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Food Analysis Lecture 24 (04/23/2013)

Food Analysis Lecture 24 (04/23/2013). GC (2). Qingrong Huang Department of Food Science. Sample Injection. Direct Injection Sample Derivatization. Column. Either packed or capillary column Packaging Materials: - Packed column: silane-treated diatomaceous earth

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Food Analysis Lecture 24 (04/23/2013)

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  1. Food Analysis Lecture 24 (04/23/2013) GC (2) Qingrong Huang Department of Food Science

  2. Sample Injection • Direct Injection • Sample Derivatization

  3. Column • Either packed or capillary column • Packaging Materials: • - Packed column: silane-treated diatomaceous earth • - Capillary column: hollow fused silica glass • Stationary phase: - polysiloxane-based (-Si-O-Si-) • -General rules: choose polar phase to separate polar compounds • and phenyl-based column to separate aromatic compounds.

  4. Detector • Thermal Conductivity Detector (TCD) • Flame Ionization Detector (FID) • Electron Capture (ECD) • Flame Photometric Detector (FPD) • Photoionization Detector (PID)

  5. Carrier Gas • In capillary chromatography, A term is small; • N2 is the most efficient carrier gas, but its minimum occur in low u; • Hydrogen is a better choice than He and N2

  6. SPME • Microextraction technique which employs a thin film of sorptive polymer on a glass fiber • Fiber • Having varying film thickness of polymer • Coatings are based on both absoption and adsorption mechanisms • Convenient, solventless extraction technique • Used to extract analytes from both liquid and solid matrices

  7. SPME Advantages • less solvent required • speed • less glassware is needed • better precision and accuracy • minimal solvent evaporation • readily automated

  8. SPME Device

  9. SPME Procedure Extraction Desorption

  10. Degradation of 2,4-Decadienal • 2,4-Decadienal is an Important Aldehyde Flavor • Contributes Deep Fat Characteristics at 10 ppm • Key Component in Flavors Such as: • Chicken • Lamb • Beef • French Fries

  11. Degradation of 2,4-Decadienal E2,E4-Decadienal Dodecane Freeze Dried → Less Degradation than Spray Dried Red Peaks: Neo FD 60°C 30 Days Blue Peaks: Neo SD 60°C 30 Days Hexanal E2,Z4-Decadienal 2-Octenal Trans-2-Nonenal Limonene Octanoic Acid Hexanoic Acid Methyloctanoate 1-Heptanol

  12. What is citral? Neral Geranial

  13. Citral Degradation Citral . Cyclization (induced by protons) Monoterpene alcohols and more…

  14. Citral Degradation Disappears in TWO weeks! Ueno, T., et al., J. Agric. Food Chem., 2006. 54(8): p. 3055.

  15. Mass Spectrometry

  16. Mass Spectrometry Basics - An analytical tool used for measuring the molecular mass of a molecule • Ionization of molecules; • Separation of ions based on mass-to-charge ratio (m/z); • Detected by mass analyzer under electrostatic field; • Routine in analytical labs; • GC-MS: interface of mass spectrometer with GC; • LC-MS: interface of mass spectrometer with HPLC.

  17. Sample Introduction • Direct Injection: for a gas or a volatile liquid; • Direct Insertion: for somewhat volatile solid, the sample is placed • in a small cup at the end of a stainless steel rod or probe; • For mixtures, we can use GC-MS or LC-MS through an • interface which removes excess GC carrier gas or HPLC solvent.

  18. Ionization • In ion source, the compound is exposed to a beam of electrons • emitted from a filament composed of rhenium or tungsten metal. • When a direct current is applied to the filament (~70 eV), it heats • and emits electrons that move across the ion chamber toward a • positive electrode; • As the electron pass through the source region, they come in close • proximity to the sample molecule and extract an electron, forming • an ionized molecule; • The ionized molecules can further fragment into smaller molecular • fragments.

  19. Ionization Process

  20. Example of Ionization Process

  21. Chemical Ionization (1) • - Chemical Ionization (CI) is especially useful technique when no • molecular ion is observed in EI mass spectrum, and also in the case of • confirming the mass to charge ratio of the molecular ion. • Chemical ionization technique uses virtually the same ion source • device as in electron impact, except, CI uses tight ion source, and • reagent gas. Reagent gas (e.g. ammonia) is first subjected to electron • impact. • Sample ions are formed by the interaction of reagent gas ions and • sample molecules. This phenomenon is called ion-molecule reactions. • -Reagent gas molecules are present in the ratio of about 100:1 with • respect to sample molecules. • -Positive ions and negative ions are formed in the CI process. • -Depending on the setup of the instrument (source voltages, detector, etc...) • only positive ions or only negative ions are recorded.   

  22. Chemical Ionization (2) •   In CI, ion molecule reactions occur between ionized reagent gas molecules • (G) and volatile analyte neutral molecules (M) to produce analyte ions. • Pseudo-molecular ion MH+ (positive ion mode) or [M-H]- (negative ion mode) • are often observed. Unlike molecular ions obtained in EI method, MH+ and • [M-H]- detection occurs in high yield and less fragment ions are observed. • Positive ion mode: (example) • CH4 + e -----> CH4+. + 2e ------> CH3+ + H. • CH4+. + CH4 -----> CH5+ +CH3. • Negative ion mode: (example) • NH3 + e -----> NH3+. + 2e • NH3+. + NH3 ------> NH4+ + NH2.

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