Single element commercial solutions of Fe, Mg, Cu and Al from four commercial sources have been analysed by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) in order to assess their performance as high accuracy calibration standards.

Each commercial standard was analysed three times to obtain mean values and quality control was maintained by repeat analysis of NIST certified reference materials. Combined uncertainty budgets have been calculated for each result to determine whether the uncertainty quoted by the manufacturers can be routinely attained.

The ID-ICP-MS procedure is based on the ‘matching’ method recommended by Henrion, but has been modified from a time and effort consuming iterative method to a single step analysis. It is designed to mitigate most of the problems associated with mass discrimination in the plasma interface. For each solution a combined uncertainty calculation was performed following Eurachem and ISO guidelines. This considers all realistically quantifiable uncertainties from the preparatory stages and repeatability of the measured isotope ratio or spectral data.

The results indicate that the majority of solutions can be accurately measured to a mean value that is statistically indistinguishable from that quoted on the certificate and with a combined standard uncertainty of <1%.

However, commercial calibration solutions continue to be sold primarily quoting the volumetric assay (microgram ml-1). Gravimetric sample preparation (microgram g-1) is the preferred method for ID-ICP-MS and so correction for density is required. In the absence of density values the addition of an on-site density measurement increases the time and cost of analysis and introduces an additional source of error.

Plasma Source Mass Spectrometry The New Millennium, Grenville Holland and Scott D Tanner, RSC, ISBN: 0-85404-895-2