Instrumentation for Solid Sample Introduction

Although most samples are commonly presented as liquids for atomic emission spectroscopy, direct solid sample analysis has the advantage that no major pretreatment or dissolution steps are required [44]. This minimises dilution errors or contamination from reagents and reduces the reagent and manpower cost per sample. In addition, improved detection Hmits may be obtained if microsamples or microanalysis are possible without any further dilution. However, the analyst has to ensure that the solid material sampled is representative of the bulk material. ICP-AES has generally a remarkable tolerance for total dissolved sohds compared to ICP-MS or flame AAS so that, depending on the overall matrix, between 2 and 25 % suspended sohds can be coped with. Therefore, most of the sohd sample introduction devices described below are dedicated for ICP-AES. 

Electrothermal vaporisation Electrothermal vaporisation, analogous to the approach utilised in AAS, can be performed by resistive heating of sample devices such as boats, ribbons, rods, or tubes from graphite or a refractory element [46]. 

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- 

---