Every drift tube ion mobility spectrometer (IMS) has
an optimum
drift voltage to reach maximum resolving power. This optimum depends,
among other things, on the temporal and spatial width of the injected
ion packet and the pressure within the IMS. A reduction of the spatial
width of the injected ion packet leads to improved resolving power,
higher peak amplitudes when operating the IMS at optimum resolving
power, and thus a better signal-to-noise ratio despite the reduced
number of injected ions. Hereby, the performance of electrospray ionization
(ESI)-IMS can be considerably improved. By setting the ion shutter
opening time to just 5 μs and slightly increasing the pressure,
a high resolving powerRP> 150 can
be
achieved with a given drift length of just 75 mm. At such high resolving
power, even a mixture of the herbicides isoproturon and chlortoluron
having similar ion mobility can be well separated despite short drift
length.