Chemistry, Third edition

ABSORBANCE AND TRANSMITTANCE OF A SAMPLE

The range and intensity of wavelengths emitted by the light source (its emission

spectrum) is found using an uninterrupted beam (i.e. an empty cell, Fig. 20.5(a))

and may be displayed either as a graph of light intensity versus light wavelength, or

in the form of a photographic emulsion in which darkareas register the emitted light

to which it has been exposed.

If a sample is exposed to the light source, and some of the photons emitted by the

light source have energies equal to the gaps between the energy levels found in the

atoms or molecules in the sample, absorption may take place. The spectrum in Fig.

20.5(b) shows troughs at three wavelengths – (i), (ii) and (iii) – indicating that three

different energy transitions have taken place within the atoms or molecules of the

sample. The light-sensitive photographic emulsion registers the absorptions as light

areas, i.e. wavelength regions in which the intensity of the transmitted light is small.

(The light areas appear black when the emulsion is developed and printed as a pho-

tograph.)

Absorbance and transmittance of a

sample

The light source in the spectrometer continuously bombards the sample cell with

photons. If the sample cell is empty of absorbing material, the maximum intensity of

light (at that wavelength) reaches the detector. We symbolize the maximum inten-

sity as I0()whereis the wavelength of light selected by the grating or prism. If an

absorbing sample is introduced into the beam and the sample absorbs light at that

wavelength, excited states may be formed. The excited states lose their energy by col-

lisions or by light emission, and the intensity of light reaching the detector along the

cell axis is reduced. We symbolize the new value of intensity as I(). The percent

transmittanceof the sample, symbolized %T, at wavelength is defined as

100 I()

%T= ————–

I0()

Alternatively, the reduction in beam intensity may be expressed as an absorbance

A. Absorbance is a unitless quantity defined as

I0()

A= log —–(

l())

In commercial spectrometers, the instrument adjusts the diffraction grating so

that successively lower (or higher) wavelengths are isolated, and then allowed to irra-

diate the sample. This is called a scanover the desired wavelength range and gives val-

ues of Iat each . Spectrometers also obtain I 0 at each , either (as in double-beam

instruments) by periodically diverting the incident beam through an empty cell dur-

ing the scan, or (as in single-beam instruments) by computer storage of the variation

of I with from a previous scan using an empty cell. Either way, the spectrometer has

now suf-ficient information to produce a plot of either %TorAagainst wavelength.

Such a plot is called an absorption spectrum. The absorption spectrum of the empty

sample cell will be a straight line since, by definition, IequalsI 0 at each (Fig. 20.6(a)).

Scanning an absorbing sample will result in the spectrum showing peaks (Fig.

20.6(b)) which indicate the wavelengths of light absorbed.

20.4

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