Design, construction, and characterization of a six-electrode, direct current, variable length plasma source for atomic emmission spectroscopy

Graduation date: 1988The variable length, six-electrode plasma source for atomic\ud emission spectroscopy described here is operated from three\ud compact, simple, and inexpensive direct current power supplies. The\ud vertical arcs formed between the three electrode pairs completely\ud entrain the sample and are typically operated at 40 V and 20 A.\ud Three concentric quartz tubes, similar to the ICP, supply argon, as\ud well as sample, to the plasma. The argon consumption rate of\ud typically 7.4 L/min is comparable to or less than the consumption\ud rates of commercial ICP and DCP systems and is achieved without the\ud need for water cooling of any of the components. The plasma\ud assembly is completely demountable to facilitate easy replacement\ud of damaged parts and to provide experimental flexibility.\ud Spatial shifts in the vertical emission profiles, similar to\ud those documented for the ICP, occur with changes in the nebulizer\ud gas flow rate and current. The MgII 280.27-nm emission and\ud signal-to-background ratio (S/B) increase by factors of 23 and 10,\ud respectively, for an increase in the current from 16 to 25A. A\ud twofold increase in the MgII 280.27-nm S/B occurs for an increase\ud in the plasma length from 14 to 21.5 mm. Detection limits for five\ud elements range from 15 to 50 times higher than those for the ICP.\ud These detection limits are achieved without the use of ceramic\ud sleeves. Movement of the region of maximum emission to positions\ud below the top of the outer quartz tube, which occur with the\ud lengthening of the plasma, is thought to be the main reason for the\ud poorer detection limits. Adding 10% nitrogen to the outer argon\ud flow causes a 50% enhancement of the Ca ion S/B.\ud The effect of Na on Ca atom emission is directly opposite to\ud that for the ICP. The Ca atom vertical profile with Na is depressed\ud in the region up to 10 mm above the sample bullet, and crosses over\ud to enhancement at higher regions. Plasma length exhibits a\ud significant effect on both the Na and P interference on Ca.\ud Careful selection of the plasma length and observation height can\ud be used to virtually eliminate the interferences