Meaning
Sample introduction techniques in gas chromatography deliver liquid extracts directly into the capillary column without a heated vaporization chamber. This process of cold on-column injection prevents the thermal degradation of heat-sensitive polymer additives such as slip agents and antioxidants during the analysis of raw materials. It establishes a direct baseline for the actual concentration of additives in the polymer batch before moulding occurs.
The technique finds its limits with highly viscous polymer extracts that cannot be safely processed through a narrow needle.
Thermal Control
Capillary columns must be maintained at temperatures below the boiling point of the extraction solvent during the initial injection phase. Using cold on-column injection suppresses solvent boiling at the needle tip, preventing droplet formation and aerosol creation. This thermal profile ensures that all components of the polymer extract enter the column as a single cohesive band.
Solvents with low boiling points evaporate gradually, allowing the targeted additives to focus tightly at the head of the column.
Sample Deposition
Mechanical delivery of the liquid extract requires a precise alignment of the syringe needle with the capillary inlet. This physical deposition allows cold on-column injection to bypass the discrimination errors typical of hot split or splitless methods. It allows for the accurate measurement of heavy oligomers that otherwise deposit on the glass wool of standard injection ports.
Moulders use the resulting chromatograms to determine the exact fraction of low-molecular-weight species that could cause surface defects in finished parts.
Quantitative Accuracy
Material specifications for medical-grade plastics demand the precise calculation of volatile residuals. In these stringent scenarios, cold on-column injection yields highly reproducible peak areas for both light and heavy migrant compounds. This precision helps quality managers distinguish high-quality virgin resin from cheaper, out-of-specification polymers.
