Instrumental Analyses

The detection limits for organic compounds by GC-MS have decreased greatly since the initial studies and are currently in the 1 ng or less per compound range. The extracts or fractions can be analyzed by temperature-programmed capillary GC. However, with current instrument capabilities all samples, total extracts or fractions, can be analyzed directly by capillary GC-MS using benchtop quadrupole or ion trap mass spectrometers.

Samples that contain wax esters (plant wax) or triglycerides (e.g., beef fat from grilling) can also be analyzed by high-temperature GC and high-temperature GC-MS using custom-made or metal-covered capillary columns coated with 0V-170-0H and GC oven temperatures to 400 °C [169]. All compound identifications should be based on comparisons with authentic standards whenever possible, GC retention time, literature mass spectra and interpretation of mass spectrometric fragmentation patterns. Homologous compound series should be defined with regard to carbon chain length by key ion searches on the GC-MS data and confirmed by mass spectrum and retention time comparison with standards.

The development of compound-specific stable carbon (also hydrogen and nitrogen) analysis (compound-specific isotope analysis) by GC combustion isotope ratio MS provides a secondary parameter in molecular characterization besides the chemical structure or elemental compositions [98,179,180, 231]. Any organic fraction that is amenable to GC analysis can be analyzed by this method. Compounds can also be measured as derivatives (e.g., methylated or trimethylsilyated), and a correction made for the isotope ratio of the derivatizing group. This is done by determining the isotope composition of suitable underivatized standards by both combustion analysis and then analyzing the standard mixture suitably derivatized versus the Pee Dee belemnite (PDB) standard for carbon, standard mean ocean water (SMOW) for hydrogen, and N2 in air for nitrogen.

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