Quenching of the OH and nitrogen molecular emission by methane addition in an Ar capacitively coupled plasma to remove spectral interference in lead determination by atomic fluorescence spectrometry

摘要

A new method is proposed to remove the spectral interference on elements in atomic fluorescence spectrometry by quenching of the molecular emission of the OH radical (A~2Σ~+→X~2Π) and N2 second positive system (C~3Π_u →B~Σ_g) in the background spectrum of medium power Ar plasmas. The experiments were carried out in a radiofrequency capacitively coupled plasma (275 W, 27.12 MHz) by CH4 addition. The quenching is the result of the high affinity of OH radical for a hydrogen atom from the CH4 molecule and the collisions of the second kind between nitrogen excited molecules and CH4, respectively. The decrease of the emission of N2 second positive system in the presence of CH4 is also the result of the deactivation of the metastable argon atoms that could excite the nitrogen molecules. For flow rates of 0.71 min~(-1) Ar with addition of 7.5 ml min~(-1) CH4, the molecular emission of OH and N2 was completely removed from the plasma jet spectrum at viewing heights above 60 mm. The molecular emission associated to CH and CH2 species was not observed in the emission spectrum of Ar/CH4 plasma in the ultraviolet range. The method was experimented for the determination of Pb at 283.31 nm by atomic fluorescence spectrometry with electrodeless discharge lamp and a multichannel microspectrometer. The detection limit was 35 ng ml~(-1), 2-3 times better than in atomic emission spectrometry using the same plasma source, and similar to that in hollow cathode lamp microwave plasma torch atomic fluorescence spectrometry.