146
M. Talebi et al.
provide similar selectivity to polar biscyanopropyl siloxane stationary phases. However, when employed in the second dimension, an incomparable group-type separation of FAMEs with different carbon numbers and degrees of unsaturation could
be obtained using ionic liquid columns. The degree of orthogonality of ionic liquid columns in comprehensive two-dimensional gas chromatography was evaluated
using a 1-phase GC × GC system. Among the commercial IL columns, SLB-IL111
showed the best results regarding thermal sensitivity and degree of difference over
the tested temperature range [98]. The orthogonality between the two separations is
usually achieved using GC columns of different polarities. The greatest spread of
FAMEs in 2D space can be achieved using a combination of the highest (IL111)
and the lowest polarity (IL59) columns. This configuration was found to be effective for separation and quantification of safflower oil FAs [100]. Using SLB-IL111
in both dimensions along with a capillary tube coated with palladium between
1 D
and
2 D offered exceptional separation of poorly resolved FAMEs that could not be
achieved by conventional GC [58]. Instead of employing two GC columns of different polarities or using different elution temperatures, separation could be performed
by modification of FAMEs’ chemical structure between the two dimensions. All the
saturated FAMEs lie on a straight diagonal line bisecting the separation plane. The
FAMEs with the same carbon skeleton but a different degree of unsaturation, the position of double bonds, or geometric configuration lie on lines parallel to the
1 D time
axis. Thanks to their upper temperature limit of 300 °C, SLB-IL60 and SLB-IL61
were effectively applied in the second dimension of GC × GC to analyze biodiesel
samples [64]. Using the IL phase along with a nonpolar phase provided adequate
orthogonality to identify suspected low-level biodiesel contamination and resolved
semi-polar FAME compounds from the nonpolar and aromatic hydrocarbons. Also,
chemical markers of biodiesel sources were identified and characterized based on
their FAME composition.
Furthermore, a recent generation of inert ionic liquid (iIL) columns was examined for separation of FAMEs [102]. A modified surface treatment of iIL phases
results in improved surface interaction and reduced adsorption toward polar or
active compounds. Although similar retention times with minor selectivity can be
observed using iIL phases, the average tailing factors and peak widths were significantly reduced compared to the corresponding conventional IL columns. Also, a
polar/nonpolar column set was found to provide the best separation of FAMEs.
6.4 Analysis of Essential Oils
Essential oils are major constituents of flavors and fragrances and are widely used
in pharmaceutical, cosmetic, and food industries. The analysis of essential oils is
essential for quality control (purity determination, adulteration, etc.) and the determination of natural or synthetic origin. Essential oils are complex mixtures consisting
M. Talebi et al.
provide similar selectivity to polar biscyanopropyl siloxane stationary phases. However, when employed in the second dimension, an incomparable group-type separation of FAMEs with different carbon numbers and degrees of unsaturation could
be obtained using ionic liquid columns. The degree of orthogonality of ionic liquid columns in comprehensive two-dimensional gas chromatography was evaluated
using a 1-phase GC × GC system. Among the commercial IL columns, SLB-IL111
showed the best results regarding thermal sensitivity and degree of difference over
the tested temperature range [98]. The orthogonality between the two separations is
usually achieved using GC columns of different polarities. The greatest spread of
FAMEs in 2D space can be achieved using a combination of the highest (IL111)
and the lowest polarity (IL59) columns. This configuration was found to be effective for separation and quantification of safflower oil FAs [100]. Using SLB-IL111
in both dimensions along with a capillary tube coated with palladium between
1 D
and
2 D offered exceptional separation of poorly resolved FAMEs that could not be
achieved by conventional GC [58]. Instead of employing two GC columns of different polarities or using different elution temperatures, separation could be performed
by modification of FAMEs’ chemical structure between the two dimensions. All the
saturated FAMEs lie on a straight diagonal line bisecting the separation plane. The
FAMEs with the same carbon skeleton but a different degree of unsaturation, the position of double bonds, or geometric configuration lie on lines parallel to the
1 D time
axis. Thanks to their upper temperature limit of 300 °C, SLB-IL60 and SLB-IL61
were effectively applied in the second dimension of GC × GC to analyze biodiesel
samples [64]. Using the IL phase along with a nonpolar phase provided adequate
orthogonality to identify suspected low-level biodiesel contamination and resolved
semi-polar FAME compounds from the nonpolar and aromatic hydrocarbons. Also,
chemical markers of biodiesel sources were identified and characterized based on
their FAME composition.
Furthermore, a recent generation of inert ionic liquid (iIL) columns was examined for separation of FAMEs [102]. A modified surface treatment of iIL phases
results in improved surface interaction and reduced adsorption toward polar or
active compounds. Although similar retention times with minor selectivity can be
observed using iIL phases, the average tailing factors and peak widths were significantly reduced compared to the corresponding conventional IL columns. Also, a
polar/nonpolar column set was found to provide the best separation of FAMEs.
6.4 Analysis of Essential Oils
Essential oils are major constituents of flavors and fragrances and are widely used
in pharmaceutical, cosmetic, and food industries. The analysis of essential oils is
essential for quality control (purity determination, adulteration, etc.) and the determination of natural or synthetic origin. Essential oils are complex mixtures consisting