A New, Validated GC-PICI-MS Method for the Quantification of 32 Lipid Fatty Acids via Base-Catalyzed Transmethylation and the Isotope-Coded Derivatization of Internal Standards

Background: Fatty acids (FAs) represent a ubiquitous class of nonpolar alkyl carboxylate metabolites with diverse biological functions. Nutrition, metabolism, and endogenous and exogenous stress influence the overall FA metabolic status and transport via the bloodstream. FAs esterified in lipids are of particular interest, as they represent promising biomarkers of pathological diseases and nutritional status. Methods: Here, we report a validated gas chromatographic-mass spectrometric (GC-MS) method for the quantitative analysis of 32 FAs exclusively bound in esterified lipids. The developed sample preparation protocol comprises three steps using only 5 µL of human serum for Folch extraction, sodium methoxide-catalyzed transesterification in tert-butyl methyl ether, and re-extraction in isooctane prior to a quantitative GC-MS analysis with positive ion chemical ionization (PICI) and selected ion monitoring (SIM). Results: The base-catalyzed transmethylation step was studied for 14 lipid classes and was found to be efficient under mild conditions for all major esterified lipids but not for free FAs, lipid amides, or sphingolipids. To minimize matrix effects and instrument bias, internal fatty acid trideuteromethyl esters (D3-FAME) standards were prepared through isotope-coded derivatization with D3-labeled methylchloroformate/methanol medium mixed with each transmethylated serum extract for the assay. The method was validated according to FDA guidelines and evaluated by analyzing NIST SRM 2378 Serum 1 and sera from three healthy donors. Conclusions: The measured quantitative FA values are consistent with the reference data of SRM 2378, and they demonstrate the application potential of the described method for general FA analysis in esterified lipids as a novel complementary tool for lipidomics, as well as for the analysis of membrane FAs in dry blood spots and red blood cells.

A Validated Method for Quantification of Fatty Acids Incorporated in Human Plasma Phospholipids by Gas Chromatography-Triple Quadrupole Mass Spectrometry

Fatty acids (FA) are important mediators of health maintenance and disease risk. Optimal quantification assays of FA in high and low abundance as well the identification of 13C-labeled tracers to monitor FA metabolism are of major interest. The article on hand reports about the development and validation of a gas chromatography (GC)-triple quadrupole mass selective detection (GC-TQMS) method for absolute quantification of FA in human plasma phospholipids (hpPL). The quantification of the calibration solution by GC-flame ionization detection (GC-FID), with the introduction of a correction factor, allows the direct comparison of individual FA concentrations in hpPL by GC-TQMS. Specificity, sensitivity, and reproducibility are achieved by optimized chromatographic separation and employment of GC-TQMS. The inter-method comparison between GC-FID and GC-TQMS concentrations revealed good comparability for 27 FA. A full validation has been performed with linearity over 4 magnitudes, a limit of detection of 0.18-38.3 fmol on column, a recovery of 83.6-109.6%, and intraday and interday precision data meeting the criteria of EMA and FDA guidelines. The method includes the absolute quantification of 58 positional and geometrical (cis/trans) isomeric FA in hpPL in the concentration range of 1-3000 nmol/mL, covering also low abundant positional cis/trans isomers. Results obtained from both methods are highly comparable, and selectivity and sensitivity are improved by using GC-TQMS. Additionally, we show here that calculation of 13C-labeled C16:0 tracer/tracee ratios in hpPL in human isotope enrichment studies is possible.

PCI-GC-MS-MS approach for identification of non-amino organic acid and amino acid profiles

Alkyl chloroformate have been wildly used for the fast derivatization of metabolites with amino and/or carboxyl groups, coupling of powerful separation and detection systems, such as GC-MS, which allows the comprehensive analysis of non-amino organic acids and amino acids. The reagents involving n-alkyl chloroformate and n-alcohol are generally employed for providing symmetric labeling terminal alkyl chain with the same length. Here, we developed an asymmetric labeling strategy and positive chemical ionization gas chromatography-tandem mass spectrometry (PCI-GC-MS-MS) approach for determination of non-amino organic acids and amino acids, as well as the short chain fatty acids. Carboxylic and amino groups could be selectively labelled by propyl and ethyl groups, respectively. The specific neutral loss of C₃H₈O (60Da), C₃H₅O₂ (74Da) and C₄H₈O₂ (88Da) were useful in the selective identification for qualitative analysis of organic acids and amino acid derivatives. PCI-GC-MS-MS using multiple reaction monitoring (MRM) was applied for semi-quantification of typical non-amino organic acids and amino acids. This method exhibited a wide range of linear range, good regression coefficient (R²) and repeatability. The relative standard deviation (RSD) of targeted metabolites showed excellent intra- and inter-day precision (<5%). Our method provided a qualitative and semi-quantitative PCI-GC-MS-MS, coupled with alkyl chloroformate derivatization.

Simple and rapid determination of unsaturated fatty acids in 1 µl of rat plasma by LC–MS/MS

Background: Deficiency or imbalance of unsaturated fatty acids will promote the pathogenesis of many diseases. In order to monitor the exposure of unsaturated fatty acids, the method based on LC–MS/MS was developed. Results: Standard calibration curves for α-linolenic acid, linoleic acid, palmitoleic acid and oleic acid were linear (r ≥0.99). The intra-and interbatch accuracy (RE%) ranged from -4.5 to 8.6%, while the intra- and interbatch precisions (RSD%) were ≤8.7%. The extraction recovery varied from 85.4 to 99.6%, and no obvious matrix effect was observed. Conclusion: The method offers a simple approach for measuring 4 unsaturated fatty acids in 1 μl rat plasma within 3.95 min.

A rapid multidimensional GC–flame-ionization detector method for determination of fatty acid methyl esters

Background: Fatty acids play important roles in the pathogenesis of many diseases. In order to investigate a wide spectrum of fatty acids in biological matrix, we developed an approach for quantitative analysis of fatty acids based on a heart-cutting 2D gas chromatographic-flame ionization detector system. Results: This system uses a Dean Switch interface between the primary polar and secondary nonpolar capillary column. Most of the standards were separated by the primary column. Unresolved components are selectively transferred to the secondary column, where they are completely separated. This method was further evaluated using plasma obtained from 11 healthy and 11 chronic coronary artery disease patients. Conclusion: Our results prove that this approach is sensitive, precise and specific, capable of measuring 37 specific fatty acids. It indicates that this approach offers a reliable and sensitive technical platform for comprehensive quantification of fatty acids.