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A highly sensitive platform coupling capillary ion chromatography (Cap IC) with Q Exactive mass spectrometer has been developed for metabolic profiling of head and neck squamous cell carcinoma (HNSCC) cells. The Cap IC allowed an excellent separation of anionic polar metabolites, and the sensitivities increased by up to 100-fold compared to reversed-phase liquid chromatography and hydrophilic interaction chromatography performed at either high- or capillary-flow rates. The detection limits for a panel of standard metabolites were between 0.04 to 0.5 nmol/L (0.2 to 3.4 fmol) at a signal-to-noise ratio of 3. This platform was applied to an untargeted metabolomic analysis of head and neck cancer cells and stem-like cancer cells. Differential metabolomics analysis identified significant changes in energy metabolism pathways (e.g., glycolysis and tricarboxylic acid cycle). These experiments demonstrate Cap IC/MS as a powerful metabolomics tool by providing enhanced separation and sensitivity of polar metabolites combined with high resolution and accurate mass measurement (HR/AM) capabilities to differentiate isobaric metabolites.

Modern separation methods in conjunction with high-resolution accurate mass (HRAM) spectrometry can provide an enormous number of features characterized by exact mass and chromatographic behavior. Higher mass resolving power usually requires longer scanning times, and thus fewer data points are acquired across the target peak. This could cause difficulties for quantification, feature detection and deconvolution. The aim of this work was to describe the influence of mass spectrometry resolving power on profiling metabolomics experiments. From metabolic databases (HMDB, LipidMaps, KEGG), a list of compounds (41 474) was compiled and potential adducts and isotopes were calculated (622 110 features). The number of distinguishable masses was calculated for up to 3840k resolution. To evaluate these models, human plasma samples were analyzed by LC-HRMS on an Orbitrap Elite hybrid mass spectrometer (Thermo Fisher Scientific, CA, USA) at resolving power settings of 15k (7.8 Hz) up to a maximum of 480k (1.2 Hz). Software XCMS 1.44, MZmine 2.13.1, and Compound Discoverer 2.0.0.303 were used for evaluation. In plasma samples, the number of detected features increased sharply up to 60k in both positive and negative mode. However, beyond these values, it either flattened out or decreased owing to technical limitations. In conclusion, the most effective mass resolving powers for profiling analyses of metabolite rich biofluids on the Orbitrap Elite were around 60 000-120 000 fwhm to retrieve the highest amount of information. The region between 400-800 m/z was influenced the most by resolution.

In this study, we have demonstrated a targeted metabolomics method for analysis of cancer cells, based on high-performance ion chromatography (IC) separation, Q Exactive HF MS for high-resolution and accurate-mass (HR/AM) measurement and the use of stable isotope-labeled internal standards for absolute quantitation. Our method offers great technical advantages for metabolite analysis, including exquisite sensitivity, high speed and reproducibility, and wide dynamic range. The high-performance IC provided fast separation of cellular metabolites within 20 min and excellent resolving power for polar molecules including many isobaric metabolites. The IC/Q Exactive HF MS achieved wide dynamic ranges of 5 orders of magnitude for six targeted metabolites, pyruvate, succinic acid, malic acid, citric acid, fumaric acid, and alpha-ketoglutaric acid, with R(2) ≈ 0.99. Using this platform, metabolites can be simultaneously quantified from low fmol/μL to nmol/μL levels in cellular samples. The high flow rate IC at 380 μL/min has shown excellent reproducibility for a large set of samples (150 injections), with minimal variations of retention time (SD < ± 0.03 min). In addition, the IC-MS-based approach acquires targeted and global metabolomic data in a same analytical run, and the use of stable isotope-labeled standards facilitates accurate quantitation of targeted metabolites in large-scale metabolomics analysis. This metabolomics approach has been successfully applied to analysis of targeted metabolites in head and neck cancer cells as well as cancer stem-like cells (CSCs), and the findings indicate that the metabolic phenotypes may be distinct between high and low invasive head and neck cancer cells and between CSCs and non-SCCs.

Data mining was performed on 28 330 unique peptide tandem mass spectra for which sequences were assigned with high confidence. By dividing the spectra into different sets based on structural features and charge states of the corresponding peptides, chemical interactions involved in promoting specific cleavage patterns in gas-phase peptides were characterized. Pairwise fragmentation maps describing cleavages at all Xxx-Zzz residue combinations for b and y ions reveal that the difference in basicity between Arg and Lys results in different dissociation patterns for singly charged Arg- and Lys-ending tryptic peptides. While one dominant protonation form (proton localized) exists for Arg-ending peptides, a heterogeneous population of different protonated forms or more facile interconversion of protonated forms (proton partially mobile) exists for Lys-ending peptides. Cleavage C-terminal to acidic residues dominates spectra from singly charged peptides that have a localized proton and cleavage N-terminal to Pro dominates those that have a mobile or partially mobile proton. When Pro is absent from peptides that have a mobile or partially mobile proton, cleavage at each peptide bond becomes much more prominent. Whether the above patterns can be found in b ions, y ions, or both depends on the location of the proton holder(s) in multiply protonated peptides. Enhanced cleavages C-terminal to branched aliphatic residues (Ile, Val, Leu) are observed in both b and y ions from peptides that have a mobile proton, as well as in y ions from peptides that have a partially mobile proton; enhanced cleavages N-terminal to these residues are observed in b ions from peptides that have a partially mobile proton. Statistical tools have been designed to visualize the fragmentation maps and measure the similarity between them. The pairwise cleavage patterns observed expand our knowledge of peptide gas-phase fragmentation behaviors and may be useful in algorithm development that employs improved models to predict fragment ion intensities.