Positive Material Identification (PMI)

Sometimes it may happen to wonder why the sky looks blue and what is the answer? It may be already known that sunlight is made up of all the colors of the rainbow: red, orange, yellow, green, blue, and violet. It may be also known that sunlight has to pass through our atmosphere before it reaches our eyes. The gas molecules in the atmosphere break up, or "scatter" the sunlight into its many parts. But they scatter some parts more effectively than others. Different colors of light have different energies or wavelengths. Red light has a long wavelength and a lower energy, blue light has a short wavelength and a higher energy. The gas molecules in the atmosphere scatter the higher-energy blue wavelengths better than the red wavelengths. So the sky looks blue.

As mentioned before, red and blue are only two of the colors which make up the light coming from the sun. This light can be seen as a source and as such it can be analyzed, but how can this be done? As a source, light has a spectral range which can be totally or selectively transmitted to an imaging system which transfers this range to a spectrometer. This spectrometer, on receiving the spectral range transmits it to a detector which eventually elaborates the data.

Spectroscopy is the study of spectra, i.e. characteristic wavelengths or colors. Optical emission spectroscopy (OES) comprises several techniques that form the most important means we have for chemical analysis.

In OES, we measure spectra emitted by atoms and ions with optical transitions in the wavelength range from about 100 nm to 900 nm. This range includes the ultraviolet, and visible light (from violet at 380 nm to red at 760 nm), and the near infra-red.

Both analysis techniques offer advantages and disadvantages. XRF analyzers are easy to use, the units are light and small in size, and the sample to be measured does not require much preparation. But, there are limitations on the number of elements that XRF units can measure. Also, traditional methods of generating X-rays have used radioactive isotopes, the use of which requires much documentation. In the latest generation of portable XRF analyzers, isotopes have been replaced by small X-ray tubes requiring much less documentation.

OES instruments are larger in size and use argon gas to improve accuracy. Sample preparation plays an important role, but on the other hand, there is practically no limitation on the instruments’ ability to analyze elements typically used in metals. One of the key reasons why OES technology is chosen instead of XRF is because of its superiority in the measurement of light elements in metals, such as carbon and aluminum. OES is the only reliable way to measure carbon outside of the laboratory, which commonly needs to be measured in samples of stainless steels, magnesium and silicon. The technology also is employed in the measurement of aluminum in aluminum alloys. OES measurements can be attained without an argon atmosphere, but will suffer degraded accuracy and precision or repeatability.

Positive Material Identification (PMI) refers to the identification and analysis of various metal alloys based on their chemical composition in nondestructive testing (NDT). Measurement results are shown in the form of elemental concentration in percentage or by specific alloy name such as SS316L or Inconel 625. PMI is a field-testing method made possible by the portability of most PMI analyzers. These instruments also can be used in the laboratory.

The two main technologies used for alloy identification in PMI are X-ray fluorescence (XRF) and optical emission spectroscopy (OES). XRF instruments work by exposing a sample to a beam of X-rays. The atoms of the sample absorb energy from the X-rays, become temporarily excited and then emit secondary X-rays. Each chemical element emits X-rays at a unique energy. By measuring intensity and characteristic energy of the emitted X-rays, an XRF analyzer can provide qualitative and quantitative analysis regarding the composition of the material being tested.

In the OES technique, atoms also are excited; however, the excitation energy comes from a spark formed between sample and electrode. In this case, the energy of the spark causes the electrons in the sample to emit light, which is converted into a spectral pattern. By measuring the intensity of the peaks in this spectrum, the OES analyzer can produce qualitative and quantitative analysis of the material composition. Although OES is considered a nondestructive testing method, the spark does leave a small burn on the sample surface.