Cool plasma conditions

Isobaric interferences (especially those arising from the plasma itself, e.g., ArO+ on Fe) can be eliminated using cool-plasma conditions, sometimes in combination with a shield torch. This option is not suitable for seawater samples because a cool plasma, in the presence of a heavy matrix, cannot fully ionize elements with high first ionization potentials, notably Zn, Cd, and Hg. Protocols have thus been established for analysis of 10-fold diluted seawater on instalments with sufficiently high resolution to separate most of the affected isotopes from their isobaric interferences [1], To circumvent the issue entirely, others have used online chemical extraction to separate analytes of interest... 

Although this technology is effective in resolving a wide range of polyatomic interferences, the increased cost associated with this type of instrumentation (more than twice the price of a quadrupole instrument) limits its use in most routine laboratories, hence alternative methods of interference reduction have been sought for. The use of chemical extraction and chromatography (in order to separate the analyte from the matrix prior to analysis) or the operation of the ICP-MS under so-called cool plasma conditions, allows the elimination of... 

For the easily ionized elements working at so-called cool plasma conditions has been shown to be very successful. From the calculation of the degrees of ionization... 

Cool plasma conditions are obtained at low r.f. power and high carrier gas flow rate. As these are conditions that favor the occurrence of secondary electrical discharges between the ICP and the sampling cone, some instruments require a grounded shield between the ICP torch and the load coil to decouple these... 

Vanhaecke, F., Saverwyns, S., de Wannemacker, G., Moens, L., and Dams, R. (2000) Comparison of the application of higher mass resolution and cool plasma conditions to... 

Using cool plasma conditions AU instruments today have the ability to use... 

The cold/cool plasma approach, which uses a lower temperature to reduce the formation of the argon-based interferences, has been a very effective way to get around some of these problems. However, this approach can sometimes be difficult to optimize, is only suitable for a few of the interferences, is susceptible to more severe matrix effects, and it can be time consuming to change back and forth between normal and cool plasma conditions. These limitations and the desire to improve performance have led to the commercialization of collision/reaction cells (CRC) and collision/reaction interfaces (CRl). Designs for CRC and CRl were based on the early work of Rowan and Houk, who used Xe and CH4 in the late 1980s to reduce the formation of ArO+ and Ar2 species in the determination of Fe and Se with a modified tandem mass spectrometer. This research was investigated further by... 

FIGURE 14.3 Spectral scan of 100 ppt and deionized water using cool plasma conditions. (From S. D, Tanner, M. Paul, S. A. Beres, and E. R. Denoyer, Atomic Spectroscopy, 16[1], 16, 1995.)... 

Unfortunately, even though the use of cool plasma conditions is recognized as being a very useful tool for the determination of a small group of elements, its limitations are well documented. A summary of the limitations of cool plasma technology includes the following ... 

FIGURE 14.5 Matrix suppression caused by increasing concentrations of HNO3 using cool plasma conditions (RF power 800 W, nebulizer gas 1.5 L/min). (From J. M. Collard, K. Kawabata, Y. Kishi, and R. Thomas, Micro, January 2002.)... 

All of the instruments on the market can be set up to operate under cool or cold plasma to achieve very low DLs for elements such as K, Ca, and Fe. Cool plasma conditions are achieved when the temperature of the plasma is cooled sufficiently low enough to reduce the formation of argon-induced polyatomic species." This is typically achieved with a decrease in the RF power, an increase in the nebulizer gas flow, and sometimes a change in the sampling position of the plasma torch. Under these conditions, the formation of species such as AC, ArH+, and " Ar is dramatically reduced, which allows the determination of low levels of " Ca and Fe respectively. ... 

Under normal hot plasma conditions (typically, RF power of 1200-1600 W and a nebulizer gas flow of 0.8-1.0 L/min), these isotopes would not be available for quantitation because of the argon-based interferences. Under cool plasma conditions (typically, RF power of 600-800 W and a nebulizer gas flow of 1.2-1.6 L/min), the most sensitive isotopes can be used, offering low ppt detection in aqueous matrices. However, not all instruments offer the same level of cool plasma performance, so if... 

The ArO+ BEC at mass 56 will be a good indication of the DL for Fe+ under cool plasma conditions. The BEC value will typically be about an order of magnitude greater than the DL. 

Although most instruments offer cool plasma capability, there are subtle differences in the way it is implemented. It is therefore very important to evaluate the ease of setup and how easy it is to switch from cool to normal plasma conditions and back in an automated multielement run. Also, remember that there will be an equilibrium time in switching from normal to cool plasma conditions. Make sure you know what this is, because an equivalent read-delay will have to be built into the method, which could be an issue if speed of analysis is important to you. If in doubt, set up a test to determine the equilibrium time by carrying out a short stability run while switching back and forth between normal and cool plasma conditions. 

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