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n-butanol, 2-pentanone, nitropropane, and pyridine. McReynolds Constants are then calculated using the retention index data of the column relative to the retention index data for the same five probes on squalane, the most non-polar GC stationary phase.
The five McReynold’s Constants are summed to obtain Polarity (P) values, which are then normalised to SLB-IL100 (set at P=100) to obtain Polarity Number (P.N.) values.
To investigate the selectivity of the various ionic liquid GC phases, a test mix composed of aliphatic, aromatic and polar compounds was analysed at one isothermal temperature. Figure 3 compares the chromatograms. The analyses showed the more polar the stationary phase is the greater retention of polar compounds and less retention of the nonpolar aliphatic compounds. Tridecane (C13) demonstrates the typical shorter retention of an aliphatic hydrocarbon as the column polarity increases. The columns are listed in order of least polar (SLB-IL59) to most polar (SLB-IL111). The least polar phases, (SLB-IL59, 60 and 61) are based on dicationic phosphonium cations. SLB- IL76 has a trigonal phosphonium cation. The three most polar ionic liquid phases (SLB-IL82, 100 and 111) are all dicationic imidazolium cations.
SLB-IL60 is an ionic liquid phase with a polarity that is similar to that of a polyethylene glycol (PEG) phase. SLB-IL60 is able to undergo the same analyte- phase interactions as polyethylene glycol (PEG) columns, but at different relative amounts. Based on its unique phase structure, the SLB-IL60 column is also able to undergo additional interactions that PEG columns cannot. With PEG columns, possible interactions appear to be dispersive, hydrogen bonding, and acid-base interactions. With the SLB-IL60 column, possible interactions appear to be dispersive, dipole-dipole, dipole- induced dipole, pi-pi, hydrogen bonding, and acid-base interactions. Due to these additional interaction mechanisms, the SLB-IL60 column will retain some polar and polarisable analytes relatively longer, and some non-polar analytes relatively less. This results in unique and alternate selectivity compared to PEG columns.
Figure 4 compares the selectivity of the SLB-IL60 to that of Supelcowax 10 performed by evaluating a series of normal alkanes (C15,16,17,18 and 20) along with 2-octanone, 1-octanol, 2,6-dimethylaniline and 2,6-dimethylphenol. The elution pattern is similar on both columns as 2-octanone elutes prior to C15 and 1-octanol elutes
Figure 3: Comparison of the Ionic liquid Column Selectivity, 30m x 0.25mm ID x 0.25µm df, Oven: 110°C, Inj, 250°C, Det. FID 250°C, Carrier Gas: Helium, 25cm/ sec at 110°C, Injection: 1µl, 100:1 Split, Sample Components: toluene, ethylbenzene, p-xylene, isopropylbenzene, cyclohexanone, 1,2,4-trimethylbenzene, 1,2,4,5,- tetramethylbenzene, n-tridecane (C13).
Figure 4: Selectivity Comparison of SLB-IL60 and Supelcowax-10 SLB-IL60 column: SLB-IL60, 30 m x 0.25 mm I.D., 0.20 µm (29505-U) Supelcowax 10: 30 m x 0.25 mm I.D., 0.25 µm oven: SLB-IL60- 130°C isothermal Supelcowax 10- 155°C isothermal
inj. temp.: 250°C
carrier gas: helium, 25cm/sec set at the isothermal temperature detector: FID, 250°C injection: 1 µL, split 100:1
between the C15 and C16 alkanes. A difference in selectivity is demonstrated as 2,6-dimethylphenol elutes prior to 2,6-dimethylaniline on the SLB-IL60 column.
FAME analyses also demonstrate the similarity and differences in the selectivity of the SLB-IL60 columns and traditional PEG columns. Traditional PEG phases typically elutes FAME isomers by degree of
unsaturation within a carbon chain length with minimal overlap of the even carbon chain lengths. Figure 5 compares the elution pattern of a PUFA 3 fish oil sample on the SLB-IL60 and an Omegawax 250 capillary column. A similar elution pattern is demonstrated for the C18 carbon number series. As the carbon chain lengths increase along with the degree of unsaturation in
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