Mass Analyzers IVQuadrupole 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/No-

credit.

• 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. The number of charges, z2. The length the quadrupole3. The distance between rods of the quadrupole4. The velocity of the ion before entering the quadrupole5. 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. z2. The length the quadrupole3. The distance between rods of the quadrupole4. The velocity of the ion before entering the quadrupole5. 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 scannedB. V is held constant, while U is scannedC. U is held constant, while V and ω are scannedD. U and V are both changedB. 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 scannedB. V is held constant, while U is scannedC. U is held constant, while V and ω are scannedD. U and V are both changedB. 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 band-pass filter

Such “rf-only” quads are an important tool for transferring ions between regions of mass spectrometers.

Often denoted with small “q”

Collisional Cooling

Ken Standing et al. JASMS, 1998, 9, 569-579

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

Collisional cooling

Triple Quadrupole Mass Spectrometer

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.

Detector

• 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 Quad. Ion Trap

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

V

U From de HoffmannLike a quadrupole mass spectrometer, ion stability described by variables related to RF and DC components.

For most operation, DC component is zero. And stability determined by qz

222 )2)(2(

8

ooz zrm

Veq

z

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

From de Hoffmann

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.

222 )2)(2(

8

ooz zrm

Veq

z

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. Definitely TrueB. Maybe (?)C. Probably Not (?)D. Definitely False

Example: Scanning V

Figure from: http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.html

•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

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

222 )2)(2(

8

ooz zrm

Veq

z

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) An ion trap requires lower vacuum than an FTICR(ii) 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

From de Hoffmann

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

Resonant Ejection

Figure from: http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.html

•fz is proportional to qz

•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

Ion at this qz will oscillate in

resonance with fz

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-and-fragmentation 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)