A new source of spectral excitation based on nitrogen microwave induced plasma with a frequency of 915 MHz for atomic emission spectrometry

An experimental prototype of a new spectral excitation source based on microwave induced plasma with an excitation field frequency of 915 MHz and a power of up to 3000 W has been created. The plasma in this source is excited by an H 011 wave in a cylindrical microwave resonator with a dielectric element installed inside and has a shape and dimensions close to those of an argon ICP. Compared to a traditional microwave plasma with an excitation frequency of 2450 MHz, the temperature of the resulting plasma is 300 K higher and amounts to approximately 5500 K. It has been shown that the intensities of analyte lines are 2 – 5 times higher compared to traditional MIP sources with a frequency of 2450 MHz, and the detection limits for most elements are lower. The influence of matrix elements on the magnitude of the analytical signal has been studied using Mg, Ca, K, and Na as examples. The matrix influence of the spectral excitation source with MIP (915 MHz) is similar to that of MIP (2450 MHz) and increases in the following order: Mg < Ca < K < Na. When 0.5% Na was introduced into the MIP (2450 MHz), the plasma was quenched. For the MIP (915 MHz), the matrix effect was less pronounced: for lines with energies >8 eV, the introduction of a 0.5% K solution reduced the analyte intensity by a factor of 5, while for the MIP (2450 MHz), it decreased by a factor of 10. The relative standard deviation was approximately 3% over 3 h of measurement. Despite some improvement in the performance of the new spectral excitation source with a plasma excitation frequency of 915 MHz, this is clearly insufficient to replace the traditional MIP source with an excitation frequency of 2450 MHz in commercial spectrometers.