Secondary cathode

This chapter deals exclusively with the methods that have been developed for the direct solids analysis of nonconductive samples by glow discharge mass spectrometry. The basic approaches to operation and sample preparation for the three primary methodologies of compaction, secondary cathode, and radio frequency powering are described. Examples of source performance and practical applications of each are taken from the analytical literature. Whereas this chapter de-... 

Electrical Secondary Contact to Cathode Secondary Cathode... 

Figure 3 Schematic representation (not to scale) of the source assembly used in the secondary cathode approach to glow discharge mass spectrometry (GD-MS) analysis of nonconductive samples. (From Ref. 21.)...

Milton and Hutton [21] evaluated aluminum, copper, silver, indium, lead, and tantalum as possible secondary cathode materials. The first three candidates (Al, Cu, and Ag) sputtered at rates too high to allow production of ions characteristic of the glass sample (i.e., tended to produce too thick a metallic layer). Indium and lead are soft materials that lead to the overcompression of the insulator-cathode-sample sandwich, consistently resulting in electrical short-circuiting between the anode and cathode. Finally, tantalum does indeed exhibit the desirable characteristics for application as secondary cathode materials. Although not explicitly required, the fact that Ta is a getter element is likely to provide some added benefits as well. 

The major advantage of using the secondary cathode method for nonconductor analysis is that the sample is analyzed directly in its native form. Because grind-... 

The most creative application of the secondary cathode approach was described by Schelles and Van Grieken [24], who investigated its ability to determine the elemental constituents of polymeric materials. Mass spectrometric analysis has almost exclusively been directed at the determination of molecular weights and disparity characteristics secondary ion mass spectrometry (SIMS) [53,54] and matrix assisted laser desorption ionization (MALDI) [55,56] have carried the major share of the workload. Growing concerns over the fate of polymeric materials in the environment and the leaching of heavy metals into ground waters have necessitated the development of methods that permit the elemental analysis of bulk polymers. In addition, the use of polymers as immobilization media for waste remediation is also pressing these developments. 

Figure 5 Temporal profile of ions produced from a polytetrafluoroethylene (PTFE) sample using a Ta secondary cathode. , C+ A, F+ X, CF+ , Fe+. (From Ref. 24.)...

The use of the sample compaction and secondary cathode sampling methodologies has certain positive attributes based on the initial sample form, analysis time constraints, and analytical performance criteria of a given application. A common shortcoming of both approaches is that a material other than the analytical specimen is subjected to the GD atomization/ionization processes, inflicting additional... 

Schelles W., De Gendt S., Muller V. and Van Grieken R. (1995) Evaluation of secondary cathodes for glow discharge mass spectrometry analysis of different nonconducting sample types, Appl Spectrosc 49 939— 944. 

Schelles W. and Van Grieken R. E. (1996) Direct current glow discharge mass spectrometric analysis of macor ceramic using a secondary cathode, Anal Chem 68 3570-3574. 

Pajo et al. (2001a) used GD-MS to measure impurities in uranium dioxide fuel and showed that these impurities could be used to identify the original source of confiscated, vagabond nuclear materials. De las Heras et al. (2000) used GD-MS to determine neptunium in Irish Sea sediment samples. The sediment samples were compacted into a disk that was used with a tantalum secondary cathode in the glow discharge. Using a doped marine sediment standard for calibration, detection limits down to the mid pg/g level were determined. 

For aluminum, the outer surface of the oxide layer in humid environments is considered to be a mixture of aluminum oxide and aluminum hydroxide. After the adsorption of chloride ions, an ion exchange can occur leading to the substitution of hydroxyl ions by chloride ions [179, 180]. After the chemical attack of the oxide, aluminum is electrochemicaUy dissolved. The chloride ions are regenerated after the dissolution of the transitory hydrox-ychloride compounds. Thus, a relatively small amount of chloride ions can result in a progressive attack of the protective layer. Within the head of the filiform filament, the anodic dissolution of aluminum leads to a local acidification of the anolyte due to the hydration of aluminum ions. It has been observed that a secondary cathodic reaction, the reduction of hydrogen ions, can occur. Hydrogen evolution has been observed within the head [166]. 

---