Development of a Structure for Lossless Ion Manipulations (SLIM) High Charge Capacity Array of Traps

Abstract

Enhancing the sensitivity of low-abundance ions in a complex mixture without sacrificing measurement throughput is highly desirable. This work demonstrates a way to greatly improve the sensitivity of ion mobility (IM)-selected ions by accumulating them in an array of high-capacity ion traps located inside a novel structures for lossless ion manipulations ion mobility spectrometer (SLIM-IMS) module. The array of ion traps used in this work consisted of seven independently controllable traps. Each trap was 386 mm long and possessed a charge capacity of ∼4.5 × 10⁸ charges, with a linear range extending to ∼2.5 × 10⁸ charges. Each ion trap could be used to extract a peak (or ions over a mobility range) from an ion mobility separation based on arrival time. Ions could be stored without losses for long times (>100 s) and then released all at once or one trap at a time. It was possible to accumulate large ion populations by extracting and storing ions over repeated IM separations. Enrichment of up to seven individual ion distributions could be performed using the seven independently controllable ion traps. Additionally, the ion trapping process effectively compressed ion populations into narrow peaks, which provides a greatly improved basis for subsequent ion manipulations. The array of high charge capacity ion traps provides a flexible addition to SLIM and a powerful tool for IMS-MS applications requiring high sensitivity.

Figure 1:

A schematic diagram of the multilevel SLIM and the array of traps. The cyan arrows within the inset depict the path ions take through the levels.

Figure 2:

(A) Leucine enkephalin charges measured through the bypass (control) and after they were stored for 500 ms. (B) A plot of the efficiencies calculated from (A) for a trap to store up to 4×10⁸ charges.

Figure 3:

(A) Signal of leucine enkephalin ions through the bypass and storage durations from 5 to 100 s. (B) The trap efficiency (I_Trapped/I_Bypass). Guard voltage = 10 V, blocking voltage = 102 V, and TW = 0 V.

Figure 4:

Mobiligrams (single unsummed) of negatively charged Agilent tune mix ions acquired with a Faraday detector. The peaks are labeled numerically in the order the ATDs were measured. (A) Reference (no trapping), (B) when peaks 4 and 6 from (A) were stored in the first and second trap nearest the bypass then released after the separation was complete, and (C) when peaks 3 – 7 from (A) were collected in the five traps nearest the bypass then released. (C) The time between the release of ions stored in adjacent traps was adjusted to affect the spacing between the pulses of released ions.

Figure 5:

(A and B) 3D heat maps (unsummed) of a bovine serum albumin (BSA) digest. The gold rectangle encloses the mobility and mass range of digest peptide ions collected in trap 7 for up to 30 collection events. (A) The gold circles highlight the ions having the m/z of 653 and 752. (B) Release of stored ions after their accumulation over 30 collection events. The gold circles highlight the reduced charge state ions (1305 and 1503 m/z). The scales of A and B were truncated for figure clarity. The trap TW was set to 2 V_PP for all collection events.