Glow-discharge sampling as coupled to spectrometric detection

Glow-discharge sampling as coupled to spectrometric detection 

With samples less than 5 mm in diameter, the flat sample holder depicted in Fig. 8.15B affords direct analyses provided the sample is nominally fiat. The holder is mounted to the end of the DIP and comprises a PTFE clamp assembly, support rods, boron nitride spacers and either stainless steel or aluminium anode orifice plates. The DIP is placed through the clamp assembly and the rf feedthrough is placed directly behind the sample (cathode), which locks it in place. The spacers separate the anode plate and the cathode in the same manner as the 0-ring seal in the rf-AES source design. The anode body in this case is the Ta ion volume enclosed by the anode plate and the ion exit orifice mounted to it the entire cell assembly is affixed to the commercial source cryo-cooling ring. 

The GD ion source gained popularity in the past two decades on account of its stability, sensitivity and operational simplicity. At least three commercial instruments were already available in the early 1990s. 

Elemental analysis by GD-MS provides substantial advantages over competing techniques in that it responds to metals and non-metals, exhibits a high sensitivity, suffers from minimal matrix effects and provides isotopic information. Unsurprisingly, it has developed into the most prominent GD coupling. 

One key difference between MS and optical detection techniques is that the sample material must be physically transferred out of the source. Analyte ions are extracted by electrostatic lenses and transferred through a mass analyser for eventual detection. Although ions are formed throughout the source cell, only those created very close to the exit orifice can survive to the high-collision environment and depart in the charged state. Both magnetic-sector and quadrupole-based instruments have been used in GD-MS, and commercial versions of each are available. The typical discharge operation conditions for GD-MS are 1-5 mA, 800-1500 V and 0.2-2.0 torr. 

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