Waste 1. Place vials of product in Hood for the Supervisor to discard 2. Place a

Question

Waste 1. Place vials of product in Hood for the Supervisor to discard 2. Place any undistilled starting material into Beaker in Hood Labeled "Starting Material Discards" Exercise 2 1. Describe the function(s) of each of the following: a. The carrier gas b. The column packing c. The solid support 2. What is the effect of the following on retention time? a. Increases in flow rate of carrier gas. b. Increases in temperature of the column. c. Decreases in molecular weights of compounds d. Increases in concentration of compounds 3. If the recording paper is moving at the rate 2.0 in./min., what are the retention times of the peaks that appear at: a. 10.0 in b. 4.0 cm c. 25.0 mm 35

Explanation / Answer

excercise2:

1. a.

A carrier gas facilitates transport of the injected sample to the column for separation of components of the mixture. Typical purity levels should be 99.995% or higher. Desirable features of carrier gas are :

• Inertness towards sample and stationary phase
• Should not have a response on the detector

Commonly used gases are Nitrogen, Helium and Argon. Helium gas is preferred as carrier with TCD detector because of high explosion hazard of hydrogen. Traps are recommended in the gas lines to prevent moisture or other impurities from reaching the columns.

b.

A capillary column for GC is basically a very narrow tube with the stationary phase coating the interior surface. In packed columns the stationary phase is coated onto the packing particles.

In all chromatography the separation of different compounds occurs because of their interaction with the stationary and mobile phases. For example in simple paper chromatography a solvent (water, alcohol etc.) flows over the paper (stationary phase) moving the sample with it. The different compounds which make up the sample will move more or less slowly depending, in simple terms, on how much they stick to the paper. The "stickier" compounds move more slowly therefore move a shorter distance in a given time resulting in separation. In gas chromatography the gas is the mobile phase, the column coating is the stationary phase and the vapourised sample is separated by how long the constituent compounds take to emerge from the other end of the column and pass into the detector. This is called the retention time. You can buy columns coated with different stationary phases depending on what type of compounds you want to analyse as the type of stationary phase will determine which compounds pass over it more quickly/slowly.

c. the purpose of solid support is to provide a large surface area for holding thin film of liquid phase. The main requirement for adequate support material are chemical inertness ans stability and large surface area,

2.a. increase in flow rate low retention time.

b. On increase in temperature of column , lesser retention time

c.less molwt less retention time

d. increase in concentration less retention time

3 given:

flow rate is 2 inch/min

a. 10 inches = 2 inch in 1 min, so 10 inches will be moved in 5 min

b. 1 cm= 0.3937 inch

so 4cm= 4*0.3937 inch =1.5748 inch

2 inches wil be covered in 1 min, so for 1.5748 inch 0.7874min

c. 1mm=0.03937inch, 25 mm=25*0.03937=0.9842inch , it takes 0.4921 min.

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