General Application Method Development and Validation

Identification of sources of analytical bias in method development and method validation is another very important application of reference materials in geochemical laboratories. USGS applied simplex optimization in establishing the best measurement conditions when the ICP-AES method was introduced as a substitute for AAS in the rapid rock procedure for major oxide determinations (Leary et al. 1982). The optimized measurement parameters were then validated by analyzing a number of USGS rock reference samples for which reference values had been established first by classical analyses. Similar optimization of an ICP-AES procedure for a number of trace elements was validated by the analysis of U S G S manganese nodule P-i (Montaser et al. 1984). 

Another excellent example of the use for method development and validation appears in Morrison and Richardson (1996). Their laboratory was analyzing many samples of Li ore and related samples for Ba, among other elements, using a routine XRF procedure. The reference sample chosen as a control sample for the run was the zinnwaldite ZWC (Govindaraju et al. 1994), for which analyses produced a value approximately twice the reference value. Investigation of that result identified a Rb overlap in the X-ray spectrum that had not previously been observed in use of the method. 

Similarly, some INAA data contributed to the derivation of a reference value for Ba in SDO-i were biased high by an interference from Ru (Wandless 1993). The Ru is a fission product of U, whose concentration of 40 qg/g is relatively high in SDO-1. In this case, no appropriate reference sample was available for analysis to control the SDO-1 results the interference was identified through the disagreement between INAA data and data produced using XRF and ICP-AES methods on the same sample. A bias-free method again resulted when analysis of an atypical type led to detection of a rarely encountered but sizeable spectral overlap. Once identified, correction was straightforward. 

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