GD Sources

In addition to the analysis of relatively thick metallic coatings and diffusion profiles, dc GD-OES has also been successfully applied to the analysis of thin protective layers (thickness typically 100 nm), e.g. phosphate and chromate layers on steel [4.185]. 

4 Chw Discharge Optical Emission Spectroscopy (CD-OES) 229 Tab. 4.2. Some typical applications of GD-OES depth-profile analysis. 

Oxide scales Oxide scale on alloyed steel 4.218 

Thin layers Anodic alumina films on aluminum 4.211 

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

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Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

Glow Discharge Sources. Glow discharge (GD) ion sources and some of their applications using different mass analyzers have been discussed in earlier chapters of this volume. Virtually all work that couples these sources to FT-ICR mass analyzers has involved dc discharges (see Chapter 2 for further discussion of the types of GD sources). 

Figure 20 Ultrahigh mass resolving power mass spectrum of 58Fe+ from a glow discharge (GD) source obtained with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a 7-tesla superconducting magnet.

Although the coupling of steady-state discharges to the TOF-MS has proved successful, the TOF-MS is perhaps better suited for the investigation of pulsed GD sources. Here, the high temporal resolution and multielement capabilities of the TOF-MS can be fully exploited, with the coupling of a pulsed source to a pulsed... 

One application of great interest is depth profiling by means of GD-TOF-MS. Particularly in conjunction with pulsed GD sources, the simultaneous multi-elemental capabilities of TOF-MS permit the analysis of solid samples as a function of depth (or time) as the discharge operates. In such a case, one might envision complete trace elemental analysis of solid materials with resolution on the nanometer scale. 

Radio frequency-powered GD sources have been coupled with various analytical spectrometries for more than two decades [167]. Analyses are not limited to conducting samples in fact, electrical insulators such as glass, ceramics and geological materials — some of the most difficult samples to dissolve for solution methods of elemental analysis — can be readily examined. 

These factors clearly influence plasma formation, the requirements for which depend on whether the GD source is of the rf or dc type and also on the detection technique to which the GD is coupled. As early as 1976, Gough [177] reported improved direct solid atomic absorption analysis achieved by using a flowing gas instead of a static cell and... 

Sputtering rates and penetration rates In order not to elaborate excessively on these concepts, this section deals only with the theoretical aspects in relation to pulsed GD sources. Interested readers are referred to the original references for more details. 

In the recently developed atmospheric electrolyte-as-cathode GD sources, a liquid serves as the cathode for the discharge, which facilitates direct analysis of continuously flowing liquids [302]. The device is potentially applicable to the continuous analysis for metals in water and wastewater. 

A modified version of the Grimm-type GDL has been described by Shao and Horlich [594], The source has a floating restrictor and is designed so as to replace an ICP torch in an ICP-MS. Therefore, the anode is slightly positive with respect to the earthed skimmer interface of the MS system. The simultaneous analysis of an unknown sample and a reference material was carried out by means of a system based on two pulsed GD sources housed within the same tube [595], Optimization of the relative position of the two cathodes was achieved by evaluating the signals produced in GD-MS when using the same specimen for each of them. 

A tandem system consisting of pulsed dye laser ablation and ionization in a GD source for MS have been described by Barshick and Harrison [649]. The role played by the working gas (Ar, He, Ne) on redeposition of sputtered material has also been clarified. Removal of interfering species in GD-MS is possible through the use of getters such as Ag, C, Ta, Ti and W [650]. This approach has been applied successfully in the determination of rare earths so as to avoid oxide ion formation. 

In speciation, glow discharges are excellent detectors for GC work as shown earlier. In addition to the low power and pressure ICPs they can be used successfully for element-specific detection for gas chromatography. An rf-GD-MS system has been used as a detector for GC by Olson et al. [661], The set-up should consist of a temperature-controlled transfer line of stainless steel from the exit of the GC to the inlet of the GD source. The system has been tested with tetraethyl-Pb, tetraethyl-Sn and tetrabutyl-Sn and provided useful structural information for the identification of these compounds through the observation of fragment peaks the detection limits were down to 1 pg. 

Elements with low intensity fluorescence lines (e.g. Eu, Tm and Y) have been determined in aqueous solutions by depositing and drying nanoliter amounts of sample on the Ni cathode of a miniature GD source used as the atom reservoir [665], The atomic cloud thus formed was extited by a Cu-vapor laser-pumped dye laser to detect fluorescence directly. Absolute detection limits of 2 fg for Eu, 0.08 fg for Tm and 1.2 pg for Y were achieved and the total time for analysis from sample probing to data acquisition did not exceed 5 min. 

A schematic DC GD source is shown in Fig. 7.43. The gas is present at a pressure of a few torr. The DC GD source can be operated with a DC potential of 800-1200 V applied between the electrodes. The sample is in electrical contact with and serves as the cathode as seen in Fig. 7.43. The applied potential causes spontaneous ionization of the... 

Figure 7.43 A DC GD source. A flat conducting sample serves as part of the cathode. [Courtesy of Jobin Yvon, Inc., Horiba Group, Edison, NJ (www.jyhoriba.com).]...

The first application of GD-OES using a DC source was for direct multielement analysis of solid metals and alloys, much like a spark source. The bulk composition of the sample was determined. The DC GD source for analysis of conductive solids has several advantages... 

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