Optimization of a Direct Sample Insertion Into a Stabilized Capacitive Plasma for Optical Emission Spectrometry (SCP-OES)

 A new sample insertion device for the stabilized capacitive plasma (SCP) has been developed, which enables it to analyze dry residues of micro amounts of liquid samples. Insertion was applied into an SCP as plasma source because of its good stability and excitation properties as well as its low instrument and operation costs. The plasma is sustained at a frequency of 27.12 MHz and an RF power of 150 W. For analysis the liquid samples are positioned at the tip of a quartz rod with the aid of a μL syringe. Then the sample is dried and the sampling rod inserted into the plasma. After optimization of the carrier gas flow (5 L/h) and the sample volume (20 μL) the detection limit for Pb with Ar as plasma gas is 200 pg. By further improving the guidance of the insertion detection limits for Pb, Cu, Cd and Mg in the 1 to 30 ng/mL range or 20 to 600 pg range absolute were obtained. It was found that the detection limits in the case of He are better than those obtained with Ar. The matrix interferences caused by changes in the concentration of the easily ionizable element Na were found to be below 10% for Na concentrations of up to 0.45 μg/mL. Ethanol concentrations of up to 14% in the analyte solutions did not cause any interferences. Received December 17, 1998. Revision June 4, 1999.