THE EFFECT OF 2 GASES FORMING SUPERCRITICAL FLUIDS (XE AND CO2) ON THE SPECTRAL CHARACTERISTICS AND ANALYTICAL CAPABILITIES OF MICROWAVE-INDUCED PLASMAS

A comparative study of the effect of CO2 and Xe added along with the p lasma gas to He and Ar microwave induced plasmas (MIPs), simulating po ssible conditions to be used when a MIP is employed as specific detect or for supercritical fluid chromatography (SFC), has been carried out. The proportions of CO2 and Xe to the plasma gas investigated are comp arable to the typical percentages used for SFC-MIP couplings. The stud y has been performed with two different MIP systems: an atmospheric pr essure discharge held in a Beenakker cavity TM(010) and a reduced pres sure surfatron-MIP. The influence of CO2 and Xe addition on the spectr ochemical properties of the discharge has been studied by using the at omic emission of mercury and some typical non-metals (chlorine, carbon and sulfur) at different wavelengths (atomic and ionic lines). Result s showed that ion line emission intensities are always reduced more si gnificantly than atom line emissions by both dopant gases on study, wh atever the pressure. In general terms, however, the effect of adding X e is less severe, both for atom and ion lines, than that of CO2; in mo st cases the detection limits (DLs) observed are better for Xe than fo r CO2 as dopant gas. In fact, the DLs obtained for the selected lines of mercury measured were practically unaltered by the addition of 0.2% Xe to atmospheric pressure Ar or He MIPs. CO2 addition (0.2%) produce d about 1.5 times worsening of the observed DLs for mercury. For non-m etal analyses better DLs were also obtained, in general terms, with Xe than with CO2 as dopant gas.