final exam

Question 1

molbile phase

Answer 1

gas or liquid
in tlc it’s the solvent
in gc it’s the inert gas (helium typically)

Question 2

stattionary phase

Answer 2

liquid or solid
in TLC it’s moving past silica gel
in GC it’s going through the column

Question 3

best seperation of TLC

Answer 3

if polarity of the solvent and the target compounds are similar an optimum seperation can be achieved because even small differences in polarity lead to different degrees of retention by stationary phase

Question 4

what does GC create

Answer 4

molecules come out at different times and then signals are generated by the help of a computer

Question 5

the injector of GC

Answer 5

mobile phase
carrier gas is also within this usually helium or H2 (help push sample through)
injector is kept at a temperature that higher than the boiling point of the sample because to make sure the sample vaporizes instantly (can’t be too low or high though)

Question 6

The column of GC

Answer 6

stationary phase
where molecules seperate
molecules with strongest attractive forces will move through column the slowest while non polar moves through faster

Question 7

what happens when column temperature is either too high or too low

Answer 7

too low - kinetic energy of molecules will be very low, one or more compounds that enter the gc column may be absorbed by the stationary phase, compounds do not move because they do not reach the detector

too high - kinetic energy of molecules is too high, compounds will barely interact in stationary phase, they remain in mobile phase, no seperation

Question 8

detector in GC

Answer 8

different types - TCD, MS, and FID
all have different sensitivies but doesn’t change the result

we use GC-MS - it separates, quantifies, and identifies compounds at the same time

Question 9

gas chromatogram features

Answer 9

retention time - time needed for compound to pass through the compound

peak area - is proportional to the amount of the compound that is detected

Question 10

parameters that could affect the outcome of gc

Answer 10

*most affective one is temperature

longer column - longer retention time
higher column temperature - shorter retention time
higher helium flow - shorter retention time
higher polarity of the stationary phase - longer retention time

Question 11

peak area with GC and integrals/ratios

Answer 11

the area under the peak is called the integral, it’s proportional to the amount of compound that moves through the detector

detectors are sensitive and might not show all the same result

Question 12

response ratios of GC

Answer 12

it’s the response of a given compound relative to the response of another compound that was chosen as a standard
A = 20
B= 30
C = 50

RRa = 20/30 = .67
RRc = 50/30 = 1.67

keep b the same but a and c can be divided further

Question 13

GC theory

Answer 13

Sn2 reaction

Question 14

CC adsorption/mobile and stationary phase

Answer 14

when solvent moves down the stationary in a column filled with silica gel, the strongest intermolecular phases are those between the surface of the silica gel and the solvent or sample molecules because the silica gel is usually the most polar component in the system

while the solvent sample mixture moves down the column all components in the mobile phase are in competition for the silica gel surface

Question 15

process behind CC

Answer 15

a solid adsorbent (silica gel) is used as the stationary phase, it is packed into a column and a solvent (the molbile phase) containing a mixture of compounds is moved past the solid material. The solvent used as a mobile phase is often referred to as an eluent

Question 16

seperation of CC

Answer 16

more polar molecules will move slower throughout the column than non polar molecules

Question 17

slurry packed method vs dry pack method

Answer 17

slurry pack method - formed from a mixture of silica and solvent

after addition of the slurry to the column, the stopcock is opened to allow the eluent to drain as the adsorbent bed packs. the stop cock is closed when the absorbent is settled.

air bubbles mist not be allowed to form in the bed, bubbles cause the bed to be irregular and interfere with the uniform movements of mixture of the compounds, this leads to a decreased efficiency of the seperation

dry pack method - silics is added to an empty column with a cotton plug, the eluent is added to column and pushed (with pressure) through the stationary phase until all silica is wet

Question 18

applying compounds to the column (as liquid or solid)

Answer 18

as a liquid - sample is dissolved in minimal amount of solvent and is taken up by pipette and then put into the top of the column. The solution is allowed to absorb into the silica gel by opening up the stopcock and letting all liquid disappeared into the packing

as a solid - the compounds that have to be separated are dissolved in a solvent, the solution is mixed with a small amount of adsorbent. the solvent is removed by evaporation, the resulting powder has the sample absorbed to the surface of silica gel. The dry mixture of sample and adsorbent is added to the top of the column bed

this application takes more time than as a liquid, so only applies to when material that has to be purified by column chromatography has pnly a low solubility in the preffered eluent

Question 19

detecting separated compounds

Answer 19

when compounds move through silica gel down a column, they form bands (disc shaped areas in which compounds are present)

most of the time collected into test tubes then dotted on TLC plate

Question 20

optimizing seperation