Nicolet Nexus 670 FT IR

Infrared (IR) spectroscopy is a chemical analytical technique, which measures the infrared intensity versus wavelength (wavenumber) of light. Based upon the wavenumber, infrared light can be categorized as far infrared (4 ~ 400cm^-1), mid infrared (400 ~ 4,000cm^-1) and near infrared (4,000 ~ 14,000cm^-1).

Infrared spectroscopy detects the vibration characteristics of chemical functional groups in a sample. When an infrared light interacts with the matter, chemical bonds will stretch, contract and bend. As a result, a chemical functional group tends to adsorb infrared radiation in a specific wavenumber range regardless of the structure of the rest of the molecule. For example, the C=O stretch of a carbonyl group appears at around 1700cm^-1 in a variety of molecules. Hence, the correlation of the band wavenumber position with the chemical structure is used to identify a functional group in a sample. The wavenember positions where functional groups adsorb are consistent, despite the effect of temperature, pressure, sampling, or change in the molecule structure in other parts of the molecules. Thus the presence of specific functional groups can be monitored by these types of infrared bands, which are called group wavenumbers.

The early-stage IR instrument is of the dispersive type, which uses a prism or a grating monochromator. The dispersive instrument is characteristic of a slow scanning. A Fourier Transform Infrared (FTIR) spectrometer obtains infrared spectra by first collecting an interferogram of a sample signal with an interferometer, which measures all of infrared frequencies simultaneously. An FTIR spectrometer acquires and digitizes the interferogram, performs the FT function, and outputs the spectrum.

An interferometer utilizes a beamsplitter to split the incoming infrared beam into two optical beams. One beam reflects off of a flat mirror which is fixed in place. Another beam reflects off of a flat mirror which travels a very short distance (typically a few millimeters) away from the beamsplitter. The two beams reflect off of their respective mirrors and are recombined when they meet together at the beamsplitter. The re-combined signal results from the �interfering� with each other. Consequently, the resulting signal is called interferogram, which has every infrared frequency �encoded� into it. When the interferogram signal is transmitted through or reflected off of the sample surface, the specific frequencies of energy are adsorbed by the sample due to the excited vibration of function groups in molecules. The infrared signal after interaction with the sample is uniquely characteristic of the sample. The beam finally arrives at the detector and is measure by the detector. The detected interferogram can not be directly interpreted. It has to be �decoded� with a well-known mathematical technique in term of Fourier Transformation. The computer can perform the Fourier transformation calculation and present an infrared spectrum, which plots adsorbance (or transmittance) versus wavenumber.

When an interferogram is Fourier transformed, a single beam spectrum is generated. A single beam spectrum is a plot of raw detector response versus wavenumber. A single beam spectrum obtained without a sample is called a background spectrum, which is induced by the instrument and the environments. Characteristic bands around 3500 cm^-1 and 1630 cm^-1 are ascribed to atmospheric water vapor, and the bands at 2350 cm^-1 and 667 cm^-1 are attributed to carbon dioxide. A background spectrum must always be run when analyzing samples by FTIR. When an interferogram is measured with a sample and Fourier transformed, a sample single beam spectrum is obtained. It looks similar to the background spectrum except that the sample peaks are superimposed upon the instrumental and atmospheric contributions to the spectrum. To eliminate these contributions, the sample single beam spectrum must be normalized against the background spectrum. Consequently, a transmittance spectrum is obtained as follows.

Where %T is transmittance; I is the intensity measured with a sample in the beam (from the sample single beam spectrum); Io is the intensity measured from the back ground spectrum

The absorbance spectrum can be calculated from the transmittance spectrum using the following equation.

The final transmittance/absorbance spectrum should be devoid of all instrumental and environmental contributions, and only present the features of the sample. If the concentrations of gases such as water vapor and carbon dioxide in the instrument are the same when the background and sample spectra are obtained, their contributions to the spectrum will ratio out exactly and their bands will not occur. If the concentrations of these gases are different when the background and sample spectra are obtained, their bands will appear in the sample spectrum.

In Attenuated Total Reflectance (ATR) spectroscopy all that is required for analysis is that the sample of interest be brought into contact with the ATR crystal. The infrared beam is passed into the ATR element such that its angle of incidence exceeds the �critical� angle. Under this condition total internal reflection of the beam occurs and a standing evanescent wave is established at the ATR crystal/sample interface. The amplitude of this wave decays rapidly with increasing distance from the reflecting interface thus sample concentration and thickness are not a concern for these measurements. Minimal to no sample preparation is required for this technique and a wide variety of solids and some liquids (dependent upon crystal material) can be analyzed using ATR. This technique is available using the Hyperion microscope and the Nexus spectrometer.

Attenuated total reflection infrared (ATR-IR) spectroscopy is used for analysis of the surface of materials. It is also suitable for characterization of materials which are either too thick or too strong absorbing to be analyzed by transmission spectroscopy. For the bulk material or thick film, no sample preparation is required for ATR analysis.

For the attenuated total reflection infrared (ATR-IR) spectroscopy, the infrared radiation is passed through an infrared transmitting crystal with a high refractive index, allowing the radiation to reflect within the ATR element several times.

The sampling surface is pressed into intimate optical contact with the top surface of the crystal such as ZnSe or Ge. The IR radiation from the spectrometer enters the crystal. It then reflects through the crystal and penetrating �into� the sample a finite amount with each reflection along the top surface via the so-called �evanescent� wave. At the output end of the crystal, the beam is directed out of the crystal and back into the normal beam path of the spectrometer.

To obtain internal reflectance, the angle of incidence must exceed the so-called �critical� angle. This angle is a function of the real parts of the refractive indices of both the sample and the ATR crystal:

Where n₂ is the refractive index of the sample and n₁ is the refractive index of the crystal. The evanescent wave decays into the sample exponentially with distance from the surface of the crystal over a distance on the order of microns. The depth of penetration of the evanescent wave d is defined as the distance form the crystal-sample interface where the intensity of the evanescent decays to 1/e(37%) of its original value. It can be given by:

Where l is the wavelength of the IR radiation. For instance, if the ZnSe crystal (n₁=2.4) is used, the penetration depth for a sample with the refractive index of 1.5 at 1000cm^-1 is estimated to be 2.0�m when the angle of incidence is 45�. If the Ge crystal (n₁=4.0) is used under the same condition, the penetration depth is about 0.664�m. The depth of penetration and the total number of reflections along the crystal can be controlled either by varying the angle of incidence or by selection of crystals. Different crystals have different refractive index of the crystal material. By the way, it is worthy noting that different crystals are applied to different transmission range (ca. ZnSe for 20,000~650cm^-1, Ge for 5,500~800cm^-1).

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) involves numerous light-sample interactions. Spectra may exhibit features associated with the transmission and/or reflection (external and/or internal) of infrared radiation. This technique is most often employed in the analysis of powders and rough surfaces. Prior to analysis it is recommended that samples be ground to a particle size of between 2 to 5 microns to reduce the amount of specularly reflected light. Additionally, samples which are highly absorbing or possess a high refractive index are diluted with a diffusely scattering matrix, e.g. KBr. This tool is useful for analyzing intractable solids by scratching the sample surface with an abrasive paper and then measuring the spectra of the particles adhering to the paper. This technique is available using the IFS 66/S spectrometer

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is a technique that collects and analyzes scattered IR energy. It is used for measurement of fine particles and powders, as well as rough surface (e.g., the interaction of a surfactant with the inner particle, the adsorption of molecules on the particle surface). Sampling is fast and easy because little or no sample preparation is required.

When the IR beam enters the sample, it can either be reflected off the surface of a particle or be transmitted through a particle. The IR energy reflecting off the surface is typically lost. The IR beam that passes through a particle can either reflect off the next particle or be transmitted through the next particle. This transmission-reflectance event can occur many times in the sample, which increases the pathlength. Finally, such scattered IR energy is collected by a spherical mirror that is focused onto the detector. The detected IR light is partially absorbed by particles of the sample, bringing the sample information.

1. Fill the micro-cup with the powder (or the mixture of the powder and KBr).
The diffuse reflectance accessory uses a focusing mirror to focus the beam on the sample surface and collect the IR energy. The micro-cup needs to be filled consistently in order to keep the focus.

2. Scratch the sample surface with a piece of abrasive (SiC) paper and then measuring the particles adhering to the paper.

3. Place drops of solution on a substrate. If colloids or powders are dissolved or suspended in a volatile solvent, you can place a few drops of the solution on a substrate, and then evaporate the solvent, subsequently analyze the remaining particles on the substrate.

It is well known that particle size is a key variable in a transmission measurement with the pellet method. Large particle will results in the scattering of the energy, leading to the shift of the spectrum baseline and the broadening of IR bands. The scenario becomes worse in a diffuse reflectance measurement, because the infrared light travels in the sample for a long period and the optics collects a large portion of the distorted energy. It is important to grind the sample particles to 5 microns or less.

Sampling Method Matrix
This table is meant to serve as a general guide for method selection based on the physical form of a sample. Upon assessment of a specific sample it is possible that none of the listed techniques will be suitable. It is up to you, as a scientist to work out your method.

	Sampling Method
Sample Form	ATR	SR	RA	DR	TR	PA
powder	x			x	x	x
particle	x			x	x	x
coating	x	x	x	x	x	x
monolayer			x
fiber	x			x	x
microtome cut	x	x			x
monolithic solid	x	x				x
thin films	x	x	x		x	x
residue			x
paper	x				x	x

The following table lists the typical IR materials for various applications. These IR materials are used for IR window or diluents. It is worthy noting that hygroscopic materials are suitable for organic samples while non-hygroscopic materials are used for water-containing samples.

1. 1 Solid samples
(1.1.1) Thin plate samples
You can use standard magnetic plate sample holder.

(1.1.2) Powder samples
You can press powder into a pellet. In this method, you can select different diluents (matrixes) for various applications. For mid-IR frequency range, KBr, KCl or diamond dust can be used. For far-infrared testing, high-density polyethylene (HDPE) or diamond dust is suitable. For near-infrared analysis, CsI or KBr can be selected. You can prepare a KBr Pellet as follows:

(a) The powder sample and KBr must be ground to reduce the particle size to less than 5 mm in diameter. Otherwise, large particles scatter the infrared beam and cause a slope baseline of spectrum.
(b) Add a spatula full of KBr into an agate mortar and grind it to fine powder until crystallites can no longer be seen and it becomes somewhat �pasty� and sticks to the mortar.

(c) Take a small amount of powder sample (about of 0.1-2% of the KBr amount, or just enough to cover the tip of spatula) mix with the KBr powder. Subsequently grind the mixture for 3-5 minutes.
(d) Assemble the die-set as shown in the drawing. When assembling the die, please add the powder to the 7mm collar. Put the die together with the powder into a Handi-Press. Press the powder for 2 minutes to form a pellet. A good KBr pellet is thin and transparent. Opaque pellets give poor spectra, because little infrared beam passes through them. White spots in a pellet indicate that the powder is not ground well enough, or is not dispersed properly in the pellets.
(e) Disassemble the die set and take out the 7mm collar. Put the collar together with the pellet onto the sample holder.
(f) Please clean the die after you finish your experiment.

It is worthy noting that Br- from the KBr can often replace ligands in the compound whose spectrum is desired.

1.2. Liquid samples
(1.2.1) Making a sandwich
To prepare a liquid sample to IR analysis, firstly place a drop of the liquid on the face of a highly polished salt plate (such as NaCl, AgCl or KBr), then place a second plate on top of the first plate so as to spread the liquid in a thin layer between the plates, and clamps the plates together. Finally wipe off the liquid out of the edge of plate. You can mount the sandwich plate onto the sample holder. After finishing your experiment, clean the plates with isopropanol (No water!) and return them to the desiccators.

NOTE: Volatile liquid can�t be prepared with this method, because it will evaporate while its spectrum is being obtained. If the liquid sample is toxic or smelly, please don�t use this method. In addition, NaCl and KBr are dissolved into water, and thus they can�t be used for aqueous samples.

(1.2.2) Using a liquid cell
The Nicolet demountable pathlength cell is designed for liquid sample. Assemble the cell as shown in the drawing. Then use a syringe to fill the liquid into the cell. Finally seal the cell.

2. Sample preparation for ATR analysis
The strongly infrared adsorbing materials such as rubbers, polymer films and aqueous solutions can be analyzed by the attenuated total reflection (ATR) technique. Minimum or no sample preparation is required for ATR analysis.

2.1 Solid samples
For solid samples, please use the flat crystal. To obtain the best results, the recommended sample dimension is 75-90mm long � 16-25mm wide � <4mm thick. The sample surface must be flat, otherwise intimate contact between the sample and the ATR crystal can not be obtained.

Firstly place a sample on top surface of the crystal. Then place the gripper plate on the sample. Adjust the pressure applied to the gripper plate to ensure the consistent contact is achieved between the crystal and the sample. Precaution: if you find a considerable force is necessary to overcome the resistance, STOP! Excessive pressure can crack and permanent damage the crystal. Also, the crystal must be keep clean and scratch-free. Scratches affect the sample/crystal contact and thus have significant influence on the depth of penetration.

2.2 Liquid samples
For liquid samples, please use the trough crystal. Fill the trough plate with your liquid sample. Then place the cover over the trough plate.
Precaution: Ensure the liquid sample does not attack the crystal.

After you have finished your experiment, clean up the crystal using proper solvents
(Acetone or water can be used for ZnSe and Ge).

3. Sample preparation for PM-IRRAS measurement
Typically, you can deposit a metal film on the regular glass slide (or silicon wafer). The metal/glass combination can be acted as the reference sample. In addition, you will deposit a monolayer (or organic film) over the metal film. Hence, you will have two samples, i.e., the metal/glass combination and the monolayer/metal/glass composite. The (typical) recommended dimension of sample is 55~75mm x 17~25mm.

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