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CHY1402 - Analytical methods and applications > Preparative chromatography > Flashcards

Preparative chromatography Flashcards

1
Q

What type of technique is TLC?

A

Analytical technique

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2
Q

What type of technique is column chromatography?

A

Preparative technique

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3
Q

Which range of values do you want the Rf values to be within?

A

0.2-0.4

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4
Q

What is the first step for running column chromatography?

A
  • Add a slurry of silica gel in the eluent to a glass column
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5
Q

What is the second step for running the column for column chromatography?

A

Open the tap to allow solvent to drain

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6
Q

What is the third step for running the column for column chromatography?

A

Add a thin layer of sand to the top of the silica to protect the surface

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7
Q

What is the 4th step for running the column in column chromatography?

A

Add a concentrated solution of the mixture

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8
Q

What is the 5th step for running the column in column chromatography?

A

Add the eluent and do not allow the column to run dry

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9
Q

What are the components to the column chromatography?

A 
Glass column 
Sand layer 
Stationary phase 
Glass Frit
Collection vessel 
Eluent
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10
Q

What is gradient elution?

A

You change the solvent during elution

to move polar compounds quicker increase solvent polarity

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11
Q

What are the two most common options for stationary phase?

A

Silica

Alumina

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12
Q

What is the pH like of silica?

A

Slightly acidic

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13
Q

What is the pH like of alumina?

A

Slightly basic

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14
Q

What does silica retain more strongly?

A

Bases

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15
Q

What does alumina retain more strongly?

A

acids

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16
Q

What are characteristics of silica?

A

Always wet

Uses compound hydrolysis

17
Q

What happens when you collect colourless compounds from the column?

A

Collect regular samples in tubes

Retain a sample of the original mixture for comparison

18
Q

How do you analyse colourless compounds?

A

Spot original alongside all of the fractions collected in the right order
Run TLC using the same solvent as the column

19
Q

How do you detect the colourless compounds on the TLC plate? What are the two methods?

A

Under UV light

By chemical means

20
Q

How does detection work by placing under a UV light?

A

The TLC plates are coated with fluorescent material

Place under a UV light and the compounds will be able to seen

21
Q

How does detection work by chemical means?

A

Spray plate or dip into a reagent

22
Q

Which general reagents are used for detection by chemical means?

A

Iodine vapour
Potassium permanganate
Vanillin
Phosphomolybdic acid

23
Q

Which reagents can be used to identify aldehydes and ketones?

A

2,4 - dinitrophenylhydrazine

24
Q

Which reagent is used to identify amino acids and amines?

A

Ninhydrin

25
Q

What happens once you have identified the spots with the same Rf values?

A

You combine those fractions and then evaporate off the solent

26
Q

What do you need to do to the flask before you add the fractions?

A

Pre-weigh it so you can work out the dry product to work out the yield for the reaction

27
Q

Why must you add the solvent when the eluent is below the sand?

A

The product will dissolve in the mobile phase which will cause poor separation and low yield for the pure sample

28
Q

What are two problems that can occur in column chromatography?

A

Changes in the solvent composition can change the Rf values

There may be overlapping of fractions that can occur

29
Q

What does the length of the column determine?

A

The separation of the compounds

30
Q

What happens if the column is too short?

A

The difference in Rf will be too small

31
Q

What does a longer column mean?

A

More time to separate effectively

32
Q

What does the width of the column determine?

A

How thin the band is

33
Q

What happens if the column is too wide?

A

There will be too much compound added

34
Q

What does a thinner band mean?

A

Better separation of compounds

CHY1402 - Analytical methods and applications (14 decks)

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