ATR-FTIR Crystal Guide
A side-by-side comparison of the four major ATR crystal materials. Choose the right crystal for your sample type and application.
Crystal Comparison
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| Property | Diamond | ZnSe | Germanium | Silicon |
|---|---|---|---|---|
| Spectral Range | 4000–400 cm⁻¹ (gap at 2300–1800) | 4000–650 cm⁻¹ | 4000–700 cm⁻¹ | 4000–1500 cm⁻¹ |
| Refractive Index | 2.4 | 2.4 | 4.0 | 3.4 |
| Depth of Penetration | ~2 µm | ~2 µm | ~0.66 µm | ~0.8 µm |
| Hardness (Knoop) | 8000 | 150 | 780 | 1150 |
| Chemical Resistance | Excellent — acids, bases, solvents | Poor — acids dissolve it | Good | Good |
| Mechanical Durability | Excellent | Fragile | Moderate | Good |
| Best For | Universal — any sample | Soft samples, routine use | Dark / highly absorbing samples | Semiconductor industry |
| Relative Price | $$$ | $ | $$ | $$ |
Which Crystal Should I Use?
Answer a few questions about your sample to get a recommendation.
STEP 1 OF 5
Is your sample highly absorbing or dark-colored?
Crystal Details
Crystal choice directly affects sample preparation requirements and spectral quality. The refractive index determines the depth of penetration at each wavenumber. See the detailed properties below.
Diamond
Diamond is the gold standard for ATR crystals. Its combination of extreme hardness, chemical inertness, and wide spectral range makes it suitable for virtually any sample type.
PRACTICAL TIPS
- The only crystal safe for acidic, basic, and abrasive samples
- Intrinsic diamond absorption creates a gap at ~2300–1800 cm⁻¹ — avoid this region for quantitative work
- Type IIa diamonds have the best IR transparency
- Can withstand high clamping pressures without damage
CLEANING
Wipe with isopropanol or ethanol on a lint-free cloth. For stubborn residues, use acetone. Diamond tolerates mild abrasive cleaning if necessary. Always verify with a background scan after cleaning.
ZnSe
ZnSe is the most common and affordable ATR crystal. Its refractive index matches diamond, but it's much softer and chemically vulnerable. Ideal for routine work with benign samples.
PRACTICAL TIPS
- Never use with acidic samples — ZnSe dissolves in dilute acid
- Low hardness means scratches from abrasive powders are common
- Same refractive index as diamond (2.4) — spectra are directly comparable
- Best value for labs doing routine polymer, organic, or pharmaceutical analysis
CLEANING
Clean gently with isopropanol or hexane on a soft tissue. Never use acidic cleaners or abrasive materials. If the crystal surface appears hazy, it may need professional polishing.
Germanium
Germanium's high refractive index (4.0) produces a very shallow penetration depth. This is a feature, not a limitation — it prevents spectral saturation in highly absorbing samples where other crystals fail.
PRACTICAL TIPS
- First choice for dark, opaque, or highly absorbing samples
- Shallow dp means surface-sensitive measurements — useful for coatings and thin layers
- Higher refractive index means the critical angle is lower — easier to achieve total internal reflection
- Temperature-sensitive: Ge becomes opaque above ~125°C
CLEANING
Clean with isopropanol or acetone on a lint-free cloth. Germanium has moderate hardness — avoid abrasive cleaning. Handle with care to prevent chipping.
Silicon
Silicon ATR crystals are primarily used in the semiconductor industry and for specialized applications. The limited spectral range (cutoff at 1500 cm⁻¹) restricts general use, but Si has good mechanical and chemical properties.
PRACTICAL TIPS
- Spectral cutoff at ~1500 cm⁻¹ means you lose the fingerprint region
- Good for C-H, O-H, and N-H stretching regions (3600–2800 cm⁻¹)
- Harder than ZnSe and Ge — more resistant to scratching
- No absorption features in the usable spectral range
CLEANING
Clean with isopropanol or semiconductor-grade solvents. Silicon is relatively durable — gentle wiping is sufficient for most residues.
Frequently Asked Questions
What is the best ATR crystal for general use?
Diamond is the best ATR crystal for general use. It offers the widest spectral range (4000–400 cm⁻¹), extreme hardness (Knoop 8000), and excellent chemical resistance. Diamond ATR crystals handle acidic, basic, and abrasive samples without damage, making them suitable for virtually any analytical application.
Can ZnSe ATR crystals be used with acidic samples?
No. ZnSe dissolves in acidic solutions and should never be used with acidic samples. Use a diamond ATR crystal for acidic or corrosive samples. ZnSe is best suited for routine analysis of benign, non-corrosive materials like polymers and organic compounds.
Why use germanium ATR instead of diamond?
Germanium has a higher refractive index (4.0 vs 2.4 for diamond), which produces a shallower penetration depth (~0.66 µm). This makes germanium the first choice for highly absorbing or dark-colored samples where diamond or ZnSe would produce saturated spectra. Germanium is also useful for surface-sensitive measurements of coatings and thin layers.
What is the spectral range of a diamond ATR crystal?
A diamond ATR crystal covers 4000–400 cm⁻¹, which spans the entire mid-infrared region. However, diamond has an intrinsic absorption band between approximately 2300–1800 cm⁻¹ that reduces sensitivity in this region. This gap typically does not affect most analyses, but avoid it for quantitative work in that range.
RELATED GUIDES
ATR-FTIR Sample Preparation
Techniques for preparing liquids, solids, powders, films, and biological samples for ATR analysis.
ATR vs Transmission FTIR
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ATR-FTIR Applications
Pharmaceutical, polymer, forensic, and food science applications with example spectra.