Fundamentals of infrared spectroscopy Elvis Weullow

Hands-on Soil Infrared Spectroscopy Training Course Getting the best out of light 11 – 15 November 2013

INFRARED SPECTRSCOPY Vibrational spectroscopy (or IR spectroscopy): measures transitions from one molecular vibrational energy level to another, and requires radiation from the IR portion of the ER spectrum. Ultraviolet-visible spectroscopy: (also called electronic absorption spectroscopy) involves transitions among electronic energy levels of the molecule, which require radiation from the UV visible portion of the electromagnetic spectrum. Such transitions alter the configuration of the valence electrons in the molecule.

Increasing Frequency 50,000 cm-1

UV

X-Ray 200 nm

12,820 cm-1

Vis

380 nm

NIR

780 nm

Increasing Wavelength

4,000 cm-1

2,500 nm

400 cm-1

FIR, Microwave

MIR 25,000 nm

IR PRINCIPLES Each molecular vibrational motion occurs with a frequency characteristic of the molecule and of the particular vibration. The energy of the vibration is measured by its amplitude (the distance moved by the atoms during the vibration), so the higher the vibrational energy , the larger the amplitude of the motion. According to quantum mechanics, only certain vibrational energies are allowed to the molecule (this is also true of rotational and translational energies), so only certain amplitudes are allowed. For a vibrational mode to absorb IR radiation, the vibrational motion associated with that mode must produce a change in the dipole moment of the molecule

Vibrations Stretching frequency

Bending frequency O

Modes of vibration Stretching

Bending

C—H

C

H

H H

H H

Wagging 1350 cm-1

Scissoring 1450 cm-1

H H

H

Symmetrical 2853 cm-1

H

Asymmetrical 2926 cm-1

H

H

Rocking 720 cm-1 H

H

Twisting 1250 cm-1

IR PRINCIPLES • Infrared radiation VNIR 350nm to 2500nm, NIR 12500cm-1 to 4000 cm-1 MIR 4000 cm-1 to 400 cm-1

• Bonds subject to vibrational energy changes => continually vibrate in different ways:

Symmetrical stretching

Antisymmetrical stretching

Scissoring

Rocking

Wagging

Twisting

• Energy absorption in IR region then occurs & translated into absorption spectrum.

Source: www.wikipedia.org/

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IR SPECTROSCOPY • Foundation The IR region spans the wavelength range of 12500 -400 wavenumbers, in which absorption bands correspond mainly to overtones and combinations of fundamental vibrations. The vibration of molecules can be described using the harmonic oscillator model, by which the energy of the different , equally spaced levels can be calculated from Evib= ( + / )h/2Π√(k/ ) Where is the vibrational quantum number, h the pla k’s constant K the force constant and the reduced mass of the bonding atoms. Only those transitions between consecutive energy levels (Δ =±1) that cause a change in dipole moment are possible. ΔE=ΔErad=h Whe e ν is the fu da e tal i atio al f e ue y of the o d that yields a absorption band in the mid IR region.

IR Principles For soils Different clay types have very distinct spectral signatures in the near infrared region because of strong absorption of the overtones of SO42- , CO32and OH-, and combinations of fundamental features of, for example, H2O and CO32 Absorption due to charge transfer and crystal field effects in Fe2+ and Fe3+ is particularly evident at 0.35 to 1.0 nm. Soil spectra are a product of many overlapping absorption features of organic and mineral materials they generally have few distinct absorption features.

This makes qualitative interpretation of individual features of limited value, but even subtle differences in shape can yield quantitative information on soil properties.

IR INSTRUMENTATION IR equipment can incorporate a variety of devices depending on the characteristics of the sample and the particular analytical conditions and needs (such as speed, sample complexity and environmental conditions), so the technique is very flexible. Classification of Modern IR Instruments • Filter based instruments: -Here filters are used as wavelength selectors, commercially available for dedicated applications. For example, an instrument for determination of the quality parameters of gasoline (Zeltex Inc.) employs 14 interference (Fabri-Perrot) filters and 14 LED (Light Emitting Diode) sources in the NIR region. •

LED based instruments:Using Light Emitting Diodes (LED) in the field , price and size of the instrument can be reduced, produce NIR radiation with a band width of about 30-50 nm. LEDs function as both the light source and the wavelength selection system, typically cover the range 400–1700 nm. They have the advantages that the measurement is very fast (e.g. one spectrum per second) and noninvasive. These features are particularly useful where a high sample throughput or ultra-rapid on-line measurements are required.

IR INSTRUMENTATION •

•

Dispersive optics-based instruments:

These are instruments based on grating monochromators, offer advantage of longer life, relatively low cost, when compared with other scanning instruments employing modern technologies. The main limitations are the slow scan speed and a lack of wavelength precision, which deteriorates for long term operation due to mechanically driven mechanism fatigue. Also, the presence of moving parts limits the use of dispersive instruments in the field and in more aggressive environments. It is mainly employed in the control of sugar and alcohol production, also on a truck to monitor the content of protein, oil and humidity in real time during grain harvest.

IR INSTRUMENTATION • •

AOTF based instruments Acousto-Optical Tunable Filters (AOTF) are modern scan spectrophotometers offer advantage of constructing instruments with no moving parts, very high scan speeds, broad NIR spectral region, random access to any number of wavelengths . The scan speed is usually limited by the detector response time. An AOTF comprises a crystal of TeO2 through which a plane traveling acoustic wave is generated at right angles to the incident light beam

IR INSTRUMENTATION •

Fourier-transform based instruments:

• These instruments are based on the use of interferometers and Fourier transform to recover the intensities of individual wavelengths in the IR region are, undoubtedly, the instruments combining most of the best characteristics in terms of wavelength precision and accuracy, high signal-to-noise ratio and scan speed .Typical wavelength accuracy is better than 0.05 nm and the resolution can achieve values below 1 nm in the NIR region.

COMMON NIR/MIR DETECTORS USED

Comparison Between Dispersion Spectrometer and Dispersion FTIR To separate IR light, a grating is used.

Grating

Detector

Slit

Spectrometer In order to measure an IR spectrum, the dispersion Spectrometer takes several minutes. Also the detector receives only a few % of the energy of original light source.

Sample To select the specified IR light,

A slit is used.

Light source An interferogram is first made by the interferometer using IR light.

Fixed CCM Moving CCM

Detector B.S. Sample The interferogram is calculated and transformed into a spectrum using a Fourier Transform (FT).

IR Light source

FTIR In order to measure an IR spectrum, FTIR takes only a few seconds. Moreover, the detector receives up to 50% of the energy of original light source. (much larger than the dispersion spectrometer.)

FOURIER TRANSFORM IR Fourier transform (FT) relates to a mathematic way to convert rapidly collected ti e do ai data to spectra that we can interpret. The FT spectrometer works by splitting an IR beam into two. When these recombine, the light waves interfere constructively or destructively, creating the i te fe og a

hi h is the ti e do ai sig al

.

Sampling of an actual interferogram

Interferometer interferogram

Output of a Laser interferometer

Primary interferometer interferogram that was sampled Optical path difference x

Instrumentation Dispersive VNIR

• Portable • Repeatability? • External service • No validation

FT-NIR

• Benchtop • Repeatability *** • Self serviceable • Validation in-built • ISO compliant • Industry proven • Multipurpose

FT-MIR

Benchtop • Repeatability*** • No gas purging • Some servicing • Robotic • Validation in-built • ISO compliant • Outperforms NIR • Liquid Nitrogen required

FT-MIR

• Benchtop • Miniaturized MIR instrument • Repeatability*** • No gas purging • Some servicing • Validation in-built • ISO compliant • Outperforms NIR • ATR accessory can be attached to it. • Does not use liquid nitrogen

Handheld NIR/MIR

• Handheld • Sample homogeneity? • Variable moisture? • Repeatability? • Still expensive • Rapidly developing • Need to prepare by developing soil reference libraries

Advantages of FTIR •

Speed: analysis done in a very short time.

•

Sensitivity: detectors used are more sensitive and offer high optical throughput.

•

Mechanical simplicity: Few moving parts (only interferometer) hence rugged.

•

Internal calibration: HeNe laser used as an internal wavelength calibration standard hence no need for external calibration.

MIR and NIR Spectra Mid-Infrared Absorption Soil Spectrum

Near-Infrared Absorption Soil Spectrum

APPLICATIONS OF INFRARED SPECTROSCOPY 

Soils – high-throughputs soil analysis, mapping and monitoring of soil properties, remote sensing, digital soil mapping, precision agriculture, monitoring schemes for good agricultural practice and environmental services payments schemes, sediment analysis, soil pollution, mobile rural IR spectroscopy soil testing services.

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Crop agronomy/breeding/plant sciences –Tissue testing and crop response, Germplasm screening /breeding, seed viability/treatment, metabolomics –relating plant tissue biochemical fingerprints to ecological factors

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Crop and livestock products quality and processing – grain quality and storage, cash crops- tea and coffee etc, fruits and vegetables, Beverages and juice, Dairy products – butter, cheese, Meat, Wood and paper, Biofuels.

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Water quality – Long term monitoring of aquatic systems and Heavy metals pollution of fresh water sediments e.g. Cd, Cu, Zn, Pb , Mn, Fe in sediments Etc.

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