2.2.4. Detection and analysis of volatiles
The various types of GC detectors provide different information about volatile compound identity, emission rates, and activity (Table 1; Tholl et al., 2006; D’Alessandro and Turlings, 2006; Skoog et al., 2007).
- Mass spectrometry (MS) detection is used to provide information about peak identity and molecular mass.
- Electron ionization (EI) mass spectrometry is used to identify unknown compounds based on the comparison of their mass spectral fragment patterns with the fragment patterns of chemical standards found in mass spectra software libraries (i.e. NIST, WILEY, and other commercial libraries).
- Chemical ionization (CI) mass spectrometry is a much milder ionization technique than EI for mass spectrometry that provides information about the molecular mass of sample compounds. It is an excellent technique to quantify known compounds in complex mixtures, especially when combined with selected ion monitoring (SIM) to filter out all ions (m/z) except the selected ions from the chromatogram.
- SIM can also be used with EI ionization if characteristic mass fragments are known for the compounds of interest (Tholl et al., 2006).
Unfortunately, EI-MS is less accurate for quantification due to differences in ionization rates between compounds. This is even more pronounced with SIM and CI where quantification is impossible unless standard curves are generated separately for each compound. For accurate compound quantification, detection methods are available that combine great sensitivity with linear responses over a wide range of concentrations (Table 1; Skoog et al., 2007). Detectors differ considerably in their sensitivities and biases toward various compounds (Núñez et al., 1984). Flame ionization detection (FID) is a technique commonly used for quantification because of its sensitive and relatively unbiased detection of organic compounds. However, FID often displays notably less sensitivity to some oxygen, nitrogen, and sulphur-containing compounds. Other less common, specialized ionization detectors have been developed to provide sensitive detection of these heteroatom-containing compounds. Researchers should generate standard curves of synthetic chemical standards to test the response of detectors to their compounds of interest over a range of concentrations.