Visualising Reagent
Definition
A spray or dip that converts a colourless TLC spot into a coloured or fluorescent one so the chemist can locate it. Marquis turns opioids purple and amphetamines orange; Dragendorff turns alkaloids orange-red; ninhydrin turns amino acids violet. The reagent panel is what makes TLC a presumptive identification rather than a generic separation.
- Purpose
- Reveals colourless TLC spots as colour or fluorescence
- Marquis on opioids
- Purple
- Marquis on amphetamines
- Orange
- Dragendorff
- Orange-red with alkaloids
- Ninhydrin
- Violet with amino acids
Common questions
Does a colour reaction on TLC confirm the drug's identity?+
No. It is presumptive only. The same colour can be given by structurally related compounds, so a positive TLC visualisation is followed by a confirmatory technique such as GC-MS or LC-MS before an identification is reported.
Can more than one reagent be applied to the same plate?+
Usually the plate is run in duplicate or triplicate lanes so different reagents, or the same reagent under UV light, can be applied to separate lanes without one spray interfering with another spot's reaction.
Related terms
- Mobile Phase
- The solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching...
- Resolution (Rs)
- Rs = 2(tR2 − tR1) / (W1 + W2). Above 1.5 means baseline separation; below 1.0 means peaks overlap and quantitation is...
- Rf Value
- The retention factor, defined as the distance the spot travels divided by the distance the solvent front travels on the same plate....
- Stationary Phase
- The immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it...
- Theoretical Plate Count (N)
- A measure of column efficiency, N = 16(tR/W)² for a column peak, where tR is retention time and W is peak width...