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

Abstract

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 N 2 second positive system (C 3Π u → B 3Σ 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 CH 4 addition. The quenching is the result of the high affinity of OH radical for a hydrogen atom from the CH 4 molecule and the collisions of the second kind between nitrogen excited molecules and CH 4, respectively. The decrease of the emission of N 2 second positive system in the presence of CH 4 is also the result of the deactivation of the metastable argon atoms that could excite the nitrogen molecules. For flow rates of 0.7 l min − 1 Ar with addition of 7.5 ml min − 1 CH 4, the molecular emission of OH and N 2 was completely removed from the plasma jet spectrum at viewing heights above 60 mm. The molecular emission associated to CH and CH 2 species was not observed in the emission spectrum of Ar/CH 4 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.