SOIL AND PLANT ANALYSIS (1+1)

UV Spectrophotometer

Ultraviolet and visible spectrometers have been in general use for the last 35 years and over this period have become the most important analytical instrument in the modern day laboratory. In many applications other techniques could be employed but none rival UV-Visible spectrometry for its simplicity, versatility, speed, accuracy and cost-effectiveness.
For convenience of reference, the Joint committee on Nomenclature in Applied Spectroscopy has set definitions of the various spectral regions as given below:


Region	Wavelenght (nm)
Far ultraviolet	10 – 200
Near ultraviolet	200 -380
Visible	380 -780
Near infrared	780 – 3000
Middle infrared	3000 – 30,000
Far infrared	30,000 – 300,000
Microwave	300,000 – 1,000,000,000

The human eye is only sensitive to a tiny proportion of the total electromagnetic spectrum between approximately 380 and 780 nm and within this area we perceive the colors of the rainbow from violet through to red. If the full electromagnetic spectrum shown in Figure below was redrawn on a linear scale and the length of one centimeter represented the visible region, then the boundary between radio and microwaves would have to be drawn approximately 25 kilometers away.


A molecule or ion will exhibit absorption in the visible or ultraviolet region when radiation causes an electronic transition within its structure. Thus, the absorption of light by a sample in the ultraviolet or visible region is accompanied by a change in the electronic state of the molecules in the sample. The energy supplied by the light will promote electrons from their ground state orbital’s to higher energy, excited state orbitals or antibonding orbital’s.


Relationship between light absorption and color



Color absorbed	Colour observed	Absorbed radiation (nm)
Violet	Yellow-green	400-435
Blue	Yellow	435-480
Green-blue	Orange	480-490
Blue-green	Red	490-500
Green	Purple	500-560
Yellow-green	Violet	560-580
Yellow	Blue	580-595
Orange	Green-blue	595-605
Red	Blue-green	605-750