Fourier Transform (FT-IR) Spectroscopy
The Nicolet Magna-IR™ 550 Spectrometer (with better than 0.1 cm-1 resolution) is suited for the characterization of liquids and solids in the mid-infrared (7400 cm-1 to 350 cm-1) spectral range. These materials selectively absorb infrared radiation to varying degrees depending upon the chemical nature of the material, producing a vibrational infrared spectrum characteristic of the material. This spectrum provides information on the presence or absence of functional groups and gives an overall characterization (chemical bonding and molecular structure) of the material being examined. FT-IR is generally a non-destructive analytical technique which provides both qualitative and quantitative (standards required) information. Unknown samples can be identified either by comparing to the spectra of known substances or to a spectra library. These spectra libraries can be constructed from materials obtained from customer's operations, or purchased from commercial sources. Even though library searches are very effective, there is still a need for accurate interpretation of the sample spectra by an experienced operator.
FT-IR Spectroscopy, with the aid of various attachments, can be used in the following applications:
- Identification of contaminates, organic stains, or coatings on steel surfaces
- Analysis of coatings on paper, fibers, containers, etc.
- Characterization of plastics, paints, varnishes, pigments, dyes, resins, adhesives, oils, greases, waxes, pastes, etc.
- Characterization of multi-layer laminates
- Surface studies (extremely thin films and surface defects)
- Identification of mill process contaminates
- Comparison of "good" versus "bad" material
- Characterization sub millimeter particles
- Analysis of a few microliters of liquid
- Investigation of regions as small as 10 μm can be achieved.
- Quality control
Microscope
Nic-Plan™ FT-IR microscope — This FT-IR microscope (microbeam FT-IR) is interfaced to an FT-IR optical bench and is used for the analysis of difficult to analyze or small samples. All mirrors and motors are controlled using the spectrometer software. This permits hands-off switching between viewing and transmittance or reflectance operations. Additionally, the View-Thru™ remote sample masking capability permits viewing of the entire sample area while aperturing for infrared analysis. View-Thru™ also enables complete photographic recording of both the full field of view and the IR analysis area. There is practically no minimum sample size, any substrate that can fit on the microscope stage can be analyzed.
Microscope accessories
Attenuated Total Reflectance (ATR) Objective — Since the sample's absorption diminishes the amount of the totally internally reflected radiation, this is commonly called attenuated total reflection (ATR). The ATR objective permits non-destructive surface analysis. Highly absorbing materials, coatings, coatings on non-reflecting substrates, polymers, soft rubbers, coated papers, biomaterials and a variety of other previously impossible samples can be analyzed with no sample preparation. The ATR objective uses a high refractive index, zinc selenide (ZnSe) crystal. Because the incident radiation penetrates a short distance into the sample when it is reflected at the interface, an absorption spectrum of the sample is produced. Its design features three viewing modes; Survey, Contact and ATR. The sample is positioned in contact with the crystal with a shallow depth of penetration (approx. 2 μm). ATR's consistent penetration depth provides accuracy for both qualitative and quantitative analysis.
Micro Compression Diamond Cell — This cell permits the crushing and flattening of materials such as minerals, rubbers, organic crystals, plastics and polymers. The cell flattens these materials to an ideal thickness for transmission analysis. A unique feature of the cell is its use of flat, parallel diamonds that have no facets. Facets are undesirable because of internal reflection losses and edges that can crack the diamond. The diamond has a 1.8 mm diameter working area for multiple samples.
Optical Bench
The FT-IR optical bench contains the IR source, interferometer, detector and sample compartment. The bench allows sample analysis with the use of various sample holders. Purified and dry air is introduced to the sample compartment providing a clean purge during sample data collection. A resolution of better than 0.1 cm-1 is achieved for near or far-infrared spectral ranges. Samples that are too large for microscope work can easily be cut or sheared to fit in the sample compartment.
SPECULAR REFLECTANCE (OPTICAL BENCH ACCESSORY)
Fixed 80 Degree Grazing Angle Accessory — This optical bench accessory simplifies surface analysis, non-destructively. Gold-coated optics provide a fixed 80 degree angle of incidence, making this technique ideal for the analysis of very thin (monolayer) coatings and adsorbed species or coatings on metal surfaces. Two masks (7 mm and 13 mm diameter) are available for isolating small areas on large samples.
Diffuse Reflectance (Optical Bench Accessory)
DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) Accessory — DRIFTS eliminates the need for tedious and time-consuming preparation of KBr pellets. Samples (powders, organics, inorganics) can be analyzed neat or easily mixed into a sampling matrix consisting of potassium bromide (KBr) or potassium chloride (KCl). The precision on-axis optics of the accessory provides the ultimate energy throughput for nanogram (low ppm) levels of sensitivity for most samples.
Liquid Transmission Sampling (Optical Bench Accessory)
Demountable Path-length Cell — This optical bench cell is ideal for analysis of liquids, mulls and films. Various path-length spacers are use in conjunction with Teflon gaskets to provide even pressure (leak proof) around the perimeter of the cell. A hypodermic syringe is used to inject the sample through a Luer fitting into the cell opening.
Disposable 3M IR Cards — The disposable (no cross-contamination) 5 cm x 10 cm cards come in two non-breakable microporous substrates: Type 61 polyethylene and Type 62 polytetrafluoroethylene (PTFE). These sample cards (holders) are complimentary accessories for qualitative mid-range (4000 cm-1 to 400 cm-1) transmission infrared spectroscopy. The substrate (sample area) of both cards are chemically inert and non-hygroscopic. They are especially useful for preparing liquids (even those containing water), pastes and semi-volatile samples. Except for the aliphatic C-H stretching that occurs between 3000 cm-1 and 2800 cm-1, Type 61 sample application substrate is highly effective from 4000 cm-1 to 400 cm-1, and work especially well for substances requiring interpretation in the fingerprint region (1600 to 400 cm-1). The type 62 sample application substrate is highly effective from 4000 cm-1 to 1300 cm-1, while providing the ability of spectral interpretation within the aliphatic C-H stretching region that occurs between 3000 cm-1 and 2800 cm-1.
Magnetic Film Holder — This accessory is used to hold thin solid film strips in position for transmission spectral analysis inside the optical bench. Magnetic rubber strips are used to secure the solid film sample to a slotted (3/8 X 1-1/8 inch) steel back plate.
FT-IR Spectral Library search databases
The FT-IR spectrometer is especially useful for identifying unknowns when reference spectra (library search databases) are available. Commercially available reference spectra are collected using FT-IR spectrometers, not dispersive IR, to obtain the signal to noise, resolution, peak location and relative peak intensities needed to differentiate between very similar spectra. This is particularly important when performing spectral subtract and quantitative analysis. However, it is best to make custom libraries for specific spectral searches of known materials utilized in a given industrial process. Even though this is time consuming, these custom library spectra are more accurate since they are collected under the same instrument conditions as the samples under investigation.
Up to ten library matches are listed in order in the search results. The library index number and compound name of each spectrum are given along with a match value between 0 and 100. The match value tells you how well the library spectrum matches the unknown. A match value of 100 indicates a perfect match. The closer the value is to 100, the better the match.
Commercial libraries and online libraries are used in the investigation of spectral matching.
