U.S. Department of Energy

Pacific Northwest National Laboratory

Collision Cross Section Calibration with Structures for Lossless Ion Manipulations

Introduction

 

Ion mobility mass spectrometry (IM-MS) is a powerful separation and structural characterization technique, providing the ability to measure collision cross sections (CCS), revealing information about the three dimensional structure of gaseous ions. In many cases, CCS can be used to identify ions in a mixture, and highly accurate and precise CCS measurements greatly expand IM-MS capabilities. Recently, long path structures for lossless ion manipulations (SLIM) traveling wave (TW) IM modules have allowed extremely high resolution IM separations. However, since SLIM do not utilize uniform low-field drift cells, CCS cannot be directly measured from experiments. To that end, we have developed a CCS calibration framework to provide high resolution CCS assignment.

 

Methods

 

Travelling wave potentials and a combination of lateral DC-only electrodes (guards) and extended RF electrodes aligned with the ion path provided for TWIM separations in several Torr nitrogen in conjunction with efficient ion confinement. Ions from nanoelectrospray ionization of mixtures of multiple classes of compounds (e.g. peptide, glycan, lipid) were injected to the SLIM module. A SLIM ion switch controlled whether ions made multiple passes through the serpentine path of the module, or were sent to the TOF MS for analysis. Multiple mixtures of calibrants of different classes overlapping in CCS space with the compounds studied were prepared and infused as both external and internal calibrants. TWIM-MS features were extracted and calibrated using in-house developed software tools.

 

 

Preliminary data

 

Recently, multi-pass SLIM separations have been reported, showing very high IM resolutions and peak capacities for a variety of compounds, including peptides, lipids, and carbohydrates. A SLIM ion switch was positioned at the end of a long (>10 meter) serpentine ion path to allow ions to either exit to a TOF MS for mass analysis, or to be shuttled to the beginning of the ion path for addition separation. Resolutions much higher than that from conventional commercially available instruments (both TW and uniform field) have been achieved (e.g., separation powers of over 1000 for singly charged ions for 200 m multi-pass separations). Due to the abundance of information from bottom-up proteomics of many protein standards (e.g. tryptic peptide accurate monoisotopic MW), the first efforts for applying CCS calibration have utilized whole protein digests. Early results have shown baseline separations of peptides in a protein digest (serum albumin) that are inseparable by conventional IM instruments. Initially, a poly-alanine mixture was used to begin evaluating CCS calibrations for peptides and was used as external and internal calibration standards. The protein digest was then run on an Agilent 6560 IM-MS to compare the calibrated CCS values against values measured directly by a uniform low field instrument. The presentation will detail the efficacy of CCS calibration in SLIM TWIM measurements as well as effects resulting from the choice of calibrant, internal vs. external calibration, and other biological compound classes.

 

 

Novel aspect

 

Development of a CCS calibration framework for long path length ultrahigh resolution SLIM TWIM separations

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