Simultaneous spectrophotometric determination of compounds having relatively disparate absorbance and concentration ranges; application to antidiabetic formulation of linagliptin and metformin.

The limited linear range of UV-Visible spectrophotometry may be insufficient to occupy multiple components with wide variations in their concentrations or absorptivities that will hinder the simultaneous spectrophotometric determination and may require spiking or measurements in subsequent dilution steps. The current work introduces the absorptivity target concentration (ATC) values, a simple way for the proper choice of the working spectral region to execute accurate and linear spectrophotometric measurements. Simultaneous spectrophotometric determination of linagliptin (LNG) and metformin (MET) that are present in a ratio of 1:400 was carried out using traditional spectrophotometric techniques such as third derivative and derivative ratio as well as recently developed techniques such as ratio difference and factorized dual wavelength. The proposed methods were able to determine MET in the concentration range of 50-1200 μg mL. On the other hand, LNG was successfully determined from its zero-order absorption UV-spectrum at λ (296 nm) in the concentration range 2.5-25 μg mL. The mentioned methods were successfully applied for the determination of the LNG and MET in their combined dosage form. The methods were validated according to the ICH guidelines. The proposed ATC value can be employed as a novel concept for the proper choice of the working spectral region where spectrophotometric measurements can be deployed accurately and precisely.