An interview with Vassili Karanassios, who is a Professor in the Department of Chemistry and the Waterloo Institute for Nanotechnology at the University of Waterloo, in Waterloo, Ontario, Canada. His research focuses on micro- and nano-analysis and the development of miniaturized instruments for on-site applications.
You have noted that the sample introduction efficiency of conventional pneumatic nebulizers for inductively coupled plasma (ICP) spectrometry is typically only 1–3% and is a key limitation. Why is it so low?
To generalize, it is not the pneumatic nebulizer but the spray chamber a nebulizer is typically attached to. The nebulizer generates a fine mist (akin to fog) from an analytical sample or from a blank. The droplets of this mist have a wide range of diameters, with many droplets being larger than others. Because the residence time of droplets in an inductively coupled plasma is 2–3 ms, larger droplets may not be completely desolvated as they travel through the plasma. Thus, any analyte contained in larger droplets will likely generate a signal disproportional to the concentration of the analyte in the droplet. Smaller droplets, on the other hand, are more likely to be completely desolvated and thus generate signals proportional to analyte concentration in the droplet. Thus, the analytical signal becomes dependent on droplet size.To ensure that a narrow and reproducible range of droplet diameters enters the ICP, nebulizers are connected to a spray chamber. Although there are many spray-chamber designs, a key function of any spray chamber is to ensure that a small, narrow range of droplet diameters is introduced into the plasma. Thus, larger-diameter droplets are excluded and are drained into a waste-collection vessel. It is during this step that large sample-introduction inefficiencies are introduced.
A field-portable rhenium coiled-filament sampling device for ICP spectrometry that was developed in your laboratory reportedly provides much improved sampling efficiency over traditional nebulizer systems. How did the idea for this device come about? Were other systems tested that met with less success?
Our near-torch vaporization (NTV) system falls in the general category of electrothermal vaporization (ETV) sample introduction for ICP spectrometry, and ETV systems have been around for decades. Although many ETV designs have been described in the literature, to generalize, a typical ETV sample introduction system uses a graphite support (for example, a graphite tube) as the sample holder. There are three main problems with graphite-based ETV sample introduction systems:
This interview has been edited for length and clarity.
To read the full interview visit: http://spectroscopyonline.com/Karanassios