The selection of internal standards for elements analyzed by a multielement direct-current argon plasma emission spectrometer was made by studying the elemental covariance of a multielement standard. Principal components factor analysis of many analytical determinations of this standard indicated that the elemental covariance clustered into two main groups. These groups were a function of instrumental variation, mainly plasma shape. The first principal factor accounted for 43.4% of the variance and had high loadings for strontium, barium, boron, manganese, nickel, titanium, silicon, calcium, and cadmium. The second principal factor accounted for an additional 36.7% of the variance and had high loadings for lithium, vanadium, magnesium, potassium, and aluminum. Compared with an external standard, internal standards dictated by the principal components factor analysis results gave improved precision. Internal standards not correlated with response variations degraded both the accuracy and the precision of the analysis.
G. E. Bentley, V. T. Hamilton, E. J. Peterson, and L. E. Wangen, "The Selection and Evaluation of Internal Standard Elements Based on Elemental/Instrumental Covariance," Appl. Spectrosc. 40, 949-953 (1986)