Dichroism measurements in forensic fibre examination. Part 3 - dyed cotton and viscose fibres.
A number of dyed cellulosic fibres were examined with plane polarized light on their dichroic behavior by microscopy and microspectrophotometry (MSP-PPL). Significant dichroic effects (mostly hypochromic effects and hypsochromic bands shifts) were reported. The effect is related to the chemical structure: some dye structures always possess dichroism (azo, stilbene, thiazole and oxazine), some dyes demonstrate sometimes dichroic effects (anthraquinoid, indigoid) while other structures never demonstrate dichroic effects (sulphur, diphenylmethanes, triarylmethanes, phthalocyanines). In some cases a different dichroic behavior was found for the same dyes applied on cotton and on viscose.
Dichroism measurements in forensic fibre examination: part 5-pigmented fibres.
A number of pigmented fibre samples were examined with plane polarized light on their dichroic behaviour by optical light microscopy (OLM) and microspectrophotometry with plane polarized light (MSP-PPL). It was found that about half of the samples show a strong dichroic effect and another 20% have a weak dichroism. Both regular (80%) and inversed dichroic effects (20%) occur. The dichroic characteristics of pigmented fibres can be compared to these of sheet polarizers. It is suggested that the dichroic behaviour of pigmented fibres depends strongly on the crystal structure (shape of the pigment grains) and the draw ratio (orientation of the polymer chains).
The standardisation of thin layer chromatographic systems for comparison of fibre dyes
Commercially available TLC development systems were compared on the basis of convenience, reproducibility and resolution. Chromatograms produced using both the Camag No 25155 tank and the Kodak No 13260 tank were reproducible, but the former system was more convenient to use. A series of standard dye mixtures, covering acid, basic, direct, disperse and reactive dyes, was prepared so that TLC systems might be assessed for performance.