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 (180-1100nm) it is considered to be UV/Visible.

• The three sub-domains of the UV-VIS spectrum are:

- near UV (185-400nm)

- 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 (Eelec)

- Energy of rotation (Erot)

- Energy of vibration (Evib)

• Alterations of Eelec will result in adjustments for both Erot and Evib based on the following relationship:

ΔEtot = ΔErot + ΔEvib + ΔEelec , with

ΔEelec > ΔEvib > ΔErot

• 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 (I0) 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/I0 or %T = I/I0 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)