct analysis in real time (DART) [10], low temperature plasma (LTP) [11],
rapid evaporative ionization mass spectrometry (REIMS) [12], paper spray ionization (PSI) [13], electrospray laser desorption ionization (ELDI) [14] and many others, which ionize analytes in their ambient, reducing or eliminating extraction and
purification steps, have been introduced. As an example, ambient conditions using
DART ionization coupled to high resolution mass spectrometry have been used in
the analysis of multiple mycotoxins in cereals [15], while ELDI has been used for a
rapid identification of herbal toxins emergency care [16].
3.3 Analyzers
Once the ions are formed in the ion source, they are accelerated towards the mass
analyzer where their separation occurs according to their m/z ratios.
Analyzers can be divided into two main groups: those based on ion separation in
space (or ion-beam analyzers: sectors, quadrupole, time of flight) and those separating ions in time (ion traps, Orbitrap, FT-ICR) (Table 3.1).
For ion separation, analyzers can use a magnetic field (B), an electric field (E)
and a radiofrequency. As an example, quadrupoles use an electric field and a radiofrequency while with Orbitrap only an electric field is used.
Analyzers differ each other for some features, such as coverage of m/z range and
resolving power. While time-of-flight has not restriction on m/z range, routinely all
the others can analyze ions up to m/z 4000÷6000.
Actually, analyzers with high resolving power are: sectors (EB, BE), time of
flight, FT-ICR and Orbitrap. A high resolving power allows to obtain some advantages, such as an increase of selectivity due to isobaric ion differentiation, elimination of interfering species and measurement of accurate mass from which the
elemental composition of an ion can be obtained. On the other hand, an increase of
resolving power implies a decrease of sensitivity and an increase of scan time.
Analyzers are involved not only in obtaining a full scan mass spectrum but also
in tandem mass spectrometry experiments. Briefly, tandem mass spectrometry uses
3 Mass Spectrometry Methods for Food Safety/Detection of Toxins in Food