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.
OF INFRARED (IR) SPECTRA
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.
- Good quality
- Proper sample
spectra for comparison
- Closely related
Polyethylene glycols - molecular weight
Petroleum based oils
Amino/Hydroxyl functionality - amino alcohols
and some polymers
Difficult to identify cations
Simple anions (halogens)
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:
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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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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.
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.
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
Limitations of the SEM
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.
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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Analysis Through Energy Dispersive X-ray Spectrometry
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
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,
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.
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.
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
Kind of Soil
Specific Soil Resistance
Total Acidity to pH = 7
Total Alkalinity to pH = 4.8
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