Infrared Absorption Spectroscopy

Purpose: The purpose of this experiment is to provide experience with the methods commonly used to obtain infrared spectra of solids and liquids. These techniques may be used for either qualitative or quantitative application. Quantitation in infrared spectroscopy is considerably more difficult than in UV-Visible absorption and emission. Therefore, while quantitation is attempted here to introduce and illustrate some of the difficulties, the application of IR for compound identification is emphasized.

For this purpose an infrared spectrum is used in two ways. It may be interpreted to provide clues to the structure of an unknown. This approach is rarely conclusive, and must be supplemented with other types of evidence. Infrared spectra may also be used to compare a known compound with an unknown. For a conclusive identification, all features of both spectra that are more intense than instrument noise must match.

References: Bauman, Absorption Spectroscopy, Silverstein and Bassler, Spectrometric Identification of Organic Compounds

Equipment: Fourier transform infrared spectrophotometer (FTIR); diffuse reflectance and attenuated total reflectance (ATR) sampling accessories; mortar and pestle; rubber or plastic gloves; micro spatula; forceps; desiccators for cells and small pieces of equipment; 2 or 5 mL volumetric flask.

Reagents: IR grade KBr; dry chloroform, n-butanol; solid & liquid samples.

Procedure

All auxiliary equipment such as mortar, pestle, and diffuse reflectance sample cups should be washed, dried and stored in the desiccator at the end of each lab period. The ATR crystal should be carefully wiped with acetone. KBr should be stored in a drying oven when not immediately needed and otherwise in a desiccator.

1. Liquid Sample: The spectrum of your liquid sample is to be obtained using the ATR sampling accessory: on the neat liquid with the dry cell as a blank, and on a CHCl₃ solution with neat CHCl₃ as the blank. Follow the ATR instructions for placing the accessory in the FT-IR sample compartment and for placing the samples in the cell.

• Record the spectrum of the empty cell (into the reference memory channel of the FT­IR).
• Record the spectrum of CHCl₃ vs. the empty cell.
• Record the spectrum of CHCl₃ into the reference channel.
• Record the spectrum of your sample in CHCl₃ solution, versus the CHCl₃ blank. Prepare the solution using a 2 or 5 mL volumetric flask and the 50 mL syringe, to cause the most intense sample peak to fall between 0.75 to 1.5 absorbance units. Select the proper concentration by trial, making the initial solution 20% (v/v). Scan manually the region of the most intense peak to ascertain whether the concentration is right before recording a pattern.
• Once you have prepared a stock solution with the requisite absorbance, prepare three dilutions of the solution, diluting by factors of 2, 5, and 10. Rinse the liquid cell with the solution to be recorded, then record the spectrum of each solution. Use the FT-IR software to determine the peak area of the peak with the highest absorbance.

2. Effect of Impurities: Record the spectrum of pure n-butanol and 95% n-butanol, both neat, in the ATR cell.

3. Solid Sample: The solid spectra are obtained using the Diffuse Reflectance sampling accessory. Follow the sampling accessory instructions for placing the accessory in the FT-IR sample compartment and for loading samples into the sample cups. Samples are ground and "diluted" with KBr powder, with pure KBr powder used as the blank. Spectral distortions are minimized by keeping the powder granule size of the KBr blank, KBr "diluent", and sample roughly the same. (i.e., don't grind the heck out of the sample plus diluent but not the blank; grind both so that the final powder grain size is roughly similar. Also, don't grind excessively; this promotes water absorption by the sample and the KBr.)

• Record a reference spectrum of dry KBr.
• Grind your sample together with KBr, in a ratio of KBr to sample of about 10:1 to 20:1. Do not use excessive amounts of sample or KBr; use only enough total amount of sample plus diluent to powder to fill the sample cup . 2-3 times. Store the mixed powder in a labelled vial in the dessicator until ready to use. Obtain the spectrum. If it is too concentrated, dilute the remaining sample with additional KBr. If too dilute, prepare a new sample mixture of higher concentration.

Report

Liquid Sample -

1. Compare your neat liquid spectrum very carefully with the reference spectrum (Sadtler index). Note on it any features which do not match. Can they be accounted for by the cell blank?
2. Identify those peaks that are characteristic of the class of compounds of which your sample is representative. The text, Chapter 18, Silverstein and Bassler and Bacumann may be useful.
3. On your CHCl₃ spectrum, note those bands which show less than 40% transmittance. Mark these regions on your solution spectrum. These may be disregarded in using such a spectrum. Compare the solution spectrum with that of the neat liquid. Note any differences.
4. Make two calibration plots, of the maximum absorbance and of the integrated peak area of the strongest peak in the spectrum versus relative concentration for the four spectra. Do either of the plots follow a simple Beer's Law (linear) relationship? Why or why not?

Effect of Impurities -
Compare your spectrum of pure n-butanol with that of the reference and note any discrepancies. Then compare the spectrum of pure n-butanol with that of the 9% sample. Mark the differences. If properly prepared, they will be minor; however, even these small deviations are sufficient to vitiate an identification if based entirely upon this evidence. Further purification would be necessary for a conclusive identification.

Solid Sample -
Compare the spectrum with the reference (Sadtler index). Identify any peaks that do not match. Determine whether they can be accounted for by the KBr blank, or by the use of diffuse reflectance as the sampling method.