ATR Accessory
ATR Accessory
Attenuated total reflectance sampling for FTIR analysis of gemstone surfaces
An ATR accessory (attenuated total reflectance accessory) is a specialised sampling module fitted to a Fourier-transform infrared (FTIR) spectrometer that allows mid-infrared spectra to be collected directly from the surface of a polished gemstone without cutting, powdering, or otherwise preparing the sample. The accessory works by pressing an infrared-transparent crystal — typically diamond, germanium, or zinc selenide — against the specimen surface; infrared radiation undergoes total internal reflection within that crystal, and the evanescent wave that penetrates a fraction of a micrometre into the sample is attenuated by the material's characteristic absorption bands. The resulting spectrum is functionally equivalent to a conventional transmission spectrum and can be matched against reference libraries for identification purposes.
Operating Principle
Total internal reflection occurs when radiation travelling through a dense optical medium strikes an interface with a less dense medium at an angle exceeding the critical angle. Rather than passing through, the beam reflects internally, yet a standing electromagnetic field — the evanescent wave — extends a short distance (typically 0.5–2 µm, depending on wavelength and refractive indices) beyond the interface into the contacting material. This wave interacts with molecular bonds in the sample and is attenuated at frequencies corresponding to their vibrational modes. Because penetration depth is so shallow, ATR sampling is inherently surface-sensitive and is largely unaffected by the bulk thickness or opacity of the specimen.
The ATR crystal must have a refractive index substantially higher than that of the material under analysis. Diamond ATR elements are preferred in gemmological laboratories because diamond is chemically inert, extremely hard, and resistant to scratching by faceted stones. Germanium elements offer a higher refractive index (approximately 4.0) and are useful for highly absorbing materials, though they are softer and more vulnerable to surface damage.
Gemmological Applications
ATR spectroscopy has become a routine tool in advanced gemmological testing laboratories, valued particularly where transmission FTIR is impractical:
- Pearls and nacre. The curved, opaque surface of a pearl cannot be placed in a standard transmission beam path. ATR contact measurement yields spectra that distinguish natural nacre from imitation materials (glass, shell bead, or polymer coating) and can reveal the presence of bleaching agents or dyes in the outer nacre layers.
- Amber and copal. ATR readily distinguishes Baltic amber (characterised by the diagnostic "Baltic shoulder" absorption near 1250 cm⁻¹ in its succinite spectrum) from younger copal resins and from polymer simulants such as polystyrene or celluloid, without any need to sample the material.
- Polymer-impregnated gemstones. Jadeite, turquoise, and certain rubies and emeralds are commercially treated with epoxy resins or other polymers to improve stability or apparent clarity. ATR spectra of the polished surface can detect the carbonyl and ether absorption bands characteristic of these fillers, complementing bulk transmission measurements.
- Coated stones. Thin surface coatings — lacquers, wax layers, or anti-reflective films — produce strong ATR signals even when they are invisible to the naked eye or under magnification, because the evanescent wave samples precisely the outermost layer.
Practical Considerations
Contact pressure between the ATR crystal and the gemstone surface must be applied carefully and consistently; most modern accessories incorporate a pressure applicator with a torque-limiting mechanism to ensure reproducible contact without risk of damaging either the crystal or the stone. Surface cleanliness is critical: residual fingerprint oils, polishing compounds, or mounting adhesives will contribute their own absorption bands to the spectrum and must be removed with a solvent wipe before measurement.
Because ATR penetration depth is wavelength-dependent (deeper at longer wavelengths), band intensities and relative ratios differ somewhat from transmission spectra of the same material. Analysts should use ATR-specific reference libraries or apply mathematical corrections (ATR-to-absorbance conversion) when making direct comparisons with transmission data.
The technique is non-destructive in the sense that no material is removed, though there is a theoretical risk of surface marking from the contact crystal on very soft or heavily included stones. In practice, a diamond ATR element pressed against a faceted gemstone of hardness 6 or above causes no observable damage under normal operating pressures.
Laboratory Context
Leading gemmological laboratories, including the Gemmological Institute of America and Gübelin Gem Lab, incorporate FTIR with ATR capability as part of their standard instrument suites. The accessory is typically used alongside transmission FTIR, Raman spectroscopy, and UV-Vis spectrophotometry rather than as a standalone technique, since each method interrogates different aspects of a stone's composition and treatment history. For pearl testing in particular, ATR FTIR has largely supplanted earlier wet-chemical and density methods for the detection of surface treatments.