Preliminary Physicochemical Characterization of Iguratimod Using UV Spectroscopy, FTIR And Melting Point Analysis

Table 1. List of Instruments used

1. 2. UV - Visible spectrophotometric analysis:
      1. Preparation of Standard Stock Solution:

An accurately weighed quantity of 10 mg of Iguratimod was transferred into a 100 mL volumetric flask. The drug was dissolved in methanol with the aid of sonication and the volume was made up to the mark with the same solvent to obtain a stock solution of 100 µg/mL.

1. 1. 2. Preparation of Working Standard Solutions:

Appropriate aliquots of the stock solution were transferred into a series of 10 mL volumetric flasks and diluted with methanol to obtain concentrations of 5, 10, 15, 20, 25, and 30 µg/mL.

1. 3. Determination of Maximum Wavelength (λmax):A standard solution of Iguratimod was scanned in the wavelength range of 200–400 nm using methanol as blank to determine the wavelength of maximum absorbance.
   4. Construction of Calibration Curve:

The absorbance of the prepared working standard solutions was measured at the selected λmax. A calibration curve was constructed by plotting concentration (µg/mL) against absorbance, and the regression equation and correlation coefficient were calculated to evaluate the linearity of the method according to the Beer–Lambert Law.

2.3. Determination of melting point:

The melting point of Iguratimod was determined using the capillary tube method. A small amount of finely powdered sample was filled into a capillary tube sealed at one end to a height of approximately 2–3 mm. The capillary tube was placed in a digital melting point apparatus and heated gradually. The temperature at which the sample started melting and the temperature at which it completely liquefied were recorded as the melting point range.

2.4. FT IR Analysis:

Fourier Transform Infrared (FTIR) spectroscopy is used to identify functional groups present in a compound based on the absorption of infrared radiation by molecular bonds. Each functional group absorbs infrared radiation at characteristic frequencies, producing a unique spectrum that serves as a molecular fingerprint for compound identification.

2.4.1. Sample Preparation:

The FTIR spectrum of Iguratimod was recorded using the direct sample method without preparation of a potassium bromide pellet. A small quantity of the powdered sample was placed directly on the sample holder of the FTIR spectrophotometer.

2.4.2. Procedure:

The sample was gently pressed against the crystal surface of the instrument and scanned over the wavenumber range of 4000–400 cm⁻¹. The spectrum obtained was recorded and analyzed to identify the characteristic absorption bands corresponding to different functional groups present in the molecule.

2.4.3. Interpretation

The observed absorption peaks were compared with standard reference values to confirm the presence of functional groups such as aromatic rings, carbonyl groups, and other characteristic bonds present in Iguratimod.

3. Results and discussion:

3.1. UV Visible spectrophotometric Analysis:

The UV spectrum of Iguratimod was recorded in methanol in the wavelength range of 200–400 nm to determine the wavelength of maximum absorption (λmax). The drug exhibited a prominent absorption peak at approximately 257 nm, which was selected for quantitative analysis.

A series of standard solutions in the concentration range of 5–30 µg/mL were prepared and their absorbance values were measured at the selected λmax. The calibration curve was constructed by plotting concentration against absorbance. The calibration plot showed a linear relationship, confirming adherence to the Beer–Lambert Law within the studied concentration range.

The regression equation obtained from the calibration curve demonstrated a high correlation coefficient (R² value close to 1), indicating good linearity and reliability of the method for quantitative determination of Iguratimod. The results suggest that the developed UV spectrophotometric method is simple, rapid, and suitable for routine analysis of Iguratimod.

Figure 1. Determination of λmax

3.2. Determination of melting point:

The melting point of Iguratimod was determined using the capillary tube method with a digital melting point apparatus.

The melting point of Iguratimod was found to be in the range of 238–242 °C, which is in close agreement with the reported literature value.

The sharp and narrow melting range obtained indicates the good purity and crystalline nature of the drug sample. A narrow melting interval generally suggests the absence of significant impurities, whereas impurities usually produce depression and broadening of the melting range. The observed melting behavior therefore confirms the identity and purity of iguratimod used for the analytical study.

The experimentally obtained melting point was consistent with previously reported physicochemical data, supporting the suitability of the received reference standard for further characterization and method development studies. The high melting point of iguratimod may be attributed to its stable aromatic heterocyclic structure and intermolecular interactions within the crystal lattice.

Reported literature melting point: 238–242 °C.

Observed melting point: 239.7 °C.

Figure 2. Determination of melting point

3.3. FTIR Analysis:

The FTIR spectrum of Iguratimod was recorded in the range of 4000–400 cm⁻¹ using the direct sample method. the observed peaks were in agreement with the reported structure of iguratimod and demonstrated the presence of aromatic, carbonyl, sulfone, and amide functionalities.

Major absorption bands observed in the FTIR spectrum included a peak around 3300–3400 cm⁻¹ corresponding to N–H stretching vibration, indicating the presence of the amide group. The bands observed near 2920–2850 cm⁻¹ were attributed to aromatic/aliphatic C–H stretching vibrations. A strong characteristic absorption peak around 1650–1700 cm⁻¹ confirmed the presence ofthe carbonyl (C=O) functional group. The intense peaks observed in the region of 1320–1150 cm⁻¹ were assigned to symmetric and asymmetric stretching vibrations of the sulfone (S=O) group. Peaks in the region of 1500–1600 cm⁻¹ indicated aromatic C=C stretching vibrations.(Table 2)

The obtained FTIR spectrum confirmed the structural integrity and purity of Iguratimod and showed no evidence of significant interference or degradation products. The characteristic functional group frequencies correlated well with the proposed chemical structure, thereby supporting the suitability of the drug for further analytical investigations and formulation studies.

Figure 3. FTIR Spectrum of iguratimod

Table 2. FTIR interpretation of iguratimod