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Direct nebulization of an aqueous sample was successfully carried out in our laboratory in a lab-built low pressure-inductively coupled plasma (LP-ICP) using a double membrane desolvator (DMD). For the particular LP-ICP used, rotational temperatures of selected diatomic molecules and excitation temperatures of Fe II and Fe I were measured. The OH radical showed two rotational temperatures, low and high. The former were obtained in the range 800 to 1100 K, while the latter were obtained between 1850 and 2800 K. The molecular nitrogen ion consistently obtained the same temperatures as those of the high rotational temperatures of the OH radical. This result suggests that the high density of rotational states of these molecules, N+2 and OH radical, allows collisional energy exchange between molecular rotational and argon translational degrees of freedom in the plasma. An increase of plasma gas flow from 0.3 to 1.1 L min−1 produced a decrease of excitation temperatures of Fe II from 10400 to 9200 K. The excitation temperature of Fe I was measured to be about 6000 K. Noticeably, the excitation temperature of Fe ion was high, and even higher than that reported in an atmospheric ICP. This experiment provides proof that the mechanism of ionization and excitation was influenced mainly by the character of electrons in the low-pressure plasma.
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