System Analysis of FTIR Equipped with ZnSe (Zinc Selenide) ATR

FTIR systems equipped with a ZnSe ATR (Attenuated Total Reflection) accessory are widely used for rapid surface analysis of various materials. This configuration allows infrared spectra to be obtained directly from solids, liquids, pastes, and films without complex sample preparation procedures such as pellet pressing or dissolution. The ZnSe ATR module provides reliable spectral data while maintaining good signal-to-noise performance and operational efficiency.

Core Principles and Structure

ATR (Attenuated Total Reflection) Mechanism

In ATR analysis, infrared light undergoes total internal reflection inside the ZnSe crystal. During this process, an evanescent wave is generated at the interface between the crystal and the sample. This evanescent wave penetrates slightly into the sample surface, typically to a depth of about 1–5 μm.

When the sample is in close contact with the crystal surface, specific molecular groups absorb infrared radiation at characteristic wavelengths. The absorption causes a reduction in the intensity of the reflected infrared beam, allowing the FTIR system to record the characteristic spectrum of the sample.

Because ATR measures surface interaction between the infrared beam and the sample, materials such as solids, liquids, pastes, and thin films can be tested directly without complicated pre-treatment.

ZnSe ATR Module

The ZnSe ATR module generally adopts a single-reflection or multi-reflection prism structure, with an incidence angle commonly around 45°. It is integrated into the FTIR sample chamber and typically includes a controllable pressure head to ensure adequate contact between the sample and the crystal.

ZnSe crystals are usually produced using chemical vapor deposition (CVD). The resulting crystal appears as a light yellow transparent polycrystalline material. A reflective coating is often applied to enhance infrared light throughput and improve the optical efficiency of the ATR system.

Know-Why

In an ATR accessory, the effective contact area between the sample and the ATR crystal is very small. Applying controlled pressure helps the sample closely contact the crystal surface. Even when the infrared light undergoes only one effective reflection inside the ATR crystal, high-quality spectra can still be obtained.

Single-reflection ATR accessories are suitable for testing a wide range of materials, including:

• Solid samples
• Fibers
• Hard polymers
• Paint films
• Thin coatings on glass or metal surfaces
• Trace liquids

The crystal platform of the ATR accessory usually exposes only a small plane of about 3 mm. Therefore, during measurement, it is recommended that the flat surface of the sample closely contacts the crystal area.

When ZnSe is used as the crystal material, the preset pressure is typically around 20 kg. Under these conditions, the liquid samples that can be tested generally fall within a pH range of 5–9. Strong acids and strong bases should not be tested with ZnSe crystals.

Diamond crystals, in comparison, can withstand greater pressure and have strong chemical resistance. They can test liquids across a wider pH range from 1 to 14 and are suitable for harder samples. Other materials such as ZnSe, AMTIR, Ge, and Si can also be used as ATR reflection crystals depending on analytical requirements.

Advantages, Applicable Scenarios and Limitations

Core Advantages

ZnSe ATR accessories offer several practical advantages:

• Lower cost compared with diamond ATR
• High mid-infrared transmittance and good signal-to-noise ratio
• No complex sample preparation required; liquids can be directly dropped onto the crystal surface and solids can be lightly pressed
• Suitable for rapid batch testing
• Good compatibility with heating or low-temperature accessories

Aplicaciones típicas

Pharmaceutical Industry

ZnSe ATR is commonly used for the analysis of ointments, gels, tablet coatings, and rapid qualitative identification of pharmaceutical raw materials.

Polymer Materials

Applications include testing plastic films, rubber materials, and surface components of adhesives.

Chemical Industry

It is widely used for quality control of oils, coatings, inks, and emulsions.

Food Industry

ZnSe ATR can be applied to detect components and spoilage in oils, sauces, and beverages.

For laboratories requiring higher measurement efficiency or improved signal interaction, accessories such as the Accesorio reflectante Multi ATR can provide enhanced reflection paths and improved analytical flexibility.

Limitations

Although ZnSe ATR crystals are widely used, several limitations should be considered:

• The crystal is relatively soft and prone to scratching
• Chemical corrosion resistance is weaker than that of diamond crystals
• Sensitivity for strongly absorbing samples, such as highly concentrated dark solutions, is lower than that of germanium (Ge) ATR
• Not suitable for highly abrasive samples such as mineral powders

Key Points for Operation and Maintenance

Preparación de la muestra

The solid surface should be flat to ensure good contact with the crystal. Liquid samples should not contain bubbles and should avoid strongly corrosive media.

Cleaning

After testing, the crystal surface should be cleaned using absolute ethanol or isopropyl alcohol and gently wiped with a soft non-woven cloth. Acetone or strong alkaline cleaning agents should not be used.

Pressure Control

The matching pressure head should be used to avoid excessive pressure on the crystal. Powder samples should be applied as a thin and evenly distributed layer.

Daily Maintenance

• Clean the crystal immediately after testing
• Seal and protect the crystal from moisture when not used for extended periods
• Avoid drastic temperature fluctuations
• Do not touch the crystal surface with hard objects

Spectrum Correction

Regular spectral calibration should be performed using polystyrene membranes or standard liquids. ATR background subtraction and wavelength calibration help maintain spectral accuracy.

Proper sample contact should also be ensured to prevent baseline drift caused by poor crystal contact.

Comparison with Other ATR Crystals

Crystal	Refractive Index	Rigidity	Applicable Samples	Price
ZnSe	2.40	Medio	Medium-low absorption liquids, soft solids, pastes	Medio
Diamante	2.41	Muy alta	Hard, highly corrosive samples	Alta
Ge	4.00	Bajo	Strongly absorbing or dark-colored samples	Moderately low

Caption: Comparison of common ATR crystal materials used in FTIR accessories.

Selection and Compatibility with Spectrometers

Mainstream FTIR instruments generally support standard ZnSe ATR modules, which can be installed directly without recalibrating the optical path.

If strongly absorbing samples need to be measured, multi-reflection ZnSe or Ge ATR should be prioritized. When analyzing samples containing strong acids or strong oxidizing agents, diamond ATR is recommended due to its superior chemical resistance.

In ATR mode, spectral peak intensity is related to the wavenumber because of differences in penetration depth. Therefore, quantitative analysis typically requires ATR correction algorithms such as the Eriksson correction.

During analysis, consistent background subtraction should also be maintained to prevent baseline drift caused by poor contact between the sample and the ATR crystal.