What is Spectroscopy?

As light passes through a medium, light is absorbed and the intensity of the transmitted light is given by, I_T (λ)=I₀e^-ε(λ)lc where ε(λ) is the wavelength-dependent molar absorptivity constant, l is the path length and c is the concentration of particles in the medium. Substituting the transmittance, T=I_T/I₀ and absorbance, A=-log⁡(T) yields, A= εlc This is known as the Beer-Lambert Law, which states that the absorption of light, at a specific wavelength, as it passes through a medium is proportional to the concentration of a substance within that medium. This implies that concentrations of chemical species in a particular sample can be related quantitatively to the spectra that are produced with an optical measurement. Quantitative measurements are parameterized by statistical methods known as chemometrics, which relate the relative intensities in the spectra to the actual concentrations found using traditional laboratory methods. These models can then be used in the process to predict the concentrations of unknown samples in real-time.

Fundamental wavelengths fall within the Mid-IR range of 2.5 - 25µm and correspond to a change from one vibrational state to the next, Δv=±1. However, molecules have harmonic overtones in addition to their fundamental frequencies, which correspond to jumps over more than one state, Δv=±2,± 3,± 4,… In the NIR range, these energies primarily correlate to overtones forming hydrogen bonds in a medium.

As light passes through a medium, light is absorbed and the intensity of the transmitted light is given by, I_T(λ)=I₀e^-ε(λ)lc where ε(λ) is the wavelength-dependent molar absorptivity constant, l is the pathlength and c is the concentration of particles in the medium. Substituting the transmittance, T=II₀ and absorbance, A=-log⁡(T) yields, A= εlc This is known as the Beer-Lambert Law, which states that the absorption of light, at a specific wavelength, as it passes through a medium is proportional to the concentration of a substance within that medium. This implies that concentrations of chemical species in a particular sample can be related quantitatively to the spectra that are produced with an optical measurement. Quantitative measurements are parameterized by statistical methods known as chemometrics, which relate the relative intensities in the spectra to the actual concentrations found using traditional laboratory methods. These models can then be used in the process to predict the concentrations of unknown samples in real-time.

Because most matter found on Earth contains hydrogen, NIRS is a powerful tool for monitoring a multitude of processes in chemical and biochemical engineering, as well in agriculture and food & beverage processing.

This is known as Stokes Raman scattering. The losses of energy can be recorded to create a spectrum in the Mid-IR range. Unlike IR spectroscopy, the Raman technique relies on a change in polarizability within a molecule and therefore primarily considers symmetric vibrational modes, where the bond lengths change in phase with one another. For this reason, Raman spectroscopy is considered complementary to IR techniques.

Raman spectra, like IR spectra, are unique to every sample, with peaks corresponding to the specific physical/chemical properties of the material. The peak intensity, i.e. peak height or peak area, is proportional to the number of molecules or the concentration in the sample, I(λ)∝σlcI where σ is the Raman cross-section, l is the pathlength, c is the concentration, and I represent various instrument parameters including, laser power, integration time, etc. This implies that concentrations of chemical species in a particular sample can be related quantitatively to the spectra that are produced with an optical measurement. Quantitative measurements are parameterized by statistical methods known as chemometrics, which relate the relative intensities in the spectra to the actual concentrations found using traditional laboratory methods. These models can then be used in the process to predict the concentrations of unknown samples in real-time.

Raman spectroscopy, due to its dependence on changes in polarizability, is considered a complementary technique to IR spectroscopy. Raman spectroscopy is particularly useful for the detection of gas-phase diatomic molecules, e.g. H2, N2, etc., as well as hydrocarbons and samples containing water. For example, in a sample containing sufficient levels of moisture, where water concentration is not of interest, Raman spectroscopy can be used to analyze the sample. Because of its strong dipole, water does not exhibit a change in polarizability as it vibrates, making it effectively invisible to Raman spectroscopy, where an IR spectrum might be consumed by absorption from water, making them less desirable for water-based sample measurements.