Real-time internal standardization with an axially-viewed inductively coupled plasma for optical emission spectrometry

An evaluation of precision improvements using real-time internal standardization with an axially-viewed inductively coupledplasma (ICP) is presented. New findings are presented with respect to the nature of the noise in the analytical signals from theaxial ICP. It is observed that a high degree of correlation exists in the line signals from the axial ICP. Using the yttrium ion lineat 371.030 nm as the internal standard, the analytical precision after the application of real-time internal standardization ismaintained between 0.1 and 0.2% relative standard deviation (RSD) for ion lines. Precision improvement factors of 3 to 4 areobtained by comparison with the uncorrected results. With atomic lines, real-time internal standardization using the yttrium ionline is less effective, yielding precision values between 0.2 and 0.7% RSD. The precision improvement factors for atomic linesare between 1.5 and. 3. Thus, real-time internal standardization provides significant improvements in the RSDs of the linesignals. The limits of these improvements are explored and an equation is presented which yields the fundamental shot noiselimit for precision. Shot noise limited precision is demonstrated. However, this is not possible for all elements using a singleinternal standard signal. The effectiveness of real-time internal standardization is shown to be dependent on the nature of thespecific spectral line. With the axially-viewed ICP, the dominant phenomenon preventing the full benefit of internal standardizationfrom being obtained is the amplitude of the noise in the line signals and not the degree of correlation between analyteand internal standard signals. A trend is observed for atomic transitions in which lower excitation energy is correlated withhigher relative noise amplitudes. This finding is in contrast with previously published work on the radially-viewed ICP. Anexplanation of this result is proposed which takes into account the influence of vaporizing sample droplets in the observationvolume