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Description of Fourier Transform (FT) Infrared (IR) Method

Most materials absorb IR energy at different wavelengths depending upon their chemical nature. This phenomenon provides a method for characterizing many materials. IR energy is passed through the sample and the absorbance and/or transmittance versus IR wavelength is measured. Output is in the form of a graph, which is called the IR spectrum. The spectrum is a "fingerprint" of the material. It can be compared to those of known materials (reference spectra) to identify the unknown material.

The spectra of long-chain hydrocarbons (mineral oils, waxes, polyethylene) will be very different from the spectra of esters (vegetable oils, synthetic oils, acrylates). Different class groups are usually easy to identify. To characterize materials within a class group more subtle differences in the spectra can be used to narrow the identity of the substance (Spectral Interpretation). For very closely related materials like vegetable oils (corn, cotton, linseed) the method can only characterize the material as a vegetable oil, but not identify individual oils.

If spectra of two materials are the same with respect to both IR band position (wavelength) and relative band intensity, then the substances are chemically similar or closely related. If the spectra differ in any way, then the materials are not the same. A number of specifications (Mil-Spec, USP, ASTM) use the IR method for material identification.

Mixtures of materials (commercial products, contaminants, additives) are more difficult to characterize, since bands from all of the components overlay one another in the spectrum. Also components at low concentration are difficult to detect, because the major component overlays the bands from the minor component. In these cases the components must be separated from one another for identification.

If a reference match cannot be found, the location and band intensity give some information about the chemical nature of the material. For example, the spectra of esters always have a carbonyl (C=O) band, while those of pure long-chain hydrocarbons will not.



FTIR Interpretation


An IR spectrum by itself does not provide an exact chemical structure of a compound, but will provide information about its functionality based on band location and intensity (see the table of functional groups). Reference spectra are required for exact structural information.

The following information identifies the requirements and limitations of FTIR analysis.

General Requirements

  • Good quality spectrum
  • Proper sample preparation
  • Relatively pure material
  • Correlation chart
  • Reference spectra for comparison


  • Closely related materials, i.e.,
    Polyethylene glycols - molecular weight
    Vegetable oils
    Petroleum based oils
    Amino/Hydroxyl functionality - amino alcohols

  • Mixtures and some polymers

  • Inorganics
    Difficult to identify cations
    Simple anions (halogens)


  • Relative intensities of bands are important
  • Any mismatch with reference spectrum negates identification
  • Lack of bands is positive information

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Aqueous samples may be analyzed for common anions including:

  • Fluoride

  • Chloride

  • Phosphate

  • Nitrite

  • Nitrate

  • Sulfate

  • Acetate

  • Formate

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Wet Methods of Analysis

Chemical analysis of aqueous solutions using Photometric (Hach) or Titration Procedures. This includes acidity, alkalinity, hardness, chlorine, triazoles,  and other analytes.

Sample preparation may include one of several extraction methods to obtain an aqueous solution for analysis from a solid or solid surface.

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Scanning Electron Microscopy

Scanning electron microscopes have been commercially available since the late 1960's. The instrument has evolved from a highly specialized research laboratory instrument into an accepted part of analytical laboratories and production facilities. With the capacity of magnifying features from 10 to 100,000X, it can be found in the businesses of semiconductor and nylon fiber quality assurance, pollution particle characterization, and equipment failure analysis. The SEM also serves as a platform for micro-analytical techniques, such as Energy Dispersive X-ray Spectroscopy (EDS). In combination with this technique, the SEM becomes a powerful tool for ferreting out and characterizing evidence for root cause failure analysis. Both mechanical and corrosion related failures often have features that can be best discerned at high magnification and identified by x-ray microanalysis.

Operating Principles

In the SEM, a very finely focused beam of electrons is scanned over the surface of the specimen. As the electron beam scans the specimen, it not only provides topographical information, but also, as it penetrates the surface, interacts with the sample to cause effects such as electron backscattering, x-ray emission, secondary electron emission, and cathode luminescence.

The imaging process of the SEM takes place when a cathode ray tube (CRT) is scanned simultaneously with the electron beam. The most common method of imaging utilizes secondary electrons. As the electron beam is scanned over the specimen, surface features in line of sight of the secondary electron detector will generate proportionally more electrons. The detector generates a signal that is proportional to the number of the electrons received as various surface features come under the electron beam. The intensity of the CRT beam is modulated proportionally to represent the magnitude of the signal arriving from the secondary electron detector. A picture is built up that represents the surface topographical features and are discerned by the effect that, for a "hill" on the surface, the side facing the detector will generate more secondary electrons that are likely to arrive at the detector, and consequently, that side will appear brighter than the side that is not in the line of sight of the detector.

Another commonly used image is produced by the detection of backscattered electrons. The intensity of these electrons is influenced by differences in atomic number. Microstructural phase with different average atomic numbers and grain boundaries in unetched specimens can be imaged with these detectors.

Limitations of the SEM

Feature resolution is limited to 70 to 100 Angstroms for most microscopes. Specimens must be resistant to vacuum; liquids with vapor pressures less than 10-3 torr cannot be analyzed. Since the scattering takes place in vacuum, the maximum size of the specimens to be examined is limited to the size of the chamber at the base of the microscope. Consequently, typical specimen dimensions are limited to less than 3 cm on a side.

Standardized Methods

The standard practice for the performance of a scanning electron microscope is covered in ASTM E 968, "Scanning Electron Microscope Beam Size Characterization."

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Elemental Analysis Through Energy Dispersive X-ray Spectrometry

Operating Principles

One of the instruments most commonly used in conjunction with the SEM is the Energy Dispersive X-ray Spectrometer (EDS). The x-ray spectrometer converts a x-ray photon into an electrical pulse with specific characteristics of amplitude and width. A multi-channel analyzer measures the pulse and increments a corresponding "energy slot" in a monitor display. The location of the slot is proportional to the energy of the x-ray photon entering the detector. The display is a histogram of the x-ray energy received by the detector, with individual "peaks," the heights of which are proportional to the amount of a particular element in the specimen being analyzed.

The locations of the peaks are directly related to the particular x-ray "fingerprint" of the elements present. Consequently, the presence of a peak, its height, and several other factors, allows the analyst to identify elements within a sample, and with the use of appropriate standards and software, a quantitative analysis can be made of elements with atomic number of 4 (carbon) or greater.

Combining the EDS system with the SEM allows the identification, at microstructural level, of compositional gradients at grain boundaries, second phases, impurities, inclusions, and small amounts of material. In the scanning mode, the SEM/EDS unit can be used to produce maps of element location, concentration, and distribution.

Limitations of EDS

The design of the equipment makes the technique incapable of detecting elements lighter than carbon. Sensitivity (ability to detect the presence of an element above background noise) is 0.1 wt% with the EDS. There is also poorer sensitivity for light elements (low atomic weight) in a heavy matrix. Resolution of the x-ray energy levels limits the positive identification of certain elements (i.e., molybdenum and sulfur) due to overlapping energy slots.

Quantitative analysis is usually limited to flat, polished specimens. Unusual geometries, such as fracture surfaces, individual particles, and films on substrates can be analyzed, but with considerably greater uncertainty.

Standardized Methods

The standard guide for the performance of energy dispersive x-ray spectroscopy is covered in ASTM E 1508, "Quantitative Analysis by Energy-Dispersive Spectroscopy."

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Soils are analyzed for corrosivity using  the Critical Parameters and Rank Numbers for Soil Aggressiveness, from the “Handbook of Cathodic Protection, The Theory and Practice of Electrochemical Corrosion Protection Techniques” by Von Baeckmann and Schwenk. The paramaters that can be analyzed in the laboratory are listed below.

1.      Kind of Soil

2.      Specific Soil Resistance

3.      Water Content

4.      pH

5.      Total Acidity to pH = 7

6.      Redox Potential

7.      Total Alkalinity to pH = 4.8

8.      Sulfide

9.      Chloride

10.    Sulfate

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