Spectroscopic techniques: why, when, where,and how Dr. Roberto GIANGIACOMO

Spectroscopic techniques: why, when, where,and how Dr. Roberto GIANGIACOMO

BASIC INFORMATION Spectroscopy uses light to analyze substances or products by describing the energy transfer between light and matter. The energy of a photon is defined as:

A spectrum is the fingerprint or the image of a sample, exhibiting absorption peaks corresponding to the frequencies of the vibrations among the bonds of the atoms constituting the matter under investigation.

The frequency of an absorption band is dependent to a first approximation upon the masses of the atoms involved and the force constant of the interatomic bond in accordance with the classical equation for a harmonic oscillator κ= 4π 2 ν 2 µ where κ is the force constant, ν the frequency, and µ the reduced mass of atoms involved in the vibration µ = m 1 m 2 /(m 1 +m 2 ) where m 1 and m 2 are the masses of the atoms involved

The force constants holding these diatomic groups vary not only with the mass of atoms but also with the type of chemical bond. The frequency of absorption is determined largely by the force constant, κ.

These molecular vibrations can be described using the model of the harmonic diatomic oscillator.

The potential energy V of the harmonic oscillator is a quadratic function of the displacement of the vibrating atoms. Precondition for the absorption of a light photon is that the frequency of the light photon equals the energy difference between two vibrational states of the bond.

The interaction of infrared radiation with a vibrating molecule only occurs when the vibration is accompanied by a change of the dipole moment. In this case the energy is succesfully transferred to the molecule.

Types of bands

The deviations from simple harmonic motion of molecular groups (degree of anharmonicity) increases with the amplitude of the oscillation, becoming very large as the amplitude approaches that required for dissociation of the group. The amplitude of molecular motion is not determined by the intensity of the radiation falling upon the sample but is a function of the masses and force constants of the atoms involved.

Symmetrical stretching

Asymmetric stretching

Rocking

Scissoring

Twisting

Wagging

The absorption of photons in one of the spectral regions determines which region is the most useful to describe a molecule or an atomic transition. Vibrational spectroscopic techniques are used to assess molecular motions and firgerprint types.

1.3 MIR 1 spectrum Abs 0.5 0 4000 3000 2000 1000 800 Numeri d onda[cm-1] Wavenumbers [cm -1 ] FUNCTIONAL GROUPS FINGERPRINT AREA (1500-600 FREQUENCIES ( >1500 cm-1) cm-1) identification of organic functional identification of simple bonds groups variable values invariable values characteristic ofeachsingle identical for molecules containing the molecule same functional group

FINGERPRINT AREA Most simple bonds absorb in this area and due to their similar energy, strong interactions with neighboring bonds occur. Absorption bands arising from these interactions and related to the global molecule structure are generated. Due to its complexity it s not possible an exact spectral interpretation ; conversely, this complexity makes this area unique and useful for an exact spectral identification

NIR, MIR and Raman comparison RAMAN M m MID-INFRARED INFRARED NEAR-INFRARED V V V Stokes Anti-Stokes q SCATTERING TECHNIQUE q ABSORPTION TECHNIQUES q SOURCE MONOCHROMATIC RADIATION (LASER VIS-NIR) SOURCES (DISPERSED) POLYCHROMATIC RADIATION (GLOBAR TUNGSTEN) INFORMATION CONTAINED IN SCATTERED RADIATION I Raman 10-8 I Source INFORMATION CONTAINED IN ABSORBED RADIATION 24

Quantitative Vibrational Spectroscopy RAMAN MIR / NIR I Raman c however, a reference intensity is required for the compensation of fluctuations in the scattering efficiency I log 0 I = A = a b c (BEER S LAW) A = absorbance a = absorptivity (cm 2 mol - 1 ) b = sample thickness (cm) c = sample concentration (mol cm - 3 ) Raman and MIR spectra can frequently be evaluated by isolated signals or absorption bands univariate method In NIR this is rarely the case and the information has to be extracted from larger spectral regions multivariate methods 25

Sample Preparation RAMAN MID-INFRAREDINFRARED NEAR-INFRARED NO SAMPLE PREPARATION NO SAMPLE PREPARATION ONLY VIA ATR NO SAMPLE PREPARATION 26

Process monitoring : Raman, MIR, NIR comparison RAMAN MID-INFRARED/ATRINFRARED/ATR NEAR-INFRARED SMALL SAMPLE VOLUME (µl) OR SAMPLE THICKNESS (µm) LARGE SAMPLE THICKNESS (UP TO cm) LIGHT-FIBER OPTICS (> 100m) LIMITED LIGHT-FIBER OPTICS (> 100m) AT-LINE / IN-LINE PROBES ATR-PROBES TRANSMISSION TRANSFLECTION DIFFUSE-REFLECTION PROBES 27

NIR & MIR For quantitative determinations the use of NIR instruments that detect the NIR predominates over MIR apparatus because fewer precautions are needed (moisture problems in the MIR). Conversely, MIR may be useful when it s necessary to define precise λ ranges in NIR region for quantitave analyses, taking advantage of higher MIR interpretative power.

WHY Spectroscopic techniques: do not require the use of solvents or carrier require little or no sample preparation are very fast and offer an immediate response lend themselves to image acquisition can be used for multiple applications have become portable

WHEN Identification of components or constituents for qualitative determinations/discriminations Quantitative measurements, on the basis of calibration (single or multiple components), for product composition/process monitoring

QUANTITATIVE ANALYSIS Calibration: High number of samples (NOT replicates) > 50 Samples must be selected following specific rules

WHERE Off-line: at the laboratory In-line: directly along the process At-line: close to the sampling point On-line: directly inside the process line/pipe

At lab

In field

On farm equipments

On drones

HOW

NIR measurements Trasmittance Reflectance Transflectance

D confocal Raman image of a fluid inclusion in garnet (Red: Garnet, Blue: Water, Green: Calcite, Turquoise: Mica). Scan range: 60 μm x 60 μm x 30 μm.

Raman depth-profiling (x-z direction) of a multilayered polymer coating. 50 μm x 100 μm scan range, 200 x 200 pixels, 24 000 spectra, acquisition time per spectra: 50 ms.