Download Some basic concepts on ion mobility in gas phase

Some basic concepts on ion
mobility in gas phase
Pietro Traldi
CNR-ISTM Padova
E
ions
+V
Molecular density and mean free path
Vacuum range
Pressure 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=KE
v = drift velocity
E = electrical field intensity
K = ion mobility
v = s/t
t = 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 / tDE760T
Where tD is the drift time, L the length of the drift
tube, P the pressure, E the electric field strength and
T the temperature
Collision cross section
1

18  2

16
1
ze
1
kbT  2
1
1 
 

m
m
B
 I
2
t D E 760 T 1
L P 273 N
Spread of identical ions due to diffusion
(space-charge and scattering effects)
∆x = (4kBTL/πEe)1/2 = (4kBTL2/πVe)1/2
Resolution
t/ ∆t = L/ ∆x = (πVe/4kBT)1/2
Ion mobility
To perform an ion mobility experiment,
ions are introduced into an atmospheric pressure region
(called “drift tube”) across which an electric field is uniformely
applied.
The uniform field is generated by connecting a series of evenly
spaced rings with equal value resistors.
Conventional ion mobility
spectrometers
From Ion
Source
To
Detector
GATE
High Mobility Ion
Low Mobility Ion
Electric Field
Protein conformation
GATE
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
1
2
Applied potential:
-V
1
V cosωt
+V
2
+V
-V
1
2
-V
1
+V
2
The passage of an ion through electrode 2 depends on:
i) Its mass, charge and cross section
ii) Intensity of V
iii) Frequency ω
iv) Distance between 1 and 2
v) 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
Passato prossimo - presente
Synapt HDMS system
ESI Mass Spectrum of -Lactalbumin
%
100
0
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
m/z
PAP_10.raw : 1
7+
2000
8+
m/z
9+
1500
10+
11+
5V
12+
13+
1000
Trap
Injection
Voltage
PAP_10.raw : 1
PAP_10.raw:1
7+
Drift Time 
P A P _10.raw :1
High efficiency ion mobility
Excitation in Trap T-Wave
results in a more open conformation
PAP_10.raw : 1
7+
2000
8+
m/z
9+
1500
10+
11+
12+
13+
1000
Trap
Injection
Voltage
25 V
PA P_10.raw:1
PAP_10.raw : 1
7+
Drift Time 
P A P _10.raw :1
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).