An overview of two similar polar phases will be examined for analysing essential oils. This overview provides a fingerprint elution order for peppermint and spearmint oils using the polyethylene glycol and ionic liquid phases and it also highlights the uniqueness in elution order when using an ionic liquid capillary column. The ionic liquid phase is a new dicationic phosphonium cation liquid stationary phase which has selectivity like a polyethylene glycol (PEG) or wax phase. A PEG type phase is traditionally used for essential oil analysis [1]. However, the PEG phase does not always perform a few key separations. Over the years, extensive evaluations of columns manufactured with ionic liquid stationary phases has occurred [1,2,3]. Their main strength was discovered to be unique selectivity, higher temperatures and minimum column bleed compared to the PEG column. These columns have the ability to perform many of the same applications as columns made with polyethylene glycol stationary phases but with slight elution order changes. Many times, this results in increased resolution and/or shorter run time.
Essential oil analyses are challenging to chromatographers because of the complexity of the various samples. The quality and authenticity of the oil needs to be evaluated by analytical means to ensure the product’s description reflects its true composition and that the capillary column is providing adequate separation. The best way to improve the resolution of essential oil samples is to choose a capillary column with a unique selectivity that is stable and reproducible. Traditionally, columns based on polyethylene glycol chemistries have been the columns of choice for these analyses due to the separation and sufficient analysis time. It is important to demonstrate the compounds resolution because different species of peppermint (Willamette vs. Yakima) or spearmint (Farwest Native vs. Farwest Scotch) oils may contain different percentages or ratios of the key flavour and fragrance compounds.
Peppermint and spearmint oils are aromatic liquids that are extracted from the plant leaves by steam distillation. The distinctive aroma of essential oils is attributed to the unique composition of chemicals in the plant which includes a wide variety of chemical compounds such as: terpenes, ketones, aldehydes, alcohols, acids, esters, and hydrocarbons [4].
Peppermint is cultivated in Oregon (Willamette Valley) and Washington State (Yakima Oil). Northern regions are considered to produce the best quality of oil due to the long daylight hours. Composition of the oil is influenced by many factors: cultivation, production, geographic location, time of planting and harvest, and the distillation process. Under short day conditions mint plants have been observed to produce oil with higher levels of menthofuran and lower levels of methone and menthol than plants in long-day conditions (5). The main components of Peppermint oil are menthol (peak 30) and menthone (peak 31) in Figures 1 and 2. Yakima Oil contains higher levels of methofuran. Methofuran has a tendency to autoxidize and may be added back into the final peppermint to restore the peppermint characteristic. p-Cymene (peak 13, Figures 1 and 2) is also of interest in peppermint oil because it is a measure of oxidation [5].
Willamette Peppermint has a fresh minty, non-herbaceous odour that is cool and refreshing while Yakima peppermint is pungent and herbal in taste and smell. There is a variety of uses for peppermint oil such as adding flavour or fragrance to foods, cosmetics, soaps, toothpastes, mouthwashes, and other products.
The aromatic fragrance of Farwest Native and Farwest Scotch spearmint oil is immediately recognised for its crispness, and is used to flavour food, popular drinks like iced tea, alcoholic beverages, and candy. Carvone is what gives spearmint its distinctive smell, which is often associated with cleanliness, making it popular for use in mouthwash, shaving creams, soaps, and shampoos.
The ionic liquid phase provides unique selectivity of essential oil compounds (i.e. terpineol-4-ol/E- Caryophyllene/neo-Menthol) that are different from the traditional PEG columns (Figure 3). Ionic liquid phases are much smaller compared to big, bulky polysiloxane polymers and polyethylene glycol phases, plus there are no active hydroxyl groups. These features lead to greater stability, even in the presence of moisture and/or oxygen. The ionic liquid phase undergoes the same analyte-phase interactions as the polyethylene glycol but at different relative amounts. The ionic liquid phase also undergoes additional interactions that the PEG phase cannot. With PEG columns, possible interactions appear to be dispersive, hydrogen bonding, and acid-base interactions and with the ionic liquid phase 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 ionic phase 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 traditional PEG columns.
While the main components of Spearmint oil are α-pinene, β-pinene, carvone, eucalyptol, linalool, limonene, myrcene,
(E)-caryophyllene and menthol (~0.5% menthol is reported in spearmint compared to the ~40.0% in peppermint oil). Pure spearmint oil contains at least 45-80% of carvone and 10-30% of limonene. Native spearmint oil (Figure 4), for example, contains an amount of trans-sabinenehydrate (peak 24) and no methone (peak 31), while the opposite is true for Scotch spearmint oil (Figure 7). There is a large amount of methone and no trans-sabinenehydrate. The ratio of trans-sabinenehydrate to methone is a useful way to distinguish between the two oils
