Diamond, ZnSe or germanium? Choosing your ATR crystal

The crystal is the ATR accessory. Everything else is optics and mechanics around it. Four properties settle the choice: refractive index, penetration depth, spectral range and chemical tolerance — and each material trades them differently.

The physics in one paragraph

For total internal reflection, the crystal's refractive index must comfortably exceed the sample's (most organics sit near 1.5, so a crystal at 2.4 or above is safe). The evanescent wave's penetration depth falls as the crystal's index rises, and rises at lower wavenumbers; at 45° incidence and 1,000 cm⁻¹ against a typical organic, diamond and ZnSe (both n ≈ 2.4) probe roughly 2 µm, while germanium (n = 4.0) probes roughly a third of that — about 0.7–1 µm.

Diamond — the default that earns its price

Chemically inert across essentially the full pH range, effectively scratch-proof, and usable to very low wavenumbers (Specac's monolithic diamonds are specified far into the far-IR). Its one quirk is a throughput dip around 2,200 cm⁻¹ from the diamond lattice itself — occasionally relevant for nitrile bands, rarely fatal. The purchase price is the highest of the three; the lifetime cost is usually the lowest, because it survives the samples and the operators. For a laboratory buying one ATR, buy diamond.

ZnSe — the economical liquids specialist

Similar refractive index to diamond, excellent throughput, and a spectral window of roughly 20,000–650 cm⁻¹ — at a fraction of diamond's cost. The constraints are physical and chemical: ZnSe is soft (it scratches under careless clamping and even under abrasive wipes — use cotton-based materials only) and tolerates roughly pH 5–9, ruling out strong acids, oxidisers and complexing amines. As the trough crystal in multi-reflection accessories for liquids and soft samples, it is exactly right.

Germanium — the strong-absorber problem-solver

Germanium's high index is a feature, twice over. First, the shallow ~1 µm penetration keeps ferociously absorbing samples — carbon-black-filled rubbers, dark filled polymers, printing inks — on scale where diamond and ZnSe simply saturate. Second, that same shallowness makes Ge a surface-selective probe: the top layer of a laminate or coating, largely free of the bulk beneath. Its window is narrower (roughly 5,500–600 cm⁻¹) and the weaker overall signal is the price of admission. If your laboratory sees rubber compounds, buy the Ge option on day one.

Silicon, briefly

Hard, robust, index 3.4, with useful far-IR transparency — and available as consumable ATR slides for contamination-critical workflows where cleaning validation costs more than the slide.

The specification shortcut

Accessories like the Quest ATR make this a non-decision at purchase time: interchangeable pucks mean one instrument runs diamond for daily duty, Ge for the black samples, and ZnSe where budget-per-measurement matters. Tell us your three most common samples and we will configure the crystal set around them — start here, or test your samples against the Selector first.