Interaction of radiation and matter If matter is exposed to electromagnetic radiation, e.g. infrared light, the radiation can be absorbed, transmitted, reflected, scattered or undergo photoluminescence. Photoluminescence is a term used to designate a number of effects, including fluorescence, phosphorescence, and Raman scattering

The complement of the absorbed light gets transmitted. The color of an object we see is due to the wavelengths transmitted or reflected. Other wavelengths are absorbed. The more absorbed, the darker the color (the more concentrated the solution). In spectrochemical methods, we measure the absorbed radiation.

The distance of one cycle is the wavelength (l). The frequency (n) is the number of cycles passing a fixed point per unit time. l = c/n (c = velocity of light, 3 x 1010 cm s-1 ). The shorter the wavelength, the higher the energy: E = hn This is why UV radiation from the sun burns you.

TYPES OF OPTICAL INSTRUMENTS

Spectroscope: uses human eye as a detector • Spectrograph: photographic emulsion used as detector • Spectrometer: has photoelectric readout 1. Monochromator: one exit slit, Greek for “one color” 2. Polychromator: multiple exit slits • Spectrophotometer:electronics takes ratio of two beams (%T), may be at same or different wavelengths, may be single beam or double beam

The Beer-Lambert law (or Beer’s law) is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:

A = a() * b * c

where A is the measured absorbance, a() is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration. When working in concentration units of molarity, the Beer-Lambert law is written as:

A =  * b * c

where  is the wavelength-dependent molar absorptivity coefficient with units of M-1 cm-1. Data are frequently reported in percent transmission (I/I0 * 100) or in absorbannce [A = log (I/I0)]. The latter is particularly convenient.

Sometimes the extinction coefficient is given in other units; for example,

A = E1% * b * c

where the concentration C is in gram per 100 ml of solution. This useful when the molecular weight of the solute is unknown or uncertain.

Limitations of the Beer-Lambert law

The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include:

  • deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity
  • scattering of light due to particulates in the sample
  • fluoresecence or phosphorescence of the sample
  • changes in refractive index at high analyte concentration
  • shifts in chemical equilibria as a function of concentration
  • non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band stray light