Food Analysis Lecture 19 (4/5/2005)

Basic Principles of Chromatography (2)

Qingrong Huang

Department of Food Science

Read Material: Chapter 27, page 437

Final Exam: April 29

Extraction

• Extraction: the transfer of a solute from one liquid phase to another.

• Batch extraction: the solute is extracted from one solvent by shaking it with a second immiscible solvent.

• Continuous extraction: requires special apparatus like Soxhlet extractor.

• Countercurrent extraction: a serial extraction process which separates two or more solutes with different partition coefficients from each otherby a series of partitions between two immiscible liquid phases.

2 phasein solute ofion concentrat

1 phasein solute ofion concentratkt coefficienPartition

Soxhlet Extraction

Example of Soxhlet Extraction

• Cereal fat extraction: - fat content is measured by weight loss of the sample or by weight of fat removed.1. Weigh, to the nearest mg, about 2g of predried sample into a predried extraction thimble, with porosity permitting a rapid flow ofethyl ether;2. Weigh predried boiling flask;3. Put anhydrous ether in boiling flask. Assemble boiling flask, Soxhlet flask, and condenser;4. Extract in a Soxhlet extractor at a rate of 5 to 6 drops per second condensation for about 4hrs;5. Dry boiling flask with extracted fat in an air oven at 100 °C for30 mins, cool in desiccator, and weight.

Chromatography

• Chromatography: a general term applied to a wide variety of separationtechniques based on the partitioning or distribution of a sample (solute) between a moving or mobile phase and a fixed or stationary phase.

• The relative interaction of a solute with these two phases is described by the partition (K) or distribution (D) coefficient (ratio of concentration of solutein stationary phase to concentration of solute in mobile phase).

• The mobile phase may be either gas (GC), liquid (LC), and supercriticalFluid.

Chromatography

Characteristics of DifferentChromatographic Techniques

Physicochemical Principles of Separation

• Adsorption (solid-liquid) chromatography: oldest, Tsvet in 1903- The stationary phase is a finely divided solid (to maximize the surface area), -The mobile phase can be either gas or liquid- The stationary phase (adsorbent) is chosen to permit differential Interaction with the components of the sample to be resolved.-The interaction forces include:

- Van der Waals forces- Electrostatic forces- Hydrogen bonds- Hydrophobic interactions

-Typical stationary phases: silica (slightly acidic), alumina (slightlyBasic), charcoal (nonpolar).

Applications of AdsorptionChromatography

• Separate aromatic or aliphatic nonpolar compounds, such as lipids,

primarily according to the type and number of functional groups present.

• Carotenoid pigments from plants;

• Analysis of fat – soluble vitamins, etc…

Ion-ExchangeChromatography

• Ion-exchange Chromatography: a separation/purification process occurring naturally, e.g. in soils, and is utilized in water softeners and deionization. Three types of separation:(1) Ionic from nonionic;(2) Cationic from anionic;(3) Mixtures of similarly charged species.

• Similar to adsorption chromatography• nature of interactions - electrostatic • Cationic exchangers: contain covalently bound negatively charged functional groups.• Anionic exchangers: contain bound positively charged groups.

Ion-ExchangeChromatography (2)

Size-Exclusion Chromatography(SEC)

• SEC, also known as Gel Permeation Chromatography (GPC), can be used for the resolution of macromolecules, such as proteins andCarbohydrates, as well as for the fractionation and characterizationOf synthetic polymers.

Chromatography