Forensic analysis of fiber colorants using TLC should be considered for single fiber comparisons only when it is not possible to discriminate between the fibers of interest using other techniques, such as comparison microscopy (brightfield and fluorescence) and microspectrophotometry in the visible range.

The extraction procedures carried out prior to TLC analysis can provide useful information about dye classification. TLC can provide useful qualitative information about dye components. Similar colors made up of different dye components can be differentiated using this technique. The application of TLC may serve to discriminate between fibers, or it may confirm their similarity.

TLC may be prohibitively difficult or undesirable in some circumstances. Short lengths of fibers or pale colored fibers may not have an adequate concentration of colorant present to be examined, dye extraction from some fibers may be impossible. The desire to preserve evidence for possible analysis by another examiner may preclude removing the color for analysis.

Dye from the known material should first be characterized and eluent systems evaluated to achieve optimum separation of the extract. Dye is then extracted from single known and questioned fibers, using an equivalent amount of material.

The development of each individual TLC plate will show some variability as a result of the coating and conditioning of the plate, solvent condition, and temperature. It is important to evaluate the performance of each TLC plate by spotting known materials along with the questioned samples. See Ref (16).

Examples for the preparation of Standard dye mixtures are given in Appendix X1.

1.1 Metameric coloration of fibers can be detected using UV/visible spectrophotometry. If spectrophotometry is restricted to the visible spectral range only, differences in dye components may remain undetected. One method of detecting additional components is to use thin-layer chromatography (TLC). TLC is an inexpensive, simple, well-documented technique that, under certain conditions, can be used to complement the use of visible spectroscopy in comparisons of fiber colorants. The principle of the method is that the dye components are separated by their differential migration caused by a mobile phase flowing through a porous, adsorptive medium.

1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.