Some basic concepts on ionmobility in gas phase

Pietro Traldi

CNR-ISTM Padova

E

+ V

ions

Molecular density and mean free path

Vacuum rangePressure in hPa (mbar)

Molecules /cm3 Mean free path

Ambient pressure

1013 2.7 × 1019 68 nm[4]

Low vacuum 300 – 1 1019 – 1016 0.1 – 100 µm

Medium vacuum

1 – 10−3 1016 – 1013 0.1 – 100 mm

High vacuum 10−3 – 10−7 1013 – 109 10 cm – 1 km

v = K Ev = drift velocityE = electrical field intensityK = ion mobility

v = s/tt = s/v

t = s/KE

K =(3e/16Nσ) (2π/µkBT)1/2

Ion mobility depends from:

- the ion charge e - the number of molecules of buffer gas volume N- the collision cross section σ- the reduced mass of buffer gas (M) and ion (m): µ=Mm/(M+m)- the Boltzmann constant kB- the buffer gas temperature T

Some definitions:

Reduced ion mobility

K0= LP273 / tDE760TWhere tD is the drift time, L the length of the drifttube, P the pressure, E the electric field strength and T the temperature

Collision cross section

N

TPL

EtmmTk

ze DBIb

1273

7601116

18 21

21

21

Spread of identical ions due to diffusion(space-charge and scattering effects)

∆x = (4kBTL/πEe)1/2 = (4kBTL2/πVe)1/2

t/ ∆t = L/ ∆x = (πVe/4kBT)1/2

Resolution

Ion mobilityTo perform an ion mobility experiment,ions are introduced into an atmospheric pressure region(called “drift tube”) across which an electric field is uniformelyapplied.

The uniform field is generated by connecting a series of evenlyspaced rings with equal value resistors.

GATE

To Detector

From IonSource

Electric Field

Low Mobility Ion

High Mobility Ion

Conventional ion mobility spectrometers

GATE

Protein conformation

Measuring the ion mobility of an ion can yield information about its structure as small, compact, ions drift quicker than large extended ions

+V -V -V +V

1 2 1 2

Applied potential: V cosωt

+V -V -V +V

1 2 1 2

The passage of an ion through electrode 2 depends on:i) Its mass, charge and cross sectionii) Intensity of Viii) Frequency ωiv) Distance between 1 and 2v) Pressure and nature of the gas and temperature

Flexibility

• Fragmentation can be induced in both TRAP and TRANSFER T-WAVES

• The system can operate in both Mobility-Tof and Tof only mode

Synapt HDMS system

Passato prossimo - presente

m/z1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

%

0

100

ESI Mass Spectrum of -Lactalbumin

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Trap Injection Voltage5 V

m/z

1000

1500

2000

Drift Time

Excitation in Trap T-Waveresults in a more open conformation

High efficiency ion mobility

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Trap Injection Voltage25 V

m/z

1000

1500

2000

Drift Time

A didactical view of the difference existing between tandem mass spectrometric experiments performed by QQQ (left side) or by resonant excitation in an ion trap (right side).