Chapter 7 - Mass spectrometry (part 2) Flashcards
The construction of a typical QIT mass spectrometer consists of
a ring electrode with a hyperbolic inner surface and two electrically common hyperbolic end-cap electrodes.
The trajectory of ions inside the QIT resembles
a figure 8 with rotating and oscillating simultaneously.
As the ions repel each other in the QIT, their trajectories
expand as a function of time.
To avoid QIT expansion, the trap is maintained with a pressure of ________ gas around _______ Torr.
Helium
10-3
A single high-vacuum pump with a flow of about ________ is sufficient to maintain QIT vacuum compared with _______ for other mass spectrometers.
40 L/s
250 L/s
Ion motion in QIT can be analyzed by______ equations.
Mathieu
When solving equations of motion for ions withing QIT analyzer, three fundamental parameters of fundamental importance emerge
a, q, and beta
Most commercial QITs operate by applying a
fundamental RF voltage to the ring at constant frequency and varied amplitude.
The quadrupole ion trap will be operated along the ______ axis.
qu (x)
At a fixed value of the RF potential V applied to the ring electrode, heavier ions will have lower ______ values.
qz
If the value of V is equal to 0.908, betaz is equal to one, and the ion has reached its _________.
stability limit.
At the stability limit, a slight increase in V will cause
the ion to have an unstable trajectory and will be expelled from the trap in the z direction.
In QIT, only _____% of expelled ions will reach the detector.
50
The maximum internal mass can be increased by
increasing V at higher values without arcing, reducing the size of the trap, or using a lower RF frequency.
In an ideal QIT, the motions of the ions in the radial and axial directions are
mutually independent.
If a supplementary RF voltage to the end-cap electrodes is applied to the end caps,
resonant ejection of ions occurs at qz < 0.908
Principle of resonant ejection
Ions are stored in the 3D trap at a voltage VA of the fundamental RF.
MS scan based on resonant ejection may either be carried out in a
forward (low to high mass) or reverse manner.
Resonant ejection can serve for precursor ion selection in
tandem MS experiments.
_______ rather than _______ MS occurs in QIT.
Time-dependent, space-dependent
QIT can perform not only MS2, but also
MS3 and MSn
QIT instruments do not support what kind of scans?
constant neutral loss and precursor ion
General sequence for MS in QIT.
- Select ions of one m/z ratio
- Let those ions fragment
- Analyze the ions by one of the scanning methods
- Alternatively, select a fragment in the QIP, and let it fragment further
To fragment ions by resonance excitation,
an RF voltage must be applied to adjust the qz value for the selected m/z ratio to the frequency of the RF generator used.
In general, fragments with m/z values lower than about _____ of the precursor ions m/z will be lost in QIT.
20%
Space charge effect
QIT has the capacity for a limited number of ions at any one time before repulsive charges cause the excess ions to be ejected.
Concentrations of ions above the space charge limit lead to
poor performance with grossly deteriorated resolution and shifts in the mass axis calibration.
The linear ion trap (LIT) adds sections
at each end of the quadrupole to create a potential well.
Once in the LIT, ions are cooled by
collision with an inert gas.
Placing electrodes of higher potential near the front and back ends of RF-only quadrupole creates
a trapping potential within the quadrupole.
Two modes for the mass selective ejection of ions used in LIT
- ions are expelled axially using fringe field effects
- slots are hollowed out in two opposite rods and mass selective radial expulsion of ions is obtained
One great advantage of LITs in comparison with QITs is a
more than 30-fold higher ion trapping capacity before space charge effects, and a more than 10 times higher trapping efficiency.
LIT advantages increase
sensitivity and the dynamic range.
The performance of LITs is highly dependent upon
mechanical construction.
2D and 3D ion traps are commonly operated at _____ resolution with scan rates on the order of _____ m/z per second.
unit
11,000
2D and 3D ion traps can trade spectral acquisition rate for
resolution.
The orbitrap mass analyzer consists of
an outer barrel like external electrode and a central spindle like internal electrode along the axial axis.
In orbitrap, both electrodes are connected to
independent voltage supplies.
In orbitrap, the outer electrode is split into
an ion excitation part and an ion detection part.
In orbitrap, the space between the internal and external electrodes forms
the measurement chamber under high vacuum conditions (~10-9).
In orbitrap, ion introduction is achieved by using a
field compressor.
Orbitrap uses a ___________ field, which is the sum of a quadrupole field of the ion trap and a logarithmic field of a cylindrical capacitor.
quadro-logarithmic
Curved linear traps function in the same way as their linear counterparts with the added advantage that
ions can be focused in narrow areas to be ejected as compact bursts of ions.
In curved linear traps, ions are focused by
altering the trapping fields of the electrodes at the front and rear of the instrument.
The curved linear ion trap is one of the most _______ ways of injection ions to the orbitrap.
efficient
Slow injection
When ions are injected equatorially.
In order to eliminate gas carryover from the ion trap (linear or curved),
the ion beam is usually deflected by a set of transfer lenses.
Fast injection
When ions are injected into the orbitrap at a position offset from its equator.
The _______ mode allows for the highest mass resolution in orbitrap.
Fourier Transform
Two modes of orbitrap mass detection
- Fourier Transform Mode
- Mass Selective Instability Mode
MSI can be achieved by
resonance excitation.
A convenient means of detection in MSI mode is
collision with a conversion dynode in the outer electrode.
Instances in which MSI mode should be used:
- Ions with certain m/z ratio can be stored for MS/MS analysis
- High intensity signals from unwanted ions can be ejected to improve dynamic range
The mass resolving power is half the
frequency resolving power.
The orbitrap provides a resolving power up to 140000 at m/z
200
