Chapter 7 - Separation and Identification Techniques

Flashcards generated from Chapter 7 notes covering Planar Chromatography (TLC), HPLC, and Gas Chromatography concepts, components, formulas, and diagrams.

How is chromatography defined in the provided notes?

It refers to the group of techniques used to separate complex mixtures into their components based on the different physical interactions between the individual compounds and the stationary phase of the system.

What are the three types of physical interactions by which mixture components interact with the stationary phase in chromatography?

Charge, relative solubility, or adsorption.

What are the two major categories of chromatography, and which specific techniques belong to each?

1. Planar Chromatography: Paper Chromatography and Thin Layer Chromatography (TLC).
2. Column Chromatography: High Performance Liquid Chromatography (HPLC) and Gas Chromatography (GC).

What are the four general components of a chromatography system?

1. Mobile phase (gas or liquid)
2. Stationary phase (solid or liquid supported on a solid)
3. Column holding the stationary phase
4. Eluate (separated components)

What are the primary components and sorbent materials used in Thin-Layer Chromatography (TLC)?

TLC uses a thin, uniform layer of sorbent (stationary phase) coated on glass, metal, or rigid plastic backing plate. Most commonly used sorbents are Alumina, Silica gel, Cellulose, and Cross-linked dextran.

Which diagram shows the standard experimental setup for Thin-Layer Chromatography (TLC)?

The Thin-Layer Chromatography (TLC) chamber diagram showing the glass tank with cover, TLC plate coated with sorbent, origin line, and mobile phase solvent.

Why do nonpolar substances move faster than polar substances on a standard TLC plate?

Nonpolar substances are weakly adsorbed to the stationary phase and are more soluble in the solvent, allowing them to travel a greater distance.

What is the formula for calculating the retention factor ($R_f$) in chromatography?

$$R_f = \frac{\text{distance traveled by the component of interest}}{\text{total distance traveled by the solvent front}}$$

According to the provided diagram, what is the $R_f$ value of a red dye that travelled $1.7\,\text{cm}$ from the baseline while the solvent travelled $5.0\,\text{cm}$?

$$R_f = \frac{1.7}{5.0} = 0.34$$

What are the main disadvantages of Thin-Layer Chromatography (TLC)?

1. It lacks reproducibility from one chromatography run to the next.
2. The degree of equilibration with the vapor phase in the separation chamber renders the $R_f$ value somewhat variable (not precise and not reproducible).

Why was High Performance Liquid Chromatography (HPLC) formerly called High Pressure Liquid Chromatography?

Because high pressures of $3000\,\text{psi}$ or more are commonly required for proper operation.

Which diagram illustrates the component layout and flow path of an HPLC system?

The HPLC system schematic showing the solvent reservoir, pump, sample injector, column, detector, data station, and waste line.

What are the functions of sample injectors and pumps in HPLC, and which types are most commonly used?

Sample injectors introduce the sample into the mobile phase (loop injector is most commonly used). The pump forces the mobile phase through the column while maintaining constant volume or constant pressure flow (mechanical reciprocating pump is most widely used).

What is the difference between Normal-Phase Columns and Reversed-Phase Columns in HPLC?

Normal-Phase Columns have a polar stationary phase (silica) and a non-polar mobile phase (e.g. hexane), causing non-polar components to elute first. Reversed-Phase Columns have a polar mobile phase and a nonpolar stationary phase, causing the most polar components to elute first. Reversed-phase HPLC is the most commonly used form.

What is Retention Time in HPLC, and what factors cause it to vary?

Retention time is the time taken for a compound to travel through the column to the detector (measured from injection to peak maximum). It varies depending on: pressure used, nature of stationary phase, exact solvent composition, and column temperature.

What is the graph recorded during HPLC analysis called, and what does the peak represent?

The graph is called a chromatogram. Peak height or area is proportional to the concentration of the compounds that produced the peak.

What are the two main disadvantages of HPLC mentioned in the notes?

HPLC is expensive and gives slower results than Gas Chromatography.

What carrier gases are commonly used in Gas Chromatography (GC), and why must they be inert?

Carrier gases include nitrogen, helium, or argon. They must be inert so that there will be no explosion.

Which schematic diagram details the internal components of a Gas Chromatography (GC) instrument?

The GC analysis diagram showing the carrier gas, two-stage regulator, sample injector, injector oven, coiled column in column oven, detector oven, and recorder/workstation.

What analytical technique is modern Gas Chromatography commonly coupled with to assist in identifying unknown compounds?

Mass spectrometers (MS).

What three behaviors can occur to a molecule injected into a Gas Chromatography column?

1. It may condense on the stationary phase.
2. It may dissolve in the liquid on the surface of the stationary phase.
3. It may remain in the gas phase.

In Gas Chromatography, what happens to a compound with a boiling point higher than the column temperature?

It will obviously tend to condense at the start of the column and will not turn into gas.