Polyolefin Resin Grade Verification via Attenuated Total Reflectance Spectroscopy
ATR-FTIR verifies polyolefin grades by measuring specific infrared absorbance ratios to quantify comonomer content, branching density, and copolymer structure.
Zinc Selenide Crystal
Meaning
Yellow, polycrystalline optical element widely used in infrared spectroscopy due to its broad transmission range and moderate refractive index. A zinc selenide crystal is transparent up to twenty micrometres, making it suitable for attenuated total reflectance analysis. This material provides a cost-effective option for routine laboratory testing.
Mechanical Limits
Physical fragility demands careful sample handling and mounting. Compared to diamond, a zinc selenide crystal is relatively soft and prone to scratching or cracking under high mechanical loads. It cannot withstand the high torque required to analyze hard, glass-filled engineering plastics.
This limitation means the crystal is best suited for softer polymers like polyethylene, rubber, and thin elastomeric films.
Optical Interaction
Refractive index contrast controls the penetration depth of the light beam. With a refractive index of two point four, the zinc selenide crystal creates an evanescent wave that penetrates about two micrometres into the polymer sample. This is deeper than the penetration achieved with germanium, giving a stronger spectral signal for typical thermoplastic films.
However, it can lead to distorted peaks if the polymer index is very close to that of the crystal, which occurs with some highly filled aromatics.
Chemical Resistance
Exposure to harsh solvents can damage the optical surface and affect measurement accuracy. The zinc selenide crystal is sensitive to strong acids and ammonium compounds, which can etch the polished face of the element. It can also release toxic hydrogen selenide gas if exposed to highly acidic mixtures.
Analysts must therefore clean the surface only with neutral solvents like isopropyl alcohol to maintain its long-term integrity.