Reference materials (RMs), including NIST Standard Reference Materials (SRMs) are used for quality assurance, traceability of measurements and instrument and method validation. The Cryogenic Reference Material Production Facility at the Hollings Marine Laboratory houses specialized equipment required to generate fresh-frozen RMs, as well as multiple innovative tools used for processing unique materials. The distinctive ISO Class 7 clean room setting helps eliminate external sources of contamination, an important consideration when generating large batches of homogenous materials. To date, the facility staff have assisted in the development and production of more than 20 different reference materials, including dietary supplements, wood products, sediment and coal dust as well as many marine species (mussels, kelp, aquacultured seafood and wild-caught seafood).
Large-batch cryogenic reference materials are produced using a Palla VM-KT Vibrating Cryomill. The mill is pre-cooled to cryogenic temperatures using liquid nitrogen. Frozen material is fed through the inlet funnel to a vibrating chamber containing 26 titanium rods. The rods pulverize the material into a fine powder. The fresh-frozen product is then extruded from the chamber through the outlet below.
The particle size distribution of the resultant material is evaluated using laser-diffraction. These data help assess homogeneity based on size and are used as a process control indicator to determine if additional milling is required. Laser diffraction particle size analysis (PSA) is also used to assign informational values to existing RMs that require particle size data.
Smaller batches of RM can be homogenized either cryogenically or at room temperature using polytetrafluoroethylene (PTFE) disk mills. Material is added to the disk mill in small pieces and loaded into a shaker. High force oscillations pulverize the material into a fine powder
Staff are also developing and implementing protocols for generating custom RMs. Using techniques such as blending of multiple materials or amending matrices with endogenous analytes, in-house RMs can be generated when natural sources of material cannot be procured. These techniques can also be used to develop ‘rapid-response’ RMs to aid the measurement community in the event standardization of a specific measurement is immediately required and there is no time to collect and process large amounts of naturally- sourced material.
*Candidate materials in progress