Question

1. What is the difference between spectrophotometry and spectroscopy?
2. Why is a background absorption measurement taken before taking a spectrum?
3. What is Beer's absorption equation?
4. What are kind transitions does UV-Vis measure? (i.e. purely vibrational, rotational, etc.)?

Answer 1

Answer 1: Spectrophotometry is a quantitative technique for the measurement of reflection or transmission properties of a material as a function of wavelength. It basically measures the relative intensity of electromagnetic radiation reflected or transmitted at a particular wavelength. For this purpose the device used is known as spectrophotometer which consists of two instruments, a spectrometer for producing light of any selected wavelength, and a photometer for measuring the intensity of light. Spectrophotometers are used to measure the absorbency of a certain wavelength of a solution, reflectance of solutions, transmittance or the transparency of solids.

Spectroscopy is a technique to study the interaction of matter with electromagnetic radiation. It basically identify which wavelength of light is absorbed or reflected by a particular compound. For this purpose we use spectrometer and one should have a proper knowledge of electromagnetic spectrum.  

Answer 2: While obtaining a spectra, there are frequently other signals detected by the detector. These interference are inevitable and influence the spectra resulting in loss of spectral detail which is known as spectral deviation. In order to avoid these deviation we run background measurement prior to taking sample measurement. Therefore after taking the sample measurement there background signals are subtracted to avoid the deviations.

Answer 3. Beer's law states that the absorbance is directly proportional to concentration of the solution having sample.

(1)

Ao

Absorbance is also directly proportional to path length of light which is width of cuvette used

$A\propto l$ (2)

Combining both equation we get

A c/

Therefore   (Beer's equation or Beer-Lambert Equation)

A= ech

Where, A = Absorbance, c = concentration, l = path length

$\varepsilon$ = molar absorptivity or molar extinction coefficient and is a measure of the probability of the electronic transition.(proportionality constant)