Download Time-of-Flight (TOF) Analyzers

CH437 ORGANIC STRUCTURE ANALYSIS
CLASS 3: MASS SPECTROMETRY 3
Synopsis. Ion selection continued: time-of-flight (TOF) analyzers. Other
analyzers: quadrupole ion trap (QIT) and ion cyclotron resonance analyzers.
Detectors: point and array detectors. Electron multipliers.
Time-of-Flight (TOF) Analyzers
Time-of-flight analyzers were present in some of the earliest commercial mass
spectrometer (1955). In its simplest form, a TOF analyzer is characterized by a
linear tube that is shielded from electric and magnetic fields. This is called the
flight tube or drift tube. At one end of the tube, ions from the source are
accelerated by electrostatic plates (source focusing lenses) (typically 20 kV).
They enter the field-free region and travel in straight lines to the detector,
situated at the other end of the tube. See below.
A short pulse of ions is necessary, rather than a continuous supply, because a
TOF analyzer uses only time to select the ions. The pulse of ions is supplied
typically either by pulsed laser (as in MALDI-TOF) at about 20 kV and using a
source focusing lens of 0 V, or by a continuous source (e.g. EI) and pulsed
potentials on the source focusing lenses (0 – 20 kV).
Equations of Motion
If an ion enters the field-free region with a velocity v after being accelerated
through the source focusing lenses of potential Vs, then
or
mv2
2
=
zeVs
v
=
2zeVs
where z is the charge on the ion
m
Hence the time taken for the ion to reach the detector is given by,
t
d
=
v
d
=
2zeVs
m
Since Z, e, d and Vs are constant,
t =
or
m
z
-1/2
x (2eVs)
t = constant x
m
z
x d
(1)
A quick analysis of this relationship will show that an ion of m/z value 100 will
take twice as long to reach the detector as an ion of m/z 25 and that the times of
arrival of ions of small mass difference (e.g. 100 and 101) will be close.
Upper Mass Range, Sensitivity and Resolution
Two advantages of TOF analyzers are:
1. There is (theoretically) no upper limit to the mass range:
m/z > 300, 000 Da have been observed by MALDI-TOF.
2. They are highly sensitive, since virtually all the ions reach
the detector: mass spectra of 10-15 mol quantities of
gramicidin have been recorded using a TOF analyzer.
However, the big disadvantage lies in resolution. The equations of motion
described previously make the following assumptions regarding ions of the same
m/z ratio:
1. They have the same kinetic energy
2. They are produced at the same time
3. They are produced at the same point in space
In practice, none of these is true and ions of the same m/z value are subjected to
energy, time and space distribution, so that they arrive at the detector at slightly
different times, resulting in low resolution. This is especially serious for ions of
larger m/z values, because the difference in time of arrival becomes smaller, as
the difference between (m+1)/z and m/z diminishes with increase in m, according
to equation (2), which is derived from equation (1):
tm _ tm+1
= t
= constant x
m
z
_
(m + 1)
z
(2)
Delayed Pulse Extraction
One way to reduce the spread of kinetic energies amongst ions of the same m/z
value is to apply a time lag or delay (of a few microseconds) between the ion
formation and extraction into the flight tube. The principle is illustrated below.
Reflectron TOF Analyzers
Another way to improve resolution of TOF analyzers involves the use of an
electrostatic reflector (reflectron). The reflectron, a series of grids and ring
electrodes, creates a retarding field that acts as an “ion mirror”: it deflects ions
and sends them back through the flight tube, as shown below.
If the electric field in the reflectron is E and ion of charge Ze and kinetic energy
Ek enters the field with velocity vix and penetrates to a depth of x in the field, then
x
=
Ek
zeE
At x its velocity vix along the x axis is 0 and hence its mean velocity in the field will
be vix/2. The time needed to penetrate the field (t0) is given by
t0
x
=
vix/2
The total time (tR) in the field is
2x
tR = 2t0 =
vix/2
=
4x
vix
Now, consider two ions of identical mass (same m/z) but with different kinetic
energies, Ek and Ek’. We define the parameter a2 as Ek’/Ek.
The velocity along the x axis during the field-free flight will be given by equations
(3) and (4):
Ek
mvix2
=
Ek'
or
vix
2
mvix' 2
=
vix'
2
=
2Ek
=
2Ek'
(3)
m
2
2Ek a
=
m
m
That is to say, combining equations (3) and (4), we have
v'ix = avix
The time for field-free flight outside the reflectron, for a path length of d is
t
=
d
t' =
;
vx
d
vx '
d
avx
=
t
a
or t ' =
In the reflectron, the ions will penetrate at a depth of x or x’, where
x =
or
Ek
;
zeE
x' =
Ek'
a2 Ek
=
zeE
zeE
x' = a2x
Hence, the time spent in the reflectron will be
tR
=
4x
vix
;
t'
R
=
4x'
vix'
=
4a2x
avix
=
4ax
vix
or t'R = atR
Hence the total flight time for the ions will be t + tR and t’ + tR’, respectively:
(4)
ttotal = t + tR
and
t' total
=
t
a
+
atR
This means that if a>1, the ion with higher kinetic energy will have a shorter
flight outside the reflectron (t/a), but a longer flight inside the reflectron (atR). The
reverse is true is true for ions with lower kinetic energy (a<1).
The variations in the flight times for ions with the same m/z ratio
but different kinetic energies compensate for each other.
It will be seen in the next class that reflectron TOF analyzers can be readily
configured to perform tandem MS (MS-MS).
Other Analyzers
Two other analyzers will be dealt with only briefly here: they are the quadrupole
ion trap (QIT) analyzer and the ion cyclotron resonance (ICR) analyzer.
Quadrupole Ion Trap (QIT) Analyzers
These are like a quadrupole bent on itself in order to form a closed loop. Indeed
the equations of motion for ions in the space between the electrodes are very
similar for those of the standard (transmission) quadrupole. The main difference
is that ions are trapped in the system, rather than transmitted through it: they
assume concentric three-dimensional orbits, depending on their m/z values. By
scanning from low to high values of U, the motions of ions with progressively
higher and higher m/z ratios develop larger and larger oscillations, until they are
ejected from the trap into the detector. A common example of a QIT analyzer,
with an EI source and EMD, is shown below. The advantages and disadvantages
of QIT analyzers are similar to those of transmission quadrupoles: they are most
frequently used in GC-MS.
Ion Cyclotron Resonance (ICR) Analyzers
ICR analyzers consist of a box (of sides a few cm in length), which is located in a
strong magnetic field. Low velocity ions are injected into the box. Under these
conditions, the ions assume small radius circular trajectories and are actually
“trapped” inside the box. See the diagram below.
Two (opposite) sides of the box (perpendicular to the direction of the magnetic
field) are subjected to a radiofrequency field, as shown in the diagram below.
When the radiofrequency matches the rotational frequency of a group of ions of
common m/z ratio, these ions absorb the energy of the wave (resonance), so that
their velocities and hence trajectory radii increase. This causes the ions to pass
closely to the plates perpendicular to the trajectory, thereby inducing an “image
current” which is detected and amplified. If the radiofreqency scan is very rapid
(~1s), then all the ions in the box can be excited simultaneously. The complex
time-dependent waveform of all the tiny “image currents” is converted,
electronically, to a frequency-dependent intensity function (the mass spectrum)
by Fourier transform. Hence this form of mass spectrometry is known as Fourier
transform ion cyclotron resonance mass spectrometry (FTICRMS). The principles
of Fourier transform are illustrated below.
FTICR mass spectrometers are sensitive and accurate, but expensive.
Detectors
All mass spectrometers require at least one ion detector (or collector), which is
usually located after the last (or only) analyzer of the instrument. Some
instruments, such as MS/MS types, have more than one detector.
Detectors measure the small ion current (e.g. 10-15A) resulting from ions as they
emerge from the analyzer via the exit slit. They must be supplied with electronic
circuitry of considerable sophistication - in particular, ones with the following
attributes:
1.
High gain (amplification).
2.
Long dynamic range (say 5 decades or powers of 10).
3.
Fast response (down to <1ns for some channel electron
multiplier detectors).
4.
Minimal noise and interference.
Detectors can be classified as those without and those with internal amplification,
that is (respectively): Faraday cup detectors and electron multiplier detectors
(EMDs)/photomultiplier detectors (PMDs)
Also they can be classified as point ion collectors (these collect ions sequentially)
or as array collectors (these collect ions simultaneously)
Faraday Cup Detectors
This type of detector, a point ion collector, is commonly used in less expensive
low-resolution instruments and basically consists of a sloping electrode in a metal
cup. Ions impacting the cup cause emission of secondary electrons, which are
repelled by the negatively charged screen at the mouth of the cup onto the
electrode, thus avoiding signal loss. The detector output is amplified by an
external high-impedence amplifier, but as there is no internal signal amplification,
the detector is relatively insensitive. However its advantages are many - it is
inexpensive, simple and offers a response that is not dependent on ion mass and
other factors. For this reason, Faraday cup detectors are often used for high
accuracy isotope ratio measurements.
Electron Multiplier Detectors
These are used in all general-purpose mass spectrometers. They have an
internal amplification system, in that secondary electrons (ejected as a result of
ion-surface impact) are accelerated to strike a further surface producing more
secondary electrons, and so on. In this way, a large number of secondary
electrons can be produced from a single ion-surface impact.
Electron multiplier detectors are of the following basic types: Dynode electron
multiplier detectors (EMDs) and array detectors.
Dynode EMDs
There are two major types in use today: discrete dynode and continuous dynode
EMDs. Discrete dynode EMDs consist of a series of plates (up to 20), called
dynodes, which are made of Be/Cu alloy or Al and are linked by resistors, with
the first dynode at a negative potential with respect to the potential of the final (or
collection) dynode. The resistors ensure there is a positive potential gradient
between successive dynodes, so that the released electrons cascade down the
chain to the final dynode. Continuous dynode EMDs are particularly common in
modern MS instruments. The channel type of continuous EMD is a thin (1mm
i.d.) hollow glass cylinder whose inside is lined with heavily lead-doped glass for
favourable secondary electron emission. A typical configuration for this kind of
detector is illustrated below.
A voltage is applied across its length and because the glass behaves as a
continuous strip resistor, this appears as a gradual potential drop as one
traverses the tube from output to input end.
The amplifying power is the product of the conversion factor (number of
secondary particles emitted by the conversion dynode upon the impact of a
single ion) and the multiplying factor of the continuous dynode system: it is
commonly 105 – 107. Hence a major advantage of this type of EMD is sensitivity,
which allows rapid scanning and so continuous dynode EMDs are often used
with quadrupole or TOF analyzers. A disadvantage is their limited lifetime (~2
years), because of surface contamination resulting from impact of too many ions,
especially if the vacuum is sub-standard. They are also less precise than
Faraday cup detectors, because the conversion factor depends on mass, charge
structure and energy of the impacting ions.
Where impacting ions have insufficient energy, which is often the case with more
massive ions (m/z > 2000), a post-analyzer accelerator is required.
Array EMDs
Array detectors are able to detect ions of different m/z values simultaneously
during the analyzer magnetic field scan. The most common types are the
microchannel plate models. Microchannel plates (MCPs) consist of thin plates
containing millions of 10 µm diameter channels, which have a particular angle
bias with respect to the surface. Two plates are commonly used in a “chevron”
arrangement, with the second plate oriented so that the bias angles are a mirror
image of the first plate. An ion striking the channel wall will set off an electron
avalanche within the channel, which is amplified via several channels in the
second plate. Array detectors are often used with magnetic sector analyzers.
Photomultiplier Detectors (PMDs)
This type of detector depends on the production of secondary electrons (as for
previously discussed detectors) as ions from the analyzer strike the conversion
dynode. However, the secondary electrons are then accelerated towards a
phosphorescent screen, where their energy is converted to photons, which are
detected by a photon multiplier. The number of photons produced is proportional
to the number of ion impacts. A thin film of Al covers the phosphorescent screen
to avoid build-up of charge. Amplification is typically 105; not quite as sensitive as
continuous dynode EMDs, but their lifetime is generally longer.