How ATR-FTIR Technology Reveals Art's Hidden Secrets
The world's most precious artworks are hiding layers of history, and a powerful scientific technique is bringing them to light, one molecule at a time.
Imagine examining the cross-section of a 500-year-old Leonardo da Vinci mural and distinguishing a sequence of organic and inorganic layers, each thinner than a human hair. For conservators and art historians, this is not a fantasy. Thanks to high-resolution Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) micro-mapping, this level of analysis is now a reality.
This advanced chemical imaging technique combines the precise identification of infrared spectroscopy with unparalleled spatial resolution, allowing scientists to create detailed molecular maps of incredibly complex, thin-layered structures.
Initially developed for materials science and biomedical research, ATR-FTIR has crossed over into the world of cultural heritage, offering a non-destructive alternative when other techniques fail. It is revolutionizing the way we preserve and understand our artistic legacy by revealing the hidden composition of priceless objects without damaging them.
To appreciate the power of micro-mapping, one must first understand the fundamentals of ATR-FTIR spectroscopy.
Fourier Transform Infrared (FTIR) spectroscopy is a technique that uses infrared light to probe the chemical bonds in a material. When infrared light hits a sample, different chemical bonds (e.g., C-O, N-H, O-H) vibrate and absorb specific wavelengths of light, creating a unique "molecular fingerprint." This fingerprint allows scientists to identify the materials present in the sample.
The Attenuated Total Reflection (ATR) part is what makes this method so convenient and powerful for analyzing solid samples. Instead of passing light through a thin slice of material, ATR works by reflecting an infrared beam inside a special crystal.
The sample is placed in direct contact with a small, hard crystal, often made of diamond.
An infrared beam is directed into the crystal at an angle that causes it to reflect completely off the internal surfaces.
When the light reflects, a tiny electrical field, called an evanescent wave, protrudes slightly beyond the crystal's surface and into the sample.
This evanescent wave is absorbed by the sample, typically penetrating only 0.5 to 2 micrometers deep. The infrared light that returns to the detector carries the unique absorption signature of the sample's molecules 2 .
Standard ATR-FTIR provides a chemical signature for a single spot. High-resolution micro-mapping takes this a giant step further by automating the process to analyze hundreds or thousands of adjacent spots on a sample. By moving the sample with microscopic precision and collecting a full spectrum at each point, the instrument builds a detailed map where every pixel contains a full chemical identity. The resulting data can be used to create images showing the precise spatial distribution of different components—like oils, resins, pigments, or decay products—across the surface.
The key breakthrough lies in the lateral resolution. In optical systems, resolution is fundamentally limited by the diffraction of light, a physical barrier known as the diffraction limit. Advanced ATR-FTIR micro-mapping systems now operate "close to the diffraction limit," meaning they can resolve details almost as small as physically possible for infrared light. This allows them to distinguish features just a few micrometers apart, a capability essential for studying the thin layers found in historic paints and coatings 1 .
Microscopic cross-sections taken from artwork edges
Embedded in resin and polished to smooth surface
Systematic scanning in grid pattern with ATR crystal
Chemical images generated from spectral data
A landmark 2017 study, "Close to the diffraction limit in high resolution ATR FTIR mapping," serves as a perfect demonstration of this technique's power. The research team applied this method to cross-sectional samples from real artworks, including a Leonardo da Vinci mural painting, to uncover their complex, layered histories 1 .
The researchers followed a meticulous process to ensure both the safety of the samples and the accuracy of the data.
Tiny, microscopic cross-sections—smaller than a pinprick—were carefully taken from the edges of existing cracks or damaged, non-visible areas of the artworks to avoid any aesthetic harm.
These cross-sections were then embedded in a resin and polished to create a perfectly smooth surface that reveals a side-view of all the layers.
The analysis was performed using a high-resolution micro-ATR-FTIR instrument equipped with a single-element detector. A diamond ATR crystal was used for its durability and excellent optical properties 1 .
The polished cross-section was placed on the microscope stage. The instrument then systematically scanned the sample point-by-point in a pre-defined grid pattern. At each point, it pressed the diamond crystal onto the surface and collected an entire FTIR spectrum.
After collecting thousands of spectra, specialized software was used to generate chemical images. The software plotted the intensity of a specific molecular vibration (e.g., the carbonyl stretch for resins at ~1710 cm⁻¹) at every location, creating a map where brightness corresponds to the concentration of that specific material 1 .
| Item | Function |
|---|---|
| Diamond ATR Crystal | Extremely hard and chemically inert crystal that contacts the sample; allows internal reflection of the IR beam to generate the evanescent wave 2 . |
| Single-Element Detector | Captures the infrared signal at each individual point on the sample to build the spectral map with high sensitivity 1 . |
| Embedding Resin | A stable polymer used to hold the tiny, fragile sample cross-sections firmly in place during the polishing and analysis process. |
| Mock-up Samples | Laboratory-created multi-layered systems with known composition; used to validate the method's accuracy before applying it to real artworks 1 . |
The application of this technique to the da Vinci mural sample yielded spectacular results. The high-resolution micro-mapping successfully reconstructed the images of the micrometric multi-layered system, revealing a complex stratigraphy that would have been invisible with other analytical techniques.
| Spectral Range (cm⁻¹) | Molecular Vibration | Common Assignment |
|---|---|---|
| ~3300 | N-H Stretch | Proteins (egg, collagen), Nylon |
| ~2920, 2850 | C-H Stretch | Oils, resins, plastics, waxes |
| ~1700-1650 | C=O Stretch (Amide I) | Proteins (animal glue, egg) |
| ~1650-1600 | C=O Stretch | Carboxylic acids (drying oils, resins) |
| ~1550 | N-H Bend (Amide II) | Proteins (animal glue, egg) |
| ~1000-1100 | Si-O Stretch | Silicates (plaster, quartz) |
The chemical maps clearly distinguished between:
The success of this experiment was profound. Firstly, it proved that ATR-FTIR micro-mapping is an effective analytical alternative when fluorescence or thermal effects prevent the use of other methods like micro-Raman spectroscopy. Secondly, it demonstrated that the method provides exceptional spectral quality and chemical image contrast, which is essential for correctly identifying materials in complex mixtures 1 .
The utility of high-resolution ATR-FTIR imaging extends far beyond art conservation. Its ability to provide detailed chemical maps of complex micro-systems has made it a vital tool in numerous fields:
Researchers use it to study the distribution of proteins and lipids in skin tissue, track the dissolution and drug release from pharmaceutical tablets, and even investigate the chemical changes in arterial walls related to atherosclerosis, all in situ and sometimes in contact with water 3 .
The technique is indispensable for tracking the stability and aggregation of proteins in biopharmaceuticals, such as antibodies, ensuring their efficacy and shelf-life 4 .
ATR-FTIR, combined with machine learning, can rapidly identify the species of edible mushrooms (boletes) based on their amino acid profiles, helping to prevent food fraud and poisoning 6 . It has also been used to accurately identify different species of aphids and other insects based on their unique biochemical composition .
| Advantage | Impact |
|---|---|
| High Lateral Resolution | Can analyze thin layers (a few micrometers) found in historic paints and coatings, revealing complex stratigraphy 1 . |
| Minimal Sample Preparation | Reduces the risk of altering or damaging precious micro-samples from artworks. |
| Non-Destructive Nature | The ATR technique is inherently non-destructive, preserving the sample for future analysis. |
| Excellent Spectral Quality | Provides clear molecular fingerprints for confident identification of unknown materials. |
| Chemical Image Contrast | Creates easy-to-interpret visual maps that show exactly where different components are located. |
This powerful fusion of physics, chemistry, and art history demonstrates that the most profound stories of a masterpiece may not be in its visible brushstrokes, but in the hidden, microscopic world of its material composition.
High-resolution ATR-FTIR micro-mapping represents a quiet revolution in the scientific study of cultural heritage.
As this technology continues to evolve, it promises to unveil even more secrets from the past, ensuring that the beauty and knowledge of our shared heritage remain vibrantly alive.
Uncovers artistic techniques and material history
Enables informed conservation decisions
Reveals hidden molecular composition