Mass Analyzers IV
Quadrupole Ion Traps
Chem 5181 – Fall 2007
J. Kimmel
Announcements
•
Jose is at a meeting through next Monday
•
Journal Skim #2 due today
–
Note that these assignments are Credit/Nocredit.
•
First presentation next Thursday (04-Oct,
Coburn)
•
Lab report due NEXT Tuesday (02-Oct) at
start of class
•
HW2 postponed (probably next Tuesday)
HW 1
• What is the mass of a 12C+ ion (with units)?
• What is the charge of a 12C+ ion (with units)?
• Typical flight times in TOFMS are on the order of
seconds, milliseconds, microseconds, nanoseconds,
picoseconds, or femtoseconds?
• What is the source of peak shapes in TOF mass
spectra?
From HW 1, #1
“(b) Derive an equation to predict ion velocity as a function of m/z for this linear
MALDI-TOF instrument, with an acceleration voltage of 10 kV and a flight
path length of 1.3 m. List units for all terms in your expression.
(c) Use Igor to plot expected ion flight times for the range m/z = 0 to 1000.
(d) In addition to the applied acceleration voltage, all ions also have an energy
component originating from the laser ionization process. Along the axis of
drift, this energy yields a normal distribution of velocities with a peak at 500
m/s and a full width at half maximum (FWHM) of 200 m/s. This
distribution is independent of m/z. Use Igor to plot the expected resolution
(FWHM) of the spectrometer for the m/z range 0 to 1000.
(e) Can this instrument resolve singly charged isotope peaks (e.g. CH4+
from CH3D+) across this entire range? If not, across what m/z range is this
possible?”
A few more thoughts on the quadrupole ….
Clicker Q
Q: To determine whether a quadrupole will transmit an ion
of known m/z, one must know:
1.
2.
3.
4.
5.
The number of charges, z
The length the quadrupole
The distance between rods of the quadrupole
The velocity of the ion before entering the quadrupole
The angular frequency of the applied Rf potential
A. All of the above
B. 2,3,4,5
C. 1,3,4,5
D. 3,4,5
E. 3,5
Clicker Q
Q: To determine whether a quadrupole will transmit an ion
of known m/z, one must know:
1.
2.
3.
4.
5.
z
The length the quadrupole
The distance between rods of the quadrupole
The velocity of the ion before entering the quadrupole
The angular frequency of the applied Rf potential
A. All of the above
B. 2,3,4,5
C. 1,3,4,5
D. 3,4,5
E. 3,5
To operate a quadrupole in a scanning mode,
where individual m/z values are transmitted one
after the other (e.g., m/z = 100; 101; 102 …)
A. U is held constant, while V is scanned
B. V is held constant, while U is scanned
C. U is held constant, while V and ω are scanned
D. U and V are both changed
B. A or B
To operate a quadrupole in a scanning mode,
where individual m/z values are transmitted one
after the other (e.g., m/z = 100; 101; 102 …)
A. U is held constant, while V is scanned
B. V is held constant, while U is scanned
C. U is held constant, while V and ω are scanned
D. U and V are both changed
B. A or B
When acquiring mass spectra with unit resolution for
ions originating from a continuous source (that is,
ions being presented to the mass spectrometer as a
steady stream) the duty cycle of a quadrupole mass
spectrometer:
A. Is nearly 100%
B. Depends on the m/z range being scanned
C. Is independent of m/z range, but depends on U, V, and ω
D. Cannot be determined
E. Two of the above
When acquiring mass spectra with unit resolution for
ions originating from a continuous source (that is,
ions being presented to the mass spectrometer as a
steady stream) the duty cycle (fraction of ions
detected) of a quadrupole mass spectrometer:
A. Is nearly 100%
B. Depends on the m/z range being scanned
C. Is independent of m/z range, but depends on U, V, and ω
D. Cannot be determined
E. Two of the above
Rf-Only Quadrupoles
Operated with U = 0, quadrupole becomes a broad bandpass filter
Such “rf-only” quads are an important tool for transferring
ions between regions of mass spectrometers.
Often denoted with small “q”
Collisional Cooling
A common application of rf-only
multipoles involves collisional
cooling.
In an ESI source, the expansion into
vacuum produces a ion beam with
broad energy distribution
Ion optics and TOFMS experiments
rely on precise control of ion
energies
Desire strategies to dampen energy
from external processes
Rf-induced trajectory in high
pressure region yield collisions, and
reduction in energy
Ken Standing et al. JASMS, 1998, 9, 569-579
Collisional cooling
Triple Quadrupole Mass Spectrometer
Detector
Q1
q2
Q3
Q1 selects parent; q2 CID fragmentation inside RF-only quad; Q3 fragment
analysis
Fragment Ion Scan: Park Q1 on specific parent m/z; scan Q3 through all
fragment m/z to determine make-up of Q1
Parent Ion Scan: Park Q3 on specific fragment m/z; scan Q1 through all
parent m/z to determine source of fragment
Neutral Loss Scan: Scan Q1 and Q3 simultaneously, with constant
difference, a, between transmitted m/z values (a = MQ1 – MQ3). Signal
recorded if ion of m/z = MQ1 has undergone fragmentation producing a neutral
of m = a.
Quadrupole
Quad. Ion Trap
• RF fields yield m/z band
of stability
• 2D Manipulation of
trajectory
• Detect those ions that are
selectively transmitted
with stable trajectories
• Continuous analysis
• RF fields yield m/z band
of stability
• 3D Manipulation of
trajectory
• Detect those ions that are
selectively ejected due
to destabilized trajectory
• Pulsed analysis
Quadrupole Ion Traps
Ring electrode (r)
End cap electrodes (z)
Fundamental RF: Fixed frequency
(1.1 MHz) variable voltage (up to 7
kV) applied to Ring Electrode
DC: An optional DC voltage may be
applied to the ring electrode, which
will affect the stability of ion
trajectories
Resonance AC: Fixed frequency
voltage applied to end caps for
resonant ejection or fragmentation
Note that ions enter and exit along z
axis
Pressure (1 mTorr) dampens extra
kinetic energy and E of repulsion
Ions from source are focused along z axis of trap by standard transfer optics
Continuous beam is gated into trap. Ionization period is set to maximize signal and
minimize space charge effects.
Cell is filled with inert gas (e.g., He) at 1 mTorr to dampen kinetic energy of ions
and contract trajectories toward center – improves resolution
Ion Traps
Hand-held dimensions
Ion Motion Inside an Ion Trap
From Lambert
•RF fields induce oscillations in r and z directions
•A “trapped” ion is stable along both axes
Stability Diagram
Like a quadrupole mass spectrometer,
ion stability described by variables
related to RF and DC components.
aU
From de Hoffmann
For most operation, DC component is
zero. And stability determined by qz
8ezV
qz 
m(ro2  2 zo2 )( 2) 2
aV
qz depends on mass, charge,
dimensions, RF frequency, and RF
amplitude (V)
Ions trapped with stable trajectory up to
qz of 0.908
m/z Dependent Stability
8ezV
qz 
m(ro2  2 zo2 )( 2) 2
Stability boundary at qz = 0.908
Stability diagrams for m/z = 10, 50, and
100 in V(RF) - U(DC) space.
Note that, like quadrupole, broadest
range of m/z stability at U=0
Increasing V will destabilize low m/z
ions. That is, high m/z stable to higher
V.
From de Hoffmann
True or False. Just like in a
quadrupole, I can
determine the intensity of
a given m/z value by
adjusting U and V so that
ions of that m/z are in the
apex the stability diagram.
A.
B.
C.
D.
Definitely True
Maybe (?)
Probably Not (?)
Definitely False
Example: Scanning V
Figure from:
http://www.abrf.org/AB
RFNews/1996/Septem
ber1996/sep96iontrap.h
tml
•U = 0
•Ions of different m/z are simultaneously trapped
•Increasing V (1000, 3000, 6000), increase qz of all ions, moving toward stability boundary
•V determines low mass cut-off at qz = 0.908 (Demos 1, 2, 3, 5)
Max m/z
8ezV
qz 
2
2
2
m(ro  2 zo )( 2)
For the scanning mode, the detected (destabilized) m/z increases with V
Pressure places an upper limit on how high V can go – Arcing!
For example, with ro = 1 cm, zo= cm, and v = 1.1 MHz, Vmax of 8kV yields an
upper mass limit of ~ 650 Th
How can we increase this value?
Clicker
How many of the following are true.
(i)
At any moment, an ion trap detects those m/z values
that are “stable”
(ii) Ion traps typically have > 60% duty cycle
(iii) In the scanning mode we discussed, the m/z range of
an ion trap is limited by the minimum voltage that can
be applied while still inducing stable trajectories
(iv) In the scanning mode that we discussed, increasing V
yields detection of higher m/z ions
(a) 0
(b) 1
(c) 2
(d) 3
(e) 4
Clicker
How many of the following are true.
(i)
At any moment, an ion trap detects those m/z values
that are “stable”
(ii) Ion traps typically have > 60% duty cycle
(iii) In the scanning mode we discussed, the m/z range of
an ion trap is limited by the minimum voltage that can
be applied while still inducing stable trajectories
(iv) In the scanning mode that we discussed, increasing V
yields detection of higher m/z ions
(a) 0
(b) 1
(c) 2
(d) 3
(e) 4
Clicker
How many of the following are true.
(i)
(ii)
An ion trap requires lower vacuum than an FTICR
Resolution in an ion trap depends critically on the
precision of the power supplies used to set the voltage
of the ring electrode
(iii) Resolution in an ion trap depends critically on the
speed of the acquisition electronics
(iv) For the scanning mode we discussed, the duty cycle
of an ion trap depends on the m/z range recorded
(a) 0
(b) 1
(c) 2
(d) 3
(e) 4
Secular Frequency
• Because of inertia, ions do not oscillate at the fundamental
frequency applied to the trap, v
• Instead, ions oscillate at a secular frequency, f, that is lower than
v
• It is possible to calculate the value of fz based on applied V
• Along the z axis, fz is proportional to qz (See text 2.2.2)
• If an RF voltage at frequency = fz is applied to the end caps, ions
with secular frequency fz will come into resonance and the
amplitude of its oscillation along z axis will increase
• If the increase is large enough, the ion will be ejected
Resonant Ion Ejection
Example:
v = 1.1 MHz causes z oscillation
with fz= 160 kHz
Apply v’ = 160 kHz to end caps
Energy transferred to ion through
resonance causes destabilization
along z
Resonant ejection allows
selective detection of ions at qz
lower than 0.908
From de Hoffmann
Resonant Ejection
•fz is proportional to qz
Ion at this qz will
oscillate in
resonance with fz
•For fixed fundamental
frequency, qz of an ion is
adjusted by varying V
•fz applied to end caps creates
a “hole” in the stability diagram
at the qz corresponding to ion
oscillation frequency fz
•Scan of V destabilizes ions of
changing m/z
Figure from:
http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.html
Resonant Ejection Extends Mass Range
In ion traps, V is limited to ~ 8 kV, which ultimately limits m/z value that can
be ejected at qz = 0.908
Resonance ejection gets around this limit
Ghost Peak
•If an ion unintentionally
fragments during analysis, it
is possible that the fragment
(mf) has an m/z with a qz
value that is higher than
resonant ejection value
•If later, V is increased and
pushes mf to stability limit, it
will be detected as wrong
(higher) m/z because system
thinks it was ejected by
resonance
•“Ghost Peak”
Resonant Ejection Enables MSN
Forward and reverse scanning of V allows
user to isolate single m/z value in trap
Isolated ions can be fragmented by
collisions with background gas
•Excite ion with resonance
•Keep amplitude low enough to avoid
ejection
The ability to repeat the isolation-andfragmentation cycle allows MSN analysis
(Demos)
Additional Notes
• Space charge effects due to repulsion of
ion in traps
– Limits the total number of ions => sensitivity
• m/z may be selected by apex methods
similar to quadrupole mass spectrometer
• Buffer gas improves resolution by
tightening ion packet (dampens initial KE
and E from repulsion)