ABSTRACT
The term ion mobility spectrometry (IMS) refers to the principles, methods, and instrumentation for characterizing chemical substances on the basis of velocity of gas-phase ions in an electric field.1 Such principles are simple in concept and, in practice, offer users convenience, high-speed analyses, portability of instrumentation with high reliability, and comparatively low cost of operation. In traditional IMS used for modern analytical measurements, a bundle of ions, known as a swarm, is introduced into a voltage gradient or electric field (E, in units of V/cm). The ion swarm attains a constant velocity through the electric field, called the drift velocity (vd, in units of cm/sec), at ambient pressure in a gas, usually air (Figure 1-1). This velocity is proportional to the electric field strength as in Equation 1-1:
vd
= KE (1-1)
The proportionality coefficient, K, is termed the mobility coefficient of the ion in units of cm2V
−1sec
−1. This relationship is valid only for the ion swarm and not for the speed of individual ions. In air at ambient pressures, swarms of ions between 14 and ~500 amu exhibit velocities of 1 to 10 m/sec in electric fields of 150 to 300 V/cm at temperatures from 25 to 250
°C. Calculated mobility coefficients of such drift velocities are 0.8 to 2.4 cm2 V
−1sec
−1 and are usually normalized to 273 K and 760 torr, yielding a reduced mobility (Ko) as shown in Equation 1-2:
Ko
= K (273/T)(P/760) (1-2)
where T is temperature in Kelvin and P is pressure in torr of the gas atmosphere through which the ions move.
