topic 18 - modern analytical techniques

Question 1

what solvent is used in NMR

Answer 1

CDCl3

Question 2

why is CDCl3 used as a solvent in NMR

Answer 2

doesn’t produce any signals that could interfere with signals produced from hydrogen atoms

Question 3

what is the name of the molecule used as a standard reference in NMR

Answer 3

tetramethylsilane

Question 4

why is tetramethylsilane used as a standard reference in NMR

Answer 4

it’s a symmetrical molecule so produces 1 strong signal, easily identifiable
unreactive

Question 5

in paper chromatography what is the stationary and mobile phase

Answer 5

stationary phase - filter paper
mobile phase - solvent

Question 6

in paper chromatography why may a substance not travel far up the filter paper

Answer 6

it is more strongly attracted to the stationary phase than the mobile phase

Question 7

in paper chromatography why may a substance travel far up the filter paper

Answer 7

it is more strongly attracted to the mobile phase than the stationary phase

Question 8

describe thin layer chromatography

Answer 8

similar to paper chromatography
instead of filter paper, a thin sheet of glass coated in a thin layer of silica is used (stationary phase)

Question 9

how do you calculate the Rf value

Answer 9

Rf value = distance travelled by component / distance travelled by solvent front

Question 10

describe column chromatography

Answer 10

-silica is packed into a tube and soaked in a solvent
-the investigated mixture is placed on top of the stationary phase (silica) and more solvent is added on top (mobile phase)
-the tap is opened, solvent drips through and components of mixture separate as they move down the tube at different speeds
-components found closer to the bottom of the tube are attracted more strongly to the mobile phase than the stationary phase

Question 11

what is an advantage of column chromatography instead of paper chromatography

Answer 11

much larger quantities of substances can be separated in column chromatography

Question 12

name 2 types of more refined methods of column chromatography

Answer 12

high performance liquid chromatography (HPLC)
gas chromatography (GC)

Question 13

describe how HPLC is different from column chromatography

Answer 13

-solvent is forced through a metal tube under high pressure, instead of passing through by gravity
-particle sizes of the stationary phase are much smaller, better separation of products
-sample is injected into column
-process is automated, a computer rapidly detects the results

Question 14

in HPLC, the metal tube is packed with silicia molecules (SO2) and a hexane solvent is used, will tetrachloromethane have a higher or lower retention time compared to phenylamine

Answer 14

-tetrachloromethane is non-polar, so is more strongly attracted to the hexane (also non-polar), so more attracted to the mobile phase
-so has a longer retention time
-phenylamine is polar, so more strongly attracted to silica (polar), so more attracted to the stationary phase
-so has a shorter retention time

Question 15

describe how HPLC is different from column chromatography

Answer 15

-the metal tube can be metres long and is coiled
-the stationary phase is a solid or liquid coated on the inside of the tube
-mobile phase is an inert carrier gas (mostly nitrogen)
-once the sample is injected, components vaporise and move through the tube
-process is automated, a computer rapidly detects the results

Question 16

describe how mass spectrometry is combined with gas chromatography

Answer 16

-gas chromatography is used to separate the different components in a sample
-each components has a different retention time so come out at different times
-one at a time, each component enters a mass spectrometer
-the m/z values and relative abundances are identified for the components, and can be compared with values for known substances to identify them