Agriculture Reference
In-Depth Information
ionized in the source (in the de
ned
m
/
z
range) in a sample without reinjecting
it [73].
Since the release of the
rst TOF
-
MS analyzer, which showed limited resolving
power (
300 FWHM) and mass accuracy, this technology has undergone numerous
≈
modi
cations. Some of these were re
ectron and orthogonal acceleration. The
first of
these solutions, re
ectron, allowed reaching mass resolving power approaching
10,000 FWHM and mass accuracy
10 ppm [74,75]. With the development of
off-axis or orthogonal acceleration TOF
<
-
MS (oa-TOF
-
MS) of ions, it has become
the catalyst for the current range of resolution of TOF
MS instrument with greatly
improved resolving power and mass accuracy. Nowadays, instruments offer a mass
resolving power of
-
18,000 FWHM [76].
Despite these improvements, early TOF
>
MS instrumentation was hindered by the
narrow dynamic range of the detector. This resulted in the notion that TOF
-
MS could
not be used for quantitative purposes. Consequently, in the past years, signi
-
cant
enhancements related to the dynamic range have been made. There have been
improvements in other parameters as well, such as mass accuracy, resolution,
sensitivity and scan speed, and type of detector [77].
The linearity of TOF
MS measurements is limited because of the way the ions are
detected. Regarding the type of detector, two types of detector systems are used in
TOF
-
MS: the analog-to-digital converter (ADC) and the time-to-digital converter
(TDC) [77]. ADC detectors suffer from inherent background noise, whereas one of
the major problems associated with TDC detectors is saturation, affecting linearity
and mass accuracy [77]. In order to overcome these problems, a traveling wave-based
radio frequency-only stacked ring ion guide (TWIG) has been used. TWIG has been
used to extend the dynamic range of TDC-equipped TOF
-
MS [77]. This technique is
called dynamic range enhancement (DRE). DRE has improved the linear dynamic
range up to four orders of magnitude.
In order to maintain the stability of the mass axis, TOF
-
MS instruments require the
use of a continuous internal lock mass or the periodic recalibration by switching to a
discontinuous lock spray [70]. The introduction of the reference compound(s)
together with the mobile phase, however, can lead to matrix ionization suppression,
thus decreasing the reference compound sensitivity and increasing mass errors.
Consequently, an additional electrospray source that orthogonally generates lock
mass ions or a baf
-
e, which periodically switches between two positions [78], is used
to eliminate the potential risk of interferences by isobaric sample compounds, as well
as the probability of signal suppression.
Therefore, the modern benchtop TOF
-
MS instruments characterized by techno-
logical advances in re
ectron technology, orthogonal injection, and DRE data
acquisition have resulted in a new generation of MS analyzers for LC that have
improved mass resolution (6,000
-
20,000 FWHM), mass accuracy (2
-
10 ppm),
dynamic range (four to
five orders of magnitude), and sensitivity (fmol). These
developments in TOF
MS instrumentation makes this analyzer a powerful analytical
tool, enabling target, posttarget, and nontarget analyses, as well as identi
-
cation,
confirmation, and quantification of comprehensive lists of analytes in a single
injection.
