Demonstration of a Nested Electrostatic Linear Ion Trap for Flexibility in Selecting Analyzer Figures-of-Merit

Abstract

An electrostatic linear ion trap (ELIT) is used to trap ions between two ion mirrors with image current detection by central detection electrode. Transformation of the time-domain signal to the frequency-domain via Fourier transform (FT) yields an ion frequency spectrum that can be converted to a mass-to-charge $\left(m/z\right)$ scale. Injection of ions into an ELIT from an external ion source leads to a time-of-flight ion separation that ultimately determines the range of $m/z$ over which ions can be collected from a given ion injection step. The $m/z$ range is determined both by the length of the ELIT and by the distance of the ELIT entrance from the ion source. A longer ELIT leads to a wider $m/z$ range while a shorter ELIT, under equivalent conditions, leads to higher resolving power due to increased ion frequencies. Hence, there is an inherent trade-off between the two important analyzer figures-of-merit of $m/z$ range and resolving power based on the length of the ELIT. In this work, we demonstrate a nested ELIT arrangement, referred to herein as an NELIT, that allows for the selection of one of two possible ELIT lengths within a single array of plates while employing a common detection electrode. While a range of ELIT lengths are possible, in principle, the geometry described herein leads to an effective length ratio of 2.40 for the two traps in the NELIT.

Keywords: electrostatic linear ion trap; enhanced resolving power; mass range; mass spectrometry; nested electrostatic linear ion trap.

Figure 1.

Schematic representation of a Fourier transform nested electrostatic linear ion trap- (NELIT) instrument. The outer trap lenses are depicted in green while the inner trap lenses are depicted in orange. The pressures indicate the base pressures in the relevant regions. The pressure in the analyzer region is 8.9 × 10⁻¹⁰ - 2 × 10⁻⁹ torr during the analysis because the base pressure is not fully reached after the pulsed admission of the bath gas to the trapping quadrupole.

Figure 2.

Mass spectrum ions derived from a mixture of PDCH and PFMD generated from a 2 ms pulse of an ASGDI source and stored in the outer trap of the NELIT. The most abundant ions correspond to [PFMD]^−•, [PFMD-F•]⁻, [PFMD-CF₂]^−•, [PFMD-CF₃•]⁻, [PDCH]^−•, [PDCH-F•]⁻and [PDCH-CF₃•]^.−. At $\text{m/z}<\text{200}$ second and third harmonics of these ions can be observed.

Figure 3:

Mass resolving power ($\left(m/z\right)/(\Delta \left(m/z\right)$, FWHM) as a function of acquisition time. Replicates of n = 4 – 8 for each time point were collected. Horizontal demarcations indicate resolving power at FWHM for 200 and 400 ms transient trapping times for both inner and outer traps.

Figure 4.

Anions derived from PDCH from the inner (orange) and outer (green) traps on the frequency scale using a 200 ms acquisition time. (top) Peak shapes of the PDCH molecular anions stored in the inner (orange) and outer (orange) traps.

Figure 5.

Schematic diagram showing the NELIT from the point of release of ions from the trapping quadrupole, $\text{ML5}$. The red arrow shows the relevant distances for determining the $m/z$ ratio range for the inner trap and the blue arrows show the relevant distances for determining the $m/z$ ratio range for the outer trap. The table summarizes the relevant distances measured by collecting mass spectra of PDCH as a function of gate time, ${\text{t}}_{\text{g}}$.

Figure 6.

Upper (dashed lines) and lower (solid lines) limits to the $m/z$ ranges for the outer (green) and inner (orange) traps of the NELIT as a function of ${\text{t}}_{\text{g}}$ as determined using relations (20) and (21).