A DRIFTS accessory — an abbreviation for Diffuse Reflectance Infrared Fourier Transform Spectroscopy accessory — is an optical attachment fitted to a standard FTIR (Fourier transform infrared) spectrometer that enables mid-infrared spectral data to be collected from rough, opaque, or cabochon-cut gemstones without the need to prepare a thin, transmissive sample. Rather than passing an infrared beam directly through a specimen, the accessory captures light that has been scattered diffusely from the sample surface and shallow subsurface, making it one of the few mid-IR techniques genuinely suited to the irregular geometries and opaque materials routinely encountered in gemmological practice.

Operating Principle

In conventional FTIR transmission mode, a gemstone must be thin enough — typically tens of micrometres — to allow infrared radiation to pass through without complete absorption. This requirement is impractical for most finished gems and impossible for opaque or heavily included stones. The DRIFTS accessory circumvents this constraint by directing the infrared beam onto the sample surface at a controlled angle and then collecting the diffusely scattered component using an ellipsoidal or parabolic mirror arrangement. The diffusely reflected radiation carries molecular absorption information from the uppermost layers of the material, typically penetrating only a few micrometres to tens of micrometres depending on wavelength and sample composition.

The resulting spectrum is mathematically transformed — most commonly via the Kubelka–Munk function — to convert raw reflectance data into a format that approximates an absorbance spectrum and can be compared directly against reference libraries. Without this conversion, band intensities would be non-linear with concentration, complicating identification.

Gemmological Applications

The DRIFTS accessory is particularly valuable in the following contexts:

• Surface coatings and fillers: Polymer resins, waxes, and glass fillers applied to emeralds, rubies, and turquoise produce characteristic mid-IR absorption bands. DRIFTS can detect these treatments directly on a polished cabochon surface without any sample preparation beyond gentle cleaning.
• Opaque and heavily included stones: Turquoise, malachite, lapis lazuli, and similar opaque gem materials cannot be analysed in transmission mode; DRIFTS provides a viable mid-IR route for species identification and treatment detection on these stones.
• Rough and unpolished specimens: Freshly mined or uncut material can be examined in situ, which is useful for rapid sorting or provenance screening at the source.
• Organic gem materials: Coral, ivory, amber, and jet — all of which present challenges for transmission FTIR — yield interpretable spectra under diffuse reflectance conditions.

Sample Preparation and Specular Reflection

The principal technical challenge of DRIFTS is the contamination of the diffuse signal by specular reflection — mirror-like surface reflection that carries no molecular absorption information and can distort or invert absorption bands in the resulting spectrum, a phenomenon sometimes described as a Reststrahlen artefact. On highly polished gemstone surfaces, the specular component can dominate entirely, rendering the spectrum uninterpretable.

Several strategies are employed to minimise specular contamination:

• Tilting the sample slightly off the focal plane to reduce the specular contribution reaching the detector.
• Using a polariser to discriminate between polarised specular and unpolarised diffuse components.
• Analysing rough or matte surfaces in preference to mirror-polished facets where the investigation permits.
• Applying a fine layer of potassium bromide (KBr) powder as a diluent when the sample geometry allows — a technique more common in analytical chemistry than in gemmology, where non-destructive constraints usually preclude it.

For this reason, DRIFTS tends to perform most reliably on cabochons, rough material, and surfaces with some natural texture, rather than on brilliantly polished faceted stones, where attenuated total reflectance (ATR) accessories or micro-FTIR in reflectance mode may be preferable.

Position Within the Gemmological Laboratory

DRIFTS occupies a complementary niche alongside other FTIR accessories. Transmission FTIR remains the gold standard for faceted, transparent stones of sufficient thinness; ATR accessories offer excellent surface contact measurements on flat or gently curved surfaces; and DRIFTS fills the gap for rough, opaque, or irregularly shaped specimens where neither of the foregoing is practical. Major gemmological laboratories — including the GIA laboratory and Gübelin Gem Lab — employ diffuse reflectance infrared techniques as part of broader spectroscopic workflows for treatment detection, particularly for polymer-filled rubies and emeralds and for the characterisation of simulants and assembled stones.

Further Reading

• GIA Gems & Gemology — peer-reviewed articles on FTIR applications in gemmology