# IM-MS background

The simplest configuration of IM-MS instrumentation utilises traditional ‘drift-time’ ion mobility spectrometry and consists of a tube located within a vacuum system, filled with a buffer gas, across which a weak electric field is applied, typically in the range of 5-100 V.cm-1. Gaseous ions are formed in the ionisation region, before being steered towards a small orifice at one end of the drift tube, through which they are injected. Ions drift through the chamber due to the electric field, which causes ions to travel in the direction of this field, whilst collisions with the buffer gas slow the ions’ progress.

If however, the energy at which the ions are injected is low enough and the gas pressure sufficiently high the ions will undergo many (>10E6) collisions in the first few mm of the drift cell and they will progress with a constant velocity. The low drift velocity and the high number of collisions means that the ions undergo many rotations on the timescale of the experiment, and hence the measurement achieved is of the rotationally average temperature dependent collision cross section (Ω).

Schematic diagram of the MoQToF. (A) z-Spray ESI ion source; (B) vacuum chamber 1 housing precell hexapole; (C) vacuum chamber 2, new chamber housing the precell Einzel lens (D); drift cell (E); and postcell hexapole (F); this chamber also contains all gas and electric feedthroughs for the drift cell; (G) vacuum chamber 3 housing the quadrupole mass analyzer and hexapole collision cell leading to the orthogonal ToF mass analyzer (not shown).

DT IM-MS is the only form of IM-MS in which the collision cross sections of ions can be determined directly from the drift time. By measuring the drift velocity of an ion and solving eqn (1), it is then possible to determine the collision cross section (Ω), which is buffer gas dependent [37, 38], using eqn (3).

$K_{0}=\frac{3ze}{16N}\left ( \frac{2\pi }{\mu k_{B}T} \right )^{\frac{1}{2}}\frac{1}{\Omega }$

Where K0 is the reduced mobility, z is the ion charge state, e is the elementary charge, N is the gas number density, μ is the reduced mass of the ion-neutral pair, kB is the Boltzmann constant and T is the gas temperature.

A number of individual research laboratories have presented IM-MS instruments which incorporate drift cells over which a linear field gradient is applied. Of particular interest are the remarkable efforts made in the design and construction of DT IM-MS instrumentation by Bowers and Kemper [15, 39, 40], Clemmer [9, 36,41-43], Jarrold [44], Hill [45,46], Barran[20], Smith[47,48] and Russell [49-51].

The main disadvantage of LDT IM-MS is that in order to achieve adequate separation it is necessary to periodically introduce a narrow pulse of ions into the drift region, necessitating a low duty cycle and decreasing the sensitivity of the technique.

Previously, the problems associated with the low duty cycle have been effectively overcome through the use of a Paul geometry ion trap which allowed ions to be accumulated while the previous mobility separation was occurring, before injecting them into the drift cell [52]. The addition of ion funnels to ion mobility instrumentation, both prior to the drift region [39] and within the drift cell housing, has increased the amount of ions that can be successfully transferred through the drift cell without scattering.

Notable efforts have been made recently which allow several pulses of drifting ions to be in the mobility separator at the same time, by then reconstructing the arrival time distributions using Hadamard transforms [53] to improve the sampling efficiencies via TOF-MS [54].