Time-of-flight mass spectrometer TOF MS

Principles, developments and applications

IntroductionTime-of-flight (ToF) mass spectrometers combine a relatively simple mechanical setup with extremely fast electronic data acquisition. Using TOF mass spectrometers, mass resolutions (m/dm)of up to 10 000 and a mass range of up to 500 000 Da together with an ion transmission of morethan 10% and large acceptance volumes is possible. Most ToF mass spectrometers employ ion detectors using multi-channel plates which have a time response < 1 ns and a high sensitivity (single ion signal > 50 mV).

The most simple mechanical construction:-easy coupling with many ion sources (electron impact, chemical ionization, SIMS, laser ablation, MALDI)-miniaturized TOF analysers have been built (of 12- 20 cm long and dia. 5 cm).

The advantage of ToF analyzers is their unlimited mass range for a sensitive analysis of large biomolecules or clusters of up to several millions of daltons and their ability to rapidly obtain mass spectra relevant for the fast analysis of transient signals. The latter property of analyzing fast transient signals is of significance for the application of coupling techniques such as gas chromatography mass spectrometry (GC-MS) or capillary electrophoresis.

Principles

D

Origin of TOF

Stephens(1946): idea of time of light instrument (US patent)Cameron, Eggers(1948): ion velocitron, experimental setup

Ion velocitron

First spectra

TOF MS: constant impuls constant kinetic energy

mv = q0ES d t0

te

∫ = q0ESte

v =q0

mESte und tD =

mq0

DESte

When the extraction field will only switchedon for a short time the ions will beaccelerated only in a short time te and willpossess a similar moment:

With tD time-of-flight and D the distancefrom the grounded grid to the detector.

Stephens, (Phys. Rev. 69 (1949) 691)

W = q0ES d s0

s0

∫ = q0ESs0 = q0U0

v = q0

m2ESs0 und tD = D m

q0

12ESs0

At relatively long extraction field pulseIons leave the acceleration distancebefore end of the extraction pulse andwill possess similar kinetic energy.

Wolff, Stephens (1953): ions were accelerated to constant momentum flight times produced a linear mass scale

Keller (1949) ions were accelerated to constant energy eV resulting in flight times across the drift region proportional to the square root of mass

Example

Mass calibration, mass resolution• Mass calibration: the measured arrival times of all ions provides a time

spectrum that is converted into a mass spectrum by calibrating the instrument. A generally accepted calibration equation is:

• Mass resolution: in TOF–MS, mass resolution is related to the temporalwidth of the isomass ions packet when that packet arrives at the detector. In the ion source, ions are accelerated out of the source region with inherent dispersion in time (instant of ion formation), space (location of ion at the time of acceleration), and velocity (owing to differences in the initial kineticenergy of ions). These are the three primary factors that limit the resolution in a TOF instrument.Mass resolution of TOF analyser can be determined from the formel

m/dm=t/2dt dt is time dispersion of isomassof ion packet, the width of mass peak at its half intensity.

TOF MS principles: ideal case

Ions formed in the gas phase by electron impact or MPI are generally located in the center of the source

In the source region, the electrical field E is used to accelerate ions to constant energy.

The drift region is field free and is bounded by an extraction grid and a second grid placed just before the detector

Ions cross this region with velocities that are inversely proportional to the square root of their mases. Lighter ions have higher velocities and arrive at the detector sooner than heavier ions

50-400 cmFew cm

10-200 µs

Time of ion flight• Ions can be formed at different

distances s in the source region• they posses thermal and spherical

velocity distribution

• Ions spend some time in the source region, where are accelerated to their kinetic energies

An ion entering field free region possess the kinetic energ which is a function of his positionin the source, strength of the electric field and ist initial kinetic energy

The flight time of the ion can be now calculated in different regions of mass analyser derived from the previous equation

detectorextractiongrid

The flight time in the source extraction region is t =s/v and is obtained by the integration between the time of ion formation t0 and the time that ion leaves the source ts

The final kinetic energy depends upon the initial kinetic energy and position

Contribution of turn around time

In the drift tube the velocity is constant:

Uncertainty of the time formation

The total ion flight time is:

The initial kinetic energy and position effect the flight times in both the extraction and drift regions, while the time around time effects only the extraction region

Effect of the initial kinetic energy distribution

Turn-around time (Umkehrzeit)

tsur =2m

q0ES

Uth + q0ESs0 − Uth( ) und

tssu=2m

q0ES

Uth + q0ESs0 + Uth( )

tu = tsur − tssu= 22mUth

q0ES

t3

t2t3t2

Minimizing ion spread

tTOF = 2mUth + q0ESs ± Uth

q0ES

+D2

1Uth + q0ESs

For ions at s diff. than s0:

To make minimal time spread:

For D = D‘, TOF-MS is space-focused and mass resolution does not depends on the initialposition of ions.

dd s

tTOF = 0 = 2m 12

1Uth + q0ESs

−D4

q0ES

Uth + q0ESs( )3 2

⇒ D ' = 2 s0 +Uth

q0ES

≈ 2s0

Space focusing of the first order

Location of the space focus plane is independent of mass, while ions of different mass are focused at this plane at different times

Mass dispersion at this short distance from the source is usually not sufficient to allow a detector to be placed at this point. The time of flight would be to short for achieving a good mass resolution.

3-12 mm

Mass resolutionm = kt 2; d m = k2t d t

md m

=kt 2

k2t d t=

12

td t

⇒ m∆m

=12

t∆t

= R =12

tTOF

tu + ∆ts + ∆ti

R =14

D + 2s0

2s0Uth

q0ESs0

+∆ss0

s0

2−

D4

+ ∆ti

q0ESs0

2m

für D ≤ D '

D + 2s0

∆ss0

s0

2−

D4

+

Uth

q0ESs0

s0

2−

D4

− s0

Uth

q0ESs0

+ ∆tiq0ESs0

2m

für D > D '

The mass scale follows a square-root law regardless of the relative sizes of the extraction and drift regions, or whether any other accelerating or decelerating regions (multiple stage extraction, Einsel lenses, reflectrons) are utilized.

Simplification

R =12

tTOF

tu

=12

2m2

Uth + q0ESs0

q0ES

+D2

1Uth + q0ESs0

2mUth

q0ES

R =Uth + q0ESs0 +

D2

q0ES

Uth + q0ESs0

4 Uth

R ≈14

q0ESs0

Uth

1+D

2s0

für q0ESs0 >Uth

One can approximate ∆t~ tu:

Two-field TOF MS

Design offers more flexibileapplication

First order space focusing for two-field extraction TOF MS

Space focus plane

It is located in the drift region at the point at which ions have spent an equal time in the extraction and drift regions

Principle of space focusing

Dual stage extraction and space focussing

Location of the focusing plane can be shifted much further from the source by using dual stage extraction; E1 is much larger than E0.

First order space focussing can be achieved at almost any distance from the source.

Second order focussing is possible for unique combinations of source and extraction fields and geometries.– Accomodate broader initial spatial distributions– Optimal conditions of the reflectron

Boesl et al. J. Mass. Spectr. Ion Processes, 112 1992, 121-126Weinkauf et al Z. Naturforsch., A, 44 1989, 1219

„Tricks“ applied for improvements of mass resolution

• Using very high accelerating voltages:

– No dependence on flight tube length, initial kinetic energy spreads play small rule– A shorter flight time is disadvatages

• Time lag focusing

Fragmentation and mass resolution

Indistinguishable in time and locationfrom the ionization event; prompt ion formation

Most detrimental to the mass spectralResolution; tailing of the molecular ion peak and increases baseline noise

Decomposition in the drift regionSimilar flight times (reflectron can be helpful in the detection

Some typical applications

Ion sourceCold cathode discharge ion source

-

U

MCP

-V

Ion source

Cn-

Is combined with TOF MS

TOF MS in spectroscopy

• Electronic transitions of neutral species

• Electronic transitions of negatively charged species: photodetachment

• Photoelectron spectrocopy (neutral, anions)

• Electronic transition of cations: photofragmentation spectroscopy

Carbon bearing molecules generated in discharges (Acetylene+Ar)

– odd-even intensity alternation– magic numbers 3, 5, 10, 44, 50, 60, 70

chains, cyclic, fullerenes

4460

1443

C24HM-

U

C7

C7–

λ1 scanned

C7– *

λ2 fixed

C7 + e–

Photodetachment experiment

C7–

λ1 λ2C7–

Ref. anions

Neutrals

Resonantly-EnhancedTwo-Color Detachment Spectroscopy

EA

Photodetachment spectrum of C7-

One-color photodetachment

Ne-matrices

Two-color photodetachment

Rotationally resolved spectra of C3-

Reflectron Time of Flight MS

Mass resolution

• Time:– Usage of short laser pulses– Pulsed extraction with very fast rise time and high extraction pulses

• Space:– Forming ions on surface or in the gas phase with a tightly focused photoionizing

laser beam– Dual stage extraction optics: pushing the space-focus plane down the flight tube

toward the detector

• Initial kinetic energy distributions: most difficult factor to deal with

– Using high extraction fields --- eV >> U0

No combination of static electrical fields for ion extraction can provide simultaneous focusing of the spatial and kinetic distributions!!!!!

Reflectron: ion mirror

Reflectron:

• Reflector compensate kinetic energy distribution of ions after they have entered the flight tube.

• It generally involves reflection of the kinetic energy distribution back upon itself by reversing the direction of ion motion.

• Reflectron does not correct or reduces the kinetic energy spreads; it ensures that ions with differing energies but the same mass will arrive at the detector at the same time.

• Reflectron reproduces the ion packet (∆t) at the source or space focus plane, but at longer distance and at a later time (t), so the mass resolution could be improved

m/∆m=t/2∆t !!! t increasesDt is reduced

Calculating tTOF

Time of flight in field free regionL1+L2=L

L1

L2

Time of flight in deccelarating fielddT

Time of flight in turn-around fielddK

with

F(k) shows the dependence of the flight time on the ion kinetic energy:

and and

Time focusing of the reflectronThe parameters of the analyser are noe chosen to minimize the energy spread:

UT and UK/dK can be now calcualted and set for the reflectron

Mass resolution for reflectron type TOF analyser

Corrections of the ion packet time spread and enhancement of the Flight distance result in improvements of mass resolution.Typically few thousand but the experiments show the spectra recorded also with resolving power of ~40 000

Contribution of the various time spreads vs ion mass

The Mamyrin Reflectron 1973

Mass resolution 3000

Details of the first reflectron

• Ions were formed by electron impact

• Extraction of ions occurs by application of a dual-stage system, first grid was pulsed

• Es/Ed ratio was reduced to produce a space-focus plane close to the source:

– Space-focus plane serves as a suitable origin for the reflectron system– Expansion of the ion packet in space and time is minimized at shorter space-focus plane

• Two linear drift regions L1 and L2

• Reflectron: set of the grids kept at increasing voltage

• Ions are deflected at 2° in order to locate the detector close to the source region.

– dT =0.001(L1+L2), UT=0.7 Uo – a retarding region: 2/3 of the average eUo was reduced– dK =0.06L, UK=0.45 Uo– a reflecting region

Single and double stage ion mirror

The penetration depth can be set at some distance of the reflectron depth by setting the voltage at the back of the reflectron to some value greater than the initial accelerating voltage at source backing plate.

The focusing action of the reflectron: the penetration depth varies with ion kinetic energy, while the drift lengths (L1 and L2) are constant.

Within a narrow range, the flight times become independant of the initial kinetic energy U0. The best resolution is achieved when L1+L2=4d

It is composed of two linear retarding voltage regions, separated by additional grid

The field strength is greater in the first region

L1+L2>>4d -> dual stage reflectrons are smaller with respect to the flight tube length than the single-stage designs

Disadvatages to dual-stage reflectrons: is thatt he addition of second grid; the transmission losses can result from ions striking the grid wires.The close spacing of two grids having very different votages results in high local electrical fields in the vicinity of the grid wires that produces additional ion deflection and scattering

Ideal reflectrons;quadratic reflectron mass spectrometer

• It provide kinetic energy correction to infinite order

• One can show that thie voltage across the reflectron length x can be described as Vx=ax2

• A mass scale is independant of kinetic energy and

t=km1/2

R-TOF MS: comercial systems posses both linear and reflectron stage options

Ion gating

• Kick out to large signals• Improves resolution

Examples:ICP source coupled with Ref-TOF MS

Examples

Mass spectrometer in Phobos mission Oct. 2011

Laser ablation mass spectrometerfor elemental analysis

Prototype of Laser Mass Spectrometer (LMS) for the Mercury Lander

Ion mirror

Detector unit

Sampling point

Prototype

Laser unit with electronics

High-voltage electronics

Thermal enclosure

Flight Design

160

mm

Mercury Rover

Phobos-Grunt

• Collect soil samples from Phobos and possibly from Mars and return them to Earth for scientific analysis

• Explore Phobos, Mars, and Martian space• In situ and remote studies of Phobos, to include

analysis of soil samples• Monitoring the atmospheric behaviour of Mars,

including the dynamics of dust storms• Studies of the vicinity of Mars, to include its

radiation environment and plasma and dust

Pfragmentation studies of mass selected ion