Infrared spectroscopy

Infrared spectroscopy IR liquid and solid technique

Infrared spectroscopy is a widely used analytical technique in the field of chemistry. It involves the interaction of infrared radiation with a sample, which causes the sample to vibrate. By analyzing the resulting spectrum, it's possible to identify and quantify the compounds present in the sample.

ūüďč Index of contents
  1. What is Infrared Spectroscopy?
  2. How Does Infrared Spectroscopy Work?
  3. Applications of Infrared Spectroscopy
  4. IR Spectrum Acquisition
    1. Care and Handling of IR Plates
    2. Thin-film IR Sampling Techniques
    3. Other Sampling Techniques
  5. IR Spectroscopy of Liquids
  6. IR Spectroscopy of Solids
    1. Visual Guide for Film Thickness
  7. Acquiring an IR Spectrum
    1. Acquiring a Spectrum

What is Infrared Spectroscopy?

Infrared spectroscopy is a non-destructive analytical technique used to identify and quantify the chemical compounds in a sample. Infrared radiation is passed through a sample, and the amount of radiation absorbed by the sample is measured. The resulting spectrum is unique to the sample, allowing for the identification and quantification of the compounds present.

Infrared radiation has a lower frequency than visible light, making it ideal for analyzing the vibrational states of molecules. The vibrational states of molecules are caused by the stretching and bending of chemical bonds, and different chemical compounds have unique vibrational frequencies.

How Does Infrared Spectroscopy Work?

Infrared spectroscopy works by passing infrared radiation through a sample and measuring the amount of radiation absorbed. The amount of radiation absorbed is proportional to the concentration of the compound in the sample.

Infrared radiation is typically produced by a source such as a filament or a laser. The radiation is then passed through an interferometer, which splits the radiation into two beams. One beam passes through the sample, while the other is used as a reference.

The two beams are then recombined, and the resulting interference pattern is analyzed to produce a spectrum. The spectrum is unique to the sample, allowing for the identification and quantification of the compounds present.

Applications of Infrared Spectroscopy

Infrared spectroscopy has a wide range of applications in the field of chemistry. It's commonly used in the analysis of organic compounds, such as alkanes, alkenes, and alcohols. Inorganic compounds, such as metal complexes and coordination compounds, can also be analyzed using infrared spectroscopy.

Infrared spectroscopy is also used in the pharmaceutical industry to identify and quantify the active ingredients in drugs. It's also used in the analysis of food and beverage products, as well as in the characterization of polymers and other materials.

Advantages of Infrared Spectroscopy Infrared spectroscopy has several advantages over other analytical techniques. It's non-destructive, meaning that the sample can be reused or analyzed further. It's also highly selective, allowing for the identification and quantification of specific compounds in a sample.

Infrared spectroscopy is also relatively quick and inexpensive, making it a popular choice for routine analysis in many industries.

Conclusion Infrared spectroscopy is a powerful analytical technique used in the field of chemistry to identify and quantify chemical compounds. It works by analyzing the interaction of infrared radiation with a sample, and the resulting spectrum is unique to the sample, allowing for the identification and quantification of the compounds present.

If you're looking to learn more about infrared spectroscopy, we recommend checking out our detailed guide on the subject. We've included a diagram below to help you visualize the process of infrared spectroscopy.

IR Spectrum Acquisition

In the organic teaching labs at CU Boulder, almost all samples for IR spectroscopy are prepared as a thin film of the organic compound on salt plates. Two things are important as you are getting ready to run an IR:

  • You need to be able tell a good salt plate from a bad salt plate and you must know how to handle these plates properly so that¬†you¬†do not turn a good plate into a bad plate. These topics are covered in the first section below.
  • You must choose the proper method to prepare the sample on the salt plate(s): the method is determined by whether the organic compound is a liquid or a solid, as explained in the second section below.

Care and Handling of IR Plates

The IR plates that you use in the organic chem labs are made of polished sodium chloride. Sodium chloride is chosen because it is transparent to infrared radiation. These plates - called "salt plates" - are rather expensive because each plate is cut from a single giant crystal; they are very fragile and sensitive to moisture, including the moisture in your fingers.

Infrared spectroscopy

What happens when you place salt in water? It dissolves. What happens when you place your sweaty fingers on a salt plate? It dissolves the salt, leaving a fingerprint on the plate. The images below show what a fingerprint on a plate looks like, and how it happens.

Infrared spectroscopy Infrared spectroscopy

The picture below shows the correct way to hold a salt plate. Alternatively, you can wear gloves when you handle salt plates, as long as the gloves are dry.

Infrared spectroscopy

Moisture in the air causes salt plates to become cloudy; moisture on your fingers causes prints (see above). We store them in desiccators to prevent them from becoming cloudy. Salt plates are very fragile and will chip or shatter if you drop them. They can be scratched by metal spatulas and Pasteur pipets. The photo below shows five plates, one in good condition and four that are slightly damaged. When you are preparing to run a spectrum, try to choose a good plate (and keep it that way!). All of the plates in the photo probably would be useable, especially in organic teaching lab applications.

Infrared spectroscopy

The damage done to a plate by fingerprints, pits, and cloudiness cause the IR spectrum run on the plate to have broad bands. This is because IR spectra look best when run as a thin, even films of compounds. Pits and fingerprints cause a thick and uneven film to be laid down on the plate, leading to scattering of IR radiation and to bands that are too intense; cloudiness causes scattering of IR radiation and prevent it from going through the sample, leading to broad bands and a spectrum that is less than 100% transmission.

Infrared spectroscopy

Can plates be used even though they are damaged? In most cases, the answer is "yes". If the last plate available for use looks cloudy or pitted, try running a spectrum and see if it the spectrum is acceptable. A plate that has a big chip off it can still be used, as long as it is big enough to sit on the holder and in the path of the irradiation. IR plates can be resurfaced, so don't throw away a plate if it looks bad.

Thin-film IR Sampling Techniques

Two different methods are used to prepare thin films on an IR plate. If the compound to be studied is a liquid, you need to use thin-liquid film IR sampling technique(see below). If it is a solid, use thin-solid film IR sampling technique(see below). Note: a solution of a solid compound in a solvent does not count as a liquid; if the compound of interest is a solid at room temperature, even if it is dissolved in a solvent like methylene chloride, the compound is a solid and must be run by the procedure for thin-solid films.

Once the sample is prepared, you can collect the spectrum(see below) in one of the IR instruments.

Other Sampling Techniques

Solid samples can be prepared for IR examination by several other methods:

  • A mull is prepared by grinding the sample with mineral oil and sandwiching the resultant paste between sodium chloride plates.
  • The solid can be dissolved in a solvent and placed in a special cell, called a solution cell, which is made of sodium chloride. This gives what is called a solution spectrum.
  • A KBr pellet is prepared by grinding the solid sample with solid potassium bromide (KBr) and applying great pressure to the dry mixture. Again, KBr is chosen because it is transparent to infrared radiation. If the pellet is prepared properly, one can actually see through it, as through a pane of glass.

IR Spectroscopy of Liquids

Liquid organic compounds can be examined directly as a thin film, "neat", between two NaCl plates. By "liquid organic compound" we mean pure organic compounds that are liquids at room temperature, not solutions of a solid organic compound in a solvent. Although it is possible to run a solution as a thin film between two plates, the resultant spectrum would show bands of both the compound of interest and the solvent.

Obtain two salt plates from the desiccator in the main fume hood in your lab. Ideally, the plates will already be clean. If necessary, clean the plates with a small amount of acetone. The plates should also be transparent, but quite foggy plates usually give acceptable spectra.
Use a Pasteur pipet to place a drop of your liquid unknown on one salt plate.
Put the second salt plate on top so that the liquid spreads into a thin film. When running a liquid sample, you need to have two plates, both to prevent the liquid from running off the plate and to prevent it from evaporating.
Pick up the plates and take them to the IR. Carry them by the edges and/or wear gloves. (Gloves are advised because sometimes the liquid compound leaks out from between the plates.)
Take the plates to one of the FT-IR instruments and place them in the V-shaped sample holder inside the instrument. Obtain the IR spectrum. Once you have obtained your spectrum, clean the salt plates again with acetone and return them to the desiccator.

IR Spectroscopy of Solids

In the organic teaching labs at CU, the preferred method of solid sample preparation is the "Thin Solid Film" method. Briefly, the solid sample is dissolved in a suitable solvent (usually methylene chloride) and the solution is dropped onto a salt plate. After the solvent evaporates, a thin-solid film of the compound remains on the plate. The IR is run directly in the FT-IR.

Obtain a single salt plate from the desiccator in the main fume hood in your lab. Ideally, the plate will already be clean. If necessary, clean the plate with a small amount of acetone. The plate should also be transparent, but quite foggy plates usually give acceptable spectra.
Put about 50 mg of solid into a beaker, vial, or test tube - no need to weigh it, just judge by the photo below.
Add couple of drops of solvent (usually acetone or methylene chloride) to dissolve the solid.
Place a drop of this solution on a salt plate. Allow the solvent to dry; you should observe a thin solid film on the plate. If you do not, add another drop of the solution and allow it to dry (see below for a visaul guide of how this film should look).
Place the plate in the V-shaped sample holder inside the instrument. Note that you use only one salt plate in this procedure, not two plates as when running a thin liquid film.

Visual Guide for Film Thickness

Infrared spectroscopy Infrared spectroscopy Infrared spectroscopy
Not enough sample Too much sample Just right!

Acquiring an IR Spectrum

The organic chemistry teaching labs at CU Boulder have 4 Nicolet FT-IR instruments: an Avatar 320, an Avatar 370, and two IR 200s. There is one in the instrument room attached to each of the lab rooms. If the instrument in your lab has a long line, you can also use the one in M1B70.

Infrared spectroscopy Infrared spectroscopy
Avatar 320 - Located in M1B25. Avatar 370 - Located in M1B70. IR 200 - Located in M1B27 and M1B73.

Acquiring a Spectrum

First, prepare your sample as directed in the liquid sampling technique or solid sampling technique pages. Most likely, the EZOMNIC program has already been launched, a background run, and is ready to go - in this case, you can skip to the sample collection step. If not, double-click the icon for the program.
Run a background. First, make sure there are no plates in the IR, then click on the Col Bkg icon in the upper tool bar.
Wait for the instrument to run four background scans. This is necessary because it provides a baseline that the instrument will subtract out of subsequent scans - it's a way to compensate for the presence of water, CO2, impurities in salt plates, etc.
If you have scanned the background properly, this is what it looks like. If it does not look like this, for instance, if it has a lot more bands, you probably left IR plates in the instrument when you took the background.
Place the salt plate(s) in the instrument. Click on the icon in the upper toolbar that says "Col Smp" to collect the sample.
Your spectrum should appear after a few seconds.
If it looks like this, for instance, if it is a straight or jaggedy line, someone probably left a sample in the instrument when the background was run. You'll need to collect a fresh background.
If you are happy with your spectrum, you can print to the default printer for your lab.

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Jose Hullgren (Laboratory Analist)

Hello to all readers, my name is Jose Hullgren, it is a pleasure to present you this website of my authorship, I am currently working as a laboratory analyst and for the last 10 years I have been working in the pharmaceutical industry. The main idea of this page is to provide relevant information in the field of the pharmaceutical industry above all. We also cover different areas of chemistry and sciences in general that we find interesting. Perfil Linkedin

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