Instrumented Chemical Analyses
LTI performs a variety of instrumented techniques including Direct
Reading Atomic Emission Spectroscopy (AES) [formerly Optical Emission Spectroscopy (OES)], Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES), Inductively Coupled Plasma
Mass Spectroscopy (ICP-MS), Combustion Furnace Method and Inert Gas Fusion.
Direct Reading Atomic Emission Spectroscopy
AES is one of the most useful instrumental based methods for performing
elemental chemical analysis. The basic principle of AES is that when
free atoms are put into an energetic environment, they emit light in
a series of wavelength bands, similar to the diffraction of light into
a rainbow. These wavelength bands or emission lines form a pattern that
is characteristic of the atom that produced it. Generally, the intensities
of the various characteristic lines are proportional to the number of
atoms that produced the lines. If an element is present in a sample,
its characteristic spectrum lines also will be present. The concentration
of that particular element can be determined by measuring the intensities
of the characteristic lines and comparing them to the same lines from
known concentration standards. Thus, the type of element can be identified
qualitatively by the emission spectrum and the amount of the element
can by measured quantitatively by the intensities of the emission lines.
The AES system that we have at Laboratory Testing has a spark source. The sample is put in the spectrometer as a cathode and a tungsten pin is the anode. The gap between the pin and the sample is filled with the inert gas Argon. When the spark strikes the sample, rapid heating of the sample occurs causing vaporization of a small amount of sample from the surface, and forming a plasma (high energy gas cloud). The plasma produces the spectrum of lines from the elements that are present in the sample. The spectra are analyzed using mirrors and a diffraction grating to separate the analytical lines, as well as a set of photomultiplier tubes to quantify the light emitted from the elements and to measure their concentrations. Then, a computer compares the amount of light from the sample to that of known standards to identify the element, and calculates the quantity of the element.
Spectrographic spark source instruments are very rapid and are often used
in production applications such as steel mills and foundries. Sample preparation
involves grinding a flat surface on the solid test piece. (Samples for
this type of analysis must be solid, at least 0.040" thick, and capable
of having a 1" x 1" area ground or machined on them so they will cover
the excitation aperture). The flat test piece is then put in the spectrometer
and the system is energized. The test results are printed for viewing
and evaluation.
Inductively Coupled Plasma Atomic Emission Spectroscopy
ICP-AES is a technique that determines elemental concentrations of trace to major, based on the principle of atomic spectroscopy. In theory, the technique can be used to detect all elements, except argon. In practice, good results are obtained for about 70 elements with detection limits at the parts per billion level.
ICP is an excitation source for atomic emission spectroscopy. The plasma
consists of argon gas operated at atmospheric pressure and inductively
coupled to a radio frequency (RF) electromagnetic field. The components
of an ICP system are the sample introduction area, the ICP torch and argon
gas supplies, the RF generator, the spectrometer, the detection electronics
and the system computer. The sample is a solution of the test material
that is introduced into the ICP as a fine aerosol of droplets produced
by a nebulizer. The torch is usually a set of concentric quartz tubes
that contain the argon used for the plasma, for cooling the torch and
for guiding the test sample to the plasma. The RF generator supplies the
high-frequency alternating current in the induction coil and sustains
the inductive coupling of energy into the plasma. The spectrometer detects
the atomic emissions produced. The computer is used to control and monitor
instrument functions and to process, store and output the results of the
analysis.
Inductively Coupled Plasma Mass Spectroscopy
Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) combines the easy sample introduction and quick analysis of ICP technology with the accurate and low detection limits of a mass spectrometer. This method is highly sensitive and capable of trace multi-element analysis, often at the parts-per-trillion level. It is capable of the determination of a range of metals and several non-metals. ICP-MS also allows for routine chemical testing for trace elements in super alloys and provides ultra-trace elemental analysis for high purity alloys like those used in the Semi-Conductor Industry.
Samples are decomposed to neutral elements in a high-temperature argon plasma to produce ions and are analyzed based on their mass to charge ratios. Aqueous samples are introduced by way of a nebulizer which aspirates the sample with high velocity argon, forming a fine mist. The aerosol then passes into a spray chamber where larger droplets are removed. Droplets small enough to be vaporized in the plasma torch pass into the torch body, where the aerosol is mixed with more argon gas. A coupling coil is used to transmit radio frequency to the heated argon gas, producing an argon plasma located at the torch. The hot plasma removes any remaining solvent and causes sample atomization followed by ionization.
Combustion Furnace Method for Carbon and Sulfur Analyses
Carbon and sulfur are easily oxidized. Therefore, when they oxidize they leave oxide gas as the metal. High temperature combustion uses this oxidation principle as the basis of obtaining the content of these elements in a material. In the combustion furnace, the introduction of oxygen with the high temperature causes the sample to combust. The gases are passed through a series of traps, absorbers and converters to remove interfering elements and to ensure the gases have the proper structure for detection. Infrared detection is used to determine the concentration of the carbon or sulfur. The infrared detector is used on the basis that various gases can absorb energy within a specific wavelength of the infrared spectrum. The amount of energy absorbed is related to the amount of the carbon or sulfur in the test sample.
Inert Gas Fusion
Inert gas fusion is used to determine the gas content in ferrous and nonferrous materials. The gases, hydrogen, nitrogen and oxygen, are found in the materials as a result of the melting processes and subsequent hot and cold working methods. Controlling the gas contents to low levels minimizes their adverse effects on mechanical properties such as strength and ductility. The inert gas method reverses the bonding between the gases and the metals, causing the dissociation of the gases. The dissociated gas is moved along a very elaborate separation chamber by an inert carrier gas. The gas to be analyzed flows into the detection system. Depending on the gas to be determined, the detection system is either an infrared system or a thermoconductivity detection system. The infrared system is used here at LTI to detect oxygen, and the thermo-conductivity system is used to detect nitrogen and hydrogen.