Sparking, atomisation

This chapter describes the basic principles and practice of emission spectroscopy using non-flame atomisation sources. [Details on flame emission spectroscopy (FES) are to be found in Chapter 21.] The first part of this chapter (Sections 20.2-20.6) is devoted to emission spectroscopy based on electric arc and electric spark sources and is often described as emission spectrography. The final part of the chapter (Sections 20.7-20.11) deals with emission spectroscopy based on plasma sources. 

Principles and Characteristics The original idea of spark-source mass spectrometry (SSMS) is due to Dempster [356], long before the first commercial instruments. In spark-source MS, atomisation and ionisation... 

Finally, direct solid analysis by optical and mass spectrometry will be presented in Section 1.5. This alternative is becoming more appealing nowadays and implemented in laboratories because of the many advantages brought about by eliminating the need to dissolve the sample. Techniques based on the use of atomiser/excitation/ionisation sources such as sparks, lasers and glow discharges will be briefly described in that section. 

Besides plasmas, which are at the forefront of thermal atomisation devices, other excitation processes can be used. These methods rely on sparks or electrical arcs. They are less sensitive and take longer to use than methods that operate with samples in solution. These excitation techniques, with low throughputs, are mostly used in semi-quantitative analysis in industry (Fig. 15.2). Compared to the plasma torch, thermal homogeneity in these techniques is more difficult to master. 

Arc and spark ablation Arc and spark ablation systems for sample introduction are similar to the respective atomisation sources described above [47]. The dc arc equipped with metal or graphite electrodes is the most widely used form of the arc. Spark sources, as depicted in Fig. 12.31, are frequently used for conducting samples due to their instantaneous high temp>erature whereas the arc temp>erature will increase during a measurement cycle, which can lead to fractionation effects. 

Laser ablation Laser ablation (LA) in combination with the ICP atomiser has become a powerful and flexible techniqvie for solid sample introduction [47]. LA-AES has found its niche primarily as a bulk sampling technique for the analysis of bulk solid materials with a large focal spot (500—1000 pm). It offers comparable detection capability to spark ablation/emission but is not dependent on the sample being conductive. The experimental set-up, revealed in Fig. 12.32, consists in its simplest form of a pulsed laser (excimer- or Nd YAG-laser) with a defined pulse energy, some focusing optics, and a sample cell with a continuous Ar flow con-... 

---