Terpenes Analysis of Essential Oils using GC-VUV An Automated, Speed-based Approach Alex Hodgson, Senior Applications Chemist VUV Analytics, Inc. VUV Absorption Spectroscopy • Simple absorption spectroscopy in a new spectral region • Vacuum ultraviolet (VUV) spectroscopy is a universal technique that provides unique spectral fingerprints • High energy, low wavelength exposure produces electronic transitions between σ→σ*, n→σ*, and π→π* molecular orbitals • Compound identification is unambiguous, even for structural isomers • Chromatographic compression leads to higher sample throughput • Software deconvolution of coelutions reduces priority on chromatographic separation • Chromatography runs can be deliberately compressed Introduction to Terpenes • Class of organic compounds produced by plants (and some insects) • Primary constituents of essential oils, which contribute heavily to fragrances and flavors • Widely utilized in the food and fragrance, cosmetics, and pharmaceuticals industries • Composed of multiple units of isoprene (C5H8) • Monoterpenes – 2 isoprene units (C10H16) • Sesquiterpenes – 3 isoprene units (C15H24) • Very high prevalence of structural isomers Monoterpenes Sesquiterpenes Monoterpenoids Sesquiterpenoids α-Pinene β-Myrcene Limonene β-Caryophyllene α-Humulene Eucalyptol p-Cymene Linalool trans-Nerolidol Guaiol Headspace/Solid-phase Microextraction (SPME) Setup Gerstel MPS2 Autosampler VUV Analytics Agilent 6890 Gas VGA-100 Chromatograph Headspace Run Conditions • Gerstel MPS2 • Incubator: 80°C for 10 minutes, 250 rpm agitation (10 sec on, 1 sec off) • Syringe: 90°C, 2.5 mL injection volume • Agilent 6890 GC • Inlet: 250°C, split 2.5:1 • Column: Restek Rxi-624Sil MS (30 m x 0.25 mm, 1.40-μm) • Flow: 4 mL/min helium, constant flow • Oven: 60°C, hold 0.1 min; 23.8°C/min to 300°C (run time - 10.2 min) • VUV Analytics VGA-100 • Makeup gas: 0.25 psi N2 • Flow cell and transfer line: 275°C • Acquisition rate: 4 spectra/sec • Acquisition range: 125-240 nm Terpenes Standard (Restek) Class Compound RT (min) Formula MW Sources Monoterpene α-Pinene 3.87 C10H16 136.23 Coniferous trees, rosemary, eucalyptus Camphene 4.03 C10H16 136.23 Camphor, neroli, valerian β-Myrcene 4.21 C10H16 136.23 Thyme, cardamom, hops, cannabis β-Pinene 4.25 C10H16 136.23 Nutmeg, rosemary, sage 3-Carene 4.43 C10H16 136.23 Turpentine, rosemary, cedar α-Terpinene 4.50 C10H16 136.23 Allspice, juniper, marjoram cis-Ocimene 4.53 C10H16 136.23 Basil, lavender, clary sage Limonene 4.58 C10H16 136.23 Citrus fruits, mint trans-Ocimene 4.64 C10H16 136.23 Basil, lavender, clary sage γ-Terpinene 4.77 C10H16 136.23 Citrus fruits, cumin, Syrian oregano Terpinolene 4.98 C10H16 136.23 Allspice, citrus fruits, juniper Terpenes Standard (Restek) Class Compound RT (min) Formula MW Sources Monoterpenoid p-Cymene 4.60 C10H14 134.22 Cumin, thyme Linalool 5.21 C10H18O 154.25 Mint, cinnamon, rosewood Isopulegol 5.67 C10H18O 154.25 European pennyroyal, corn mint Geraniol 6.27 C10H18O 154.25 Almonds, palmarosa, rose Sesquiterpene β-Caryophyllene 7.32 C15H24 204.35 Cloves, rosemary, hops, cannabis α-Humulene 7.53 C15H24 204.35 Sage, ginseng, tobacco Sesquiterpenoid cis-Nerolidol 7.85 C15H26O 222.37 Neroli, jasmine, lavender trans-Nerolidol 8.01 C15H26O 222.37 Neroli, jasmine, lavender Guaiol 8.37 C15H26O 222.37 Cypress, pine, cannabis α-Bisabolol 8.73 C15H26O 222.37 German chamomile, balsam poplar 0.5 7 1. α-Pinene 12. Terpinolene 2. Camphene 13. Linalool 125-160 nm 11 8 3. β-Myrcene 14. Isopulegol 4. β-Pinene 15. Geraniol 170-240 nm 3 5 6 9 5. 3-Carene 16. β-Caryophyllene 0.4 6. α-Terpinene 17. α-Humulene 7. cis-Ocimene 18. cis-Nerolidol 4 17 12 8. Limonene 19. trans-Nerolidol 16 9. p-Cymene 20. Guaiol 1 2 10. trans-Ocimene 21. α-Bisabolol 0.3 10 11. γ-Terpinene Last analyte Detector Response Detector 0.2 elutes before 9 min! 0.1 13 19 20 14 18 21 15 0 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 Time (min) Mass Spectra of Coeluting Isomers 41 93 β-Myrcene 69 121 136 93 β-Pinene 41 69 121 136 Absorbance Spectra of Monoterpene Isomers 1 α-Pinene β-Pinene β-Myrcene 0.9 Limonene cis-Ocimene trans-Ocimene 0.8 0.7 0.6 0.5 0.4 Normalized Absorbance Normalized 0.3 0.2 0.1 0 130 140 150 160 170 180 190 200 210 220 230 240 Wavelength (nm) Spectral Deconvolution of Coeluting Peaks 0.4 125-240 nm Limonene 0.35 α-Terpinene p-Cymene 0.3 cis-Ocimene trans-Ocimene 0.25 0.2 Detector Response Detector 0.15 0.1 0.05 Summary: • XXX 0 • XXX 4.44 4.46 4.48 4.5 4.52 4.54 4.56 4.58 4.6 4.62 4.64 4.66 • XXX Time (min) Region 1 – Single Analyte Spectral Matching 0.22 0.2 0.18 0.16 0.14 0.12 0.1 Absorbance 0.08 0.06 0.04 Summed Retention Region Spectrum 0.02 Summed Target Spectrum Best Fit; R^2 = 0.99890 alpha-Terpinene 0 130 140 150 160 170 180 190 200 210 220 230 240 Wavelength (nm) Region 2 – Summation of Spectra from Coelution #1 Summed Retention Region Spectrum 0.33 Summed Target Spectra Best Fit; R^2 = 0.99923 0.3 alpha-Terpinene 0.27 cis-Ocimene 0.24 0.21 0.18 0.15 Absorbance 0.12 0.09 0.06 0.03 0 130 140 150 160 170 180 190 200 210 220 230 240 Wavelength (nm) Region 3 – Summation of Spectra from Coelution #2 1.2 Summed Retention Region Spectrum 1.1 Summed Target Spectra Best Fit; R^2 = 0.99962 1 Limonene 0.9 p-Cymene 0.8 0.7 0.6 Absorbance 0.5 0.4 0.3 0.2 0.1 0 130 140 150 160 170 180 190 200 210 220 230 240 Wavelength (nm) Region 4 – Single Analyte Spectral Matching 0.5 0.45 0.4 0.35 0.3 0.25 Absorbance 0.2 0.15 0.1 Summed Retention Region Spectrum Summed Target Spectrum Best Fit; R^2 = 0.99771 0.05 trans-Ocimene 0 130 140 150 160 170 180 190 200 210 220 230 240 Wavelength (nm) VUV vs MS – Tale of the Tape Who has the edge? Higher flow limit Lower relative LOD Ease of use/ Ease of maintenance 3-dimensional information Unique spectra for all measurable compounds Isomer differentiation VUV Methods – Run Conditions Agilent 6890 GC VUV VUV VUV Method 1 Method 2 Method 3 • 1 µL injection Col Flow (mL/min • Inlet: 250°C, split 10-300:1 1.4 2 4 • Column: Restek Rxi-1ms He) (30 m x 0.25 mm, 0.25-µm) Oven Start (°C) 40 40 40 VUV Analytics VGA-100 Initial Hold (min) 0.2 0.2 0.1 • Makeup gas: 0.25 psi N2 Oven Ramp 9.6 20.4 30.9 • Flow cell and transfer line: 275°C (°C/min) • Acquisition rate: 5 spectra/sec • Acquisition range: 125-240 nm Oven Final (°C) 250 250 250 Run Time (min) 22.1 10.5 6.85 VUV Run Results Peak # Analyte 1 α-Pinene 2 Camphene 3 β-Pinene 4 β-Myrcene 5 3-Carene 6 α-Terpinene 7 p-Cymene 8 Eucalyptol 9 Limonene 10 cis-Ocimene 11 trans-Ocimene 12 γ-Terpinene 13 Terpinolene 14 Linalool 15 Isopulegol 16 Geraniol 17 β-Caryophyllene 18 α-Humulene 19 cis-Nerolidol 20 trans-Nerolidol 21 Caryophyllene oxide 22 Guaiol 23 α-Bisabolol VUV Methods Comparison – Tea Tree Oil VUV Method 1 VUV Method 2 VUV Method 3 VUV Average Compound (mg/mL) – (mg/mL) – (mg/mL) – CV (%) (mg/mL) 22.1min 10.5min 6.85min α-Pinene 20.6 21.5 23.0 21.7 4.48 α-Terpinene 77.9 80.4 84.8 81.0 3.57 β-Caryophyllene 2.09 2.26 2.03 2.13 4.46 β-Myrcene 7.20 7.72 7.95 7.62 5.14 β-Pinene 5.83 5.85 5.21 5.63 5.30 Eucalyptol 21.0 25.0 22.8 22.9 7.24 γ-Terpinene 161 172 193 175 7.53 Limonene 6.38 5.62 7.03 6.34 9.08 p-Cymene 25.2 27.9 33.3 28.8 11.6 Terpinolene 27 29.7 23.1 26.6 10.3 MS Methods – Run Conditions Agilent 6890 GC MS Method MS Method • 1 µL injection 1 2 • Inlet: 250°C, split 20-975:1 Col Flow (mL/min He) 1 1.4 • Column: Restek Rxi-1ms (30 m x 0.25 mm, 0.25-µm) Oven Start (°C) 40 50 • Transfer line: 280°C Initial Hold (min) 1.5 1 Agilent 5973N MS Ramp 1 (°C/min) 1 15 • EI Source: 300°C Ramp 1 End (°C) 77 -- • Quad 1: 200°C Ramp 2 (°C/min) 7 -- • Scan range: 50-250 Oven Final (°C) 250 250 Run Time (min) 63.2 14.3 MS Run Results Peak # Analyte 1 α-Pinene 2 Camphene 3 β-Pinene 4 β-Myrcene 5 3-Carene 6 α-Terpinene 7 p-Cymene 8 Eucalyptol 9 Limonene 10 cis-Ocimene 11 trans-Ocimene 12 γ-Terpinene 13 Terpinolene 14 Linalool 15 Isopulegol 16 Geraniol 17 β-Caryophyllene 18 α-Humulene 19 cis-Nerolidol 20 trans-Nerolidol 21 Caryophyllene oxide 22 Guaiol 23 α-Bisabolol VUV vs MS Comparison – Tea Tree Oil MS Method 1 Average w/ all VUV runs Compound CV (%) (mg/mL) – 63.2min (mg/mL) α-Pinene 26.3 22.8 9.56 α-Terpinene 91.2 83.6 6.04 β-Caryophyllene 1.58 1.99 12.6 β-Myrcene 7.44 7.58 3.76 β-Pinene 6.36 5.81 7.00 Eucalyptol 30.2 24.8 14.0 γ-Terpinene 205 183 9.48 Limonene 7.84 6.72 12.2 p-Cymene 35 30.4 13.1 Terpinolene 31.1 27.7 11.0 MS Methods Comparison – Tea Tree Oil MS Method 1 MS Method 2 MS Average Compound CV (%) (mg/mL) – 63.2min (mg/mL) – 14.3min (mg/mL) α-Pinene 26.3 27.3 26.8 1.75 α-Terpinene 91.2 96.2 93.7 2.71 β-Caryophyllene 1.58 2.39 1.99 20.4 β-Myrcene 7.44 9.78 8.61 13.6 β-Pinene 6.36 7.91 7.13 10.9 Eucalyptol 30.2 28.7 29.5 2.51 γ-Terpinene 205 202 204 0.788 Limonene 7.84 17.4 12.6 37.9 p-Cymene 35.0 43.0 39.0 10.2 Terpinolene 31.1 38.3 34.7 10.3 Spectral Comparison – Limonene & Eucalyptol 1 Norm Abs Norm 0 125 150 175 200 225 λ (nm) Eucalyptol Limonene Norm AbundNorm m/z Class-Type Analysis of….Gasoline? Time Interval Deconvolution (TID) • Alternative quantitation method using VUV Analyze™ software • Chromatogram is divided into equal, small time intervals (typically <0.05 min) • For each time interval, compare measured spectrum against reference spectra in designated library, best analyte(s) fit determined • Can quickly be performed to measure total response per analyte for a chromatogram; this can be converted into a relative mass percentage, relative volume percentage, or absolute concentration Essential Oil TID Run Conditions • Agilent 6890 GC • 1 μL injection • Inlet: 250°C, split 250:1 • Column: Restek Rxi-1ms