Infrared spectroscopy is based on molecular vibrations caused by the oscillation of molecular dipoles. Bonds have characteristic vibrations depending on the atoms in the bond, the number of bonds and the orientation of those bonds with respect to the rest of the molecule. Thus, different molecules have specific spectra that can be collected for use in distinguishing products or identifying an unknown substance to a certain extent.
Thermo Scientific Nicolet S10 FT-IR spectrometer has capability at rugged, precise, fast-paced operation, yet simplifies laboratory data collection to its most basic: load the sample, generate the spectrum, and press print or save. The instrument delivers data that are confidence enough in the verification and identification of materials with a minimal investment in time. The transmission compartment fits all of the typical transmission accessories, like salt-plate cells, flow cells or film holders. Sample introduction and cleaning of liquids, gels and pastes is even simpler using the attenuated total reflectance (ATR) accessory. Gases can be analyzed using standard gas cells (user has to purchase the cells at the moment), right in the transmission housing. The instrument came with powerful analysis program, OMNIC SpectaTM, gives unparalleled access to spectral interpretation tools. The OMNIC interface allows you to assign functional groups, search against libraries, deeper analysis such as analysis of mixtures against a library of over 9000 compounds.
Sample Preparation Guide
In order to obtain any meaningful information from your IR spectrum, you need to have a decent spectrum with sharp peaks, i.e., with ‘good’ S/N ratio and good resolution. Following guide is for the preparation of general samples. It may possible to tailor your sample preparation according to your research needs.
Sample preparation method depends on the physical state of the sample. Thus, collecting spectra goes about one of three general ways.
- Solids – Nujol mulls (a thick suspension of the sample with a mineral oil by Plough Inc.)or pressed KBr pellets are typically used for collecting spectra of solids
- Liquids – Thin-film cells are used for solution-phase IR spectroscopy
- Gases – Spectra of gases can also be obtained using gas cells. (At the moment the IC-FAS does not have gas cells)
If samples are prepared according to the above method, the transmission method is used to collect the spectra. In addition to that, attenuated total reflection (ATR) mode can be used directly in the solid or liquid state without further sample preparation.
- For solid samples
There are many ways to prepare the sample. If your sample dissolves in non-aqueous solvent you can use the following procedure. Remember that all solvents containing water will either dissolve the KBr plates or make them fog up. Therefore it is important to use anhydrous solvents.
Prepare a concentrated solution of your compound in a suitable anhydrous solvent (e.g. CH2Cl2). There are several ways to do this depending how much material you have at hand: either you place a small amount (2–5 mg) of your compound directly on the plates and add one drop of solvent, or you dissolve it in a small test tube first and transfer this solution with a pipet onto the salt plates. If you use a solvent it is important to obtain a spectrum of solvent as well, or run the pure solvent as a baseline to automatically subtract it. The KBr plates must be thoroughly cleaned after this procedure to prevent contamination of future samples. Wipe the windows with a tissue, then wash several times with your solvent, then ethanol
If you cannot find a good solvent to dissolve your sample you can make KBr pellets. The concentration of the sample in KBr should be in the range of 0.2% to 1%. The pellet is much thicker than a liquid film hence a lower concentration in the sample is required (Beer’s Law). Too high a concentration usually causes difficulties obtaining clear pellets. The IR beam is absorbed completely, or scattered from the sample, which results in very noisy spectra. Therefore, make sure to work fast. Transfer some KBr out of the oven (dry at 100 °C for some time before use) into a mortar. Add about 0.2 to 1 wt.% of your sample, mix and grind to a fine powder. It is true that a homogeneous mixture will give the best results but excessive grinding of the potassium bromide is not required. The finely powdered potassium bromide will absorb more humidity (it is hygroscopic) from the air and therefore lead to an increased background in certain ranges. For very hard samples, add the sample first, grind, add KBr and then grind again. Then you can fill this mixture into the stainless steel sample holder (will be provided by the scientific assistant) and press them using hydraulic press (will be provided by the scientific assistant) to get a homogeneous and slightly translucent pellet. Insert the sample holder and get the FTIR spectrum. It is common to have a cloudy KBr disks. The reasons could be
- KBr mixture not ground enough
- Sample was not dry
- Sample: KBr ratio high
- Pellet too thick
- Bolts not tightened sufficiently.
- Sample has low m.p.
Alternative methods to obtain IR spectra of solids are Nujol (mineral oil) mulls between KBr plates. Remember that Nujol by itself shows a characteristic spectrum. Good results are obtained by this method only if the average particle size of the solid is somewhat less than the wavelength of light the particles are to transmit. Samples should therefore be ground in a mortar to reduce the average particle size to 1 to 2 microns. About 5 to 10 mg of finely ground sample is then placed onto the face of a KBr plate, a small drop of mineral oil is added and the second window is placed on top. With a gentle circular and back-and-forth rubbing motion of the two windows, evenly distribute the mixture between the plates. The mixture should appear slightly translucent, with no bubbles, when properly prepared. Place the sandwiched plates in the spectrometer and obtain a spectrum. If the bands are distorted (show fronting or tailing) the particle size is too great and some radiation incident on the mull has been scattered out of the sample beam. If a better spectrum is required reduce the particle size further. The KBr plates must be thoroughly cleaned after this procedure too to prevent contamination of future samples. Wipe the windows with a tissue, then wash several times with methylene chloride, then ethanol.
- For liquid samples
Prepare a dilute solution in non-aqueous solvent. Take two KBr plates. Place a small drop of that liquid on one of the KBr plates. Place the second plate on top and make a quarter turn to obtain a nice even film. Place the plates into the sample holder and run a spectrum. If the sample is too concentrated, separate the plates and wipe one side clean before putting them back together. The KBr plates must be thoroughly cleaned after this procedure to prevent contamination of future samples. Wipe the windows with a tissue, and then wash several times with methylene chloride (or another solvent that will take off you sample), then ethanol.
- For gas samples
At the moment the we donot have gas cells. Thus, if one interested please contact us for more details.
Click following link for the reservation of the FT-IR instrument
Lecturer-In-Charge
Dr. Janitha Walpita