Open-File Report 97-382
By Carl M. Harris, Charles J. Litteral, and Donna L. Damrau
The U.S. Geological Survey National Water Quality Laboratory has developed a method for the determination of dissolved calcium, iron, magnesium, manganese, silica, and sodium using a modified ultrasonic nebulizer sample-introduction system to an inductively coupled plasma-optical emission spectrometer. The nebulizer's spray chamber has been modified to avoid carryover and memory effects common in some conventional ultrasonic designs. The modified ultrasonic nebulizer is equipped with a high-speed rinse cycle to remove previously analyzed samples from the spray chamber without excessive flush times. This new rinse cycle decreases sample washout times by reducing carryover and memory effects from salt or analytes in previously analyzed samples by as much as 45 percent. Plasma instability has been reduced by repositioning the argon carrier gas inlet on the spray chamber and by directly pumping waste from the chamber, instead of from open drain traps, thereby maintaining constant pressure to the plasma. The ultrasonic nebulizer improves signal intensities, which are 8 to 16 times greater than for a conventional cross-flow pneumatic nebulizer, without being sensitive to clogging from salt buildup as in cross-flow nebulizers. Detection limits for the ultrasonic nebulizer are 4 to 18 times less than detection limits achievable using a cross-flow pneumatic nebulizer, with equivalent sample analysis time.
The National Water Quality Laboratory (NWQL) purchased an ultrasonic nebulizer for use with a newly acquired Thermo-Jarrell Ash inductively coupled plasma-optical emission spectrometer (ICP-OES). The authors intended to investigate the capabilities of the nebulizer and determine the usefulness of this technology to the NWQL. It was hoped that this nebulizer could be used to improve the overall performance of the methods in use at the NWQL. The first phase of the project was to adapt the nebulizer for use in evaluating a low ionic-strength method to determine calcium, iron, magnesium, manganese, silica, and sodium in acid-rainwater samples. The second phase of the project was to adapt the nebulizer for use with the general 20-element ICP-OES scan for whole-water recoverable samples. The third phase would evaluate its possible use with the inductively coupled plasma- mass spectrometer systems used for ultratrace element scans. Because the ultrasonic nebulizer was to be used on a variety of systems and applications, a detailed description of all aspects of the nebulizer's performance was needed. Signal intensities, detection limits, method precision and bias, sample interferences, and sample carryover effects were investigated but a complete study was not possible. Budget cutbacks and a laboratory reorganization precluded completion of the second and third phases of the project. Only a small part of the second phase of the project was finished, consisting primarily of establishing fundamental instrument settings and preliminary detection limit data for the 20-element scan. No data were collected for the inductively coupled mass spectrometer. On the basis of the present (1997) research, the ultrasonic nebulizer could improve the performance of the mass spectrometer as well. However, much information was gained before the project was terminated. This report summarizes the results.
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