R&D Opportunities in Arc/Spark Optical Emission Spectrometry

Jun 30, 2011

A recent tutorial (1) prompted this commentary. Although generally considered a "mature" technology, arc/spark optical emission spectrometry continues to provide opportunities for spectrochemists. Here, we have listed a few suggestions for further improvement.

Arc/spark optical emission spectrometry (OES) is essential for the production of most metals and many of the products that use metals in their production. In the making of metals, a liquid sample is extracted from a furnace, quickly cooled, surfaced on a belt or disk sander, lathe, or milling machine, and analyzed in seconds for all elements of interest by arc/spark OES. That analysis informs the melter about what further refining is needed or what alloys to add to make the desired alloys for aluminum foil, aluminum for engine blocks, or low carbon steel for car hoods.

Before World War II, detectors did not exist to effectively support arc/spark optical emission spectrometers. Spectrochemical analyses were performed with spectrographs that photographed the spectrum of standards and samples, and the spectra were then measured using densitometers or microphotometers. Skilled spectroscopists could perform a multielement analysis in about 1 h, which was considerably faster than classical "wet" chemical methods.

World War II fostered the development of the photo-multiplier tube (PMT) as a radar detector. Applied Research Laboratories (Glendale, California) along with Alcoa (Pittsburgh, Pennsylvania) and Baird Associates (Cambridge, Massachusetts) working with Dow Chemical (Midland, Michigan) saw the potential of the PMT as a spectrometer detector and somewhat concurrently began developing direct reading arc/spark spectrometers. The first commercially available arc/spark OES system appeared circa 1947 and was a major technological advance for the metal-producing industries because the analysis time was reduced from approximately 1 h to less than 10 min. These new instruments came to be colloquially referred to as direct readers.

In the ensuing years, other improvements were made in arc/spark OES, including the development of vacuum optical systems allowing the determination of P, S, and better C; computers and microprocessors allowing faster and more precise analyses; improved excitation sources allowing better precision of analysis; lower detection limits; and high energy pre-spark (HEPS).

By 1990, a relatively unskilled operator could perform a complete multielement analysis of any major metal in about 30 s, including sample preparation. In fact, the whole analytical process could be automated so that an operator was not needed for routine spectrometer operation.

At that time arc/spark OES was considered a mature technology and generally unworthy of the attention of the spectrochemical research community. There have always been too many other exciting avenues to pursue: inductively coupled plasma (ICP) spectroscopy, ICP–mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, gas chromatography (GC)–MS, and many others. (Note: ICP users might be interested to know that the first ICP-OES system used for analyses was an Applied Research Laboratories [ARL, now part of Thermo Fisher Scientific, Austin, Texas] arc/spark OES system retrofitted with an ICP excitation source by Stanley Greenfield at the Albright and Wilson R&D Centre in the United Kingdom circa 1961.)

However, this viewpoint should have changed significantly with the advent of several new technologies developed between 1985 and 2000:

  • proved spark creation
  • Charge-coupled device (CCD) optics
  • Gas-filled vacuum ultraviolet (VUV) optical chamber
  • Digitally controlled arc/spark excitation sources
  • Advanced electronic time-resolved spectroscopy (TRS)
  • Single-spark evaluation and the mapping of inclusions in metals

Any one of these new technologies should have produced a flurry of papers. Instead, the research was mainly carried out in industrial laboratories by manufacturers of OES instrumentation, primarily Spectro Analytical Instruments (now part of Ametek, Berwyn, Pennsylvania) and ARL. Few papers were published.

We suggest that significant opportunities still exist to improve analyses by arc/spark OES. Some have been listed below.