Table 2.

Basic characteristics of NIR spectroscopy in comparison with competing techniques.

	NIR	IR	Raman
Spectral region wavelength [nm] wavenumbers [cm−1]	1000–2500 10,000–4000	2500–25,000 4000–400	2500–200,000 4000–50
excitation mechanism	absorption	absorption	inelastic photon scattering
relative complexity of instrumentation	low	medium	high
selection rule (chemical sensitivity)	change in dipole moment (polar moieties, enhanced signal of X–H groups, e.g., O–H, N–H, C–H)	change in dipole moment (polar moieties)	change in polarizability (non-polar symmetrical bonds, e.g., C–C, skeletal vibrations)
sampling (i.e., spectra acquisition modes)	transmission; diffuse reflection; transflection	transmission; diffuse reflection (only after sample preparation); transflection; attenuated total reflectance (ATR)	reflection (scattering)
remarks about sample preparation	no/minimal sample preparation needed; moderate suitability of water as solvent or glass as container/optics	optimal sample thickness (in transmission mode); sample dilution (e.g., KBr pellet) for diffuse reflectance mode; optimal/stable sample-IRE contact surface (in ATR mode)	suitability of water as solvent or glass as container/optics
chemical specificity	low to moderate	high	high
major issues and challenges	low sensitivity; overlapping contributions in the spectra; difficult spectral interpretation;	limited suitability of moist samples; unsuitability of glass optics and materials (absorption of glass); interfering signal from atmospheric H2O and CO2	Raman signal obscured by autofluorescence (stronger for excitation lasers with shorter emission wavelengths); laser heating, danger of destruction of molecular structure, e.g., proteins; or sample thermal decomposition (particularly of dried material)

Abbreviations: IRE - internal reflection element.