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Conventional pneumatic nebulizer-spray chamber combinations for ICP spectrometry have a sample introduction efficiency of only 1-3%. A unique electrothermal vaporization device developed by Vassili Karanassios of the University of Waterloo uses field-portable rhenium filament coils with a very small vaporization chamber and increases the sample introduction efficiency to 100%.
An interview with Vassili Karanassios, who is a Professor in the Department of Chemistry and with the Waterloo Institute for Nanotechnology (WIN), University of Waterloo, Waterloo, Ontario, Canada. His research focuses on micro- and nano-analysis and the development of miniaturized instruments for on-site applications. The January 2014 issue of Spectroscopy includes an article coauthored by Prof. Karanassios that discusses one such instrument, an electrothermal vaporization device for inductively coupled plasma (ICP) sample introduction (1).
You have noted that the sample introduction efficiency of conventional pneumatic nebulizers for 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:
• The use of graphite promotes carbide formation. Because a large number of elements across the periodic table form carbides and because carbides typically have very high vaporization temperature, such elements either do not vaporize at all or they have incomplete vaporization. This incomplete vaporization occurs because a typical graphite-based ETV system can reach a maximum temperature of only about 2800 ºC. There are ways of alleviating the adverse effects of carbide formation, but they typically include chemical modification, thus complicating method development and instrumentation. In general, applicability of graphite-based ETV systems is limited to volatile, non-carbide-forming elements (for example, Pd, Cd, and Zn).
• Because the graphite support of ETV systems has a relatively large mass (for example, 250 mg), a relatively large power supply must be used (for example, one delivering up to 5 kW). This requirement increases purchasing and operating cost of instrumentation.
• Because the size of the vaporization chamber for a typical ETV system is relatively large (roughly, the size of one’s fist), such systems cannot be brought very close to an ICP torch. They are typically operated 0.5–1 m away from the torch, with a tube connecting the ETV vaporization chamber to the torch. The length of the tube dictates transport losses. As a rule of thumb, the longer the tube, the higher the sample loss. In many cases and depending on vaporization power used, black deposits (from carbon vaporizing from the graphite support) can be seen collecting in the tube after only a few hours of operation. Therefore, frequent tube cleaning or tube replacement is required, thus also necessitating recalibration. For ICP–mass spectrometry (ICP-MS) and in addition to tube replacement, carbon entering the ICP forms “gas phase carbides” that often cause isobaric interferences and spectral interference effects.
In our NTV system, we use a metallic surface, so carbide formation is not an issue. And because our filaments are lightweight, they can be heated up very rapidly (for example, at 6000 ºC/s; in other words, we reach about 3000 ºC in 0.5 s) using a small and relatively inexpensive power supply. To provide an example here, the maximum power we use with Re coils is about 70–80 W and the typical power is 40–50 W (which is less than a typical incandescent light bulb). Furthermore, because we use a small vaporization chamber (roughly the size of an adult person’s thumb), the NTV vaporization chamber can be brought very close to the base of an ICP torch. Hence the name “near-torch vaporization.” This way, transport losses are minimized, thus making our sample introduction about 100% efficient. Finally, to my knowledge, my research group is the first to use coiled filaments (or any other support) for use in the field and analysis in the laboratory.
What are the major advantages of the coiled-filament NTV sample introduction system compared with traditional ICP sample introduction systems?
The main advantages of NTV over traditional graphite-based ETV are described above. As compared to conventional sample introduction (such as via a pneumatic nebulizer) there are three key advantages:
• Improved detection limits, which result from improved sample introduction efficiency,
• The ability to allow analytical determinations from limited amounts of sample (microliter or even nanoliter volumes). This ability is critical because there are many cases in which only a limited amount of sample is available for analysis (for example, in the analysis of samples of clinical or biological origin).
• Loading our portable coiled-filament assemblies with samples in the field and analyzing them in the lab opens up unique opportunities for applications that involve bringing part of the lab to the sample. This is something new that (at present and to my knowledge) no other sample introduction system can do.
What types of ICP spectrometry applications have benefited from the use of the NTV sample introduction system?
For field sampling, we primarily targeted difficult sample types (for example, sea water because of its high salt matrix and hard tap water also because of its high salt content) and elements for which ICP–atomic emission spectrometry (ICP-AES) does not have adequate detection limits (for example, Pb). We have demonstrated that using portable and lightweight coiled-filament assemblies, we can do electrochemical preconcentration and matrix cleanup and speciation (for example, Cr) essentially in the field, followed by measurement in the laboratory. Admittedly, thus far our research efforts have been primarily toward instrumentation development and toward understanding of the underlying principles of operation of NTV, rather than on analytical method development. If there is research funding available, I see no reason why method development cannot be targeted (and it should), thus enlarging the scope and range of the application of NTV-ICP.
What are the next steps in your laboratory’s research?
Although we have interfaced NTV to a number of commercially available ICP-MS systems, most of our work has been primarily focused on ICP-AES. Our next efforts will be on the analysis and characterization of nanomaterials using NTV and ICP-MS. For ICP-MS in particular, because we dry a sample that has been placed on a coiled filament before vaporizing it into an ICP (for detection by MS), there is no constant flow of water-solvent into the ICP (as would be the case if a pneumatic nebulizer was used). Thus we expect that a number of oxygen-related isobaric interferes in ICP-MS will be reduced or eliminated because of the use of “dry” sample introduction.
In complementary developments, we have developed a smaller-size vaporization chamber and coiled-filament sample introduction system that we use with our battery-operated microplasmas. The scalability of NTV expands the utility of this sample introduction approach, especially for applications in which we need to “bring a larger part of the lab to the sample.”