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Analyte excitation and ionization in the argon inductively coupled plasma

University of British Columbia

A new approach to evaluating departures from local thermal equilibrium (LTE) in the inductively coupled plasma (ICP) is described. Experimentally evaluated electron density (n[sub e]) is used as the basis for an LTE framework. Spatially resolved, radial electron density profiles were measured under a variety of plasma operating conditions. The variation of n[sub e] with spatial position, aerosol flow rate, input power and presence of easily ionizable elements was determined. The measured n[sub e] was used to calculate an LTE temperature, T[sub e,LTE]. This temperature was higher than most spectroscopically measured temperatures in the plasma. LTE ion-atom emission intensity ratios for Sr, Ca, Mg, Cd and Zn were calculated from n[sub e] and T[sub e,LTE] compared to experimental values. Ion-atom ratios were within an order of magnitude of LTE values and, in most cases, less than the LTE values. The infrathermal ion-atom ratios were interpreted as being due to an overpopulation of the atom levels relative to LTE populations. Experimental degree of ionization was evaluated for a number of elements and compared to LTE values. Elements were underionized relative to LTE values. Degree of ionization and departure from LTE were correlated with ionization potential of the element. Elements with a high ionization potential had lower degrees of ionization and exhibited greater departures from LTE than elements of lower ionization potential. The underionization and infrathermal ion-atom intensity ratios are attributed to overpopulation of atom levels. The experimental observations and results of other plasma diagnostic studies are discussed in terms of a partial LTE (p-LTE) model for the ICP. The ion ground state and upper energy levels of atoms are in Saha equilibrium. Lower atomic levels are over populated with respect to these levels. The overpopulation of the lower levels is a consequence of radiative depopulating processes, such as radiative decay and radiative recombination, not being balanced by their inverse absorption processes. For low lying atomic levels where radiative processes make significant contribution to the total depopulation rates this improper balance causes deviations from LTE populations. The atom ground state is the most overpopulated level and overpopulation decreases with increasing energy of the level until p-LTE is reached for the upper atomic levels.

Text

2010-07-30

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http://hdl.handle.net/2429/27038

Chemistry

University of British Columbia

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