1) Which of the following gases is unsuitable for use as a GC carrier gas?(10 )
ID: 969869 • Letter: 1
Question
1) Which of the following gases is unsuitable for use as a GC carrier gas?(10 )
a. nitrogen
b. helium
c. oxygen
2) Which type of column has the greater efficiency and resolution, packed or capillary?(10)
a.Packed column
b. Capillary column
c. None
d. Both have equal efficiency
3) In Gas Chromatography (GC) what is the stationary phase? What is the moving phase?(10)
4) Volatile compounds are best analyzed using gas chromatography having (10)
a. Thermal conductivity based detector (TCD)
b. Flame ionization detector (FID)
c. Both
d. None
5) Which of the following detectors is known as Universal detector in GC(10)
a. Thermal conductivity based detector (TCD)
b. Flame ionization detector (FID)
c. Both
d. None
6) What does the retention factor, k', describe? (10)
a)The distribution of an analyte between the stationary and the mobile phase
b) The migration rate of an analyte through a column
c) The velocity of the mobile phase
7) What two factors affect the movement of mixture during TLC chromatography? (10)
a. Solubility and affinity
b. Capillary action and Retention factor
c. Temperature and column length
d. None
8) Of the following compounds, which one will elute from the chromatography column last?(10)
a) a will elute first and b will elute last
b) d will elute first and c will elute last
c) b will elute first and a will elute last
d) b will elute first and d will elute last
9) The retention factor Rf is (10)
a. Distance traveled by solvent divided by the distance by the compound
b. Distance traveled by fastest compound divided by the solvent
c. Distance traveled by the compound divided by the distance traveled by the solvent.
d. Distance traveled by slowes compound divided by the distance traveled by fastest compound.
10) If the retention factor of compound A is 0.76 and B is 053. Which of the two compounds is more polar (10)
a. Compound A is more polar than Compound B
b. Compound B is more polar than Compound A
c. Both have same polarity
d. Polarity cannot be determined by the given data.
11.
Explain why this statement is true: "If two spots in a chromatogram have the same Rf values, they may be the same compound. If they do not have the same Rf, they are not the same compound." (20)
12.
Why is it difficult to separate compounds that have similar structures? (10)
13.
Which of these compounds would have the largest Rf if they were chromatogrammed with a polar stationary phase and a nonpolar mobile phase? And why ? (10+10)
11.
Explain why this statement is true: "If two spots in a chromatogram have the same Rf values, they may be the same compound. If they do not have the same Rf, they are not the same compound." (20)
12.
Why is it difficult to separate compounds that have similar structures? (10)
13.
Which of these compounds would have the largest Rf if they were chromatogrammed with a polar stationary phase and a nonpolar mobile phase? And why ? (10+10)
Explanation / Answer
1)c.oxygen
2)b. Capillary column
3)The mobile phase (=carrier gas) is comprised of an inert gas i.e., helium, argon, or nitrogen. The stationary phase consists of a packed column in which the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (=high boiling polymer).The separation of compounds is based on the different strengths of interaction of the compounds with the stationary phase (“like-dissolves-like”-rule).
4)b. Flame ionization detector (FID)
5)a. Thermal conductivity based detector (TCD)
6)b. The migration rate of an analyte through a column
7)a. Solubility and affinity
8)compounds not given
9)c. Distance traveled by the compound divided by the distance traveled by the solvent.
10)b. Compound B is more polar than Compound A
11)Rf is simply a measure of polarity. Many compounds have very similar/identical polarities.There are a near infinite number of different compounds. If you are able to measure an Rf value to within an accuracy of 0.1% this means that there are only 1000 possible values.
Logically many substances must have the same Rf value.that does not mean they are same compound (may or maynot be) and also if their polarity is different definately they are not of the same compound.
12) similar structures mean similar affinities so the spots will move the similar distances and not be resolved hence difficult to separate compounds
13) no compounds given
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