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

Degradation of 2,4-Decadienal

Red Peaks: Neo FD 60°C 30 Days Blue Peaks: Neo SD 60°C 30 Days

Hex

anal

1-H

epta

nol

Hex

anoi

c A

cid

2-O

cten

al

Lim

onen

e

Met

hylo

ctan

oate

Tra

ns-2

-Non

enal

Oct

anoi

c A

cid

Dod

ecan

e

E2,

Z4-

Dec

adie

nal

E2,

E4-

Dec

adie

nal

Neral Geranial

What is citral?

Citral DegradationCitral

Cyclization (induced by protons)

OH OH

and/or

Monoterpene alcohols

and more…OH O

OHOH

.

,p-dimethylstyrene p-methylacetophenone

O

p-cresol

OH

p-cymene

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

Citral Degradation

Ueno, T., et al., J. Agric. Food Chem., 2006. 54(8): p. 3055.

Disappears in TWO weeks!

Mass Spectrometry

Mass Spectrometry Basics

• 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.

- An analytical tool used for measuring the molecular mass of a molecule

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

+. + 2eNH3

+. + NH3 ------> NH4+ + NH2

.