The Identification of an Unknown Substance


Organic Chemistry 3510

1. Read pages 104-105, 534-535, 544-547.

2. In the laboratory, infrared (IR) analysis involves both sampling techniques and instrumental techniques to obtain the spectrum as well as the analysis of the spectral data to determine what the sample is. It is assumed that the student has encountered the theory of infrared spectral analysis either in a lecture course or in other readings. This document will focus on the experimental aspects.

A critical part of the infrared experiment is getting the infrared radiation to interact with the sample without losing a significant part of the infrared radiation from non-sample interactions (mirrors which absorb light rather than reflect, scratches on any optical surface which reflect light in the wrong direction, sample surfaces which reflect light, etc.). Classically, liquids were analyzed either neat (suspended between two sodium chloride plates (sodium chloride does not absorb infrared radiation in the spectral range of concern)) or in solution in a solvent such as carbon tetrachloride which does not absorb too much infrared radiation in the spectral range of concern. Solids, with problems associated with crystal surfaces reflecting away most of the radiation, were analyzed either in solution (at least the few solids which dissolved in suitable IR solvents) or more commonly as a mull, a KBr pellet, or a melt (if the melting point was low enough). Such sampling techniques were necessary to provide adequate sample for the classical dispersive optical-null double-beam prism and (later) grating spectrometers. However, the ready availability of low cost powerful laboratory computing has allowed the routine utilization of more sensitive nondispersive Fourier Transform Infrared Spectrometers (FT-IR). FT-IR allows for the collection of all the spectral data in seconds compared to 3-10 minutes for the classical grating spectometer. However, FT-IR requires powerful computing to mathematically analyze the collected data. Computers also allow for the collection of many spectra in a short time and the averaging of the spectra to eliminate most random noise. We now use only FT-IR in this course.

While classical sampling techniques are used with FT-IR, the great reduction of noise in FT-IR permits other sampling techniques that are faster, use less sample, and avoid hazardous solvents. Attenuated Total Reflectance (ATR) is a sampling technique in which the infrared radiation is passed into a crystal in contact with the sample. The radiation hits the crystal surface and bounces back through the crystal and, ultimately, to the detector of the spectrometer. Due to the wave nature of radiation, when the radiation is reflected from the crystal surface, part of the wave extends outside of the crystal for a very short distance. If the sample is within that distance, the sample can absorb the same infrared energy photons as if the infrared light was simply passed through the sample resulting in the same infrared spectrum as obtained by classical methods.

The ATR sampling accessory used in this course uses a single reflectance diamond crystal. The solid or liquid sample is placed on the diamond crystal and a compression clamp (swivel tip anvil) is lowered onto the sample forcing the sample next to the diamond. The pressure of the compression clamp is controled by a torque limiter so that adequate pressure is applied without dislocating the diamond from its holder.

Diamond is a suitable reflecting crystal since it is a very rugged optical material. It will resist scratching from most solid samples. However, diamond does have one shortcoming. It absorbs infrared radiation in the 2300-1800 cm-1 region which limits the signal to noise in this region. If this region is critical to your analysis, see your instructor about replacing either the diamond crystal with a zinc selenide crystal (careful, ZnSe scratches easily) or removing the ATR accessory so that classical sampling techniques can be used.

3. Suggestions (detailed instructions for the operation of our spectrometer will be given by your instructor in lab).

a. Before coming to lab become familiar with saving and retrieving files on the computer network.

b. Get a numbered liquid unknown from your instructor. It is one of the compounds listed below. You will obtain the spectrum using the diamond ATR accessory and the procedure described below. The "capillary film technique" described on page 545 is also described below just in case you need it in later courses or for problem samples. You will use an interferometric nondispersive spectrometer. (For now, do not worry about differences in spectrometers. Instead, concern yourself with sample handling and spectum analysis.)

c. Normally you will not have to run a "background". (A background measures the amount of energy that actually gets to the detector without any sample in the sampling device. The energy reaching the detector is not constant at all wavelengths due to: absorption by spectrometer mirrors and windows; scattering by flaws, scratches, and dirt; absorption by condensed compounds and the atmosphers; variable output by the infrared source; etc. The computer in the spectrometer subtracts the background from your sample data to produce the spectrum.) However, if you are the first person to use the spectrometer for the day, or if the spectrometer has been run for quite awhile, or if your spectrum is problematic, you should run a background.

4. A. Obtaining a spectrum of a solid or liquid using the diamond ATR.

a. Clean the diamond crystal and the stainless steel mounting plate with several drops of 95% alcohol on a small piece of cotton. Also, clean the bottom of the swivel tip anvil with 95% alcohol. (The alcohol should evaporate quickly.)

b. Place a drop of a liquid sample on the diamond crystal or a few milligrams of a solid sample in the center of the diamond crystal. Do not touch the diamond or base plate with the pipet or spatula.

c. Lower the swivel tip anvil by rotating the knob of the torque limiter in a clockwise fashion.

d. Continue to turn the knob clockwise until the torque limiter audibly clicks at the maximum allowable torque.

e. Obtain your spectrum. Print a hard copy and save your spectrum on the network with a recognizable name.

f. Rotate the torque limiter knob counter clockwise until the swivel tip anvil is about five milliimeters above the diamond.

g. Clean the diamond crystal, stainless steel mounting plate, and swivel tip with 95% alcohol on cotton.

h. Go to another networked laboratory computer with the IR software and retrieve your spectrum. Locate the nine largest and other signifacant peaks by either manual annotation or have the software find peaks for you. Print a copy of either the annotated spectrum or the computer data table.

i. Record the nine largest and other signifacant peaks in your notebook. (You may also attach the original spectrum in your notebook.)

4. B. Obtaining a spectrum of a liquid using salt plates.

a. Obtain the salt plates, holder, and O-rings from the desiccator.

b. Clean the salt plates by wiping with a Kim-wipe moistened with absolute alcohol. Be extremely careful not to touch the surface of the salt plates with your fingers. (Your fingers are wet enough to dissolve the salt!)

c. Place an O-ring on the salt plate holder.

d. Place a clean salt plate on the O-ring. Again, do not touch the flat surface of the salt plate!!

e. Place a drop or two of a dry liquid sample on the salt plate.

f. Place the other clean salt plate on top of the sample. Be sure there are no air bubbles in the sample.

g. Place the other O-ring on the upper salt plate.

h. Place the other metal holder on the O-ring.

i. Lightly tighten the four knurled nuts. If you tighten too hard you may crack the salt plates or squeeze all of your sample out from between the salt plates.

j. Place the holder in the spectrometer. Obtain your spectrum. Print a hard copy and save your spectrum on the network.

k. Disassemble the sample holder and clean the salt plates again by wiping with a Kim-wipe moistened with absolute alcohol.

l. Return the clean salt plates in their container to the desiccator. Also, return the salt plate holder and O-rings to the deciccator.

5. From the IR spectrum, determine the functional group of your unknown. Your unknown contains carbon, hydrogen, and perhaps oxygen. (Feel free to share spectra with your fellow students or to examine the Aldrich Library of FT-IR Spectra by Charles Pouchert [the index is their catalog].) Once you have determined the functional group, determine the identity of the unknown by matching IR fingerprints and using whatever other information you may have acquired. If you are having difficulty examining the "fingerprint", you may have to expand your spectrum. Some spectra are also available on the Web at NIST. These spectra are gas phased and won't be perfect matches. When you obtain spectra, you may have to "click" Reverse X to get large wavenumbers on the left. Spectra are also available at AIST in Japan. You can then print these reference spectra but first change your printer to "landscape".

Possible unknowns: (Since this experiment does not involve chemical changes, your Reagent Table (Notebook Prelab Write-up, Item 6, "Table of Reagents"), need only contain the name, color, and boiling point of each unknown. Also, there is no rule that you have to look up every compound, you could do several, and other members of your study group could do several. Just don't let one person do all of them and simply copy their work! {Liquids which are colorless like water will not have a color listed. Some editions of the CRC Handbook do not include color. All of the compounds in this experiment are colorless.}) While the list below is alphabetical, you may want to organize them by functional group.)

amyl acetate@
butyl ether&&
ethyl acetate&
ethyl benzoate*
ethyl ether^
isopropyl alcohol$
methyl benzoate$$
methyl ethyl ketone%

Compounds are arranged in reference books in a variety of ways. Some are strictly alphabetical, some use older names, some alphabetize using a parent grouping followed by the substituent. Listed below are some of the various names of these unknowns as found in the literature and the Aldrich Catalog.

#acetone is also called: ## acetophenone is also called: @ amyl acetate is also called: ** p-anisaldehyde is also called: &&butyl ether is also called: @@trans-cinnamaldehyde is also called: & ethyl acetate is also called: * ethyl benzoate is also called: ^ ethyl ether is also called: @ isopropyl alcohol is also called: $ methyl benzoate is also called: % methyl ethyl ketone is also called:

6. Be sure to record information about your spectrum in your notebook. (A good rule of thumb is to include the wavelengths of the nine largest peaks and any smaller, but significant, peaks as well as their relative transmittances {s, m, or w for strong, medium, or weak [or the actual transmittance values]}.) Also, turn in a hard copy of your spectrum along with your notebook pages. The spectrum will not be returned.

7. Be able to answer the following:

a. Define: absorption spectrum; spectra; spectrum; transmittance spectrum; cm-1.

b. Explain what problems a "wet" sample would give using the diamond ATR technique and what problems a "wet" sample would give using salt plates. Explain why care must be taken not to touch the surface of the salt plate.

c. What is an IR fingerprint?

| Chemistry Home | Dr. Sundin Home | Chemistry 3510 | sundin@uwplatt.edu |