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1st Year sem 2: Forensic Identification > D L3: Thin layer Chromatography > Flashcards

D L3: Thin layer Chromatography Flashcards

1
Q

where was TLC first developed and when?

A

Ukraine 1937

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2
Q

what samples can be sued for TLC?

A

Amino acids, drugs, lipids

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3
Q

what do samples in TLC need to be in?

A

need to be in a solution with an organic solvent

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4
Q

In TLC samples may have to be?

A

dissolved in a solvent or extracted from a substrate

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5
Q

what materials re TLC plates?

A

glass, plastic or foil

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6
Q

what are TLC plates coated in?

A

fine grade silica (SiO2)

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7
Q

How big are TLC plates?

A

purchased at 200x200 mm2

cut to 20x50 mm2 or 50x80 mm2 rectangles

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8
Q

what is important to make sure the TLC is run accurately?

A

important make sure that the silica at the edges of the plates is not extensively chipped

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9
Q

what does the silica on TLC plates need to be like?

A

robust enough to write on using a soft pencil

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10
Q

how is a sample appleid to a TLC plate?

A

The sample is applied using a TLC spotter which is a fine glass capillary

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11
Q

what size should the sample spots me?

A

4-5mm in diameter

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12
Q

How should you increase the amount of solvent applied?

A

repeat spot, waiting for the solvent dry in between applications

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13
Q

where does the TLC plate need to be placed to run it?

A

in a sealed developing tank.

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14
Q

what is filter paper used for during TLC?

A

plate in a sealed developing tank.

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15
Q

where do the spots of sample need to be ?

A

above the level of the solvent

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16
Q

How does the solvent move up the TLC plate?

A

by capillary action

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17
Q

when should the plate be removed from the tank?

A

when the solvent front is 3-4 mm from the top of the plate and the position of the solvent front marked

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18
Q

what ways can a TLC plate be viewed?

A
  1. under a UV light
  2. Coloured spots
  3. Liquid developers
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19
Q

what wave are used when suing a UV light to look at a TLC plate?

A

short wave (254 nm) and long wave (365 nm)

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20
Q

Why can a UV light be used to develop a TLC plate?

A

TLC plates contain fluorescent compounds which are bright under UV.
shows up as a dark purple spots on a white background

21
Q

When using a UV light what does a compound nee dot be visible?

A

a chromophore which absorbs the UV light

22
Q

How can you tell which compound is more present when using UV light on a TLC plate?

A

The relative intensity of a spot when compared to other spots of the same compound gives an indication of the amount of compound present

23
Q

what is chromophore?

A

This is the part of the molecule which is responsible for the light absorbing properties of a molecule

24
Q

what does chromophore consist of?

A

consist of a series of conjugated double bonds

25
Q

How is a liquid developer used to develop a TLC plate?

A
  • sprayed on or the plate can be dipped in

- plate heated to initiate the reaction between developer and compounds

26
Q

are stains formed form liquid developers permanent?

A

not always, can fade overtime

27
Q

what is the retention factor?

A

distance moved by compound

28
Q

if two spots have the same Rf what can this mean?

A

they are likely to be the same compound

29
Q

what is adsorption?

A

interaction between the analyte and the surface of the stationary phase

30
Q

The more strongly the analyte is adsorbed the _________ it travels through the stationary phase

A

slower

31
Q

what is silica surface?

A

its polar

32
Q

When using a Silica stationary phase the solvents chosen are usually ______ in nature

A

organic

33
Q

if the analyte is polar what does the solvent need to be

A

polar

34
Q

when is optimal separation achieved?

A

when all the spots are separated

35
Q

what happens when the solvent is too polar ?

A

all the spots are crowded at the top of the plate

36
Q

what happen when the solvent is too non-polar?

A

the spots are crowded at the bottom of the plate

37
Q

what should you do when analysing basic molecules?

A

add a small amount of base to solvent system such as NH3 or Et3N

38
Q

what should you do When analysing acidic molecules ?

A

add an acid to the solvent mixture

39
Q

what dictates polarity of an organic molecule?

A

functional groups

40
Q

what do functional groups contain that make a compound polar?

A

polarised bonds

41
Q

what does it mean if The compound runs as a streak rather than a spot ?

A

The sample was overloaded

42
Q

what does it mean if the The sample runs as a smear or a upward crescent ?

A

Compounds which possess strongly acidic or basic groups tend to smear

43
Q

what does it mean if the sample runs as a downward crescent ?

A

The adsorbent was disturbed during the spotting

44
Q

what does it mean if the plate solvent front runs crookedly ?

A

Either the adsorbent has flaked off the plate or the sides of the plate are touching the paper

45
Q

what does it mean No spots are seen on the plate ?

A

Not spotted enough compound, or use a different developer or the solvent is over the baseline

46
Q

what is 2D-TLC?

A
  • sample is run in one direction and then the plate is turned around 90o and run in the other direction
  • Each time the plate is run a different solvent system is used
  • Spots that overlap in the first run may separate out in the second
47
Q

what are some advantages of TLC?

A
  • High through-put
  • Low cost
  • Easy sample preparation
  • Requires the minimum amount of equipment
  • Limited training of analyst required
  • An excellent ‘look-see’ technique which gives you an idea of what is in the sample
  • Good for a whole variety of compounds, though basic compounds can be problematic
  • Range of visualisation techniques
48
Q

what are disadvantages of TLC?

A
  • Poor chromatographic resolution
  • Constant need to run standards alongside analytes on the same plate
  • Quantification difficult and may not be reproducible

1st Year sem 2: Forensic Identification (15 decks)

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