Nanotechnology: From Microelectronics to Health Care
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 - 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. Detector Thermal Conductivity Detector (TCD) Flame Ionization Detector (FID) Electron Capture (ECD)
Flame Photometric Detector (FPD) Photoionization Detector (PID) 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 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
SPME Advantages less solvent required speed less glassware is needed better precision and accuracy minimal solvent evaporation readily automated SPME Device SPME Procedure
Extraction Desorption 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
Red Peaks: Neo FD 60C 30 Days Dodecane E2,Z4-Decadienal E2,E4-Decadienal Octanoic Acid Trans-2-Nonenal Methyloctanoate
2-Octenal Limonene Hexanoic Acid 1-Heptanol Hexanal Degradation of 2,4-Decadienal Blue Peaks: Neo SD 60C 30 Days
What is citral? Neral Geranial Citral Degradation Citral Cyclization (induced by protons) OH OH
OH and/or p-cresol p-cymene Monoterpene alcohols OH O
OH O ,p-dimethylstyrene p-methylacetophenone . OH and more Citral Degradation
Disappears in TWO weeks! No. Compounds No. Compounds 1 2,3-dehydro-1,8-cineole
p-cymen-8-ol 14 8-hydroperoxy-p-cymene Ueno, T., et al., J. Agric. Food Chem., 2006. 54(8): p. 3055. Mass Spectrometry 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. 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. 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. Ionization Process Example of Ionization Process
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. 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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