Testing Carbon in Alloys
LIBS – How SciAps turned spark OES into a handheld
The traditional method to test carbon in alloys is spark optical emission spectroscopy (spark OES). Spark OES, uses a high voltage spark source to continuously spark the alloy. The heat from the sparking vaporizes the surface of the metal, stripping the outer valence electrons from the atoms that comprise the alloy. These valence electrons form an electron plasma. As the plasma cools, the electrons recombine with the atoms, causing characteristic photons (light) to be emitted from the electrons at known wavelengths. For example if an electron recombines with a carbon atom, then the light emitted will be unique (i.e. characteristic) of carbon. In the case of carbon one of the common wavelengths of this light is 193.1 nm. The same holds for other elements like Si, Cr, Mn, etc.
The light from the plasma is piped through fiber optic cables and into a spectrometer. The spectrometer separates all of the various wavelengths of light via a diffraction grating and a system of lens and mirrors. The different wavelengths of light are fanned out across a CCD detector, and onboard computer determines the intensity of light at each wavelength. The intensity of light of a given wavelength is related to the concentration of that element in the alloy. For example a higher intensity of light at 193.1 nm (carbon) means more carbon is in the material.
Testing in an inert gas atmosphere, typically argon, is a requirement to achieve low limits of detection, good precision and quantification with spark OES. The volume defined by the location of the spark and where the light enters the fiber should be purged of oxygen. Oxygen quenches the light from the plasma, particularly in the deep UV region of the spectrum where many elements including carbon are measured. Doing so with argon increases the signal strength (intensities) by a factor of 10x-50x depending on the element.
These requirements – argon, large console, power supply to generate electric spark – all combine to make spark OES systems cart-mounted, plus a large argon gas tank. Hardly portable. And in many areas compressed gas regulations make transporting the large argon tank difficult or impossible. For example, for testing carbon steels in a pipeline, the operator must have a crane that lowers the system and the argon tank into the ditch for testing. When moving to the next location, the crane lifts and transports the spark OES unit.
Until recently, despite the mobility limitations, spark OES was the only technique available to measure carbon content in alloys, in the field. Handheld X-ray fluorescence (HHXRF) cannot analyze elements as light as carbon. In fact handheld XRF measures atomic elements from magnesium (atomic number 12) and higher. Elements including carbon, boron, beryllium, lithium cannot be measured in alloys by HHXRF.
Laser Induced Breakdown Spectroscopy (LIBS)
SciAps has introduced a new technology – laser induced breakdown spectroscopy (LIBS) for the analysis of carbon and other elements. The Z-200 C+ unit is also optical emission spectroscopy. However the power hungry spark source has been replaced by a miniature, pulsed laser.
It is fair to ask how does this small, battery operated pulsed laser replace the power of the high voltage spark source used in spark OES? One key parameter is power density i.e. dumping enough energy, over a short enough period of time, in a small enough area to vaporize a portion of the sample. The Z’s laser produces a pulse containing about 5 mJ of energy. This energy is delivered over a very short time scale of about 2 billionths of a second (2 ns). The diameter of the laser is about 50 um, so the area is about 2,000 um2. The laser therefore delivers approximately a gigawatt of power per square cm of surface area, but over a very short amount of time. This high instantaneous power allows for instant vaporization of the sample surface. The small yet powerful laser is a critical component that yields a handheld form factor for the LIBS technology.
Argon consumption is another defining feature of the LIBS technology compared to spark OES. Like spark OES, LIBS also requires argon gas purge. However because the laser beam is such a small diameter, the purge volume for LIBS is about 1,000 times smaller than spark OES. This means that operators can obtain a reasonable number tests (600 for general testing, about 150 for carbon analysis) using a small, user replaceable argon canister located in the handle of the device.
The key to handheld form factor for carbon testing is thus the laser and the reduced argon requirements. The pulsed laser replaces the large, power hungry high voltage spark source. The nature of the laser requires far less argon to achieve an acceptable purge.