Instant Notes: Analytical Chemistry

through the fluorescence of complexed species using fluorogenic reagents are

given below in Table 3.

Table 3. Examples of UV-visible and fluorimetric analysis

Analyte Reagent Wavelength(s)/nma Application

Absorptiometry
Fe o-Phenanthroline Water, petroleum
Mn Oxidation to MnO 4 - 520 Steel
Aspirin – Analgesics
Sulfonamides Diazo derivatives Drug preparations

Fluorimetry
Al Alizarin garnet R 470/580 Water, soil
Borate Benzoin 370/450 Water, soil
Quinine – 250/450 Water, drugs
Codeine – 200/345 Drugs, body fluids
Vitamin A – 250/500 Foods
Polyaromatic hydrocarbons – 200/320- 550 Environmental

Many organic compounds and inorganic complexes may be determined by

direct absorptiometry using the Beer-Lambert Law (Topic E2). It is important to

recognize that for the most accurate work, or determination of trace amounts,

three criteria must be observed.

(i) The absorptivity of the species to be determined must be reasonably large.

While it is possible to determine metals such as copper or cobalt in water as

the aquo complex, this will give accurate results only down to about 1%

since e~10 m^2 mol-^1. However, for anthracene, C 14 H10, which has three fused

aromatic rings, e=18 000 m^2 mol-^1 and, thus, even a solution of about

0.5 ppm will give an absorbance of approximately 0.1 in a 1 cm cell.

(ii) The species must be stable in solution. It must not oxidize or precipitate or

change during the analysis (unless the analysis intends to study that change).

(iii) Calibration must be carried out over the range of concentrations to be deter-

mined. Agreement with the Beer-Lambert law must be established.

(iv) In complex matrices, it is not possible to analyze for all the species present

using a few spectra. It is necessary to separate the components using one of

the techniques described in Section D, or to use combined methods such as

those in Section F.

It should be noted that it is possible to determine two (or more) species in an

analytical sample by measuring the absorbance at several wavelengths.

Calibration and measurements at two wavelengths enables two components to

be determined simultaneously, but if more wavelengths are measured, a better

‘fit’ of the experimental data is achieved.

Example

Two organic components X and Y have absorption maxima at 255 and 330 nm,

respectively.

For a pure solution of X, e(255) =4.60; e(330) =0.46

For a pure solution of Y, e(255) =3.88; e(330) =30.00

For a mixture of X and Y in a 0.01 m cell, A(255) =0.274 and A(330) =0.111

Quantitative
analysis

E9 – Ultraviolet and visible molecular spectrometry: applications 231