Optimization of the technique of standard additions for inductively coupled plasma mass spectrometry

The use of standard additions in the presence of instrument drift and the o ptimum size of the added spike relative to the unknown analyte concentratio ns have been investigated for ICP-MS. In particular, a bracket approach, wh ere the spiked sample is measured between two different measurements of the sample, has been investigated. The average of the two sample measurements is used in the standard additions formula to estimate concentration. Severa l multi-element analyte solutions with single element matrices (Na, Cs, Ba) , were analyzed using both the bracket approach and regular standard additi ons. It was found that the bracket approach led to better results where dri ft was significant. In addition, optimum spike size was investigated. Simpl e models predict that determinations would be more precise with larger spik es if the instrument response was linear and RSD was constant. These result s show that while the use of larger spikes (from 7 to 50x the unknown conce ntration) did not yield the better precision predicted by the models, the p recision was no worse than for spikes of size equal to the unknown concentr ation. The Autonomous Instrument project is an approach to the automation o f ICP-MS based on choosing an appropriate analytical calibration methodolog y for an unknown sample. The method of standard additions is the most accur ate analytical methodology considered by the Autonomous Instrument. These r esults have implications for the Autonomous Instrument, suggesting that bra cket standard additions should be considered the best method, followed by r egular standard additions. In addition, the spike size results imply that i n automatic determination, the long linear range of ICP-MS allows the addit ion of large analyte spikes with minimal prior knowledge of the sample.