Analytical Evaluation of a Reduced-Pressure Microwave-Induced Plasma Studied by Optical Emission Spectrometry Method

ABSTRACT

The authors describe a system that utilizes a reduced-pressure (RP) air-cooled microwave-induced plasma (MIP) torch to interface an ultrasonic nebulizer (USN) with an optical emission spectrometer (OES). Argon was investigated as plasma gas. The analytical potential of such techniques was illustrated for the determination of elements. A univariate approach and simplex optimization procedure was used to achieve optimized conditions and derive analytical figures of merit. Analytical performance of the RP-MIP was characterized by determination of the limits of detection (LODs) and precision (RSDs) with the RP-MIP-OES observed at flow rate of 10 µL min⁻¹ without removal of any matrix. The experimental concentration detection limits for simultaneous determination, calculated as the concentration giving a signal equal to three times the standard deviation of the blank (LOD, 3σ_blank criterion, peak height), were 15, 4.5, 6.2, 2.9, 31, 6.3, 3.1, 13, 5.4, and 33 n g mL⁻¹ for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. Absolute limits of detection were 167, 50, 68, 32, 350, 69, 34, 143, 59, and 363 pg for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. The method offers relatively good precision (RSD ranged from 7 to 12%) for liquid analysis and microsampling capability. The accuracy of the method was verified by the use of digested certified reference materials (SRM 1648 (Urban Particulate Matter), IAEA 336 (Lichen), SRM 2710 (Montand Soil), INCT-SBF-4 (Soya Bean Flour)) and by aqueous standard calibration technique. The analyte concentrations in reference materials were in satisfactory agreement with the certified values. The method requires small amounts of reagents and reduces contamination and losses. In general, low-pressure argon discharges proved to be superior, in terms of detection limits (DLs), to atmospheric pressure MIPs for the excitation of the analyte atomic or ionic emission.

KEYWORDS:

• Échelle spectrometer
• elements’ determination
• reduced-pressure microwave-induced plasma
• reference materials

ACKNOWLEDGMENTS

The authors are grateful for financial support by the Ministry of Science and Higher Education, Poland (Grant No. N N204 130935). The assistance and cooperation of ERTEC, Wrocław, Poland, in obtaining vacuum plasma torch is acknowledged.

Notes

^a Response, peak height of the element emission intensity.

^a For sample weights of 300 mg, sample volume 10 mL.

^b Sample solution flow rate of 10 µL.

^c Sample solution flow rate of 14 µL.

^a Non-certified value.

^b Below limit of detection.