Transcript 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 60°C 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 60°C 30 Days
What is citral?
Neral
Geranial
Citral Degradation
Citral
Cyclization (induced by protons)
OH
OH
OH
and/or
p-cresol
p-cymeneMonoterpene 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
8
p-mentha-1(7),2-dien-8-ol
2
p-cymene
9
a-terpineol
3
p-cresol
10 (2R, 5R)-tetrahydrofuran isomer
4
a,p-dimethylstyrene
11 (2S, 5R)-tetrahydrofuran isomer
5
p-mentha-1,5-dien-8-ol
12 Trans-p-menth-2-ene-1,8-diol
6
p-methylacetophenone
13 Cis-p-menth-2-ene-1,8-diol
7
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.