ATR (Attenuated Total Reflectance)
Definition
An FTIR sampling mode in which the sample is pressed against a diamond or ZnSe crystal and the evanescent wave probes the surface. No pellet, no mull, no preparation. The default sampling mode in modern forensic FTIR.
- Sampling depth
- 0.5-2 micrometers (surface layer)
- Common crystals
- Diamond, zinc selenide (ZnSe), germanium
- Sample prep required
- None (direct contact method)
Common questions
What is ATR and why is it useful in forensic FTIR analysis?+
ATR (attenuated total reflectance) is an FTIR sampling mode where you press the sample directly against a crystal, usually diamond or zinc selenide. It requires no sample preparation like pellets or mulls, making it fast and the default method in modern forensic labs. The evanescent wave samples just the top surface layer.
Can ATR sample paint evidence? How deep does it go?+
Yes. ATR works well on paint layers using a germanium or diamond crystal pressed against the surface. It samples approximately the top 0.5 to 2 micrometers and produces high-quality undistorted spectra, making it a reliable tool for examining paint composition in forensic cases.
What is the main advantage of ATR over traditional FTIR sample prep?+
ATR eliminates the need for manual sample preparation. You skip the time and mess of making pellets or mulls. Just press the sample against the crystal, collect the spectrum, and you are done. This speed and simplicity have made it the standard in forensic labs.
Related terms
- ASTM E2937
- ASTM International standard guide for identification of paint binders by pyrolysis-GC-MS; specifies pyrolysis conditions, GC column, oven program, and diagnostic ion reporting.
- Backscattered Electron (BSE) Imaging
- An SEM imaging mode sensitive to atomic number and crystallographic texture; can reveal grain-boundary elongation patterns in the sub-surface cold-worked zone after...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Electron Impact (EI) Ionisation
- A hard ionisation mode at 70 eV used in GC-MS that produces extensive, reproducible fragmentation. The fragmentation pattern is matched against libraries...
- ENFSI EPG Protocol
- European Network of Forensic Science Institutes Expert Working Group on Paint protocol; the European standard for paint comparison examination sequence and reporting.
- Fingerprint Region
- The 1500 to 400 cm⁻¹ stretch of an infrared spectrum where bending and combination bands produce a pattern unique to a given...
- FTIR Microspectroscopy
- Fourier-transform infrared spectroscopy collected from a microscope-focused spot of 10-20 µm diameter; the primary technique for paint binder characterisation.
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- IRUG Database
- Infrared and Raman Users Group spectral database; covers historic pigments, artists' materials, and museum-object coatings. Primary reference for art-investigation paint analysis.
- Likelihood Ratio Reporting
- A Bayesian framework for reporting paint-comparison conclusions, expressing the probability of the observed data given the hypothesis of common source versus the...
- Limit of Detection (LOD) and Quantitation (LOQ)
- LOD is the lowest concentration at which the signal-to-noise ratio is 3:1; LOQ is where it is 10:1. LC-MS/MS LOQs for drugs...
- Microtome Cross-Section
- A thin section of a paint chip cut perpendicular to the layer planes, allowing each layer to be examined separately by microscopy...
Explained in these topics
- Paint Layer Examination: Cross-Section, FTIR and Py-GC-MSAn FTIR collection mode where a germanium or diamond crystal is pressed against the sample surface, sampling the top 0.5-2 µm. Produces high-quality undistorte...
- Spectrometric Detection: UV-Vis, FTIR and Mass SpectrometryAn FTIR sampling mode in which the sample is pressed against a diamond or ZnSe crystal and the evanescent wave probes the surface. No pellet, no mull, no prepa...