U.S. Department of Energy

Pacific Northwest National Laboratory

Structures for Lossless Ion Manipulations Device as an Ion Mobility Filter

Introduction

 

Structures for Lossless Ion Manipulations (SLIM) allow confining and manipulating ions utilizing a combination of radio frequency (RF) and direct current (DC) fields or traveling waves (TW). TW can be employed in SLIM devices to separate ions based on their mobility. We have been exploring concepts for the continuous filtering of ions for the selection of specific and narrow mobility ranges. Such a device would be an IM analog of a e.g. quadrupole mass filter. In this presentation we show the supporting simulations and experiments demonstrating the filtering capability of the SLIM device.

 

Methods

 

The SLIM filter module (30.5 cm) was designed having two parallel arrays of electrodes, namely the rung and guard electrodes. Ions are confined laterally by the applied DC voltage to the guard electrodes, while confined between the surfaces by effective potentials created by applying alternating 1800 out of phase RF voltages. In the current design, ions are guided by a combination of TW and opposing DC drift fields. The SLIM was segmented into two mirror-image sections where the TW and opposing DC are applied. By choosing the suitable combination of DC gradient and TW parameters for the two sections, it is possible to transmit ions of certain mobility while filtering out other ions.

 

Preliminary data

 

In this presentation, we demonstrate a SLIM ion mobility filter allowing ions of specific mobility to be efficiently transmitted. Ion trajectories simulations showed the SLIM devices can filter ions according to their mobilities when opposing TW and DC drift fields are combined. By choosing the suitable combination of DC gradient and TW parameters for the two sections, we found it is possible to transmit ions of certain mobility while filtering out other ions. The SLIM filter is operated by combining a positive DC gradient in the first half and a negative DC gradient in the second half of the SLIM. Two TW were used, one moving forward in the first section, with the other moving in the reverse direction in the second section of the SLIM module. The filtering is determined by DC gradient and the TW parameters, such as frequency, amplitude and the sequence (or in other words, the duty cycle of the TW). Experiments show that filtering with minor loss of ions can be achieved by proper selection of TW frequencies. The difference in frequency, frequency window, determines the range of mobilities transmitted through the filter, which can be explained by the relative ion velocity obtained from the applied DC and TW potentials. The sequence of the TW was found to affect the sensitivity of the device. The velocity of the ions due to TW and that due to the DC field were extracted from the simulations. The filtering is due to the opposing effects of the TW and the DC gradient. Those ions whose mobility due to TW is higher than that due to the DC gradient will successfully pass the first section. While in the second section ions having a higher mobility due to DC gradient will be transmitted.

 

Novel aspect

 

A novel high duty cycle SLIM module for efficient ion transmission based on mobility

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