CH 908: Mass SpectrometryLecture 6

Mass Analyzers

Prof. Peter B. O’Connor

Objectives

• Types of mass spectrometers and how they operate– Time-of-flight– Quadrupoles– Ion traps

• Mathieu stability diagram analyss

– FTICR – Orbitrap

Electron Multiplier

Notes: channeltron microchannel plates chevron

Mass Spectrometers• Time of Flight

• Magnetic Sector

• Quadrupole

• Triple Quadrupole

• Quadrupole Ion Trap

• FTICRMS

•Orbitrap

Mass Spectrometers DO NOT measuremass. They measure mass/charge ratio.

Understanding how mass spectrometers work is understanding how ions move in electric and magnetic fields.

Ions in a DC Electric Field

F = qE = m d2x/dt2

+

10 KV

Time of Flight Mass Spectrometry

• MALDI-TOF

• EI-TOF

• ESI-TOF

The most simple of all mass spectrometers, at least conceptually.

Linear versus reflectron

Delayed extraction (time lag focusing)

Detection electronics

PSD scan

Orthogonal injection

Basic TOF mass spectrometer

Laser

V D (field free drift region)

Source

SOscilloscope

++

++

Figure 3. The principle of MALDI time-of-flight mass spectrometry.

1. TOF requires a pulsed ion source

2. TOF requires a small kinetic energy distribution in the ions

3. Radial dispersion causes signal loss

4. TOF requires a detector/oscilloscope/digitizer that’s MUCH faster than the ion flight time.

TOF fundamental limitationsResolution limited by:

length of TOF flight tube

kinetic energy distribution

- delayed extraction

- reflectron

- orthogonal injection

propagation delay in detector

Laser

Vs

D1 (first field free drift region)

Source

S

Oscilloscope

++

Figure 4. Combined Linear/Reflectron MALDI time-of-flight mass spectrometer.

D2 (second field free drift region)

First Detector

Second Detector

Vr ≈ Vs

deflector

++

+

+

Figure 14. Quadrupole Time-of-Flight Hybrid Vr ≈ Vp

Laser

V

D (field

free drift region)

Source

S

Oscilloscope

++

+

Pusher (Vp)

+

+

Delay Generator

Q0 Q1 Q2

(RF-only) (mass filter) (RF-only)

+ +

Focusing

++++

++

+

+

Collision Cell

++

+

second field free drift region

first field free drift region

Figure 6. MALDI tandem time-of-flight mass spectrometer.

Laser

Vs

Source

Oscilloscope

++

Detector

Vr ≈ Vs

deflector

+ +

+

+

+

++

++++

Collision Cell (Vc)

Delay Generator

TOF Parameters

Simple, cheap (in theory), robust, sensitive.

A good modern TOF should give:

>10k Resolving power

~1-10 fmol sensitivity (single scan)

~10 ppm mass accuracy internally calibrated (5 ppm if the peak is particularly large or clean).

>1000 scans/second

Unlimited mass range

TOFMS CalibrationEquationm = At2+B

TOF fundamental limitationsResolution limited by:

length of TOF flight tube

kinetic energy distribution

propagation delay in detector

Sensitivity limited by:

ion stability

ion transfer efficiency

MS/MS is difficult

Ions in a Magnetic Field

F=qv x B +B

V

F

Magnetic Sector Mass Spectrometry

• MALDI

• EI

• ESI

Large, expensive, obsolete.

Swept beam instrument

The first “High Resolution” mass spectrometer (> 10k RP)

Lousy sensitivity (~1 nmol)

High energy collisional fragmentation

Extremely linear detector response (isotope ratio mass spectrometry)

Sector CalibrationEquationm = AB0

2r2/V

Jeol and Thermo-Finnigan MAT

Ions in a magnetic field

Sector Fundamental Limitations

Resolution/sensitivity tradeoff by using a mass filtering slit

Resolution limited by:

magnetic/electric field homogeneities

slit width

Sensitivity limited by:

ion transfer efficiency

slit width

metastable decay

Scan speed / scan stability tradeoff

Quadrupoles

• MALDI

• EI

• ESI

Small, cheap, ubiquitous.

Swept beam instrument

Resolution typically 1000, mass accuracy typically 0.1%

Sensitivity depends on the source. Typically in the 100 fmol range.

1989 Nobel Prize in Physics for development of ion trapping techniques

Wolfgang Paul(quadrupole ion traps)

Hans Dehmelt(Penning ion traps)

Quadrupole mass spectrometer

Wiring of a quadrupole

The potential energy diagram of a quadrupole showing the saddlepoint in the electric field (generated using Simion 7.0)

3D - Quadrupole ion traps

•linear ion traps

•3D ion traps

•They follow exactly the same rules as quadrupoles

Figure 11. The shape of Paul ion trap mass spectrometers.

r

z

A. a cross-section of a hyperbolic quadrupole ion trap

B. a potential energy diagram of the QIT showing the saddlepoint in the electric field (generated using Simion 7.0)

Quadrupole Ion Traps

Capillary

Skimmer LensesOctopole Ion Guide

Lenses

Entrance Endcap

Ring Electrode

Exit Endcap

Quadrupoles

• qz V/m• qz fion • az U/m

z stability

r stability0.5 1.0 1.5

qz

Operating Line

=1.0qz=.908

Stablez & r

az

0.2

0.0

-0.2

-0.4

-0.6

0.4

+

+

+

-

-

“Matthieu eqn”

A± = U ± Vsin(ωt)

2 2

4z

eVq

m r

2 2

8n

eUa

m r

Quadrupole Ion Traps

• qz V/m• qz fion • az U/m

z stability

r stability0.5 1.0 1.5

qz

Operating Line

=1.0qz=.908

Stablez & r

az

0.2

0.0

-0.2

-0.4

-0.6

0.4

+

+

+

-

-

“Matthieu eqn”

A± = U ± Vsin(ωt)

2 2 2

8

( 2 )z

eVq

m r z

2 2 2

16

( 2 )z

eUa

m r z

Figure 12. Mathieu stability diagram with four stability points marked. Typical corresponding ion trajectories are shown on the right.

0.5 1.0 qz

az0.2

0.0

-0.2

-0.4

-0.6

0.0

z stable

r stable

r and z stable

qz = 0.908

A

A B

C D

B

C

Daz = 0.02, qz = 0.7 az = 0.05, qz = 0.1

az = -0.2, qz = 0.2 az = -0.04, qz = 0.2

QITMS: Mass-Instability Ion Ejection

0.5 1.0 1.5

qz

Operating Line

=1.0qz=.908

az

0.2

0.0

-0.2

-0.4

-0.6

0.4

+

+

+

-

-

Highm/z

Lowm/z

• Mass Analysis: Ramp RF Volt. on ring electrode

• Ions increase in qz value

• Ions become axially unstable at qz = 0.908

• Ions are ejected from ion trap

• Low m/z ions are detected first

2 2 2

8

( 2 )z

eVq

m r z

QITMS: Resonant Ejection• Mass Analysis:

Ramp RF Volt. on ring electrode

• As RF increases ions increase in qz

• Apply dipolar AC signal to endcap electrodes for resonant ejection

• Ions are ejected radially from trap

• Low m/z ions are detected first

0.5 1.0 1.5

qz

Operating Line

=1.0qz=.908

az

0.2

0.0

-0.2

-0.4

-0.6

0.4

+

+

+

-

-

Highm/z

Lowm/z

Res. Ejectionat z=2/3

QITMS Parameters

• MALDI

• EI

• ESI

Small, cheap, ubiquitous.

Ion trap instrument

Resolution typically 1000, mass accuracy typically 0.1%

Sensitivity depends on the source. Typically in the 100 fmol range.

MSn compatible

Operates in 10-4 mbar Helium.

Ion Molecule Reactions (e.g. gas phase H/D Exchange) Why is this problematic?

QITMS CalibrationEquationm = AV/r2f2

Quadrupole MS Fundamental Limitations

Resolution:

homogeneity of the electric field (charging of the electrodes, or inaccurate machining distorts this)

scan speed

Sensitivity:

scan speed

ion transfer efficiency

Mass range:

limited on high end by size of trap and potentials available

limited on low end by stability diagram

Octopole ion guide/trap

Octopole ion guide/trap

Hexapole ion trap

Fourier Transform Mass Spectrometer

• MALDI

• EI

• ESI

Big, expensive, but superior performance.

Ion trap instrument

Resolution typically >50000 broadband, >1,000,000 narrowband

Mass accuracy typically 1 ppm internally calibrated 5-10 ppm externally calibrated

Sensitivity depends on the source. Typically in the 100 fmol range.

MSn compatible

Ion Molecule Reactions (e.g. gas phase H/D Exchange)

Electrospray FTMS

Actively Shielded 7T Superconducting

Electromagnet

Turbo pump

Turbo pump

Turbo pump

Electrospray Ion Source

RF-only QuadrupoleIon Guide

CylindricalPenning Trap

How Does FTMS Work?

VqtrapVftrap

Vinner-rings

ORSKQ0IQ1STQ1IQ2

Q2

IQ3GR

Gate Valve(ground)

Shutter RNG

RF-Only Hexapole

ESI qQq-FTMS Diagram

How Does FTMS Work?

The Penning Trap

The ions’ view of the cell

How Does FTMS Work?

+

-

Ions are trapped and oscillate with low, incoherent, thermal amplitude

Excitation sweeps resonant ions into a large, coherent cyclotron orbit

Preamplifier and digitizer pick up the induced potentials on the cell.

How Does FTMS Work?

600 800 1000 1200 1400 1600m/z

RF Sweep

Transient Image current detection

Mass Spectrum

FFT

10 MHz 10 kHz

RP f≅ •t/2Sensitivity f•t

Calibrate

High Resolution (~50,000 FWHM)

High mass accuracy (~1 ppm)

High sensitivity (femtomoles)

Good FTICR review article

Effect of transient duration

700 800 900 1000 1100 1200 1300 1400 1500

700 800 900 1000 1100 1200 1300 1400 1500

1080 1090 1100

700 800 900 1000 1100 1200 1300 1400 1500 1600

[M+2H]2+

Beta Casein Tryptic digest, 2 pmol/ul

T15

y7* y8y9

y10

y11

X

X

XX

Xy12 y13 y14b7

b8

*

b9 b10 b11***b12

Y132+

?1+

MS

Isolation

MS/MS

FTMS Calibration Equation

Theory: ω± = ωc/2 ± (ωc2/4 – 2eVα/ma2)1/2

Practice: m = A/f + B/f2 + C m = A/(f-B-CV-DI)

ωc = qB0/m

1. Zhang, L. K.; Rempel, D.; Pramanik, B. N.; Gross, M. L. Accurate mass measurements by fourier transform mass spectrometry Mass Spectrom Rev 2005, 24, 286-309.

FTMS Fundamental Limiting Factors•Resolution

•Pressure

•Magnetic field (strength and homogeneity)

•Electric field (homogeneity)

•Space charge

•Sensitivity

•Preamplifier Noise

•Magnetic field strength

•Space charge

•Mass range

•Magnetic field

•Frequency performance of electronics

A new instrument – the orbitrap

Self Assessment

• In TOF-MS, which ions arrive at the detector first? Why?

• In a QIT, what q-value corresponds to the low m/z cutoff in RF-only mode?

• What part of the Mathieu stability diagram is used in mass filtering mode in a quadrupole or QIT?

• In FTICR, doubling the detection time will result in what change to the resolving power? Doubling the magnetic field will result in what change?

Fini…

CH908: Mass spectrometryLecture 6 – Mass Analyzers