Thin Layer Chromatography

Question 1

Thin Layer Chromatography

Answer 1

An analytical technique allows for the separation of the component dyes in a colourant.

Question 2

The colour of a fabric depends on

Answer 2

which wavelength of visible spectrum are absorbed and which are reflected by the dye in the fabric that form the fabric

Question 3

Why would a fabric appear white or black?

Answer 3

If the dye has total reflection of the visible spectrum: fabric appears white
and If the dye has total absorption: fabric appears black

Question 4

What is a dye?

Answer 4

A dye is a colored substance that has an affinity to the substrate to which it is being applied.

Question 5

All dye contain

Answer 5

A chromophore
An auxochrome

Question 6

What is a chromophore?

Answer 6

A chromophore is a part of a molecule that absorbs light.

Question 7

What is auxochrome?

Answer 7

Auxochrome a functional group that influences the solubility of a molecule and increases its ability to bond to a fibre, hence intensifying the colour.

Question 8

Fibre dyes are classified according to…

Answer 8

1. Dyeing process (influenced by the solubility of the dye in water
2. Chemical class
3. Type of fibre to which they are applied (also influenced by the solubility of the dye in water)

Question 9

Dyeing process is

Answer 9

The dyeing process is accomplished by dissolving or dispersing the colorant in a suitable solution (usually water)

Question 10

Nitro Dyes are

Answer 10

It contain nitro (-NO2) as chromophore and hydroxyl group (-OH) as auxochrome

Question 11

Nitroso Dyes

Answer 11

contain the nitroso(-NO)group as chromophore and Phenolic (-OH) as auxochrome.

Question 12

Azo Dyes

Answer 12

Azo group (-N=N-) present between two aromatic rings is the primary chromophore.

Question 13

Triarylmethane Dyes

Answer 13

Central carbon bonded to three six membered carbon ring structures( two aromatic), one in the quinoid form, is the chromophore.
E.g. malachite green, crystal violet, phenolpthalein

Question 14

Anthraquinone Dyes

Answer 14

The anthraquinone ring is the chromophore and -OH, SO3H, and –NH2 groups are the auxochrome.
E.g. Alizarin, Remazol brilliant blue.

Question 15

Indigo Dyes

Answer 15

• Have the indigoid structure (-CO-C=C-CO-)
• Used to dye cotton used mainly for the production of denim cloth for blue jeans.

Question 16

Non- reactive/Reactive Dyes

Answer 16

Acid, Disperese, Azo, Basic, Mordant, Direct, Reactive

Question 17

Non Extractable Dyes

Answer 17

Vat & Sulfur

Question 18

Acid dyes

Answer 18

*Are water-soluble anionic dyes (negatively charged) that are applied to fibers using neutral to acid dye baths.
*Useful for fibres like silk, wool, polyamide and polypropylene

Question 19

Fibres like wool have many amino groups and therefore

Answer 19

Fibres like wool have many amino groups and therefore dye penetration is high (dark shades achievable)

Question 20

Fibres like silk and polyamides have fewer amino groups hence

Answer 20

Fibres like silk and polyamides have fewer amino groups hence difficult to achieve dark colours.

Question 21

Basic Dyes

Answer 21

• are water-soluble cationic dyes (positively charged)
• Applied to modified acrylic, polyacrylonitrile, polyester and polypropylene fibers.
• These dyes are salts of ammonium, sulphonium or oxonium.
• Glacial acetic acid is added to aid their solubility and hence uptake into fibres.

Question 22

Examples of Basic Dye

Answer 22

methylene blue, crystal violet.

Question 23

Direct Dyes

Answer 23

• Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dye bath.
• Used on cotton, paper, rayon, linen, leather, wool, silk and nylon.

*Applied directly to the fibres from aqueous medium containing an electrolyte such as sodium chloride (NaCl) or sodium sulfate (Na2SO4).

Question 24

Metaballized Dyes

Answer 24

• No natural affinity to fibre.
• Require a mordant, which improves the fastness of the dye against water, light and perspiration.
• Fibre is first treated with mordant then dye solution
• Used to dye wool

Question 25

The mordant in metallised dyes

Answer 25

The mordant: Chromium (Cr) oxides/ sodium/potassium dichromate

Question 26

Vat Dyes

Answer 26

• Rarely seen in forensic fibre examination except indenim garments
• used for dyeing of cellulosics
• Large wooden vats are used to apply dye-hence name
• Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly.

Question 27

Subsequent oxidation reforms

Answer 27

the original insoluble dye.

Question 28

Reactive dyes utilises

Answer 28

Reactive dyes utilize a chromophore attached to a substituent that is capable of chemically reacting with the functional groups of the fibre polymer forming covalent bonds.

Question 29

The covalent bonds that attach reactive dye to natural fibers make them

Answer 29

among the most permanent of dyes.

Question 30

Reactive dyes are used to dye

Answer 30

Used to dye wool, cellulosics and sometimes polyamides

Question 31

In a weakly alkaline solution, procion dye becomes

Answer 31

attached through hydroxyl groups to the fibre

Question 32

Disperse dyes are

Answer 32

• Are water insoluble.
• The dyes are finely ground in the presence of a dispersing agent .
• The aqueous dispersion consists of the dye, water and the surface agent (dispersing agent).
• The dispersing agent aids the dye by promoting penetration and uniform distribution of colour.
• The very fine particle size of dye gives a large surface area that aids dissolution to allow uptake by the fibre.

Question 33

Disperse dye is used to dye

Answer 33

polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic fibres

Question 34

Azoic dyes are used to dye

Answer 34

cotton and viscose

Question 35

Why is azoic dying process importance?

Answer 35

This technique of dyeing is unique, in that the final color is controlled by the choice of the diazoic and coupling components.

This method of dyeing cotton is declining in importance due to the toxic nature of the chemicals used.

Question 36

Sulfur dyes are used to dye

Answer 36

Used to dye cellulosics.

Question 37

Sulfur dyes must be reduced to produce

Answer 37

These dyes must be reduced to produce the leuco form of the dye conventionally using sodium sulphide in alkaline dye bath. Gives blue, black, mustard, yellow and olive green colours

Question 38

Largest selling dye by volume?

Answer 38

Sulfur Black 1 is the largest selling dye by volume.

Question 39

TLC for non-reactive dyes

Answer 39

[Dye classes: acid, azoic, basic, direct, disperse, metallized]

First classify the fibre in control garment

Evaluate extraction solution

Evaluate eluent system for optimum separation of dye

Extract dye from single recovered and control fibres

Prepare TLC chromatograms

First classify the fibre in control garment

Evaluate extraction solution

Evaluate eluent system for optimum separation of dye

Extract dye from single recovered and control fibres

Prepare TLC chromatograms

Question 40

Choosing appropriate extraction solution

Answer 40

The dye class and the fibre type will dictate the extraction solution to be used

In preparing extraction solutions, pyridine/water 4:3 v/v and formic acid/water 1:1 v/v can be prepared ahead of time

2% oxalic acid (0.2g in 10ml water) needs to be used immediately after it is made