Accumulation of Large Ion Populations with High Ion Densities and Effects Due to Space Charge in Traveling Wave-Based Structures for Lossless Ion Manipulations (SLIM) IMS-MS

Abstract

The accumulation of very large ion populations in traveling wave (TW)-based Structures for Lossless ion Manipulations (SLIM) has been studied to better understand aspects of "in-SLIM" ion accumulation, and particularly its use in conjunction with ion mobility spectrometry (IMS). A linear SLIM ion path was implemented that had a "gate" for blocking and accumulating ions for arbitrary time periods. Removing the gate potential caused ions to exit, and the spatial distributions of accumulated ions examined. The ion populations for a set of peptides increased approximately linearly with increased accumulation times until space change effects became significant, after which the peptide precursor ion populations decreased due to growing space charge-related ion activation, reactions, and losses. Ion activation increased with added storage times and the TW amplitude. Lower amplitude TWs in the accumulation/storage region prevented or minimized ion losses or ion heating effects that can also lead to fragmentation. Our results supported the use of an accumulation region close to the SLIM entrance for speeding accumulation, minimizing ion heating, and avoiding ion population profiles that result in IMS peak tailing. Importantly, space charge-driven separations were observed for large populations of accumulated species and attributed to the opposing effects of space charge and the TW. In these separations, ion species form distributions or peaks, sometimes moving against the TW, and are ordered in the SLIM based on their mobilities. Only the highest mobility ions located closest to the gate in the trapped ion population (and where the highest ion densities were achieved) were significantly activated. The observed separations may offer utility for ion prefractionation of ions and increasing the dynamic range measurements, increasing the resolving power of IMS separations by decreasing peak widths for accumulated ion populations, and other purposes benefiting from separations of extremely large ion populations.

Figure 1

Schematic diagram of the SLIM TWIMS-MS platform and the SLIM module arrangement, having three independently controllable TW regions. The red electrodes indicate the location of the gate that can be provided with a DC blocking potential at the end of region B. The ion funnel trap (IFT) was used to either direct ions from an ESI source into the SLIM or to stop ion accumulation.

Figure 2

Arrival time distributions (ATD) for the three peptide precursor ions (at m/z 386, 523, and 558; peaks a-c in Figure 3) and the total ion signals (TIS) for ion accumulation times of (A) 49 ms, (B) 163 ms, (C) 326 ms, and (D) 652 ms using 8 V_p-p amplitude TW. Ions in (A) have insufficient time to reach the gate at the end of region B (see Figure 1); ions in (B)–(D) have peaks due to accumulating close to the gate, with long “tails” yet to reach the gate vicinity. See also Figure 3 for the corresponding full mass spectra.

Figure 3

Mass spectra integrated over the range of arrival times for the data shown in Figure 2 for accumulation times of (A) 49, (B) 163 s, (C) 326, and (D) 652 ms using an 8 V_p-p TW amplitude. The three peptide precursor ions (Kemptide (2+), Angiotensin II (2+), and Neurotensin (3+) at m/z 386, 523, and 558 (peaks a, b, and c) are indicated in A, where they dominate the spectrum. (A) 49 and (B) 163 s accumulation times provided efficient ion utilization and conventional spectra, while longer accumulation times result in decreased relative abundances of the three peptide precursor ions and the appearance of additional ion species resulting from their charge loss or fragmentation (e.g., peaks (d)–(j); see text for additional description). The product fragment ion labels are color coded to match their precursor ions.

Figure 4

ATD for ions accumulated in region B from the ESI of the peptide mixture. TIS is shown for accumulation times of (A) 326 and (B) 652 ms using TW amplitudes of 8, 15, and 20 V_p-p. The smaller ion population sizes evident for larger TW amplitudes is a combined result of ion losses and undetected lower m/z species (see Experimental Section).

Figure 5

ATD on an absolute scale and a normalized scale (insets i and ii to C and D) for Neurotensin (3+) and (2+), m/z 558 and 836, as well as two of its more prominent fragment ions (y,m/z 643 and y,m/z 724; peaks i and f in Figure 3) for different ion accumulation times. For the two shortest accumulation times, 49 ms (A) and 163 ms (B), where space charge effects are minimized, the Neurotensin (3+) charge state (blue) dominates. For longer accumulation times, 326 ms (C) and 652 ms (D), the Neurotensin (2+) charge state (black) and several fragment ions become increasingly prominent, forming peaks with distinct arrival times.

Figure 6

(Left) ATD for accumulation times of (A) 49, (B) 163, (C) 326, and (D) 652 ms for Angiotensin II (2+), m/z 523, a green trace, and three of its fragment ions (b⁵m/z 647, y⁴m/z 676, and b⁶m/z 784). (Right) Normalized ATD for an ion accumulation time of 326 ms and additional storage times of (E) 8, (F) 41, (G) 57, and (H) 163 ms for Angiotensin II (2+) and its m/z 784 b⁶ fragment (black trace).