6 Gas Chromatography Columns Using Ionic Liquids …
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of a large number and variety of compounds with different physicochemical properties. The compounds include hydrocarbons (monoterpenes, sesquiterpenes, and aromatics), oxygenated compounds (aromatics, phenols, alcohols, sesquiterpene alcohols, aldehydes, ketones, esters, lactones, ethers, oxides, etc.), and other compounds.
Most of the compounds are isomers or analogs of each other and generate the same
mass spectra. Hence, chromatographic separation of these compounds is important.
The separation demands stationary phases with broad selectivities. Traditional wax
(polyethylene glycol-based) and cyanopropyl polysiloxane-based phases are used for
the analysis of essential oils. However, these stationary phases produce coelutions
of many of the compounds. Multidimensional GC can also be used for the analysis
of such complex mixtures, but such approaches involve operational complexity and
expensive instrumentation. However, the IL GC stationary phases show dual-nature
behavior and broad selectivities, which provide them advantages in the separation
of flavors and fragrances. Applications of different IL GC stationary phases in the
analysis of essential oils are discussed in the following section. Benefits of new
inert ionic liquid (iIL) GC stationary phases in the analysis of essential oils are also
considered.
The advantages of IL stationary phases in the separation of flavor and fragrance
compounds were observed during the initial development of dicationic IL GC stationary phases [7–9]. Furthermore, a dicationic IL 1,9-di(3-vinylimidazolium)nonane
bis(trifluoromethanesulfonyl)imide [C 9 (vim) 2 ][NTf 2 ] 2 , now known as SLB-IL100
was used in the analysis of fennel, cinnamon, and nutmeg essential oils [104].
This study involved the use of four different IL columns: (1) a (1:1:1) mixture
of dicationic IL [C 9 (vim) 2 ][NTf 2 ] 2 , a monocationic IL 1-vinyl-3-nonylimidazolium
bis[(trifluoromethyl)sulfonyl]imide [vC 9 im][NTf 2 ], and OV-1701 polysiloxane, (2)
the dicationic [C 9 (vim) 2 ][NTf 2 ] 2 IL, (3) a 5% phenyl-methylpolysiloxane (HP-5
MS) column, and (4) a polyethylene glycol (HP-Innowax) column. Comparison of
the analyses of three essential oils on the four columns showed that column 1 (a
mixture of dicationic IL and monocationic IL) showed better efficiency and wider
selectivity compared to the other three columns. Column 1 had better selectivity in
the separation of both nonpolar and polar compounds compared to column 2, and
not all the hydrocarbons in the essential oils were separated on column 2. The dicationic IL is polar (polarity number 100) and showed lower retention of nonpolar
compounds and retained the polar compounds longer. However, the polysiloxane
part of column 1 played a role in the better separation of hydrocarbons. Also, the
dicationic IL column displayed an affinity for oxygen-containing compounds, while
the polyethylene glycol stationary phase retained the “hydrogen-bond acidic” compounds (i.e., alcohols). This study used a polar IL [C 9 (vim) 2 ][NTf 2 ] 2 , (PN = 100)
and showed coelution for some hydrocarbons.
Later, a medium-polarity commercialized column SLB-IL59 (PN= 59) was used
in the analysis of essential oils and it showed a better separation of essential oils
components (both polar and nonpolar compounds) [32]. Separations of four test
compounds (p-cymene, 1,8-cineole, limonene, and (Z)-β-ocimene) were compared
on three columns (5% phenyl PDMS, PEG, and SLB-IL59). The PEG column and
147
of a large number and variety of compounds with different physicochemical properties. The compounds include hydrocarbons (monoterpenes, sesquiterpenes, and aromatics), oxygenated compounds (aromatics, phenols, alcohols, sesquiterpene alcohols, aldehydes, ketones, esters, lactones, ethers, oxides, etc.), and other compounds.
Most of the compounds are isomers or analogs of each other and generate the same
mass spectra. Hence, chromatographic separation of these compounds is important.
The separation demands stationary phases with broad selectivities. Traditional wax
(polyethylene glycol-based) and cyanopropyl polysiloxane-based phases are used for
the analysis of essential oils. However, these stationary phases produce coelutions
of many of the compounds. Multidimensional GC can also be used for the analysis
of such complex mixtures, but such approaches involve operational complexity and
expensive instrumentation. However, the IL GC stationary phases show dual-nature
behavior and broad selectivities, which provide them advantages in the separation
of flavors and fragrances. Applications of different IL GC stationary phases in the
analysis of essential oils are discussed in the following section. Benefits of new
inert ionic liquid (iIL) GC stationary phases in the analysis of essential oils are also
considered.
The advantages of IL stationary phases in the separation of flavor and fragrance
compounds were observed during the initial development of dicationic IL GC stationary phases [7–9]. Furthermore, a dicationic IL 1,9-di(3-vinylimidazolium)nonane
bis(trifluoromethanesulfonyl)imide [C 9 (vim) 2 ][NTf 2 ] 2 , now known as SLB-IL100
was used in the analysis of fennel, cinnamon, and nutmeg essential oils [104].
This study involved the use of four different IL columns: (1) a (1:1:1) mixture
of dicationic IL [C 9 (vim) 2 ][NTf 2 ] 2 , a monocationic IL 1-vinyl-3-nonylimidazolium
bis[(trifluoromethyl)sulfonyl]imide [vC 9 im][NTf 2 ], and OV-1701 polysiloxane, (2)
the dicationic [C 9 (vim) 2 ][NTf 2 ] 2 IL, (3) a 5% phenyl-methylpolysiloxane (HP-5
MS) column, and (4) a polyethylene glycol (HP-Innowax) column. Comparison of
the analyses of three essential oils on the four columns showed that column 1 (a
mixture of dicationic IL and monocationic IL) showed better efficiency and wider
selectivity compared to the other three columns. Column 1 had better selectivity in
the separation of both nonpolar and polar compounds compared to column 2, and
not all the hydrocarbons in the essential oils were separated on column 2. The dicationic IL is polar (polarity number 100) and showed lower retention of nonpolar
compounds and retained the polar compounds longer. However, the polysiloxane
part of column 1 played a role in the better separation of hydrocarbons. Also, the
dicationic IL column displayed an affinity for oxygen-containing compounds, while
the polyethylene glycol stationary phase retained the “hydrogen-bond acidic” compounds (i.e., alcohols). This study used a polar IL [C 9 (vim) 2 ][NTf 2 ] 2 , (PN = 100)
and showed coelution for some hydrocarbons.
Later, a medium-polarity commercialized column SLB-IL59 (PN= 59) was used
in the analysis of essential oils and it showed a better separation of essential oils
components (both polar and nonpolar compounds) [32]. Separations of four test
compounds (p-cymene, 1,8-cineole, limonene, and (Z)-β-ocimene) were compared
on three columns (5% phenyl PDMS, PEG, and SLB-IL59). The PEG column and