Spectroscopic Methods of Analysis

WAVE PROPERTIES OF ELECTROMAGNETIC RADIATION Electromagnetic radiation needs no medium through which it can travel. Electromagnetic radiation travels in waves in individual bundles of energy, called photons or quanta. Waves have amplitudes (A), wavelengths (λ), and frequencies (ν).

Electromagnetic radiation comes in a wide variety of energies, amplitudes, wavelengths, and frequencies.

ABSORPTION AND TRANSMISSION When a photon of radiation passes near an elementary particle, absorption can occur only if the energy of the photon matches exactly the energy difference between two orbitals in the element.

Amplitude is the

maximum wave

height

λ is the linear distance between

successive maxima

or minima of a

wave.

ν is the number of

oscillations per

second.

Absorbance of a

substance in a

transparent medium

occurs when the

intensity of

specific

frequencies of

electromagnetic

radiation decrease.

Spectroscopic Methods of Analysis

Colorimeter

Some substances can absorb radiation in specific wavelengths of the visible range of the electromagnetic spectrum and this is practical for obtaining quantitative chemical information through the use of a spectrophotometer or a colorimeter. For absorbing species, the intensity of visible light before passing through the chemical sample is higher than the intensity of visible light that has passed through the sample. This ratio of incident radiation (I) to outputted radiation (Io) is called transmittance (T).

T = Io/I

Transmittance (T) is a measure of the quantity of light that passes through a medium. Absorbance, A, is related to transmission by the equation

A = – log10 T

BEER’S LAW AND BEER’S LAW PLOT If a sample absorbs visible light, the amount of absorbing particles is related to the absorbance by

A α moles

Moles in solution are measured by molar concentration, c, therefore,

A α c A plot of absorbance versus concentration is called a Beer’s Law Plot and the equation describing this relationship is called the Beer—Lambert Law or Beer’s Law, expressed as

A = εbc where ε is a proportionality constant and the slope of a Beer’s Law Plot, called the molar absorptivity. b is the pathlength of the radiation through the absorbing medium in cm. A Beer’s Law Plot and Beer’s Law is used in quantitative analysis to determine the concentration of an unknown sample from absorbance data.

I Io

Spectroscopic Methods of Analysis

Analyte is the

substance being

measured in an

analytical

procedure.

Sample Problem Since absorbance is a unitless value, calculate the units of the molar absorptivity constant, ε.

L IMITATIONS 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