Structural Characterization of Phospholipids and Peptides Directly from Tissue Sections by MALDI Traveling-Wave Ion Mobility-Mass Spectrometry

Abstract

Ion mobility-mass spectrometry (IM-MS) provides rapid two-dimensional separations based on analyte apparent surface area or collision cross section (CCS, Å 2 ) and mass-to-charge, respectively. Recently, traveling-wave (t-wave) IM-MS was developed which uses electrodynamic rather than electrostatic fields commonly used in drift cell IM-MS instruments. The underlying theory for obtaining CCS data is well developed for drift cell IM-MS, while strategies for obtaining CCS values from t-wave IM-MS data remains an active area of research. In this report, methods were developed and validated to obtain CCS values of phospholipids and peptides directly from thin tissue sections by MALDI t-wave IM-MS using CCS calibrants measured by MALDI drift cell IM-MS. Importantly, the average percent difference between t-wave and drift cell CCS measurements is minimized by calibrating with the same biomolecular class. Calibrating t-wave phospholipid CCS values with drift cell peptide CCS measurements results in an average percent difference of ca. 7% between the same lipids measured using t-wave and drift cell IM-MS, while this improves to <0.5% when drift cell phospholipid CCS values are used for calibrating t-wave data. A suite of CCS values are reported for lipids and peptides that were determined directly from tissue, i.e. without the need for tissue extraction and further purification steps.

Commercial Intermediate Pressure MALDI Ion Mobility Spectrometry Mass Spectrometer Capable of Producing Highly Charged Laserspray Ionization Ions

“…The relative drift times of the diagonal lines drawn through the different components in the sample are highly charged protein ions þ6, þ5, and þ4 < multiply charged peptide and protein ions þ3 and þ2 < singly charged matrix ions < singly charged peptide and lipid ions. The separation trend is in accord with previous studies using vacuum MALDI-IMS-MS. [40][41][42][43][44][45] Here, the extent of separation between the highly charged and the singly charged ions is of notable analytical utility.…”

Structural Separations by Ion Mobility-MS for Glycomics and Glycoproteomics

“…Published absolute collision cross sections can be obtained from several databases, including: (i) peptide collision cross sections determined by ESI (78, 79), (ii) intact protein collision cross sections determined by ESI (80) (iii) peptide collision cross sections determined by MALDI (40), and (iv) biologically relevant carbohydrate, lipid, and oligonucleotide collision cross sections determined by MALDI (56). For these comparative measurements, it is necessary to have standards in the same biomolecular class as the samples being measured (81). …”

Assessment of altered lipid homeostasis by HILIC-ion mobility-mass spectrometry-based lipidomics

“…When ion mobility is coupled with mass spectrometry, a two-dimensional separation is achieved on the basis of the CCS-to-charge (/z) and the mass-to-charge (m/z), respectively. Several ion mobility (IM)-MS techniques have been applied to lipidomic analysis, including drift tube ion mobility (DTIM) (25)(26)(27), traveling wave ion mobility (TWIM) (28,29), and differential mobility spectrometry (DMS) (30)(31)(32). In DTIM-MS, the ions travel through the neutral gas-filled drift tube under a low and static electric field, which leads to separation of ions on the scale of micro-to milliseconds (21).…”

“…TWIM offers higher sensitivity than does traditional DTIM and a faster duty cycle, but has slightly lower IM resolution and does not allow direct measurement of CCS values. However, CCS of unknown ions can be indirectly calculated by calibrating against appropriate ions with known CCS values (29). Calibrants that are of similar physical properties to the analytes are desired to achieve the highest accuracy of CCS measurements in TWIM.…”