Lecture 5. UV-VIS Spectroscopy

Ultraviolet and Visible Absorption Spectroscopy When matter absorbs electromagnetic radiation in the domain ranging from the near ultraviolet to the very near infrared ( nm) it is considered to be UV/Visible.

The three sub-domains of the UV-VIS spectrum are: - near UV ( nm) - visible (400 – 700nm) - very near infrared (700 – 1100nm) UV-VIS Spectral Region

Absorption in the UV-VIS region Absorption occurs in the UV-VIS region when photons from a source of light interact with ions or molecules in a sample. Molecules that absorb photons in the UV- VIS region, transfer the corresponding energy from that photon to one or several of its outermost electrons.

This transfer of energy results in a change in the components of the total mechanical energy which are: - Electronic energy (E elec ) - Energy of rotation (E rot ) - Energy of vibration (E vib)

Alterations of E elec will result in adjustments for both E rot and E vib based on the following relationship: ΔE tot = ΔE rot + ΔE vib + ΔE elec, with ΔE elec > ΔE vib > ΔE rot

The concentration of an analyte in solution can be determined by measuring the absorbance at some wavelength and applying the Lambert-Beer Law.This method is known as colorimetry. Colorimetry utilizes the relationship between concentration and optical density or absorbance. UV-VIS Method

Colorimetry can be used on compounds which have absorption spectrum in a specific spectral region. Compounds that do not have absorption properties can be modified with specific reagents that lead to derivatives which permit absorption measurements. A wide range of instruments such as basic colorimetric devices to automated spectrophotometers can achieve the analyses of these various compounds.

UV-VIS spectrometers collect data over a required range and generate the spectrum of the compound under analysis as a graph representing the transmittance or absorbance as a function of wavelength along the abscissa given in nanometers.

The transmittance (T) is a measure of the attenuation of a beam of monochromatic light based upon the comparison between the intensities of the transmitted light (I) and the incident light (I 0 ) when the compound is placed in the optical pathway between the source and the detector. T is expressed as a fraction or percentage: T = I/I 0 or %T = I/I 0 x 100

The absorbance, formerly known as optical density is definded by: A = - log T (insert pic of UV-VIS spectra)

UV-VIS Applications (insert info from journal article pertaining to biomaterial/wood)