Thin Layer Chromatography Flashcards
Thin Layer Chromatography
An analytical technique allows for the separation of the component dyes in a colourant.
The colour of a fabric depends on
which wavelength of visible spectrum are absorbed and which are reflected by the dye in the fabric that form the fabric
Why would a fabric appear white or black?
If the dye has total reflection of the visible spectrum: fabric appears white
and If the dye has total absorption: fabric appears black
What is a dye?
A dye is a colored substance that has an affinity to the substrate to which it is being applied.
All dye contain
A chromophore
An auxochrome
What is a chromophore?
A chromophore is a part of a molecule that absorbs light.
What is auxochrome?
Auxochrome a functional group that influences the solubility of a molecule and increases its ability to bond to a fibre, hence intensifying the colour.
Fibre dyes are classified according to…
- Dyeing process (influenced by the solubility of the dye in water
- Chemical class
- Type of fibre to which they are applied (also influenced by the solubility of the dye in water)
Dyeing process is
The dyeing process is accomplished by dissolving or dispersing the colorant in a suitable solution (usually water)
Nitro Dyes are
It contain nitro (-NO2) as chromophore and hydroxyl group (-OH) as auxochrome
Nitroso Dyes
contain the nitroso(-NO)group as chromophore and Phenolic (-OH) as auxochrome.
Azo Dyes
Azo group (-N=N-) present between two aromatic rings is the primary chromophore.
Triarylmethane Dyes
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
Anthraquinone Dyes
The anthraquinone ring is the chromophore and -OH, SO3H, and –NH2 groups are the auxochrome.
E.g. Alizarin, Remazol brilliant blue.
Indigo Dyes
- Have the indigoid structure (-CO-C=C-CO-)
- Used to dye cotton used mainly for the production of denim cloth for blue jeans.
Non- reactive/Reactive Dyes
Acid, Disperese, Azo, Basic, Mordant, Direct, Reactive
Non Extractable Dyes
Vat & Sulfur
Acid dyes
*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
Fibres like wool have many amino groups and therefore
Fibres like wool have many amino groups and therefore dye penetration is high (dark shades achievable)
Fibres like silk and polyamides have fewer amino groups hence
Fibres like silk and polyamides have fewer amino groups hence difficult to achieve dark colours.
Basic Dyes
- 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.
Examples of Basic Dye
methylene blue, crystal violet.
Direct Dyes
- 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).
Metaballized Dyes
- 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
The mordant in metallised dyes
The mordant: Chromium (Cr) oxides/ sodium/potassium dichromate
Vat Dyes
- 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.
Subsequent oxidation reforms
the original insoluble dye.
Reactive dyes utilises
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.
The covalent bonds that attach reactive dye to natural fibers make them
among the most permanent of dyes.
Reactive dyes are used to dye
Used to dye wool, cellulosics and sometimes polyamides
In a weakly alkaline solution, procion dye becomes
attached through hydroxyl groups to the fibre
Disperse dyes are
- 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.
Disperse dye is used to dye
polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic fibres
Azoic dyes are used to dye
cotton and viscose
Why is azoic dying process importance?
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.
Sulfur dyes are used to dye
Used to dye cellulosics.
Sulfur dyes must be reduced to produce
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
Largest selling dye by volume?
Sulfur Black 1 is the largest selling dye by volume.
TLC for non-reactive dyes
[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
Choosing appropriate extraction solution
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
