2.3　Presentation of Ultraviolet and Visible Spectrum

The UV spectrum is universally recorded as a plot of absorbance versus wavelength.It is customary with either ε or lg ε plotted on the ordinate^[56] and wavelength plotted on the abscissa^[57]，as the melamine spectrum shown below.However，very few electronic spectra are reproduced in the scientific literature^[58]； most are described by indications of the wavelength maxima and absorptivities of the principal absorption peaks.For example：237 nm（ε10⁴）or 237 nm（lg ε 4.0）.Fig.2-3 shows a UV spectrum of melamine^[59].

Fig.2-3　UV spectrum of melamine

圖2-3　三聚氰胺的紫外光譜

Because the absorbance of a sample will be proportional to the number of absorbing molecules in the spectrometer light beam^[60]（e.g.their molar concentration^[61] in the sample tube^[62]），it is necessary to correct the absorbance value for this and other operational factors if the spectra of different compounds are to be compared in a meaningful way.The corrected absorption value is called “molar absorptivity^[63]”，and is particularly useful when comparing the spectra of different compounds and determining the relative strength of light absorbing functions（chromophores）.Molar absorptivity（ε）is defined as：

Where A——absorbance；

c——sample concentration in moles/liter^[64]；

l——length of light path through the sample in cm^[65].

Molar absorptivities may be very large for strongly absorbing chromophores（＞10000）and very small if absorption is weak（10 to 100）.