FTIR-ATR Spectroscopy Lab

1. Michelson Interferometry & ZPD Centerburst

Unlike dispersive spectrometers that measure each wavelength sequentially, a Fourier-Transform Infrared (FTIR) spectrometer passes broadband radiation through a Michelson Interferometer. A beam splitter divides radiation between a fixed mirror and a moving mirror driven by an air-bearing carriage:

At Zero Path Difference (ZPD, δ = 0), all wavelengths constructively interfere simultaneously, generating an intense central spike known as the centerburst. A co-axial 632.8 nm Helium-Neon (HeNe) laser monitors mirror position with sub-nanometer precision, providing the Connes advantage of superior wavenumber accuracy across the entire spectrum.

2. ATR Evanescent Wave Optics & Depth of Penetration

Attenuated Total Reflectance (ATR) bypasses laborious sample preparation (such as KBr pellets) by pressing the solid or liquid sample directly onto an internal reflection crystal with high refractive index ($n_1$). Above the critical angle $\theta_c = \arcsin(n_2/n_1)$, an evanescent wave decays exponentially into the sample:

Because $d_p$ is proportional to wavelength ($\lambda = 1/\bar{\nu}$), the evanescent field penetrates roughly three times deeper at $1000\text{ cm}^{-1}$ ($d_p \approx 1.6\ \mu\text{m}$) than at $3000\text{ cm}^{-1}$ ($d_p \approx 0.5\ \mu\text{m}$). This wavelength dependence naturally strengthens low-frequency fingerprint bands relative to transmission spectra.

3. Crystal Element Selection: Diamond vs Ge

The choice of ATR internal reflection element dictates chemical compatibility, mechanical durability, and spectral window:

• Monolithic Diamond ($n_1 = 2.40$): Exceptional hardness and chemical resistance (pH 1–14). Allows high clamping pressures up to 1000 psi without scratch risks; ideal for hard polymers and tablet excipients.
• Germanium ($n_1 = 4.00$): Very high refractive index drastically suppresses penetration depth to $0.4 - 0.9\ \mu\text{m}$. Indispensable for strongly absorbing samples like carbon-black filled tires and dark composites.
• Zinc Selenide ($n_1 = 2.40$): Cost-effective optical element for routine general analysis, but vulnerable to strong acids/bases and mechanical scratches.

4. Quantitative Beer-Lambert & Fingerprint Analysis

Quantitative analysis in FTIR converts transmission $\%T$ into Absorbance $A = -\log_{10}(T/100)$, which scales linearly with analyte concentration $c$ and effective pathlength $d_{\text{eff}} = 2 d_p$:

• Carbonyl Index in Polymers: Ratio of $1718\text{ cm}^{-1}$ ($\text{C=O}$ ketone/acid) to $1463\text{ cm}^{-1}$ ($\text{CH}_2$ reference) quantifies environmental degradation and UV photo-oxidation.
• Protein Secondary Structure: Deconvolution of the Amide I band ($1600 - 1700\text{ cm}^{-1}$) into $\alpha$-helix ($1654\text{ cm}^{-1}$) and $\beta$-sheet ($1632\text{ cm}^{-1}$) subcomponents tracks biopharmaceutical stability.
• Pharmaceutical Polymorphism: Sharp shifts in hydrogen-bonded $\text{O-H}$ and $\text{N-H}$ stretches distinguish metastable polymorphs and amorphous dispersions.