Describe the purpose of the mobile phase in chromatography: a. What is the main

Question

Describe the purpose of the mobile phase in chromatography:

a. What is the main function of the mobile phase?

b. What are the requirements for the mobile phase in GC? Give an example for a mobile phase in GC.

c. What is the main difference between the function of the mobile phases in GC and LC?

2.Consider the analysis of arctic air samples for parts-per-billion of Freon 11

(CCl3F), a chemical that can damage the earth's ozone layer. If you had a gas

chromatograph, which of the following detectors would be most useful for the

analysis of the air samples?

a. electron capture detector

b. thermal conductivity detector

c. flame ionization detector

f. all of the above could be used for analysis of parts-per-billion of CCl3F in air

e. it is not possible to detect parts-per-billion of CCl3F with any of these detectors

3.Consider the effect of mass diffusion on band broadening in a gas

column. How does this effect depend upon the flow rate of the

mobile phase?

a. The height equivalent to a theoretical plate decreases with an increase in the

flow rate of the mobile phase.

b. The height equivalent to a theoretical plate increases with an increase in the flow rate

of the mobile phase.

c. The height equivalent to a theoretical plate is independent of the flow rate of the

mobile phase.

d. There is insufficient information to answer the question

Explanation / Answer

1a) A chromatographic system consists of a stationary phase on a solid support and a mobile phase. The stationary phase consists of solid adsorbents while the mobile phase is a gas or a liquid. Analyte molecules in a sample mixture interact with the stationary adsorbent particles and are loosely held by the particles. The function of the mobile phase is to push these analyte molecules through the bed of packed stationary particles so that the analyte molecules are separated into bands depending on the strength of their interactions with the stationary and the mobile phase. In short, the analyte molecules interact with the molecules of the mobile phase. Depending on whether the analyte/stationary phase or analyte/mobile phase is stronger, the analyte molecules are separated into bands and are eluted as pure fractions.

b) The mobile phase in GC is usually a gas like He, Ar or N2. The requirements for the mobile phase in GC can be summarized as :

i) The carrier gas must be chemically inert. This is important so that the carrier gas does not react with the analytes being analyzed.

ii) The carrier gas must be thermally stable. GC separations are usually performed at high temperatures and the carrier gas must remain stable at such high temperature.

iii) The carrier gas must be very pure. Impurities in the carrier gas can react with the analytes or decompose the analytes or give unwanted signals in the GC.

iv) The carrier gas must be economically priced to keep the cost of operation within limits.

c) The main difference between the mobile phases in LC and GC relates to the mode of operation of the two techniques. GC is usually used for analysis of non-polar hydrocarbons while LC can be used for non-polar, medium polar or highly polar compounds. In GC, the carrier gas (mobile phase) is an inert gas. The sample is vaporized at the inlet and travels along the GC column with the mobile phase. The main function of the carrier gas is therefore to make the compounds travel through the column. Depending on the volatility of the compounds and the strength of interaction of the analytes with the stationary phase, the compounds are separated into bands and are eluted.

LC mobile phases play a more important function. The most widely used LC technique is Reversed Phase LC where the stationary phase is less polar than the mobile phase. Depending on the polarities of the compounds being analyzed, different compounds in the sample matrix interact to different extent with the stationary phase and are loosely bound to the stationary phase. The mobile phase is more polar than the stationary phase and interact with the analyte molecules. Depending on whether the analyte/mobile phase interaction is stronger or weaker than the analyte/stationary phase interaction, the analyte molecules are separated into bands and elute as pure fractions.

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.