Characterizing a nitrogen microwave inductively coupled atmospheric-pressure plasma ion source for element mass spectrometry

Abstract

A high-power nitrogen-based microwave inductively coupled atmospheric-pressure plasma was coupled to a quadrupole mass spectrometer to investigate its characteristics as an ion source for element mass spectrometry. The influence of operating conditions on analyte sensitivity, plasma background, and polyatomic ion formation was investigated for conventional solution-based analysis. By varying the forward power and the nebulizer gas flow rate, the plasma background ions were found to decrease with increasing gas flow rates and decreasing operating power. Analyte ions showed different trends, which could be related to the physical–chemical properties of the elements. We could identify three groups based on the location of maximum intensity in the power vs. flow rate contour plot. Atomic ions of elements with low first ionization energy and low oxygen bond strength were found to maximize at a high nebulizer gas flow rate and lower microwave power. Elements with intermediate ionization energy and higher oxygen bond strength required higher power settings for optimum sensitivity, while elements with the highest ionization energies required the highest power and lowest gas flow rates for their optimization. The latter group showed a substantial suppression in sensitivity compared to elements of similar mass, which is considered to result from the high abundance of NO in the plasma source, whose ionization energy is close to that of these elements. Metal oxide ions were found at similar or higher abundances than in the conventional argon-based ICP and could be minimized only by using a low gas flow rate and high power settings. These general trends were also observed when the vacuum interface was modified. To change the dynamics of the supersonic expansion, different sampler cone orifice sizes and sampler–skimmer distances were investigated and the interface pressure was lowered through an additional pump. These modifications did not yield significant differences in ion transmission but lowering the interface pressure reduced the relative abundance of metal oxide ions. The limits of detection were evaluated for optimized plasma conditions and found comparable to those of an argon ICP source with the same mass spectrometer.