Validation and accreditation questions

You asked

1)    must the process of sample size reduction for analysis be included in the validation process for accreditation?

I understand it the sample size reduction is part of sample preparation, not sampling (from source), so yes that step must be included in your method validation. The sample preparation variability must be evaluated. 

You also asked

2)    can you validate this as one method, even though different size reduction requirements and dilutions are used prior to analysis?"

You can validate as one method if the same in-house or standard testing method is followed, and the dissolution and analytical part of the validation is common,

As an example. an accredited would be stated as Procedure for the dissolution of crushed ore, waste ore and concentrate (final product) with analysis by ICP-OES. For accreditation you must specify the Material or product tested, the Standard Specifications, Techniques / Equipment used, the Type of Tests / Properties Measured, and Range of Measurement (if applicable, for example your limit of detection is a restriction).