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FTIR and ATR-FTIR for forensic identification: vibrational principle, the 4000 to 400 cm-1 fingerprint region, KBr pellet and ATR sample modes, and how Indian SFSLs use IR for white-powder triage, paint chip layers, suicide-note tablets and explosive residue.
Infrared spectroscopy is the technique that takes an unknown solid sitting on the chemistry bench and gives it a name in under a minute. The analyst presses a tablet onto a diamond crystal, the spectrum draws itself from 4000 to 400 cm-1, and the library search returns paracetamol, ammonium nitrate, RDX or PVC with a match score above 0.95. No solvent. No dilution. No sample loss. That is why ATR-FTIR has quietly displaced every other presumptive identification step at the Indian SFSL chemistry bench since about 2010.
The physics behind that one-minute answer is older than most of the instruments that exploit it. An infrared photon at the right frequency drives a normal mode of molecular vibration if the vibration changes the dipole moment of the molecule. Each functional group has its own characteristic absorption band, and the 1500 to 400 cm-1 fingerprint region is unique to the whole molecule. Two compounds with the same fingerprint region are the same compound, period.
The contrarian point students miss is that IR is not a quantitation technique, it is an identification technique. The bench reaches for UV-Vis or HPLC when the question is "how much". The bench reaches for FTIR when the question is "what is this". A single ATR-FTIR run answers the second question for solids and viscous liquids faster than any other instrument in the lab.
What an IR photon actually does to a molecule, and why N2 is invisible.
A molecular bond is a spring. It has a natural frequency of vibration set by the masses at each end and the stiffness of the bond. An IR photon whose energy matches that natural frequency is absorbed and the bond vibrates harder. Absorbance at the matched wavenumber is what the spectrum draws.
There is one selection rule worth committing to memory. A vibration is IR active only if it changes the dipole moment of the molecule. Homonuclear diatomics like N2, O2 and H2 have no permanent dipole and no change in dipole on stretching, so they are completely IR invisible. Heteronuclear bonds (C=O, O-H, N-H, C-Cl) are strongly IR active. This is why ambient air does not interfere with the spectrum despite being mostly nitrogen and oxygen, and why CO2 at 2350 cm-1 and water vapour at 3700 and 1600 cm-1 do show up if the optical path is not purged.
The infrared spectrum splits into three regions and the working bench cares about one of them.
| Near-IR | 14000 to 4000 cm-1 | Overtones and combination bands | Pharma quality control, agriculture, in-line process monitoring |
| Mid-IR | 4000 to 400 cm-1 | Fundamental vibrational modes | Forensic identification, organic and many inorganic compounds (the workhorse) |
| Far-IR | 400 to 10 cm-1 | Lattice modes, heavy atom bonds, torsions | Inorganic crystallography, organometallics, low frequency lattice studies |
The wavenumber convention is worth being comfortable with. cm-1 is the reciprocal of wavelength expressed in centimetres. Higher wavenumber is higher photon energy and shorter wavelength. A C-H stretch at 3000 cm-1 is more energetic than a C-O stretch at 1100 cm-1. The axis runs right to left on a conventional spectrum, with the high wavenumber stretching modes on the left and the low wavenumber fingerprint region on the right.
The two halves of an IR spectrum and how a chemist reads each.
A mid-IR spectrum reads in two halves. Above about 1500 cm-1 sit the group frequencies, where individual functional groups absorb at predictable wavenumbers regardless of the rest of the molecule. Below 1500 cm-1 sits the fingerprint region, where coupled and skeletal vibrations produce a pattern unique to the whole molecule.
The group frequency half is what the analyst eyeballs first. A broad band at 3200 to 3600 cm-1 means hydrogen-bonded O-H, almost always alcohol or carboxylic acid or water of crystallisation. A sharp band at 3300 to 3500 cm-1 with one or two prongs is N-H, primary or secondary amine. A strong absorption between 1680 and 1750 cm-1 is the carbonyl region, the single most diagnostic band in the whole spectrum, distinguishing aldehydes, ketones, esters, amides and acids.
| O-H stretch (H bonded) | 3200 to 3600 | Broad, strong | Alcohol, carboxylic acid, water of crystallisation |
| N-H stretch | 3300 to 3500 | Sharp, medium, often double | Primary amine (two prongs), secondary amine (one) |
| C-H stretch (sp3) | 2850 to 3000 | Sharp, medium, multiple bands | Aliphatic CH3, CH2, CH |
| C-H stretch (sp2) | 3000 to 3100 | Sharp, weak | Aromatic or vinylic CH |
| C≡C stretch |
Why the modern instrument is twenty times faster and gives a cleaner spectrum.
Older dispersive IR instruments used a monochromator with a grating that scanned wavelengths one at a time onto a slit. Acquisition was slow, signal-to-noise was poor at the low energy mid-IR end, and a single survey scan took several minutes. Anyone who used a Perkin Elmer 137 or 257 in the 1970s remembers waiting for the chart recorder to crawl through 4000 to 400 cm-1.
FTIR replaced the monochromator with a Michelson interferometer. A beamsplitter divides the source beam into two arms, one with a fixed mirror and one with a moving mirror. The recombined beam carries every wavelength simultaneously, modulated at a different audio frequency depending on its wavenumber. The detector reads an interferogram (intensity versus mirror displacement) and a Fourier transform converts that into the conventional spectrum.
Two specific advantages drove the technology shift. The Felgett or multiplex advantage means every wavenumber is measured during every moment of the scan, so signal-to-noise improves with the square root of the integration time. The Jacquinot or throughput advantage means the optical path has no narrow slit, so much more light reaches the detector. Together these give a 20 to 100 fold improvement in scan time and signal-to-noise, and they are why FTIR has been the default since the late 1980s.
A typical bench FTIR scans 4000 to 400 cm-1 in 10 to 30 seconds at 4 cm-1 resolution. Co-adding 16 or 32 scans (still under a minute total) gives a clean spectrum on milligram quantities. Compared to the dispersive era, this collapses what used to be a 30-minute exercise into a casework workflow you can run on a hundred samples a day.
Five ways to put a sample in front of the IR beam, and why ATR has quietly won.
The classical solid mode is the KBr pellet. Potassium bromide is IR transparent across the entire mid-IR range, ductile enough to flow under pressure, and chemically inert with most analytes. About 1 to 2 mg of dry analyte is ground with roughly 200 mg of dry KBr in an agate mortar, transferred into a 13 mm stainless steel die, and pressed at 8 to 10 tonnes for two to three minutes. The disc that pops out is glass clear if the work was done well, milky if the powder was damp.
Nujol mull is the alternative for moisture sensitive solids. The analyte is ground with mineral oil into a paste, smeared between two NaCl or KBr plates and run as a suspension. The trade-off is that Nujol itself absorbs in the C-H stretch region (about 2900 cm-1) and the C-H bend region (1460 and 1377 cm-1), so those windows are blocked. For a chlorinated or oxidised analyte where Nujol is silent, the mull is fine; for a hydrocarbon, the mull is useless and a fluorocarbon oil (Fluorolube) is substituted.
Liquid mode runs neat liquids as a thin film between two NaCl or KBr plates with no spacer, or in a fixed pathlength solution cell of 0.025 to 1 mm built from KBr or CaF2 windows. Gas cells with 10 cm or longer optical paths handle vapours for atmospheric monitoring. None of these is used much in routine forensic chemistry.
What changed everything is ATR. A diamond, ZnSe or Ge crystal of refractive index well above the sample is mounted with the sample pressed against its top face. The IR beam hits the crystal-sample interface at greater than the critical angle and undergoes total internal reflection, but a small evanescent wave penetrates 0.5 to 2 µm into the sample and is attenuated at the wavenumbers the sample absorbs. The reflected beam, now carrying the IR spectrum of the surface layer, goes to the detector.
| KBr pellet | Solid powder, milligram quantity | 5 to 10 minutes | Hygroscopic KBr, sample dilution, possible ion exchange with KBr |
| Nujol mull |
From a white powder triage to suicide note tablets to RDX residue.
The casework that lands on a forensic chemistry bench is dominated by unknown solids whose first question is "what is it". IR answers that question faster than anything else, which is why the same instrument turns up in half a dozen different investigation streams.
Unknown white powder triage is the bread and butter. A constable walks in with a polythene packet seized from a roadside bag and the bench needs an answer before the investigation team commits to a narcotic versus non-narcotic line. ATR-FTIR distinguishes paracetamol from caffeine from sucrose from talc from ammonium nitrate from urea in seconds, and from methamphetamine, MDMA, cocaine and heroin against a controlled-substance library inside a minute. The result triages the sample to the toxicology bench for confirmation, the explosives bench for a different workup, or back to the IO with a non-suspicious finding.
Suicide-note tablets recovered at autopsy are an almost daily case at any tertiary medical college in India. A tablet with no markings, half dissolved in stomach contents, gets placed on the diamond and identified against a pharmaceutical library: aluminium phosphide, chloroquine, dapsone, paracetamol, organophosphorus tablet formulations. A confirmed identification in 30 seconds points the post-mortem chemistry workflow at the right specific assay (GFAAS for the metallic phosphide residue, GC-NPD for the organophosphate parent, HPLC for the hepatotoxin).
Counterfeit pharmaceuticals fall under CDSCO investigations and reach the FSL when state drug controllers seize a suspect batch. ATR-FTIR fingerprints the active pharmaceutical ingredient (API) against the bulk-drug reference, and any deviation in the fingerprint region flags adulteration, substitution or absence of the labelled API. NIPER Mohali maintains one of the better Indian bulk-drug ATR-FTIR libraries for exactly this work.
Paint chip layer-by-layer analysis is the classic hit-and-run case. A paint flake from a victim's clothing or a recovered vehicle is examined under a stereo microscope, the layers are separated mechanically (clear coat, base coat, primer, electrocoat, original factory layers), and each layer is mounted on a diamond ATR or on an IR microscope stage. The layer sequence and the IR fingerprint of each layer is matched against the reference paint flake from the suspect vehicle. A match across all layers in the same order is strong source attribution evidence, particularly for older multi-coat finishes.
What sits on the bench at CFSL Chandigarh, FSL Madhuban and CFSL Pune.
A modern FTIR ships with a software search engine and one or more spectral libraries. The NIST IR library, the Sadtler commercial libraries (now Bio-Rad KnowItAll), the Mainlib drug library and instrument-vendor in-house libraries between them cover most of the molecules a forensic bench will see. A typical search returns the top 10 hits ranked by a similarity score (Euclidean, first-derivative or correlation algorithm depending on software). A score above 0.95 is the working confirmatory threshold for routine identification, although a careful analyst still eyeballs the overlay rather than trusting the number alone.
Limitations matter. Aqueous samples are awkward because water absorbs strongly across most of the mid-IR range and overwhelms most analyte bands. Mixtures are the harder case, because overlapping bands from multiple components confuse both the eye and the search engine; the spectrum of a 50:50 paracetamol-caffeine tablet is not the sum of two clean spectra and a single library match is unreliable. Quantitation is possible by Beer-Lambert in the IR but is much less precise than UV-Vis quantitation, so the bench reaches for HPLC or UV-Vis when a number rather than an identity is needed.
The IR microscope couples a polarising or stereo microscope to the FTIR optical path with a small ATR objective or transmission stage. Spatial resolution is about 10 µm at the diffraction limit, which is enough to read a single paint layer in cross section, a single fibre, a single ink line on a document or a single particle in a heterogeneous sample. The IR microscope is what converts FTIR from a bulk technique into a microspectroscopy capable of evidence-grade single-particle identification.
The instrument map at common Indian Indian SFSLs settles around four vendors. The Bruker ALPHA II is a compact bench ATR-FTIR widely deployed because of its size, speed and price, and the Bruker TENSOR is the larger research-grade unit. Agilent Cary 630 is the other compact ATR favourite. PerkinElmer Spectrum Two and Shimadzu IRTracer-100 round out the routine choices. CFSL Chandigarh runs a Bruker ALPHA II with an IR microscope for paint and fibre work. FSL Sector 14 Madhuban uses ATR-FTIR for daily white-powder triage. CFSL Pune handles explosive-residue ATR-FTIR for the western India workload. NIPER Mohali keeps a pharmaceutical ATR-FTIR with a curated bulk-drug library for counterfeit pharmaceutical investigations.
Which selection rule determines whether a vibrational mode is IR active?
| 2100 to 2260 |
| Sharp, weak to medium |
| Alkyne, nitrile region overlap |
| C=O stretch | 1680 to 1750 | Sharp, very strong | Carbonyl: ester, ketone, amide, acid (most diagnostic single band) |
| C=C aromatic | 1450 to 1600 | Multiple bands, medium | Aromatic ring presence |
| C-O stretch | 1000 to 1300 | Strong, can be multiple | Ester, ether, alcohol C-O |
| Aromatic C-H bend | 700 to 900 | Sharp, medium | Substitution pattern (ortho, meta, para) |
The fingerprint region from 1500 to 400 cm-1 is where the molecular identity actually lives. Two compounds can share every group frequency above 1500 (paracetamol and a structurally related impurity often do) and still be distinguishable in the fingerprint pattern. A library search algorithm scores the match across the entire spectrum but weights the fingerprint region heavily, which is why a match score above 0.95 against a NIST or Sadtler library is the working confirmation threshold.
| Moisture sensitive solid |
| 3 to 5 minutes |
| Nujol bands obscure 2900, 1460 and 1377 cm-1 |
| Liquid film between plates | Neat liquid | 1 minute | Volatile liquids evaporate, plates fog with humid samples |
| Solution cell | Dilute liquid | 5 minutes | Solvent absorption windows, fixed pathlength |
| Diamond ATR | Solid, viscous liquid, paste, fibre, film | 30 seconds | Surface layer only (0.5 to 2 µm), poor for very dilute analytes |
The ATR advantage is operational rather than spectroscopic. A tablet placed on the diamond and pressed with a torque-limited anvil gives a usable spectrum in under a minute with no sample destruction, no dilution, no skill in pellet pressing, and no clean-up beyond a wipe of the crystal with isopropanol. Every modern Indian SFSL chemistry bench has standardised on diamond ATR for routine identification work, with the KBr pellet kept for the occasional case where the surface layer is not representative or where a longer effective pathlength is needed.
Fibre identification distinguishes synthetic from natural and tells synthetics apart. Polyester (PET) shows a strong ester C=O at 1715 cm-1 and aromatic bands at 1410 and 720 cm-1. Nylon shows the secondary amide N-H at 3300 and amide I and II at 1640 and 1540 cm-1. Acrylic shows nitrile C≡N at 2240 cm-1. Cotton, viscose and other cellulosics show the broad O-H, C-H and ring-stretch pattern of cellulose. An IR microscope coupled to FTIR with about 10 µm spatial resolution handles single fibre cross sections without dissecting the fibre.
Plastic identification at recycling, smuggling and contamination cases distinguishes polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), PET and ABS in seconds. The wavenumber positions of the C-H stretches and the strong skeletal bands below 1500 cm-1 are diagnostic. A counterfeit pharmaceutical blister, a tampered food packaging, or a bag holding contraband is identified by polymer type before any further chemistry.
Explosive residue is a non-trivial application. TATP (triacetone triperoxide), RDX, HMX, PETN, ANFO (ammonium nitrate fuel oil) and urea nitrate each give characteristic IR spectra. ATR-FTIR on a swab extract or a recovered solid is used at CFSL Pune and the explosive units of state SFSLs as a presumptive identification. Confirmation rides on LC-MS/MS or ion chromatography for nitrate and nitrite, but the ATR-FTIR call is what triggers the confirmatory workflow.
Forged document examination uses IR to distinguish toner inks (carbon black with polymer binder) from liquid inks (dye in solvent), to compare the toner of a questioned page against a known printer source, and to identify the polymer of a security feature on a counterfeit currency note. An IR microscope examines the toner layer in situ on the document.