Chemistry Reference
In-Depth Information
% Absorbance
Mass of added
standard
Mass of drug
in sample
Figure 7.17
A calibration graph using method of standard additions.
The method of standard additions is widely used in atomic spec-
troscopy (e.g. determination of Ca
2
ions in serum by atomic emission
spectrophotometry) and, since several aliquots of sample are analysed to
produce the calibration graph, should increase the accuracy and precision
of the assay.
Derivative spectroscopy
Occasionally during the assay of a medicinal product, excipients present in
the formulation mask the absorbance of the active drug. This often occurs
in liquid formulations such as syrups and linctuses where a small amount
of a highly coloured dye is used to colour the mixture. If a simple dilution
of a coloured mixture was carried out, the resulting absorbance measure-
ments would not obey Beer's law and a non-linear graph of absorbance vs
concentration would result.
In cases such as these, it is often possible to use the
mathematical
derivative
of the absorbance spectrum to calculate the content of active
drug. In calculus, the derivative of a function is the instantaneous rate of
change of that function with respect to a variable. This can also be thought
of as the slope of the graph or function at any point on the graph.
Derivatising a simple plot of absorbance vs wavelength allows certain peaks
within the total spectrum to be 'sharpened' and separated from the back-
ground absorbance of the excipients. The original spectrum of the drug is
called the fundamental (or D
0
) spectrum, the first derivative D
1
, second
derivative D
2
, and so on until useful information becomes lost in the elec-
tronic noise of the spectrophotometer. If the original spectrum is a plot of
absorbance vs wavelength (the D
0
spectrum) then the first derivative, D
1
, is
written as d
A
/dk, the second derivative as d
2
A
/dk
2
and so on. A diagram of
D
0
, D
1
and D
2
spectra is shown in Figure 7.18.
