Plasma sources with multiple electrodes have been developed and
characterized in this research for atomic emission spectroscopy.
Multi-electrode plasma sources use electrodes to shape the plasma in
order to continuously entrain a sample stream in the center of hot
plasma, in a manner similar to an inductively coupled plasma (ICP)
source. By changing the electrode spacings, the residence time can be
readily adjusted even during plasma operation. Fora 25-mm plasma
length, a relatively long residence time of the analyte species (23
msec) in the plasma was calculated, indicating sufficient time for
desolvation, vaporization, and atomization of the sample.
For a 30-mm plasma length, the interference effect of either
phosphate or aluminum on calcium emission at 393.37 nm is
insignificant over the entire vertical region of the plasma. The
effect of an easily ionized element (EIE) is to generally enchance
Ca(II) emission in the lower regions, and to depress the emission in
the higher regions in the plasma, which is in general agreement with
observations in an ICP. Under an assumption of local thermal
equilibrium (LTE), the Fe excitation temperature and the electron
number density in the center of the plasma at 8 mm above the top of
the quartz tube are 5000 K and 1.0x10¹⁵ cm⁻³ for a 30-mm plasma.
When current, gas flow rate, and plasma length are varied, the
spatial emission profiles reflect many of the characteristics
associated with an ICP. The short term stability of the plasma with a
blank solution is better than 2% (relative standard deviation). A
total of less than 6 L/min of argon gas is required to operate the
plasma. With the vertical 4-electrode plasma source, 4 to 5 orders of
magnitude of linear dynamic range are obtained and detection limits
for 12 elements are comparable to those obtained with an ICP