INSTRUMENTAL TECHNIQUE

ION MOBILITY DRIFT TUBE

PAPRI CHAKRABORTY02.07.2016

Ion-mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized molecules in the gas phase based on

their size,shape and charge, depending on their mobility in a carrier buffer gas. 

What is Ion Mobility Spectrometry ?

HistoryIMS was first developed primarily by Earl W. McDaniel of Georgia

Institute of Technology in the 1950s and 1960s when he used drift cells with low applied electric fields to study gas phase ion mobilities and

reactions.

 In the following decades, he coupled his new technique with a magnetic-sector mass spectrometer, with others also utilizing his techniques in new

ways. IMS cells have since been attached to many other mass spectrometers and high-performance liquid chromatography setups.

Instrumentation for Ion Mobility Spectrometry

Primarily, three IMS techniques are used in IM-MS: Drift time ion mobility spectrometry (DTIMS)

Travelling-wave ion mobility spectrometry (TWIMS) Field-asymmetric ion mobility spectrometry (FAIMS)

DTIMS TWIMS FAIMS

Francesco Lanucara et.al, Nature Chemistry. 2014, 6, 281-294

Travelling-wave ion mobility spectrometry (TWIMS)

TWIMS DRIFT TUBE A TWIMS device comprises a series of ring electrodes called a stacked ring ion guide (SRIG), to which a travelling voltage is applied.

These voltages radially confine the ions, while application of a transient direct current (DC) voltage to each electrode in succession from one end of the device to the other propels the ions axially.

Radio-frequency voltages of opposite phases are applied to adjacent electrodes.

Schematic of ring electrodes in Drift cell

Perspective view of an IMS Drift tube

RadiofrequencyVoltage Supply

High Voltage DC Power Supply

Segmented electrode

Gate electrodeRing Electrode

Ion Collector

Resistor

CapacitorVoltage Divider

Disc Shaped Electrode

Ion mobility spectrometer with high ion transmission efficiencyUS 200301 32379 A1

U+

Vcos(ω

t)U

-Vcos(ω

t) U, V are amplitudes of respective D.C. and R.F. sources

RF sources frequency Ѡ ~ 10 kHz to 100 MHz

A Direct Current (DC) voltage can be applied to a pair of adjacent rings, to produce a potential barrier that the ions cannot cross. As the DC potential is stepped to an adjacent set of rings the ion barrier moves forward, causing any ions in front of it to be propelled forwards. Depending on the velocity and voltage of the wave, the ions will get separated as more compact ions will move forward and less compact ions will roll over the wave.

Travelling Waves through the Drift Tube

Model traveling wave profiles: triangular (a), bitriangular(b), and half-sinusoidal (c).A. Alexandre et. al, Anal. Chem. 2008,

80, 9689–9699

Mathematical Description of TWIMS OperationThe physical quantity ion mobility K is defined as the proportionality factor of an ion's drift velocity vd in a gas and an electric field of strength E,

Ion mobilities are commonly reported as a reduced mobilities, correcting to standard gas density n0, which can be expressed in standard temperature T0 = 273 K and standard pressure p0 = 1013 mbar :

The ion mobility K can be experimentally determined by measuring the drift time tD of an ion traversing within a homogeneous electric field, the potential difference U in the drift length L :

A. Alexandre et. al, Anal. Chem. 2008, 80, 9689–9699

Determination of Collission Cross section

The recorded drift time of an ion allows calculation of Ω, according to the Mason –Schamp equation :

The proportional relationship between Ω and K0 is only true at or below the ‘low-field limit’, where the ratio between electric field strength and buffer gas density is small. (≤ 2 × 10–17 V cm2).

The TWIMS device is operated below the low-field limit and, following calibration, determination of CCS is therefore possible.

Calibration of the drift time through the TWIMS cell under defined conditions (gas type/pressure, travelling wave speed or height, and so

on) is necessary as the direct relationship between Ω and K0 is no longer applicable, owing to the constantly changing electric field.

Separation of ions in the drift tube

https://www.youtube.com/watch?v=LTLtLsgkgLY

A drift tube’s resolving power RP can be calculated as :

where tD is the ion drift time, ΔtD is the Full width at half maximum.Pressure inside the drift tube is maintained around 0.025 – 3 mbar.

Low pressure results in strong heating even at moderate E. Low gas pressure in TW IMS means strong fields in E/N terms and thus substantial heating of ions. This may cause fragmentation or distortion of macromolecular structures, complicating theirdetection and characterization.CCS determination requires calibration of the drift time through the TWIMS cell. Advantage of TWIMS is that it can be used for

mobility separation of product ions generated either by collision induced dissociation or by electron-transfer dissociation. The geometric configuration of current commercial DTIMS-MS and FAIMS – MS instruments means that they can only be used to

separate analytes immediately post-ionization.

Synapt G2Si Travelling Wave Mass Spectrometer

IMS Cell of Synapt G2-Si 25.2 cm long drift cell 168 electrodes with 0.5 mm thickness and 1.0 mm spacing p ≈ 1-3 mBar IMS Resolution tD/ΔtD ≈ 40