Chemistry Extended Response Chromatography

Question 1

Stationary Phase in TLC?

Answer 1

fine layer of adsorbent material e.g. alumina or silica, coated on an inert solid surface such as a glass plate

Question 2

What does the stationary phase in TLC separate components by?

Answer 2

Polarity, Affinity; Physisorption; Strength and number of Intermolecular forces - more for polar, less for non-polar.

Question 3

What is the mobile phase in TLC?

Answer 3

Suitable organic or water solvent

Question 4

How does the mobile phase move up the TLC plate?

Answer 4

Capillary action

Question 5

By what features does the mobile phase separate components in TLC?

Answer 5

Polarity, Affinity, Solubility

Question 6

Retention Factor Definition

Answer 6

Retention factor is the distance travelled by an individual component along the stationary phase divided by the elution distance of the mobile phase.

Question 7

Polarity interactions in TLC?

Answer 7

In normal phase:
Polar compounds are more attracted to the stationary phase and less attracted to the mobile phase; higher Rf

Non-polar compounds are less attracted to the stationary phase but more attracted to the mobile phase; lower Rf

and vice versa. for reverse-phase

Question 8

Solubility and affinity for stationary and mobile phases

Answer 8

As solubility in and affinity for the mobile phase increases; the elution of the compounds becomes slower; lower Rf

As affinity for the stationary phase increases, slower elution of compounds; higher Rf

Question 9

Steps of TLC

Answer 9

1. The stationary phase is prepared with adsorbent
2. The sample is applied towards the bottom of the place
3. The plate is placed vertically in a developing chamber
4. Separation takes place due to combined action of factors such as polarity, affinity, solubility, and adsorption levels for the stationary and mobile phases
5. The plate is taken out of the developing chamber
6. The samples resolution is visualized directly for colored samples or alternatively for invisible samples that have fluorescent properties, UV-light may be used for the visualisation.

Question 10

What is the purpose of TLC

Answer 10

To qualify substances by visualisation; no quantification

Question 11

Limitations of TLC

Answer 11

Requirement of pre-known Rf comparative values

Almost continuous possibility for separation by controlling plate length

Since TLC is generally conducted within an open system, environmental factors such as temperature and humidity can change the separation efficiency

Question 12

Forensic and analytical application of TLC?

Answer 12

Forensic application:
- Drug Testing

Analytical applications:
- Identification of pesticides
- Lipid analysis

Question 13

What is the stationary phase in gas chromatography?

Answer 13

The stationary phase is a very thin layer of inert high-boiling point liquid on an inert solid surface such as silica beads compressed in a long skinny tube. This tube is coiled multiple times inside a column oven which is kept at a constant temperature.

Question 14

What is the mobile phase in gas chromatography

Answer 14

The mobile phase is an inert gas. This gas carries the sample.

Question 15

Retention time definition GC?

Answer 15

Retention time refers to the time taken for a compound in a substance to pass through the thin tube called a column and reach the detector.

Question 16

What is the purpose of GC? How is it done?

Answer 16

To identify and quantify the substances in a sample; by comparing to standard reference chromatograms

Question 17

Features of the inert carrier gas

Answer 17

• Inert gas
• Low molecular weight to reduce resistance to components

Question 18

Gas Chromatography (GC) Process

Answer 18

1. Gaseous or liquid sample is injected into the vaporisation port
2. The sample vaporisation port is then heated up for liquid samples.
3. The vaporised sample is then introduced into the column by the action of the carrier gas from the cylinder.
4. Separation occurs by polarity and other physical properties of components.
5. Components exit column and enter into detector
6. Detector outputs electrical signals, quantifying components
7. Data processing unit plots a gas chromatogram (intensity of signal on vertical axis, retention time on horizontal axis). Qualifies and quantifies the sample components

Question 19

How is the quantity of a substance read from a chromatogram

Answer 19

Area under substance peak

Question 20

What factors does the stationary phase separate the components by in GC?

Answer 20

Volatility, Polarity, Solubility, and affinity

Question 21

What factors does the mobile phase separate the components by in GC?

Answer 21

Contrasting volatility, solubility, polarity and affinity.

Question 22

Analytical and Forensical application of GC

Answer 22

Forensic Applications:
- Forensic toxicology
- Drug Analysis
- Arson Investigation

Question 23

What is the stationary phase in HPLC?

Answer 23

The stationary phase in HPLC in normal-phase is a polar solid, while in reverse-phase HPLC, it is a non-polar solid. The part of the solid packed in the column that reacts interacts with the target compounds are more accurately referred to as the stationary phase

Question 24

What is the mobile phase in HPLC?

Answer 24

In normal-phase HPLC the mobile phase is a non-polar or relatively less polar liquid compared to the stationary phase. In reverse-phase HPLC, it is a polar liquid.

Question 25

What is the purpose of HPLC?

Answer 25

To identify and quantify different components in a sample.

Question 26

Compounds with the highest retention time in HPLC mainly due to polarity…

Answer 26

Elute last

Question 27

Compounds with the lowest retention time in HPLC mainly due to polarity…

Answer 27

Elute first

Question 28

Limitations of Gas Chromatography

Answer 28

• GC only has the ability to separate volatile (low boiling point) and thermally stable compounds. Highly sensitive equipment
• The sample retention times of a substance are usually required to be compared to a standard run with chemically pure substances of the compound suspected to be analysed in a chromatogram and signified.

Question 29

HPLC process?

Answer 29

1. A high pressure pump is attached to the column
   - The column is filled with adsorbent material such as silica or alumina, that has a very small particle size, to increase surface area for interaction
2. The mobile liquid phase is taken from the solvent reservoir and pumped through the high-pressure pump and injected into the column by the sample injector, alongside the sample
3. Separation occurs by polarity, affinity and molecular weight.
4. The separated components are then introduced to the UV-Vis detector, where it exposed to UV light
5. The absorbance values are recorded
6. Detector outputs signals which quantify and provides the purity of components.
7. Along with the retention times in the column, the absorbance values are recorded on the vertical axis on the chromatogram

Question 30

Limitations of HPLC

Answer 30

• Lack of Universal detector
• Lower separation efficiency than capillary gas chromatography due to lower peak capacity
• Very expensive system and large quantities of expensive organic compounds are use
• Not suitable for gradient elution

Question 31

In HPLC and GC, the detection of which types polarity of compounds are reverse phase, and normal phases suitable for respectively

Answer 31

Reverse phase is suitable for detecting polar compounds

Normal Phase is suitable for detecting polar compounds

Question 32

Factors that lead to better of more efficient separation in HPLC

Answer 32

• Increased column temperature
• Wider and longer column
• Higher concentration of solvent
• Sample molecular size

Question 33

HPLC forensic and analytical applications

Answer 33

Analytical Applications:
- BAC analysis
- Drug analysis
Forensic Applications:
- Trace analysis