Cleaning ATR crystals without wrecking them
An ATR crystal is the most-touched optical surface in the laboratory — pressed against hundreds of samples a month and wiped after every one. Cleaned properly, it lasts years. Cleaned carelessly, the damage shows up not as a dramatic failure but as quietly falling throughput and baselines you stop trusting. The routine takes sixty seconds; the chemistry rules take two minutes to learn.
The sixty-second routine
- Remove the bulk of the sample with a dry, soft, lint-free wipe — no pressure, no circular scrubbing.
- Moisten a fresh wipe with isopropanol (or acetone for stubborn organics, where the crystal chemistry allows — table below) and pass it gently across the crystal and surrounding plate.
- Let the solvent flash off, then collect a fresh background. A clean crystal shows a flat 100% line and the energy throughput you recorded on day one.
What each crystal tolerates
| Crystal | Safe cleaning | Keep away | Character |
|---|---|---|---|
| Diamond | Water, IPA, acetone — chemically inert and extremely hard; even dilute acids are tolerated | Abrasives (they won't scratch the diamond, but will ruin the surrounding plate and optics) | The forgiving one — which is why it is the default |
| ZnSe | IPA or acetone on a soft wipe, used promptly | Acids and strong alkalis — hold the crystal to pH 5–9; abrasive wipes and hard samples, because ZnSe is comparatively soft and scratches | The one that pays for carelessness fastest |
| Germanium | IPA or acetone, feather-light pressure | Mechanical shock and over-clamping — Ge is brittle | Handle like glass; it is the carbon-black specialist, not the rough-service crystal |
| Silicon | IPA or acetone | Abrasives; aggressive hot alkalis | Robust and hard — a sensible consumable-slide material |
Full optical and chemical selection data — refractive index, range, penetration depth — lives in the crystal table in our Sample Preparation Hub, and the choosing-a-crystal question has its own guide.
Stubborn residues
Polymer films and greases lift with repeated thin solvent passes, not one soaked assault — change the wipe each pass so you are removing material rather than redistributing it. Dried aqueous residues come off with a water-damped wipe followed immediately by IPA and a dry pass. Cured or baked-on deposits are a stop-and-think moment: escalate the solvent cautiously within the crystal's chemistry, and if it will not lift, call us before you reach for anything mechanical.
Pressure discipline is crystal care
Half of "cleaning" problems are actually clamping problems. The pressure anvil on a quality ATR exists to deliver repeatable, limited force — use it as designed, and match the crystal to the sample: hard granular powders belong on diamond, not ZnSe. If your work is rough service by nature, consumable silicon ATR slides (Quest Arrow) turn crystal damage from a repair into a line item.
When the crystal is already tired
Falling energy throughput, a background that will not flatten, visible fogging or scratches under raking light — these are repolish-or-replace signals, not cleaning problems. As the authorised Specac channel we arrange crystal and top-plate service with genuine parts: describe the symptoms and we will advise whether it is a clean, a repolish or a replacement.