Volatile oils of plants, also known as essential oils, have been used for many years for their flavour, fragrance and medicinal properties. However, their extreme complexity and diversity presents analysts with a significant challenge when it comes to quality control. They are generally composed of monoterpenes and sesquiterpenes, as well as their oxygenated derivatives, such as aliphatic aldehydes, alcohols and esters.
In many cases, trace components can impart a distinctive fragrance character to the oil, meaning that rigorous quality control procedures are required to ensure that essential oils do not vary significantly from the initial reference batch, or contain any unwanted adulterants. Furthermore, because essential oils are raw materials for the perfume industry, analysts must now also ensure that they adhere to new legislation on fragrance allergens.
Traditionally, gas chromatography (GC), coupled with mass spectrometry (MS) or flame ionisation detection (FID), have been the standard methods for qualitative and quantitative analysis of such samples, relying on the combination of mass spectra and Kovats retention indices to confirm the identity of individual terpenes.
However, in recent years, comprehensive two-dimensional GC with time‑of‑flight MS (GC×GC–TOF MS) has become an attractive technique for characterisation of essential oils. The enhanced separation capacity offered by the coupling of two columns with different selectivities, combined with highly sensitive mass spectral identification, provides a high-performance solution for rapid screening of essential oils. Nevertheless, the structurally similar terpenes found in essential oils can be difficult (or impossible) to speciate when using conventional 70 eV electron ionisation, even with the added power of retention indices in two dimensions.
Here we examine the use of the Tandem Ionisation®  to provide two complementary data sets from a single analysis – one containing 70 eV data, and the other with soft EI for enhancement of molecular ions and improved speciation of isomers. This capability is illustrated by comparing a range of essential oils by flow-modulated GC×GC–TOF MS combined with multivariate statistical analysis for fast evaluation of compositional differences.
Sample preparation: 1% (v/v) dilutions of eight essential oils were prepared in ethyl acetate.
GC: Injector: Split/splitless; Liner: Single taper with wool, 4 mm (i.d.); Carrier gas: Helium, constant-flow at 0.6 mL/min; Mode: Split 100:1; Temperature: 280°C; Septum purge: On, 1 mL/min.
2D column set: 1st dimension: BPX5, 20 m × 0.18 mm × 0.18 μm; 2nd dimension: DB1701, 2 m × 0.25 mm × 0.15 μm.
Temperature program: Main oven: 40°C (1 min), 3°C/min to 260°C, 10°C/min to 280°C (10 min).
Modulator: Insight flow modulator (SepSolve Analytical); Loop dimensions: 0.53 mm i.d. × 230 mm (loop volume: 50 μL); Fill time: 3600 ms: Flush time: 200 ms; Modulation period (PM): 3.8 s.
TOF MS: Instrument: BenchTOF-Select (Markes International); Filament voltage: 1.7 V; Ion source: 300°C; Transfer line: 280°C; Mass range: m/z 45–400; Data rate: 100 Hz in Tandem Ionisation mode at 70 eV and 14 eV.
Software: Image processing: GC Image (GC Image, LLC); Statistical analysis: MATLAB® (R2011a, MathWorks).