The intensity of the calcium citrate absorbance between 1518 and
1637 cm1 increased with increasing concentration, while absorbance
intensity for the free acid decreased with decreasing concentration
in the same series of standards between 1637 and
1776 cm1 with an isosbestic point near 1637 cm1
. Sodium
dicyanamide was chosen as an internal standard because of its
unique absorption bands at 2100e2300 cm1
. Its well-defined peak
at 2228 cm1 remained constant in all calibration standards and
was unaffected by varying concentrations of chelated metals and
free acids. The areas of the peaks in the regions of the spectrum
corresponding to the free acid and metal chelate were individually
determined by defining a baseline connecting the minima on each
side of the respective region and subsequent integration. The
spectral peak for the internal standard was integrated similarly.
Resulting areas for each region of chelate and free acid were divided
by the area of the internal standard to normalize spectral differences
and allow independent determination of both free acid and
metal chelate.
Fig