Gas chromatography needs compounds to survive vaporisation. Many rubber additives do not. For those, liquid methods are used.
Thin-layer chromatography (TLC)
A drop of extract is placed on a coated plate. A solvent climbs the plate and carries each component a different distance. Spots are visualised under UV light or with a chemical stain.
- Fast and cheap. A good screen for antioxidant and antiozonant types such as the PPD family, hindered phenols and quinoline types.
- Qualitative or semi-quantitative. Spot size and intensity give rough amounts.
- Easy to compare against standards run on the same plate.
High-performance liquid chromatography (HPLC)
The extract is pumped through a column under pressure. Components are detected by UV absorbance, diode array or mass spectrometry.
- Quantitative. With calibration standards, concentrations are accurate to a few per cent.
- Handles non-volatile and heat-sensitive additives.
- LC-MS adds identification power for unknowns.
Accelerators: the hardest ingredient
Accelerators are consumed. A sulfenamide such as CBS or TBBS breaks down to a mercapto compound and amine fragments. A thiuram such as TMTD becomes dithiocarbamate salts and zinc complexes. After cure, what you detect are degradation products, not the original chemical. Experienced analysts read the family from the fragments (for example "a sulfenamide-type accelerator") and report the specific chemical only when the evidence supports it.
Antioxidants survive better
Many antioxidants remain largely unreacted and extractable, so their identification and quantification are reliable. A very low antioxidant level in an old part is also a result: it tells you the part is depleted.
Practical guidance
- Pair TLC screening with HPLC for confirmation and quantification.
- Run extraction from fresh cut samples, not surface layers that have already lost additives.
- Always mention that the result is the residual content after cure and ageing, not the original loading.
