Spectrophotometer is an instrument for measuring the intensity of light of a definite wavelength transmitted by a substance or a solution,this way provide the measure of the amount of material in the solution absorbing light. It is applied by putting a small amount of a known or unknown solution in a cuvet then put it in the machine.
Most spectrophotometers are used in the UV and visible regions of the spectrum, the concentration of a protein can be measured wave lenght 280 nm due to the presence of tyrosine, tryptophan, and phenylalanine but this method is not very accurate since the composition of protein varies greatly and the proteins with none of these amino acids do not have maximum absorption at 280 nm.
There are two classes of spectrophotometers: single and dual beam. The single beam measure absolute light intensity, while the dual beam measures the ratio of light intensities on two seperate light paths. The spectrophotometer can be applied to measure the concentration of different substances include proteins, DNA or RNA, growth of bacterial cells, and enzymatic reactions.