Quantitative Analysis of Xylene Mixtures by Fourier Transform Infrared Spectroscopy#
Theory#
Absorption in the infrared region of the spectrum is due to transitions involving vibrational energy levels. By far the most important application of infrared spectroscopy is qualitative analysis. Functional groups can be identified by their characteristic absorption bands in the infrared. Also, infrared spectra are unique for a particular compound, which means that if a spectrum for an unknown matches a known spectrum, the unknown has been identified.
For several reasons, infrared absorption is not well suited for quantitative analysis. Because sources are weak and detectors insensitive, it is necessary to record spectra using wide bandwidths. Since IR peaks are relatively narrow, this means it is difficult to avoid instrumental deviations from Beer’s Law. Many of these problems have been circumvented by the use of Fourier transform instruments where multiple scans can be rapidly obtained and the signals averaged. Other problems include solvent absorption and the lack of suitable optical materials for quantitative measurement of absorption. Since glass and quartz absorb in the IR, it is necessary to use salt plates to make IR cells. Fixed path cells also assist the chemist in improving the determination.
In this experiment, a quantitative IR analysis of a mixture of xylenes is performed. This experiment illustrates the use of an internal standard to compensate for some of the measurement problems in the IR. The internal standard is added in equal amount to both standards and sample and their infrared spectra obtained using standard salt plates. The ratio of absorbance of compounds of interest (m- and p-xylene in this experiment) to internal standard (o-xylene) is measured and compared to absolute absorbance values. The idea is that variations in sample volume, cell transmittance, etc., will affect both internal standard and compound of interest equally, and, therefore, the ratio of absorbances will be independent of these variables. In a second experiment, the same samples spectra will be obtained using a fixed path cell with similar data analysis.
Experimental#
Apparatus#
Fourier Transform IR (Perkin Elmer Spectrum 400 IR/NIR)
2 salt plates and holder
Fixed path cell
Disposable pipets
Luer-lock syringe without needle
Chemicals#
cyclohexane
o-, m-, p- xylene
Methylene chloride or chloroform for cleaning salt plates between trials
Procedure#
Prepare, in 10 mL volumetric flasks, eight standard solutions all of which have a composition consisting of 20.0% by volume o-xylene and using cyclohexane as solvent with 0.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0, and 35.0% by volume of both standards m-xylene and p-xylene.
Example
The last sample has 35.0 % m-xylene, 35.0 % p-xylene, 20.0 % o-xylene, and made to mark with cyclohexane.
Obtain an unknown consisting of a mixture of m-xylene and p-xylene in cyclohexane. Record the unknown number!
Important
Return the unknown sample to the refrigerator after use.
Pipet 2 mL of o-xylene internal standard into a 10 mL volumetric and dilute to the mark with unknown.
Tip
Use Pasteur pipet to dilute.
Using a Pasteur pipet, place two drops of the standard solution between the salt plates.
Caution
The salt plates are easily etched by moisture on your hands. Hold them by the edges and rinse them thoroughly with methylene chloride between samples and when finished. Store in a desiccator.
Record the spectra of the standards and unknown using the available Fourier transform infrared instrument. Keep in mind that data analysis requires being able to measure absorbances. If the absorbance peaks are too large — this operation can not be accomplished. Recently using a PE 2000, all samples had to be diluted 1:10 to accomplish this analysis!
Obtain the spectra of the standards and samples using a fixed pathlength liquid cell. This cell must be loaded with a needle-less luer lock syringe obtained from the instructor. The faces of this cell are again made of salt and the same rules as above still apply. Between trials, flush with methylene chloride.
All spectra should be save in a directory named after your lab group (limited to 8 characters of course). When you are done with the lab, all files should be deleted from the hard drive.
Using one (1, uno, singular) sample, run a single scan vs a single background scan, 10 scans vs 10 background scans, and 100 scans vs. 100 background scans. Compare the signal to noise ratio relative to number of scans. Report and discuss this in your report.
Treatment of Data#
The bands used for the analysis are:
o-xylene
13.5 microns (micrometers)
m-xylene
13.0 or 14.5 microns
p-xylene
12.6 microns
Use the instruments computer to obtain the absorbance values at the above frequencies (what are these frequencies in wavenumbers?)
Plot working curves of both absorbance of each standard and the ratio of the absorbance of the desired constituent to that of the internal standard versus concentration (in %) for both sample holders. Determine the composition of the unknown using these curves.
Compare the two sample holders. Is it necessary or desirable to use the internal standard approach with both, either, or neither of the sample holders. Comment.
Questions#
How does a dispersive IR instrument differ from a Fourier transform IR instrument? Include both instrumental differences and relative advantages/disadvantages.
What are the specific advantages and disadvantages of a double-beam dispersive instrument vs a single beam dispersive instrument? With this in mind, why don’t companies make a double beam Fourier transform instrument?
Discuss the particular advantages of optics such as beam splitters for infrared instruments: NaCl, KBr, CaF2, LiF, CaBr2, and quartz. Compare specifically such quantities as useful wavelength range and resolution.
Historically, IR has been plagued by poor stability of sources and detectors. Based on your results, does the PE FTIR suffer from the difficulties? This is not a simple yes/no question. Explain!
CHEM322 Spring ‘23 Procedure#
Make sure the power is on to the spectrometer.
Open the Spectrum IR software and log in as Administrator.
Locate the dessicator around the FT-IR and find two salt plates (they’re kind of hidden in with all of the mess) and place on the bench top on a Kimwipe.
Clean the salt plates with methylene chloride (or other non-aqueous solvent)
Using a Pasture Pipet, place about 2-3 drops of your standard solution between both of the salt plates and record its spectra. There is a special slip for the salt plates and it has a triangular ledge to it to rest the salt plates on.
Once you have the sample loaded make sure your parameters are set to:
scanning from 600 cm-1 - 800 cm-1
accumulations: 3 scans
blank has been measured before hand using 0% m/p-xylene solution
you may now begin to run through your samples ensuring that the plates were cleaned with DCM.
Once the samples have been run through using the salt plates, you may now switch the sample holder to a fixed path length cell. This is hidden in one of the drawers that you are going to have to rummage through… sorry the instrumental lab is a mess. It looks like a vintage, black cassette tape (do y’all still know what that looks like?) with a clear center and two little spikes sticking out of it to flush the solution through.
This whole cell goes into the FT-IR so make sure the salt plate ledge has been removed.
Flush the cell with a needleless syringe with DCM and load to run a new blank with your standard solution .
Tip
Ensure that a mL or more of DCM is passed through the cell before introducing new samples.
Tip
Flushing is not necessary if analyzing a series dilution from low concentration to high concentration as long as enough sample is added to push the previous sample out.
Load your parameters to:
parameter
value
abscissa units
wave #
ordinate units
A
resolution
4 cm-1
accumulations
3 scans
start
800 cm-1
end
600 cm-1
scan type
sample
Now one can compare number of scans to background for the last part of the experiment.
Change the parameters at the bottom for a 1 v 1, 10 v 10, and 100 v 100 scans v background.
Save all data and export to a USB and shut down the software.