Identification
FTIR Spectroscopy
Reads a substance's infrared "absorption fingerprint" by pressing it against a crystal. The natural partner to Raman — strong exactly where Raman is weak (dark, colored, and aqueous samples).
TECH bulk unknown identification, aqueous samples, mixture interpretation, Raman/FTIR selection
How it works
FTIR stands for Fourier-Transform InfraRed spectroscopy. It shines a broad band of infrared light through (or into) the sample and measures which infrared wavelengths the sample absorbs. Molecular bonds vibrate at characteristic frequencies and absorb IR at those frequencies, so the pattern of absorption is a molecular fingerprint. (The "Fourier transform" is just the math that turns the raw signal into a readable spectrum — you don't need the math to use it.) The instrument matches the fingerprint to an onboard library and reports the identity plus a match quality.
Most handheld field FTIRs sample by ATR — Attenuated Total Reflectance. You press a small amount of the unknown directly onto a hard ATR crystal (e.g., diamond); the IR beam skims the crystal surface and probes just the material in contact with it. This makes solids, pastes, gels, and liquids easy to run with a tiny sample.
Unlike Raman, ATR-FTIR cannot work through a container — the sample must physically touch the crystal. That means you open the container and handle a small amount of the unknown, with all the exposure, contamination, and reactivity risk that implies. Do it in appropriate PPE, in the right zone, with a plan for a reactive or energetic material. If you can safely avoid opening the container, scan with Raman first.
What it's good for
- Identifying bulk solids and liquids — powders, oils, solvents, and unknown residues, with a very large IR library covering industrial chemicals, TICs, and CWAs.
- Aqueous solutions and water-containing samples — FTIR handles many water solutions that Raman can't (though very high water still complicates the spectrum). This is a key reason to carry it alongside Raman.
- Dark and colored samples — no fluorescence problem, so the dyed/dark materials that defeat Raman are often readable by FTIR.
- No laser ignition risk — FTIR doesn't focus a laser into the sample, so it lacks Raman's dark/energetic ignition hazard (though sample handling risk remains).
- Strong on polar/organic functional groups — many organics, acids, and functionalized compounds give rich, distinctive IR spectra.
What it CANNOT do / limitations
- Needs direct contact — no through-container scanning; you must expose and handle the material.
- Weak on pure metals and simple ionic salts — materials without IR-active molecular bonds (e.g., pure elemental metals, and simple salts like sodium chloride that lack IR-active vibrations) give little or no useful spectrum.
- Very high water can dominate the spectrum for some analytes (water absorbs strongly in parts of the IR), so extremely dilute aqueous samples still have limits — but FTIR generally beats Raman here.
- Library-limited — only identifies what's in its library; novel compounds return poor/no matches.
- Mixture limits — see below.
- Bulk sample needed — it's not a trace or vapor-monitoring tool.
Mixtures & deconvolution limits
Real-world unknowns are often mixtures. FTIR (and Raman) instruments include mixture-analysis algorithms that try to break a spectrum into multiple library components. They work, but within limits:
- They reliably identify major components (typically present above roughly ~10%); minor/trace constituents are easily missed.
- The more components, the harder the deconvolution — a complex cocktail may return only the one or two dominant chemicals, or a low-confidence result.
- Overlapping spectra can hide one chemical behind another.
Treat a mixture result as "these are the major components," not a complete inventory. If a small amount of something highly hazardous could be hiding under a dominant carrier, corroborate with orthogonal tools (colorimetric tubes, IMS, or GC-MS for volatiles) and context.
Sampling, crystal cleaning & background scans
- Background scan first. Before running a sample, the instrument takes a background (clean-crystal) scan so it can subtract the crystal/optics and ambient contribution. Run a fresh background per the manual — a stale or contaminated background corrupts the result.
- Good contact. ATR needs the sample fully touching the crystal; some units have a pressure clamp/anvil for solids and powders. Poor contact = weak, noisy spectrum.
- Clean the crystal thoroughly between samples with the manufacturer-specified solvent — residue from the previous sample is a classic source of cross-contamination and false ID. Confirm a clean background after cleaning.
- Watch reactivity. The sample sits on a small diamond/crystal in the instrument — an aggressively reactive or energetic material in contact is a handling hazard; follow SOP.
Raman vs. FTIR — decision table
These two techniques are deliberately complementary; well-equipped teams carry both. Use this to pick the right one first.
| Question / sample type | Reach for… | Why |
|---|---|---|
| Can I ID it without opening the container? | Raman | Scans through clear glass/plastic; no contact or handling. |
| Sample is dark, colored, or dyed | FTIR | Raman fluoresces on dark/colored material; FTIR doesn't. |
| Sample is an aqueous / dilute solution | FTIR | Raman is weak on dilute aqueous (<~10%); FTIR handles water solutions. |
| Sample is a symmetric/non-polar compound or many oxidizers | Raman | Strong Raman signal for symmetric bonds. |
| Sample is a polar organic / functionalized compound | FTIR | Rich, distinctive IR absorption. |
| Suspected dark/energetic explosive | Neither by contact first — Raman only with standoff/scan-delay per SOP; FTIR requires handling | Raman laser can ignite dark energetics; FTIR requires opening/handling. Follow energetics SOP. |
| Pure metal | Neither (use elemental/other methods) | Metals reflect the Raman laser; no IR-active bonds for FTIR. |
| First tool returned fluorescence / "no match" | Switch to the other | Their blind spots don't overlap — the second often succeeds. |
Raman: through-container, light-colored, symmetric bonds — but blinded by fluorescence (dark/colored), weak on dilute aqueous, laser-ignition risk. FTIR: dark/colored and aqueous samples, polar organics — but needs contact/handling and is weak on metals/simple salts. Run one, and if it struggles, the other is usually the answer.
"Calibration" — performance verification with polystyrene
Like Raman, handheld FTIR isn't span-calibrated with gas. It runs an automated performance verification, classically against a polystyrene reference film whose IR absorption peaks are precisely known, to confirm the wavelength axis and detector are accurate. Run the manufacturer's verification per schedule and before critical IDs; the unit reports pass/fail. Also run a fresh background scan before each sample — that's the routine "readiness" step for FTIR.
Field care & storage
- Keep the ATR crystal clean and undamaged; clean thoroughly between samples and verify a clean background.
- Protect the crystal and clamp from scratches and aggressive chemicals per manual; some crystals have chemical-compatibility limits.
- Let cold instruments acclimate; run performance verification after big environmental swings.
- Store per manual; manage consumables (cleaning solvent, wipes, sampling accessories).
Common rookie mistakes
- Not cleaning the crystal and carrying over the previous sample → cross-contaminated false ID.
- Skipping the background scan or using a stale/contaminated one.
- Poor contact on a powder/solid (no clamp pressure) → weak, unmatchable spectrum.
- Expecting FTIR to read a pure metal or simple salt, or to work through a container.
- Treating a mixture result as a complete inventory instead of "major components only."
- Opening and handling a reactive/energetic unknown when Raman through the container was the safer first move.
Representative instruments
Generic examples include the Thermo TruDefender FT/FTX, Smiths HazMatID Elite, and Agilent 4300 / 4500 handheld FTIR analyzers. These are routinely paired with a handheld Raman so the team can attack an unknown from both directions. FTIR is not part of a typical RAE fleet — often a regional team or partner-agency asset. Brands are illustrative; your model and SOPs govern.
FTIR names bulk unknowns by their IR fingerprint via direct contact on an ATR crystal — strong on dark, colored, and aqueous samples where Raman fails, with no laser-ignition risk, but it requires opening and handling the material and is weak on metals/simple salts. Clean the crystal, run a background, verify with polystyrene, and use it as Raman's partner, not its replacement.
Next: the confirmatory gold standard — Field-Portable GC-MS →