Ion Mobility Spectrometry–Mass Spectrometry Performance Using Electrodynamic Ion Funnels and Elevated Drift Gas Pressures

The ability of ion mobility spectrometry coupled with mass spectrometry (IMS-MS) to characterize biological mixtures has been illustrated over the past eight years. However, the challenges posed by the extreme complexity of many biological samples have demonstrated the need for higher resolution IMS-MS measurements. We have developed a higher resolution ESI-IMS-TOF MS by utilizing high-pressure electrodynamic ion funnels at both ends of the IMS drift cell and operating the drift cell at an elevated pressure compared with that conventionally used. The ESI-IMS-TOF MS instrument consists of an ESI source, an hourglass ion funnel used for ion accumulation/injection into an 88 cm drift cell, followed by a 10 cm ion funnel and a commercial orthogonal time-of-flight mass spectrometer providing high mass measurement accuracy. It was found that the rear ion funnel could be effectively operated as an extension of the drift cell when the DC fields were matched, providing an effective drift region of 98 cm. The resolution of the instrument was evaluated at pressures ranging from 4 to 12 torr and ion mobility drift voltages of 16 V/cm (4 torr) to 43 V/cm (12 torr). An increase in resolution from 55 to 80 was observed from 4 to 12 torr nitrogen drift gas with no significant loss in sensitivity. The choice of drift gas was also shown to influence the degree of ion heating and relative trapping efficiency within the ion funnel.