Caffeine Infrared Spectrum

The quantitative analysis of a multicomponent system is illustrated by its application to the simultaneous determination, of aspirin, phenacetin and caffeine in tablets. These drug components can be determined by dissolving the sample in chloroform, with each of the components showing distinct carbonyl bands in the infrared spectrum. 

A solution of caffeine in chloroform is provided for analysis, with the concentration of caffeine in the solution being required. Caffeine in chloroform shows a distinct carbonyl band at 1656 cm in the infrared spectrum (recorded by using a 0.1 mm NaCI cell). The infrared information provided by standard solutions of caffeine in chloroform is listed below in Table 3.2. Given that the unknown sample produces an absorbance of 0.166 at 1656 cm" in its infrared spectrum, determine the concentration of this caffeine sample. Assume that there is no interference from other components in the sample. 

At the option of your instructor, obtain the infrared spectrum of the caffeine as a dry film by dissolving a sample in dry acetone (Technique 25, Section 25.4). Alternatively, a KBr pellet of the caffeine can be prepared to obtain the spectrum (Technique 25, Section 25.5). Include the infrared spectrum with your laboratory report, along with an interpretation of the principal peaks. A spectrum is included for comparison purposes. 

Fig. 6 Spectral differences in mid-infrared spectra of caffeine measured under different conditions. Diffuse reflectance spectrum of the pure crystalline powder (top trace), diffuse reflectance spectrum of 1% of caffeine in KBr-powder (middle trace, intensity data for both spectra in K-M units), and absorbance spectrum as recorded using the KBr pellet technique (bottom trace).

Fig. 5 Comparison of mid-infrared spectra of caffeine obtained by diffuse reflectance and transmission spectroscopy. (A) Diffuse reflectance spectrum of the pure powdered substance with transformed intensity data in K-M units. (B) Same diffuse reflectance spectrum, but using —log(i ) transformation (top trace), the lower spectral range was limited by the cut-off of the MCT detector used the bottom trace shows a transmission spectrum using the conventional KBr pellet technique transformed into absorbance, i.e., —log(transmittance).

Attach your infrared spectra to your report and label the major peaks. If you determined the mass spectrum, try to identify the important fragment ion peaks (Technique 28). Include the melting point, if it was required. Report the weight percentage of the caffeine recovered from the tea or coffee sample before and after sublimation. 

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