One of the less common forms of spectroscopy is based on the principle of glow discharge (GD), which is the same principle that makes neon lights possible. The process is essentially electrical in nature, with the sample used as an electrode. A voltage causes gas atoms, typically argon, to ionize and strike the sample surface to complete the electrical circuit, resulting in the liberation of atoms from the surface. This process is called sputtering, which frees atoms from the sample for analysis.

Instruments that depend on GD come in two basic varieties: those that use optical emission spectroscopy (OES) and those that use MS. This makes glow discharge similar to inductively coupled plasma (ICP) spectroscopy.

In GD-OES, the sputtered atoms are energized by the plasma discharge. When the excited atoms return to lower energy states, energy is released in the form of light. An optical spectrometer analyzes the different frequencies of light in order to identify the characteristic signal of particular elements. GD-OES instruments may use CCD-type detectors to measure an entire spectrum at once, or may use multiple photomultiplier tubes (PMTs). Each PMT is situated to analyze a specific frequency that is characteristic of a particular element, providing simultaneous measurement of multiple elements.

In GD-MS, the sputtered ions are directed to a mass analyzer, which identifies the elements present in the sample by the mass of the ions. Quadrupole MS provides excellent performance, roughly equivalent to GD-OES systems. However, the most sensitive GD-MS systems make use of magnetic sector analyzers, and some can detect trace elements at concentrations of less than one part per billion.

An advantage of GD spectroscopy is that solid samples can be directly analyzed, unlike ICP spectroscopy, which generally requires an aqueous solution of the sample, which can be difficult to prepare for samples like ceramics and other advanced materials. Another advantage of GD is that the sputtering process naturally provides an even removal of atoms from the sample surface, enabling depth profiling as the instrument analyzes the sample layer by layer. This makes the technique particularly useful for coatings and thin layers on metal and semiconductor substrates.

Competing techniques include more complex surface analysis technologies, which are generally more expensive, or ICP instruments outfitted with sample introduction methods adapted for solids, such as laser ablation or electrothermal vaporation.

Very few participants are involved in the GD market. HORIBA Jobin Yvon is the leading vendor, followed closely by LECO and Spectruma. Spectro Analytical (AMETEK) owns a controlling interest in Spectruma, a very focused player without any involvement in ICP. Thermo Fisher Scientific is the leader in GD-MS, and there are also a few niche players in GD-MS, such as Nu Instruments and MSI.

Glow Discharge Spectroscopy at a Glance:

Leading Suppliers

• HORIBA

• LECO

• Spectruma

Largest Markets

• Metals

• Semiconductors & Electronics

• Ceramics

Instrument Cost

• $80,000–$500,000