Lab - Spectroscopy

Ge151: Spring 2011

Infrared Spectroscopy

The objective of this exercise is to explore some of the capabilities and complexities of infrared spectroscopy.

Reading: Instrumental methods of Analysis by Willard, 1988, Call number QD79.I5 I52 1988

I. INTRODUCTION

Read the description of Fourier Transform Infrared (FTIR) spectroscopy handed out in class.
Obligatory "motherhood" statement for using borrowed equipment: We are very lucky to have access to the FTIR instrument in Arms; please treat the instrument with care, clean up after yourself, etc.
If you run into any difficulties then you can call me at my office (6447).

II. DATA COLLECTION

The following steps describe how to collect infrared reflectance data from mineral samples and how to record the results. Please do not change the default settings for the FTIR. If you run into problems, please talk to your TA or Prof. Rossman. Be careful of the other equipment in the lab.

Start the program: Open the 'Run FTIR' folder on the desktop. Click on the 'Run the FTIR' icon. This will open the OMNIC program.
In the main drop down menu visible scroll down to the bottom and select 'Caltech Diffuse Reflectance Mid-IR'
Make sure the purge guards are closed (up position), open the front main lever by turning 90^o CCW, you can now open the main compartment. The purge guards are important because they prevent water vapor from entering the main body of the machine where it is difficult to remove it from.
Align the gold mirror in the sample chamber. You can look at it from above and you should be able to see the laser spot on it (this is a guiding laser only the real illumination is not at visible wavelengths). Close the compartment and lock it with the front lever. Leave the top window open a crack (about 1 mm) to make the air purges more efficient. The purging gas will eventually push out all the water vapor from the sample chamber. Wait about 2 minutes for this process to work, and then open the purge guards on both sides of the chamber (levers in the down position).
To check the alignment of the sample holding carriage open the menu collect-experiment setup-bench. Look at the interferogram. The max/min values should be 7-8. If they are much less, the sample holder is out of alignment and needs to be adjusted. When satisfied with the alignment, click ok to exit the menu.
To collect a background spectrum click the 'Col Bkg' button. The default setting is 32 scans; you will see the spectrum add up on the screen. If there are large fine lines in the spectrum, there is still a lot of water in the chamber. If this is the case, wait a minute, then collect a new background. Print out a copy of the background and turn it in. The printer is in the next room. You should see what basically looks like a black body curve; gold has few absorption features here (which is why it's the standard).
Go through the same procedure with the purge guards and put your sample in the chamber. Use the sample holders provided; again look for the spot of the guiding laser. Be careful not to bump the sample holder out of alignment. Close the chamber, wait 2 minutes and open the purge guards.
Click the 'Col Smp' button; this will go through the same procedure as before. The spectrum will show up in a new window. The background has already been divided out of the spectrum. List two reasons why we still need to remove the background spectra if all the contaminants (like water vapor) have already been removed.
Chemists and most other people working in labs tend to use wavenumbers for their work. Astronomers and planetary scientists tend to use microns instead. If you'd like to convert the x-axis to microns you can run through these steps. Go to the 'process' menu on the top menu bar, select 'other conversions'. Scroll down to microns. Accept the default settings. The scale is still backwards (in order of increasing energy rather than increasing wavelength). Go to the view menu and select 'Display limits', change the x-axis limits to 2 - 25 microns.
The background spectrum also appears on the screen you can remove it by clicking on that spectrum (it should change colour). Go to the 'edit' menu and select 'clear'.
Print out the spectrum. You can print it out in chunks by altering the display limits as described above.
Please be careful with the samples, especially the powder. Leave the lab area clean and orderly.

III. INVESTIGATIONS

Before taking the spectra below, read about calcite, albite, and gypsum in Farmer: The Infrared Spectra of Minerals, in the Geology library. Identify the characteristic lines for each mineral within 5200-400 cm^-1. Try to identify these lines as you collect the spectra with the FTIR.

A: Different compositions.

1. Collect spectra and identify the major spectral features in each of the following samples, make printouts showing your results:

Calcite (CaCO3).
Albite feldspar (NaAlSi3O8).
Gypsum (CaSO4•2H2O).

B: Effects of particle size

1. Collect spectra from the solid (one piece), the fine (powdered), and the coarse (granular) samples of calcite (CaCO3).

2. Identify the major spectral features. There should be some you did not anticipate. (Use Farmer: The Infrared Spectra of Minerals (in the Geology library) or Nakamoto: Infrared and Raman Spectra of Inorganic and Coordination Compounds (in Millikan) or Salisbury et al. IR (2.1-25 microns) Spectra of Minerals.)

3. Describe and explain the differences between the three spectra.

C: Effects of packing density

1. Collect spectra from loosely packed, powdered calcite and heavily packed, powdered calcite (take your sample and compress it in the holder with your pen).

2. Describe and explain any differences between the spectra.

D: Palagonites and Mars.

Many authors have suggested that a palagonite such as the Hawaiian pahala ash used in this exercise is a good spectral analog for Mars. Note that palagonite is not a true mineral, but rather a mixture of volcanic glasses, iron oxides, smectite clays, and other minerals.

1. Collect spectra from the palagonite. Attempt to identify the major spectral features.

2. Below is a 2 to 6 micron spectrum of Mars from Calvin, 1996 (based on Mariner data). Compare the Mars spectrum with the data you collected. Would you consider palagonite a good spectral analog for Mars? Why or why not? Print out the palagonite spectra only in the two to six micron region for this comparison, label features that agree/disagree with the Martian spectrum.

3. People have been searching for evidence of carbonates on Mars for decades. Try an experiment, take the calcite sample and sprinkle a little palagonite on top and see how its spectra changes, try this for a few different amounts of palagonite. How much do you need to make the carbonate spectrally unrecognizable? Knowing what you know about the Martian environment would you expect to find cleanly exposed samples of carbonate anywhere?

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